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APPROVED:
Oliver M. R. Chyan, Major Professor Justin Youngblood, Committee Member Rob Petros, Committee Member William E. Acree, Committee Member and
Chair of the Department of Chemistry Mark Wardell, Dean of the Toulouse Graduate
School
FUNDAMENTAL STUDIES OF COPPER BIMETALLIC CORROSION IN ULTRA
LARGE SCALE INTERCONNECT FABRICATION PROCESS
Simon Kibet K oskey, B.Sc.
Dissertation prepared for the Degree of
DOCTOR OF PHILOSOPHY
UNIVERSITY OF NORTH TEXAS
May 2014
Koskey, Simon Kibet. Fundamental Studies of Copper Bimetallic Corrosion in Ultra
Large Scale Interconnect Fabrication Process. Doctor of Philosophy (Chemistry-Analytical
Chemistry), May 2014, 113 pages, 2 tables, 55 figures, chapter references.
In this work, copper bimetallic corrosion and inhibition in ultra large scale interconnect
fabrication process is explored. Corrosion behavior of physical vapor deposited (PVD) copper on
ruthenium on acidic and alkaline solutions was investigated with and without organic inhibitors.
Bimetallic corrosion screening experiments were carried out to determine the corrosion rate.
Potentiodynamic polarization experiments yielded information on the galvanic couples and also
corrosion rates. XPS and FTIR surface analysis gave important information pertaining inhibition
mechanism of organic inhibitors. Interestingly copper in contact with ruthenium in cleaning
solution led to increased corrosion rate compared to copper in contact with tantalum. On the
other hand when cobalt was in contact with copper, cobalt corroded and copper did not. We
ascribe this phenomenon to the difference in the standard reduction potentials of the two metals
in contact and in such a case a less noble metal will be corroded.
The effects of plasma etch gases such as CF4, CF4+O2, C4F8, CH2F2 and SF6 on copper
bimetallic corrosion was investigated too in alkaline solution. It was revealed that the type of
etching gas plasma chemistry used in Cu interconnect manufacturing process creates copper
surface modification which affects corrosion behavior in alkaline solution. The learning from
copper bimetallic corrosion studies will be useful in the development of etch and clean
formulations that will results in minimum defects and therefore increase the yield and reliability
of copper interconnects.
Copyright 2014
by
Simon Kibet Koskey
ii
ACKNOWLEDGEMENTS
First and foremost I would like to thank the Almighty God for his favor upon my life and
for making this entire journey to be possible. Secondly, I wish to express my heartfelt gratitude
to my major advisor, Professor Oliver Chyan, for he has been more than an advisor to me. He
gave me an opportunity to work with him, provided research guidance, kept looking out for my
professional career and above all offered a caring ear. His professionalism, technical expertise
and personal integrity are some of his qualities I will always strive to emulate. I am truly grateful
that I have been part of his research family. I’m also grateful to my committee members for
taking their time and being there to see me through this journey. I would also like to give thanks
to Dr. Kanwal Jit Singh of Intel for his guidance and encouragement throughout and specifically
trusting me with projects and ensuring that I get everything I needed to succeed during my
internship at Intel. Many thanks go to my present and former research colleagues Dr. Yu and Dr.
Pillai, Pofu, Sirish, Tamal, Jafar, Nick and Arindom for the encouragement, intellectual help and
support throughout these years.
Finally, I want to thank my family members for without them this would not have been
possible. I am deeply indebted to my family for their material, moral and spiritual support during
my entire time in school. To my parents, Samson and Grace, the sacrifices you made for me and
my siblings cannot be repaid and no thank you is enough, my friend and loving sister Hellen
Chelangat for always being there for me, believing in me and always encouraging me to aim
higher, my cousin Eunice Kimunai, thanks you for your love and kindness, your kindness will
forever be in my heart. To my siblings, John, Mary, Julius, Ben and Mark, thank you so much for
your unconditional love and support. May God bless you all.
iii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ........................................................................................................... iii
LIST OF FIGURES ..................................................................................................................... viii
CHAPTER 1: INTRODUCTION AND INSTRUMENTATION .................................................. 1
1.1 Introduction ........................................................................................................................... 1
1.2 Instrumentation...................................................................................................................... 5
1.2.1 Electrochemistry ............................................................................................................. 5
1.2.1.1 Electrochemistry of Corrosion ................................................................................ 5
1.2.1.2 Tafel Plots ............................................................................................................... 8
1.2.1.3 Electrochemical Impedance Spectroscopy (EIS) .................................................... 9
1.2.1.4 Rotating Disk Electrode System (RDE) ............................................................... 11
1.2.2 X-ray Photon Spectroscopy .......................................................................................... 11
1.2.3 Contact Angle ............................................................................................................... 15
1.2.4 Thin Film Deposition.................................................................................................... 16
1.2.4.1 Chemical Deposition ............................................................................................. 17
1.2.4.2 Physical Vapor Deposition (Sputtering) ............................................................... 18
1.2.5 Micropattern Corrosion Screening Technique ............................................................. 20
1.3 References ........................................................................................................................... 22
CHAPTER 2: COPPER INTERCONNECT PROCESSING ....................................................... 25
2.1 Introduction ......................................................................................................................... 25
2.2 Interconnect Integration ...................................................................................................... 26
iv
2.3 Process Flow ....................................................................................................................... 27
2.3.1 Tungsten Vias Fabrication ............................................................................................ 27
2.3.2 Dual Damascene Copper Process ................................................................................. 29
2.3.3 Dielectric Patterning ..................................................................................................... 32
2.3.4 Low k Dielectrics .......................................................................................................... 34
2.3.5 Metallization ................................................................................................................. 36
2.3.5.1 Copper Diffusion Barrier Deposition.................................................................... 36
2.3.5.2 Copper Deposition ................................................................................................ 39
2.3.6 Chemical Mechanical Polishing (CMP) ....................................................................... 41
2.3.7 Cu Interconnects Reliability ......................................................................................... 44
2.3.7.1 Electromigration ................................................................................................... 44
2.3.7.2 Stress Induced Voiding ......................................................................................... 46
2.4 Summary ............................................................................................................................. 46
2.5 References ........................................................................................................................... 47
CHAPTER 3: BIMETALLIC CORROSION BEHAVIOR OF COPPER ON RUTHENIUM
AND COBALT ON COPPER THIN FILMS IN POST CMP CLEANING SOLUTIONS ........ 53
3.1 Introduction ......................................................................................................................... 53
3.2 Metal Corrosion................................................................................................................... 54
3.3 Experimental ....................................................................................................................... 58
3.3.1 Micropattern Corrosion Screening ............................................................................... 58
v
3.3.2 Tafel Plots ..................................................................................................................... 60
3.3.3 Optical Profilometry ..................................................................................................... 61
3.4 Results and Discussion ........................................................................................................ 61
3.4.1 Bimetallic Corrosion of Cu on Ru in Post CMP Cleaning Solution ............................ 61
3.4.2 Bimetallic Corrosion of Co on Cu in Acidic Post CMP Cleaning Solution ................. 64
3.4.3 Activation Studies of Inhibitors 5 and 6 in Acidic Post CMP Cleaning Solution ........ 66
3.5 Summary ............................................................................................................................. 69
3.6 References ........................................................................................................................... 69
CHAPTER 4: STUDY OF PYRAZOLE AS COPPER CORROSION INHIBITOR IN MODEL
ALKALINE POST CHEMICAL MECHANICAL POLISHING CLEANING SOLUTION ..... 71
4.1 Introduction ......................................................................................................................... 71
4.2 Experimental ....................................................................................................................... 75
4.3 Results and Discussion ........................................................................................................ 77
4.3.1 Effect of Substrate on Cu Corrosion............................................................................. 77
4.3.2 Cu Micropattern Corrosion and Inhibition ................................................................... 79
4.3.3 Electrochemical Analysis ............................................................................................. 82
4.3.3.1 Tafel Plots ............................................................................................................. 82
4.3.3.2 Electrochemical Impedance Spectroscopy (EIS) .................................................. 83
4.3.4 Water Contact Angle Measurement ............................................................................. 84
4.3.5 Surface Analysis ........................................................................................................... 86
vi
4.3.5.1 XPS Analysis ........................................................................................................ 86
4.4 Proposed Mechanism of Cu Corrosion Inhibition .............................................................. 89
4.5 Summary ............................................................................................................................. 89
4.6 References ........................................................................................................................... 90
CHAPTER 5: INTERFACIAL CHARACTERIZATION OF PLASMA TREATED COPPER
SURFACES RELATED TO ADVANCED COPPER INTERCONNECTS ............................... 94
5.1 Introduction ......................................................................................................................... 94
5.2 Experimental ....................................................................................................................... 97
5.3 Results and Discussion ...................................................................................................... 100
5.3.1 Micropattern Corrosion Study .................................................................................... 100
5.3.2 Bimetallic Contact Effect ........................................................................................... 102
5.3.3 Direct Galvanic Current Measurements ..................................................................... 103
5.3.4 Water Contact Angle Measurements .......................................................................... 104
5.3.5 XPS Analysis of Plasma Treated Cu .......................................................................... 106
5.4 Effect of Corrosion Inhibitor-Benzotriazole ..................................................................... 109
5.5 Summary ........................................................................................................................... 110
5.6 References ......................................................................................................................... 111
vii
LIST OF FIGURES
Figure 1.1 Schematic of one level of transistor and two levels of interconnect ............................. 2
Figure 1.2 Schematic of via and metal line..................................................................................... 3
Figure 1.3 SEM micrographs of interconnect architecture with 6 levels of Cu lines/vias, W contacts/local interconnects and SiO2 ILD by (a) IBM [4] and (b) Motorola [5] ........................... 4
Figure 1.4 Schematic of electrochemical cell showing anodic and cathodic sites [17] .................. 6
Figure 1.5 Three electrode system electrochemical cell ................................................................. 7
Figure 1.6 Tafel plot showing Ecorr, Icorr, cathodic and anodic curves [12] ................................... 9
Figure 1.7 DC and AC currents in corroding systems (Wikipedia) .............................................. 10
Figure 1.8 Nyquist plot and equivalent circuit [21] ...................................................................... 11
Figure 1.9 Basic components of XPS system (Wikipedia) ........................................................... 12
Figure 1.10 PHI 5000Versa Probe ................................................................................................ 14
Figure 1.11 Schematic of contact angle of liquid droplet [27] ..................................................... 16
Figure 1.12 Dual magnetron sputtering system ............................................................................ 19
Figure 1.13 Schematic of principle of sputtering process [31] ..................................................... 19
Figure 1.14 Micropattern corrosion screening structure. .............................................................. 21
Figure 2.1 Device delay and interconnect delay as a function of feature size [4] ........................ 26
Figure 2.2 Typical cross section of hierarchical scaling of Cu[6] ................................................ 27
Figure 2.3 Tungsten vias fabrication process flow [9] ................................................................. 28
Figure 2.4 SEM micrograph of Cu interconnect architecture by IBM [15] .................................. 30
Figure 2.5 Single and dual Damascene processes [4] ................................................................... 31
Figure 2.6 Dual Damascene schemes for defining trenches and vias: (a) buried etch stop, (b) clustered approach (c) partial via first (d) full via first (e) line first [17-21] ................................ 33
Figure 2.7 Ideal and typical step coverage of barrier material deposited by PVD [32] ............... 38
viii
Figure 2.8 Schematic of Cu electroplating system [42] ................................................................ 40
Figure 2.9 Schematic of chemical mechanical polishing system [45] .......................................... 43
Figure 2.10 Possible failure mechanism for Cu interconnect [55] ............................................... 45
Figure 3.1 Dual damascene process .............................................................................................. 54
Figure 3.3a Micropattern corrosion screening sequence .............................................................. 59
Figure 3.3 b Micropattern corrosion screening time lapsed images of Cu on Ru in 0.1M NH4OH pH 2 solution ................................................................................................................................. 60
Figure 3.4 Corrosion rate of Cu/Ru in alkaline post CMP cleaning solution without inhibitors [12]. ............................................................................................................................................... 62
Figure 3.5 Corrosion rate of Cu/Ru in alkaline cleaning solution with 15 potential inhibitors [12]....................................................................................................................................................... 63
Figure 3.6 Time lapsed images of Cu/Ru in acidic post CMP cleaning solution with inhibitors and graph of corrosion rate vs. inhibitors ..................................................................................... 64
Figure 3.7 Time lapsed images of Co/Cu in acidic post CMP cleaning solution ......................... 65
Figure 3.8 Corrosion rate vs. inhibitors (a) and time lapsed images of Co/Cu in acidic test solution with inhibitors 1-6 (b) ..................................................................................................... 66
Figure 3.9 Micropattern corrosion of Co/Ru in acidic post CMP cleaning solution after pretreatment with inhibitors 5 and 6 for 2, 5 and 30 minutes ....................................................... 67
Figure 3.10 Activation time and Co removal during pretreatment from optical profilometer ..... 68
Figure 4.1 Structures of (a) benzotriazole (BTA) and (b) pyrazole .............................................. 75
Figure 4.2 Micropattern corrosion screening structure ................................................................. 76
Figure 4.3 Time lapsed images of Cu microdots deposited on Ru, Ta and glass in 8 wt.% TMAH solution .......................................................................................................................................... 78
Figure 4.4 Tafel plots of Ru, Cu and Ta measured in TMAH pH 14 solution ............................. 79
Figure 4.5 Inhibitor concentration dependent etch rate of Cu in 8 wt.% TMAH ......................... 81
Figure 4.6 Time lapsed images of 50nM Cu/Ru immersed in 8 wt.% TMAH with additional 1mM pyrazole and 10mM BTA .................................................................................................... 81
ix
Figure 4.7 Tafel plots of Cu in 8wt.% TMAH and with pyrazole and BTA ................................ 82
Figure 4.8 EIS data (a) Nyquist plot of Cu in TMAH (black), TMAH +BTA (red) and TMAH+pyrazole (blue) and inset equivalent circuit used to fit data. .......................................... 84
Figure 4.9 Variation in DI water contact angle of Cu in TMAH and TMAH+ inhibitor ............. 85
Figure 4.10 XPS Cu 2p spectra of; (a) bare Cu, (b) BTA modified Cu and (c) pyrazole modified Cu and Cu LMM spectra of; (d) bare Cu, (e) BTA modified Cu and (f) pyrazole modified Cu .. 87
Figure 4.11 XPS N1s spectra of: (a) bare Cu, (b) BTA modified Cu and (c) pyrazole modified Cu....................................................................................................................................................... 88
Figure 5.1 Plasma etching of dielectrics and Cu plasma exposure scheme. ................................. 96
Figure 5.2 Micropattern corrosion screening structure ................................................................. 99
Figure 5.3 Progressing corrosion with time images.................................................................... 100
Figure 5.4 Corrosion rate and time of plasma treated Cu/Ru ..................................................... 101
Figure 5.5 Comparison of Cu/Ru and Cu/Ta corrosion after plasma treatment ......................... 103
Figure 5.6 Direct current measurements of Cu Vs Ru in TMAH pH 14 solution ...................... 104
Figure 5.7 Time dependent water Contact angle measurements after progressive immersion in TMAH, pH 14 solution ............................................................................................................... 105
Figure 5.8 XPS analysis of plasma treated Cu (a) C1s peak (b) F1s peak and inset F1s peak after one minute Ar+ sputtering ........................................................................................................... 107
Figure 5.9 XPS analysis of Cu 2p peak ...................................................................................... 107
Figure 5.10 Corrosion results in TMAH and TMAH+10mM BTA (a) and image of Cu(1)BTA complex (b) ................................................................................................................................. 109
x
CHAPTER 1
INTRODUCTION AND INSTRUMENTATION
1.1 Introduction
The past 40 years, microelectronic industries have evolved tremendously with some of
the greatest innovations being realized. The trend will most certainly continue in the coming
years as the companies are investing greatly in research and development of novel methods and
materials. Chemistry has been playing an important role in the development of integrated circuits
(IC) and other microelectronic devices processes. Some of the greatest challenges are not in
device or design but the ability to manufacture the components which are highly reliable while at
the same time keeping the manufacturing cost low. Introduction and integration of new materials
has proved to be inevitable as the need for high performance and low energy devices by the
consumers increase. New materials bring new challenges that have to be addressed in order to
avoid reliability issues and increase yield. Some of the challenges facing the microelectronic
industry can only be addressed by applying chemical principles like chemical kinetics,
thermodynamics, quantum mechanics and surface chemistry. Metal corrosion is among the big
challenges that must be addressed in order to manufacture high performance and reliable logic
and memory devices.
Manufacture of ICs involves making transistors first and then interconnects. The process
of making transistors is termed as front end of line (FEOL) and is made in one level (one layer).
The process of making interconnects is termed as back end of line (BEOL) and is made in
several levels as the connections are very complex.
Scaling of ICs and increasing the number of transistors to nearly 1 billion has led to a
high degree of complexity in circuit design. Copper replaced aluminum as the interconnect metal
1
of choice by the end of last century because of its high conductivity and electromigration
resistance [1]. The introduction of copper as the wiring material led to the adoption of Dual
Damascene process as etch back process used in aluminum interconnect was practically
impossible for copper because of lack of volatile by-products (Cu halides) at the processing
temperatures [1-2]. A simple schematic of one level of transistor (device) and two levels of
interconnect is shown in figure 1.1.
Figure 1.1 Schematic of one level of transistor and two levels of interconnect
Electrical signals from the source drain and gate of the transistor are transported to
different interconnect levels through vias. The latest Intel processor released in 2012 has nine
metal layers [3] and porous low k as the material for interlayer dielectric (ILD). The vertical
wires touching the transistor in ICs are called contacts and mostly made of tungsten while all
other vertical wires connecting the different levels are known as vias. The horizontal wires are
2
called metal lines. Vias and metal lines are made of copper. Figure 1.2 is a schematic showing
via and metal
Figure 1.2 Schematic of via and metal line
IBM and Motorola in 1997 integrated copper into their CMOS logic technology (Figure
1.3) [4-5] and shortly after that the all other IC manufacturers switched to copper interconnects
because of the advantages of copper and high performance achieved in microprocessors. The
performance improvement by copper interconnects was also achieved at a lower cost [4].
(a)
3
(b)
Figure 1.3 SEM micrographs of interconnect architecture with 6 levels of Cu lines/vias, W contacts/local interconnects and SiO2 ILD by (a) IBM [4] and (b) Motorola [5]
Copper metallization is achieved by dual damascene process whereby copper is deposited
into patterned trenches and vias [6]. In this process, the pattern is first made on ILD by
anisotropic plasma etching process producing vertical sidewalls. Plasma etching has the
disadvantage of forming polymer residues on the sidewalls and bottom of vias resulting from the
reaction of the gases with the etched material which can cause reliability issue. These polymer
residues must be cleaned prior to the next steps as it can cause problems with subsequent layers
such as poor adhesion and coverage, fluoride contamination and poor electrical contact [7].
Many dry chemistries have been explored [7-9] as well as wet chemistries [10-11] for cleaning
post etch residues. Each cleaning type has the potential of causing reliability issue. The next
steps are barrier deposition, copper seed layer deposition and finally electrochemical deposition
4
of copper [8]. Chemical mechanical polishing (CMP) process is incorporated in the
manufacturing step to remove excess copper deposited. CMP process can lead to Cu corrosion as
the CMP slurry and chemicals come in contact with Cu which can cause reliability issue and
must be addressed. Chapters 3, 4 and 5 in this thesis address Cu corrosion in CMP and post CMP
environments.
1.2 Instrumentation
Electrochemical corrosion techniques as well as thin film deposition tools and several
surface characterization tools were utilized in this work. The working principles of the
instruments/techniques used will be discussed separately in following subsections.
1.2.1 Electrochemistry
Electrochemistry is the study of chemical reactions that take place in a solution at the
interface of an electrode and an electrolyte. Electrochemical reactions involve transfer of
electrons between the electrode and the electrolyte. The anode loses electrons and becomes
oxidized while the species in the electrolyte gains the electrons and become reduced. Oxidation
and reduction occur simultaneously in electrochemical reaction and is either driven by an
externally applied voltage as in electrolysis or the reaction creates a voltage as in batteries [12].
1.2.1.1 Electrochemistry of Corrosion
Corrosion is the gradual destruction of a material (metal or ceramic) by the reaction with
the environment [13]. Metal corrosion in aqueous solution involves the transfer of electrons from
a metal surface to the aqueous electrolyte solution hence corrosion is an electrochemical
5
reaction. Metal corrosion occurs because of the high tendency of the metals to react
electrochemically with oxygen, water and other substances in the aqueous solution [14-15].
Corrosion of metals takes place as a result of exposure to the aqueous solution. The exposed
metal surface usually possesses oxidation sites that produce electrons in the metal (anodic
reaction) and a reduction site that consumes the electrons (cathodic reaction). The anodic
reaction results in dissolution of metal which could be soluble ionic products or metal oxide.
Several possibilities of cathodic reactions can occur depending on the reducible species present
in the solution which could be reduction of dissolved oxygen or hydrogen evolution. Anodic and
cathodic reactions occur simultaneously on a metal surface thereby creating an electrochemical
cell as shown in figure 1.4. [16-17]
Figure 1.4 Schematic of electrochemical cell showing anodic and cathodic sites [17]
Anodic and cathodic sites are not necessarily in a fixed location, they could be adjacent
or far apart. If two metals are in contact, one metal can be the anode and the other the cathode
depending on the nobility of the metals i.e. the less noble metal becomes that anode. This could
result in galvanic corrosion of the less noble metal hence corrosion at the anode. Since corrosion
6
involves transfer of electrons, the electrons flow from the anode to the cathode and form the
corrosion current. Corrosion current is determined by the rate of production of electrons by the
anode reaction and their consumption by the cathodic reactions. This is then used to determine
the corrosion rate of the metal.
For electrons to flow, a driving force is needed. In an electrochemical cell, the driving
force is the difference in potential between the anodic and cathodic sites. The difference in
potential exists because every oxidation and reduction reactions have specific potential
associated with the tendency of the reaction to take place spontaneously [18]. Figure 1.5 shows
schematic representation of three electrode system electrochemical cell
Figure 1.5 Three electrode system electrochemical cell
All the electrochemical analyses in this thesis were accomplished in a three electrode
system. In this system, a reference electrode, a working electrode and a counter electrode are
used. The use of three electrode system has an advantage of offering a precise potential control
during the measurement over a two electrode system and is used widely in electrochemistry.
7
1.2.1.2 Tafel Plots
Tafel plot is an electroanalytical technique that gives information relating to the corrosion
of a metal in an electrolyte. It utilizes Tafel equation which relates the rate of an electrochemical
reaction to overpotential (η). Tafel plot is the graph of the logarithm of the current density (i)
against overpotential. A polarized electrode regularly produces a relationship between current
and potential in a region which can be approached by [12]:
η = ±B log (I/I0)
Where η is applied overpotential with respect to the open circuit potential, I is the measured
current density, B and I0 are constants, I0 is defined as the equilibrium current density, and B is
defined as the Tafel slope [12]. A typical Tafel plot is show in figure 1.5. When collecting Tafel
plot data, the initial potential is set close to the open circuit potential (OCP) of metal in the
electrolyte and potential is scanned from -250mV to +250mV with respect to OCP.
Extrapolation of the cathodic and anodic curve gives Ecorr and Icorr at OCP. The potential and
current information extracted from the Tafel plot is used to calculate corrosion rate and reaction
kinetics of the corrosion or passivation of metal [12].
8
Figure 1.6 Tafel plot showing Ecorr, Icorr, cathodic and anodic curves [12]
1.2.1.3 Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) is a method of evaluating corrosion
process based on measurement of alternating current (ac) impedance over a range of applied
frequencies. In EIS, a sinusoidal voltage is applied at varying frequency which could be as high
as 100 KHz and as low as 1 mHz to an electrode system under test. The corrosion process
usually forces the measured current to be out of phase (denoted by the phase angle) with the
input voltage [12]. Dividing the input voltage by the output current furnishes the impedance. The
variation in impedance is used for interpretation. Figure 1.6 illustrates the relationship of direct
current and alternating current in terms of corrosion.
9
Figure 1.7 DC and AC currents in corroding systems (Wikipedia)
The response is analyzed in terms of the resultant current amplitude and phase. EIS data
is typically represented in Nyquist or Bode plots. The impedance spectrum reflects oxidation-
reduction reactions and migrations across the electrochemical cell. These are determined by the
electrical and chemical properties of the corrosive medium and electrode material [19-20]. An
EIS spectrum is then modeled using an equivalent circuit to describe the electrochemical system.
The information with regard to electrochemical corrosion can then be extracted through
appropriate interpretation of the variables to predict the corrosion rate of the material under
investigation in the specific environment [21]. Figure 1.7 is a Nyquist plot and a corresponding
equivalent circuit. Nyquist plot is plotted with x axis being the real impedance (ZRe) and y axis
negative imaginary impedance (-ZIm). From the data, the solution resistance (Rs) can be found by
reading the real axis value at high frequency (next to the origin). The real value on the low
frequency region is the sum of solution resistance and charge transfer resistance (Rct). The
diameter of the semicircle is the polarization resistance and this can be used to determine the
corrosion rate of a metal.
10
Figure 1.8 Nyquist plot and equivalent circuit [21]
1.2.1.4 Rotating Disk Electrode System (RDE)
A rotating disk electrode (RDE) can be used as a double hydrodynamic working
electrode in a three electrode system [22]. The working electrode rotates during experiments
creating a flux of electrolyte flow to the electrode. The continuous conversion of reactant to
product requires the steady supply of reactant to the electrode surface and the removal of
product. The RDE working electrodes are usually used in electrochemical studies when
investigating reaction mechanisms related to redox chemistry that involve the transfer of
electrons across the interface between a solid and an adjacent solution phase.
1.2.2 X-ray Photon Spectroscopy
X-ray photon spectroscopy (XPS) is a sensitive surface analysis technique that is used in
measuring the surface composition, chemical state and electronic state of the elements that exist
11
on the surface of material [23]. To obtain XPS spectra, a material is irradiated with a beam of x-
rays and simultaneously measuring the kinetic energy and number of electrons that are ejected
from the top 10nm of the surface being analyzed. This requires ultra-high vacuum (UHV)
conditions so as to avoid collisions of electrons and loss of energy. Figure 1.8 illustrates the basic
components of XPS system. XPS is applied in analysis of surface chemistry of as is materials as
well as after some treatment. XPS can detect all elements with the exception of H and He
because the binding energy of electrons in these elements is so small compared to the excitation
energy of the x-ray photon therefore the absorption efficiency is very small [24]. XPS has found
wide range of application including microelectronic area for the evaluation of surface cleaning,
corrosion inhibition mechanisms as well as composition and bonding structures of dielectric and
barrier films. In this work, XPS is utilized as surface analysis technique in Cu corrosion
inhibition and also study of Cu after plasma treatment to evaluate the formation of fluorocarbon
residues on Cu during plasma etching.
Figure 1.9 Basic components of XPS system (Wikipedia)
12
XPS data is acquired through a plot of number of electrons (intensity) versus the binding
energy of the electrons detected. Binding energy is a measure of the attractive forces between the
electron and the nucleus. The magnitude of this attractive force depends on the charge of the
nucleus therefore each atom has a characteristic binding energy and XPS utilizes this principle to
identify atoms. In a typical XPS experiment, each element produces a set of XPS peaks at a
characteristic binding energy values that directly identifies the elements present on the surface of
the material under analysis. The intensity of each of the characteristic peaks can be directly
related to the amount of element within the area irradiated. Atomic percentages are generated
through correction of raw XPS data. This is accomplished by dividing the signal intensity by a
relative sensitivity factor (RSF) and normalizing over all elements detected. [24- 25]
To minimize loss of kinetic energy of electrons, the analysis is done in UHV to avoid
collisions of electrons with other electrons and with residual gas molecules since the detectors in
XPS instruments is approximately one meter away from the material irradiated with x-rays. An
electron analyzer collects the photoelectrons and measures their kinetic energy while the detector
counts the number of photoelectrons emitted. Binding energy of electrons is related to kinetic
energy by the following expression
EB = hν -Ek
Where EB is the binding energy of the electron needed to escape the vacuum energy level where
the atom cannot exert influence on the electron, hv is the x-ray photon energy and Ek is the
kinetic energy of the emitted electron. Since the X-ray photon energy (hν) and Ek are known, EB
can be calculated [26]. Because of the low kinetic energy of the photoelectrons, the electrons
attenuation lengths are very small ~ 3nm which make XPS a highly surface sensitive. The only
13
electrons reaching the detector are those emitted within 3-5 times the attenuation length hence
the top ~10nm of the surface is probed.
The binding energy also provides important information about the bonding characteristics
of the environment. A chemical shift towards lower binding energy would be seen if a chemical
element was to be in an electron-donating environment whereas the higher binding energy would
be observed when an element was in oxidation state. If an atom is bonded to a more
electronegative atom, the outer shell electrons are pulled toward electronegative atom leading to
a slight positive charge on the nucleus [25]. This results in the core electrons being held strongly
by the nucleus and the binding energy of the core electrons shifts to higher energy. The shift in
binding energy can give the bonding information of the material being analyzed.
Figure 1.10 PHI 5000Versa Probe
14
All XPS measurements in this thesis were accomplished using a PHI 5000Versa Probe
Scanning XPS shown in figure 1.9. It is equipped with a standard Al-Kα X-ray source at 280
watts and electrostatic analysis in constant pass energy mode of 114.7eV for survey scans and
23.5 eV for detail scans.
The Versa Probe scanning XPS provides a highly focused monochromatic X-ray beam,
(10µm to 100µm) which can precisely focus on the area under study. A 100V to 5kV
differentially pumped Ar ion gun with regulated leak valve is available for specimen cleaning
and sputter depth profiling with monolayer resolution. Also, the Ar ion gun was used to
neutralize the insulating materials to prevent the electronic field from emitting photoelectron on
local area during the X-ray irradiation. Data collection was done using PHI Explorer software
(Physical Electronics, v 3.4) and analyzed using Multipak software (Physical electronics v5.0A).
C1s peak was used as a reference (284.8 eV) so as to maximize the photoelectron count by
adjusting the position of the sample relative to the source and the detector.
1.2.3 Contact Angle
Contact angle is the angle between the tangent to the droplet of liquid placed on a flat
surface and the surface. Figure 1.10 is a schematic of contact angle of a liquid droplet. It
quantifies the wettability of a solid surface by a liquid. The contact angle of a small drop on the
surface is a function of surface free energy that is defined by Young-Dupree equation [27].
Where θ is the angle contact, γ is the interfacial free energy, and SG, SL and LG refer to solid-
gas, solid-liquid and liquid-gas interfaces respectively
15
Figure 1.11 Schematic of contact angle of liquid droplet [27]
The angle of a liquid drop on the solid surface forms as a result of balance between the
cohesive forces in the liquid and the adhesive forces between the solid and the liquid. When
there is no interaction between the solid and the liquid, the contact angle will be 180o. As the
interaction increases, the liquid spreads until the angle becomes near 0o.
Hydrophobic and hydrophilic nature of the surface can be determined using water to
measure contact angle. In this thesis contact angle measurements were utilized in the study of
copper corrosion inhibitors to determine the hydrophobicity of the surface after treatment with
inhibitors and fluorocarbon plasma etch gases.
1.2.4 Thin Film Deposition
A thin film is a layer of material with a thickness ranging a fraction of a nanometer
(monolayer) to several micrometers. Microelectronic devices are generally manufactured by
layers of thin films. Thin film deposition generally refers to the action of applying thin film to
the surface. There are several deposition techniques and each of them is customized so as to
16
control the layer thickness. Two techniques, chemical deposition and physical vapor deposition
are discussed.
1.2.4.1 Chemical Deposition
Chemical deposition is a technique in which a fluid (gas or liquid) precursor undergoes a
chemical change at the solid surface. The fluid surrounds the solid object and therefore
deposition takes place on every surface with little regard to direction. Thin films from chemical
deposition tend to be conformal rather than directional. Chemical deposition is divided into
several categories:-
• Plating - This technique relies on liquid precursors, most of the time a salt of the metal to
be deposited. The most common is electroplating. Electroplating is a plating process
where metal ions in aqueous solution are deposited on conductive substrate by an electric
field. Electroplating has been done for hundreds of years, but recently it has gained
popularity in terms of use in the semiconductor industry for metal deposition to high
aspect ratio features.
• Spin coating - It is also known as spin casting and uses a liquid precursor of a sol-gel
precursor deposited onto a smooth flat substrate that is subsequently spun at high velocity
to centrifugally spread the solution over the substrate. The spinning speed of the solution
and the viscosity of the sol determine the final thickness of the deposited film. Deposition
can be repeated as needed to increase the thickness of the film. In most cases thermal
treatment is carried out in order to crystallize the amorphous spin coated film [28].
• Chemical vapor deposition (CVD) - This technique generally uses a gas precursor mostly
a halide or hydride of the element to be deposited. Deposition occurs by the reaction or
17
decomposition of the precursor on the substrate surface. The gas phase by products that
are produced are removed by gas flow though the reaction chamber.
• Atomic layer deposition (ALD) – This is similar to CVD and uses gaseous precursor to
deposit conformal thin films with an advantage of depositing one layer at a time. The
process is split into two half reactions that are run in sequence and repeated for each layer
to ensure complete layer saturation before depositing the next layer. By exposing the
precursors to the surface repeatedly, atomic layer control of film growth rate can be
obtained as fine as ~0.1 Å (10 pm) per monolayer. Recently, there has been a rapidly
growing interest in ALD of materials used in microfabrication processes, especially in
integrated circuits (ICs) [29-30].
1.2.4.2 Physical Vapor Deposition (Sputtering)
Physical vapor deposition (sputtering) utilizes a plasma, (usually noble gas like Ar) to
knock material off from a target a few atoms at a time. The ejection of atoms from the target is as
a result of energetic particles [31] and only occurs when the kinetic energy of the incoming
particles is much higher than the conventional thermal energies. The following principle pertains
to dual magnetron sputtering system shown in figure 1.10 that was utilized in all thin film
deposition in this thesis.
In dual magnetron sputtering system (figure 1.11), a substrate (the item to be coated) is
placed in a vacuum chamber opposite a target (made of the coating material being sputtered).
The chamber is evacuated and then backfilled with a process gas (Argon). The gas is ionized
with a positive charge, which creates plasma. Resulting ions are strongly attracted to the target,
which carries a negative charge. As the relatively large argon ions knock the target,
18
atoms/molecules of target material are physically removed from the target. Due to its close
proximity, a majority of sputtered atoms/molecules land on the substrate. The intent is for this
material to arrive at the substrate with enough energy to form a thin, strongly attached film, one
monolayer at a time as illustrated in figure 1.12.
Figure 1.12 Dual magnetron sputtering system
Figure 1.13 Schematic of principle of sputtering process [31]
19
The use of an inert gas has the advantage of not decomposing in the plasma glow
discharge. Argon, having a relatively high atomic weight, provides a suitable source of ions for
effective bombardment of the target material. The effectiveness is dependent on the "mean free
path" (m.f.p.) which is inversely proportional to pressure. If the m.f.p. is too short, insufficient
energy will be gained for effective bombardment and will inhibit movement of sputtered material
from the target. If the m.f.p. is too long, insufficient collisions occur and, in addition, the flow of
sputtered material may change from diffusion in the gas to free molecular flow with a reduction
in the effectiveness of omni-directional deposition [31].
1.2.5 Micropattern Corrosion Screening Technique
Micropattern corrosion screening technique is a method that is used to study bimetallic
corrosion as a result of two different metals being in contact. It employs the use of microdots of
~130 microns diameter and varied thickness depending on experimental needs that are deposited
on various substrates through a contact mask using standard magnetron sputtering machine. The
microdots deposited form a micropattern on the substrate of choice to form a bimetallic contact
that can be studied. The samples are then immersed in a corrosive solution and in situ
investigation of corrosion behavior is done by visual inspection using a metallurgical
microscope. Figure 1.13 illustrates micropattern corrosion screening structure.
20
Figure 1.14 Micropattern corrosion screening structure.
Micropattern corrosion screening provides an efficient and rapid method for studying
bimetallic corrosion. Different combinations of metals can be fabricated easily into bimetallic
micropattern for corrosion study. Several parameters like thickness of microdots can be
controlled so as to study a wide range of corrosion rates within a reasonable experimental time.
The data from this method can be used to get the relative corrosion rate of a galvanic couple as
well as the actual visual inspection of the actual corrosion process in real time. The direct
imaging of micropattern is useful in identifying surface chemistry that might me taking place
during the corrosion process. This method is also used to identify effective corrosion inhibitors
that will find application in Cu interconnect processing [32-33].
21
1.3 References
1. K. W. Chen, Y. L. Wang, L. Chang, S. C. Chang, F. Y. Li, and S. H. Lin, Electrochem.
Solid-St. Lett., 7, G238 (2004).
2. A. Jindal and S. V. Babu, J. Electrochem. Soc., 151, G709 (2004).
3. D. Ingerly et al., Interconnect Tech. Conf. (IITC), 2012 IEEE International, 1 (2012).
4. D. Edelstein, J. Heidenreich, R. Goldblatt, W. Cote, C. Uzoh, N. Lustig, P. Roper, T.
McDevitt, W. Motsiff, A. Simon, J. Dukovic, R. Wachnik, H. Rathore, R. Schulz, L. Su,
S. Luce, and J. Slattery, International Electron Device Meeting Technical Digest, 773
(1997).
5. S. Venketesan, A. V. Gelatos, V. Misra, B. Smith, R. Islam, J. Cope, B. Wilson, D.
Tuttle, R. Cardwell, S. Anderson, M. Angyal, R. Bajaj, C. Capasso, P. Crabtree, S. Das,
J. Farkas, S. Filipiak, B. Fiordalice, M. Freeman, P. V. Gilbert, M. Herrick, A. Jain, H.
Kawasaki, C. King, J. Klein, T. Lii, K. Reid, T. Saaranen, C. Simpson, T. Sparks, P. Tsui,
R. Venkatraman, D. Watts, E. J. Weitzman, R. Woodruff, I. Yang, N. Bhat, G. Hamilton,
and Y. Yu, International Electron Device Meeting Technical Digest, 769 (1997).
6. T. Licata, E. G. Colgan, J. M. Harper, and S. E. Luce, IBM J. Res. Develop., 39, 419
(1995).
7. Q. Han, B. White, I. L. Berry, C. Waldfried, and O. Escorcia, Solid State Phenomena.,
103, 341 (2005).
8. A. Somashekhar, H. Ying, P. B. Smith, D. B. Aldrich, and R. J. Nemanich, J.
Electrochem. Soc., 146, 2318 (1999).
9. G. S. Oehrlein and Y. H. Lee, J. Vac. Sci. Technol. A, 4, 1585 (1987).
22
10. M. Shikida, K. Sato, K. Tokoro, and D. Uchikawa, J. Micromech. Microeng., 10, 522,
(2000).
11. M. Kohler, Etching in Microsystem Technology, John Wiley & Sons, Chichester (1999).
12. L. R. Faulkner, J. Chem. Ed,. 60, 262, (1983).
13. D. A. Jones, Principles and Prevention of Corrosion, Prentice Hall, Upper Saddle River
(1996).
14. L. L. Shreir, R. A. Jarman, and G. T. Burstein, Corrosion, Butterworth-Heinemann,
Oxford, (1994).
15. S. N. Popova, B. N. Popov, and R .E. White, Corrosion, 46, 1007 (1990).
16. T. Tsuru and S. Haruyama, Boshoku Gijustsu, 27, 573 (1978).
17. D. A. Jones, Corros. Sci., 8,19 (1968).
18. D. D. MacDonald, Corrosion, 45, 30 (1989).
19. D. D. MacDonald, Electrochim. Acta, 51, 1376 (2006).
20. National Information Services Corp. Feb 08 2004. (www.nisc.com)
21. Princeton Applied Research, Basics of corrosion measurement-Application note 2004
22. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and
Applications, John Wiley & Sons, Chichester (2000).
23. D. Briggs, and M.P. Seah, Practical Surface Analysis: Auger and X-Ray Photoelectron
spectroscopy, John Wiley & Sons, Chichester (1996).
24. J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of X-ray
Photoelectron Spectroscopy, Physical Electronics Inc., Eden Prairie (1995).
25. G. Beamson and D. Briggs, High Resolution XPS of Organic Polymers. The Scienta
ESCA 300 Database, Wiley, Chichester (1992).
23
26. M. F. Ebel, J. Electron. Spectrosc. Relat. Phenom., 8, 213 (1976).
27. T. S. Chow, J. Phys.: Condens. Matter, 10, L445 (1998).
28. D. Hanaor, G. Triani, and C. C. Sorrell, Surf. Coat. Tech., 205, 3658 (2011).
29. T. Suntola, Handbook of Crystal Growth, Elsevier Science B. V., Amsterdam (1994).
30. M. Leskela and M. Ritala, Thin Solid Films, 409, 138 (2002).
31. R. Behrisch, Sputtering by Particle bombardment, Springer, Berlin (1981).
32. K. K. Yu, K. S. M. Pillai, P. R. Nalla, and O. Chyan, J. Appl. Electrochem., 40 (2010).
33. P. R. Nalla, K. S. M. Pillai, K. K. Yu, S. Venkataraman, and O. Chyan, Proceeding of
2009 Advanced Metallization Conference, 83 (2009).
24
CHAPTER 2
COPPER INTERCONNECT PROCESSING
2.1 Introduction
Manufacture of integrated circuits (ICs) has developed tremendously in the last 50 years
with the minimum feature size going from 10 microns to 22 nm by 2012. The manufacturing
cost per transistor has greatly dropped while the maximum number of transistors per chip has
exceeded 1 billion [1]. The progress in ICs has been fueled by the expected improvements in
density (Moore’s law-The number of transistors in ICs will double approximately every two
years) and performance. Both increase in density and performance were achieved through device
scaling and/or increase in chip size [2]. Transistors’ performance improves as gate length,
junction depth and gate dielectric thickness is reduced in size [3]. On the other hand, chip wiring
(interconnects) on shrinking the size suffers from increased resistance because of decrease in
conductor cross-sectional area and could also result in increased capacitance if metal spacing and
height are not decreased simultaneously [4]. Therefore resistance capacitance (RC) delay has a
huge impact on the overall chip performance as the size scales down and has been reported by
ITRS [5] as one of the chief concerns in the performance of future technology nodes as shown in
figure 2.1 [4].
Many materials and processes have been introduced over the past decade in the back end
of line (BEOL) process in order to achieve improved density and performance of ICs. Some of
them include low k dielectric materials as interlayer dielectrics (ILD), novel dielectric and metal
planarization techniques using chemical mechanical polishing (CMP), chemical vapor deposition
25
(CVD), physical vapor deposition (PVD) and electrochemical deposition (ECD) techniques for
metals.
Figure 2.1 Device delay and interconnect delay as a function of feature size [4]
2.2 Interconnect Integration
Interconnects in ICs are incorporated after the front end of line process. As the number of
transistors increase, the relay of signals form one device to another, from one circuit block to
another and so on up the hierarchy becomes a challenge. The interconnections needed are
becoming increasingly more complex. Microprocessor designs utilize hierarchical metallization
schemes in which larger wires are used in upper levels of interconnects in order to minimize RC
delay and voltage drop as illustrated in figure 2.2 [6]. The lower level (metal 1) is thinner and is
used for local routing while intermediate layers are of medium thickness used for semi-global
routing and finally the top layers are wide and ‘fat’ wires used for global routing. ITRS [7]
predicted acceleration in the need for new materials in order to meet the ever increasing need for
higher performance devices. These include the need for increased current handling capability,
26
lower permittivity dielectrics and a reduction in metal barrier thickness and these new materials
will need new processes and designs in order to fully integrate then in ICs.
Figure 2.2 Typical cross section of hierarchical scaling of Cu [6]
2.3 Process Flow
2.3.1 Tungsten Vias Fabrication
Fabrication of tungsten via is accomplished by first depositing a thick layer of SiO2 on a
planer surface basically by plasma enhanced chemical vapor deposition (PECVD) with a
tetraethylorthosilicate (TEOS) precursor at about 350-400oC to form the oxide ILD. The next
step is oxide patterning that is accomplished by photoresist followed by etching to expose the
underlying metal layer. Photoresist stripping is then done and via opening is cleaned followed by
deposition of a thin layer of Ti by physical vapor deposition (PVD). The Ti film serves as an
27
adhesion layer and also decreases contact resistance to underlying conductors by reducing
interfacial oxides. A layer of Titanium nitride is subsequently deposited in situ, either by PVD or
by CVD followed by conformally filling the hole void-free with CVD tungsten by SiH4
reduction of WF6. The excess W, TiN, and Ti in the field regions are finally removed by
chemical-mechanical polishing (CMP).
Development of tungsten via technology has greatly improved to the extent where void-
free and untapered vias with aggressive aspect ratios exceeding 3:1 are easily formed, thus
enabling increases in wiring density and reducing parasitic capacitance from under- and
overlying wires.
Figure 2.3 Tungsten vias fabrication process flow [9]
Advances in lithography alignment have also enabled borderless vias to be formed,
thereby permitting even further improvements in wiring density. Damascene tungsten has been
28
adapted as planar local interconnects for strapping source/drain and gate contacts [8]. Tungsten
vias fabrication flow process is shown in figure 2.3 [9] One major drawback of tungsten via
technology is the high cost involved. Tungsten vias processing also introduces particles and
defects on the wafer which can compromise yield and reliability. Development of methods of
removing these particles and defects is therefore essential.
2.3.2 Dual Damascene Copper Process
Copper integration into ICs as the wiring metal of choice introduced more challenges in
the manufacturing process. The conventional subtractive etch that was used with aluminum
metallization became impractical because of low volatility of Cu halides (chlorides and
fluorides) that form during plasma etching at low temperatures [10-11] making etching very
slow. At the same time photoresist cannot withstand temperatures required for practical Cu etch
rates (>200oC). Figure 2.4 [12] demonstrates a cross section of Cu interconnect technology. For
this particular example, W contacts are fabricated using the damascene process described in
section 2.3.1. It has six levels of Cu wiring integrated with Cu vias between successive metal
layers. The ILD used for both via- and wire-level dielectric is SiO2.
Cu is well known to be a fast diffuser in silicon where it can act as deep level acceptor in
the silicon bandgap [13]. Deep level states degrade minority carrier lifetime leading to high
junction leakage in transistors and short term retention time in DRAMs and therefore introducing
the need for faster refreshing which in turn leads to higher power use. Under electrical bias, Cu
also diffuses through SiO2 [14]. These are some of the facts that raised many concerns about
device contamination if Cu was used as wiring material. The success of Cu interconnects
implementation therefore depended on the prevention of any trace amounts of Cu from migrating
29
to silicon substrate. This not only involved added process complexity but also influenced tools
designs and wafer handling.
Figure 2.4 SEM micrograph of Cu interconnect architecture by IBM [15]
The challenges involving Cu integration were overcome by deposition of diffusion
barriers before Cu deposition in dual damascene process developed by IBM [15]. Dual
damascene process is a modification of single damascene where by trenches and via holes are
first patterned prior to deposition of metal barrier/ Cu seed/Cu. Therefore, only one metal fill
and one Cu CMP step are needed for each level of interconnect resulting in lower cost of
processing as compared to single damascene [4]. Figure 2.5 compares single and dual damascene
processes
30
Figure 2.5 Single and dual Damascene processes [4]
In addition to reducing manufacturing cost, dual damascene process also provides lower
via resistance and improved reliability. This is achieved in two ways, one is by reducing the
number of interfaces in the via that is one bottom via for dual damascene and two (top and
bottom) for single damascene with W plugs and secondly by providing full wire via overlap at
the top of the via [4]. Many dual damascene schemes have been demonstrated but generally all
approaches fall into either via first or trench first depending on which pattern is etched first. The
individual steps in the dual damascene process are discussed in the following subsections.
31
2.3.3 Dielectric Patterning
Dual damascene has generally been adopted for Cu interconnects technology because of
its lower cost and improved reliability advantages as compared to single damascene process.
Several approaches and schemes have been proposed for the formation of vias and trenches in
dual damascene process [16]. Five different approaches have been summarized in figure 2.6 and
their advantages and disadvantages are listed in table 2.1 [17-21]. The adoption of a specific
approach for manufacturing process differs from company to company depending on their
specific technological needs, capabilities and ILD materials.
32
Figure 2.6 Dual Damascene schemes for defining trenches and vias: (a) buried etch stop, (b) clustered approach (c) partial via first (d) full via first (e) line first [17-21]
33
Process Flow Advantages Disadvantages
Buried Etch stop Topography minimized Etch process selectivity and
control are critical
Clustered Process types grouped Poor resist adhesion and
pattern transfer
Partial via first Cleaner Structure, less critical etching Lithography process difficulty
increased
Full via first Lithography and etch process slightly
easier; stacked via trivial
Lithography rework and resist
cleaning process difficult
Line first Easier etch process, less topography for
lithography
Resist cleaning process critical
Table 2.1 Advantages and disadvantages of dielectric patterning schemes
2.3.4 Low k Dielectrics
Resistive-capacitive (RC) delay is one of the major obstacles in downscaling of ICs.
Shrinking the device increases the resistance due to the decrease of conductor cross-section and
increase of wiring length while at the same time inter-line capacitance increases because of
reduction in interline spacing. The need to decrease the RC delay has led to the introduction of
new materials to the BEOL processes. The conventional SiO2 (k = 4.2) has been replaced with
lower dielectric constant k materials (k ~ 2-3) in order to decrease capacitance. These materials
are known as ‘low k’ dielectrics [22-23]. Generally, there are two approaches that are used to
lower the k-value of interconnect dielectrics: reduce the polarizability by use of low-polar bonds
34
like C-C, C-H, Si-CH3, etc. and/or making it porous to reduce their density. Many low k
materials (organic or inorganic) spanning a wide range of dielectric constants for example k=1
(air) to k~3.6 (fluorinated oxides) have been studied for use in interconnect systems and are
listed in table 2.2 [4]
Table 2.2 List of low k dielectric materials [4]
Integration and reliability issues have impeded the implementation of low k dielectric
materials into the ICs. Some of these issues include thermal and mechanical induced cracking or
adhesion loss, poor mechanical strength, moisture absorption, chemical interactions (that may
occur during lithography, etch/clean and metal deposition) and poor thermal conductivity [4].
Low k dielectric patterning faces major challenges like etch rate uniformity across the wafer or
profile control. Photoresist stripping following dry etching is one of the most harmful and
35
challenging patterning steps for low k materials. This is because strip chemistries target to
remove organic polymers (hydrocarbons) and low k dielectric materials utilize hydrocarbon
groups for hydrophobicity. This result in removal of photoresist by plasma chemistry as well as
hydrophobic groups (-CH3) from low k materials, therefore making them hydrophilic. A
hydrophilic surface can absorb moisture causing an increase in dielectric constant which can be
detrimental. Several processes have been optimized to minimize damages from resist stripping
like changing resist strip chemistry [24] and use of silylation to repair damage [25].
2.3.5 Metallization
Copper metallization process sequence is more complex than that for aluminum
metallization. This is because in addition to using electroplating to fill high aspect ratio vias and
trenches, Cu must be surrounded by a diffusion barrier to prevent Cu diffusion into the dielectric
[26-27]. Nevertheless, electroplating of both vias and trenches is done with the same
metallization step resulting in lower cost and small high aspect-ratio features can be filled void
free resulting in high reliability.
2.3.5.1 Copper Diffusion Barrier Deposition
Diffusion barrier in Cu interconnects is needed to ensure that no trace of Cu diffuses
through the dielectric material into silicon substrate. Immediately after dielectric etch and
cleaning to remove post etch residues, a conductive barrier material must be deposited on the
sides and bottom of trenches and vias. Barrier materials are generally more resistive compared to
Cu and therefore their thickness must be kept to the minimum in order to maintain the effective
high conductivity of Cu over Al. Barrier materials must also demonstrate low contact resistance
36
to Cu and this is achieved by an effective clean of the vias following dielectric etch.
Furthermore, barrier materials should exhibit low stress and good adhesion to the low k
dielectric.
Via etch exposes underlying Cu wires and deposit polymer residues; therefore a cleaning
step is required in order to achieve low contact resistance of the barrier to Cu. The cleaning step
must be optimized so that it does not cause copper corrosion which in turn can redeposit Cu onto
the ILD surface and via sidewalls [28]. Cu corrosion and corrosion inhibition is the focus of the
strategies developed in this thesis, chapters 3, 4 and 5.
Many studies have focused much interest on refractory metals like Ti, W, Ta and their
nitrides as barrier materials based on the general requirements discussed [29]. A layer of TaN/Ta
has been adopted for Cu integration [30]. TaN and Ta are both very good diffusion barriers for
Cu [29]. TaN provides good adhesion to the dielectric material while Ta provides a surface with
good wettability for the Cu seed layer. A good wetting of Cu seed layer on the barrier is essential
in achieving a smooth continuous Cu seed layer which is required for void free Cu plating [31].
Deposition of barrier layer is preferentially done by PVD because it can produce high purity
films which are necessary for good wetting of Cu and also at relatively low cost [31]. The main
challenge for barrier deposition is ensuring adequate conformity in high aspect ratio vias and
trenches. Non-conformal coverage of barrier materials can lead to failure in areas where the
barrier is thinnest, usually at the lower corners and sidewalls of the via. Figure 2.7 demonstrates
ideal and typical step coverage of a barrier material deposited by PVD [32].
37
Figure 2.7 Ideal and typical step coverage of barrier material deposited by PVD [32]
Sputter deposited films of metal barrier with good step coverage is possible with the use
of ionized PVD [33-34] and it involves a two-step process. Step one uses magnetron sputtering
whereby the sputtered metal is ionized and directionally deposited onto the substrate. This results
in thicker film deposition at the bottom of the trenches and vias than the sidewalls. The second
step utilizes Ar plasma to resputter some of the barrier material from the bottom of the features
on to the sidewalls. Good barrier coverage has been demonstrated by using ionized PVD for
35nm wide trenches with approximately 5:1 aspect ratio [35].
Other materials have been proposed as alternatives to Ta based barriers and include Ti and Ru.
Ti maybe used instead of Ta to reduce cost [35] but a multilayer of Ti/TiN/Ti is required. Cu
wetting on TiN is poor and so a thin layer of Ti is needed on top of TiN for good wetting of Cu
[36]. Ru has been of great interest as a replacement of Ta because of its lower resistivity than Ta
and Cu can de plated directly onto Ru which can eliminate the need for Cu seed layer [37-38].
38
However, Ru is not a good diffusion barrier for Cu and therefore a bilayer structure is still
needed such as TiN/Ru [39]. Successful Cu integration in ICs is therefore dependent on the
development of the processes in metal barrier technologies. The implementation and specific
details of metal barrier technologies remains undisclosed and proprietary to the companies
involved.
2.3.5.2 Copper Deposition
Trenches and vias are filled with Cu following deposition of Cu barrier. High aspect ratio
features filling without voids has been a challenge to the industry. Many technologies have been
explored in order to identify a cost effective solution. Technologies like PVD, CVD, electroless
plating and electrochemical deposition (electroplating) have been extensively studied. Among
them all, electroplating of Cu has been identified to be capable of providing a void free fill in
high aspect ratio features with low resistivity and high reliability [40-41].
In electroplating, a thin layer of Cu seed is deposited on top of barrier material typically
by PVD and then the wafer is immersed in a solution containing Cu2+, sulfuric acid and trace
organic additives [41]. Electrical contact is made to the seed layer and current is passed which
drives the reaction shown below at the surface of the wafer.
As the Cu2+ is reduced out of the solution onto the wafer, the Cu anode simultaneously
undergoes oxidation to replenish the supply of Cu2+ in the solution. Figure 2.8 shows a schematic
of Cu electroplating system [42].
39
Figure 2.8 Schematic of Cu electroplating system [42]
Cu plating bath consists of organic additives (suppressors, accelerators) which each play
a role in the successful void free filling of Cu into the trenches and vias. Suppressors reduce the
plating rate at the top of the features while accelerators enhance the plating rate at the bottom of
the features. Bottom up void free filling of the trenches and vias commonly known as
‘superfilling’ is achieved as a result of correct combination of the additives. Dimercaptopropane
sulfonic acid (SPS), an accelerator contains a sulfide and a thiol-like functional group which
strongly adsorb on Cu surfaces. The presence of SPS on Cu surface may act as a charge transfer
site for the reduction of Cu2+ to Cu+ resulting in enhancement of Cu deposition [41].
Furthermore, SPS has high solubility in the plating bath therefore it continues to accelerate the
40
reaction at the bottom of the trenches and vias rather than be incorporated in the growing film.
Suppressors are polymers such as polyethylene glycol that slow down the plating reaction.
Slowing down mechanism could be as a result of blocking of growth sites on the surface of Cu
and slower diffusion of Cu ions to the surface.
Shrinking of device dimensions has also led to the thinning of the Cu seed layer in order
to avoid pinching off the top of the trenches and vias. Thinning of Cu seed layer makes Cu
plating very difficult. One reason for this is that plated Cu uniformity across wafer is harder to
achieve with thinner seed layer. This is as a result of reduction of plating current (and hence
deposition rate) in the center of the wafer if the seed layer resistance is comparable to the bath
resistance [40]. One method of avoiding this problem is to increase the resistivity of the plating
bath by reducing the acid concentration or adding a second cathode around the perimeter of the
wafer to draw current away from the very edge of the wafer [41]. The other problem with
thinning of Cu seed is that it is difficult to ensure continuity of the seed layer and if there are pin
holes in the seed layer there will be delayed plating in these areas and voids may be trapped in
the structure leading to reliability issues. In order to avoid this problem approaches like
increasing the concentration of the acid in the plating bath has been proposed [43] and also
applying plating current as soon as the wafer is immersed in the plating bath to avoid dissolving
the seed layer in the bath [41].
2.3.6 Chemical Mechanical Polishing (CMP)
Chemical mechanical polishing (CMP) is used to remove excess Cu and barrier layer
after metallization of the dual damascene structure. CMP is the key enabling technology in Cu
damascene integration [44]. As the name suggests, CMP involves both chemical action and
41
mechanical abrasion in the selective removal of the film from the wafer surface. The chemicals
in the slurry react with the film surface and form an oxidized layer. The layer is removed by the
mechanical action because of the fine particles in the slurry and the downward pressure of the
polishing pad. This results in the wafer surface becoming progressively planar with time.
To achieve optimum planarization, a good balance must exist between chemical and
mechanical components. If the mechanical action dominates, surface scratches and
nonuniformity may result. On the other hand, if the chemical component is dominating,
overpolishing can occur, which can lead to severe surface topography due to selectivity of the
slurry chemistry against dielectric removal. Mechanical abrasion generally depends on the size
and concentration of the slurry particles, hardness and surface roughness of the pad, pad pressure
and the rotational speed of the wafer and pad. The chemical action on the other hand is
controlled by the chemistry concentration and pH of the slurry. Optimization of CMP process is
needed in order to minimize pattern density and feature size effects hence avoiding dielectric
erosion and metal dishing.
During CMP process, the wafers are placed face down on a rotating pad on which the
slurry is dispensed as illustrated in figure 2.9 [45]. Cu CMP is done in two steps, the first step is
Cu removal stopping at the barrier and the second step is the barrier removal stopping at the
dielectric [46]. In order to ensure that all metals are removed from the field regions in all parts of
the wafer, overpolishing is required. Overpolishing can lead to thinning of regions with high Cu
pattern density and could lead to variation in wire resistance which can be minimized by design
rules that restrict local Cu pattern density [47]. Furthermore, low downward processes are
required to minimize Cu corrosion during the overpolish step [48].
42
Figure 2.9 Schematic of chemical mechanical polishing system [45]
CMP faces a number of challenges especially with the integration of porous low k
materials. These includes Cu dishing and insulator erosion, cracking and adhesion loss in the
dielectric stack and scratching or contamination of the low k material by slurry or reaction by
products [49-50]. CMP therefore has a big impact in the continued complexity of the IC
technology. Polishing has enabled multilevel metallization, use of optical lithography, copper
damascene technology and great improvement in the yield and reliability over the last decade. As
researchers progress to develop new materials for low k and Cu diffusion barrier, new CMP
slurries will be needed that will be compatible with all the materials in use.
43
2.3.7 Cu Interconnects Reliability
Performance of ICs is affected by the delays arising from the relay of signal across the
circuit. Traditionally, this was associated with transistor switching. The reduction in transistor
geometries, specifically gate lengths lead to great improvement [51]. The switch from Al to Cu
wiring has enabled great advances in the miniaturization of interconnects as well as stacking of
several layers of metal.
Despite the great advantages of Cu interconnects, several challenges causing yield and
reliability issues associated with the manufacturing and integration of dual damascene structures
have been encountered. A major challenge with the shrinking of devices is the reduction of
electromigration lifetime of Cu and stress induced voiding [51]. In addition, integration of low k
materials in Cu interconnects causes problem in terms packaging reliability due to low
mechanical strength and brittleness of these materials [52].
2.3.7.1 Electromigration
Electromigration is the movement of metal atoms in a conductor due to electrical current
[53]. The electrons moving towards anode causes movement of atoms in the lattice toward the
anode. At higher current density which is typical in ICs, the electron movement towards the
anode impacts enough momentum to the atoms of Cu causing a net diffusion towards the anode.
The movement of atoms changes the atomic density along the Cu interconnects and leads to
build up of stress. Diffusion barrier layer at the bottom of the trenches and via act as blocking
boundaries and therefore during electromigration stress, Cu atoms will be depleted at the cathode
side of the wire and eventually voids will form. If the voids grow large enough covering the wire
or via, the resistance will increase and can lead to circuit failure. At the anode end of the wire,
44
Cu will accumulate resulting in hydrostatic stress [54]. Hydrostatic stress causes a back flux of
atoms that is counter to the direction of electromigration, hence for short wires, the back flux of
atoms prevents formation of killer voids and the wires are immortal [54]. Figure 2.10 illustrate
possible failure mechanism for Cu interconnects in low k dielectrics [55].
Figure 2.10 Possible failure mechanism for Cu interconnect [55]
The problem of electromigration is further enhanced by use of low k dielectrics. Low k
materials have lower thermal conductivity compared to traditional SiO2 therefore there will be
more joule heating in low k materials per current density [56]. This results in higher temperature
for the wire and therefore a faster rate of electromigration. Several approaches have been
proposed to improve electromigration lifetime of Cu and one of them is doping of Cu with
impurities such as Mn [57]. The disadvantage of this approach is that the impurity will increase
the resistivity of Cu.
45
2.3.7.2 Stress Induced Voiding
Manufacturing processes of Cu interconnects (ILD deposition and cap) is done at higher
temperatures and can cause voids due to tensile stress in the metal. Voids in Cu will form if the
tensile stress is above the critical stress [58]. This could happen as a result of thermal expansion
mismatch between the metal and the dielectrics or due to grain growth in the metal [58-59].
Voids formed can grow very larger causing increase in resistance that can lead to circuit failure.
At the same time, if there is a weak adhesion between the barrier metal and the underlying Cu at
the bottom of via, void formation will be more favorable because of the reduction in critical
stress. After a void forms, the stress field surrounding the void becomes less tensile which the
favors vacancy diffusion towards the void resulting in further growth. Voids tend to grow in vias
because the tensile stress in vias and narrow lines is lower than tensile stress in wide lines and
vacancies tend to move to the region of low tensile stress [59]. Stress induced voids can be a
cause a huge reliability issue and must be addressed especially with the dimensions of Cu lines
becoming smaller and smaller.
2.4 Summary
This chapter is a review of process integration of dual damascene Cu interconnects. The
various steps in involved in Cu interconnect processing were described at the same time
highlighting the key issues as pertaining to integration and manufacturing. It establishes the
context for understanding damascene integration of Cu and other new materials like low k
dielectrics and diffusion barriers.
46
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52
CHAPTER 3
BIMETALLIC CORROSION BEHAVIOR OF COPPER ON RUTHENIUM AND COBALT
ON COPPER THIN FILMS IN POST CMP CLEANING SOLUTIONS
3.1 Introduction
In modern interconnect systems, copper is the metal of choice for wiring because of its
higher electrical conductivity and improved electromigration resistance [1-2]. Cu interconnects
are fabricated using dual Damascene technique as Cu cannot be etched by conventional reactive
ion etching process due to lack of volatile by-products (Cu halides) at processing temperatures.
Dual Damascene process involves first creating trenches and vias in the interlayer dielectric
(ILD) followed by cleaning and diffusion barrier/Cu seed deposition. Subsequently, Cu is
electroplated and finally removal of Cu overburden is achieved by chemical mechanical
polishing (CMP) [3-4]. CMP exposes Cu and the underlying barrier to the corrosive chemicals
which could lead to bimetallic corrosion as illustrated in figure 3.1. This has brought attention to
the phenomena of thin film corrosion that must be addressed in order to ensure the maximum
device performance, reliability and longevity. It has also accelerated research and developments
of wet etch and clean chemistries that will not erode Cu while still performing the intended
purpose.
As devices are scaled to sub 22nm domains, the threat of corrosion related reliability is
increasing. In addition, ruthenium (Ru) has been studied and proposed as potential liner material
for Cu as it is more conductive and Cu can be directly plated to Ru thereby eliminating the need
for Cu seed layer [5-7]. The increased noble character of materials like Ru when integrated in Cu
interconnects can further intensify Cu bimetallic corrosion in CMP chemical environment.
53
Figure 3.1 illustrate dual Damascene process and specific areas where corrosion concerns are
shown.
Figure 3.1 Dual damascene process
Copper corrosion is a big problem not only to microelectronic industry but also to other
industries as copper has many applications. This chapter will review the basic concepts of
corrosion and bimetallic corrosion of Cu on Ru denoted as Cu/Ru and Co on Cu denoted as
Co/Cu in commercial cleaning solutions using micropattern corrosion screening and
electrochemical techniques.
3.2 Metal Corrosion
Corrosion is the destructive degradation of a metal as a result of a chemical reaction
between a metal and the environment. Corrosion is estimated to cost about 4.9% of the GNP of
54
the industrialized nations [8]. Corrosion also depletes natural resources as it is estimated that
40% of the Cu in production is used to replace copper lost to corrosion [9]. Corrosion can be
classified based on the environment in which corrosion takes place such as electrochemical
corrosion, low temperature and high temperature corrosion. Generally, corrosion is categorized
into localized corrosion and general corrosion. General corrosion is the most common type of
corrosion which results in greater destruction of materials by weight loss. It occurs due to
chemical and electrochemical reactions that proceed uniformly over the entire exposed area of
the material. Localized corrosion on the other hand limits its destruction to specific area. Several
forms of corrosion are discussed below [10].
• Galvanic corrosion – when two dissimilar metals are coupled in the presence of a
corrosive electrolyte, one of them is preferentially corroded than the other. The driving
force for galvanic corrosion is the potential difference between the two metals.
• Crevice corrosion – corrosion rate of a metal is often greater in the small volume of the
crevice created by contact with another material. It is believed that crevice corrosion is
driven by concentration differences in metal ions or dissolved oxygen between the
crevice interior and its surroundings.
• Pitting corrosion – localized attack on an otherwise resistant surface produces pitting
corrosion. The pits may be deep, shallow or undercut. Pitting corrosion usually is
initiated by the breakdown of a protective native film on the metal.
• Intergranular corrosion – grain boundary or adjacent regions are often less corrosion
resistant and preferential corrosion at the grain boundary may be severe enough to drop
grains out of the surface due to reactive impurities segregating out or passivating
elements such as chromium may be depleted at the grain boundaries.
55
• Dealloying (selective corrosion) – an alloying element that is active (negative
electrochemically) to the major solvent element is likely to be preferentially corroded.
Several corrosion prevention technologies have been proposed and include material
selection, alteration of application environment, design, cathodic protection and use of corrosion
inhibitors [11]. Use of corrosion inhibitors is generally the preferred choice for microelectronic
applications [11].
Corrosion inhibitor is a substance that is added to the environment and acts to reduce the
corrosion rate. Since corrosion involves both anodic and cathodic reactions, inhibitors are
classified by their mechanism of corrosion inhibition. Those that reduce corrosion by retarding
anodic reaction are anodic inhibitors and those that retard cathodic reaction are cathodic
inhibitors while those that retard both anodic and cathodic reaction are mixed inhibitors.
Inhibitors can also be classified based on their chemical nature i.e organic, inorganic etc.
Inhibitors must interact strongly with the metal surface in order to moderate the reactivity of
metal in corrosion reactions. The metal is protected by several mechanisms, including changes in
the electric double layer, the formation of surface barrier layers, the passivation of the metal
surface and the intervention in the partial reactions of corrosion.
For applications in Cu interconnect systems, it is important to understand the mechanism
of copper corrosion in order to be able to design, select and use corrosion inhibitors that are
effective in inhibiting corrosion. In aqueous solution, Cu corrosion occurs as a result of both
anodic and cathodic reactions. These reactions are dependent on the pH and this is illustrated by
pourbaix (E-pH) diagram for Cu-water system (Figure 3.2) [11]. Anodic reaction during Cu
corrosion occurs by the following equation;
56
Cu Cu+ + e-
Cu+ Cu2+ +e-
Figure 3.2 Pourbaix diagram of Cu-water system [11]
The cathodic reaction involves the reduction of oxygen as hydrogen evolution is not part
of the dissolution process based on the pourbaix diagram. Thermodynamics can also be used to
predict the likelihood of corrosion by comparing the standard potentials for Cu reduction for
example
57
Many studies in corrosion and corrosion inhibition have been done but up to date there is
still insufficient knowledge to predict how a specific metal or alloy will behave in a given
environment. The chemical properties of a given metal are dependent on the environmental
condition. The importance of chemical properties of the metal will also depend on the specific
application and the desired results for example in Cu interconnects electrical conductivity,
adhesion, electromigration resistance and corrosion resistance are of major importance.
3.3 Experimental
3.3.1 Micropattern Corrosion Screening
Micropattern Corrosion Study was done by depositing circular micro dots of Cu and Co
(ca. 50nm thick, 130 µm in diameter) on selected substrates through a contact mask using a
standard DC magnetron sputter (Desktop Pro, Denton Vacuum). Figure 3.3 depicts the
fabrication sequence of the micropattern corrosion screening. The Ru and Cu metal substrates
were sputter-deposited on a silicon substrate pre-cleaned by standard organic clean and HF etch
[12]. In-situ corrosion processes was investigated visually using a metallurgical microscope
(Nikon, Eclipse ME600) by immersing Cu micropattern and Co micropattern structures in
testing solutions. This visual inspection approach requires an optical clear solution being used.
58
Figure 3.3a Micropattern corrosion screening sequence
Micropattern corrosion screening technique provides Cu corrosion trend influenced by
different bimetallic contacts (Cu/Ru. Cu/Ta, Co/Cu e.t.c). Hence, the effect of bimetallic contact
can be determined readily under different chemical environement conditions. The micropattern
screening not only provides semi-quantitative trend of relative corrosion rate, but also affords a
direct visual inspection of actual corrosion processes in real time. The direct imaging of the
micropattern is useful in identifying different metastable surface transformations involved in the
corrosion process. It is also a fast corrosion screening technique as Cu microdots were designed
to be thin enough to permit screening many corrosion inhibitors within a short period of time.
Figure 3.3b show example of time lapsed images of Cu on Ru corrosion in 0.1M NH4OH pH 2
solution.
59
Figure 3.3b Micropattern corrosion screening time lapsed images of Cu on Ru in 0.1M NH4OH pH 2 solution
Two different commercial cleaning solutions (Acidic and Alkaline solution) from ATMI
were used. The samples with the microdots were submerged in testing solution and the time-
lapsed images of the Cu and Co micropattern were recorded by a computer-controlled digital
camera connected to microscope. The time it took to completely corrode the microdots was used
to determine the corrosion rate.
3.3.2 Tafel Plots
A potentiostat (CH Instruments, USA) was employed to acquire the open circuit
potentials and Tafel plots. Both sputtered Cu, Ru and Co metal substrates and solid metal shots
were used as electrodes for all electrochemical data. The metal electrodes (d = 5mm) were
polished down to 0.5 micron mirror polishing and sonicated in 18.2 MΩ de-ionized water.
Three-electrode system with Pt counter and Ag/AgCl as reference electrodes were employed in a
60
glass cell to obtain Tafel plots data (IV curves) in corresponding solutions used in micropattern
corrosion screening.
3.3.3 Optical Profilometry
Optical profilometry is a technique that allows rapid and accurate three dimensional
measurements of microscopic features. After chemical exposure and galvanic etching, the exact
volume change of the metal dots was analyzed using a Zygo NewView 7000 series optical
profiler. Wafer coupons are imaged with a 2.5x objective using a 2x zoom as well as with the
10x objective. The resulting images are then processed to exclude the background and partial
dots allowing for a volume per dot measurement. Volume changes that were not visible via
standard optical microscope become evident and enabled collection of precise corrosion rate
data. By applying this technique to determine bimetallic corrosion, large numbers of formulation
components in varied combinations may be screened quickly enhancing the development of
cutting edge wet cleans products.
3.4 Results and Discussion
3.4.1 Bimetallic Corrosion of Cu on Ru in Post CMP Cleaning Solution
Cu corrosion in post CMP cleaning solution was studied using micropattern corrosion
screening technique. Kyle et al. [12] studied bimetallic corrosion of Cu in alkaline post CMP
cleaning solution. Figure 3.4 shows corrosion rates of Cu in alkaline solution as a function of pH
without inhibitors [12].
61
Figure 3.4 Corrosion rate of Cu/Ru in alkaline post CMP cleaning solution without inhibitors [12].
The results illustrated that increase in pH of the solution leads to enhanced corrosion of
Cu at room temperature. This observation is particularly very crucial with the continued scaling
of Cu interconnect beyond 22 nm since any small Cu loss could be detrimental. Micropattern
corrosion screening was also used to screen corrosion inhibitors in alkaline post CMP cleaning
solution [12-13]. Corrosion screening was done on 15 different potential corrosion inhibitors.
From the results shown in figure 3.5, 9 compounds were found to reduce the corrosion rate. The
solution used as background had a corrosion rate of 12.5 Å/min (pH 11.8) [12].
62
Figure 3.5 Corrosion rate of Cu/Ru in alkaline cleaning solution with 15 potential inhibitors [12]
Cu corrosion in acidic cleaning solution (pH =3) was also investigated using micropattern
corrosion screening using five inhibitors that were very effective for Cu corrosion in alkaline
solution. The investigation was done at 50oC in order to mimic industrial processing condition.
The corrosion rate of Cu/Ru at this condition was extremely high (>500 Å/min) as the microdots
were completely eroded within the first 10 second of immersion. Addition of corrosion inhibitors
resulted in decrease in corrosion rate but development of better corrosion inhibitors is needed for
this solution as the rate is extremely higher than the industrial standard of < 1Å/min . Figure 3.6
shows time lapsed images of Cu/Ru samples immersed in the test solution with five different
inhibitors.
63
Figure 3.6 Time lapsed images of Cu/Ru in acidic post CMP cleaning solution with inhibitors and graph of corrosion rate vs. inhibitors
3.4.2 Bimetallic Corrosion of Co on Cu in Acidic Post CMP Cleaning Solution
Cobalt corrosion was investigated as it is one of the metals used in Cu interconnects as a
capping layer. Cobalt contact with Cu could result in galvanic corrosion. Cobalt microdots
corrosion on Cu was studied in acidic solution at 50oC. This condition was also used in order to
mimic industrial processing conditions. Background corrosion rate of Co/Cu in acidic solution
was found to be very high (150 Å/min) based on micropattern corrosion screening. The time
lapsed images and corrosion rate are shown in figure 3.7.
64
Figure 3.7 Time lapsed images of Co/Cu in acidic post CMP cleaning solution
Several inhibitors (1-6) were investigated for Co corrosion inhibition using micropattern
techniques and among then two were identified as potential corrosion inhibitors for Co. Figure
3.8 shows graph of corrosion rate vs. inhibitors and time lapsed images of Co/Cu in acidic post
CMP cleaning solution with 6 different inhibitors
(a)
(b)
65
Figure 3.8 Corrosion rate vs. inhibitors (a) and time lapsed images of Co/Cu in acidic test solution with inhibitors 1-6 (b)
The inhibitors studied were azole compounds (10mM concentration in test solution) with
similar molecular structure so that the difference in inhibition efficiency is due to the difference
in electronic structure. From the data, it can be concluded that two compounds 1 and 2 are
potential inhibitors of cobalt. The samples were immersed in the test solution and after 22 hours
there was no observable change. Therefore the compounds are very effective corrosion inhibitors
of cobalt.
3.4.3 Activation Studies of Inhibitors 5 and 6 in Acidic Post CMP Cleaning Solution
Activation studies on inhibitors 5 and 6 were carried out after they were identified as
efficient corrosion inhibitors for Co/Cu through micropattern corrosion screening. The objective
of this study was to find out how long it takes for the inhibition of Co corrosion to start. The
66
study was needed since the size of Cu interconnects are becoming smaller and smaller and any
corrosion however small might lead to serious reliability issue.
Figure 3.9 Micropattern corrosion of Co/Ru in acidic post CMP cleaning solution after pretreatment with inhibitors 5 and 6 for 2, 5 and 30 minutes
This study was accomplished by pretreating the Co/Cu microdots in the test solution
containing 10mM of each of the two effective inhibitors for 2, 5 and 30 minutes. Micropattern
corrosion screening was subsequently used to determine how long it takes to completely corrode
the pretreated microdots in the test solution without inhibitors. From the results shown in figure
3.9, it can be concluded that the activation time for both the inhibitors is less than 2 minutes.
This is because the corrosion rate was the same irrespective of the pretreatment time. Optical
profilometry was used to determine the thickness lost during pretreatment before the action of
the inhibitor fully kicked in. This was accomplished by pretreating the samples with test
67
solutions containing inhibitors 5 and 6 and subsequent analysis of Co microdots thickness
remaining. Figure 3.10 shows Co microdots thickness and top down images after 30 min
pretreatment with acidic solution containing inhibitor 6.
Figure 3.10 Activation time and Co removal during pretreatment from optical profilometer
The results indicate that for inhibitor 5 the thickness lost is about 15nm while for
inhibitor 6 the thickness lost is about 5nm. This shows clearly that the activation time for
inhibitor F is less than the activation time for inhibitor 5 and therefore Inhibitor 6 is more
effective than 5.
68
3.5 Summary
In summary, bimetallic corrosion of Cu/Ru and Co/Cu was studied using bimetallic
corrosion screening technique. This method proved to be very useful in determining the
corrosion rates and also finding effective corrosion inhibitors. It is also fast and effective in
studying several corrosion conditions like presence of oxygen, pH and temperature can be
monitored. Corrosion of metals is dependent on the solution chemistry and experimental
condition. For Cu interconnects, device scaling makes corrosion a big problem because the
amount of metal that can be corroded to cause a circuit failure is also becoming very small.
Therefore efficient inhibitors with fast activation time need to be developed in order to avoid
reliability issues and increase device yield.
3.6 References
1. T. J. Spencer, T. Osborn, and P. A. Kohl, Science, 320, 756 (2008).
2. T. Osborn, A. He, N. Galiba, P. A. Kohl, J. Electrochem. Soc.,155, 308 (2008).
3. G. Banerjee, and R. L. Rhoades, ECS Trans., 13, 1 (2008).
4. S. Armini, C. M.Whelan, M. Moinpour, and K. Maex, J. Electrochem. Soc., 156, 18
(2009).
5. T. N. Arunagiri, Y. Zhang, O. Chyan, M. El-Bouanani, M. J. Kim, K. H. Chen, C. T. Wu,
and L. C. Chen, Appl. Phys. Lett., 86, 083104 (2005).
6. O. Chyan, T. N. Arunagiri, and T. Ponnuswamy T, J. Electrochem. Soc., 150, 347 (2003).
7. R. Chan, T. N. Arunagiri, Y. Zhang, O. Chyan, R. M. Wallace, M. J. Kim, and T. Hurd,
Electrochem. Solid State Lett., 7, 154 (2004).
69
8. J. T. N. Atkinson and H. Vandrofflear, Corrosion and its control, NACE, Houston
(1985).
9. H. H. Uhlig and R.W. Revie, Corrosion and Corrosion control, John Wiley & Sons,
Chichester (1985).
10. D. A. Jones, Principles and Prevention of Corrosion, Prentice-Hall, London (1996).
11. T. Desmond, J. Electrochem. Soc., 145, 3 (1998).
12. K. K. Yu, K. S. M. Pillai, P. R. Nalla, and O. Chyan, J. Appl. Electrochem., 40 (2010).
13. P. R. Nalla, K. S.M. Pillai, K. K. Yu, S. Venkataraman, and O. Chyan, Proc. of
Advanced Metallization Conference, 83 (2009).
70
CHAPTER 4
STUDY OF PYRAZOLE AS COPPER CORROSION INHIBITOR IN MODEL ALKALINE
POST CHEMICAL MECHANICAL POLISHING CLEANING SOLUTION
4.1 Introduction
Recently, in the microelectronic fabrication process, copper has replaced aluminum as the
metal of choice for wiring due to its high electromigration resistance and low electrical resistance
[1-2]. Despite substantial reduction in resistance-capacitance (RC) delay, several challenges were
encountered in the integration of copper as wiring metal. One of the major challenges is the lack
of volatile copper compounds at temperatures less than 1000C making it difficult to directly
pattern copper lines, rendering reactive ion etching (RIE) process impractical [3]. A more
pragmatic approach to the Cu integration process involves deposition of a diffusion barrier (Ru,
Ta, TaN) and Cu metal into interlayer dielectrics (ILD), known as Damascene process, followed
by removal of overburden metals by chemical mechanical polishing (CMP) technique [4-5].
CMP process and post CMP cleaning exposes Cu interconnect and barrier to the corrosive
chemicals relevant to these processes. This results in bimetallic corrosion that can be detrimental
to the yield and reliability of integrated circuits devices. Furthermore, CMP process leaves Cu
residues, organic residues, abrasive particles and other contaminants on the surface that can
degrade the electrical properties of ILDs, lower the conductivity of Cu and lead to poor adhesion
of the subsequent layers [6].
To avoid reliability issues, an effective post CMP cleaning step is therefore required to
remove the residues and contaminants. Both acidic and alkaline cleaning solutions have been
developed in recent years that effectively remove Cu residues and contaminants. Recent study
has shown alkaline post CMP cleaning solution to be more effective to achieve a good cleaning
71
performance and low surface roughness [7]. One of the advantages of alkaline cleaning
chemistry is the selective dissolution of CuO at high pH, leaving Cu2O passivating layer on Cu
surface. Also at high pH conditions, negative charge on the slurry particles supposedly increases
to result more efficient particle removal [8]. Alkaline post CMP cleaning solution containing
TMAH, a strong base, has been used in proprietary chemical agents for effective inhibiting and
preventing re-adhesion of particles that are removed during the cleaning process [9]. Although
the TMAH based solution is efficient in removing the residues and contaminants, it has been
documented to be corrosive towards Cu and could therefore cause serious reliability issues [9].
This is a significant drawback as the Cu/diffusion barrier contacts are inevitably exposed to the
corrosive chemicals used during the cleaning process. Ruthenium (Ru) has been explored as
alternative barrier/liner materials to replace tantalum/tantalum nitride (Ta/TaN) bilayer stack
because of the scaling difficulty [10-12]. However, Cu/Ru bimetallic corrosion could be of
concern since Ru belongs to the platinum group metals that is more noble than Cu. Previously,
we reported a novel bimetallic corrosion testing technique, which is an effective methodology for
evaluating bimetallic corrosion of Cu interconnects when exposed to CMP and post CMP
cleaning conditions and showed that Cu/Ru exhibit enhanced corrosion compared to Cu/Ta [13].
Micropattern corrosion testing technique has the advantage of allowing actual monitoring of
corrosion of Cu when in contact with a barrier metal unlike other methods hence the bimetallic
effect can be evaluated.
Reliable Cu metallization process is achieved when metal loss is minimized during post
CMP cleaning and is achieved by addition of a corrosion inhibitor into the cleaning solution.
Significant progress had been made in the study of copper corrosion inhibitors and nitrogen
heterocycles have been found to be effective metal corrosion inhibitors. This is because of their
72
chelating action and formation of physical barrier on the surface of the metal that prevent
corrosion [14]. Among them, benzotriazole (BTA) has been extensively investigated and used as
Cu corrosion inhibitor in both acidic and alkaline post CMP cleaning solutions [15-18]. Neutral
BTA exists in two tautomeric forms in equilibrium – 1 H-benzotriazole and 2 H-benzotriazole,
where the former is the predominant species (99.9%) in both solution and gas phases. Due to the
presence of multiple electronegative nitrogen atoms in the ring, BTA has appreciable NH
acidicity (pKa ~8.2) and at high pH conditions, mostly exists in its highly resonance stabilized
conjugate base form, benzotriazolyl anion. BTA has been widely used as metal corrosion
inhibitor due to its strong metal-chelating capability and presence of hydrophobic benzene ring
(water solubility is 1.8-2.5 wt%). BTA physisorption at the metal interface, formation of BTA-
metal complexation monolayer and subsequent molecular self assembly via strong π-π
interaction between BTA aromatic rings combine to result a hydrophobic protective film, ([Cu+
BTA-]n), on the metal surface rendering it inaccessible for any attack from corroding chemicals
[19-21]. Inhibition efficiency of BTA for Cu corrosion is a function of the temperature,
concentration of BTA, immersion time, oxidation states on Cu surface and the pH. BTA
adsorption is much faster on Cu2O layer than on CuO or pristine Cu surface and therefore
thickness of Cu2O underlayer determines thickness of the Cu-BTA protective layer. The
adsorption of BTA on an oxide-free Cu surface, which is the case in very high pH conditions, is
suggested to be improbable or minimal [22]. According to Pourbaix diagram, at higher pH, Cu
surface comprises less oxide layer and mostly exists in Cu0 state and at pH approaching 14, Cu
directly transforms into CuO22- ion [23]. Moreover, due to its poor aqueous solubility and
inherent hydrophobicity, BTA contributes to increased organic residue defects on water surface
and therefore not efficient when used as Cu corrosion inhibitor in aqueous alkaline post CMP
73
cleaning solution. Therefore, BTA needs to be replaced by an efficient corrosion inhibitor that is
effective in highly alkaline (> pH 12) post CMP cleaning formulations and the treated surface is
more hydrophilic in nature.
One of the potential substitutes is pyrazole which is readily water soluble unlike BTA.
Owing to annular tautomerism, unsubstituted NH-pyrazoles can exist in two equally contributing
tautomeric forms. It is a weaker NH acid (pKa ~14.21) than BTA partly due to dimer formation
via intermolecular hydrogen bonding. The conjugate base, pyrazolyl anion, is stabilized by two
equally contributing resonance forms [24]. The inhibition effect of pyrazole and pyrazole
derivatives on the corrosion of Cu has been studied in hydrochloric acid using impedance
spectroscopy and polarization methods [25-26]. It was found that inhibitors adsorbed on Cu
surface to form pyrazole-Cu (II) complex protective layer without changing Cu dissolution
mechanism. Research of pyrazole as Cu corrosion inhibitor has largely been done in acidic
solutions and very little in alkaline solutions [27] using electrochemical methods. Figure 1 shows
molecular structures of pyrazole and BTA.
In this study, we report the effectiveness of pyrazole in the inhibition of Cu corrosion in 8
wt.% TMAH (pH 14) solution and compare it to BTA under the same conditions. The
investigation was carried out in-situ using the micropattern corrosion screening technique.
Corrosion potentials and currents measured by electrochemical technique, and XPS analyses
complemented well with micropattern corrosion screening results to demonstrate efficiency of
pyrazole in Cu corrosion inhibition in TMAH based alkaline post CMP cleaning solution.
74
Figure 4.1 Structures of (a) benzotriazole (BTA) and (b) pyrazole
4.2 Experimental
Corrosion measurement in this study was done by micropattern corrosion testing
technique described elsewhere [13]. As demonstrated in figure 2, copper microdots (ca. 50 nm
thick, 130 µm in diameter pattern transferred via contact mask on to Ru, Ta and glass substrates)
as well as Ru and Ta substrates on Si were deposited using magnetron sputtering (Desktop Pro,
Denton Vacuum). The actual corrosion process of micropattern array is recorded using an optical
microscope (Nikon, Eclipse ME600). The corrosion rate (Å/min) can be estimated by the time of
complete disappearance of Cu dots after immersion in the probing chemical solution. Bimetallic
corrosion screening technique has the advantage of allowing the direct observation of corrosion
of a metal in direct contact with another metal hence the effect of bimetallic contact can be
determined instantly under different conditions. It is also a fast corrosion screening technique as
Cu microdots were designed to be thin enough to permit screening many corrosion inhibitors
within a short period of time.
75
Figure 4.2 Micropattern corrosion screening structure
Pyrazole (98% Sigma-Aldrich), BTA (97% Sigma-Aldrich), and TMAH (25 wt.%
Sigma-Aldrich) were used as received. Model aqueous cleaning solution containing 8 wt.%
TMAH (pH 14) in water, as a representative of mainstream proprietary alkaline cleaning
solution was prepared using pre-purified water (>18.2 MΩ, Millipore integral 3) [6,28]. All
electrochemical measurements were done using CHI 760D (CH Instruments) potentiostat. Cu
shot, 5mm in diameter was used as the working electrode. The metal electrode was polished
down to 0.5 micron mirror polishing and sonicated in de-ionized water. A three-electrode system
with Pt as the counter electrode and Ag/AgCl as reference electrode was used in an
electrochemical cell. Electrochemical impedance spectroscopy (EIS) measurements were
completed by superimposing an ac signal with amplitude of 5 mV peak to peak and frequency
range from 100 kHz to 50 mHz. The EIS results were analyzed using ZSimpWin software.
XPS samples were prepared by sputter depositing Cu on Ru substrate (1cm2) and immersing
them in 8 wt.% TMAH solution containing 1mM pyrazole and 10mM BTA for 20 minutes. The
samples were rinsed with de-ionized water and blow-dried with dry nitrogen purge. XPS
76
analyses were conducted ex-situ using a PHI 5000 VersaProbe, a multi-technique surface
analyses instrument equipped with Al Kα (1486.7 eV) radiation and dual-gun charge
compensation system for analysis of all sample types.
4.3 Results and Discussion
4.3.1 Effect of Substrate on Cu Corrosion
The nature of bimetallic contact was studied to determine the effect on the rate of Cu
corrosion. Figure 3a shows the time-lapsed images of Cu micropatterns on three different
substrates; Ru, Ta and glass submerged in the alkaline TMAH (8 wt.%, pH=14). Ta is currently
used as part of the diffusion barrier for Cu interconnects in integrated circuit devices [29-30]
while Ru is a new promising candidate for liner metal because Cu can be directly plated onto Ru
without Cu seed [31-32]. Glass was chosen as a non-conductive dielectric substrate. As shown in
Figure 3, Cu microdots on Ta substrate required over 4X the amount of time as compared to Cu
microdots on Ru substrate to corrode completely in TMAH pH=14 solution. Based on the
micropattern screening, the Cu corrosion trend follows Cu/Ru (27 min) > Cu/Glass (56 min) >
Cu/Ta (120 min). Tantalum has a strong tendency to be oxidized and form tantalum oxide
(Ta2O5) especially when exposed to aqueous medium, Eq. 1.
2Ta + 5H2O = Ta2O5 + 10H+ + 10e- ------ (1)
Ta exhibits a thermodynamically favorable oxidation reaction that donates electrons
through the Cu/Ta bimetallic contact. This results in cathodic protection of Cu microdots. On the
other hand, Ru is nobler than Cu therefore Cu oxidation is facilitated through Cu/Ru bimetallic
77
couple. Cu microdots on glass was used to represent Cu only corrosion case and as expected it
exhibited a corrosion rate that is in between Cu/Ru and Cu/Ta [13].
Figure 4.3 Time lapsed images of Cu microdots deposited on Ru, Ta and glass in 8 wt.% TMAH solution
To confirm the trend of galvanic corrosion, Tafel plots (figure 4.4) were recorded for Cu,
Ru and Ta in TMAH pH 14 solution. The corrosion potentials mostly followed a general trend of
Ecorr, Ru > Ecorr, Cu > Ecorr, Ta which correlated well with the expected metal nobility trend.
78
Figure 4.4 Tafel plots of Ru, Cu and Ta measured in TMAH pH 14 solution
4.3.2 Cu Micropattern Corrosion and Inhibition
Chemical composition of post CMP cleaning solution has a direct effect on bimetallic Cu
corrosion process. The continued shrinking of Cu interconnects features in order to satisfy
Moore's law has led to increased need to minimize Cu corrosion during post CMP cleaning to an
industrially accepted rate of <1Å /min. The Pourbaix potential-pH diagram indicates that
alkaline solution condition could facilitate Cu to corrode more readily. Cu bimetallic contact
with Ru leads to accelerated corrosion which was confirmed using micropattern corrosion
screening technique. The nobility of Ru in TMAH solution and with addition of pyrazole and
BTA was determined using Tafel plots. The icorr and Ecorr were found to have negligible
difference for the three plots therefore confirming that Ru in TMAH is not affected by the
presence of corrosion inhibitors .The proposed mechanism of Cu/Ru corrosion is shown below.
79
Ru cathode
)(442 22 ORROHeOHO Slow −− →++ Oxygen reduction reaction
Cu anode
22
22
2
22
)(
222
2222
OHCuOHCueOHCuHOCu
eHOCuOHCu
Slow→+
++→+
++→+
−+
−++
−+
Figure 4.5 shows corrosion rate of Cu microdots on Ru in 8 wt.% TMAH with respect to
increasing concentrations of pyrazole and BTA. Figure 4.6 displays the time lapsed images of Cu
microdots after immersion in the same TMAH solution and with 10mM BTA and 1mM
pyrazole. The time required to completely erode 50nm Cu dots can be used to gauge the relative
rate of corrosion. The corrosion rate can therefore be estimated to be inversely proportional to
corrosion time. The relative Cu corrosion rate of Cu/Ru micropattern in 8 wt.% TMAH (27Å
/min) only partially diminished after addition of up to 10mM BTA (8Å /min). Comparatively, Cu
corrosion rate of Cu/Ru micropattern was significantly reduced (<1Å /min) after addition of
1mM pyrazole in 8 wt.% TMAH solution. Further increase in pyrazole concentration did not
result any substantial corrosion rate decrease. From the results, we conclude that 1mM pyrazole
is needed to achieve the industry acceptable Cu corrosion rate of <1Å /min. Furthermore, it is
desirable to keep the inhibitor concentration very low because the excess inhibitor on Cu surface
becomes the source of organic contamination that can affect the conductivity and interlayer
adhesion of the subsequent layers. This demonstrates that pyrazole is a better candidate for Cu
corrosion inhibition in highly alkaline TMAH solution as compared to the industrial standard,
BTA. All subsequent experiments in this paper were carried out using 1mM pyrazole and 10 mM
BTA.
80
Figure 4.5 Inhibitor concentration dependent etch rate of Cu in 8 wt.% TMAH
Figure 4.6 Time lapsed images of 50nM Cu/Ru immersed in 8 wt.% TMAH with additional 1mM pyrazole and 10mM BTA
81
4.3.3 Electrochemical Analysis
4.3.3.1 Tafel Plots
The trend of Cu corrosion was also monitored using Tafel plots. The rate of corrosion can
be theoretically calculated from corrosion current which is obtained from extrapolation of anodic
and cathodic curves. As shown in Figure 4.7, the corrosion current follow the trend of icorr, no
inhibitor> icorr, 10mM BTA > icorr. 1mM pyrazole which correlated well with the results from micropattern
corrosion screening while the Ecorr remain relatively the same. The disadvantage of Tafel plot
method is that the corrosion rate is obtained from fresh electrode/solution and the study is limited
to short term study only. Furthermore, corrosion study by Tafel plots is based on a single metal
electrode and cannot be directly applied to bimetallic corrosion which are both addressed by
micropattern screening technique.
Figure 4.7 Tafel plots of Cu in 8wt.% TMAH and with pyrazole and BTA
82
4.3.3.2 Electrochemical Impedance Spectroscopy (EIS)
EIS is an effective technique that is used in the analysis of various steps involved in an
electrochemical reaction by measuring the response of impedance system to a small ac potential
in a wide frequency range [33]. It provides a method for measuring the resistance against the
transfer of ionic species to the metal surface and has been used to evaluate the barrier properties
of corrosion inhibitors [34-35]. Corrosion of Cu in 8 wt.% TMAH solution in the presence of
BTA and pyrazole inhibitors was investigated by EIS to substantiate the aforementioned
effectiveness of pyrazole as Cu corrosion inhibitor. The results of the successive impedance
scans are shown in figure 4.8 in the form of Nyquist plots. All the impedance data were fitted
using an equivalent circuit shown in figure 4.8 (inset) where Rs represents the solution resistance,
Rdl is the charge transfer resistance, Qdl is double layer constant phase element (CPE), Rf is layer
resistance and Qf (CPE) represent the protective properties of the film on Cu. This equivalent
circuit has been used to fit impedance data in studies of corrosion inhibition by organic coatings
[34,36-37].
As seen in the Nyquist plot (figure 4.8), addition of BTA and pyrazole increases the
charge transfer resistance of the Cu electrode in TMAH solution. This is arrived at by evaluating
the diameter of the semicircular Nyquist plots which clearly increases with the addition of BTA
and pyrazole to TMAH. The increase in charge transfer resistance could be due to the adsorption
of inhibitor molecules on the Cu surface which modifies the surface by decreasing the electrical
capacity because of displacement of water molecules and other ions originally adsorbed on the
surface. It is particularly important to note that the charge transfer resistance with addition of
pyrazole is ~ 10X higher than that of the TMAH solution contains BTA. This confirms the
formation of a denser protective layer on the surface of Cu by pyrazole compared to the one
83
formed by pyrazole which explains why pyrazole exhibited effective corrosion inhibition shown
by Micropattern corrosion screening.
Figure 4.8 EIS data (a) Nyquist plot of Cu in TMAH (black), TMAH +BTA (red) and TMAH+pyrazole (blue) and inset equivalent circuit used to fit data.
4.3.4 Water Contact Angle Measurement
Surface tension or wettability of Cu surface is an important aspect in post CMP cleaning.
It has direct impact on the cleaning results as a hydrophilic surface enables relatively easy
flushing away contaminants and minimizes watermarks [38]. Cu samples were submerged in
TMAH solution and TMAH with 1mM pyrazole and different concentrations of BTA for 20
minutes followed by DI water rinse and air-dry and subsequent contact angle measurement on
84
the treated surfaces. As shown in Figure 4.9, significant differences of wettability of Cu were
observed among the test solutions. The TMAH solution containing BTA produced hydrophobic
surface and hydrophobicity increased with concentration. Surface hydrophibicity of
BTA/TMAH-treated Cu surface is commonly attributed to the specific orientation of BTA
molecules relative to the Cu surface where hydrophobic benzene ring is facing away from Cu
surface and therefore forming a protective hydrophobic barrier [39]. The TMAH solution
containing pyrazole produced a relatively hydrophilic Cu surface after immersion compared to
the ones treated with BTA/TMAH solution. This is likely due to relatively smaller size of
pyrazole molecule owing to the absence of large hyrdrophobic benzene moiety. As a result, a
thin hydrophilic Cu-Pyrazole complex is adsorbed Cu surface.
Figure 4.9 Variation in DI water contact angle of Cu in TMAH and TMAH+ inhibitor
85
4.3.5 Surface Analysis
4.3.5.1 XPS Analysis
XPS analysis was done on three substrates; bare Cu, Cu immersed for 20 minutes in
TMAH with 10mM BTA and TMAH with 1mM pyrazole. Figure 4.10 present the spectra of Cu
2p and auger spectra (Cu LMM). From the results, it is clear the chemical surface states are
different for the three substrates. Bare Cu contains CuO and CuOH which is indicated by Cu
2p3/2 at 933.8 eV and Cu LMM at 569.4 eV (Figure 4.10a and b).The intense shake up satellites
around 938-946 eV indicates the presence of Cu2+ which is evoked by the availability of unfilled
d-orbitals (d9) [40-42]. The peak at 932.5 eV is attributed to either Cu2O or metallic Cu. The
difference in energy between Cu2O and Cu is about 0.1eV [43] and it was difficult to distinguish
the two with the resolution of our instrument. CuLMM spectra on the other hand has a difference
of about 2.6 eV between Cu2O (570.5 eV) and Cu (567.8 eV). This is as a result of the relaxation
energy difference between the materials [44]. In bare Cu analysis, Cu2O is not present because of
the absence of the characteristic CuLMM peak at 570.5 eV.
The Cu 2p3/2 component of Cu immersed in TMAH solution with 10mM BTA (Figure
4.10c) shows a sharp peak at 932.5 eV which is attributed to Cu2O or metallic Cu. This is
confirmed by the CuLMM peak at 570.5 eV (Figure 4.10d) which is typical of Cu2O. This
indicates that a protective film is present on Cu surface, but protection is not effective based on
micropattern corrosion screening results. Cu substrate immersed in TMAH solution containing
1mM pyrazole showed a sharp peak on Cu 2p spectra at 932.5 eV (Figure 4.10e). The actual
identification was confirmed by CuLMM spectra with 2 peaks, the main peak at 567.8 eV (Figure
4.10f) that is typical metallic Cu and another peak at 570.5 eV that is Cu2O [45-46]. This reveals
the absence of Cu2+ on the substrate treated with pyrazole and could be as a result of formation
86
of Cu(1)-pyrazole complex on Cu surface. The presence of the Cu2O on the surface after
treatment with pyrazole may suggest the adsorption process involves oxidation of Cu atoms to
Cu+ followed by chemisorption of the inhibitors.
Figure 4.10 XPS Cu 2p spectra of; (a) bare Cu, (b) BTA modified Cu and (c) pyrazole modified Cu and Cu LMM spectra of; (d) bare Cu, (e) BTA modified Cu and (f) pyrazole modified Cu
Figure 4.11 is the N1s spectra of three Cu substrates. As expected, bare Cu (Figure
4.11a) depicts absence of N1s peak. The substrate containing BTA in TMAH (Figure 4.11b) has
a single and significantly narrower peak located at 400.2 eV. The narrower N1s peak indicates
that the charge is evenly distributed by the conjugated π structure delocalized over the two N
87
atoms and both N1 and N3 are equivalent [47-48]. This could signify absence of Cu-N bonding
and explanation of ineffective corrosion inhibition as depicted by micropattern corrosion
screening. The sample treated with pyrazole (Figure 4.11c) has two peaks; one at 400.2 eV and
the other one at 399.6 eV. This shows that the two N atoms are not equivalent which may
indicate that one of the N atoms in pyrazole molecule is bonded to Cu(1) or Cu as bonding
changes the electron environment of one N atom compared to the second one [47]. This therefore
demonstrates that the effectiveness of pyrazole in inhibition of Cu corrosion is through formation
of Cu-pyrazole complex to form the inhibition film.
Figure 4.11 XPS N1s spectra of: (a) bare Cu, (b) BTA modified Cu and (c) pyrazole modified Cu
88
4.4 Proposed Mechanism of Cu Corrosion Inhibition
Based on the data obtained we propose a mechanism of Cu corrosion in TMAH solution
by pyrazole. This involves first an irreversible ionization of pyrazole (Pz) in solution as shown in
the following equation
Pz- then reacts with Cu+ from the anode reaction to form Cu(1)-Pz
The dissolution of Cu in 8 wt.% TMAH is therefore controlled by the electrochemical
reaction on the surface of Cu since ionization of pyrazole is an irreversible reaction. The Cu(1)-
Pz complex formed on the surface creates a physical barrier that prevents further contact of Cu
with TMAH and hence prevents Cu corrosion.
4.5 Summary
In this study, pyrazole was demonstrated using various experimental methods to be an
effective Cu corrosion inhibitor compared to BTA in alkaline post CMP cleaning solution
containing TMAH. 1mM of pyrazole showed effective inhibition of Cu corrosion while BTA up
to 10mM resulted in Cu Corrosion. From the XPS and ATR-IR analysis it was depicted that
pyrazole inhibit Cu corrosion in TMAH by the formation of Cu-pyrazole complex. The
reproducible micropattern corrosion screening confirmed that pyrazole is more effective
corrosion inhibitor that BTA in TMAH solution.
89
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CHAPTER 5
INTERFACIAL CHARACTERIZATION OF PLASMA TREATED COPPER SURFACES
RELATED TO ADVANCED COPPER INTERCONNECTS
5.1 Introduction
As the size of functional elements in integrated circuits (IC) decreases to follow Moore’s
law [1], reduction in the dimensions of components has been successful only to a certain extent
beyond which several problems arose. For example, decrease in the thickness of gate dielectrics
resulted in more leakage current through thin oxide layer. Also decrease in dimension of
metallization lead to increase in the resistance (R) and capacitance (C) of interconnections and
thereby increasing the RC delay [2-3]. To solve these problems, high conductivity metal wiring
and lower capacitance insulating materials have been developed. Copper was found to be the
best choice for wiring metal and replaced conventional aluminum because of its low bulk
resistivity and high reliability against electromigration [4]. Furthermore low-k interlayer
dielectrics (ILDs) were preferred over silicon dioxide because of low capacitance [5].
Plasma processes based both on reactive species and intense ion bombardment are widely
used in the semiconductor manufacturing [6]. The reactive species plasma is used for deposition
of dielectrics and photoresist removal whereas the ion-induced processes find wide use in
precision patterning processes. The main advantage of using plasma etching to make patterns on
dielectrics as opposed to wet etching is its high anisotropy i.e. the ability to provide vertical
etching in the direction normal to substrate and practically no lateral etching in the direction
parallel to the substrate. Though Cu is more conductive and highly favorable than Aluminum, it
has significant drawback with respect to patterning ability. Copper halides are non-volatile and
hence it is difficult to pattern copper by plasma etching [7]. Several approaches have been
94
investigated to improve the volatility of copper halides, such as wafer heating [8] and exposure
to ultraviolet (UV) radiation [9]; however, none of these methods led to a controllable
manufacturing process. Thus, an alternative method known as ‘Damascene’ processing is used
instead, to pattern copper [7]. In this method, the dielectric is first patterned, thereby forming
holes or vias to the underlying conductor layer; copper is then deposited into the vias to form
interconnect between conductor (metal) layers. In the ‘dual Damascene’ method, both via and
the metal line are patterned at the same time to reduce the number of processing steps. Copper
deposition is performed by electroplating, which effectively fills the vias after they are defined in
the dielectric. Excess copper is then removed by chemical mechanical polishing (CMP) [7].
Intermediate layers such as etch stop layers and hard mask layers are used to assist in obtaining
precise end or stop points (during plasma etching) and planar surfaces.
Generally fluorocarbon gas plasmas such as CF4, C2F6, CHF3, and C4F8 are used for the
etching of dielectrics [10]. Some of the undesired results of this plasma patterning process are the
formation of fluorinated layer [11], diffusion of ions into dielectrics etc. All of these undesired
processes results in the increase in dielectric constant. When CF4 gas is used, it results in
formation of fluorocarbon polymer deposits on the dielectrics sidewall. Oxygen is used along
with CF4 so that the O atoms decompose CF polymer on the porous silica surface during plasma
etching, thereby increasing etch rate. The amount of F radicals generated in CF4 plasma depends
on the amount of O2 addition because the density of F radicals reaches its maximum value at a
certain amount of O2 [12].
After the low-k dielectric has been etched away for creating vias and trenches, SiN or SiC
which is a stopper material to suppress Cu diffusion into the low-k materials was etched by
fluorocarbon plasma. During this over etching process, the surface of Cu is also exposed to
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plasma. Although the plasma will not etch the Cu surface but these fluorocarbon radicals and
ions interact with Cu and change the Cu surface chemistry. Since the Cu surface is very reactive,
the F ions induce degradation of Cu and results in reliability problems when packaging the
electronic components. Figure 5.1 shows plasma etching scheme of dielectrics and Cu exposure
to plasma.
Figure 5.1 Plasma etching of dielectrics and Cu plasma exposure scheme.
After the plasma etching of dielectrics, the photoresist and other plasma residue removal
is accomplished by oxygen- and nitrogen- containing plasma ashes but the problem with ash
process is that the surface becomes carbon depleted and becomes more hydrophilic [13-14]. An
alternative process for post-etch cleaning is done by organic solvents which shows a complete
removal of photoresist without carbon depletion and low-k increase [14-16]. It has been shown
that the presence of additives in organic solvent has a large impact in photoresist stripping
efficiency [17-21]. In order to improve removal efficiency, chemical additives containing
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fluorine and ammonium hydroxide were considered. Dilute HF and ammonium fluoride (NH4F)
were used due to their different degree of dissociation (in aqueous solution). Tetra methyl
ammonium hydroxide (TMAH) was chosen as a source of OH-. During this cleaning, the
underneath Cu surface which has been modified by the interaction of plasma etch gases, is
exposed to TMAH, a most commonly used chemical. To our knowledge, the plasma
modified Cu surface chemistry in post-etch cleaning solutions has not been extensively
documented in the literature. In order to help creating highly reliable interconnects, knowledge
on the behavior of this plasma modified Cu in post-etch cleaning solution is required.
In order to help create highly reliable interconnects, knowledge of the behavior of plasma
modified Cu in post-etch cleaning solution is essential. In this study, we evaluate the corrosion
behavior of copper in TMAH after five different plasma treatments i.e CF4, CF4+O2, CH2F2,
C4F8 & SF6 and analyze copper surface state after fluorocarbon plasma treatment using
micropattern corrosion screening, direct current measurement, water contact angle measurements
and XPS. In addition we evaluate the effect of benzotriazole (BTA ca. 10mM) addition to
TMAH on the corrosion behavior of copper.
5.2 Experimental
In this work, the galvanic effect between Cu and Ru in TMAH solution was investigated
using micropattern corrosion screening technique. Copper microdots of approximately 50 nm
thick and 130 μm diameter were sputter deposited on ruthenium substrate (denoted as Cu/Ru)
though a contact mask using a standard RF magnetron sputter (Desktop Pro, Denton Vacuum).
Ruthenium substrate (ca 100 nm thick) was sputter deposited on silicon having a 5nm thick
titanium adhesion layer deposited on it. Figure 5.2 illustrate micropattern corrosion screening
97
structure.The Cu/Ru and Cu/Ta micropattern coupons are treated with CF4, CF4+O2, CH2F2,
C4F8 and SF6 Plasma etch gas. The plasma treatments of these samples were done in Intel lab
unit using an OXFORD PLASMALAB 100 system. It is an ICP system where the top and
bottom plate bias can be controlled independently. The plasma treatment time was 30 sec for all
samples. The pressure maintained was 5 mTorr and temperature was 20 oC with 400W top and
50 W RF bias applied. The etch gas at each run are maintained at following flow rate (sccm): 20
C4F8/30 Ar, 50 CF4, 50 CF4/5 O2, 10 CH2F2/30 Ar, 20 SF6/30 Ar. In-situ corrosion investigation
of corrosion was done visually using a metallurgical microscope (Nikon, Eclipse ME600) by
immersing Cu micropattern samples in testing solution (8%TMAH, pH 14, Aldrich). Figure 2
shows the steps in Micropattern corrosion screening. Based on the time it took to corrode the Cu
microdots completely, the rate of corrosion was determined for each of the samples.
Corrosion trend was verified by direct galvanic current. This was done by connecting two
electrodes i.e. plasma-Cu sample and Ru shot to the two terminals in Keithley source meter and
immersing them in TMAH solution at the same time. The state of Ru surface was kept the same
by freshly polishing before measuring the current for each plasma treated Cu samples. Water
contact angle measurements to determine the surface wettability of the plasma treated samples
was done using water droplets. A droplet of DI water was slowly let sit on the surface of plasma
treated Cu.
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Figure 5.2 Micropattern corrosion screening structure
XPS analysis were conducted ex-situ using a PHI 5000 VersaProbe, a multi-technique
surface analysis instrument equipped with hemispherical analyzer, multi-channel detector and
sputter etching and electrons flood gun capabilities which are useful in studying charge shifts.
The x-ray source used in this study was an Al Kα 1486.7 eV excitation source in a high vacuum
system with a base pressure under 1×10−7 Pa. High resolution spectra was collected for each
sample with a pass energy of 23.4 eV, energy step of 0.1eV and data was collected at a take-off
angle of 45o.
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5.3 Results and Discussion
5.3.1 Micropattern Corrosion Study
The Cu surface chemistry change due to the plasma processing has been a great issue in
terms of their behavior in corrosion. To our knowledge, there is no literature published on how
these plasma radicals/ions modified Cu behaves especially in post etch cleaning solutions. For
the first time we studied the corrosion properties of these plasma treated Cu samples in 8%
TMAH solution at a pH of 14, using micropattern corrosion technique. Figure 5.3 shows
progressing corrosion images with time of Cu microdots deposited on Ru substrate and
immersed in 8 wt.% TMAH solution pH 14.
Figure 5.3 Progressing corrosion with time images
The time it took to corrode 50nm of Cu microdots was used to estimate the corrosion rate
in Å/min. The relative Cu corrosion rates observed was different for all plasma treatments and
100
followed the trend CF4+O2 (13.9 Å/min) < CH2F2 (15.6Å/min) < CF4 (17.9Å/min) < C4F8
(20.0Å/min) < SF6 (27.7Å/min). Figure 5.4 shows corrosion rate and time of plasma treated
Cu/Ru
Figure 5.4 Corrosion rate and time of plasma treated Cu/Ru
The results from corrosion screening indicate that SF6 plasma treatment induced fastest
corrosion of Cu in TMAH compared to the other four plasma treatments. This could be because
of the high fluorine concentration in the plasma that led to formation of CuF2 that easily
hydrolyze in TMAH solution. CF4+O2 plasma treatment was found to have the lowest corrosion
rate among all the plasma treatment even though the fluorine content is relatively high. This
could be because of the presence of oxygen plasma which could bombard fluorine radicals
lowering the amount of fluorine left to react with Cu surface hence decreasing formation of
101
CuF2. Another reason for the lower corrosion rate could be formation of a passivating native
oxide layer due to the presence of oxygen plasma which formed a barrier on Cu surface and
prevented corrosion. Different plasma treatment tends to form different surface oxides on Cu
which act as a barrier between Cu and corrosive TMAH solution hence the different corrosion
rates. Figure 5.4 is shows corrosion rate and time of plasma treated Cu in TMAH solution.
5.3.2 Bimetallic Contact Effect
The nature of bimetallic contact was also investigated after plasma treatment and found
to have a great effect the corrosion on Cu. Ta which is currently used as diffusion barrier for Cu
interconnects [22-23] was used as a substrate for Cu microdots and corrosion rate after plasma
treatment in TMAH solution was compared to that of Cu on Ru. Figure 5.5 shows the corrosion
rate of Cu/Ta and Cu/Ru. Corrosion rate of Cu on Ta substrate was approximately 3-4 times
lower than that of Cu on Ru substrate in TMAH solution which could be due to substrate
inducing effect. Since Ru is nobler that Cu, then bimetallic contact between them enhances
galvanic corrosion. On the other hand, Ta has a strong and thermodynamically favorable
tendency to be oxidized to tantalum oxide in aqueous solution by donation of electrons through
bimetallic contact of Cu and Ta providing cathodic protection to Cu microdots [24]. It was
noticed that in both substrates (Ru and Ta), high fluorine content plasma etch gases (C4F8 & SF6)
interaction with Cu surface led faster corrosion in TMAH solution.
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Figure 5.5 Comparison of Cu/Ru and Cu/Ta corrosion after plasma treatment
5.3.3 Direct Galvanic Current Measurements
In order to verify the corrosion trend from micropattern testing, actual galvanic corrosion
current was measured from each of the plasma modified Cu samples coupled with Ru.
Comparable current values were obtained by trend of galvanic corrosion by maintain the area of
the plasma-Cu electrodes exposed to solution the same for all samples. The measurements were
taken as soon as the electrodes are immersed in the solution to record the initial corrosion
current. The results showed that the initial corrosion current varied for different plasma treated
Cu samples. Figure 5.6 shows the galvanic corrosion current over the time of immersion in
TMAH against Ru shot electrode. From the results, CF4+O2 treated Cu sample had the least
corrosion current and followed the trend CF4+O2 < CH2F2 < CF4 < C4F8 < SF6.This supported
micropattern corrosion data since the rate of corrosion is directly proportional to the corrosion
current.
103
Figure 5.6 Direct current measurements of Cu Vs Ru in TMAH pH 14 solution
5.3.4 Water Contact Angle Measurements
Water contact angle is a simple technique used to quickly identify the nature of any
surfaces. A droplet of deionized water was slowly introduced on to the surface of plasma treated
Cu samples in order to characterize its hydrophilic and hydrophobic nature by contact angle.
Figure 5.7 shows the trend of water contact angle on the plasma modified Cu samples over
different immersion time in TMAH solution and totally for 15 minutes. The contact angle
measured was only on Cu surface and there was no exposure of Cu/Ru interface.
Prior to TMAH treatment, the contact angle for all plasma treated samples was ~75o-85o.
After 2 minutes immersion in TMAH solution, Cu samples exposed to CF4+O2, CH2F2, CF4 and
104
SF6 plasma treatments became hydrophilic. This could be an indication that these plasma
treatments deposited less CFx polymer residues on Cu surface and any residues on the Cu surface
is easily removed by alkaline TMAH solution. It was also observed that the contact angle of C4F8
stayed high relative to the other plasma treatments even after treating the sample for 15 minutes
in TMAH solutions. This could indicate the presence of CFx polymer residues on the surface that
is a lot and cannot be easily removed by TMAH. The presence of polymer layer coating did not
provide corrosion protection as shown by micropattern corrosion testing and direct galvanic
current measurements.
Figure 5.7 Time dependent water Contact angle measurements after progressive immersion in TMAH, pH 14 solution
105
5.3.5 XPS Analysis of Plasma Treated Cu
XPS was used to identify the chemical bonding structure on the plasma treated Cu
surfaces. High resolution C 1s, F 1s and Cu 2p is shown in figure 5.8a and b. In the analysis of
oxidation states and chemical bonding on Cu surface after plasma treatments, all peaks were
aligned using C 1s peak at a binding energy of 284.8 eV. In figure 5.8a, a peak at 284.8 eV that
corresponds to carbon that is not bonded to fluorine of CFX group is present in all the five plasma
treatments. This peak originates from two sources: ubiquitous carbon found on all surfaces and
fluorocarbon gas used for plasma treatment. It was also noted that all the plasma treatment
resulted in a peak at 288.5 eV which correspond both CF and C=O. CH2F2 plasma treatment
contains only C=O at 288.5 eV and this is supported by the absence of any fluorine peak on F1s
shown in Figure 5.8b while all the other four plasma treatments contain both CF and C=O.
Among all the different plasma etch gas treatments, C4F8 treated sample distinctively showed
multiple carbon peaks at higher binding energies that corresponds to C-CF/C-O (286.6 eV),
CF/C=O (288.5 eV), CF2 (291.3 eV) and CF3 (293.4 eV). The absolute binding energies stated in
this work for fall within the range (CF3-292.6 to 295, CF2-290.3 to 292.8, CF/C=O-288 to 290,
C-CF/C-O-285.5-287 and C-C/C-H-284 to 285.5) of published literature values [25- 28]. The
high electronegativity of fluorine causes a large shift in the binding energy of carbon. This is
probably due to the high amount of fluorine contained in C4F8 gas. The Cu samples treated by
other plasma etch gases mostly contained carbon peaks at 284.8 eV that belongs to C-C or C-H
bonding and 288.5 eV which belongs to C-F/C=O bonding.
106
Figure 5.8 XPS analysis of plasma treated Cu (a) C1s peak (b) F1s peak and inset F1s peak after one minute Ar+ sputtering
Figure 5.9 XPS analysis of Cu 2p peak
107
In figure 5.9, analysis of Cu 2p region, all the samples showed a Cu peak at 932.5 eV
which is in good agreement with the reported values in literature [29-31]. This peak location is
also associated with Cu(I) oxidation state of Cu with a very small shift of about 0.1eV [32].
Moving to higher binding energies, the samples treated with CF4, CF4+O2, and SF6 revealed
peaks at 934.7 eV and 935.8 eV that are assigned to Cu(OH)2 and CuF2 respectively. These are
also in agreement with the literature values reported [29, 33]. Samples treated with CH2F2 and
C4F8 only reveal presence of Cu(OH)2 depicted by the peak at 934.7 eV. The low intensity of Cu
2p peaks and absence of CuF2 peak after C4F8 plasma treatment confirms the presence of
fluorocarbon polymer layer on Copper surface. This is also in observed and confirmed on F 1s
(fig 8a) spectra with a peak at 688 eV for C-F bonding and absence of peak at 684.5 for Cu-F
bonding. This suggests that the fluorocarbon residue is an overlayer and not a chemical reaction
with Cu. Similarly CH2F2 plasma treatment on Cu did not show CuF2 peak at 935.8 eV meaning
no fluorination of Cu took place. This is also verified by the absence of peak on F1s region
which further suggested absence of fluorocarbon polymer coating on copper surface. This could
be attributed to the low fluorine concentration in the plasma gas and presence of hydrogen. The
peaks related to Cu2+ (934.8 eV and shake up satellite peak) could be as a result of storage and
transportation to ex-situ XPS spectrometer and both disappear after one minute of sputter
cleaning leaving Cu2O peak at 932.5 eV. Samples treated with CF4, CF4+O2, and SF6 further had
visible shakeup satellite associated with Cu (II) oxidation state which is an indication of the
presence of Cu2+ and also Cu-F bonding as shown in Fig 8a indicating fluorination of Cu by
these plasma treatment. It was noted that CF4, CF4+O2 and SF6 plasma treated samples after one
minute of Ar+ sputtering contained mixture of CuO, Cu2O and Cu(OH)2 while C4F8 and CH2F2
are primarily made of Cu2O.
108
5.4 Effect of Corrosion Inhibitor-Benzotriazole
Minimization of metal loss during chemical mechanical polishing (CMP) is an important
procedure in achieving successful Cu metallization. This is accomplished by use of a corrosion
inhibitor. One of the commonly used inhibitor in Cu CMP formulations is benzotriazole (BTA)
[34-35] which protects the recessed Cu lines from corrosion while Cu overburden is removed by
mechanical polishing in order to achieve the overall planarization. Corrosion inhibition is made
possible by formation of a protective layer through chemical reaction of BTA and Cu (+1) [36-
37]. Figure 5.10 shows corrosion results in TMAH and TMAH + 10mM BTA.
Figure 5.10 Corrosion results in TMAH and TMAH+10mM BTA (a) and image of Cu(1)BTA complex (b)
109
Micropattern corrosion method was used to determine the effect of BTA on corrosion of
plasma treated Cu in TMAH. It was observed that there was an overall drop in the corrosion rate
in all the plasma treatment. A significant decrease was noticed on CH2F2 and C4F8 plasma
treatment and this was attributed to the fact that it was primarily composed of Cu2O based on
XPS results which favored the formation of Cu (1) BTA complex.
5.5 Summary
New insights have been provided on the behavior of Cu corrosion treated with different
plasma etch gases in TMAH alkaline solution. The corrosion rate of plasma gas treated Cu on Ru
and Ta substrates was observed using micropattern corrosion screening method. It shows that the
corrosion rate of all the fluorocarbon gas plasma treated Cu samples is higher than that of non-
treated blank Cu. Though the samples were treated by etch gas plasma, the corrosion rate is
found to be affected by Cu/Ru and Cu/Ta interface. Enhanced corrosion was observed for Cu/Ru
than Cu/Ta which was attributed to higher nobility of Ru. XPS results confirmed the presence of
Cu oxides (Cu+ & Cu2+) and Cu fluoride in Cu samples treated with CF4+O2, CF4 and SF6. Cu
samples treated with CH2F2 plasma was found to contain mainly Cu2O and without fluorocarbon
polymers or Cu fluorides. Compared to other gas plasma treatments, C4F8 resulted in deposition
of a large amount of fluorocarbon residues on the surface. This is attributed to the higher amount
of C and F in C4F8 compared to other gases. From the contact angle measurements, it was
evident that C4F8 treated Cu showed greater resistance to cleaning with TMAH solution while
other plasma treatments like CF4, CF4+O2 CH2F2and SF6 on Cu can be cleaned within 2 minutes.
110
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