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APPROVED: Oliver M.R. Chyan, Major Professor Guido Verbeck, Committee Member Michael G. Richmond, Chair of the Department of
Chemistry Sandra L. Terrell, Dean of the Robert B. Toulouse
School of Graduate Studies
FTIR-ATR CHARACTERIZATION OF HYDROGEL, POLYMER FILMS,
PROTEIN IMMOBILIZATION AND BENZOTRIAZOLE
ADSORPTION ON COPPER SURFACE
Karthikeyan Pillai, B.Tech
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
December 2007
Pillai, Karthikeyan. FTIR-ATR Characterization of Hydrogel, Polymer Films, Protein
Immobilization and Benzotriazole Adsorption on Copper Surface. Master of Science (Analytical
Chemistry), December 2007, 62 pages, 3 tables, 26 figures, 81 references.
Plasma polymerization techniques were used to synthesize and deposit hydrogel on
silicon (Si) substrate. Hydrogel is a network of polymer chains that are water-insoluble and has
a high degree of flexibility. The various fields of applications of hydrogel include drug release,
biosensors and tissue engineering etc. Hydrogel synthesized from different monomers possess a
common property of moisture absorption. In this work two monomers were used namely 1-
amino-2-propanol (1A2P) and 2(ethylamino)ethanol (2EAE) to produce polymer films deposited
on Si ATR crystal. Their moisture uptake property was tested using FTIR-ATR technique. This
was evident by the decrease in –OH band in increasing N2 purging time of the films.
Secondly, two monomer compounds namely vinyl acetic acid and glycidyl methacrylate
which have both amine and carboxylic groups are used as solid surface for the immobilization of
bovine serum albumin (BSA). Pulsed plasma polymerization was used to polymerize these
monomers with different duty cycles. Initial works in this field were all about protein surface
adsorption. But more recently, the emphasis is on covalent bonding of protein on to the surface.
This immobilization of protein on solid surface has a lot of applications in the field of
biochemical studies. The polymerization of vinyl acetic acid and glycidyl methacrylate were
shown as successful method to attach protein on them.
Chemical mechanical polishing (CMP) of Cu is one of the processes in the integrated
chips manufacturing industry. Benzotriazole is one of the constituents of this CMP slurry used
as corrosion inhibitor for Cu. Benzotriazole (C6H5N3) is a nitrogen heterocyclic derivative
having three nitrogen atoms, each with an unshared pair of electrons, forming five-membered
ring structure. This molecule coordinates with Cu atoms by loosing a proton from one of its
nitrogen atom and thereby forming a film which is polymeric in nature that prevents further
oxidation of Cu. In this work, aqueous solution of BTA was used to study the adsorption
characteristics on Cu when a Cu electrodeposited Si was immersed in to it. This study was
performed using ATR-FTIR technique. The adsorption of BTA on Cu surface was evident by
the presence of aryl C-H stretching band at 3061 cm-1.
ii
Copyright 2007
by
Karthikeyan Pillai
iii
ACKNOWLEDGEMENTS
First of all, I would like to express my gratitude to my advisor Dr. Oliver Chyan for his
effective and encouraging guidance during my course of graduate study. I greatly appreciate his
immeasurable patience in teaching me about all aspects of doing research. His sincerity,
intelligence, patience and industriousness are those which I hope to emulate in my professional
and personal life. Special thanks are to Dhiman Bhattacharyya whose contributions are very
significant in my thesis. I have learnt a quite about plasma polymerization by working with him
and thanks to him we have a paper published in Chemistry of Materials Journal. I greatly thank
Dr. Praveen Nalla for his sincere efforts in teaching me about FTIR. My innumerable thanks are
to my ever friend Shyam S. Venkataraman for his great support in difficult situations and for his
help in conducting my research work. I would specially like to thank Kyle Yu for being a great
friend, and for his encouraging philosophical words with which I was able to keep up my spirit to
complete my thesis work. I also thank my past and present group members Sarah Flores and Fan
Yang for their useful discussions.
I greatly thank my committee member Dr. Guido Verbeck for his help in finishing up my
thesis. I would like to express my admiration towards his great enthusiasm and optimistic spirit
in professional life. I would like to thank Dr. Teresa D. Golden for being a good mentor of my
graduate courses. I thank my family members for their love and support that encouraged me a
lot to accomplish my degree.
iv
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS........................................................................................................... iii LIST OF TABLES......................................................................................................................... vi LIST OF ILLUSTRATIONS........................................................................................................ vii Chapters
1. INTRODUCTION ...................................................................................................1
1.1 Silicon ..........................................................................................................2
1.1.1 Properties of Semiconductors ..........................................................2
1.1.2 Structure of Silicon ..........................................................................4
1.2 Silicon ATR Preparation..............................................................................6
1.3 Surface Preparation of Si .............................................................................6
1.3.1 Cleaning of Silicon Surface .............................................................7
1.3.2 Metal Deposition on Silicon ............................................................9
1.4 FTIR-ATR Spectroscopy...........................................................................10
1.4.1 Total Internal Reflection ................................................................10
1.4.2 Attenuated Total Reflection...........................................................11
1.5 Plasma Polymerization...............................................................................14
1.6 Chapter References ....................................................................................17 2. SYNTHESIS OF HYDROGEL VIA PLASMA POLYMERIZATION AND
IMMOBILIZATION OF BIOMOLECULES ON POLYMER SURFACES .......19
2.1 Introduction................................................................................................19
2.2 Experimental ..............................................................................................21
2.2.1 Cleaning of Silicon ATR Crystal...................................................21
2.2.2 Rotating Pulsed Radio Frequency (RF) Plasma Reactor ...............22
2.2.3 Hydrogel Preparation by Plasma Polymerization ..........................23
2.2.4 Poly Vinyl Acetic Acid Film Preparation......................................24
2.2.5 ATR-FTIR Instrumentation ...........................................................25
2.2.6 Plasma Polymerization...................................................................25
2.3 Results and Discussion ..............................................................................26
v
2.3.1 FTIR Characterization of Plasma Polymerized 1A2P ...................26
2.3.2 FTIR Characterization of Plasma Polymerized 2EAE...................29
2.3.3 Immobilization of BSA on to Vinyl Acetic Acid (VAA) Polymer Film................................................................................................32
2.3.4 EDC Chemistry..............................................................................33
2.3.5 Immobilization of BSA on Plasma Polymerized Glycidyl Methacrylate ..................................................................................37
2.4 Conclusion .................................................................................................40
2.5 Chapter References ....................................................................................42 3. STUDY OF BENZOTRIAZOLE ADSORPTION ON COPPER SURFACE BY
FTIR-ATR SEPCTROSCOPY ..............................................................................46
3.1 Introduction................................................................................................46
3.2 Experimental ..............................................................................................48
3.3 Results and Discussion ..............................................................................49
3.3.1 Microscopic and FTIR Results of Si-H .........................................49
3.3.2 Optimization of Electroless Cu Deposition ...................................51
3.3.3 Contact Angle Measurement..........................................................54
3.3.4 FTIR results of Cu/Si and BTA/Cu/Si ...........................................55
3.3.5 Concentration Effect of BTA on Adsorption on Cu ......................56
3.4 Conclusions and Future Investigations ......................................................59
3.5 Chapter References ....................................................................................61
vi
LIST OF TABLES
Page
1.1 List of commonly used IRE ...............................................................................................14
3.1 Series of experiments to produce uniform Cu coverage on Si...........................................51
3.2 Contact angle measurements of Cu on Si surface with different times of deposition .......53
vii
LIST OF ILLUSTRATIONS
Page
1.1 Energy bands for (a) metals (b) semiconductors (c) insulators ...........................................3
1.2 Miller indices structure for Si (100) and Si (111) planes.....................................................4
1.3 The lattice structure of a) (100) and b) (111) orientation planes .........................................5
1.4 Schematic representation of ATR spectroscopy ................................................................12
1.5 Schematic of the bell-shaped RF plasma polymerization reactor......................................16
2.1 Chemical structures of 1-amino-2-propanol and 2(ethylamino)-ethanol...........................20
2.2 360° rotating pulsed RF plasma reactor.............................................................................23
2.3 FTIR spectra of 1A2P films; a) 1800 Å, 2 side coated; b) 1800 Å, 1 side coated; c) 180 Å, 2 side coated; d) 180 Å, 1 side coated ..............................................................................27
2.4 FTIR spectra of 1A2P films synthesized under different duty cycles a) 10/30 b) 10/10 and c) CW for different purging times .....................................................................................29
2.5 FTIR spectra of 2EAE synthesized under different duty cycles a) 10/30 b) 10/10 and c) CW with different purging times .......................................................................................31
2.6 Schematic representation of the process of BSA immobilization on surface....................32
2.7 Mechanism of EDC reaction with carboxylic group to produce amide.............................34
2.8 IR spectrum subtracted from that of VAA showing amide carbonyl peaks after 6-aminohexonoic acid has reacted with –COOH group of VAA..........................................35
2.9 FTIR spectra of PP VAA, spacer reacted PP VAA and BSA reacted PP VAA................35
2.10 PP VAA spectrum was subtracted from that of BSA showing only the BSA spectrum with different interval of 1% sodium dodecyl sulfate (SDS) washing ..............................36
2.11 Structure of glycidyl methacrylate.....................................................................................37
2.12 (a) FTIR spectra of 50nm (A), 150nm (B), 300nm (C) thick GM polymer films (b) FTIR spectra of PP GM, rinsed PP GM film and BSA reacted PP GM film..............................38
2.13 FTIR subtracted spectra of BSA after covalent attachment to the GM film .....................39
3.1 Structure of benzotriazole ..................................................................................................47
viii
3.2 Microscopic image of hydrogen terminated silicon in bright field, 100x magnification ............................................................................................................................................50
3.3 FTIR spectrum of silicon hydride sample..........................................................................50
3.4 Microscopic dark filed images of copper deposited on silicon substrate for different times of immersion ......................................................................................................................53
3.5 FTIR spectra of three samples with different durations of Cu deposition in NH4F electrolyte A) 60 minutes B) 25 minutes C) 5 minutes .....................................................55
3.6 FTIR spectra of BTA adsorbed on Cu for 5 minutes in the region 4000 – 2000 cm-1 showing aryl C-H stretching at 3061 cm-1 .........................................................................56
3.7 FTIR spectra of Cu sample immersed in different concentrations of BTA (a) 0.01 M BTA; (b) 0.05 M BTA; (c) 0.5 M BTA .............................................................................58
3.8 Structures of BTA derivatives ...........................................................................................59
CHAPTER 1
INTRODUCTION
This introductory chapter is written with an objective of explaining the purposes,
analytical techniques, and methods etc. for a better understanding for the reader. The
synergetic effect of attenuated total reflection (ATR) in sampling with Fourier transform
infrared (FTIR) spectroscopy forms the foundation for my research work. For ATR
analysis, the element silicon (Si) was chosen as internal reflection element (IRE) due to
its diverse applications [1]. Chapter 1 begins with an overview of preparing, cleaning
and surface modifying of silicon ATR crystals and explaining about different techniques
used to characterize the silicon surface modifications. The core of this research work can
be broadly divided in to two chapters. Chapter 2 discusses about the method of formation
of thin film polymers on silicon substrate using plasma polymerization and its properties
characterized by FTIR-ATR technique. This plasma polymerization technique [2] was
used for two purposes; to produce thin film of hydrogel on Si substrate with the starting
monomer 1-amino-2-propanol; to produce a vinyl acetic acid polymer film on Si leaving
the surface with carboxylic group and this functionalized surface was then used to
immobilize the bio-molecule bovine serum albumin (BSA) studied by FTIR-ATR.
Chapter 3 deals with the modification of Si ATR crystal surface with copper (Cu)
by electroless deposition technique [3] and various parameters followed for the Cu
deposition on Si. The Cu deposited substrate was then used for the study of adsorption of
benzotriazole ( BTA) molecule which is an aromatic heterocyclic molecule with five
membered triazole ring is a well known corrosion inhibitor for Cu and its alloys. The
adsorption of BTA on Cu surface was characterized by FTIR-ATR. The primary
1
substrate which binds these studies together is the use of silicon as the foundation from
which the research is conducted.
1.1 Silicon
Since the past several years, it can be said that silicon is one of the most important
elements whose application has a tremendous effect in the improvement of technological
world. Silicon and silicon based materials form backbone of the integrated circuit (IC)
chips. With its semi-conducting characteristics upon doping, silicon has attracted a
colossal interest in various research fields like optoelectronics, sensors, microelectronics.
1.1.1 Properties of Semiconductors The materials involved in this work fall into one of the categories of metals,
semiconductors, and insulators. The discussion of their properties is necessary in order to
better understand their behavior as a whole. Metals are conductors of electricity because
of their possession of high density charge carriers. Insulators are non-conductors of
electricity which refers to their zero charge density. Semiconductors become highly
useful and important because of their conducting nature that lies between metals and
conductors.
All these three types of materials have different properties in terms of charge
conduction which is attributed to the energy band difference. The energy band structure
of these materials is illustrated in figure 1.1. Crucial for conduction process is whether or
not there are electrons in the conduction band. In insulators, the electrons in the valence
band are separated by a large gap from the conduction band which is attributed to non-
2
conduction in them. In metals or conductors, the valence and the conduction bands are
overlapped. But in semiconductors, the energy gap between the valence and conduction
band is small enough so that any thermal or other excitations can bridge the gap. An
important parameter is the Fermi level, the top of the available electron energy levels at
low temperatures. The position of the Fermi level with respect to the conduction band is
a critical factor in determining the electrical properties.
Conduction Band
Valence Band Valence Band
Conduction Band
Valence Band
Conduction Band
E
EfEg Ef
(a)
(c) (b)
Fig. 1.1 Energy bands for (a) metals (b) semiconductors (c) insulators.
For non-doped, intrinsic semiconductors like silicon and germanium, the Fermi
level is essentially in the halfway between valance and conduction band. There will not
be any conduction at 0 K, but at elevated temperatures, the electrons in the valence band
get excited in to the conduction band. The structure of Si plays an important role in
having different properties.
3
1.1.2 Structure of Silicon
Since all the work done described in this thesis have foundation on silicon as the
primary substrate, it is necessary to discuss about the properties of silicon based on its
structure. Si forms four covalent bonds with other Si atoms resulting in tetrahedral shape.
It has diamond structure that has fcc Bravais lattice where each silicon atom is sp3
hybridized [4]. Single crystal is a crystalline solid where the crystal lattice of the entire
sample is continuous. Polycrystalline solid is made up of number of small crystals
known as crystallites. Single crystal Si wafers are commercially available and are widely
used in microelectronic industry [1]. The (100) and the (111) planes are the most
common and popular orientations used in single crystal silicon wafers for IC processing.
Fig. 1.2 Miller indices structure for Si (100) and Si (111) planes.
Fig 1.2 illustrates the structure of Si (100) and Si (111) orientation. The set of integers
used in the bracket are called Miller indexes that define the crystal orientations. These
integers are obtained from the reciprocal of the intercepts made by the atomic plane to a
three dimensional x, y and z axes at corresponding distances of the unit vectors a, b and c.
4
Note that the (100) plane has a square shape and (111) plane is in triangular shape. See
Fig. 1.3 which illustrates the lattice structures of (100) and (111) orientation planes.
Fig.1.3 The lattice structure of a) (100) and b) (111) orientation planes [5].
The (100) oriented crystals are generally used for making metal-oxide
semiconductor integrated circuit (MOS IC) chips and the (111) crystals for making
bipolar transistors. The reason that (111) crystals are used to make bipolar transistors is
that the surface of (111) crystal is stronger which is because the atom surface density is
higher and is a better fit for higher power devices. The fragments of the (100) crystals
normally have right angles and that of (111) crystals will have 60 triangles [5]. In
chapter 2, Si (100) ATR crystal was used merely as a substrate for plasma polymerization
but in chapter 3, Si (111) ATR crystal was used to develop smooth hydride surface that
serves as a source of electrons for Cu ions reduction on Si surface.
a)
Basic lattice Silicon atom Basic lattice cell Silicon atom
b)
5
1.2 Silicon ATR Preparation
Double polished silicon N-type phosphorus doped (100) (Wacker Co.) wafers
with a resistivity of 0.6 – 1.0 Ohm cm and N-type (111) wafers (from PCA) with a
resistivity of 1.0 – 10.0 Ohm cm were used to make ATR crystals. The silicon ATR
crystals were cut from those wafers with 5.8 cm × 1 cm × 0.7 cm. The two ends of the
crystal were ground and polished to a fineness of 1 micron with the help of a polishing
machine from High Tech Allied Products. The silicon ATR crystal was covered all
around with Teflon tape in order to protect any scratch/damage to the surface and was
clamped by sample holder at an angle of 45 to the plane of the rotating platen of the
polishing machine. The platen was rotated with a speed of 100 rpm in the direction along
the bevel face. Continuous flow of water was provided to flush away any particles that
were grinded by the polishing pad. At first, 30 micron grit size pad was used to coarse
polish the bevel face. Subsequently, 15, 9, 3, and 1 micron pads were used to further
polish the bevel face to have fineness. Blue lube solution was used in lieu of water when
3 and 1 micron grit size polishing pads were used. All polishing pads were from Allied
High Tech Products (www.alliedhightechproducts.com). Distilled water rinsing and
observation under an optical microscope was used to ensure the quality of polishing
1.3 Surface Preparation of Silicon
Since all the results obtained from this work are dependent on the nature of the Si
surface, it requires that the surface has to be clean of metal and organic contaminants.
Silicon ATR crystal after polishing at both the ends at 45 angle is subjected to wet
cleaning in order to remove the contaminants on the surface. A renowned cleaning
6
method called RCA cleaning was employed to clean the Si surface to remove the metal
and organic contaminants present on the surface. Similar standard procedures are
followed in preparing the Si surface for further adsorption or metallization studies. The
RCA cleaning was developed by Radio Corporation of America that includes many
chemical steps. The following are the different cleaning steps;
1.3.1 Cleaning of Silicon Surface 1.3.1. A. Standard Cleaning - I (SC1) The SC1 cleaning is done to remove the organic contaminants present on the Si
surface. This is a basic cleaning method required for Si surface before using it for further
treatments. The SC1 solution is a mixture of ammonium hydroxide (NH4OH), hydrogen
peroxide (H2O2) and ultra pure water (UPW) in a ratio of 1:1:5. The silicon crystal after
polishing is sequentially rinsed and sonicated with UPW, isopropyl alcohol (IPA) and
immersed in the SC1 solution for 10min at a temperature of 70-80 C. The hydrogen
peroxide is a strong oxidizing agent which oxidizes the silicon and forms a layer of
surface oxide. The ammonium hydroxide etches away the oxide layer formed by
hydrogen peroxide. This process of oxide formation and etching removes all the organic
contaminants and particles from the surface. This process leaves the silicon surface with
chemically formed clean oxide surface. The SC1 cleaned Si (100) crystal was used as a
substrate for polymer deposition which is described in chapter 2.
7
1.3.1. B. Standard Cleaning - II (SC2) The SC2 cleaning is done to remove the metallic impurities present on the Si
surface. This solution is a mixture of hydrochloric acid (HCl), H2O2 and UPW in the
ratio of 1:1:5. The silicon substrate is immersed in this solution for about 10 minutes at a
temperature of 70-80 C. HCl acid present in the solution dissolves all the metal particles
present on the surface. Simultaneously, the silicon surface is oxidized by the presence of
H2O2. This process removes alkali ions and residual metals that are left after SC1
cleaning.
1.3.1. C. Piranha Cleaning The Piranha solution is a mixture of sulfuric acid (conc.) and hydrogen peroxide
used to clean organic residues off the substrate. This treatment can be done to Si in place
of SC1, in order to minimize the surface roughness which will be high in SC1 treatment.
The mixture is a strong oxidizer which helps in removing organic matters and it will also
makes the surface more hydrophilic. Many different mixture ratios like 4:1 or even
7:1are commonly used, but a typical mixture is 3:1 conc. sulfuric acid and hydrogen
peroxide (30%). The following reaction equation explains the ions formed in the Piranha
solution.
H2SO4 + H2O2 -------> H3O+ + HSO4- + O ---------------- (1)
The atomic oxygen species formed in the solution allows the Piranha solution to dissolve
the elemental carbon. The sulfate ions help in removing metals as soluble complexes.
The substrate is exposed for about 10-15 minutes at a temperature of 100 C.
8
1.3.1. D. Diluted Hydrofluoric Acid Etching (DHF) HF is used to remove the oxide from the silicon surface and contamination in the
oxide from the wafer surface [6,7]. HF is extremely dangerous and should be handled
with great care. The concentration of HF typically used is 4.9%. It reacts with silicon
dioxide and the proposed mechanism for the etching mechanism is given by
4HF + SiO2 ------> SiF4 + 2H2O ---------------- (2)
(OR)
SiO2 + 6HF ------> 2H+ + SiF62- + 2H2O ---------------- (3)
The SiF4 is formed by the interaction of SiO2 with HF. As a result, the Si-F bond
polarizes the Si-Si bond in the second layer of the substrate. Hence the surface silicon is
removed as SiF4 which is basically induced by these polarizations. This process renders
the silicon surface to be hydrogen terminated that is extremely hydrophobic. This Si-H
surface behaves as a source of electrons for reduction of Cu ions which is described in
chapter 3.
1.3.2 Metal Deposition on Silicon by Electroless Method Various techniques such as electron beam evaporation, electrochemical deposition
physical and chemical vapor deposition, laser ablation are available for depositing a
metal film on a substrate. Electroless deposition is one of the techniques widely used in
the semiconductor-devices industries. It is useful to discuss some of the important
concepts of metal deposition on silicon because in chapter 3, the electroless deposition of
Cu on silicon substrate is an important experiment carried out for making Cu/Si substrate
9
for benzotriazole adsorption studies. Metal deposition on hydrogen terminated silicon
surface has been studied extensively by researchers [8,9]. The metal that is reduced on
H-terminated silicon has to have more positive reduction potential (0.34 V vs. SHE) with
respect to hydrogen (0.00 V vs. SHE) and be noble. Metals such as Cu2+, Ag2+, Au2+, and
Pt2+ will fall under this category. The deposition of metal on H-silicon surface without an
external applied potential is known as immersion or electroless deposition. Such kinds of
depositions are essentially based on the electrochemical phenomenon. This involves
reduction of metal ions on silicon leading to deposition and simultaneous oxidation of
silicon resulting in its dissolution [8]. The metal ion reduction reaction can be written as
Mn+ + ne- ------> M ---------------- (4)
In electroless deposition process, a metal is deposited on the surface of a substrate
through the transfer of electron between a metal ion and a reducing agent (e.g.
formaldehyde) in the solution. In this work, HF is used instead of a reducing agent in the
electroless deposition of Cu on Si. Under such conditions, the metal is deposited at the
expense of etching the substrate material.
1.4 Background and Theory of FTIR-ATR Spectroscopy
1.4.1 Total Internal Reflection A phenomenon of refraction can be explained as when a ray of light passes from a
denser to a rarer medium, it bends away from the normal in to the rarer medium. But
above a certain angle of incidence which is called as critical angle, if the light is
completely reflected back in to the denser medium, it refers to the phenomenon of total
internal reflection.
10
1.4.2 Attenuated Total Reflection
Attenuated total reflection is a sampling technique used in conjunction with
infrared spectroscopy which enables samples to be examined directly in the solid or
liquid state without further preparation [9]. Since FTIR-ATR spectroscopy has been
extensively used in this work that I have undertaken, it is worth to know about the theory
and background of this technique. ATR uses a property of total internal reflection called
the evanescent wave, the phenomenon which was observed by Newton in the early
1700s. Extensive research has been done after 1970 in this field with the arrival of
ameliorated instrumentations like FTIR etc. The advantages that make this technique
unique refer to its simplicity in sampling and the amount of information derived from it.
Fig. 1.4 depicts the basic principle of ATR spectroscopy.
In FTIR-ATR spectroscopy technique, the IR radiation gets propagated through
the internal reflection element and produces an evanescent electrical field that decays
exponentially with distance Z. For the internal reflection to occur, two mediums must be
in contact with each other; a denser medium with high refractive index (Si, 3.42) and a
rarer medium which is the thin film of sample on the surface of the silicon. The angle of
incidence of IR radiation must be greater than the critical angle θc, which is a function of
refractive index of both the sample and IRE (Si crystal) [10,11]
11
dp Depth of few microns
IR in
IR Out Internal Reflection Element
Evanescent Wave
Sample
Fig. 1.4 Schematic representation of ATR spectroscopy.
θc = Sin-1(n2/n1) ---------------- (5)
where n2 = refractive index of sample and n1 = refractive index of ATR element. The
number of reflections of IR radiation depends on the dimensions of ATR element.
Typically, an ATR element has dimensions of 6×1×0.7 cm with the bevel edge polished
to an angle of 45. Normally, the number of reflections that can be achieved in our lab
will be up to 100 reflections. The total number of reflections (N) of a given length (L)
and width (W) of an ATR element is given by;
N = (L / W) Cot θc ---------------- (6)
The evanescent wave which is basically an electrical field (E), is produced normal to the
reflecting surface. The exponential decay of the evanescent wave is in the order of
microns and depends on the energy of the IR radiation. If Eo is considered to be the
amplitude of the incident energy, then E, the amplitude of the evanescent wave can be
expressed as [12]
12
E = Eo exp [-(2π/λ1) (sin2θ – (n2/n1)) 1/2Z ---------------- (7)
Where λ1 = λ/n1 is the wavelength of radiation in Si ATR, λ is the wavelength of
radiation in air and Z is the distance from the surface. Harrick [10] modified this equation
by taking in to account the energy absorption in the rarer medium as,
E = Eo exp [-γZ] ---------------- (8)
Decay Constant,
1/2π (sin2θ – (n2/n1)2) 2 ---------------- (9)
γ = λ1
The penetration depth of the evanescent wave is expressed as dp = 1/γ . That is,
2π (sin2θ – (n2/n1)2) 1/2
λ1 dp = ---------------- (10)
where λ1 is the wavelength of IR radiation, n1 is the refractive index of ATR element and
n2 is that of sample medium. A sample calculation of penetration depth is shown.
At wavenumber 400 cm-1 (25 µm) with angle of incidence 45,
dp = 25
2 * 3.14 * [(sin2 45 – (1/3.42)2 ] 1/2
= 6.18 µm
13
Hence the penetration depth can be controlled by varying angle of incidence or by
choosing IRE element. Various IRE elements are there with different refractive index. A
list of elements with their refractive indexes is provided in the following table.
IRE Refractive Index
KRS 5 (Thallium Bromide/Thallium Iodide)
2.35
AgCl 1.98
AgBr 2.16
Silicon 3.42
Germanium 4
Table 1.1 List of commonly used internal reflection elements.
In ATR spectroscopy, the bevels of ATR element crystals were shaped in such a way that
the angle of its surface be 45. The main advantage of using this ATR crystal is that it
undergoes multiple reflections with IR radiation which enables amplification of weak
signals as a result of which the sensitivity is increased. It can detect up to sub-monolayer
of substance.
1.5 Plasma Polymerization
Since chapter 2 deals with the synthesis of hydrogel and other polymer films
through plasma polymerization, it is important to discuss the functions of plasma
polymerization. Plasma polymerization is a unique technique used to fabricate thin
polymer films from a variety of organic and organometallic starting materials [13-20].
14
The principle behind this technique is that it uses plasma sources to generate a gas
discharge which provides energy to activate or fragment gaseous or liquid monomer
mostly that contains vinyl group and initiates polymerization. It is a procedure wherein
the gaseous monomers are stimulated through plasma and condensed on selective
substrates as highly cross-linked layers.
The functionalization of polymer surfaces with specific reactive groups has
attracted a lot of research interest into it. Plasma polymerization is a specific type of
chemistry between plasma species, between plasma and surface species or between
surface species [13]. Two types of reaction mechanism may be postulated 1) plasma-
induced polymerization 2) plasma-state polymerization. Former one is the conventional
polymerization of molecules containing carbon-carbon bonds and the latter depends on
the presence of electrons and other species in plasma, energetic enough to break bonds.
Unlike conventional polymers, plasma polymers do not consist of regular repeated units,
but they tend to form an irregular three dimensional cross-linked network.
Plasma polymers generally are chemically inert, stable, insoluble, mechanistically
tough, and have been used in wide variety of applications protective coatings, electrical,
optical and biomedical films. The several advantages of plasma polymerization over
conventional one are;
1. The starting gases may not contain functional groups that are normally associated
with conventional polymerization
2. Such films are highly coherent and adherent to various substrates
3. This process can be used to produce films of thickness 500 Å to 1µm easily
4. An ultrathin, pin-hole free films can be produced by plasma polymerization
15
Pressure Transducer
Monomer Inlet
Vacuum System Matching Unit
Substrate Bidirectional
Coupler
Function Generator
Pulse Generator
Oscilloscope Amplifier
Fig.1.5 Schematic of the bell-shaped radio frequency plasma polymerization reactor.
5. It is possible to tailor the films with respect to specific chemical functionality,
thickness and other physical and chemical properties by carefully controlling the
polymerization parameters
In this work 1-amino-2-propanol and vinyl acetic acid were used as starting monomers
and introduced into the plasma chamber to get polymerized and deposited on silicon ATR
crystal. These films were further characterized by FTIR-ATR spectroscopy.
16
1.6 Chapter References
1. Runyan, W.R.; Bean K.E; Semiconductor Integrated Circuit Processing
Technology; Addison-Wesley Publishing Company: Reading, 1990.
2. Y. Vickie Pan, Roger A. Wesley, Reto Luginbuhl, Denice D. Denton, Buddy D.
Ratner, Biomacromolecules 2001, 2, 32-36.
3. L.A. Nagahara, T. Ohmori, K. Hashimoto, and A. Fujishima; J. Vac. Sci.
Technology A 1993,11,4, Jul/Aug.
4. Cotton, F.A.; Murillo, C.; Advanced Inorganic Chemistry, John Wiley & Sons;
New York, 1999.
5. Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Prentice-
Hall Inc., Upper Saddle River, New Jersey, 2001.
6. V.A. Burrows, Y.J. Chabal, G.S. Higashi, K. Raghavachari and S.B. Christman,
Appl. Phys. Lett., 1988, 53, 998.
7. Y.J. Chabal, G.S. Higashi, K. Raghavachari and S.B. Christman J. Vac. Sci
Technol., 1989, A7, 2104.
8. In-Churl Lee, Sang-Eun Bae, Moon-Bong Song, Jong-Soon Lee, Se-Hwan Paek,
and Chi-Woo J. Lee* Bull. Korean Chem. Soc. 2004, 25, 2.
9. Shen Ye, Taro Ichihara, and Kohei Uosaki, Journal of the Electrochemical
Society, 2001,148, 6, C421-C426.
10. N.J.Harrick, Internal Reflection Spectroscopy, Interscience Publishers, NY, 1967.
11. FT-IR Spectroscopy—Attenuated Total Reflection (ATR). Perkin Elmer Life and
Analytical Sciences, 2005.
12. F.M. Mirabella, N.J. Harrick, Internal Reflection Spectroscopy; Review and
17
Supplement, Harrick Scientific Corporation, N.Y. 1985.
13. N. Morosoff, An introduction to Plasma Polymerization, in R. d’iigostino (ed.),
Plasma Deposition, Treatment, and Etching of Polymers, Academic Press, 1990.
14. Y. Takahashi, M. Iijima, K. Inagawa and A. Ito, J. Vac. Sci. Technol. A, 1987, 5,
2253.
15. Y. Kagami, T. Yamauchi and Y. Osada, J. Appl. Phys., 1990, 68, 610.
16. R. Hollahan and A.T. Bell, (eds.), Techniques and Applications of Plasma
Chemistry, Wiley, New York, 1974.
17. A.T. Bell and M. Shen (eds.), Plasma Polymerization, American Chemical
Society, Washington, DC, 1979.
18. W. Gombotz and A. Hoffman, J. Appl. Polym. Sci.: Appl. Polym. Symp. 1988, 2
285.
19. W. Gombotz and A. Hoffman, J. Appl. Polym. Sci., 1989, 37, 91.
20. H. Yasuda, Glow DischargePolymerization, in J. Vossen and W. Kern (eds.),
Thin Film Processes, Academic Press, New York, 1978.
18
CHAPTER 2
SYNTHESIS OF HYDROGEL VIA PLASMA POLYMERIZATION AND
IMMOBILIZATION OF BIOMOLECULES ON POLYMER SURFACES
2.1 Introduction
Hydrogels are defined as a network of polymer chains that are water-insoluble
and possess a degree of flexibility very similar to natural tissue due to their significant
water content. The swelling and de-swelling property of these polymeric materials is
dependent on nature of both intermolecular and intramolecular cross linking and also the
extent of hydrogen bonding in the polymer network [1]. The applications of hydrogel
are varied including drug release [2-4], biosensors [5-7], tissue engineering [8-10] and pH
sensors [11,12]. Different monomers are nowadays in utilization for the synthesis of
hydrogel. Some of the monomers used for this hydrogel synthesis are
N-isopropylacrylamide(NIPAM)[13-16], vinyl alcohol [8,17], ethylene glycol [18,19]
and N-vinylpyrrolidone [20]. The techniques that have been employed to immobilize
these thin films on solid surfaces are electron beam radiation [24,25], photo-initiated
grafting [8,26,27], use of activated and functionalized substrates [28,29] and plasma
polymerization [13,14,30-32].
This work was done in collaboration work with Mr. Dhiman Bhattacharyya from
Dr. Richard B. Timmons Lab, University of Texas, Arlington (UTA); to prepare surface-
immobilized hydrogel films using plasma polymerization technique and this forms the
first part of this chapter. The monomers used here to synthesize hydrogel film are 1-
amino-2-propanol (1A2P) and 2(ethylamino)-ethanol (2EAE) which are low-molecular-
weight, volatile and contain both amine and hydroxyl functional groups.
19
H3CNH2
OH
1-amino-2-propanol
OHNH
H3C
2-(ethylamino)ethanol
Fig. 2.1 Chemical structures of 1-amino-2-propanol and 2(ethylamino)-ethanol.
These monomers are plasma polymerized and deposited as thin films on
substrates placed inside the plasma reactor chamber. The synthesis of these polymer
films were carried out under variable duty cycle pulsed plasma conditions to control film
compositions obtained during plasma polymerization [33]. This method of
polymerization under variable duty cycle conditions will not only yield hydrogel but are
also energetic to produce films that are strongly grafted on to the surface. What is
reported here is the study of rapid moisture uptake property of these films which in turn
concerns about the type of substrates used. It is known that the amount of O-H functional
group present on these films relates to the moisture uptake property. Specifically, FTIR
technique is used to analyze quantitatively the O-H functional group present on the film.
Hence, ATR crystals made from Si wafers serve as a suitable substrate for depositing
these polymer films and the technique ATR-FTIR spectroscopy [34,35] is used to study
the moisture uptake properties. It describes the single-step, plasma polymerization
synthetic route to prepare these uniform and conformal thin films of thermoresponsive
hydrogel using 1A2P and 2EAE monomers.
20
The second part of this chapter deals with plasma polymerization of vinyl acetic
acid (VAA) and glycidyl methacrylate (GM) monomers, and the immobilization of
proteins on these polymer surfaces. FTIR-ATR technique was used to characterize these
films. Such biochemical studies include probing cell-surface interactions [36-38], and
inflammatory responses to implants [39,40]. Initial work in this area dealt primarily
about the simple protein surface adsorption. Since protein denaturation occurs readily
upon adsorption [41,42], the emphasis has shifted to studies in which the proteins are
covalently bound to the surfaces and retain their inherent structures and functions.
Plasma enhanced chemical vapor deposition (PECVD) was used to synthesize thin
polymeric films on substrates. Certain plasma conditions were followed under which the
polymeric films formed will retain their original functional groups. These functional
groups are available for the attachment of bio-molecules. The polymer film made from
vinyl acetic acid had –COOH group on the surface and subsequent attachment of BSA
was achieved using a spacer molecule, 6-aminohexanoic acid and carbodiimide
chemistry.
2.2 Experimental
2.2.1 Cleaning of Silicon ATR Crystal
The silicon ATR crystals after polishing contain organic, inorganic and metallic
contaminants on the surface. In order to have a clean surface, the crystals had to undergo
certain cleaning steps. The lab wares used here for all the experiments were made of
polyfluoroalkoxy material. These lab wares were cleaned many times in boiling
solutions of 10% HNO3 before had they employed in cleaning of silicon crystals.
21
Ultra pure water with resistance > 18.2 MΩ/cm was supplied via a Millipore Milli-Q®
Elix5® ultra pure water purification system (Millipore Corp., Bedford, MA,
www.millipore.com) for rinsing purposes as well as for any dilution of chemicals. The
silicon crystal was initially rinsed and sonicated in UPW followed by rinsing with
isopropyl alcohol and dichloromethane. High purity electronic grade HF, H2O2, NH4OH
and HCl (the chemicals were from Air Liquide, Dallas Chemical Center, 13456 N.
Central Expwy, Dallas, Texas, 75243, www.airliquide.com) were used in cleaning of
silicon crystals.
All silicon ATR crystals used were cleaned in hot solutions (70-80 C) of SC1
(Standard Cleaning-I; 1:1:5; H2O2 : NH4OH : H2O) and SC2 (Standard Cleaning-I; 1:1:5;
H2O2 : HCl : H2O) for 10 minutes. The crystals after cleaning from SC1 and SC2 were
rinsed with UPW and dried with a stream of N2 gas.
2.2.2 Rotating Pulsed Radio Frequency (RF) Plasma Reactor
The rotating pulsed RF plasma reactor shown in the fig 2.2 was used to synthesize
the hydrogel and other polymer films that were described in this chapter. A simple
working principle of this plasma reactor was described. The AC signal from the power
outlet is converted in to radio frequency by function generator. The radio frequency
generated is equivalent to 13.56 MHz. This RF signal is amplified by using RF
Amplifier. This instrument has the following components; a cylindrical reactor made of
Pyrex in which polymerization reaction occurs, a hot electrode connected to RF
amplifier, a ground electrode connected to ground, a low pressure monomer inlet which
is used to introduce low volatile monomers in to the reactor, a high pressure monomer
22
inlet which is used to introduce high volatile monomers, gas cylinders of oxygen,
hydrogen, argon which are used to produce plasma inside the reactor.
vv
vvv
To Vacuum
Monomer Input Port IIFor Low Vapor Pressure
Monomer
Flow Direction
Pyrex PlasmaReactor
360 de
gree ro
tating
Ferro-Fluid
ic Valve
Hot electrode
Pressure
Controller
MassFlowmeter
MKS BaratronPressure Transducer
Butterfly Valve
Monomer Input Port IFor High Vapor
Pressure Monomer &Oxygeb, Hydrogen or
Argon
Fig 2.2 360 rotating pulsed RF plasma reactor.
2.2.3 Hydrogel Film Preparation by Plasma Polymerization
This preparation of hydrogel film was done in Dr. Timmons lab in UTA. The
1A2P and 2EAE monomers were obtained from Sigma Aldrich, St Louis, MO and had a
stated purity of +98%. The Si ATR crystals were sonicated in acetone, methanol and
hexane to clean the atmospheric adsorbed organic contaminants from the crystal surface
prior to use. The crystals were placed inside the reactor to have the monomer deposited
23
on to the surface. All the substrates inside the reactor chamber were subjected to oxygen
plasma at 100W average power input to remove any carbonaceous residue left on the
surface. A background pressure of 6 mtorr was achieved for each run. Before
deposition, these monomers were freeze-thawed to remove any dissolved gases.
Monomer vapors were subjected to a radio-frequency plasma glow discharge, at room
temperature, 130 mtorr pressures and at 150W peak power input. Two different duty
cycles namely 10/30, 10/10 (ton/toff ms) and a continuous wave (CW) operational mode
were employed.
2.2.4 Preparation of Polymer Thin Films by Pulsed Plasma Polymerization
The chemicals required for this experiment such as vinyl acetic acid, glycidyl
methacrylate, 6-aminohexanoic acid and BSA were obtained from Sigma-Aldrich, St.
Louis, MO. The chemicals 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride (EDC) and 2-(N-morpholino ethanesulphonic acid (MES) buffer were
from Pierce, Rockford, IL. The cleaned Si ATR crystal was placed inside the reactor
chamber. The reactor was evacuated to a background pressure of 2 mtorr. After
introducing the vinyl acetic acid monomer in to the reaction chamber, plasma was ignited
at a constant monomer pressure of 80 mtorr. VAA was plasma polymerized under pulsed
conditions. The optimum plasma duty cycle was determined to be 0.75 ms/20 ms (ton/toff
ms) and 200 W power input. Typically, 50 nm thick films were developed.
24
2.2.5 ATR-FTIR Instrumentation
A Bruker Equinox 55 FTIR instrument (Bruker Optics Inc., 19 Fortune Drive,
Manning Park Billerica, MA 01821 – 3991, www.brukeroptics.com) having ATR
accessory was used to characterize the all the silicon samples and their surface
modifications. The silicon ATR crystal was placed in a variable angle ATR accessory
from Pike Technologies such that the angle of IR light incidence is normal to the ATR
bevel face. Before taking the spectrum, the sample was adjusted for its correct position
to get maximum throughput of IR beam. A liquid nitrogen cooled mercury cadmium
telluride (MCT, high D*, narrow band, EG&G Inc. 200 Orchard Ridge Drive, Suite 100,
Gaithersburg, MD 20878, www.egginc.com) detector was used to detect the IR signal.
2.2.6 Plasma Polymerization
Plasma polymerization technique is used to synthesize novel materials in the
form of thin films and the surface modifications of those films have become increasingly
active research area in recent years. Those polymeric films possess different properties
that have many applications [43,44]. The primary approach of this plasma
polymerization technique is focused on the film chemistry controllability during
deposition. The goal here is to produce surface active coatings and/or molecularly
tailored surfaces. The advantages of plasma technique include all-dry, single step,
relatively rapid process which provides pin-hole free, conformal and adhesive films of
uniform thickness [43]. The important feature of this approach is that it offers an
opportunity to use low total power input. This tells the fact that the power is turned off
25
during polymerization process in the film. The average power given under pulsed
conditions is calculated from the equation:
<P> = [ton / (ton + toff)] * peak power.
Where ton – time duration when power is on
toff – time duration when power is off
peak power – power input during the plasma on periods.
2.3 Results and Discussion
2.3.1 Characterization of Plasma Polymerized 1A2P by FTIR
The plasma polymerized 1A2P thin film was deposited on Si ATR crystal and
characterized by FTIR-ATR. As an initial step, four samples of 1A2P were prepared
with two different thick films (1800 Å and 180 Å) each with one and two side coatings
on ATR crystal. Fig.2.3 shows the IR spectra of these films from which the peak
assignments are as follows: (a) N-H and O-H stretching modes at the region from 3600
cm-1 to ~3000 cm-1; (b) –CH3 asymmetric stretch at 2970 cm-1; (c) –CH2 asymmetric
stretch at 2930 cm-1; (d) –CH3 symmetric stretch at 2866 cm-1; (e) –C≡N stretch at 2240
cm-1; (f) -N≡C stretch at 2180 cm-1; (g) amide C=O stretch at ~1650 cm-1. It was
observed that the intensity of the each stretching band is proportional to the amount of
film present on the sample.
26
4000 3500 3000 2500 2000 1500 1000-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
2240
2866
29302970
a b c d
1650
2180
3330
Abs
orba
nce
Wavenumber cm-1
Fig. 2.3 FTIR spectra of 1A2P films; a) 1800 Å, 2 side coated; b) 1800 Å, 1 side coated;
c) 180 Å, 2 side coated; d) 180 Å, 1 side coated.
The 1A2P polymer films are prone to absorb large amounts of moisture from the
atmosphere. These polymer films synthesized under 10/30, 10/10 and CW conditions
were characterized by ATR-FTIR as a function of atmospheric exposure to purging time
with dry stream of N2 gas. The FTIR spectra were recorded after the films had been
exposed sufficiently to atmosphere so that the film will be in equilibrium with the
atmospheric moisture. From the spectra (fig 2.4), it was observed that the thickness of
the films under 10/30 and 10/10 duty cycle conditions, varied linearly with time of
deposition except that the film thickness formed by CW run indicates, the rate of
formation of the film was decreased with time of deposition .
27
Secondly, for each of the film the spectra were recorded for 0, 2, 5, and 10
minutes of dry nitrogen gas purging that refers to spectra A, B, C and D respectively.
28
Fig. 2.4 Reproduced with permission from American Chemical Society. FTIR spectra of
1A2P films synthesized under different duty cycles a) 10/30; b) 10/10 and c) CW for
different purging times.
It was observed that there was a substantial decrease in –OH band intensity for all the
films as the purging time was increased. The absorptions of atmospheric CO2 and H2O
were eliminated by subtracting a background spectrum of uncoated ATR crystal from
each of the polymer film absorptions.
2.3.2 FTIR Characterization of Hydrogel Synthesized from the Monomer
2(ethlyamino)ethanol (2EAE)
One of the general properties of hydrogel films is moisture absorption. In order to
test that generality, a hydrogel from another monomer 2EAE was synthesized via plasma
29
polymerization and characterized by FTIR. Figure 2.5 shows the IR spectra of hydrogel
films synthesized from 2EAE with three different deposition conditions namely 10/30,
10/10 and CW. The 2EAE monomer contains structural features (N-H and O-H groups)
in common with 1A2P.
4000 3500 3000 2500 2000 1500 1000
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
W avenum ber cm -1
A B C D
(a)
4000 3500 3000 2500 2000 1500 1000
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Abs
orba
nce
W avenum ber cm -1
A B C D
(b)
30
4000 3500 3000 2500 2000 1500 1000
-0.2
0.0
0.2
0.4
0.6
0.8
1.0 A B C D
Abs
orba
nce
W avenumber cm -1
(c)
Fig. 2.5 FTIR spectra of 2EAE synthesized under different duty cycles a) 10/30
b) 10/10 and c) CW with different purging times.
The depositions were carried out at 150 W peak RF powers, which were the same
for 1A2P. It turns out that the IR spectra were essentially identical to those of 1A2P,
with absorption bands. Like 1A2P, 2EAE polymer films also posses the water absorption
property when exposed to atmosphere. This was evidenced by the decrease in the peak
intensity at the region from 3600 cm-1 to ~3000 cm-1, which corresponds to –OH band as
the nitrogen gas purging time was increased. In spite of the fact that the absorption bands
were same as the 1A2P, it was observed that there was a decrease in the peak intensity at
the region between 3500 cm-1 and 3100 cm-1 relative to the peak centered around 1600
cm-1. Comparing to 1A2P, this decrease was more in 2EAE.
31
2.3.3 Immobilization of Bovine Serum Albumin (BSA) on to Vinyl Acetic Acid (VAA)
Polymer Film
The use of plasma enhanced CVD is unique, to deposit thin polymeric films on
the surfaces in order to have the functional groups available for protein attachment onto
it. With this motto, VAA was coated on Si ATR crystal for a film thickness of 50 nm.
CONH
CONH
CONH
CONH
COOH
OH
OH
OH
CO
CO
CO
PP VAA CO
CO
CO
CO
OH
OH
OH
OH
6-aminohexanoic acid H2NCH2(CH2)3CH2-COOH
EDC, in MES at pH 4.7
EDC, in MES at pH 4.7
BSA Solution BSA
CONH
CONH
CONH
CONH
CO
CO
CO
CO
NH
NH
NH
NH
Fig.2.6 Schematic representation of the process of BSA immobilization on solid surface.
The plasma polymerized VAA (PP VAA) film was characterized using FTIR, and
the spectra were observed to have peaks at ~1700 cm-1 and at the region between 3500
cm-1 and 3000 cm-1, indicating the presence of C=O and O-H functional groups
respectively. The PP VAA coated Si ATR crystal was then washed with 2-(N-
32
morpholino)ethanesulphonic acid (MES) buffer solution at pH 4.7, to remove any
physisorbed oligomers/monomers.
Fig 2.6 shows the steps followed in the process of BSA immobilization on VAA
polymer film. The film was then dipped in the 6-aminohexanoic acid that acts as spacer
and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) chemistry
was followed for reaction between spacer and VAA. EDC chemistry is known to activate
–COOH group of VAA to react with –NH2 terminal of spacer to form amide carbonyl (-
CONH) [43]. After dipping into the spacer, the film was left with amide carbonyl
functional groups by the reaction of –NH2 with –COOH of VAA polymer. This was
confirmed by the presence of amide carbonyl peaks at the regions of 1654 cm-1 and 1530
cm-1 in the (see fig. 2.8)
2.3.4 EDC (Carbodiimide) Chemistry
The carboxylic acid (1) will react with carbodiimide to produce the key
intermediate O-acylisourea (2) which can be defined as a carboxylic ester with an
activated leaving group. The O-acylisourea will react with amines to produce the desired
product amide (3) and undesired side-product urea (4). The key intermediate reacts with
an additional carboxylic acid to produce acid anhydride (5) which in turn reacts with
amines to produce amide (3) and urea (4). The main undesired reaction involves the
rearrangement of O-acylisourea to the stable N-acylurea (6).
33
OH
O
R1
N C N
R R
O
O
R1
N C
R R
N
H
O
O
R1
C
HNR
NR
2
1
O
O
R1
O
R1
OH
O
R1
NH
NH
O
R RNH
NH
O
R R
H2N R2
H2N R2
4 4
NH
O
R1
R2
3
NH
O
R1
R2
3OH
O
R1 1
+
5
NH
N
O OR
R1
R6
Ref: http://en.wikipedia.org/wiki/Carbodiimide
Fig 2.7 Mechanism of EDC reaction with carboxylic group to produce amide.
34
1750 1700 1650 1600 1550 1500
0.025
0.030
0.035
0.040
0.045
0.0501530
1654A
bsor
banc
e
W avenumber cm -1
Fig. 2.8 IR spectrum subtracted from that of VAA showing amide carbonyl peaks after 6-
aminohexonoic acid has reacted with –COOH group of VAA.
Fig. 2.9 Reproduced with permission from American Chemical Society. FTIR spectra of
PP VAA, spacer reacted PP VAA and BSA reacted PP VAA. Inset showing the amide
carbonyl peaks after reaction with spacer molecule.
35
The spacer reacted film was then tested for the attachment of protein onto it by
dipping it in to BSA solution (1 mg/ml BSA in MES buffer solution). The presence of
BSA on the surface was confirmed by the appearance of strong amide carbonyl (>C=O)
peaks at 1654 cm-1 and 1528 cm-1. The characteristic N-H stretching frequency due to
the presence of amide bonds in BSA was observed at 3305 cm-1 (fig. 2.9).
Fig. 2.10 Reproduced with permission from American Chemical Society. PP VAA
spectrum was subtracted from that of BSA showing only the BSA spectrum with
different interval of 1% sodium dodecyl sulfate (SDS) washing.
The covalent bonding of BSA was checked by further washing the BSA attached
film with 1% (SDS) for 1 hr, 2 hr and 4 hrs of duration and the FTIR spectra were taken
after each washing step. It turns out that the >C=O peak intensity remains essentially
constant even after 4 hours of washing with SDS. See fig 2.10.
36
2.3.5 Immobilization of BSA on Plasma Polymerized Glycidyl Methacrylate (GM)
To test the generality of BSA covalent bonding on to the polymer film surfaces,
this work was extended to include one more monomer, glycidyl methacrylate (C7H10O3).
See fig. 2.11 for the structure of GM.
H3C
CH2
O
O
O
Fig 2.11 Structure of glycidyl methacrylate.
Three films were synthesized through plasma polymerization with different thicknesses,
50nm, 150nm, 300nm. Fig 2.12 shows the ATR-FTIR spectra of these three polymer
films; it was observed that the absorbance is proportional to the thickness of the film and
that the spectra were essentially identical with that of VAA. The peaks are assigned as
(a) C=O at the region of 1710 cm-1; (b) –CH3 asymmetric stretch at 2976 cm-1; (c) –CH2
asymmetric stretch at 2930 cm-1; (d) –CH3 symmetric stretch at 2866 cm-1; The C=O
functional group present on the film surface was observed to react with BSA.
37
4000 3500 3000 2500 2000 1500 1000-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.41728 A
B C
Abs
orba
nce
Wavenumber cm-1
(a)
4000 3500 3000 2500 2000 1500 1000
0.0
0.2
0.4
0.6
0.8
1.0
2972
33011656
1727
1539
(b) PP GM Water rinsed B SA attached
Abs
orba
nce
Wavenumber cm-1
Fig. 2.12 (a) FTIR spectra of 50nm (A), 150nm (B), 300nm (C) thick GM polymer films
(b) FTIR spectra of PP GM, rinsed PP GM film and BSA reacted PP GM film.
38
.
4000 3500 3000 2500 2000 1500 1000-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
3313
1539
1656 BSA 1 hr SDS washing 2 hr SDS washing 4 hr SDS washing
Abs
orba
nce
Wavenumber cm-1
Fig 2.13 FTIR subtracted spectra of BSA after covalent attachment to the GM film.
In order to test the protein attachment to the surface, the films were dipped in to the BSA
solution and IR spectra were taken. It was observed that the amine functional group
present in protein reacts with the carbonyl terminal of the film and forms amide carbonyl
functional group. This was detected through FTIR and their respective stretching bands
are at 1645 cm-1 and 1528 cm-1. The characteristic N-H peak at 3313 cm-1 was observed
(see fig. 2.13)
39
2.4 Conclusion
A thin film hydrogel was prepared using plasma polymerization of new class of
monomers. The monomers included in the study were 1A2P and 2EAE and the films
formed by the plasma polymerization of those monomers were characterized using ATR-
FTIR technique for the molecular structures and for the moisture uptake property of these
films. It was observed that the hydrogel films formed possess moisture retention property
which was revealed in FTIR spectra. The increase and decrease in O-H absorption band
with changing the films purging time tells about the moisture absorption from the
atmosphere. Variable duty cycle in plasma polymerization was useful to study the
different characteristics of hydrogel film produced under different duty cycle conditions.
From the hydrogel films, it was noted that the thickness of the films under of 10/30 and
10/10 conditions varied linearly with deposition time which was not true in films
produced under continuous wave run, instead, the rate of film formation was decreased
with increase in deposition time. The reason for this presumably arises from an
increasing contribution from film extirpation at relatively high power conditions, an
observation that has been made in prior plasma-polymerization studies.
As said in this work, plasma polymerization technique is a simple approach for
the functionalization of the surfaces with reactive functional groups. These functional
groups can further be used for the attachment of target molecules like protein. This
process involves single, solventless coating method and provides a conformal, pin-hole
free surface. The film can be coated on any solid surface. The polymerization of vinyl
acetic acid and glycidyl methacrylate were shown to have reactive surface species, and
thus it is convenient for the covalent coupling of target biomolecule via simple
40
derivatization reaction. This is a general approach where not only protein but, a host of
other compounds can also be used for covalent attachment. Finally, it should be noted
that this method of plasma polymerization can be used for a range of other monomers and
the technique ATR-FTIR can be very well used to characterize the attached
biomolecules.
41
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45
CHAPTER 3
STUDY OF BENZOTRIAZOLE ADSORPTION ON COPPER SURFACE BY
FTIR-ATR SPECTROSCOPY
3.1 Introduction
In integrated circuit fabrication industry one of the processes is “removing” which
means removing material from the silicon wafer either chemically, physically or a
combination of both. The process chemical mechanical polishing (CMP) is the one that
has been followed to remove materials from wafer surface by both chemical reaction and
mechanical grinding to achieve planarization [1]. The choice of process left for the
copper metallization is dual damascene process as copper is very difficult to dry etch.
One of the applications of CMP is the copper interconnection. The components involved
in CMP process are polishing pad, a rotating wafer carrier that holds the wafer face down
against the polishing pad and a slurry dispenser unit. Slurry plays a very important role
in CMP process. The chemicals in the slurry solution react with surface particulates and
dissolve the materials or form chemical compounds that can be removed by abrasive
particles.
Two main kinds of CMP slurries are there for oxide removal and metals removal.
Additives in the slurry control the pH, which affects the chemical reactions in the CMP
processes and helps to achieve optimal process results. During CMP process, the Cu
surface is exposed to different kinds of solutions and corrosion happens. To eliminate
this problem, one of the additives called benzotriazole (BTA) is added in the slurry which
acts as the corrosion inhibitor for copper during CMP process. Benzotriazole (C6H5N3) is
a nitrogen heterocyclic derivative having three nitrogen atoms, each with an unshared
46
lone pair of electrons, forming a five-membered ring structure and widely known as a
good corrosion inhibitor for Cu and Cu-based alloys in different environments [2].
HN
N
N
Fig. 3.1 Structure of benzotriazole.
Extensive studies have been conducted on the adsorption of BTA on Cu surfaces
using various techniques like XPS, XRD, and FTIR etc. Cotton’s studies indicated that
the BTA reacts with Cu and forms protective layers that are inert, thermally stable and
long-lasting on the metal surfaces. The protective action of BTA is because of its ability
to react at Cu surfaces to form a chemisorbed, two dimensional polymeric barrier layers
[3]. He discovered that the triazole ring plays an important role in the formation of
complex with Cu. The Cu(I)BTA complex is formed from cuprous salts with a basic
formula C6H4N3Cu. Cotton suggested that the bonding between cuprous ion and BTA
molecule was achieved by the formation of coordination complex. The proton from the
NH group is replaced by the Cu(I) ion forming a covalent link. A coordination bond
from the lone pair of electrons is also formed with one of the nitrogen atoms and thus it
forms a polymeric chain like structure. In this chapter, the adsorption of benzotriazole on
Cu surface has been characterized using ATR-FTIR technique. In continuance of this
work, the corrosion inhibition effect of BTA on Cu surface will be studied.
47
3.2 Experimental
N-type (111) Si single crystal double polished wafers from polishing corporation
of America, 430 Martin Ave. Santa Clara, CA 95050 (www.pcaSi.com/ - PCA#7493)
were cut in to pieces with dimensions of 6×1 cm and polished at the ends with an angle of
45 using allied high tech products’ techprep/multiprep polishing machine. These Si
crystals are used as attenuated total reflection (ATR) crystals which undergo multiple
internal reflections with IR light. These ATR crystals were used as substrates for copper
deposition. They were cleaned by the procedure described in chapter 2 and after cleaning
they were immersed in 4.9% HF solution for 5 minutes. HF etching of cleaned silicon
substrates removes oxide and terminates the surface with hydride. The hydride
terminated silicon substrate was used for copper deposition through electro-less plating
technique. The IR spectra were taken after each experimental step.
After the IR measurements of Si-hydride crystal were taken, the bevel plane of the
ATR crystal was covered with a tape before immersing it into the copper containing
solution. This was done to prevent the bevel plane from copper deposition on it so that
IR light can pass through in to the crystal and get reflections. Then the silicon hydride
substrate was immersed in 50 ml of 4.9% HF solution containing cupric ions of
concentration 1.5mM for a time period of 20 sec. Cu ions present in the solution were
reduced to metallic Cu at the silicon surface. The crystal was rinsed well with ultra pure
water after removing it from the copper solution. The electroless plated copper film on
silicon surface was observed through optical microscope (Nikon, Japan) to study the
morphology of the surface and then the IR spectrum of Cu/Si crystal was taken which
serves as a reference for BTA adsorption. The crystal was immersed in an aqueous
48
solution of benzotriazole of concentration 0.05M for 5 minutes, rinsed and dried with a
stream of dry air. ATR-FTIR measurements were made after each successive step using
the same equipment and parameters as described in chapter 2. All the experiments were
conducted using PFA lab wares that were cleaned in hot solution of 10% HNO3. The
ultra pure water (UPW) was supplied via a Millipore Milli-Q® Elix 5® ultrapure water
(Millipore Corporation, Bedford, MA, www.millipore.com/ ) purification system
(R>18.2 MΩ/cm) and was used to rinse the samples as well as any dilution of chemicals.
3.3 Results and Discussion
3.3.1 Microscopic and FTIR Results of Hydrogen Terminated Silicon Surface The silicon (111) ATR crystal after SC1 treatment was terminated with hydrogen
using 4.9% HF solution. The oxide for about a thickness of 30 Å developed on the
surface due to SC1 cleaning has been etched away in the process of hydrogen
termination. The hydrogen terminated silicon surface was microscopically smooth as can
be seen from images below at 100x magnification, bright field, and fig 3.2. It has been
reported in the literature that the mean roughness of HF etched silicon surface will be
<0.15 nm [4]. A silicon wafer exposed to HF reacts with the surface silicon oxide, SiO2
as per the following reactions [5],
SiO2 (s) + 6 HF (aq) H2SiF6 (aq) + 2H2O --------------- (1)
H2SiF6 (aq) SiF4 (g) + 2 HF --------------- (2)
49
Fig.3.2 Microscopic image of hydrogen terminated silicon in bright field, 100 x
magnifications.
2300 2250 2200 2150 2100 2050 2000 1950 1900-0.005
0.000
0.005
0.010
2072
2138
2104
2085
Abs
orba
nce
W avenumber cm -1
Fig. 3.3 FTIR spectrum of silicon hydride sample.
The ATR-FTIR characterization of hydrofluoric acid passivated silicon surface
exhibits a complex vibrational spectrum as shown in Figure 3.3. The Si-H vibrational
50
stretching frequency was observed in the region 2050 cm-1 – 2150 cm-1. The
monohydride stretching modes are at 2085 cm-1 and 2072 cm-1 corresponding to the
coupled monohydride (H-Si-Si-H) symmetric and asymmetric vibrational stretching
frequencies. The silicon dihydride band exists at 2104 cm-1 and the trihydride at 2138
cm-1. After the hydrogen termination process, the surface is highly hydrophobic such that
the water contact angle is ~70
3.3.2 Optimization of Electroless Copper Deposition Parameters
Metal deposition on substrates by electroless technique is more advantageous over
the other techniques in terms of parameters like uniformity, super-filling, simplicity and
cost. The application of electroless deposition is vital in semiconductor-devices
industries. A series of experiments were done for the electroless Cu deposition in order
to obtain smooth, fully covered and small nuclei Cu particles on the Si surface. See table
3.1 for the different parameters followed to optimize the deposition. For the optimization
of Cu deposition, a number of small Si chips (1cm×1cm) were used.
The parameters that were under optimization include a) electrolyte for Cu
deposition viz. 4.9% HF and 40% NH4F solutions, b) concentration of Cu ions, c) time of
deposition d) surface pretreatments of Si. In the event of optimizing these parameters, it
was observed through optical microscope that the Si crystal being treated with SC1
solution, annealed at 850C for 30 minutes that undergone Cu deposition for 20s using
4.9% HF containing 1.5mM Cu (highlighted in the table) had smooth and fully covered
Cu particles on the surface as can be seen from microscope images in fig. 3.4. The main
concern for this optimization of Cu deposition parameters is towards the goal of
51
achieving good “IR throughput”. The IR throughput will be diminished totally if the
deposited Cu film is too thick on the Si ATR crystal.
Cleaning Method Hydrogen Termination Cu Deposition
Time of Cu Deposition
SC1 4.9% HF 0.6mM Cu/NH4F 30 min, 60 min, 90 min
Piranha Solution 10 min 40% HN4F 0.6mM Cu/NH4F 5 min
Piranha Solution 10 min 4.9% HF 0.6mM Cu/NH4F 5 min
Piranha Solution 15min 4.9% HF 0.6mM Cu/NH4F 5 min
Cu Deposition Using HF in lieu of NH4F
Piranha Solution 10 min 4.9% HF 0.6mM Cu/HF 5 min
Hot Piranha Solution 10min 4.9% HF 0.6mM Cu/HF 5 min
Piranha Solution (2:1) 4.9% HF 0.6mM Cu/HF 5 min
Piranha Solution (2:1) - Hot 4.9% HF 0.6mM Cu/HF 5 min
Growing Thick Oxide on Si before HF Dipping
SC1, Annealing 850C, 30 min 4.9% HF 0.6mM Cu/HF 5 min
SC1, Annealing 850C, 30 min 4.9% NH4F 0.6mM Cu/NH4F 5min
SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 90 sec
SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 40 sec
SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 30 sec
SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 20 sec
SC1, Annealing 850C, 30 min 4.9% HF 1.5mM Cu/HF 10 sec Table 3.1 Series of experiments conducted to produce uniform Cu coverage on Si.
52
The process of electroless deposition indicates the mechanism of direct
displacement reaction that results in the dissolution of Si by fluoride ions present in the
solution with simultaneous reduction of Cu ions on the Si surface by obtaining the
electrons released by Si-H. The possible oxidation/reduction reaction for the Cu
deposition on HF-treated silicon surfaces can be represented as,
2Cu2+ + Si + 6HF 2Cu(s) + SiF62- + 6H+ --------------- (3)
a) Immersion time: 90s b) Immersion time: 40s
c) Immersion time: 30s d) Immersion time: 20s
Fig. 3.4 Microscopic dark filed images of copper deposited on silicon substrate for
different times of immersion.
53
3.3.3 Contact Angle Measurements
The selection of annealing process as an initial treatment, to grow thermal oxide
good coverage of Cu particles as the surface
ecome
d.
.
Si
easurements of Cu on Si surface with different times of
eposition.
on Si surface was found suitable for a
b s smooth and homogeneous after the HF etching. Along with the next step of
optimizing the Cu deposition time, contact angles of the Cu/Si surface were measure
Contact angles were measured using a contact angle goniometer which is a very useful
tool in studying the nature of the surface. Table 3.2 gives the contact angles of Cu/Si
surfaces having different times of Cu deposition. It was observed that the contact angle
of Cu deposited for 90s was 95 which is pretty close to that of a pure Cu metal surface
From this information, it can be said that the Si surface was fully covered with Cu
particles which was evidenced from microscopic results. The contact angle was
decreased as the deposition time was decreased indicating that the Cu coverage on
surface was decreasing.
Table 3.2 Contact angle m
Time of deposition in 1.5mM Cu
Nature of Cu film from Microscope observation
Contact angle with water
90 sec Uniform, Dense particles 95
40 sec Uniform, Less dense 68-75
30 sec Uniform 50-55
20 sec Uniform and Good Coverage 40
10 sec Dispersed particles 30
d
54
3.3.4 FTIR-ATR Characterization of Cu/Si and BTA/Cu/Si Samples
Fig. 3.5 depicts the FTIR spectra of Si ATR crystals that had Cu deposition for 5,
d that the increase
in the t
4F
lectrolyte A) 60 minutes B) 25 minutes C) 5 minutes
0.05M of BTA for 5 min and IR
pectra were taken. Fig. 3.6 shows the FTIR spectrum of BTA adsorbed on Cu/Si
substrate. It has been reported that the BTA molecule loses the hydrogen atom from its
4000 3500 3000 2500 2000 1500
0.0
0.2
0.4
0.6
0.8
1.0
25 and 60 minutes in 40% NH4F solution. From the spectra, it is note
ime of Cu deposition results in the increase in the absorbance of the spectra. The
Cu film deposited using 4.9% HF was found to be uniform, smaller sized particles and
having good IR signal. The signal was diminished for all the copper deposited Si ATR
crystals except for the one that had 20s Cu using 4.9% HF.
1.2 A B
C
Abs
orba
nce
W avenumber cm-1
Fig. 3.5 FTIR spectra of three samples with different durations of Cu deposition in NH
e
The Cu/Si sample after rinsing was immersed in
s
55
triazole
on
m-1 showing aryl C-H stretching.
the corrosion protection on Cu was due to the
rmation of a complex polymeric surface film on Cu. The bonding structure and
ied by many investigators
using various techniques [9-14]. However, the effect of concentration of BTA plays an
4000 3500 3000 2500 2000
0.00
0.01
0.02
0.03
ring and the Cu is bonded to the nitrogen atom. The spectrum is in agreement
with Poling’s [5] work in that the signature IR band for the BTA molecule adsorbed
the Cu surface in IR spectrum is aryl C-H stretching at 3061 cm-1. The evidence that the
BTA molecule was chemisorbed on the Cu surface is the appearance of aryl C-H
stretching band at 3061cm-1.
0.04
3061
Abs
orba
nce
Wavenumber cm-1
Fig. 3.6 FTIR spectra of BTA adsorbed on Cu for 5 minutes in the region 4000 – 2000
c
3.3.5 Concentration Effect of BTA on the Adsorption on Cu
Many papers indicated that
fo
adsorption behavior of Cu and BTA has been extensively stud
56
0.0156
0.0158
0.0160
3100 3075 3050 3025 30000.0146
0.0148
0.0150
0.0152
0.0154
30611mAbs
(a)
im nt role in corrosion inhibition of Cu. The concentrations of BTA used in m
cases were about 1-50 mM, and people have studied the properties of the treated
solutions such as pH, corrosion anion concentrations like Cl
ceso
rban
Ab
Wavenumber cm-1
porta any
.
. With
and
.6
- and SO4- etc. J-H Chen et al
have studied the growth kinetics and structure of CuBTA film as a function of BTA
concentration and in further the inhibition mechanism. They found that the inhibition
effect is poor in low concentration of BTA. The critical concentration is 0.17 mM
this concept, the adsorption of BTA on Cu surface was studied through FTIR-ATR as a
function of BTA concentration. Three concentrations were used viz. 0.01 M, 0.05 M
0.5 M. The time of immersion of Cu sample in BTA aqueous solution is 5 min. Fig. 3
shows the result. It was observed that the intensity of aryl C-H peak at 3061 cm-1 is
increased as the BTA concentration is increased.
57
1mAbs
3100 3050 30000.015
0.016
0.017
0.018
0.019
Abso
rban
ce
Wavenumber cm -1
(b)
ig. 3.7 FTIR spectra of Cu sample immersed in different concentrations of BTA
) 0.01 M BTA; (b) 0.05 M BTA; (c) 0.5 M BTA.
3200 3150 3100 3050 3000 29500 .030
0 .032
0 .034
0 .036
0 .038
W avenum ber cm -1
3061
Abs
orba
nce
1 mAbs
(c)
F
(a
58
3.4 Conclusions and Future Investigations
Benzotriazole adsorption on Cu surface was investigated with FTIR-ATR
ing at 3061 cm-1. BTA concentration
ace have been conducted. It was observed
.
Benzotriazole-5-carboxylic acid
1-(2-Pyridylcarbonyl)benzotriazole
technique. It is known that the BTA forms bond with Cu surface through its triazole ring
and IR results show the presence of aryl C-H stretch
dependent studies for adsorption on Cu surf
that the absorbance of aryl C-H band was proportional to the concentration of BTA
My future investigations are going to be on the study of corrosion inhibition effect
of BTA on Cu surface in low concentrations and various pH conditions. The corrosion
inhibition effect of many BTA derivatives such as in the figure 3.8
Benzotriazole-1-carboxamide
Fig. 3.8 Structures of BTA derivatives.
59
will also be studied. It was reported that the bu tached to the BTA lky group at molecule
eric effect and thereby prevents further oxidation of Cu [13-15].
Sec ruc re, th surface of Cu is left with
organic impurities like BTA which is used as one of the constituents in the slurry. These
inhibitors and other organic impurities have to be removed from the Cu surface before the
successive processes. Hence, my focus would be finding a method to remove BTA from
Cu surface without changing the nature of Cu. Thirdly, my aim is to establish a
methodology to deposit ruthenium (Ru) on Si as Ru can be used as a barrier layer for Cu
diffusion into Si at higher temperatures. The effect of BTA on Ru will also be studied.
causes st
ondly, in the process of dual damascene Cu st tu e
60
3.5 Chapter References
. Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Prentice-
Hall Inc., Upper Saddle River, New Jersey, 2001
. J.B.Cotton, Proc, 2nd Int. Congr. Metallic Corrosion, NACE, New York,1963.
3. J.B. Cotton and I.R. Scholes, Br. Corrosion Jnl., 1967, 2, 1.
4. Oliver M. R. Chyan, Jin-Jian Chen, and Hsu Y. Chien; J Electrochem. Soc., 1996,
143, 1.
5. Fred Meyer and John B White, Semiconductor International, Silicon Engineering
Technology Center Laboratory, 1999, July, 137.
6. G. W. Poling, Corros. Sci. 1970, 10, 359.
7. S. Thibault, Corros. Sci. 1977, 17, 701.
8. F. Manfeld and T. Smith, Corrosion, 1973, 29, 105.
9. D. Chadwick and T. Hashemi, Corros. Sci., 1978, 18, 39.
10. S. L. Cohen, V. A. Brusic, F. B. Caufman, G. S. Frankel. S. Motakef and B. Rush.
J. Vac. Sci. Technol., 1990, A8, 2317.
11. R. R. Thomas, V. A. Brusic and B. M. Rush, J.Electrochem. Soc., 1992, 139, 678.
12. Chen Jin-Hua, Lin Zhi-Cheng, Chen Shu, Nie Li-Hua and Yao Shou-Zhuo*
Electrochemica Acta, 1998, 43, 3-4, 265-274.
13. C. Tornkvist, D. Thierry, J. Bergman, B. Liedberg and C. Leygraf; J.
Electrochem. Soc., 1989, 136, 1.
14. L. Tommesani, G. Brunoro, A. Frignani, c. Monticelli and M. Dal Cole Corrosion
Science, 1997, 39, 7, 1221-1237.
1
2
61
15. N. Huynh, S.E. Bottle, T. Notoya, D.P.; Schweinsberg Corrosion Science, 2000,
42, 259-274.
62