Raghavan
1NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Wet Etching and Cleaning: Surface Considerations and Process Issues
Dr. Srini Raghavan
Dept. of Chemical and Environmental Engineering
University of Arizona
1999 Arizona Board of Regents for The University of Arizona
Raghavan
2NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Outline
• Etching and cleaning solutions/processes
• Particle adhesion theory
• Surface charge and chemistry
• Contamination
Raghavan
3NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Etching and Cleaning Solutions
• HF Solutions
– Dilute HF (DHF) solutions - prepared by diluting 49% HF with dionized water
– Buffered HF solutions - prepared by mixing 49% HF and 40% NH4F in various proportions
• example: Buffered Oxide Etch (BOE) - patented form of buffered HF solution
– May contain surfactants for improving wettability of silicon and penetration of trenches containing hydrophobic base
• nonionic or anionic
• hydrocarbon or fluorocarbon
Raghavan
4NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Etch Rate of SiO2E
tch
Rat
e (Å
/min
)
at c
onst
ant t
emp.
Weight % HF0 100
Etc
h R
ate
(Å/m
in)
NH4F/HF Ratios
Tem
pera
ture
Etch rate of SiO2 increases with increasing weight % of HF in the etch solution, as well as higher ratios of NH4F buffer in BHF solutions. Etch rate also directly increases with increasing temperature.
More NH4F Less NH4F
Raghavan
5NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Etching and Cleaning Solutions (cont’d)
• Piranha
– H2SO4 (98%) and H2O2 (30%) in different ratios
– Used for removing organic contaminants and stripping photoresists
• Phosphoric acid (80%)
– Silicon nitride etch
• Nitric acid and HF
– Silicon etch
Raghavan
6NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Etching and Cleaning Solutions (cont’d)
• SC-2 (Standard Clean 2)
– HCl (73%), H2O2 (30%), dionized water
– Originally developed at a ratio of 1:1:5
– Used for removing metallic contaminants
– Dilute chemistries (compositions with less HCl and H2O2) are being actively considered
Raghavan
7NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Alkaline Cleaning Solutions
• SC-1 (Standard Clean 1)– NH4OH (28%), H2O2 (30%) and dionized water– Classic formulation is 1:1:5– Typically used at 70 C– Dilute formulations are becoming more popular
• Tetramethyl Ammonium Hydroxide (TMAH)– Example: Baker Clean
• TMAH (<10%), nonionic surfactant (<2%), pH regulators for a range of 8-10, and chelating/complexing agents
• Could possibly be used with H2O2 to replace SC1 and SC2 sequence
Raghavan
8NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Surfactants
• Alkyl phenoxy polyethylene oxide alcohol– Nonionic compounds– Alkyl group: 8 - 9 carbons– 9 - 10 ethylene oxide groups– Examples: NCW 601A (Wako Chemicals), Triton X-100 (Union Carbide)
• Alkyl phenoxy polyglycidols– Nonionic surfactants– Example: Olin Hunt Surfactant (OHSR)
• Fluorinated alkyl sulfonates – Anionic surfactants– Typically 8 carbon chain– Example: Fluorad FC-93 (3M)
Raghavan
9NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Surfactants (cont’d)
• Acetylenic alcohols
– Unsaturated triple bond in the structure
– Nonionic
– Example: Surfynol 61 (APCI)
• Betaines
– Zwitterionic in nature
– Used mostly in alkaline clean
– Example: Cocoamidopropyl betaine
Raghavan
10NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
RCA Cleaning
Two-step wet cleaning process involving SC-1 and SC-2:
1) 1:1:5 NH4OH-H2O2-H2O at ~70 C
• Oxidizing ammoniacal solution
• Ammonia complexes many multivalent metal ions (e.g. CU++)
• Treatment leaves a thin “chemical” oxide
• Without H2O2, Si will suffer strong attach by NH4OH
2) 1:1:5 HCl-H2O2-H2O at ~70 C
• HCl removes alkali and transition metals (e.g. Fe)
Raghavan
11NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Problems with SC1 Clean
• Some metals (e.g. Al) are insoluble in this oxidizing, highly basic solution and tend to precipitate on the surface of Si wafers
• High Fe contamination of the wafer surface after a SC1 clean
• Rough surface after cleaning
– SC1 solutions with lower ammonia content (X:1:5, X<1) are being actively investigated
Raghavan
12NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Particle Removal During SC1 Clean
• H2O2 promotes the formation of an oxide
• NH4OH slowly etches the oxide– In a 1:1:5 SC1, the oxide etch rate is ~0.3 nm/min at 70 ºC.
At the alkaline pH value of SC1 solution, most surfaces are negatively charged. Hence, electrostatic repulsion between the removed particle and the oxide surface will prevent particle redeposition.
Raghavan
13NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Particle Removal Efficiency vs. Immersion TimeSC1 solutions w/ varying NH4OH concentration
Par
ticl
e R
emov
al E
ffic
ienc
y
0
1.0
Immersion Time
The efficiency curve is steeper with a higher concentration of NH4OH in the SC1 solution.
1:1:5 NH4OH:H2O2:H2O
Raghavan
14NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Standard Clean for Silicon
• Step 1 - Piranha/SPM– 4:1 H2SO4 (40%):H2O2 (30%) @ 90 C for 15 min
– Removes organic contaminants
• Step 2 - DI water rinse
• Step 3 - DHF– HF (2%) for 30 sec
• Step 4 - DI water rinse
• Step 5 (SC-1/APM)– 1:1:5 NH4OH (29%):H2O2 (30%) H2O at 70 C for 10 min
– removes particulate contaminants
– desorbs trace metals (Au, Ag, Cu, Ni, etc.)
Raghavan
15NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Standard Clean for Silicon (cont’d)
• Step 6 - DI water rinse
• Step 7 - SC-2– 1:1:5 HCl (30%):H2O2 (30%):H2O at 70 C for 10 min
– dissolves alkali ions and hydroxides of Al3+, Fe3+, Mg3+
– desorbs by complexing residual metals
• Step 8 - DI water rinse
• Step 9 - Spin rinse dry
Raghavan
16NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Adhesion of Particles to Surfaces
• Attractive Forces (AF)– van der Waals forces (short range)– Electrostatic (if the charge on the particles is opposite to the charge
on the surface (typically longer range)
• Repulsive Forces (RF)– Electrostatic (charge on the particle has the same sign as that on the
surface)– Steric forces (due to absorbed polymer layers on the surface of the
particles and wafer) (short range)
When AF > RF, particle deposition is favorable
Raghavan
17NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Particle Deposition Model
• Parameters controlling deposition
– zeta potential of wafers
– size and zeta potential of particles
– ionic strength and temperature of solution
• Transport of particles towards the wafer requires diffusion through a surface boundary layer (particles move along the flow in the solution and deposit by diffusion).
Along the flow
Diffusion layer
Sub
stra
te
Raghavan
18NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Surface Charge and Surface Electricity
• Development of surface charge
– Adsorption of H+ and OH- ions (oxides)
– Selective adsorption of positive or negative ions (hydrophobic materials)
– Ionization of surface groups (polymers such as nylon)
– Fixed charges in the matrix structure exposed due to counter ion release
• example: positively charged modified filters used in DI water purification
Raghavan
19NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Surface Charge Development on SiO2
Immersed in Aqueous Solutions
-O-Si...
-Si-O...
-Si-O...
-O-Si...
-O-Si-OH2+
-O-Si-OH2+
-O-Si-OH
-O-Si-O-
-O-Si-O-
-O-Si-OH
Bulk Solid Solution
Bulk Solid Solution
H+
OH-
Bulk SiO2
Aqueous Solution
Acidic Solutions (low pH)
Basic Solutions (high pH)H+ OH-
Raghavan
20NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Point of Zero Charge (PZC) of Materials
• PZC = the solution pH value at which the surface bears no net charge; i.e. surf = 0
0su
rf
(mic
roco
ulom
bs/c
m2 )
pH
20
-20
PZCMaterial pHPZC
SiO2 2-2.5
TiO2 5.5-6
Al2O3 ~9
Si ~4
Ny lon ~6
Development of + or - charge at a given pH depends on the nature of the metal-oxygen bond and the acid/base character of the surface MOH groups. Acidic oxides have a lower PZC than basic oxides.
Raghavan
21NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Surface Potential (o) and Zeta Potential ()
Solid Liquid
+ - - +
+ - + -
+ - - -
+ -
0
o
Surface Potential (o ):
• Not experimentally measurable
• Oxides immersed in aqueous soln’s, o = 0.059 (PZC-pH) volts
Zeta Potential ( ):
• Potential in the double layer at a short distance (typically the diameter of a hydrated counter ion) from the solid surface
• Experimentally measurable through electrokinetic techniques
• Decreases (more negative) with increasing pH
Raghavan
22NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Zeta Potential Electrophoretic Method
E K
= dielectric constant of liquid
= viscosity of liquid
K = constant dependent on particle size >> 1/ or << 1/
(1/ is the electrical double layer thickness)
• Technique useful for particles suspended in aqueous or non-aqueous media
E
Raghavan
23NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Zeta Potential from Streaming Potential
V
(+) and (-) chargesLIQUID IN
P
LIQUID OUT
• Generation of an electrical potential due to the flow of liquid past a charged surface
• Potential generated = streaming potential (Estr), which is related to zeta potential
4k EP
s
, , and k are viscosity, dielectric constant, and conductivity of solution; Es/P is the slope of the streaming potential vs. pressure drop.
Raghavan
24NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Streaming Potential CellSchematic Sketch - 6” wafers
BlockCell
LIQ OUTLIQ IN
Channel
Electrode LIQ IN LIQ OUT Electrode
Raghavan
25NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Zeta Potential vs. pHOxide Wafer - Activation Etch
0
(-)
Zet
a P
oten
tial
, mV
pH
Raghavan
26NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Contamination Mechanisms
• Liquid film draining (liquid/air interface)
• Bulk deposition from liquids
• Contaminant pick-up from air
A
L
Hydrophilic Hydrophobic
A
L(OR)