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
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Outline
• Etching and cleaning solutions/processes
• Particle adhesion theory
• Surface charge and chemistry
• Contamination
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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
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0
Weight % HF
100
Temperature
Etch Rate (Å/min)
at constant temp.
Etch Rate (Å/min)
Etch Rate of SiO2
More NH4F
Less NH4F
NH4F/HF Ratios
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.
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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
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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
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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
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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)
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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
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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)
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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
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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.
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Particle Removal Efficiency vs. Immersion Time
SC1 solutions w/ varying NH4OH concentration
1.0
Particle Removal Efficiency
1:1:5 NH4OH:H2O2:H2O
The efficiency curve is
steeper with a higher
concentration of
NH4OH in the SC1
solution.
0
Immersion Time
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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.)
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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
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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
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Particle Deposition Model
• Parameters controlling deposition
– zeta potential of wafers
– size and zeta potential of particles
– ionic strength and temperature of solution
Substrate
• 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
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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
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Surface Charge Development on SiO2
Immersed in Aqueous Solutions
-O-Si...
Bulk
SiO2
H+
-Si-O...
OH-
Aqueous
Solution
-O-Si...
Acidic Solutions
(low pH)
-Si-O...
H+
OH-
-O-Si-OH2+
Bulk
Solid
-O-Si-OH2+
-O-Si-OH
Basic Solutions
(high pH)
-O-Si-OSolution
Bulk
Solid
-O-Si-O-
Solution
-O-Si-OH
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Point of Zero Charge (PZC) of Materials
• PZC = the solution pH value at which the surface bears no net charge;
i.e. surf = 0
pHPZC
SiO2
2-2.5
TiO2
5.5-6
Al2O3
~9
Si
~4
Ny lon
~6
surf
Material
(microcoulombs/cm2)
20
PZC
0
pH
-20
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.
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Surface Potential (o) and Zeta Potential ()
+--+
Solid
+ - Liquid
+-++- -o
0
Surface Potential (o ):

Zeta Potential ( ):
• Potential in the double layer at a
short distance (typically the
diameter of a hydrated counter ion)
from the solid surface
• Not experimentally measurable
• Experimentally measurable
through electrokinetic techniques
• Oxides immersed in aqueous
soln’s, o = 0.059 (PZC-pH) volts
• Decreases (more negative) with
increasing pH
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Zeta Potential
Electrophoretic Method



E K
E

 = 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
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Zeta Potential from Streaming Potential
LIQUID IN
LIQUID OUT
(+) and (-) charges
P
V
• 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
k E
  4
 P
s
, , and k are viscosity, dielectric
constant, and conductivity of solution;
Es/P is the slope of the streaming
potential vs. pressure drop.
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Streaming Potential Cell
Schematic Sketch - 6” wafers
Electrode
LIQ IN
LIQ OUT
Electrode
Cell
Block
Channel
LIQ IN
LIQ OUT
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Zeta Potential vs. pH
Oxide Wafer - Activation Etch
Zeta Potential, mV
0
(-)
pH
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Contamination Mechanisms
• Liquid film draining (liquid/air interface)
A
A
(OR)
Hydrophilic
Hydrophobic
L
L
• Bulk deposition from liquids
• Contaminant pick-up from air
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