silicon-based surface treatments for improved vacuum system throughput, inertness, and corrosion...
TRANSCRIPT
Silicon-Based Surface Treatments for Improved
Vacuum System Throughput, Inertness, and Corrosion
Resistance
David A. Smith
SilcoTek Corporation
112 Benner Circle
Bellefonte, PA 16823
www.SilcoTek.com
Bruce R.F. Kendall
Elvac Associates
100 Rolling Ridge Drive
Bellefonte, PA 16823
Research Focus: Surface Modification
• Surface treatments to improve performance of ordinary materials– Stainless steels / carbon steels– Glass– High performance alloys
• Focus on silicon / functionalized silicon– Inert– Corrosion resistant– Diffusion barrier– Tailor properties (i.e. surface energy)
2
New Technology?
• Kipping – silicon materials in 1920’s– Reductive coupling of silicon chlorides– Functional polysilanes – [SiR2]n
– Functional polysilynes – [SiR]n
– Solubility issues• Semiconductor industry (1960’s)
– High purity silicon depositions– Controlled doping, etching, implanting
3
Focus: Bulk surface modification
• Regardless of– Configuration
• 3D• Coiled tubing
– Part count– Size (within reason…)
• Engineering surface performance beyond original design
4
Why bother? Powerful Example…
5
• Silver texture on copper with heptadecafluoro -1-decanethiol coating
• Air layer between water and metal coupon
• Critical viewing angle = 48.6° (same as water/air reflection boundary); <1% water in contact with surface (CA = 173°)
Larmour, I.A.; Bell, S.E.J; Saunders, G.C. Angew. Chem. Int. Ed.2007, 46, 1710-1712.
Thermal CVD Process
• Diffusion in to stainless lattice• Native oxide formation on surface upon
atmospheric exposure6
stainless steel1. vac, heat2. SinH2n+2
a-silicon hydride
SiSi
SiSi
Si
Si
H H
H
Si
H H
stainless steel
AES Depth Profile
7500 1000 1500 2000 2500 3000
0
10
20
30
40
50
60
70
80
90
100
Sputter Depth (Å )
Ato
mic
Con
cent
ratio
n (%
)
Oxygen
Cr
Ni
Iron
Silicon
Mo
Diffusion Zone
In-Situ Surface Chemistry• Functionalize via thermal hydrosilylation
8
steel, glass,ceramic
a-silicon hydride
SiSi
SiSi
Si
Si
H H
H
Si
H H
SiSi
SiSi
Si
SiH
Si
CH2 HCH2 CH2
CH2RCH2R CH2R
a-silicon hydride
steel, glass,ceramic
CH2 CH R Si
H
+ Si
CH2 CH R
H
CH2=CH-R
US Pat. #6,444,326
DRIFTS Illustration of Func.
9
Raw 5um Silica
a-Si deposition
on Silica
Hydrocarbon
functionalization
on a-Si / silica
Surface Energy Measurements
10
Bare 316ss37.2° advancing 0° receding
a-Silicon coated53.6° advancing 19.6° receding
Functionalized a-Si 87.3° advancing 51.5° receding
Force vs Position
Position [mm]
Force [m
N]
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
Tubing Drydown Example
11
Conditions:
100’, ¼” tubing,
0.35 slpm, 22C
1ppm Equilibration Time:• Commercial seamless:
180 min. (96% DD) • E-polished seamless:
60 min. (98% DD)• Func. a-Si, e-polished
seamless: 30 min. (98% DD)
Data courtesy of O’Brien Corporation, St. Louis, MO
Func. a-Si
Anti-Corrosion Benefits Example
12
ASTM G48 Method B: Pitting and Crevice Corrosion6% Ferric Chloride solution, 72hrs, 20ºC, Gasket
wrap~10X Improvement (weight loss)
Untreated 316 SS a-SiH coated 316 SS
Tubing Inertness Example
• What does this mean?– Activity at metallic interfaces can be
minimized or avoided 13
Sulfur Flow-Through Data:
• 100’ 1/8” x .020” 316 SS tubing
• 0.5ppmv methyl mercaptan in He
• SCD detection
Data courtesy of Shell Research Technology Centre, Amsterdam
Func. a-Si
EP Tubing
Vacuum System Issues• Long evacuation times / poor base vacuum
– Leaks– Volatile Contamination
• Water vapor– Atmospheric– Gas lines
• Organic
• Metallic / non-volatile contamination• Chamber material• Prior process remnants
• Root cause: Surface Interactions
Seasoning• Systems require time / dummy
runs / process exposure before steady state is achieved
• Time and cost intensive• Root cause: Surface Interactions
15
Heat-Induced Outgassing• How to measure a potential benefit?• Outgassing rate (F) in monolayers per sec:
F = [exp (-E/RT)] / t’t’ = period of oscillation of molecule perp.
surface, ca. 10-13 secE = energy of desorption (Kcal/g mol)R = gas constant
source: Roth, A. Vacuum Technology, Elsevier Science Publishers, Amsterdam, 2nd ed., p. 177.
• Slight elevation of sample temperature accelerates
outgassing rate exponentially16
Experimental Design: Heated Samples
• Turbo pump for base pressures to 10-8 Torr– pumping rate between gauge and pump: 12.5 l/sec (pump
alone: 360 l/sec)– system vent with dry N2 between thermal cycles
• Ion pump for 10-10 Torr (thermal cycles) • Comparative evaluation parts
– equally treated controls without deposition17
Outgassing Data – Heated Samples at HV
• Turbopump, 1 x 10-7 Torr base pressure• 10hr under vacuum
18
Pressure increase with heat
0
5
10
15
20
25
Min.
(Tem
p ºC
)
0.5
(112
)
1 (1
45)
1.5
(167
)
2 (1
86)
2.5
(195
)
3 (2
01)
a-Si CVD 1st gen.
Control
As received
Outgassing Data – HV Heated Samples
• 7.5 fold improvement at 112ºC 19
Pressure increase with heat
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Min. (TempºC)
0.5 (112) 1 (145) 1.5 (167) 2 (186) 2.5 (195) 3 (201)
∆P
un
its o
f 1
0-7 T
orr
a–Si CVD 1st gen.
Control
As received
Outgassing Data – HV Realistic Evacuation Times
• Turbopump, 4.6 x 10-7 Torr base pressure• 1hr under vacuum (∆P1)
20
Pressure increase with heat – 1 hour evacuation
0
5
10
15
20
25
Seconds(Temp ºC)
15 (61) 30 (105) 45 (137) 60 (161)
a-Si Coated
Control
Outgassing Data – HV Realistic Evacuation Times
• Turbopump, 7.5 x 10-8 Torr base pressure• 10hr under vacuum (∆P2)
21
Pressure increase with heat – 10 hour evacuation
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Seconds(Temp ºC)
15 (61) 30 (105) 45 (137) 60 (161)
a-Si Coated
Control
Outgassing Calculations
• For the system (PA), sample area = 125cm2, conductance = 12.5 l/sec; therefore, ∆Q = ∆P(12.5/125) = ∆P/10
• At 1 hour, 61ºC:∆Q1 (control) = 5.4 x 10-8 Torr l sec-1 cm-2;∆Q1 (a-silicon) = 0.2 x 10-8 Torr l sec-1 cm-2
27x improvement
• At 10 hours, 61ºC:∆Q10 (control) = 0.14 x 10-8 Torr l sec-1 cm-2; ∆Q10 (a-silicon) = 0.01 x 10-8 Torr l sec-1 cm-2
14x improvement22
UHV comparison – B/A ion gauge housings
• Ion pump, 1.2 x 10-10 Torr base pressure• 156 days under vacuum (5th baking cycle)• 3.3-fold improvement at 105ºC
(no measurable ΔP for a-Si at 61ºC, 7.0 x 10-12 Torr ΔP at 105ºC)23
Pressure increase with heat
0
5
10
15
20
25
30
Sec. (TempºC)
15 (61) 30 (105) 45 (137) 60 (161) 75 (180) 90 (201)
un
its
of
10-1
0 T
orr
control
a-Si coated
24
Chamber Comparison; No Heat
Untreated a-Si
G
V4
roughing pump
Turbo pump
V1V2
V3• Common pumping
line• Valve isolation• Alternating chamber
measurements• Roughing pump for
first 44 min.
25
Chamber Comparisons; No Heat
• System conductance: 7.4 l/sec• 360 l/sec turbomolecular pump• Cold cathode gauge
26
Chamber Comparisons; No Heat
• Alternate-pumpdown system pressures• 80-84 minute range: 2.4-fold improvement
Comparative Evacuation Rates
0
10
20
30
40
50
60
70
80
Sys
tem
Pre
ssu
re (
10-7
To
rr)
Untreated Chamber
a-Si Chamber
27
Corrected Comparison
• Alternate pressure drop system measurements (true outgassing of isolated chambers)
• 80-84 minute range: 9.1-fold improvement
Corrected Evacuation Rates
0
10
20
30
40
50
60
Pre
ssu
re D
rop
(10
-7 T
orr
)
Untreated Chamber
a-Si Chamber
Current Research: Carbosilane Materials
• C, Si, H in CVD-deposited matrix• Excellent inertness• Improved corrosion resistance• High hydrophobicity• Si-H functionality for additional
chemistry
28
29
Carbosilane FT-IR
SilcoTekCarbosilane on Si
745.
1082
3.52
1002
.97
1253
.44
2102
.38
2891
.71
2951
.86
70
75
80
85
90
95
100
105
110
115
120
125
130
135
%T
500 1000 1500 2000 2500 3000 3500
Wavenumbers (cm-1)
30
AES Depth Profile
0 20 40 60 80 100 120 140 160 180 2000
10
20
30
40
50
60
70
80
90
100
Sputter Depth (nm)
Ato
mic
Con
cent
ratio
n (%
)
O
C
Si
Cr
Fe
Ni
Diffusion Zone
31
Acid / Base Resistance
• ASTM G31 screening:– 6M HCl, 24 hrs, 316 SS
coupons, 22°C
• High pH Inertness– 18% KOH, 19 hrs, 316 SS sample cylinder, 22°C– No weight loss – need further assessment– Inert to 10ppmv H2S static storage over 48 hrs.
Surface mpy Enhancement
316 SS control 91.90 ----
a-Si corr. res. 18.43 5.0 X
carbosilane 3.29 27.9 X
32
Hydrophobicity / Appearance
Surface Advancing / Receding
a-Silicon 53.6 / 19.6
Funct. a-Silicon (HC) 87.3 / 51.5
carbosilane 100.5 / 63.5
Funct. Carbosilane (HC) 104.7 / 90.1
Funct. Carbosilane (F) 110.5 / 94.8
-narrowing the hysteresis gapto Cassie-Baxter state
33
Contact Angle Illustration
Close to Release…
• DI water CA: 127°• On 304 stainless corrosion
coupon; no topography modification
34
Conclusions / Future
• Continuing research in to bulk surface modifications for the vacuum science and semiconductor industries
• Focus on silicon and carbosilane materials• Outgassing control• Inertness• Contaminant control• Anti-corrosion