Silicon-Based Surface Treatments for Improved Vacuum System Throughput, Inertness, and Corrosion Resistance David A. Smith SilcoTek Corporation 112 Benner.

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