Silicon-Based Surface Treatments for Improved Vacuum System - - PowerPoint PPT Presentation

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
• f 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)

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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

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Focus: Bulk surface modification

• Regardless of

– Configuration

• 3D
• Coiled tubing

– Part count – Size (within reason…)

• Engineering surface

performance beyond original design

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Why bother? Powerful Example…

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• 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 exposure

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stainless steel

• 1. vac, heat
• 2. SinH2n+2

a-silicon hydride

Si Si Si Si Si Si H H H Si H H stainless steel

AES Depth Profile

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500 1000 1500 2000 2500 3000 10 20 30 40 50 60 70 80 90 100

Sputter Depth (Å ) Atomic Concentration (%)

Oxygen Cr Ni Iron Silicon

Mo

Diffusion Zone

In-Situ Surface Chemistry

• Functionalize via thermal hydrosilylation

8 steel, glass,ceramic a-silicon hydride

Si Si Si Si Si Si H H H Si H H Si Si Si Si Si Si H Si CH2 H CH2 CH2 CH2R CH2R 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.

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Raw 5um Silica a-Si deposition

• n Silica

Hydrocarbon functionalization

• n a-Si / silica

Surface Energy Measurements

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Bare 316ss 37.2° advancing 0° receding a-Silicon coated 53.6° advancing 19.6° receding Functionalized a-Si 87.3° advancing 51.5° receding

Force vs Position

Position [mm] Force [mN]

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

0.5 1 1.5 2 2.5 3 3.5

Tubing Drydown Example

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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

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ASTM G48 Method B: Pitting and Crevice Corrosion 6% 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

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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

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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 sec E = 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
• utgassing rate exponentially

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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 deposition

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Outgassing Data – Heated Samples at HV

• Turbopump, 1 x 10-7 Torr base pressure
• 10hr under vacuum

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Pressure increase with heat

5 10 15 20 25 a-Si CVD 1st gen. Control As received

Outgassing Data – HV Heated Samples

• 7.5 fold improvement at 112ºC

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Pressure increase with heat

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

• Min. (Temp

ºC) 0.5 (112) 1 (145) 1.5 (167) 2 (186) 2.5 (195) 3 (201)

∆P units of 10-7 Torr

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)

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Pressure increase with heat – 1 hour evacuation

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

0.5 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 improvement

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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

5 10 15 20 25 30

• Sec. (Temp

ºC) 15 (61) 30 (105) 45 (137) 60 (161) 75 (180) 90 (201) units of 10-10 Torr control a-Si coated

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Chamber Comparison; No Heat

Untreated

a-Si

G V4 roughing pump Turbo pump V1 V2 V3

• Common pumping

line

• Valve isolation
• Alternating chamber

measurements

• Roughing pump for

first 44 min.

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Chamber Comparisons; No Heat

• System conductance: 7.4 l/sec
• 360 l/sec turbomolecular pump
• Cold cathode gauge

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Chamber Comparisons; No Heat

• Alternate-pumpdown system pressures
• 80-84 minute range: 2.4-fold improvement

Comparative Evacuation Rates

10 20 30 40 50 60 70 80 Minutes 56 64 72 80 88 96 104 112 System Pressure (10 -7 Torr) Untreated Chamber a-Si Chamber

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Corrected Comparison

• Alternate pressure drop system measurements

(true outgassing of isolated chambers)

• 80-84 minute range: 9.1-fold improvement

Corrected Evacuation Rates

10 20 30 40 50 60 M i n u t e s 5 6 6 4 7 2 8 8 8 9 6 1 4 1 1 2 Pressure Drop (10 -7 Torr) 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

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Carbosilane FT-IR

SilcoTek Carbosilane on Si 745.10 823.52 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)

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AES Depth Profile

20 40 60 80 100 120 140 160 180 200 10 20 30 40 50 60 70 80 90 100 Sputter Depth (nm) Atomic Concentration (%)

O C Si Cr Fe Ni

Diffusion Zone

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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

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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 gap

to Cassie-Baxter state

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Contact Angle Illustration

Close to Release…

• DI water CA: 127°
• On 304 stainless corrosion

coupon; no topography modification

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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