Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

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552 Chapter 4.8: Aluminum-Based

Vacuum

Systems

of A2219, A5083, and A6063 aluminum alloys.

Welding

in the World (Pergamon Press) 31(2)

(1993) 128.

97.

J. H. Dudas and F. R. Collins, Preventing weld cracks in high-strength aluminum alloys.

Weld-

ing

Journal,

June 1966, p. 3.

98.

Ibid.

99.

Ibid., p. 10.

100.

Welding Aluminum Theory

and Practice, p. 5.3.

101.

Ibid., p. 10.14.

102.

Daniel Van Lanen, Senior Welder, Argonne National Laboratory, Verbal interview Novem-

ber 4, 1994. Provided most of the specific suggestions and insights regarding welding tech-

niques based on his many years of experience.

103.

Joseph Gagliano, manager of

APS

welding, Argonne Laboratory, private communication, 1994.

104.

ANSI/AWS D1.2, Structural welding code —

aluminum,

American Welding Society, 550 N.W.

LeJeune Rd., Miami, FL 33126 (Tel: 800/334-9353).

105.

ANSI/AWS A5.10-92, Specification for bare aluminum and aluminum alloy welding elec-

trodes and rods, American Welding Society, 550 N.W. LeJeune Rd., Miami, FL 33126 (Tel:

800/334-9353).

106.

Qualification Standard for Structural Welding of Aluminum, Publication 25, the Aluminum

Association, 900-19th St., N.W, Washington, DC 20006 (Tel: 301/645-0756).

107.

MIL-STD-2219, Fusion welding for aerospace applications, U.S. Department of Defense,

Available at no charge through Naval Publications (Tel: 215/697-2179).

108.

Van Lanen.

CHAPTER

4.9

Preparation and Cleaning

of Vacuum Surfaces

Donald M. Mattox

Management

Plus,

Inc.

Surfaces that are in contact with the vacuum environment are called vacuum sur-

faces.

These surfaces may be associated with the vacuum system, fixturing asso-

ciated with a vacuum-related processing or the parts related to vacuum processing.

Thus they may be a metal, ceramic, glass, semiconductor, or polymer. Surfaces

used for vacuum applications should be prepared such that they do not contribute

to contamination in the system

[1-5].

Some surfaces, such as the fixtures, can be

easily or routinely removed for cleaning, while others, such as the vacuum cham-

ber and associated feedthroughs, are nonremovable and must be cleaned in place.

Often the surface areas of the fixtures and parts are greater than that of the vac-

uum system, and these surfaces may determine the pumping performance and the

residual gas and vapor contamination in the vacuum system. This type of contam-

ination can be called "introduced contamination" or "process-related" contami-

nation as converse to "system-related" contamination associated with nonremov-

able surfaces.

Surface preparation includes cleaning and surface modification. Cleaning

means reducing the contamination level on the surface to an acceptable level for

application of the vacuum environment. Surface modification can involve chang-

ing the surface morphology to be more rough or smooth, changing the chemical

composition of the surface, changing the outgassing or outdiffusion properties of

the material, or changing the mechanical properties of the surface.

^^^•^^ All rights of repnwJuclioH in any form reserved.

553

554 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

Surface roughness is important to the vacuum performance of a vacuum mate-

rial.

Surface roughness not only increases the surface area available for adsorp-

tion of contaminants, it also traps small particulates, making them more difficult

to remove. A rough or porous surface also allows capillary condensation of water,

making it more difficult to remove water vapor from the system. During fabrication

of surfaces for use in a vacuum environment, the surface should be made as smooth

as possible. For metals this means light machine cuts, sharp machining tools, cut-

ting lubricants, etc. Fully dense ceramics and glasses can be formed with smooth

surfaces by using fine-grit grinding and polishing materials in the final stages of

forming. After fabrication the surface can be further smoothed by mechanical [6],

chemical, electrochemical [7,8] or chemical/mechanical [9] polishing.

Mechanical polishing can be done by machine or by hand using a wet or dry

polishing procedure. The polishing material may be in the form of a powder, con-

tained in a slurry, loosely held in a polishing cloth, or rigidly held on a solid ma-

terial such as a paper. Table 1 lists a number of common abrasive and polishing

compounds and their size and nomenclature.

The cleaning and treatment of the surfaces should be spelled out in the opera-

tion and maintenance instructions for the system. This is particularly true for

aluminum.

4.9.1

SURFACE MODIFICATION

4.9.1.1 Nonremovable Surfaces

Nonremovable surfaces are vacuum surfaces that, once assembled, cannot be re-

moved or even reached without great difficulty.

STAINLESS STEEL

Stainless steel is the preferred material for nonremovable metal surfaces since it

forms a natural passive oxide and is resistant to corrosion. There are many stain-

less steel alloys, such as

• 304. Common machinable alloy, nonmagnetic—carbides can precipitate in

weld areas, which can result in galvanic corrosion if electrolytes are present.

• 304L. Low carbon — used for better intergranular corrosion resistance than

obtained with 304. Use for fluid lines and gas lines containing moisture.

• 316. For general corrosion resistance—do not mix 304 and 316 when used

in fluid transport because of galvanic corrosion at joints.

4.9.1 Surface Modification

555

Grit/Mesh

Size Emery

Coarse Abrasive Polish

100

120

150

Fine Abrasive Polish

180

220

240

280

320

400

600

800

1000

1200

1500

2000

1/0

2/0

3/0

4/0

Table 1

Average Particle Size

AI2O3,

SiC,

Garnet

1035

930

810

508

430

406

328

266

216

173

142

122

70-86

54-63

29-32

20-23

12.5-17

9-12

5-9

1.5-5

1-1.5

1.0

(microns)

B4C

710

590

500

350

297

250

210

177

149

125

105

42.5

27.7

16.3

8.3

5.5

3.7

2.5

Diamond

1036

712

508

407

267

213

142

122

0.5

0.25

• 316L. Better intergranular corrosion resistance than 316.

• 303. Has a high sulfur content and a higher tendency for porosity—do not

use,

it cannot be welded very well.

• 440. Hardenable but magnetic and more prone to corrosion than the 300

series.

Stainless steel is available as mill plate with several finishes.

• Unpolished

—very dull finish produced by hot-rolling the steel followed

by annealing and descaling. The surface is very rough and porous. Used

where surface finish is not important.

5^6 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

• Unpolished #2D —Dull finish produced by a final cold roll after the hot

rolling but before annealing and descaling. Used for deep drawing where the

surface roughness retains the drawing lubricant.

• Unpolished #2B—Bright finish obtained by a light cold roll after anneal-

ing and descaling. Grain boundary etching due to descaling still present.

General-purpose finish.

• Polished #3—Intermediate polish using 50 or 80 grit abrasive compound.

^max of 140 microinches. Heavy polishing grooves.

• Polished #4—General-purpose surface obtained with 100-150 grit abra-

sives.

Z^niax

microinches. Lighter polishing grooves.

• Buffed #6—Polished with 200 grit abrasive.

• Buffed #7—Polished with 200 grit abrasive with a top dressing using chrome

oxide rouge. R^ of 8-20 microinches.

• Buffed #8—Polished with 320 grit abrasive (or smaller) with an extensive

top dressing using chrome oxide rouge. /?a of ^-14 microinches.* To the

eye,

the surface appears to be free of grinding lines.

Stainless steel plate is often ground to form a flat surface and then plates are

welded to form a chamber. Stainless steel plates may also be shaped by deforma-

tion, which will create a wrinkled surface morphology and may trap lubricants in

the subsurface region. Deformation or machining of stainless steel will work-

harden the surface. This is desirable where the surface must provide a cutting

edge such as when used on CF flanges or for providing a deformation and wear-

resistant surface. Heating to above 450°C will anneal the work-hardened surface,

and the hardness will be lost.

The surface of the steel can be mechanically polished to improve the smooth-

ness.

It can then be chemically polished or electropolished to make it more smooth.

Electropolishing [10] decreases the R^ by about a factor of

and eliminates many

the

microcracks, asperities, and crevices in the polished surface. Typically elec-

tropolishing is done in an electrolyte containing phosphoric acid and the smooth

areas are protected by a thin phosphate layer, causing the peaks to be removed.

This phosphate layer should be removed using an HCl rinse and then the surface

rinsed to an acid-free condition prior

use.

Directed streams of electrolyte ("jets")

can be used to selectively electropolish local areas of a surface [11]. Commercial

suppliers provide electropolishing services to the vacuum industry either at their

plants or on site at the customer's plant.

*The center line average (CLA) or

is the average peak height above the center line of a profile

over the surface. The center line is defined as the line that divides the profile in such a way as the net

deviation is zero. The

tells nothing about maximum peak heights (/?,„ax)' slopes or the lateral di-

mensions of

the

peaks (waviness).

4.9.1 Surface Modification 557

Typical electropolishing solutions for stainless steel are

. Phosphoric acid

Sulfuric acid

Glycerin (USP)

Current density, amps/ft^

5 parts

4 parts

1 part

450

H2S04(1.84s.g.) 1000 ml

H2O 370 ml

Glycerin (USP) 1370 ml

Note: Add acid slowly to water (avoid overheating), then add glycerin. Use car-

bon or lead cathode. Polish at 7.5 volts for about 30 sec.

Stainless steel can also be chemically polished. A chemical polish for stainless

steel is [12]:

• Clean in a hot alkaline solution.

• Rinse.

• Activate in a hot 5% sulfuric acid solution for 5 minutes before polishing.

• Chemically polish at 75 °C in a solution of

Nitric acid 4 parts

Hydrochloric acid

part

Phosphoric acid

part

Acetic acid 5 parts

Electropolishing decreases the actual surface area available for adsorption and

reduces the contamination retention of the surface. The electropolished surface

generally exhibits a lower coefficient of friction than a polished surfaces. The var-

ious surface treatments can alter the outgassing properties of the stainless steel

surface [13-17]. The surface condition can be determined by scanning electron

microscopy (SEM,

1,000-50,000

X) for surface morphology and auger electron

spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS or ESCA) for

558 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

surface composition. The chemical composition of electropolished surfaces can

be specified for critical applications [18,19].

Electropolishing, as well as acid treatments, "charge" the steel surface with hy-

drogen, and for UHV applications the stainless steel should be vacuum-baked at

1000°C for several hours to outgas hydrogen taken up by the surface [13]. The

surface of stainless steel will form a natural passive oxide layer when dried and

exposed to the ambient. The passive layer can be improved by heating in air. How-

ever, control of the temperature and dewpoint is very important. A smooth oxide

film is formed on 316L stainless steel at 450°C and a dewpoint of -100°C but

small nodules and surface coarsening result when the oxidation is done above

550°C in air with this dew point [20]. Type 304 and 316 stainless steels are more

easily passivated than are the 400 series (hardenable) stainless steels [21].

The natural oxide on stainless steel can be removed thus:

• Vapor clean in trichloroethane (5 minutes).

• Rinse in cold water.

• Hot alkaline cleaner 5 minutes.

• Rinse in hot water.

• Potassium permanganate (100 ml DI water -f 50 g NaOH + 5 g KMn04 at

°C)

— soak to condition oxide scale.

• Hydrochloric acid dip to sensitize surface (remove natural oxide passivation).

• Pickling solution (30 vol% HNO3 4- 3 vol % HF) at room temperature for

30 minutes.

• Rinse in hot, deionized water.

LOW-CARBON STEEL

Low-carbon or mild steel (such as 1018 steel) is low cost with respect to stainless

steel and is often used in applications involving large vacuum vessels. Sometimes

the steel is porous, usually due to "stringers" in the metal plate. This problem is

generally masked by painting the exterior of the chamber to reduce the real leak

rate.

Removal of this paint can open up the porosity and increase the leak rate.

Generally, when using such steels, the surface is used in the as-machined condi-

tion. Low-carbon steel can be cleaned by wiping with neutral pH perchloroethane

or trichloroethane, followed by an acetone wipe followed by drying with anhy-

drous methanol. Water-based cleaning should be avoided because it can contrib-

ute to rusting of the steel.

Carbon steel and low alloy steels can be cleaned by pickling in a hydrochloric

acid bath (8-12 wt %) at 40°C for 5-15 min to strip the oxide from the surface

[22].

A simple technique for removing localized iron rust is (1) solvent clean,

(2) soak (or wet) in acetic acid, (3) brush away residue, and (4) repeat as neces-

sary. Low-carbon steel can also be electropolished, but this is not typically done.

4.9.1 Surface Modification 559

ALUMINUM ALLOYS

Aluminum alloys are not normally used for vacuum processing systems because

they are soft and easily corroded, especially by chlorine-containing chemicals.

With proper fabrication, aluminum has proven to be a good high- and ultra-high-

vacuum material [23]. However, mill-rolled aluminum has an outgassing rate

-100 times that of mill-rolled stainless steel [24]. A dense, thin oxide with good

outgassing properties can be formed on aluminum surfaces by (1) machining un-

der a dry, chlorine-free argon/oxygen gas, (2) machining under pure anhydrous

ethanol, or (3) extrusion under a dry, chlorine-free argon/oxygen gas [25-27].

The natural oxide on aluminum can be removed (stripped) before polishing. A

chemical strip for the oxide on aluminum is as follows:

• Soak in solution of

NaOH by weight at 70-75°C.

• Soak in a solution of 1 part concentrated HNO3 to 1 part deionized water

at 20°C, followed by a dip in a solution of

part concentrated HNO3 with

64 g/liter NH4HF2 at 20°C (desmutting procedure—Cu and Si).

• Rinse well.

The aluminum can be chemically polished thus:

• SoakinlO%HCL

• Rinse in deionized water.

• Make a solution of

H3PO4 80%

CH3COOH 15%

HNO3 5%

• Raise to temperature 90 to 110°C.

• Dip for 2-4 min.

Heavily corroded aluminum surfaces can be electrocleaned thus:

• Pickle in 5% NaOH solution at 75°C

• Washin30%HNO3.

• Dip in 12% H2SO4;

• Follow with an anodic electroetch at 90°C in a solution of 100 g H3BO3 and

0.5 g borax in 1 liter deionized water starting at 50 volts and increasing to

600 volts.

560 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

Anodized Aluminum

In special cases where the surface hardness must be increased or chemical corro-

sion resistance is necessary (e.g., plasma etching with chlorine), anodized alu-

minum surfaces can be useful [28,29]. Alloying elements, impurities and heat

treatment can influence the nature and quality of the anodized coating — typi-

cally, the purer the aluminum alloy, the better the anodized layer. To build up a

thick anodized layer on aluminum, it is necessary for the electrolyte to continu-

ously corrode the oxide, giving a porous oxide layer. ASTM Specification B-580-

73 designates seven thicknesses (up to 50 microns) for anodization with letter

designations of A through G. Anodization baths for the various thicknesses are

• Oxalic anodize — very thick films (50 microns)

• Sulfuric acid — thick films (80% aluminum oxide, 18% aluminum sulfate,

water—15% porosity)

• Chromic acid—thin films (1-2 microns)

• Phosphoric acid—very porous films (base for organic coatings)

After formation, the porous aluminum oxide can be "sealed" by hydration, which

swells the amorphous oxide. Sealing is done in hot (95-100°C) deionized water

(sulfuric acid anodize only) or by the use of sodium dichromate solution (improve

corrosion resistance) or with nickel or cobalt acetate solutions. Sealing reduces

the hardness of the anodized film. Steam sealing can be used to avoid the use of

nickel-containing hot water to prevent the possibility of nickel contamination in

semiconductor manufacturing. For vacuum use, the anodized surface should be

vacuum-baked before use. To increase the corrosion protection or lubricity of the

anodized surface, other materials can be deposited in the porous surface. Examples

are the Magnaplate® coating to improve corrosion protection and Tufram® coat-

ing using to improve the frictional properties of anodized aluminum surfaces.

Anodized aluminum does not provide a good surface for sealing with elas-

tomer seals. In anodized systems, the sealing surfaces are often machined to re-

veal the underlying aluminum. These surfaces can be protected from corrosion

with a thin layer of a chemically resistant grease such as Krytox®.

Aluminum can be anodized with

dense oxide (barrier anodization) [30] but this

technique has not been evaluated for vacuum application, because it is rather thin.

COPPER

Copper is often used in vacuum systems as an electrical conductor or as a shear-

sealing material. The polishing of copper often means the removal of the oxide

layer. Copper can be polished (smoothed) thus:

• Inmierse in a solution of

60 ml phosphoric acid (s.g. 1.75)

4.9.1 Surface Modification 561

10 ml nitric acid (s.g. 1.42)

10 ml acetic anhydride and

8 ml water

for 4 minutes at room temperature

The oxide can be removed from copper thus:

• Clean in perchloroethylene.

• Ultrasonic clean in alkaline detergent (pH = 9.7) at 60T for 5 to

10 minutes.

• Rinse.

• Deoxidize in 50 vol % HCl at room temperature for 5 to 10 minutes.

• Rinse.

• Solvent clean.

• Inmierse in solution of

60 ml phosphoric acid (s.g. 1.75)

10 ml nitric acid (s.g. 1.42)

10 ml acetic anhydride

8 ml water

for 4 minutes at room temperature

• Rinse.

CERAMICS AND GLASSES

Ceramics and glasses develop surface microcracks when ground or polished.

These microcracks reduce the strength of the material as well as contribute to sur-

face retention of contamination. Oxide ceramics and glasses can be etched in a

solution of hydrofluoric acid (HF) or ammonium bifluoride, which will mildly

etch the surface and blunt the microcracks.

POLYMERS

The use of polymers in the vacuum system should be minimized. If used, the

polymers should be outgassed by vacuum baking before being installed in the sys-