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

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562 Chapter 4.9: Preparation

and

Cleaning of Vacuum Surfaces

tern. Vacuum baking of polymeric materials should be done at temperatures below

the degradation temperature of the material. The time necessary for outgassing

will vary with the history of the material. In order to determine the necessary time

and temperature for outgassing studies should be made using weight loss as a

function of time and temperature [31,32]. Water vapor and solvents are the most

common vapors outgassed from polymers but in some cases low-molecular-

weight polymers can be evolved. A material can be said to be "outgassed" when it

has less than 1% weight loss after being held at 25 °C above the expected operat-

ing temperature for 24 hours at 5 X 10"^ torr (ASTM E595-90), though some ap-

plications may require better outgassing. Another technique is to monitor the out-

gassing products as a function of time and temperature.

4.9.1.2 Removable Surfaces

Removable surfaces are those that are routinely removed, are being processed, or

that can be removed easily for cleaning such as fixtures, tooling, liners, and

shields. In some cases the removable surface can be discarded, such as when alu-

minum foil is used in the vacuum system. Note that typical aluminum foil is

coated with a vegetable oil and must be cleaned before being used in the vacuum

system.

Common materials for removable surfaces are stainless steel and aluminum

with some use of titanium. Occasionally a polymer may be used for fixturing and

often a polymer is being processed in a vacuum system. If

possible,

the removable

surfaces should be smoothed, as has been discussed. If there has been no contam-

ination of the vacuum surface during processing, the surface that

has

been removed

from the chamber can be cleaned in the manner discussed in Section 4.9.2.3.

When using the vacuum system for film or coating deposition, plasma, or low-

pressure chemical vapor deposition (CVD), the surface will have to be "stripped"

of the deposit buildup.

LINERS AND SHIELDS

The nonremovable surface should be protected from corrosion or abrasion. This

may necessitate the use of liners and shields in the system to protect the surface

from the processing environment or minimize the need for subsequent cleaning.

STRIPPING

Stripping is the term given to the removal of large amounts of materials from

a surface, usually by chemical or mechanical means. The most simple stripping

technique is to apply an adhesive tape and pull the deposit buildup from the sur-

4.9.1 Surface Modification 563

face.

This can be assisted by having a release agent on the surface. Common re-

lease agents are carbon [33] and boron nitride (e.g.. Combat®) applied to the vac-

uum surface in a water slurry. Release agents can also be applied to nonremovable

surfaces and can be applied by glow discharge decomposition of a hydrocarbon

vapor [34]. The oxide on the surface of stainless steel acts as a natural release

agent for films of deposited materials such as copper or gold that are not very

oxygen active.

Deposit buildups can also be removed by abrasion with grit blasting and dry or

wet glass bead blasting [35-37] being common techniques. Dry glass bead blast-

ing is a commonly used cleaning technique, but, as with other grit abrasive tech-

niques, it can leave shards of glass embedded in soft surfaces. The bead blasting

can also flow the surface and trap oil contamination if the surface is not clean be-

fore bead blasting. The amount of grit embedded depends on how long the glass

beads have been used, i.e., how much they have been fractured. Water-soluble

particles can be used for abrasive cleaning and allow easy removal of the water-

soluble embedded particles. For example, 5 micron sodium bicarbonate (baking

soda) particles entrained in a high-velocity water stream can be used for mild

abrasive cleaning. Polymer beads can be used in some cases [38].

Grit blasting uses grit such as fractured cast iron, alumina, silica, plastic, etc. of

varying sizes and shapes accelerated in a gas stream to deform and gouge the

surface [39]. Particles can be entrained in a high-velocity gas stream by using a

siphon system or a pressure system such as used in sand blasting equipment. In

addition to removing gross contamination, grit blasting roughens the surface. The

Society of Automotive Engineers (SAE) has specifications on grit size and type,

e.g., SAE Specification J444 for cast iron grit in the range GIO (2820 microns

maximum size) to G325 (120 microns maximum size), which gives the percent-

age by weight allowed on standard screens. Bombardment of a surface by grit is

like "shot peening" and places the surface in compressive stress, which may pro-

duce unacceptable distortion of thin materials.

In some cases the surfaces of fixtures are deliberately roughened so as to pre-

vent the easy removal of deposit buildup, since flaking of deposited material can

be a source of particulates in the vacuum system. Roughening is typically done

using grit blasting.

Chemical etching can often be used to remove the deposit buildup [40] without

attacking the underlying material. Table 2 lists a number of etchant solutions that

can be used to remove the material indicated. Also listed are some plasmas

that can be used to remove the material indicated. Chemical etching is also used

to remove coatings from coated parts to "rework" the parts.

In extreme cases the surface must be cleaned by aggressive techniques such as

mechanical abrasive cleaning. Wet or dry mechanical abrasion using an abrasive

pad such as Scotchbrite®, or emery paper can be used, although these techniques

abrade the surface and increase the surface area.

564

Chapter

4.9:

Preparation

and

Cleaning

Vacuum Surfaces

Table

Chemical Etchants

for

System/Fixture/Substrate Stripping

Material

Removed

Ti-W

TiN

NiCr

Si02

Zr02

MgF2

SiO

plating

Etchant

H3PO4/HNO3/H2O

NaOH

BCI3 (plasma)

H2O2

KOH/H2O

(plasma)

HCl/glycerine

KMn04/NaOH/H20

HNO3/H2O

HCI/HNO3 (aqua regia)

HCI/H2O

HNO3/H2SO4/H2O

H2O2

HNO3/C2H4O2/C3H6O

HCI/HNO3

NH4OH/H2O2-30%

HF/HNO3

NH4OH/H2O2-30%

HF/HNO3

H2O2

CF4 + O2

(plasma)

HF/HNO3

CF4 + O2

(plasma)

H2O2

H2O2/NH4OH (30%)/H2O

HF/H2O

CF4 + O2

(plasma)

HNO3/HCI/H2O

HF/H2O

CF4

(plasma)

H2SO4/H2O

HF/H2O

HNO3/H2O

HF/H2O

NH4NO3/H2O

HCI/H2O

Ratio

(vol.)

20:2:5

Molar

10-30%

saturated/hot

1:1

5gm:

7.5gm:

30mL

1:1

3:1

1:1

1:1:3

10-30%

1:1:1

3:1

1:1

1:2

1:1

30%

1:1

30%

1:1:1

1:1

1:1:3

1:1

120g/liter

120ml/liter

Useful

These

Surfaces

stainless

steel

(SS),

glass

(G),

ceramic

(C)

SS,G,C

G,C

SS,G,C

SS,G,C,Cu

SS,G,C

G,C

SS,G,C

G,C

SS,G,C,Cu

SS,G,C

SS,G,C,A1

SS,Cu

SS,G,C,Cu

SS,Cu

G,C

SS,Cu

G,C

SS,Cu

steel,

brass,

Cu brass.

alloys

Can

Damage

Ti,Ag

Cu,Fe

Ag,Cu

SS,Cu

Cu,Fe

SS,Cu

Cu,Fe

Cu,

G, C

Cu,

G, C

G,C

Cu,

G, C

G,C

G.C

G,C

4.9.1 Surface Modification 565

SURFACE MECHANICAL PROPERTIES

The mechanical properties of a surface can affect the generation of particulates in

the vacuum system. Friable (easily fractured) surfaces will generate particulates

due to abrasion and wear in the system. System vibration can contribute to this

source of particulates.

Surface wear is a function of the porosity, surface hardness, friction coefficient,

and fracture toughness of the surfaces in contact. To reduce wear, the surfaces

should be fully dense, be as hard as practicable, and have

compressive

stress.

Sur-

faces of metals can be hardened by shot peening, which also introduces a com-

pressive stress, by coating with a hard material, and by chemical conversion such

as nitriding and bonding [41].

Surfaces can be hardened and dispersion strengthened by forming nitride, car-

bide,

or boride dispersed phases in the near-surface region by thermal diffusion of

a reactive species into the surface. Steels that contain aluminum, chromium, mo-

lybdenum, vanadium, or tungsten can be hardened by thermal diffusion of nitro-

gen into the surface. Typically, nitriding is carried out at 500-550°C for 48 hours

in a gaseous atmosphere, giving a "case" depth of several hundred microns. In

carburizing, the carbon content of a low-carbon steel (0.1 to 0.2%) is increased to

0.65 to 0.8% by diffusion from a carbon-containing vapor at about 900°C. Car-

bonitriding can be performed on ferrous materials by diffusing both carbon and

nitrogen into the surface. Nitrogen diffuses faster than the carbon, so a nitrogen-

rich layer is formed below the carbonitrided layer and, if quenched, increases the

fatigue strength of the carbonitrided layer. Hardening by boronizing can be done

on any material having a constituent that forms a stable boride such as Fe2B,

CrB2,

MoB, orNiB2.

Using a plasma for ion bombardment enhances the chemical reactions and

dif-

fusion rates and also allows heating of the substrate and in situ cleaning by sput-

tering. Typically a plasma containing NH3, N2, or N2-H2

(9:1,

i.e., forming gas)

is used along with substrate heating to 500-600°C to nitride steel. The term "ion-

itriding" has been given to the plasma-nitriding process. Bombardment from a ni-

trogen plasma can be used to plasma-nitride a steel surface before the deposition

of a PVD-TiN coating. Plasma carburizing is done in a carbon environment.

Table 3 shows some properties of materials that have been nitrided and borided.

The coefficient of friction can be lowered by coating with a low-friction, low-

vapor-pressure solid material such as molybdenum disulfide [42]. In high-torque

applications, the surfaces can be coated with a low-shear metal, such as gold, sil-

ver, or lead, which will act as a lubricant. It should be noted that graphite is not a

good lubricant in vacuum because of the lack of moisture. Oils will creep away

from the point of contact.

566 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

Table 3

Hardening of Surfaces by Diffusion

Treatment

Carburizing

Nitriding (ion)

Carbonitriding

Bonding

Substrate

Steel: Low C, Med C, C-Mn, Cr-Mo, Ni-Mo,

Ni-Cr-Mo

Steel: Al, Cr, Mo, V or

austinic stainless

Steel: Low C, Med C, Cr, Cr-Mo, Ni-Cr-Mo

Steel: Mo, Cr, Ti, cast Fe; Cobalt-based

alloys; Nickel-based alloys

Microhardness

(kg/mm-)

650-950

900-1300

550-950

1600-2000

Case

Thickness

(microns)

50-3000

25-750

25-500

4.9.1.3 Parts to Be Processed

When a vacuum system is to be used for processing, the parts introduced will

contribute to the gas load and contamination in the system. This contribution

should be consistent from run to run if the processing is to be reproducible. The

gas load and contamination from the processing with a specific number of parts

and associated fixturing should be determined and factored into the initial design

of the vacuum system. The surface of parts to be processed in the vacuum system

can be modified to change their morphology, chemistry, or mechanical properties.

SURFACE MORPHOLOGY

The surface morphology of the parts being introduced into the vacuum system

can be important to the "introduced contamination." Rough surfaces will have a

high surface area, which will affect the removal of water vapor during pumping

and will tend to retain particulates, which may be released in the vacuum system.

SURFACE CHEMISTRY

Outgassing and outdiffusion to the surface from the bulk can be a major concern

to the performance of the vacuum system, particularly if there is heating from the

operation of the system or from vacuum processing. Metals typically outgas hy-

drogen, which is often of no importance to the vacuum application.

Polymers outgas water vapor, solvent vapors, and absorbed gases. These can

often be removed to a tolerable level by vacuum baking before inserting the poly-

mer into the vacuum system. In some cases, the vacuum pumping system will have

to tolerate the gas/vapor load from this source during processing. Polymers can

also diffuse (outdififuse) low-molecular-weight fractions to the surface without

4.9.2 External Cleaning 567

volatilization. These surface contaminants can then affect the processing by vola-

tilizing when heated. In some cases, these contaminants can be sealed in by using

a "basecoat" on the polymer surface.

4.9.2

EXTERNAL CLEANING

Cleaning is the reduction of contamination to a tolerable level for the application

for which the vacuum is needed. External or ex situ cleaning is defined as the use

of cleaning processes that are performed in the ambient environment or are asso-

ciated with a separate piece of cleaning equipment. In situ cleaning processes are

done in the vacuum system used for processing. Generally in situ cleaning is used

to remove recontamination that occurs after ex situ cleaning. However in some

cases,

such as when the system has been contaminated by oil backstreaming,

plasma etching, or plasma-enhanced chemical vapor deposition (PECVD) pro-

cessing, in situ cleaning can be used for cleaning very contaminated vacuum sur-

faces without disassembly.

External cleaning can vary from very benign cleaning, such as wiping with

ethanol, to the very aggressive cleaning where the surface is etched [43]. It is not

feasible nor economical to try to achieve ultra-high-vacuum type cleanliness if

the requirements do not require it. In some cases a vacuum component, such as a

fixture used in vacuum deposition, may be cleaned many times. Over time, this

can result in changes in the surface, such as increasing the surface roughness. Of-

ten, when using pieces, parts, and components purchased from a supplier, the

cleaning process can be simplified by specifying cleaning, handling, and packag-

ing to be provided by the supplier. This may be an increased cost but can reduce

the problems of the system builder. Commercial cleaning and packaging services

for vacuum applications are available.

4.9.2.1 Contaminants and Contamination

A vacuum system is often used for some type of processing such as vacuum heat

treating, freeze drying, or physical vapor deposition (PVD). A contaminant is a

material on the surface or in the gas phase that interferes with processing or re-

sults in an unacceptable product. As a practical matter, a "clean" surface is one

that contains no significant amounts of contamination; thus what constitutes a

clean surface depends on the requirements. For PVD processing, general contam-

ination, such as a surface layer, can cause a low nucleation density of the deposit-

ing film on a surface, causing poor overall adhesion of

film to a surface and pre-

568

Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

Rg.l.

VAPOR

FLUX

PARTICLE OR

INCLUSION

VAPOR FLUX

,/ LARGE

SMALL >^^ |K PINHOLE

PINHOLE

Pinhole formation and pinhole flaking of material deposited on a particle or inclusion on a

surface.

venting good electrical contact in the case of deposited electrical contacts. Local

contamination (film or particle) can result in locally poor adhesion of a film to a

surface giving pinholes in the film. For example, Figure

shows the effect of par-

ticulate contamination on the formation of pinholes in a vacuum-deposited thin

film such as a mirror coating or semiconductor metallization. If the particle flakes

off in the vacuum system ("pinhole flaking"), it produces particulate contamina-

tion in the system. Cleaning should address local surface conditions such as po-

rosity, embedded particles, steps, roughness, etc., that affect film properties, pro-

duce pinholes, and local loss of adhesion. Contaminants in the vacuum system

can be categorized as

• Gaseous contamination (e.g., residual gases)

• Vapor contamination (e.g., water vapor, oil vapors)

• Fluid contamination (e.g., oils on surfaces)

• Solid contamination ( e.g., organic and inorganic particulates)

System-related contamination is contamination whose origin is the vacuum

system. Process-related contamination is contamination associated with the mate-

rial being processed and materials or

fixtures

used in the processing and

"brought

into"

the vacuum system.

4,9.2.2 Cleaning Nonremovable Surfaces

One hopes that nonremovable surfaces will not require drastic cleaning tech-

niques even with repeated use. Particulates can be removed from the vacuum

chamber using a dedicated vacuum cleaner with an efficient filter material such as

HYSURF® (DuPont).

STAINLESS STEEL

Stainless steel vacuum surfaces that have been contaminated by casual contact

with the environment can be cleaned without disturbing the natural oxide by wash-

4.9.2 External Cleaning 569

ing with a neutral detergent solution followed by a thorough rinse in deionized

water and drying using anhydrous ethanol or by wiping with neutral pH per-

chloroethane or trichloroethane, followed by an acetone wipe followed by drying

with anhydrous ethanol or methanol [4].

OTHER MATERIALS

Aluminum [23], copper, glass, and ceramics can be cleaned by using neutral pH

perchloroethane or trichloroethane, followed by acetone followed by a rinse with

anhydrous ethanol. Polymers such as neoprene, Viton®, and Teflon® should be

cleaned with methanol or isopropanol and, if low outgassing is a requirement,

they should be vacuum baked before use. For elastomer sealing, it may be desir-

able to very lightly grease the seal with a low-vapor-pressure lubricant before

assembly.

4.9.2.3 Cleaning of Removable Surfaces

Surfaces should be cleaned as thoroughly as possible external to the vacuum

system. External cleaning includes "gross cleaning" to remove large amounts of

contaminants, often by removing some of the surface material, and "specific

cleaning," which is directed toward removing specific contaminants such as par-

ticulates or hydrocarbons. A clean processing environment and proper handling

and storage after the external cleaning are important to minimizing recontamina-

tion of the cleaned surface before it is placed in the vacuum system.

REMOVAL OF PARTICULATES

The ability to remove particles from a surface depends on the size, shape, and

composition of the particle as well as the surface to which it adheres [44-46]. Re-

moval of particulate contaminants from a surface is best done by mechanical dis-

turbance in a flowing fluid environment [47]. The mechanical disturbance should

be done in a fluid environment containing detergents and wetting agents, and

the fluid should be continually filtered. Dry or wet brushing is often used for pai-

ticle removal. Camel hair and mohair arc used for dry brushing. Polypropylene,

Teflon®, and Nylon® are used for wet brushing. Mechanical scrubbing is often

combined with high-pressure fluid jets (2000-3000 psi) as a cleaning procedure.

Blowoff techniques for particle removal have the advantage that they can be

done after the parts have been placed in fixtures and even in the deposition sys-

tem. The best means of blowoff is to use filtered gas from a liquid nitrogen tank.

The gas is filtered with a 0.2-micron or smaller filter in the nozzle, and the nozzle

should allow ionizing of the gas with a radioactive or electronic source. Ionized

570 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

gas should be used when blowing off insulator/organic surfaces to prevent elec-

trostatic charge buildup on the insulator surface.

Generally fluid sprays are effective for removing large particles but are not ef-

fective on submicron-sized particles [47]. In ultrasonic cleaning (see later sec-

tion),

the

jetting action by the collapsing cavitation bubble acts as a high-pressure

fluid spray that displaces the particle. Ultrasonic jetting is good at removing large

particles but as the particle size decreases to submicron, the cleaning effective-

ness decreases.

Particles can be removed from surfaces by covering the surface with a liquid

polymer, allowing it to solidify, then stripping the polymer from the surface. This

technique is used by the optics industry to remove particles from mirror surfaces

and protect surfaces from abrasion during assembly

[48].

There are many types of

strip coats, each coating leaving different residues after stripping and having

dif-

fering corrosion compatibility with surfaces.

REMOVING CONTAMINANT FILMS

Abrasives

Abrasive powders in a paste or fluid carrier are useful in removing contaminant

films. Precipitated calcium carbonate (CaCO^), CeO [49], and Snow-Floss® (dia-

tomaceous earth with the calcium carbonate removed, leaving a friable silica net-

work) are used to clean glass surfaces with a mild abrasive action.

The use of CO2 "snow" as a mild abrasive is useful for reducing particulates

and films (50-52). Carbon dioxide "snow" can be formed by adiabatic cooling of

compressed CO2 gas. The size of the snow particles depends on the geometry of

the expansion nozzle and the gas flow and can be varied over a wide range. The

C02-snow is soft and has been shown to be a good abrasive cleaner that does not

damage sensitive surfaces such as mirrors and other optics. The C02-snow ex-

hibits some solvating action, which is probably due to a liquid phase between the

impinging particle and the surface. If the snow is made from high-purity CO2, it

will leave no residue on the surface. This technique removes particulates, finger-

prints,

and silicone oil from surfaces and has been found effective as a solvent

cleaner for the removal of hydrocarbons in many cases. It does not remove heav-

ier oils very well. A major processing variable is the purity of the compressed

CO2 gas and a purifier should be included in the cleaning system. Commercial

C02-snow cleaning units are available.

Chemical Etching

Chemical etching can be used to remove surface material along with the contami-

nants.

This is a very useful technique for achieving a "known" surface condition

4.9.2 External Cleaning 571

[53-55].

Chemical etchants can be highly selective in their action. This can result

in preferential etching of grain boundaries or certain crystallographic orienta-

tions.

Etching removes surface layers such as oxides, eUminates or blunts surface

cracks in brittle materials, and removes strongly adherent contaminates. Common

wet chemical etchants for glass include sodium or ammonium bifluoride (e.g.,

100 grams of ammonium bifluoride salt in 800 cc deionized water), trisodium

phosphate (a mild etchant), and hydrofluoric acid (a very strong etchant). These

materials are dangerous and should be handled with proper precautions to prevent

injury. When using etchants for cleaning, care must be taken to prevent selective

removal of surface constituents that are important to further processing [56].

Sometimes chemical etching does not remove some constituents from a surface

and leaves a "smut" that must be removed by further etching. For example, etch-

ing aluminum alloys with sodium hydroxide (NaOH) leaves a copper smut and/or

a silicon smut on the surface. The copper smut can be removed by a nitric acid

(HNO3) etch, and a copper/silicon smut can be removed with a HNO3/HF etch.

Reactive plasma cleaning [57] can be called dry chemical cleaning in that a

volatile reaction product is formed and leaves the surface. Typically a chlorine or

fluorine-containing gas provides the reactive species. This technique is used for in

situ cleaning of some vacuum processing systems such as those used for PECVD

and plasma etching (Section 4.9.4.2).

Solvent Cleaning

Some contaminants can be removed from surfaces by solvents that dissolve (take

into solution) specific contaminants. Polar solvents such as water and water/

alcohol mixtures are used to dissolve ionic materials (salts) that are polar contam-

inates.

Nonpolar solvents such as the chlorinated hydrocarbon solvents, are used

to remove nonpolar contaminates such as grease. Often there is a mixture of sol-

vents to dissolve both polar and nonpolar contaminates. Solvents can vary greatly

as to their ability to dissolve contaminants, and their effectiveness needs to be

determined by determining the "solubility parameter" for specific contaminants.

The solubility parameter is the maximum (saturation) amount of a specific conta-

minant that can be dissolved in a specific amount of the solvent. Many nonchlori-

nated hydrocarbon-based or petroleum-based materials are used as solvents. One

of the most common is acetone (CH3COCH3). Acetone removes heavy greases

quite effectively but tends to leave a residue and it is quite flammable. Acetone

cleaning or "wipedown" should be followed by a methanol rinse or wipedown

to remove the residue. For instance, a solvent wipedown cleaning sequence is

trichloroethylene-acetone-methanol-isopropanol.

Chlorinated hydrocarbon nonpolar solvents such as trichloroethylene are often

preferred to hydrocarbon-based or petroleum-based solvents because of their

lower flanmiability (i.e., higher flashpoint, as determined by ASTM D1310-63).