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

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572 Chapter 4.9: Preparation

and

Cleaning of Vacuum Surfaces

However, because of the toxicity and carcinogenic properties of some of these

materials, they should be used in well-ventilated areas or the vapors should be

contained in a closed recycling system. Volatile organic compounds (VOCs) are

those that have boiling points below 138°C. The discharge of VOCs into the en-

vironment is regulated by local, state, and federal laws. To comply with these reg-

ulations it may be necessary to recycle the material by condensation of the vapors

or to thermally destroy the vapors by burning [58], that is, instead of releasing

them into the atmosphere.

Chlorofluorocarbon (CFC) solvents are more stable and less toxic than the

chlorinated solvents; however, the well-substantiated atmospheric ozone deple-

tion and the controversial increase in the greenhouse effect have caused their use to

be banned. Since January 1, 1996, CFC solvents have not been produced in the

United States. Other possible solvents for removal of greases are N-methyl-2-

pyrrolidone-based solvents [59,60], terpene-based solvents, as well as the Fluoro-

inerts®, which do not contain chlorocarbon bonds. Reports indicate that the ter-

penes may be as effective as the CFCs in many instances, though they have a

greater tendency to leave residues. Terpenes suffer from the fact that they have

low flashpoints (about 120°F) and reduced lower explosive limits (LELs) than

the CFCs. Other approaches use nonlinear alcohols and purely aqueous cleaning

[61].

This area of solvent development is rapidly changing.

Another type of solvents are the supercritical fluids. If a gas such as CO2 is

compressed to its "critical pressure" (CO2 = 1077 psi), it liquefies to become

a "critical fluid." If it is also heated above its "critical temperature" (CO2 =

31.1°C), it becomes a supercritical fluid (SCF), as shown in Figure 2. Critical

fluids and supercritical fluids are good solvents for many medium-molecular-

weight, nonpolar or slightly polar organics. The more dense they are, the better

their solvency power.

Solvents can be densified most easily when they are in the supercritical state.

Carbon dioxide has been shown to have a Hildebrand solubility parameter that

can vary from 0 in the gas to 10 under high-pressure supercritical conditions [62].

Values of 6 to 8 are typical, which is about the same as hexane and carbon tetra-

chloride. Supercritical CO2 fluid (SCF-CO2, critical point 31°C, 74 bar pressure)

has the advantage that it is stable, has low toxicity, minimal cost, and is a solvent

for many organic materials and has shown promise as a solvent cleaning tech-

nique [63]. Table 4 shows typical operating parameters for SCF cleaning.

Saponifiers

Alkaline cleaners (generally silicate- and phosphate-based) are saponifiers that

convert organic fats to water-soluble soaps. Alkaline cleaners have a pH of about

11 and are generally used hot

[64].

After using alkaline cleaners, the surface should

receive an acid dip prior to the water rinse since alkali salts adhere strongly to sur-

4.9.2 External Cleaning

573

Rg.2.

10,000

8.000

6.000

4.000

2.000

1,000

600

400

^ 200

f 100

^ 60

3 40

8 20

^ 10

6.0

4.0

2.0

1.0

0.6

0.4

0.2

0.1

Solid Region

/^ Liquid Region

V -<— Triple Point

1 Supercritical

1 Fluid Region

Critical Point

Tc =

88 '^F

Pc= 1077 psig

Vapor Region

(Superheated)

-120 -80

-40

40 80

160 200

Temperature (°F)

PHASE DIAGRAM FOR PURE CO2

Phase diagram for pure CO2.

faces.

Clean oxide surfaces strongly adsorb hydrocarbons, and detergents or sol-

vents normally do not completely remove the hydrocarbons; alkaline or oxidative

cleaners must be used to remove the remaining hydrocarbons. Strong alkaline

Table 4

Operating Conditions for CO2-SCF Cleaning

Parameter

Range

Pressure

Temperature

SC CO

density

SC CO2 flow rate

Cleaning time

1450-4350 psi

100-185°F

30-50 Ib/ft^

2-lllb/hr

0.5-3 hours

574 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

cleaners can etch aluminum and oxide surfaces, particularly glasses, so solution

strength (pH), temperature, and exposure times should be carefully controlled.

Detergent Cleaning

Detergent (soap) cleaning is a comparatively mild cleaning technique

[65].

In de-

tergent cleaning, the detergent surrounds contaminants, taking them into suspen-

sion without actually dissolving the material. This action is assisted by wetting

agents and surfactants, which loosen the contaminants from the surface. Liquid

dishwasher soap is an excellent detergent for many applications such as cleaning

polymer surfaces. A major problem with soaps is that metal ions, such as the cal-

cium and magnesium, which are found in hard water, make the soaps insoluble,

thus leaving a residue. Deionized (DI) water should always be used for residue-

free detergent cleaning. Many detergents contain phosphates, which can be envi-

ronmentally harmful and subject to pollution regulations.

Surface Tension

The ability to wet a surface depends on the relative surface tensions of the surface

and the fluid. The lower the surface tension, the better the fluid can wet a surface

and penetrate into small spaces. The surface tension of clean oxide surfaces is

close to 1000 dyne/cm. The surface tension of water can be lowered by the addi-

tion of chemicals. Table 5 shows the surface tension of some water solutions:

Table 5

Surface Tension of Fluids

Material

Pure H2O

/z-propanol

H2O + 30 vol% «-propanol

Ethyl alcohol

H2O

-H

50 vol% ethanol

1000 g H2O -f 34 g NH4OH

1000gH2O+ 17.7 gHCl

1000 g H2O + 14 g NaOH

1000gH2O + 6gNaCl

at 18°C

at50°C •

at 100"C

25X

at 18°C

at 30°C

at 18X

at 20°C

at 18°C

20X

Surface Tension (in air)

mJ/m^ (dyne/cm)

= 73.05

= 67.91

= 58.9

= 23.32

= 26.9

= 21.5

= 27.5

= 57.05

= 65.75

= 101.05

= 82.55

4.9.2 External Cleaning 575

Surfactants

Surfactants are the generic name for surface-active agents that reduce the inter-

facial energy of materials in contact. Surfactants used with water have both

hydrophobic ("water hating") and hydrophilic ("water loving") groups. They dis-

solve in water by virtue of their hydrophilic groups and lower the surface energy

of water to about 30 mJ/m^. The surfactant collects at the interface between im-

miscible substances, such as oil and water, and lowers the interfacial energy. Sur-

factants should best be used in deionized water.

pH Adjusters and Chelating Agents

In solutions, pH adjusters are used to aid in the cleaning action. Generally it is

found that basic solutions clean better than acidic solutions if chemical etching is

not involved. The pH of the cleaning solution is often adjusted to basic using am-

monia or anMnonium hydroxide. Chelating agents keep in solution the normally

insoluble phosphates that are formed in hard-water detergent cleaning. Glass-

cleaning solutions often use chelating agents such as ethylene diamine tetraacetic

acid (EDTA) and citric acid with salts containing hydroxy

and amine substitutes.

Reactive Cleaning

Reactive cleaning uses liquids, gases, vapors, or plasmas to react with a contami-

nant to form a volatile or soluble reaction product.

Fluids

Reactive cleaning liquids are often oxidizing solutions. Many acid-based systems

can be used as oxidants. One system commonly used in the semiconductor indus-

try is the "piranha solution." The piranha solution is hot (>50°C) concentrated

sulfuric acid plus ammonium persulfate [66]. The addition of the solid anrnio-

nium persulfate to the hot sulfuric acid produces peroxydisulfuric acid, which re-

acts with water to form H2SO5 (Carols acid) which further decomposes to form

free atomic oxygen. The anmionium persulfate should be added just prior to the

immersion of the part into the solution. The effectiveness of this oxidation tech-

nique can be shown by first placing a piece of paper in the hot sulfuric acid where

it is carbonized, then adding the ammonium persulfate and watching the carbon

disappear. This treatment is sometimes followed by a brief dip in a 10:1 solution of

water and HF or inmiersion for 20 minutes in a hot solution of hydrogen peroxide

and ammonium hydroxide in the ratio H2O : H2O2 (30%) : NH4OH (29%) at

80°C.

Another similar oxidizing solution uses stabilized sulfuric acid and hydro-

gen peroxide.

^f> Chapter 4.9: Preparation and Cleaning of

Vacuum

Surfaces

Hydrogen peroxide (H2O2) is a good oxidizing solution for cleaning glass. Of-

ten boiling 30% unstabilized H2O2 is used. The most common hydrogen peroxide

has been stabilized, which reduces the release of free oxygen. Unstabilized H2O2

must be stored in a refrigerator to slow decomposition. Hydrogen peroxide is

sometimes used with ammonium hydroxide, to increase the complexing of sur-

face contaminants, and is used at a ratio of

8 (30% H2O2): 1 (NH4OH): 1 (H2O)

However, the decomposition rate of the H2O2 is greatly increased by combina-

tion with ammonium hydroxide.

Oxidative cleaning can be performed using chlorine-containing chemicals. For

example, a water slurry of sodium dichloroisocyanurate (i.e., swinmiing pool chlo-

rine),

which has 63% available chlorine, can be used to scrub an oxide surface to

remove hydrocarbon contamination. This combines mechanical scrubbing with

oxidation and improves the cleaning action.

Gases

Reactive gas cleaning uses a reactive gas at high temperature to form a volatile

material. For example, air firing of an oxide surface oxidizes all the hydrocarbons,

and they are volatilized. High-temperature air fire is an excellent way to clean sur-

faces that are not degraded by high temperature [67,68]. For example, alumina

can be cleaned of hydrocarbons by heating to 1000°C in air. Some care must be

taken in furnace firing, because particulate generation from the furnace liner can

be a source of undesirable particulates and sodium from the insulating material

may be an undesirable contaminant for semiconductor device fabrication.

Self-

cleaning kitchen ovens clean by oxidation at about 425°C.

The oxidation by ozone (O3) created by ultraviolet radiation (UV) at atmo-

spheric pressure and low temperature is useful for removing hydrocarbons. The

ultraviolet radiation also causes bond scission of the hydrocarbon contaminants,

which aids in cleaning. The UV/O3 cleaning has greatly simplified the produc-

tion, storage, and maintenance of hydrocarbon-free surfaces [69,70].

The UV is produced by a mercury vapor lamp in a quartz envelope so that both

the 1849 A and the 2537 A radiation is transmitted. The short-wavelength radia-

tion generates ozone and causes bond scission in the hydrocarbon contaminants.

The mercury lamps can be custom made to a variety of shapes for specific appli-

cations. Ozone adsorbs UV so the surfaces should be as close as possible to the

UV source. UV radiation intensity should be maintained to about 1-10 milli-

watts/cm^ at the surface. In the UV/O3 chamber the air may be stagnant or flow-

ing. If flowing air is used, the air should be filtered. In a correctly operating sys-

tem, ozone can be detected by smell when the chamber is opened. The smell is

similar to that of the air after a lightning storm and indicates that the ozone con-

4.9.2 External Cleaning 577

centration is less than 10 parts per million by volume (ppmbv). Higher concen-

trations of ozone deaden the olfactory nerves and are harmful.

Typical exposure times for UV/O3 cleaning are from a few minutes to remove

a few monolayers of hydrocarbon contamination to hours or days or weeks for

storage of cleaned surfaces. The UV/O3 cleaning technique has the advantage

that it can be used as a dry in-line cleaning technique [71]. The UV/O3 cleaning

technique is also useful for cleaning holes (vias) in surfaces [72].

Plasmas

Plasma cleaning can be done in a plasma system separate from the deposition sys-

tem. Reactive plasma cleaning uses a reactive species in the plasma to react with

the contaminants to form a volatile species, which leave the surface at much

lower temperatures than those necessary for reactive gas cleaning. The additional

requirement on reactive plasma cleaning is that it does not leave a residue. Oxy-

gen (from pure ["medical"] air) and hydrogen (pure or as "forming gas") [73] are

the most common materials used for plasma-cleaning vacuum surfaces both as an

external cleaning process and as an in situ cleaning technique (Section 4.9.4.2)

[74].

Oxygen (or air) plasmas are very effective in removing hydrocarbons and ab-

sorbed water vapor from surfaces. The reaction of the oxygen with carbon on the

surface can be monitored using a mass spectrometer to monitor the CO and CO2

that is produced [75]. Hydrogen plasmas can be used to remove hydrocarbon con-

tamination when oxygen plasmas are unacceptable. This technique has been used

to clean vacuum surfaces (stainless steel) in nuclear fusion reactors [76]. Hydro-

gen plasmas have been used to clean metals [77] and semiconductor materials [78].

Figure 3 shows a typical plasma cleaner where the plasma is generated by an

RF discharge and the surfaces to be cleaned are in a "remote" or "afterglow" lo-

cation and not in the plasma generation region. Figure 4 shows the processes that

occur on a surface exposed to a plasma. The surface attains a potential (sheath po-

tential) that is negative with respect to the plasma, and ions are accelerated from

the plasma to the surface. For the case of a "cold plasma," which has low energy

particles, this sheath potential will only be a few volts. When the plasma particles

are more energetic or the electrons are accelerated to the surface, the sheath po-

tential can be tens of

volts.

In addition to being bombarded by ions, the surface in

contact with the plasma will be bombarded by "activated species," excited spe-

cies,

and other high-energy species. Ions and excited species release their ener-

gies of ionization or excitation when they impinge on the surface. For example,

when a singly charged argon ion impinges on a surface, it will give up the kinetic

energy it attained by acceleration through the sheath potential and the ionization

energy, which is 15.7 eV. For a high ion flux, this release of heat can result in an

appreciable temperature rise at the surface.

578

Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

Fig.

LOW PRESSURE CHAMBER

HIGH VOLTAGE

rf ELECTRODES

^^j

^^ ^^ P^^

C PLASMA GENERATION REGION 1

^-^^^^^^

SUBSTRATES

PLASMA CLEANER

Plasma cleaner using parallel plate electrodes. The surfaces to be cleaned are placed in the

"afterglow region" outside of the plasma generation region.

RINSING

After any wet cleaning process, the surface should be thoroughly rinsed in an

ultra-pure liquid, usually water, before being allowed to dry. This avoids leaving

residues on the surface. The most common rinsing technique is to use successive

rinses (cascading rinsing) in ultrapure water until the rinse water retains a high re-

sistivity (> 12 megohm/cm). This is called "rinse to resistivity." After rinsing, the

surface should be dried as quickly as possible, because the residual water film on

the surface will cause particles to stick to the surface and, on drying, the particles

will adhere very tenaciously.

Ultra-pure or deionized (DI) water is used for rinsing since it is easily prepared

in large quantities. Water purity is measured using a conductivity cell that mea-

sures the ionic concentration in the water. Spontaneous dissociation of the water

molecule limits the resistance of water to 18.2 megohms between electrodes

spaced 1 centimeter apart (resistivity of 18.2 megohm/cm). Conductivity mea-

surements do not measure the organic or biological contamination and some type

4.9.2 External Cleaning

579

Fig.

PLASMA

(+)

SURFACE

(-]

^ "^

PARTICLES

IONS (+)

ELECTRONS

(-)

NEUTRALS

Atoms/Molecules

Excited Species

Metastable Species

Radicals

PHOTONS

Optical

Soft X-ray

lt«^^^

r-^ RECOMBINATION

Jon

* Electron

Q ADSORBED SPECIES

«9\s Cb REACTED SPECIES

O ABSORBED SPECIES

PLASMA SHEATH

SHEATH POTENTIAL

PLASMA — SURFACE INTERACTION

(LOW ENERGY IONS)

Plasma-surface interactions.

of residue analysis must be used to measure these impurities. There are a number

of techniques for determining the nature of the contaminants in water [79]. A

simple technique is to let some of the water evaporate on a clean glass surface and

then look for a "haze" or for loss of contact angle (discussed later).

Typical semiconductor specifications for ultra-pure water for end use are

• Resistivity—18 megohm/cm continuous at 25°C

• Particle count—less than 500 particles (0.5 microns or larger) per liter

• Bacteria count—less than one colony (cultured) per cc

• Organics—less than one part per million

580 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

• Total electrolytes—less than 5 parts per billion NaCl equivalent

• Quantity requirements

• Peak-level usage

High volumes of ultrapure water are made by

• Pretreatment—pH adjustment, coagulation, filtration

• Reverse osmosis — semipermeable membrane (pore size of

10 ~-^

10 ~^

microns), which rejects salts, dissolved solids (90-98%) and organics (99%),

and requires 400 to 600 psi feedwater

• Degasification—removes dissolved CO2

• Ion exchange (anion and cation)—ion exchange resins remove ions by ex-

changing

H "^

for cations and OH

for anions

• Absorption materials (activated carbon)—remove organics

• Filtration—removes particulates and biological matter (0.2 microns filter

pore size, for bacterial, 1.0 microns pore size for prefilter)

• Ultraviolet radiation or ozone bubbling—kills bacteria on filters

• Endpoint filtration—0.2 micron filter pore size

Smaller amounts of ultra-pure water can be prepared by the same process steps

beginning with the ion exchange process. Triply distilled water can also be used

but it is relatively expensive. Note that in the production of "soft water" anions

and cations are exchanged for Na"^ and Cl~, and the water will leave a residue.

APPLICATION OF FLUIDS

Fluids are often used in cleaning processes. Fluid baths should be continuously

filtered and monitored so as to replace or replenish the active ingredients as they

are used or become contaminated. In cases of contaminants that are less dense

than the fluid, the surface of the solution can be "skimmed" as contaminants rise

to the surface, or the container can be of an "overflow" type where the fluid flows

over the lip of the container. There are a number of ways to apply the fluids to the

surface to be cleaned.

Soaking

Soaking involves immersing the part in a solution for an extended time, and there-

fore soaking has not been a desirable technique for production. This may change

in the future when less aggressive cleaning chemicals must be used because of en-

vironmental concerns. Immersion of a surface in a stagnant solution is generally

a poor technique, because the contaminants that are taken into solution are con-

centrated near the surface and must diffuse away.

4.9.2 External Cleaning 581

Mechanical Disturbance

Mechanical disturbance uses agitation, wiping, brushing, or scrubbing in a fluid

environment to break up the stagnant fluid layer near the surface, loosen particles,

and aid in carrying contamination away from the surface. Care must be taken to

ensure that any material that is used in a fluid does not produce particulates and is

compatible with the fluid and surfaces it contacts. When using any mechanical

rubbing, care should be taken to prevent contamination by abrasive transfer from

the rubbing media—gentle pressure should be used. There are a variety of brush

materials used in fluids including polypropylene. Teflon®, and Nylon®. If wiping

or scrubbing with a cloth is used, care should be taken that the cloth is lint-free

and desized by multiple washing before use. Special particulate-free sponge, and

cloth materials are available for wiping. In semiconductor technology, mechani-

cal scrubbing combined with high-pressure fluid jets (2000-3000 psi) and spin-

ning are standard cleaning procedures.

Spraying

Liquid sprays can be used to remove contaminants from surfaces. Spray pressures

can be low, at less than a hundred psi, or high, at several thousand psi. Spraying

parameters include the type of fluid, pressure, angle of incidence, and volume of

fluid. Sprays should be directed at an oblique angle to the surface. Spray systems

often use copious amounts of material so the fluid should be recycled. The spray-

ing fluid can also contain suspended particles that aid in cleaning by adding a

mild abrasion to the cleaning. For example, CeO polishing compound is often

added to fluids in cleaning glass for mirrors [49].

The fluid should be monitored by residue analysis, and when it is contaminated

above a given level it should be replaced. With increasing concern about solvent

vapors, many of the newer solvent spray systems are self-contained with con-

densers to trap the solvent vapors as used with vapor cleaning. Some systems al-

low the purification of the solvents by distillation. It should be noted that spraying

can induce resonant vibrations that can cause component failure or deterioration.

Vapor Cleaning

In vapor cleaning, a cold surface is held in a hot vapor of the solvent. The solvent

condenses on the surface, dissolves the contaminants and flows off into a hot

"sump"

[80,81]. When the part reaches the temperature of the vapor, condensa-

tion ceases and the cleaning action stops. The vaporization of the solvent in the

sump provides continuous distillation of the cleaning fluid. Parts should never

be immersed in the sump fluid. Fluid in the sump should be changed when it