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

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592 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

4.9.4.1 Origin of In-Chamber Contamination

SYSTEM-RELATED CONTAMINATION

System-related contamination is contamination whose origin is the vacuum sys-

tem

itself.

System-related contamination can originate from

• Residual atmospheric gases and vapors, such as O2, N2, water vapor

• Desorption of vapors from vacuum surfaces, such as water vapor, vapors from

previous processing

• Backstreaming from pumping system, such as pump oils, lubricating oils

• Wall creep from the pumping system

• Outgassing of gases and vapors from construction materials, such as water

vapor from organics, hydrogen from metals

• Vaporization of constmction materials or solid contaminants

• Leakage of gases and vapors from real leaks

• Leakage of gases and vapors from virtual leaks

• Permeation of gases and vapors through construction materials such as rub-

ber 0-rings

• Particles generated by wear between moving surfaces

• Particles from previous processing

Many of these sources have been addressed in other sections of this book.

Minimization of system-related contamination can also be attained by having

the vacuum surfaces heated when the system is open to the ambient in order to

minimize water vapor adsorption. This can be done by having heater coils, or

flood panels on the exterior surfaces of the vacuum system and flowing hot water

through them when the system is open. Operational procedures such as having the

system opened to the ambient for as short a period of time as possible and venting

using dry gas also minimize contamination in the system. In load-lock system de-

sign, the processing chamber is kept under vacuum and materials are introduced

through a separate chamber where they can be preprocessed to minimize contam-

inants brought into the chamber. This procedure aids in reducing contamination

and in allowing a process to be reproducible.

Oil-sealed and oil-lubricated mechanical pumps are commonly used to reduce

the gas pressure in a vacuum chamber to the range of 100 mtorr. An important

factor in using these pumps is to minimize the "backstreaming" and "wall creep"

of the mechanical pump oils into the vacuum chamber and high-vacuum pump. If

oil gets into the processing chamber, it can contaminate the surfaces before pro-

cessing begins or be decomposed in a plasma to deposits contaminants such as

carbon. If the oil gets into a cryopump, it will fill the pores of the adsorbing me-

dia and decrease the pumping speed and capacity. If the mechanical pump oil

4.9.4 //? 5/ti/Cleaning 593

gets into an oil diffusion pump, the high-vapor-pressure mechanical pump oil will

quickly make its way into the deposition chamber.

To minimize backstreaming and wall creep, keep the manifold to the mechani-

cal pump in the turbulent (viscous) gas flow pressure regime as much as possible.

If at any time the flow should fall below this value, oil will make its way up the

pumping manifold, depositing on the walls and valves, and if the valves are open,

it will enter the vacuum chamber and high-vacuum pump. There is viscous flow

in a pipe of diameter D, when P (torr) X D (inches) > 0.18. For example, for a

2-inch diameter pipe, the gas flow is viscous above 90 mtorr pressure. The "cross-

over pressure" from roughing to high-vacuum pumping should be close to this

pressure to minimize backstreaming through the roughing line.

Another source of backstreaming is when there is a power failure and the me-

chanical pump stops. The oil seal in the mechanical pump is not effective in hold-

ing a large pressure differential and air will "suck" back through the pump, carry-

ing oil with it into the pumping manifold. This backstreaming can be eliminated

by having a ballast orifice or ballast valve on the roughing pump manifold to give

a continuous gas flow through the mechanical pump even when the roughing and

foreline valves are closed so as to keep the manifold pressure in the viscous flow

range. In the event of a power failure, this leak brings the pumping manifold up

to ambient pressure, thereby preventing air (and oil) from being sucked back

through the mechanical pump. This permanent leak in the roughing manifold

adds a pumping load to the mechanical pump, which must be allowed for in the

system design. If such a permanent leak is not used, a normally open (NO) (when

power is off) "leak valve" or "ballast valve," which opens when there is a power

failure, can be used in the manifold between the mechanical pump and the rough-

ing valve. However, this arrangement lets the pumping manifold have a low pres-

sure during normal operation, which allows oil to migrate through the pumping

manifold.

The roughing, backing, and high-vacuum valves should be pneumatic or sole-

noid-operated normally closed (NC) (when power is off), which will close on

power failure and not reopen until the proper signal is sent from the microproces-

sor. The roughing valve and backing valve are activated from a preset vacuum

signal to prevent lowering the manifold pressure below the viscous flow range. It

is also advisable to have the microprocessor programmed so that the roughing

valve will not open if the pumping manifold is at a much higher pressure than

the high-vacuum side of the

valve.

For

example,

if there is

short power

outage,

the

manifold will be brought to ambient pressure through the permanent leak or the

actuated leak valve, but the diffusion pump and/or the vacuum chamber can still

be under a good vacuum. If power returns and the roughing valve or backing

valve opens, then the gas flow will be reversed and gas will flow from the me-

chanical pump manifold toward the high-vacuum pump or chamber.

594

Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

Rg.6.

MECHANICAL

PUMP MANIFOLD

VACUUM GAUGE

EXHAUST

< ^^

OIL-SEALED

MECHANICAL

PUMP

GAUGE INPUTS

f t t

MICROPROCESSOR

CONTROLLER

. CHAMBER

HIGH/LOW

VACUUM

GAUGE

HIGH VACUUM VALVE

COLD TRAP

TEMPERATURE GAUGE

LN,

COLD TRAP

- DIFFUSION PUMP

-VOLTAGE GAUGE/SWITCH

I i I I

OUTPUTS TO VALVES

FAIL-SAFE DESIGN FOR DIFFUSION PUMPED SYSTEM ,^,.^,,

Vacuum manifolding for a fail-safe design of a diffusion-pumpepd system in case of a power

outage.

Figure 6 shows ways that the vacuum manifolding can be designed to minimize

oil contamination from the mechanical pumping system in a diffusion-pumped

system. In the diffusion-pumped system, the diffusion pump can be interlocked so

as to not heat up until the liquid nitrogen (LN2) cold trap has been cooled. Also

shown in the figure is a high-vacuum gauge between the high-vacuum pump and

the high-vacuum valve. This gauge allows monitoring the status of the pumping

system in a "blanked off" mode. A major change in the pump performance in the

blanked-off mode indicates a problem in the pumping system, such as oil con-

tamination of a cryopump, a low oil level in the oil-sealed mechanical pump, a

low oil level in the diffusion pump, an incorrect oil sump temperature in the

dif-

fusion pump, and so on.

Oil contamination from the pumping system can be completely eliminated by

using "dry pumps" that contain no oil. Such a system is a turbomolecular pump

with a molecular drag stage backed by a diaphragm pump. This type of system is

used in the load locks in semiconductor manufacturing.

If a vacuum system has been contaminated with oil, it is is often possible to

4.9.4 In

Situ

Cleaning 595

clean the system with an oxygen plasma. The oxygen plasma will volatilize hy-

drocarbon oils and will oxidize silicone oils to silica, which will then present a

particulate cleaning problem.

When a volume of gas is expanded rapidly, the gas cools. If the gas has a high

relative humidity, the gas can be cooled past its dewpoint and become supersatu-

rated. The supersaturated water vapor will condense on particles and ions in the

gas to form water droplets. This droplet formation is similar to the formation of

contrails behind aircraft and has been used to detect ionization tracks in cloud

track chambers. The droplets can coalesce to form drops that "rain" down on sur-

faces in a vacuum chamber [104,105]. When the drops evaporate, they leave a

residue that can affect film adhesion and cause pinholes in the deposited film.

Laser particle counters are available for detecting particles in the vacuum sys-

tem during pumpdown. To prevent this type of particle formation mechanism, the

vacuum system should be filled with low-humidity air, flushed with dry air before

pumping, or evacuated slowly.

In processes where the parts have appreciable outgassing of vapors such as wa-

ter vapor, solvents, or low-molecular-weight organics, it may be desirable to have

large-area cold-panels in the processing chamber. These traps should be cooled to

the appropriate temperature to reduce the vapor pressure of the contaminant to

the desired level. For example, to trap water vapor, the surfaces should be cooled

—

150°C.

These panels can be "defrosted" or removed periodically in order to

minimize contaminant carryover from previous processing.

Structures, such as fixtures, that are put into and taken out of the deposition

chamber often represent the largest surface areas in the vacuum system. In many

applications, particularly in sputter deposition and in transfer chambers, the fix-

ture may "crowd" the volume to such an extent that it interferes with the pumping

action, particularly of

vapors.

Often this problem can be overcome by using cold

panels in the chamber.

Fixtures and tooling often have moving surfaces in contact as the fixtures are

moved to randomize positions or move from one point to another. This moving

contact can generate particulates in the system. Wear can be minimized by using

smooth, hard surfaces in contact and vacuum-compatible lubricants such as liquid

lubricants, solid film lubricants (e.g., sputter-deposited or burnished M0S2) [42]

or low-shear metals (e.g., Pb, Sn, or Ag).

In PVD processing, buildup of excess film material on the vacuum surfaces

can cause an increased surface area that exacerbates the water vapor adsorption/

desorption problem. This film buildup also can generate particulate contamina-

tion in the system by flaking from the surface. This problem is compounded when

depositing highly stressed films. The system should be designed so that flakes do

not fall on part surfaces before deposition and fall to areas where they can easily

be removed. This means that systems subject to such problems should not be base-

pumped, because particulates will fall into the pumping system.

596 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

Turbulence has been shown to "stir up" particles in the vacuum system and

"soft pump" and "soft vent" valves have been developed to limit the flow of high-

velocity gas into the processing chamber. Rapid venting has been shown to be a

greater problem than rapid roughing. If particulates are a problem, the system

should be "vacuumed" with a vacuum cleaner on a regular schedule.

PROCESS-RELATED CONTAMINATION

Process-related contamination such as gases, vapors, and particulates in the vac-

uum processing chamber can originate from

• Particulates brought in with the parts, fixture, tooling, gases, or deposition

sources

• Particulates generated during processing (e.g., from sputtering targets, spits

from evaporation sources, wear particles from moving fixturing)

• Surface films "brought in" on surfaces

• Desorption, outgassing, and vaporization from introduced fixtures, tooling,

parts,

deposition source materials, etc.

Some types of vacuum processing produce very fine particles. Sputter deposi-

tion can produce "ultra-fine particles" [106] and plasma etching [107] can pro-

duce copious amounts of particulate matter in the system. These particles should

be swept away through the pumping system.

Contamination from desorption should be minimized by cleaning and out-

gassing of the materials before insertion into the vacuum system as described in

Section

4.9.1.1.

Where there is still appreciable contamination brought into the

vacuum system, such as high outgassing materials, the system and processing

procedures must be designed to accommodate the contamination.

4.9.4.2 Conditioning of Vacuum Surfaces

One type of

situ cleaning is sometimes termed "conditioning" and is used to re-

move the recontamination on surfaces that has occurred after ex situ cleaning or

after the system has been opened to the ambient. These contaminant species are

predominantly water vapor and hydrocarbon vapors. Vacuum system surface con-

ditioning can consist of one or a combination of procedures as follows:

DESORPTION

Desorption is the nonchemical removal of a species from the surface. In thermal

desorption, the process is due to heating. When a system is evacuated, the water

4.9.4

Situ Cleaning

597

Rg.7.

20 Torr

(Saturation)

Dry Surfaces

4 6 8 10

PUMPING TIME (hours)

WATER VAPOR PUMPING CURVES

Starting with "wet" surfaces and "dry" surfaces

Removal of water vapor from a vacuum system that initially had wet surfaces and from one

that had dry surfaces.

vapor begins to desorb from the vacuum surfaces. The rate of desorption depends

on the surface area, surface condition, temperature, and amount of water on the

surfaces. Water vapor is the most common vapor contamination in a vacuum sys-

tem. Figure 7 shows the removal of water from a vacuum system that initially had

a wet surface and one that initially had a dry surface. Note that it takes a very long

time to remove most of the water vapor from a system, particularly if it starts in a

wet condition.

One technique for increasing the thermal desorption rate is to flush the system

with a hot, dry gas

[108].

Another is to heat the system ("bakeout"), preferably to

>400°C [109-111]. Bakeout for ultra-high-vacuum applications can take days

or weeks. When using the bakeout procedure, there should be no "cold spots" to

condense volatilized contaminants.

An effective way to desorb species from a surface is to use inert gas "ion scrub-

bing," which uses low-energy ions accelerated across the plasma-wall sheath to

enhance desorption as shown in Figure 4. The plasma is established by having a

598 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

high-voltage negative electrode ("glow bar") in the vacuum system and establish-

ing a gas pressure of 10-100 mtorr of inert or reactive gas. A DC gas discharge is

established, and all surfaces in contact with the plasma attain a negative potential

with respect to the plasma across the "wall sheath." Bombardment by ions accel-

erated across the wall sheath enhances desorption of adsorbed contaminants from

the wall. To obtain the most uniform bombardment, the glow bar should extend

throughout the vacuum vessel. Because the gas throughput of the system is low

during many types of plasma cleaning, often a conditioning cycle of "pump-

plasma clean-pump" is used to flush desorbed contamination from the system.

Desorption can be stimulated by the adsorption of short-wavelength radiation.

This process is called photodesorption and is stimulated by the short-wavelength

radiation from

quartz-ultraviolet lamp [ 112,113] or plasma in the vacuum system.

REACTION AND VOLATILIZATION

Ion scrubbing with a reactive gas such as oxygen or hydrogen results in oxidation

or hydrogenation of contaminants [76,114,115]. If the reaction products are vola-

tile,

they are pumped away. The cleaning can be monitored by residual gas analy-

sis such as CO and CO2 in an oxygen plasma and CxHy in a hydrogen plasma. If

the reaction products are nonvolatile, they will be left as a residue on the surface.

Caution: A vacuum system using a compression-type pump with hydrocarbon

oil should not be used to pump pure oxygen, because an explosion can occur—

instead, use air

(21%

O2) as the plasma gas.

The UV/O3 cleaning process (Section 4.9.2.3) can be used for in situ cleaning.

This procedure requires a partial pressure of oxygen and a UV source in the vac-

uum chamber.

Instead of oxygen or hydrogen plasmas, more chemically reactive materials

such as chlorine and fluorine can be used for reactive plasma cleaning [57,107].

The chlorine or fluorine reacts with the contaminants or surface material to form

a volatile fluoride or chloride compound. This process is similar to plasma etch-

ing and usually generates a lot of particulate matter in the system. For example,

chlorine from

BCI3

is used to etch aluminum and chlorine from CCI4 and fluorine

from CF4 is used to etch silicon. Fluorine from NF3 is used to etch glasses and ce-

ramics such as that deposited by PEC YD processing

[116].

SPUTTER CLEANING

The chamber surfaces of a vacuum system can be sputtered using the "glow bar"

as a high-voltage positive electrode and the vacuum surfaces as a negative ground

[117].

Figure 8 shows a conditioning cycle for a stainless steel chamber used to

sputter molybdenum.

4.9.5 Documentation

599

Rg.8.

ATMOS

PRESSURE

0.3

1 X 10-2

liJ

5 1

10-3

(/i

1 X

lO-'*

lij

1 X 10-5

1 X 10-6

—I—I—1—\—1—1

\MECHANICAL

PUMPING

\coRPTION PUMPING

IcRYOSORPTION PUMPING

microns

\ 130 sec

LEAK

PLASMA

CLEAN

CHAMBER

IS ]

CATHODE

^OVERNIGHT

ULTIMATE

1 1 1 1 1 1 1

2-7

microns

310

sec

\ LEAK

10-8

MAGNETRON

SPUTTER

MOLY

IcRYOSORPTION

GETTERING

minute

\ LEAK

* OVERNIGHT

\ ULTIMATE

^ 2

10-8

10 20

30 40

TIME

(min)

Conditioning

stainless steel chamber

for

the sputter deposition

molybdenum. Condition-

ing

is by

sputter cleaning

the

vacuum surfaces. Deposition

molybdenum provides getter

pumping

in the

system. Effectiveness

the cleaning

shown

by the

leak-up rate.

[117]

4.9.5

DOCUMENTATION

The key

reproducible cleaning and surface preparation

documentation. Doc-

umentation is also important

the transfer

a process

product from research

and development

manufacturing,

improving

the

process over time,

and

qualifying

for

the ISO 9000 certifications.

many instances, lack

proper doc-

umentation

has

resulted

loss

product yield

and

even

loss

the process

itself,

critical information has not been documented.

Documentation starts with

the

laboratory/engineering (L/E) notebook, where

the experiments, trials, and results

development work are documented. Where

the data

are not

amenable

direct entry,

sunmiary

the findings

can

en-

tered into

the L/E

notebook

and

reference made

particular charts, graphs,

memos, etc. To ensure unquestionable entries, the

L/E

notebook should be hard-

bound

and

have numbered pages,

and

entries should

handwritten, dated,

and

initialed.

an entry

made about

patentable process, product,

idea,

the en-

try should be read

another person, then initialed and dated with

die

statement

600 Chapter 4.9: Preparation and Cleaning of Vacuum Surfaces

"Read and understood" beside the entry. Patents are developed from L/E note-

books, and dated entries arc important if questions are ever raised about when and

where an idea was conceived or a finding made.

Process specifications ("specs") are essentially the "recipe" for the process and

aie the goal of a focused R&D process development effort. Specifications define

what is done, the critical process parameters, and the process parameter limits

that will produce the desired product. The specification can also define the sub-

strate material, materials to be used in the processing, handling and storage con-

dition, packaging, process monitoring and control techniques, safety considera-

tions,

and any other important aspect of the processing. Specifications should be

dated, and there should be a procedure available that allows changes to the speci-

fications. Reference should be made to the particular "issue" (date) of specifica-

tions.

Specifications should be based on accurate measurements, so it is important

that calibrated instrumentation be used to establish the parameter windows for the

process. Specifications usually do not specify specific equipment and noncritical

process parameters. Specifications can also be used to define the functional and

stability properties of the product, and associated test methods.

Manufacturing processing instructions (MPIs) are derived from the specifica-

tions as they are applied to specific equipment and manufacturing procedures. Of-

ten the MPIs contain information that is not found in the specifications but is

important to the manufacturing flow. This may be something such as the type of

gloves to be used with specific chemicals (e.g., no vinyl gloves around alcohol,

rubber gloves for acids). The MPIs should be dated and updated in a controlled

manner. The MPIs should also include the appropriate manufacturing safety data

sheets (MSDSs) for the materials being used. In many cases the MPIs should be

reviewed with the R&D staff who have been involved in writing the specifica-

tions,

to ensure that mistakes are not made. The R&D staff should be included in

process review meetings for the same reason. In some cases, MPIs and specifica-

tions must be written from an existing process. Care must be taken that the oper-

ators reveal all the important steps and parameters.

In manufacturing, it is important to keep equipment logs for the equipment and

instmmcntation being used. These logs contain information as to when and how

long the equipment was used, its performance, any modifications that are made,

and any maintenance and service that has been performed. For example, for a

vacuum system used for thin-film deposition, the log should include entries on

performance such as

• Time to crossover pressure (roughing to high-vacuum pumping)

• Time to the basepressure specified

• Leak-up rate between specified pressure levels

• Process being performed

• Chamber pressure during processing

• Fixturing used

References ^01

• Nnumber and type of substrates being processed

• Mass spectrometer trace at basepressure and during processing

• Total run time

In critical applications, the system performance can be evaluated by statistical

analysis

[118].

The equipment logs can be used to establish routine maintenance schedules

and determine the cost of ownership (COO) of that particular equipment. When

the equipment is being repaired or serviced, it is important to log the date, action,

and person doing the work. The equipment log should also contain the calibration

log(s) for associated instrumentation, if applicable.

4.9.6

CONCLUSION

Surface preparation and cleaning are necessary to provide the best possible vac-

uum in the least amount of time. This is important to determining the "cycle

time"

in vacuum-based processing. Maintaining the surface condition is impor-

tant to obtaining a reproducible processing environment with repeated use.

TRADE NAMES

Snow Floss®—Manville Corporation

Scotch Brite® —3M Company

Soft-scour®—3M Company

GAF NMP®, GAF M-PYROL® —GAF Corporation

Bioact EC-7®—Petroform Company

Genesolv 2000® —Allied Signal Company

Fluoroinert®—3M Company

Krytox®—E. I. DuPont Company

Teflon®

—E.

I. DuPont Company

Viton®—E. I. DuPont Company

Combat®, boron nitride — Carbonmdum Corporation

REFERENCES

N.V. Cherepnin,

Treatment

of Materials for Use in High

Vacuum

(Ordentlich, Frankfurt, 1976).

Fred Rosebury, Handbook of

Electron

Tubes and

Vacuum

Techniques (Addison-Wesley, New

York, 1965), (AVS reprint), p. 1.