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2.5.3 Advanced Control Systems 167

Now the increased vaporization rate overwhelms the ability of the charcoal

to adsorb any more gas, and the pressure rises to above 1 atmosphere. A spring-

loaded pressure relief valve opens to vent the cold gases and maintain internal

pump pressure at a few pounds/in^ above atmosphere. If the pressure relief valve

were not present, 1000 liters of condensed argon could raise the pressure in a

10-liter vacuum vessel volume to 100 atmospheres, causing damage to the pump,

high-vacuum valve, or the vacuum system

itself.

For this reason, it is impor-

tant to allow free flow from the pressure relief valve for gases released during

regeneration.

Many processes use or generate flammable, toxic, or otherwise hazardous

gases.

Exhausted gases from the cryopump should generally be vented to ap-

proved stacks or treatment systems. Since 1000 liters of gas may be vaporized in

a few minutes during pump regeneration, the exhaust system must be sized for

peak flow rates. Where the gases of several processes may be combined in a vent

system, attention must be paid to possible adverse chemical reactions and flam-

mability. For example, during regeneration pumps used on ion implanters may

release tens of liters of hydrogen as well as toxic by-products of boron, arsenic,

or phosphorous. These gases should not be released into the room where pumps

are located.

Eventually the inlet array and thermal radiation shield of the pump rise above

0°C (273 K), and water frozen on the arrays begins to melt. The water flows to the

low spots and begins to evaporate. The charcoal on the second-stage array ad-

sorbs a great deal of water vapor and even liquid droplets that reach it, although

now it may be approaching room temperature. The inside surfaces of the chilled

vacuum vessel also condense water vapor to droplets or ice. The pressure in the

pump drops to

atmosphere as the final Type II gases escape and the pressure re-

lief valve closes automatically. Without any purge gas added, all the pump parts

eventually return to room temperature. If there is considerable water in the pump,

the charcoal may take on half its own weight in water vapor at 100% relative

humidity. Before the pump can be restarted, the residual gases and any water in

the pump must be completely removed. This can be done by warming the pump,

purging with dry nitrogen gas, and rough-pumping the vacuum vessel.

2.5.3

ADVANCED CONTROL SYSTEMS

2.5.3.1 Temperature Indicators

Although stable operation of cryopumps is obtained over a very wide range of ar-

ray operating temperatures, knowledge of the actual array temperatures is useful.

Older pumps used a hydrogen vapor pressure (H2VP) gauge system with a gas-

'^ Chapter 2.5: Cryogenic Pumps

filled bulb mounted on the low-temperature stage. Since the normal boiling point

of hydrogen is 20.2 K, an indication of 14.7 psig or 760 torr (1 atmosphere) on a

mechanical pressure gauge showed the array had reached 20 K. As the tempera-

ture dropped below 12 K, the gauge reading went to less than 0.2 psig or 10 torr.

Although simple and reliable, the H2VP gauge is less useful when other tempera-

ture ranges and electrical outputs are desirable.

Silicon diodes are used as temperature sensors in most cryopumps. When ex-

cited by a few microamps of direct current, the junction voltage drop provides

an accurate, repeatable indication of temperature. High sensitivity, stability over

time,

and low cost make them particularly attractive for control circuits. Although

their response is nearly linear over the 25-330 K range, the lower-temperature re-

sponse is very nonlinear. Digital electronic circuits make it relatively simple to

convert diode voltage to very accurate temperatures.

2.5.3.2 Pressure Sensors

A thermocouple vacuum gauge can be attached to the vessel of the cryopump to

measure pressure over the range of about 1 torr to 10"-^ torr. Although pressure

sensors tend to have absolute accuracy no better than ±20-30 percent at the end

of their ranges, this is sufficient for good operation of the pump. Since they actu-

ally measure the thermal conductivity of the gas that is present, they do measure

the property that is most important to the regeneration of the cryopump. Given

their mechanical ruggedness and low excitation power, thermocouple gauge tubes

have proved very stable and reliable. It is normally sufficient to re-zero the gauge

occasionally when the pump is at operating temperature and the high-vacuum

valve is closed, a condition certain to produce a vacuum well below the sensor's

lowest limit of detectability.

Normally, a gauge tube design is selected to have a filament temperature below

the ignition point of hydrogen to prevent explosion in case the pump is started

into regeneration with the gauge turned

on.

For similar safety reasons, it

strongly

recommended that hot-filament ion gauges not be placed on the pump side of the

high-vacuum valve.

2.5.3.3 Electrical Heaters

Attached to the two heat stations of the refrigerator, electrical heaters may be

used to obtain two important functions: maintaining exact temperatures during

operation and quickly warming the interior parts of the pump during regenera-

tion. Since in most cases it is desirable to hold the first-stage temperatures near

65 K, a closed-loop control circuit linked to the diode temperature sensor is used

to add a few watts of power to the first-stage heater. Argon hangup problems are

2.5.3 Advanced Control Systems 169

thus prevented. It is not normally necessary to drive the second-stage temperature

up,

since other thermal radiation loads will provide the watt or so needed to put

the refrigerator into the 10-14 K range.

During regeneration, the heaters can be driven at full power to provide 100-

200 watts or more to bring the arrays to well above room temperature in a few

minutes. Again, closed-loop control is important so that the refrigerator and array

parts are not overheated. Damage to the refrigerator can occur if the pump is

heated above 80-100°C for prolonged periods. The heaters provide the joules

necessary to vaporize the liquid argon or nitrogen frozen on the second stage and

melt the water ice on the first stage. The heated arrays also warm the flow of nitro-

gen purge gas coming into the pump during regeneration. Heat is transferred to

the outer vacuum vessel by conduction and convection. The control circuit slowly

reduces the power delivered to the heaters as the various parts of the pump come

up to temperature, typically about 40-60°C.

Because the arrays, temperature sensors, and heaters are thermally well coupled

to the refrigerator heat stations, the heater sheaths do not reach very high temper-

atures. That

is,

they remain very close to the 40-60°C maximum temperature set-

ting of the refrigerator process. The sheaths are hermetically sealed to prevent the

entry of water or gases, and also to avoid exposure of the heater filament to the

pump vacuum space. These heater characteristics are important to ensure that

the heater itself cannot become an ignition source for flammable gases during

regeneration.

2.5.3.4 Automated Regeneration

The latest generation of cryopumps use automated regeneration processes incor-

porating internal heaters for the arrays, pressure and temperature monitoring and

control, purge gas, and a controlled roughing valve. The purpose of automated re-

generation is to complete the process in a minimum amount of time while ensur-

ing that all the residual water and gases are removed from pump surfaces and ac-

tivated carbon. By roughing to a predetermined base pressure and rate of

rise,

all

water can be removed and a repeatable level of cleanliness of the activated carbon

can be obtained.

At the beginning of the automated regeneration cycle, a flow of nitrogen purge

gas is started to quickly bring the vacuum vessel to atmospheric pressure for en-

hanced heat transfer from the room. In addition, the 20- to 60-liter/minute purge

gas flow dilutes hydrogen during the initial stages of regeneration and carries it

from the pump. The electrical heaters attached to the refrigerator heat stations, or

to the arrays themselves, quickly melt the frozen gases and assist in rapid vapori-

zation. The heaters bring the arrays — and the charcoal —

a temperature near

40°C.

The heated arrays warm the incoming purge gas to speed the vaporization

of water and the warming of the vacuum vessel walls. Purge gas flow must con-

170

Chapter 2.5: Cryogenic Pumps

tinue until all the liquid water is evaporated, the array and pump vessel surfaces

are dry and warm, and most of the water vapor has been dried from the activated

carbon. When only a little water is present, as for pumps on vacuum process

chambers of load-locked systems, 20 minutes may provide sufficient purging.

However, for cryopumps on chambers regularly exposed to atmosphere and hu-

midity, such as the load locks themselves, many tens of cubic centimeters of wa-

ter may be present in the pump, requiring an hour or more of purge time. For best

performance, purge gas lines should be made of metal tubing and in-line dryers

should be used to avoid water vapor contamination of the purge gas.

Rough-pumping of the vessel begins when all surfaces are warm and dry. A

base pressure of

.05

torr is typically selected as a requirement. Roughing removes

gases and water adsorbed into the charcoal at atmospheric pressure and strips

most of the water vapor from the metal surfaces. The heaters for the arrays are

kept on during the rough process to prevent evaporative cooling of the activated

carbon. Water will be evolved at a much faster rate at higher temperatures. For ex-

ample, the vapor pressure of water is nearly four times higher at 40°C than it is at

20°C [7]. Figure 10 shows the vapor pressure of water over the 0 to 80°C range.

Fig.

10.

Effect of Temperature on Vapor Pressure of Water

oou -

'\r\f\

250

onn -

i»%n -

100 -

10 20 60 70 30 40 50

Temperature (''C)

A cryopumped large-vacuum furnace with thermally shielded, high-temperature hot zone.

2.5.3 Advanced Control Systems 171

In some regeneration routines, it is possible to perform simultaneous heating,

purging, and roughing to enhance removal of water and heavy organic deposits in

the pump, such as photoresist by-products in ion implanters. By keeping the pres-

sure above 10 torr, rapid volatilization of the water is possible without the possi-

bility of freezing the water to ice by evaporative cooling. The simultaneous flow

of purge gas carries the water through the rough pump, and prevents recondensa-

tion in dry rough pumps. If the pressure in the cryopump is reduced below about

10 torr when liquid water is present, ice quickly forms and does not sublime. A

false indication of good base pressure might then be obtained with a large-enough

rough pump, leading to an incomplete regeneration.

On reaching base pressure, the rough valve closes and the rate of rise of pres-

sure is monitored. If all liquid water is gone and the charcoal is sufficiently clean,

the rate of rise will be less than about 10 microns/minute. If the target rate is not

met, the rough valve opens again. If the number of rough cycles exceeds a preset

count, an additional purge cycle is performed to evaporate any remaining water.

The rough cycle then begins again.

Once both base pressure and rate-of-rise values are met, the heaters are turned

off, the refrigerator is turned on, and the cooling-down process of the cryopump

starts.

At the internal pump pressure of 50-100 microns, most of the residual gas

is water vapor. Once the refrigerator reaches about 150 K, the metal surfaces be-

come very efficient for pumping water vapor and the pressure starts to drop. In ad-

dition, the charcoal also begins to efficiently adsorb gases, removing any residual

air or nitrogen purge gas. As the pressure again falls below 1 micron

(10 "^

torr),

an insulating vacuum is achieved, removing thermal conduction loads from the

pump arrays and speeding the cooldown process. When the second-stage array

reaches about 17 K and the first stage is below about 130 K, the pump is ready to

use again. In most cases, the vacuum process chamber has been vented to air dur-

ing the cryopump regeneration process, so high-vacuum pumping can begin even

as the pump itself continues to cool to its ultimate low temperatures.

For ultra-high-vacuum chambers that have not been vented to atmosphere dur-

ing cryopump regeneration, it may be desirable to wait to reopen the high-vacuum

valve until some time after the pump reaches its lowest temperatures. This is to be

sure that any residual water vapor has been released from the cryopump vessel and

high-vacuum valve surfaces and is condensed on the cold surfaces of the pump.

2.5.3.S Partial Regeneration

In many applications, such as sputter chambers on semiconductor production sys-

tems,

rapid regeneration of a cryopump is economically appealing. Fortunately

these systems have load locks that prevent the ingress of water into the process

chambers and cryopumps located on them. In that case, regeneration of the 12 K

172 Chapter 2.S: Cryogenic Pumps

surfaces and condensed loads of argon or nitrogen process gases and removal of

adsorbed Type III gases in the charcoal is all that is required. Since there is little

water present, it is not necessary to raise the first-stage arrays from 65 K back to

room temperature to melt ice.

For partial regeneration, the refrigerator is kept running to maintain the first

stage near 65 K. The heater on the second stage is turned on to raise the second-

stage array temperature to about 180 K. At the same time, a short burst of nitro-

gen purge gas is added to bring up the pressure in the pump. As the second-stage

arrays rise above 60-80 K, frozen nitrogen and argon melt and fall to the thermal

radiation shield (still near 65 K) or the vacuum vessel wall. In a few minutes, the

liquid nitrogen or argon vaporizes. The temperature of the inlet array and thermal

radiation shield is kept low enough to prevent sublimation of water vapor from

these surfaces into the activated carbon. The vapors help to keep the thermal radi-

ation shield cold in spite of heat conduction through the vapors from the warmer

vacuum vessel. Vapor exits the pressure relief

valve.

Activated carbon quickly fills

with large amounts of nitrogen and argon. After a few minutes, the rough valve is

opened. If liquids remain, the pressure remains high. If all the liquids are gone,

the pressure will quickly drop below

ton*. If a rapid pressure drop is not detected

when the rough valve opens, the valve is closed again and a short burst of purge

gas is injected to raise the pressure again and encourage the vaporization of the

liquid.

Once the pressure drop verifies the absence of liquids, rough pumping is con-

tinued to base pressure, while the activated carbon array is maintained at about

180 K. Release of nitrogen and argon from the activated carbon at this tempera-

ture and a base pressure of 50 microns is sufficient to completely restore the

hydrogen capacity to its initial value. The second-stage heater is turned off, and

the refrigerator returns the second-stage array to 10-20 K for operation. In most

cases,

the fast regeneration process can be completed in about one hour or less.

When properly completed, 10 or more sequential fast regenerations may be done

without any measurable loss in hydrogen speed or capacity. If higher water loads

are present in the system, then perhaps only two to four fast regenerations may be

done between full regenerations, since the increasing water burden adds to the

thermal load on the pump. A consequence of using fast regeneration is that purge

times must be extended during full regeneration, because 4-10 times as much

water may be present in the pump as compared to when full regenerations are per-

formed at every cycle.

Variations of the fast regeneration process may be appropriate for specific ap-

plications, such as ion implantation. The high amounts of hydrogen evolved by

the implant process [8] compared to relatively low amounts of nitrogen or other

Type II gases, may require additional purge-rough cycles during fast regenera-

tion to remove all the hydrogen. Full regeneration may be required more fre-

quently than in sputtering to eliminate the larger amounts of water and photoresist

by-products.

2.5.4 Cryopump Process Applications 173

2-5.4

CRYOPUMP PROCESS APPLICATIONS

2.5.4.1 Chamber Evacuation

Even the most basic vacuum chambers require attention to the high-vacuum

pump if they are to quickly reach their intended operating pressures. When start-

ing from atmospheric pressure, a mechanical pump is used to produce initial rough

vacuum. The point of crossover to the high-vacuum pump depends on the type of

rough pump used, the speed of the pump, and the crossover rating of the cryo-

pump. Consider a 20-inch-diameter, 30-inch-high bell jar (5.5 ft^ or 155 liters)

with an

8-inch

(200-mm) cryopump. The 150 torr-liter crossover rating of the

cryopump indicates that the high-vacuum valve can be opened when the 155-liter

chamber gets below

torr. Many chambers of this size use rough pumps of about

17-35 ft^/minute

(—30-60

m^/hour) displacement, which equates to a speed of

only 8-16 liters/second. Clearly, it is advantageous to switch to the cryopump's

1200-4000 liter/second speeds as soon as possible. In addition, oil-sealed rough

pumps tend

backstream oil at pressures below 200 microns (0.2

torr).

Dry pumps

avoid the backstreaming problem, but still suffer from relatively low speeds. At

crossover, the pressure will quickly drop to the point where the outgassing of the

chamber determines the pressure. For chambers that have been exposed to room

air, this means that the chamber will probably reach a pressure in the

10 ""*

torr range in the first minute.

After crossover, the pressure will slowly decrease in the absence of

leaks.

Most

outgassing phenomena, especially water vapor, follow a j decrease in outgassing

rate

[9].

That means that it takes ten times as long to get to one-tenth the pressure.

For example, if it takes 1 minute from crossover to reach

10 ""^

torr and 10 min-

utes to reach

10 ~^

torr, it will take 100 minutes to reach 10"^ torr and 1000 min-

utes (17 hours) to get to

10 ~^

torr. If there are system leaks or large amounts of

0-rings to allow permeation, low base pressures may never be obtained.

The value of

mass spectrometer-type residual gas analyzer on all vacuum sys-

tems cannot be overemphasized. It is the only way to determine what type of gas

is responsible for determining the pressure [10]. The good news for cryopump

users is

this:

If the pump is cold, it is probably pumping at its full rated speed. The

causes of poor pressure are generally outgassing and leaks in the chamber, not a

decrease in the speed of the cryopump. The chamber pressure P is determined by

the relationship of outgassing rate Q and pumping speed for a particular gas S.

That is,

P = Q/S

Sometimes Q can be estimated by assuming the pumping speed is constant, ac-

counting for all the conductance losses between the pump and the chamber, and

174 Chapter 2.5: Cryogenic Pumps

plotting the decrease in pressure over time. Unfortunately, when water vapor out-

gassing predominates, as it usually does, the outgassing rate cannot be deter-

mined by closing the high-vacuum valve and noting the pressure rise with time.

This is because the water outgassing rate will immediately decrease as the cham-

ber pressure rises, resulting in a gross underestimate of the outgassing rate.

Leak checking of cryopumped systems can sometimes be difficult. When a he-

lium leak detector is connected to the chamber with the gate valve open to the

cryopump, only a very low sensitivity is obtained. That is because the net pump-

ing speed of the leak detector may be only

to 2 liter/second, whereas the helium

pumping speed of the cryopump is 1500-1800 liter/second. This results in a

1000:1 reduction in leak sensitivity. Closing the high-vacuum valve between the

cryopump and the chamber improves the sensitivity if the leak detector itself can

maintain a low-enough pressure in the vacuum system. Because the helium ca-

pacity of the cryopump is only a few standard cm^, the cryopump should be re-

generated after any helium leak checking.

On ultra-high-vacuum systems, base pressures in the 10"^ to less than 10~^^

torr range may be obtained with baking of the chamber for 24 hours or more at

temperatures of 100-200°C while the pump is running. Peak temperatures may

be limited by the ability of the pump to maintain its own temperature below

20-25 K during chamber bakeout. Although high pressures may be observed

during bakeout, the pressure will drop as the chamber (and cryopump) cools. It

may be beneficial to then close the high-vacuum valve and regenerate the cryo-

pump to rid the pump of any hydrogen outgassed by the chamber. When base

pressures below 2 X

10 ~^^

torr are required, the 0-ring sealed pressure relief

valve of standard cryopumps is sometimes replaced by an all-metal burst disk. As

long as provisions are made to prevent the pump vessel from rising above atmo-

spheric pressure during regeneration, the burst disk may permit slightly lower ul-

timate pressures. It is not practical to use gas purge or automated regeneration

when a burst disk is used, however.

The connections of the purge gas supply, rough pump line, and other utilities

are shown in Figure 11. The compressor supplies helium to the cryopump, and

may also provide electrical power.

2.5A.2 Evaporation Systems

Thin-film deposition by evaporation is performed for optical and electronic coat-

ings.

Evaporation can be used for a wide variety of coating materials and on a

greater range of substrate shapes than sputtering. Cryopumps work well on evap-

oration systems because they are usually not bothered by the vapors of the de-

posited materials, and they produce clean vacuum conditions. The evaporation of

reactive metals, such as aluminum, produces a conversion of residual water vapor

2.5.4 Cryopump Process Applications

175

Fig.

II.

User's

vacuum

chamber

High-vacuum gate valve

Roughing pump -

Air pressure (60 ~ 80 psig)-

Nitrogen (40-80 psig)-

Cryopump power cable

Connections of the cryopump to the vacuum chamber and utilities. (Connection diagram for

cryopump and compressor courtesy of CTI-Cryogenics.)

into metal oxide and hydrogen at a prodigious rate. It is therefore beneficial to

pump the chamber to a base pressure below 10"^ torr before beginning deposi-

tion to avoid filling the hydrogen capacity of the cryopump prematurely. In addi-

tion, thermal and optical baffling of the pumping port may be required to limit the

thermal radiation from electron beam guns and other sources from overloading

the cryopump.

If ion-assisted deposition (IAD) is used with oxygen at partial pressures in the

"^ to low

10 ""*

torr

range,

traces of ozone may be detected in the exhaust gases

during pump regeneration. Although only parts per million of ozone may be

generated during IAD processes, the cryopump regeneration exhaust should be

vented to a separate stack to make sure that the ozone is not vented into the room

air. This will also prevent reaction with oil vapor or other potential fuels in a com-

mon facility stack. Ozone destruction units may be required to meet local codes.

Vapors of

metals,

oxides, and odier evaporated compounds may reach the inlet

array of the cryopump. There they will deposit on the ice layer growing on the in-

let array and thermal radiation shield. Some may reach the outer surfaces of the

second-stage (charcoal) array. In general, these deposits remain after regeneration.

Although they may slowly build up into an ugly deposit, they will have little or no

effect on pumping speed or capacity. Ordinarily, these deposits may be removed

from the inlet array at annual (or more frequent) system maintenance intervals by

removing the array from the pump and scrubbing it with a brush in detergent and

water.

176 Chapter 2.5: Cryogenic Pumps

Because many evaporation chambers have quite large volumes, it may be nec-

essary to rough-pump them to lower base pressures before crossover to the cryo-

pump. This invites oil backstreaming from the mechanical pump into the rough

lines and process chamber. Over time, any oil condensed in the chamber will even-

tually migrate into the cryopump. A possible result is the contamination of the ac-

tivated carbon of the second-stage array, reducing the hydrogen capacity in ex-

treme cases. The use of Roots-type blowers between the oil-sealed pump and

chamber reduces backstreaming when crossover pressures of 10-100 microns

are needed. Otherwise, promptly crossing over at the appropriate pressure will at

least reduce the time that the rough pump runs in transitional flow.

2.5.4.3 Sputtering Systems

For semiconductor wafer metallization, sputter deposition is the process of choice

for aluminum and titanium. Oxide and nitride layers are also sputtered. Cryo-

pumps perform well in these applications because of their inherent cleanliness,

reliability, and high speeds for both water vapor and process gases. Since sputter-

ing is normally done in the range of 1-5 X 10"-^ torr, gas flow is increased

to raise the chamber pressure to the desired level. For a closely coupled pump,

the 1200 liter/second argon speed would require a flow of 285 std. cmVminute

(3.6 torr-liter/second) to maintain 3 X 10"-^ torr. To obtain long times between

regeneration and reduce the expenses for process gas, conductance limiters are

used to reduce gas flow to the 50-150 std. cm^/minute range [11].

A warm (room temperature) conductance limiter, such as a baffle in the pump-

ing port in front of

the

gate valve, reduces water vapor speed as well as process gas

speed by about the same ratio. An alternative is to use a sputter plate. The sputter

plate replaces the standard 65 K chevron array with a flat copper plate having a

number of small holes. At sputtering pressures, a ^-inch-diameter hole has an ar-

gon conductance of about 20 liters/second. By selecting a plate with an appropri-

ate number, size, and location of holes, a lower net argon speed will be obtained

while maintaining full water vapor pumping speed. A reduction in argon speed to

20-40%

of the base speed is conmion. Although this will also reduce the speeds

of air (oxygen and nitrogen), little negative effect is noticed on chamber pump-

down, since water vapor is the principal gas load at that time. Hydrogen speeds

are slightly reduced by the sputter plate, but not to the same proportion as argon

speeds.

Sputtering with RF magnetron sources of high power in oxygen or oxygen-rich

gas mixtures should be avoided with cryopumps. The creation of atomic oxygen

and excited O2 states leads to the formation of ozone in the oxygen ice condensed

on the second stage. At low ozone concentrations, the gases exhausted from the