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

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2.5.2 Cryopump Basics

157

An aspect of molecular flow that is important for all high-vacuum pumps is

that the pump does not draw or "suck" molecules toward the entrance. The pump

must wait for the arrival of a molecule in its quasi-random motion, and then trap

it. Because the cryopump traps the molecule by adsorption and condensation onto

a cold surface, it can be considered a piece of cryogenic "flypaper." A molecule

lands on the cold surface and never takes off again. Other molecules land on the

surface, to form progressively thicker layers of condensate.

2.5.2.2 Vapor Pressure

Cryopumps are omnivorous: They condense all gases at very high speeds. The

ability of a surface to trap a particular gas depends on the vapor pressure charac-

teristic of the gas. Nearly all gases follow the basic Clausius-Clapeyron relation

[4] of the vapor pressure P and the temperature T in Kelvins.

log P = -A{l/T) + constant

The slope of the line, -A, is the heat of vaporization of the gas. An example of

the Clausius-Clapeyron plot is shown in Figure 5. Many readers are more familiar

Fig.

Gas Vapor Pressure vs. Temperature

Argon,

Nitrogen, and Hydrogen in Clausius-Clapeyron Form

20 10 5

2.5

lA.^

Ll^

1 Argon L'

Nitrogen

-379.48X • 8.0

y - -420.58X • 7

S74

ri86

Hydrogt

\. y-<

J.624X •5.1161

^< 1

Inverse Temperature (1/K)

158

Chapter 2.5: Cryogenic Pumps

Fig.

Gas Vapor Pressure vs. Temperature

Hydrogen, Nitrogen, Argon, and Water in Honig & Hoolc Form

1 -'

5 ^

-13

-15

Hydr

age

\7]

Nitrog

7^-

jn i

jon

Watelr

10 100

Temperature (K)

with an alternative presentation of vapor pressure data by Honig and Hook [5].

The data from Figure 5 is replotted in Honig and Hook form in Figure 6. For a

surface at a given temperature in a closed volume, equilibrium will be attained at

the equilibrium vapor pressures for the various gases in the volume. The resulting

total pressure will be the sum of the equilibrium partial pressures of the individ-

ual gas components. For system pressures more than 100 times the equilibrium

vapor pressure of the cryopump array, full speed is obtained. As the system pres-

sure drops below 10 times the equilibrium pressure, the speed begins to drop off.

Zero speed occurs when the base pressure gradually approaches the equilibrium

vapor pressure of the array surface, or the ultimate pressure for the gas in the

cryopump. In most cases, these ultimate pressures are well below 10"^^ torr for

condensable gases.

Normal dry room air is about 78% nitrogen,

21%

oxygen, 1% argon, and other

gases down to about 7 ppm of helium. Depending on the relative humidity, water

vapor may make up

or 2% of the air. In a vacuum system, nitrogen, oxygen, and

other gases are rapidly removed by the initial pumping process, leaving water va-

por as the major constituent of the vacuum atmosphere. The water vapor is polar-

bonded to the surfaces in 50-100 monolayers, while other gases are easily stripped

2.5.2 Cryopump Basics '59

away. Water may make up 98% of the gas load of a vacuum system at 10""^ torr.

Consequently, high pumping speeds for water vapor are crucial in all vacuum

systems.

2.5-2.3 Condensation

Water is relatively easy to pump onto cryogenic surfaces. At 130 K, an equi-

librium vapor pressure of 10"^^ torr is obtained, ensuring nearly 100% pump-

ing efficiency to pressures below 10"^ torr. A surface below 113 K will provide

greater than 99% efficiency to below 10'^^ torr. Consequently, operation of a

cryopump surface in the 65-90 K range is 100% efficient for pumping water and

high-molecular-weight gases and vapors in any practical vacuum system. Tem-

peratures below 20 K are necessary for condensing nitrogen, oxygen, argon, and

most other condensable gases. At these temperatures, the condensates are dense

solids, like ice. It should be noted that it is not practical to condense hydrogen, he-

lium, and neon, even at temperatures in the 6-20 K

range.

These three gases must

be pumped by special adsorption surfaces within the cryopump.

For all other gases, however, the condensation surfaces in the 10-20 K range

provide 100% efficiency pumping at constant speed over the entire pressure range

from below

10 ~ ^^

torr to above

10 ""^

torr. As the pressure rises into the low

torr range, the pumping speeds increase by about 20-40% due to the onset of

transitional flow. In most applications, the speed of the cryopump can be treated

as essentially constant at all pressures.

The process of condensation requires removal of heat from the gas molecules

through contact with the cold surfaces. Remarkably, the quantity of heat removed

from gases near room temperature is quite low. For nitrogen, the conversion of

gas at 300 K to a solid at 20 K is a complex though instantaneous process.

Nitrogen Condensation Budget

Cooling Process

Gas cooled 300 K to 77 K

Liquefaction at 77 K

Cool liquid 77 to 63 K

Transition liquid to solid I

Cool solid I 63 K to 36 K

Transition solid I to solid II

Cool solid II to 20 K

Total

Enthalpy

(calories/gram-mole)

1600

1300

189

172

267

123

3700

160 Chapter 2.5: Cryogenic Pumps

This translates to about

watt of cooling required for each 100 std. cmVminute

of nitrogen gas flow into the system, about the flow in a sputtering system at 2 X

10 ~^

torr. For the same pump operating in the

"^ range or below, the total con-

densation load is less than 1 milliwatt. It is for this reason that even very large

cryopumps can be constructed with a relatively small refrigeration capacity at

20 K and below. For comparison, the mechanical work involved in the transport

of 100 std. cmVminute (0.2 torr-L/sec) of gas in a vacuum system requires only

about 30 milliwatts of power.

It is frequently advisable to control the operating temperature of the inlet array

and thermal radiation shield at a preset value. Generally, the temperature is set in

the range of

to 90 K to prevent the partial adsorption of

Ar,

N2, CO, and other

gases on the first-stage array parts. For example, the first-stage temperature can

drop to 40-45 K in a cryopump with very low thermal loads. If the pump is then

used for sputtering with Ar at

10 ~^

torr, some argon is adsorbed in a very thin

layer on inlet array surfaces. When the gas flow is shut off at the end of the pro-

cess,

the chamber pressure may take a long time to recover to the 10"^ to 10"^

range as the lightly adsorbed gas is evolved from the inlet array. The presence of

a thick water ice deposit on the inlet array can make the problem worse due to the

larger effective surface area of the microcrystalline ice deposits. Raising the first-

stage array temperature to the 65-90 K range reduces the adsorption of Ar and

speeds the recovery to base pressures.

In principle, a first-stage array temperature in the range of 90-105 K might be

desirable for some processes requiring repetitive cycling over a large dynamic

pressure range. However, since the first and second stages of the refrigerator are

thermodynamically linked, it is not normally possible to drive up the first-stage

temperature to these high values without also dragging the second-stage tempera-

tures well above the optimum 10-14 K range.

2.5.2.4 Adsorption

The three light gases H2, He, and Ne cannot be pumped by condensation and

must be adsorbed. Activated carbon or charcoal from coconut shells, wood, or

other organic materials is used for this purpose. Treatment of the charcoal with

steam at high temperatures produces huge effective areas of 1000 m^/gram or

more. The carbon then has an interconnected network of minute channels or

fissures with tightly controlled pore size in the range of 10-30 angstroms. A gas

molecule landing on the outer surface of the carbon loses kinetic energy as it

cools to 10-12 K. Such loosely held molecules then diffuse rapidly into the inner

volume of the charcoal. At these temperatures, hydrogen molecules can be re-

tained at very low equilibrium vapor pressures as up to a monolayer of gas is ad-

2.S.2 Cryopump Basics

1^1

sorbed onto the carbon. Although this may not seem impressive, at approximately

atoms/cm^ a gram of charcoal (about a teaspoon) can hold 200 std. cm^^ of

hydrogen. A handful of charcoal can hold 10-20 liters. A layer of charcoal gran-

ules or pellets is attached to copper sheets with vacuum-compatible epoxy or other

adhesives.

The remarkable ability of cold, activated carbon to adsorb gas is demonstrated

by the reduction in vapor pressure. A flat copper surface at 12 K would hold only

a small fraction of

monolayer of hydrogen while coming to an equilibrium pres-

sure of about 20 torr. By adding 50-100 grams of charcoal to the copper surfaces,

many liters of gas can be adsorbed before reaching an equilibrium pressure of

about 10"^^ to 10"^ torr. This is a reduction in pressure by 12 orders of magni-

tude with a simple technique.

Carbons are used in preference to zeolites because carbons are hydrophobic.

That is, when the cryopump is eventually regenerated, any adsorbed water vapor

can be removed from the charcoal by heating at 30-40°C, instead of the 200-

250°C required of zeolites. The activated carbons used in cryopumps have been

processed at very high temperatures to obtain nearly pure carbon with few impu-

rities.

Since both the carbon and the adhesive are at about 12 K whenever the

high-vacuum valve is open to the rest of the vacuum system, they are always

"sinks"

for gases and never sources of outgassing.

The operating temperature of the charcoal affects how much gas can be re-

tained at a given equilibrium pressure. At 10-15 K, a gram of charcoal will hold

about 200 std. cm^ of hydrogen at 5 X

10 ~^

torr or about 100 std. cm^ at 5 X

10 ~^

torr. These are the normal conditions of use of a cryopump at maximum

rated capacity. Temperatures below 10 K do not significantly improve effective

capacity, although lower ultimate pressures may be obtained in clean, baked UHV

systems with light hydrogen loads. At high hydrogen gas loads that produce par-

tial pressures of 10"^ torr and above, very low carbon temperatures are harmful.

The hydrogen may not be able to diffuse to the inside of the carbon granules

rapidly enough to maintain low pressures. Instead, operation of the charcoal in

the 12-14 K range is optimum to balance good capacity with maintainable, con-

stant pumping speed [6]. Above 25 K, though, the capacity of carbon to adsorb

hydrogen falls rapidly. It has the ability to adsorb (pump) many other gases, such

as nitrogen or argon, at temperatures well above 90 K.

Helium is more difficult to adsorb than hydrogen. A pump that can adsorb up

to 20 liters of hydrogen while holding pressures below 5 X 10"^ torr, can adsorb

only 5 std. cm^ of helium at the same maximum pressure level. It is fortunate that

helium is only a minor constituent in our atmosphere. The very low capacity for

helium precludes the use of cryopumps for any process that bleeds helium into

the vacuum chamber. It also requires that helium remain as only a trace impurity

in any other process gas, a condition not always obtained. In particular, conmier-

162 Chapter 2.5: Cryogenic Pumps

cial bottled gas suppliers and on-site air separation facilities may not count he-

lium or neon as impurities when they certify argon and nitrogen gas purity. The

presence of 10-100 ppm of helium in a process gas stream can quickly cause

base pressure problems in cryopumped systems.

2.5.2.5 Thermal Considerations

Thermal radiation loads also play a role in the design and operation of cryogenic

high-vacuum pumps. Pumps are usually mounted on a port of the vacuum cham-

ber with a thin gate valve at the opening. The vacuum chamber acts as a large

optical integrating cavity or "black body" at the wall temperature if there are

no other heat loads in the chamber. With a room temperature (295 K) wall, the

thermal load from infrared radiation on the inlet of the pump is approximately

0.05 watt/cm^, or about 16 watts across a standard

8-inch

diameter pump. With

hotter chamber surfaces, the thermal load goes up as

T"^

(in Kelvins). This limits

most cryopumps to operating on chambers with wall temperatures in the range of

50-100°C.

Most of the thermal load is absorbed by the first stage of the refrigerator as the

radiation strikes the inlet array of the pump. The inlet array of the pump is typi-

cally nickel plated to reduce the absorbed thermal load on the pump during the

cooldown process when the pump is empty of all gases. Once the pump has oper-

ated for a few hours or days, a thin layer of infrared-absorbing water ice forms on

the inlet array. This effect raises the thermal load on the inlet array. Some thermal

radiation passes through or around the inlet array to fall on a black thermal radia-

tion shield, which is also cooled to about 65-80 K by the first stage of the refrig-

erator. Because the refrigerator typically has three times the cooling capacity on

the

first

stage as on the

K second

stage,

the inlet array also serves as a shield

for the second-stage array to prevent thermal overloading of the low-temperature

surfaces.

Thermal sources in the vacuum chamber, such as heat lamps and deposition

sources, add other heat loads to the cryopump. Stainless steel vacuum chamber

walls are 90% reflective in the infrared, whereas aluminum surfaces may approach

98%

reflectivity at long optical wavelengths. This allows many bounces of the

radiation around the chamber with little attenuation. Infrared radiation, unseen by

the human eye, winds up in the "black hole" of the cryopump. Consequently,

attention to thermal shielding of the heat sources in the chamber or at the pump-

ing port may be necessary. That this shielding process can be effective is demon-

strated by the use of cryopumps on vacuum furnaces, such as the one shown in

Figure 7, having hot zones operating at 2000°C and above.

2.5.2 Cryopump Basics

\63

Fig.7.

A 12-inch (320-mm) diameter cryopump with 2000 liters of argon ice and some water vapor.

(Photo of vacuum furnace with cryopump courtesy of CTI-Cryogenics.)

2.5.2.6 Crossover

Another heat load on the cryopump is the transient load from the initial opening

of the gate valve to the vacuum chamber. Typically, a vacuum chamber is rough-

pumped to 0.1 to 1 torr before opening the valve to the high-vacuum pump. The

sudden inrush of air and water vapor, as measured by the product of chamber

pressure and volume, produces an impulsive heat load on the cooled surfaces of

the pump as the gas is condensed. The pressure in the chamber drops to

torr

164 Chapter 2.5: Cryogenic Pumps

or less in a few seconds. The joules of heat extracted

from

the gas during cooling

momentarily exceed the ability of the refrigerator to remove heat and maintain

temperature. Now, the thermal mass of

the

cooled parts determines how high the

temperature will rise. The

crossover rating

of the pump, usually given in torr-

liters,

is that product of chamber pressure and volume that can be acconmiodated

without raising the second-stage array parts above 20

This ensures that any ad-

sorbed hydrogen will not

released back

into

the system, causing

the

pressure to

up instead of

down.

Interestingly, continued use of

the

cryopump improves the

crossover rating because the condensed gas loads have significant thermal capac-

ity of their own. For example, 300 standard liters of argon gas condensed into

about

375

cm^ of argon ice weighing about

535

grams.

This ice at

12 K

has

heat

capacity approaching that of the mass of copper and stainless steel of the cryo-

pump parts at

K, effectively doubling the crossover rating.

2.5,2.7

Capacity

The capacity of

cryopump to store condensed gases depends on the dimensions

of the

pump.

Water vapor—and other

Type

I gases that

have

low vapor pressures

K—condense on the top surfaces of

the

inlet array and on the inside of the

thermal radiation shield. For

200-mm (8-inch) cryopump, nearly a liter of water

can be stored as ice, spread out over these surfaces in layers up to a few centime-

ters

thick.

The

ultimate limit

water capacity

that

the

ice will eventually choke

off

the

ability of other gases to reach the interior of

the

pump,

thus reducing their

pumping speed. In most applications, a few tens of cubic centimeters of water

will

collected over

few weeks of operation. Under

UHV

conditions,

the

pump

will not collect a thimble full of water or other gases in a

year!

A cryopump with

only a little condensed water

shown in Figure 8.

Argon, nitrogen, oxygen, and other Type II gases condensable at 20 K, collect

on the second-stage surfaces at 10-20 K. As they form ice layers of several cen-

timeters thickness, several things begin to happen. The ice grows upward until it

reaches the

65 K

inlet

array.

Since

the

Type

II gases

will

not condense at

K, the

ice continues to grow up through the inlet array without actually touching the

warmer surfaces. The Type II ice ball also begins to block access of

the

Type III

gases,

which do not condense at 20 K (hydrogen, helium, neon) to the carbon-

coated surfaces. Furthermore, the Type n ice is no longer thermally shielded by

the inlet, so the thermal loads increase on the second stage of

the

refrigerator. Be-

tween the increased thermal load and the increasing thermal gradient through the

ice,

the equilibrium pressure obtainable begins

rise.

This sets the practical limit

on capacity for Type II gases at about 1000 standard liters of gas for a 200-mm

cryopump before the equilibrium pressure rises into the

torr

range.

For sys-

tems that must cycle rapidly and frequently from above 10 "^ for sputtering

be-

ZS.2 Cryopump Basics

165

Rg.8.

A 12-inch cryopump showing argon ice thickness of 2-3 cm as regeneration continues and ice

begins to melt. (Photo of cryopump inlet array with ice courtesy of CTI-Cryogenics.)

low 10"^ torr for chamber

cleanup,

reduced efifective capacities may be observed

as the time to recover to base pressure increases.

In practical

terms,

a cryopump can be used for a long time before reaching ca-

pacity. For static, high-vacuum pumping at 10"^ torr, an

8-inch

pump could run

for one to three years before requiring regeneration. On load-lock or batch ap-

plications without additional process gas loads, 2000-5000 pumpdown cycles

may be

obtained between

regenerations.

Sputtering

systems have

typical argon gas

loads of 50-150 std. cmVminute coupled with requirements for rapid recovery,

limiting time between regenerations to

-2

weeks.

Similarly, the heavy hydrogen

gas loads of ion implanters require regeneration at intervals of 5-15 days in full-

production situations.

2.5.2.8

Regeneration

As the effective capacity is reached, it

necessary to regenerate the cryopump to

restore full pumping speed and the ability to recover rapidly to

base

pressure.

Be-

cause three types of adsorption processes have been used to collect the gases, a

166

Chapter 2.5: Cryogenic Pumps

number of effects take place during regeneration. The simplest way to regenerate

a pump is simply to close the high-vacuum valve and turn off the refrigerator.

Since the pump will initially be under high vacuum, with all the surfaces cold, the

arrays and condensed gases will warm slowly as heat is conducted through the

cylinder of the refrigerator to the second-stage array. As the second-stage array

temperature (and the temperature of the charcoal attached to the array) climbs

above 25 K, hydrogen and other Type III gases are released into the pump vessel,

raising the pressure. As the pump pressure rises above

10 "-^

torr, the thermal con-

ductivity of the gas increases, providing more heat load on the arrays. Above 30 K,

the equilibrium vapor pressures of the Type II gases condensed on the second

stage begin to rise, and nitrogen and argon add to the rising pressure in the pump.

For a few minutes, the still-cold charcoal adsorbs the Type II gases, suppressing

the pressure rise. Eventually, the second stage warms sufficiently to liquefy the

Type II gases, which then flow to the warmed thermal radiation shield and outer

vacuum vessel surfaces. A cryopump in the middle of the regeneration process

is shown in Figure 9. In some pump orientations, the liquid cryogens may flow

to the gate valve or other parts of the chamber. The outer vessel of the pump be-

comes very cold, condensing water vapor from the room air onto the outside sur-

face to liquid droplets or frost.

Fig.

Vapor pressure of water versus temperature. (Photo of interior of cryopump during regenera-

tion with melting ice courtesy of CTI-Cryogenics.)