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Part IIICryopumps

Johan E. de Rijke

Cryopumping is a means of creating a vacuum through the use of low temperatures. It occurs when gas moleculesstriking a surface lose enough of their incident kinetic energy to remain absorbed on the surface by so-called dispersionforces or van der Waals forces. Dispersion forces exist between any pair of molecules; and in the case of cryopumping,they are the important forces of attraction that hold a pumped molecule on a surface.

The amount of molecules that can be held on a surface is dependent on a number of physical factors: the temperature ofboth gas and surface, the chemical nature of gas and surface, the microscopic roughness of the surface, and the incidentflux of molecules. Typically, the dispersion forces existing between a surface and a gas molecule are greater than thosebetween the gas molecules themselves. We speak of cryosorption pumping when these larger forces are needed to holdmolecules on surface to the extent necessary to reach the desired vacuum level. Only several monolayers of gas can beaccrued on the surface before the effect of the surface becomes negligible and the pressure above the surface willincrease. We speak of cryocondensation pumping when the dispersion forces mutually existing between gas moleculesare sufficient to keep them on the surface to the degree necessary to maintain the desired pressure levels. In this case,typically a very large number of monolayers can be built up. The result is that much more gas can be accumulated thanin the case of cryosorption pumping.

The majority of cryopumps presently in use on vacuum systems for high- or ultrahigh-vacuum applications havepumping surfaces cooled by mechanical closed-loop refrigerators utilizing helium as a working fluid. The refrigerationcycle generally

Foundations of Vacuum Science and Technology, Edited by James M. Lafferty.ISBN 0-471-17593-5 1998John Wiley & Sons, Inc.


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used is the GiffordMcMahon cycle. This cycle employs two stages of refrigeration. The first refrigeration stagenormally operates between 50 K and 80 K, whereas the second stage operates between 12 K and 20 K. The temperatureof the second stage is low enough to pump all gases except neon, hydrogen, and helium by cryocondensation. Thesethree gases are pumped by cryosorption on a sorbent attached to this stage.

Cryopumping can also be used when clean rough pumping is required. In this case, liquid nitrogen is used to cool acanister of sorbent material. Gas is pumped by cryosorption, and pressures of 103 Pa can be reached.

At sufficiently low temperatures, almost all gas molecules incident on a surface are captured and the speed of acryopump will approach theoretical limitsthat is, limits imposed by molecular velocities. Therefore, cryopumps canhave large pumping speeds as compared to other pumping mechanisms. This is especially the case for water vaporbecause it is pumped at easily achievable, relatively high temperatures. Also, cryopumping is a clean pumping method;that is, no fluids internal to the vacuum envelope are used in cryopumps. Therefore, cryopumps will be found inapplications where clean vacuum production and high water-vapor pumping speed is needed.

Because cryopumps are capture pumps, regeneration or the periodic removal of accumulated gases is required. Inprinciple, this is a simple process, consisting of warming all pumping surfaces to room temperature and allowing gasesto escape through a valve mounted on the pumpbody. Then the pump is evacuated to sufficiently low pressures to createan insulating vacuum between the pumpbody and the pumping surfaces, after which the refrigerator is turned on to coolthe pump to operating temperatures. Correct regeneration is key to maintaining optimum cryopump performance.Regeneration is typically performed by automatic controllers; and because the pump cannot be used duringregeneration, considerable effort has been expended in developing efficient and fast regeneration procedures.

5.12AdsorptionDesorption

The operating principle of cryopumps can be best explained by the theory of adsorptiondesorption. This theorydescribes (a) the interactions between gas atoms and/or molecules and a surface and (b) the resulting balance betweenadsorption and desorption. For the first monolayer building up on a surface, gas molecules striking the surface arebound by the dispersion forces existing between gas and surface. These forces are generally larger than the dispersionforces that exist between the gas molecules themselves. This means that as monolayers of gas are built up on the surfaceand the effect of the gassurface interaction diminishes, the equilibrium between adsorption and desorption will change.The pressure above the adsorbed layer will increase as the magnitude of the dispersion forces decreases. Whenapproximately five monolayers have been built up, the effect of the dispersion forces from the surface will have becomenegligible. Molecules are bound only by the forces mutually interacting between them, and the pressure above acondensate will no longer increase. It will remain constant so long as the temperature of the outermost adsorbed layerdoes not increase. To summarize: As a layer of gas is built up on a surface, the pressure will increase until the effect ofthe surface has become negligible. Then the


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pressure will remain constant as long as the temperature remains constant. The relationship between pressure andamount of gas accumulated on a surface at a given constant temperature is defined as the adsorption isotherm.

To numerically describe the form that an adsorption isotherm will take, we look at molecules striking a (cold) surface.In general, a fraction of these molecules will be captured by that surface. This fraction (c) is called the capturecoefficientor the sticking coefficient. It is dependent on a number of physical factors: the temperature of both gas andsurface, the chemical nature of gas and surface, the microscopic roughness of the surface, and the incident flux ofmolecules. A quantitative value for the amount of molecules captured by a surface can be derived as shown below [180]:

If the number of molecules (per square centimeter per second) of a gas with pressure p (Pa) approaching a surface is n,then n can be expressed as

whereMis the molecular weight of the gas and Tis the temperature in Kelvin.

If a fraction c of the flux of molecules is captured, then nc molecules per square cm per second will be captured. For thetotal surface area, ncAp molecules will be pumped. The areaAp is the projected areathat is, the area of the cryopumpseen by the approaching molecules. It is not the microscopic surface area Am, which usually is many times greater.

After a certain time, a total ofNmolecules, uniformly distributed over the microscopic surfaceAm, will reside on thesurface. If the average time they remain on the surface is seconds, then the mean rate of departure from the surfacewill beN/(Am) molecules per square centimeter and per second.

From the above, the net pumping rate for the cryopump can be stated:

In Eq. (5.35), the first term represents adsorption whereas the second term represents desorption. In order to achieveeffective pumping, the first term needs to be much higher than the second. Initially, as gas starts to accumulate on thesurface,Nwill be zero and the second term of Eq. (5.35) will be zero. The first term, cnAp, then expresses themaximum pumping speed. An efficient cryopump will have a high value ofc. Sticking coefficients for many gases havebeen measured [180], and in many cases [181] they have been shown to have values approaching 1. For the case when cis one, pumping speeds as shown in Table 5.9 are achieved. These values can also be calculated from the KineticTheory of Gases.

The cryopump becomes inefficient and can be said to have reached its capacity when the total numberNof moleculesresiding on the surface causes the desorption term in Eq. (5.35) to becomes unacceptable. If the value ofc is known, thedesorption term can be determined at any stage of pumping by halting of the inflow of gas,


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Table 5.9. Maximum Theoretical Pumping Speeds for Cryosurfaces

Atomic MassUnits

Maximum Speed (liters1cm2)with Gas Temperature at:

Gas Species 295 K 77 K

H22 44.2 22.6

He4 31.2 16.0

H2O18 14.7 7.5

N2, CO28 11.8 6.0

O232 11.0 5.6

A40 9.9 5.1

CO244 9.4 4.8

causing dN/dtto become zero and then allowing the system to reach the equilibrium pressure:

In Eq. (5.36), is the concentration of molecules expressed in molecules per square centimeter. We define m as thenumber of molecules that form a monolayer, which has an approximate value of 1015 molecules per square centimeterfor adsorbates [180].

If the second term in Eq. (5.36) becomes too large while is less than 5 m, it means that only pumping bycryosorption is practical. For cryocondensation pumping, can become much larger than 5 m.

From the above, it can be seen that the general shape of an adsorption isotherm will show increasing pressure as the first

several monolayers are built up. Then as the effect of the surface becomes negligible, the pressure will reach amaximumnamely, the (saturation) vapor pressureand will no longer rise as the amount of gas accumulating increases.Figure 5.71 [182] shows typical adsorption isotherms showing the transition from less than monolayer adsorptionthrough multi-monolayer adsorption to cryocondensation.

Isotherms have been extensively studied, because cryosorption is often required in order to reach the necessary vacuumlevels. There are many adsorption isotherm configurations; as many as 13 different types have been categorized [183],and at resent no unified theoretical model exists which explains the shapes of various isotherms. Langmuir was one ofthe first who attempted to model isotherms in terms of gassurface physics. His work dealt with surface coverages of lessthan one monolayer. The equation that he developed for an isotherm takes the form [183]

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Fig. 5.71Adsorption isotherms of Xe, Kr, and Ar on a porous silver adsorbent

at a temperature of 77.4 K.

where kis a constant,p is the pressure, s is the number of sites per square centimeter, and m depicts the number ofsurface sites which are occupied.

Another important adsorption model was derived by Brunauer, Emmett, and Teller [184]. They expanded on Langmuir'stheory to include its applicability to gas coverages exceeding one monolayer and derived the following equation:

where kis a constant, m is the number of molecules in one monolayer, andpv is the vapor pressure. The above methodhas proved to be very successful in characterizing certain isotherm configurations and has become an industrial standardfor specifying surface areas of porous materials used in vacuum applications. In recognition of the authors, it is knownas the BET method for determining the areas of sieve materials.

One case of particular interest is cryosorption of hydrogen by charcoal. Hydrogen commonly occurs in vacuum systemsand is generated by many vacuum processes. Charcoal is the material most used in the two-stage, high-vacuum pumpcooled by a mechanical refrigerator. Figure 5.72 shows the adsorption of hydrogen on charcoal for various temperatures[185].

The pressures represented by the vertical portions of the isotherms shown in Fig. 5.71 represent the (saturation) vaporpressures of the indicated gases at 77.4 K. The vapor pressure is obviously a crucial value, because it represents thetheoretical ultimate pressure that can be achieved by cryocondensation for a given gas at a given temperature. The vaporpressure of a gas is derived from the ClausiusClapeyron equation and is usually presented in the form


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Fig. 5.72Adsorption of hydrogen on coconut charcoal at low pressures.

Table 5.10. Vapor Pressure of Common Gases as a Function of Temperature in K [187]

Vapor Pressure (Pa)

1011 109 107 105 103 101 10 103 105

Helium1.0 1.7 4.5

Hydrogen2.9 3.0 3.5 4.0 4.8 6.1 8.0 12 21

Neon5.5 6.1 6.9 7.9 9.2 11 14 18 28

Nitrogen18 20 22 25 29 34 42 54 80

Argon20 23 25 29 33 39 48 63 90

Carbon monoxide21 23 25 28 33 38 46 58 84

Oxygen22 24 27 30 34 40 48 63 93

Krypton28 31 35 39 46 54 66 86 124

Xenon39 43 48 54 63 74 92 119 170

Carbon dioxide 60 65 72 81 92 106 125 154 198

Water113 124 137 153 173 199 233 284 381

where the value forA is proportional to the sublimation enthalpy (Hs) and the factorB contains the entropy change associated with the phaseransition. The values ofA, B, and Chave been summarized by Haefer [186] for various solid gas condensates. Vapor pressure curves haveeen developed by Honig et al. [187] in one of the classical vacuum technology papers. Vapor pressures for some common gases are given in

Table 5.10.

.13Cryotrapping

Cryotrappingis defined as the concurrent or sequential cryopumping of two or more gases for the purpose of trapping a less readily pumped gasn the sorbate of a more

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Fig. 5.73

Hydrogen speed as a function of hydrogen accumulated on an argon sorbate at 12 K.(Courtesy of Ebara Technologies, Inc.)

readily pumped gas [183]. In other words, a gas such as hydrogen can condense on a surface of a sorbate such as argon,when hydrogen and argon are simultaneously introduced in a cryopumped system. Or hydrogen can be pumped onfreshly condensed argon sorbate. Figure 5.73 shows the normalized pumping speed for hydrogen as a function of theamount of hydrogen pumped for a pump used in physical vapor deposition applications where the standard second stagearray using charcoal as a sorbent has been replaced by a similar array without sorbent.

5.14Pumping Speed and Ultimate Pressure

In general, when calculations regarding gas flow, conductance, or pumping speed are performed, it is assumed that theall components of the vacuum system have the same temperature. This is clearly not the case when using a cryopumpedsystem. Usually it is not possible to directly observe pumping performance on a cold surface; instead, it has to bederived from measurements of pressure or throughput in a second chamber held at a different temperature. Therefore,we need to examine the flow of gas between two chambers held at different temperatures (Fig. 5.74). One chamber isheld at a temperature Tw, and the second is held at a much lower temperature Tc. The chambers are connected by anaperture with areaA. From the Kinetic Molecular Theory, the flow of particles from one chamber to the other can thenbe equated to


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Fig. 5.74Thermal transpiration.

Assume first that no pumping is taking place, so there is no net flow of gas from one chamber to the other and thepressure in both chambers is constant. Under these conditions, nw (the flow from the warm chamber to the coldchamber) will be equal to nc (the flow from the cold chamber to the warm chamber) and Eq. (5.40) can be reduced to

Equation (5.42) shows that even if there is no flow of gas between two chambers held at different temperatures, thepressures in the chambers will not be the same. This effect is called thermal transpiration (see Section 1.10).

To determine the effective speed at the entrance of the cryopump or, in the above case, the speed at the aperturebetween the warm chamber and the cold chamber, assume that there is a net flow of gas from the warm chamber to thecold chamber. From Eq. (5.40) the net flow of particles through the aperture, nnet, assuming a sticking coefficient ofunity, is

Equation (5.43) may be simplified by noting that the term preceding the brackets is nw.


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The maximum particle flow into the pump occurs when no gas flows back from the pump, ornc = 0. Then nw = nmaxand Eq. (5.43) can be written as

If we now define (from Eq. (5.42))

then Eq. (5.44) can be expressed as

wherepw(ult) is the pressure at the pump entrance when gas flow is halted. Equation (5.46) related the net flow ofparticles to the maximum flow of particles; in other words, it relates the net speed to the maximum speed. For acryocondensation pump,pc is the saturated vapor pressurepsat. Equation (5.45) shows thatpw(ult) will remain constantas long aspsat does not change and so will the net pumping speed at the pump entrance. For a cryosorption pump,pccan be obtained from the corresponding adsorption isotherm. Pressurepc will rise as the surface coverage increases.This means thatpw(ult) will also rise. As it approaches the operating pressurepw, the net pumping speed will decreaseand become zero whenpw(ult) reachespw. The pump can no longer accumulate gas at that pressure. However, from Eq.

(5.46) it can be seen that the pump will still retain 90% of its maximum speed if the operating pressure is raised by afactor of 10.

To summarize: For a cryocondensation pump, the ultimate pressure will not change as long as the temperature above theadsorbate does not change. Also, the speed will be near its maximum value as long aspsat is much smaller thanpw. Inthe case of a cryosorption pump, the ultimate pressure will rise as the equilibrium pressure over the sorbent increaseswith the amount of gas adsorbed. From Eq. (5.46) it follows that for cryosorption pumping, speed will decrease whenthe equilibrium pressure approaches the working pressurepw.

5.15Capacity

Cryopumps are capacity pumps, and thus only a finite amount of gas can be stored before the pump has to beregenerated. The need for regeneration is typically determined by the user and will be performed when pumpingperformance for a particular gas has degraded to such a point that it becomes unacceptable in the particular application.The degradation in two pumping characteristics is used to determine when regeneration is required: (a) the decrease inpumping speed that occurs as gas accumulates and/or (b) the increase in time needed to reach base pressure.


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In other words, capacity is defined as (a) the quantity of gas that can be stored on the arrays at a given pressure whilethe pump can still maintain a pumping speed igher or equal to a defined percentage (usually 50%) of its initial value atthat pressure or (b) the quantity of gas that can be stored with the pump maintaining the ability to reach a requiredpressure in a required time.

There is a third definition of capacity that also has to be consideredthat is, the capacity of the pump when a condensablegas and an adsorbable gas are being pumped simultaneously. The geometry of the pumping surfaces is designed so thata majority of gas molecules will strike the first stage array and/or the outside of the second stage before reaching thesorbent. Condensable gases will be removed, and only adsorbable gases will reach the sorbent. In essence, this shieldingis a compromise between preventing condensable gas from reaching the sorbent (see Fig. 5.75) and maintaining a highpumping speed for adsorbable gas. However, some condensable gas will reach the sorbent and occupy sites on it,thereby decreasing the ability of the sorbent to accumulate adsorbable gas.

In this case, capacity is defined as the quantity of condensable gas that can be stored at a given pressure whilemaintaining a pumping speed for the adsorbable gas at that pressure that is higher or equal to a defined percentage

(usually 50%) of its initial value after regeneration.

From the above it is obvious that the operating pressure of the pump is a key factor in determining capacity. From theadsorption isotherm and Eq. (5.46) it can be seen that the equilibrium pressure gradually rises as gas is adsorbed on asurface.

Fig. 5.75Cross section of cryopump used for high-

vacuum applications.

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A cryopump can have reached capacity at, for instance, 106 Pa, while still operating efficiently if only a pressure of 104Pa is needed.

5.16Refrigeration Technology

Most modern cryopumps in use today are cooled by a closed-loop mechanical refrigerator using helium as a workingfluid. The cryopump system consists of a compressor and an expander on which the arrays are mounted. Compressorand expander are connected by flexible hoses. The thermodynamic cycle generally used is based on a cycle developedby Gifford and McMahon [188] and by Longsworth. This cycle is used because it has proven to be simple and reliableand has a long service life, and the compressor can be remotely located from the expander and therefore the pump. Aschematic of a one-stage GiffordMcMahon (GM) refrigerator is shown in Fig. 5.76. General-purpose highvacuumcryopumps have two stages of refrigeration in order to achieve temperatures low enough for effective use.

The schematic shows that the GM machine consists of a cylinder, which contains a cylindrical piston called a displacer.

The displacer is connected to a drive mechanism, so it can be moved up and down in the cylinder. There are twovolumes, one above and one below the displacer. They are varied from maximum size to zero during the cycle, but thetotal volume remains constant. The two volumes are connected

Fig. 5.76Refrigerator schematic.


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through a thermal regenerator (inside the displacer) and to the inlet and exhaust valve. These valves are coupled to thedrive mechanism so their operation is synchronized to the position of the displacer. At the high-temperature side, thedisplacer is also equipped with a gas-tight sliding seal to prevent leakage from one side of the cylinder to the other andto ensure that helium flows through the regenerator. With two-stage machines the first-stage regenerator consists oftightly packed, high heat conductivity metal screens, the second stage regenerator is packed with a lead alloy shot.Essential is that the material of the regenerator has a high heat capacity at cryogenic temperatures. The screens and/orshot also have high surface area to volume ratios.

This construction means that the regenerator can efficiently transfer thermal energy between the screens and theincoming or outgoing helium. Also there will be very little pressure difference between the two volumes, whichminimizes the demand on the seals. Because the pressure is essentially the same in the spaces above and below thedisplacer, except for a small drop when helium is flowing through the regenerator, no work is done on the gas and thegas does no work on the displacer. So, essentially no work is required to move the displacer in the cylinder.

The operation of the refrigerator can best be understood by reviewing a cycle with the expander at operating

temperature (see also Fig. 5.76):

1.Pressure Rise. When the displacer is at the top (low temperature) end of the stroke, the exhaust valve is closed andthe inlet valve is opened. This increases the pressure from the exhaust pressureP1 to the inlet pressureP2. Helium willenter the inlet valve and fill the regenerator and the volume V1.

2.Intake. The inlet valve is kept open while the displacer is moved toward the bottom (high temperature) end of thestroke. This displaces helium from the volume V1 to V2. The helium is cooled while passing through the regenerator.This causes its pressure to decrease, and more helium will enter the system from the compressor.

3.Pressure Drop (Expansion). When the displacer has reached the bottom end of the stroke, the inlet valve is closedand then the exhaust valve opened. The helium will expand and the pressure will drop fromP2 toP1. This reduction inpressure causes a reduction in temperature. The decrease in temperature in V2 is the useful refrigeration of the cycle.

4.Exhaust. The exhaust valve is kept open while the displacer is moved toward the top (low temperature) end of thestroke. This displaces helium from the volume V2 to V1. The helium is heated while passing through the regenerator. Inturn, the helium cools the regenerator so as to refrigerate the helium passing through on the next pressure rise and intakestroke.

Two stages are required to achieve low enough temperatures to pump all gases except neon, helium, and hydrogen bycryocondensation. The added second stage is essentially providing two engines operating at two temperature levels.This multistaging is desirable because it provides a more efficient process in achieving cryogenic temperatures. Forinstance, multistaging relieves the operating regimen of the regenerators [188].


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We define a refrigeration loss Qr:

whereMis the mass flow of the gas, Cp is the heat capacity of the gas, and Tris the temperature difference betweenthe gas entering and leaving the regenerator at the low temperature end.

Assume that the temperature of the gas decreases by Te when expanding. Te must be greater than Trin order toachieve useful refrigeration. Te is proportional to the absolute temperature Tat which the expansion occurs, andtypically it is equal to 0.3 T. In a two-stage (GM) machine, temperatures of 10 K can be reached. This means Te willbe 3 K. To achieve useful refrigeration and handle other losses, Trwould need to be no greater than 1 K. This isvirtually impossible in a single-stage machine operating between 300 K and 10 K because it would require heatexchanger efficiencies that cannot be reached in practice. However, a Trof 1 K can be achieved in a two-stage systemwhere the second stage operates between 50 K and 10 K.

The above briefly describes the principles of operation of the mechanical refrigerator used to cool cryopumps tooperating temperatures. They are known as closed-loop systems because the helium continually circulates between thecompressor and expander. So-called open-loop cryopumps are also used. In these, liquid nitrogen and liquid helium areused to cool surfaces to 77 K and 4.2 K, respectively. The geometry of the arrays is similar to those described above.The first stage consists of a bowlshaped structure and a frontal louver, both cooled by liquid nitrogen. The first stageforms a radiation shield for the second stage, which is cooled by liquid helium. The cryogens are fed into cooling coilsattached to the arrays, and this type of pump is therefore called a boiling pool cryopump. They are called open-loopcryopumps because the cryogen is allowed to escape into the atmosphere after use. Sometimes gas collection systemsare used in the case of rare gases in order to recycle them.

5.17

Pump Configuration

For mechanically refrigerated cryopumps, array design configurations are, out of necessity, based on the characteristicsof the refrigerator. Two stages are needed to achieve sufficiently low temperatures to pump all gases except neon,hydrogen, and helium by cryocondensation. Because these light gases cannot be pumped by cryocondensation, acryosorbing surface of sufficient size for practical pumping performance must be provided on the arrays. The geometryof the arrays must be designed so that all gases pumped must impinge on a cold surface before reaching the sorbent.This will remove those gases being pumped by cryocondenstion before they reach the sorbent, whose full pumpingcapacity will be used for removing neon, hydrogen, and helium.

The efficiency of the refrigeration cycle decreases rapidly as temperatures approach absolute zero. Therefore, it isimportant that the available refrigeration power for the older second stage array be used for condensing gas and is notdissipated through parasitic loads. A key design feature for the first-stage array, which operates at a much higher

temperature and which has much more refrigeration power available, is that its geometry is such that it shields thesecond stage from radiation heat loads.


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The above considerations lead to an array design as shown schematically in Fig. 5.76. The first-stage array has a can- orbowl-shaped configuration. Attached to the can is the frontal array which for pumps used in high vacuum applicationsconsists of a louver. The design has to be a compromise of providing efficient radiation shielding for the second stage,while providing as high a conductance as possible for gases that will be pumped on that stage. The second-stage usuallyconsists of a set of cones as shown in Fig. 5.76. A sorbent, usually charcoal, is attached to the underside of the cones.The second-stage design is also a compromise between ensuring efficient shielding for the sorbent and still providinghigh conductance for neon, hydrogen, and helium, along with high pumping speeds.

Indium gaskets are used in connections between the frontal array and the first-stage can, as well as in connectionsbetween the arrays and the refrigerator, so that high thermal conductance is provided over the joints. Charcoal is themost commonly used sorbent because water vapor can be removed from it at room temperature. It has a greater capacitythan man-made molecular sieve material and is less affected by impurities. Molecular sieve has to be heated to 250C inorder to remove water vapor. This temperature would melt the indium gaskets and would damage internal refrigeratorcomponents, such as the seals on the displacer.

The temperature of the arrays is determined by the total heat load imposed on the pumping surfaces. The loads imposedare from radiation, convection, and condensation. As discussed below, loads from convection and condensation can beignored when the pump is operating at a pressure below 101 Pa. The main load under these conditions are radiationfrom the pumpbody and from the vacuum chamber. Additional radiation loads can be imposed by sources inside thevacuum chamber, such as bake-out heaters, plasmas, and so on.

An estimate of the radiant heat flow on to the arrays can be derived from calculating the heat flow ( Q) between twoconcentric cylinders at different temperatures [189]:

where the subscripts c and w refer to the inner, colder cylinder and the outer, warmer cylinder, respectively, and where

= StefanBoltzmann constant,

e = coefficients of emissivity,

A = surface areas,

T = temperatures in Kelvin.

If both emissivity coefficients ew and ec are equal to their maximum value 1, this reduces to


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For a cryopump with an inlet diameter of 200 mm, the area of the first stage is approximately 0.130.15 m2. If thepumpbody and vacuum chamber are at 295 K and the average first-stage temperature is 60 K, this means that themaximum heat load the first-stage would have to absorb would be approximately 60 W. This heat load is usually muchlarger than the first-stage refrigeration power of this size pump. It is clear that the radiant heat load must be reduced.This is done by reducing the emissivity of the internal pumpbody surface and the outside of the first stage can to levelsbelow 0.1 through electropolishing or nickel-plating. This treatment will reduce the heat load to a value of 12 W.

Many gases, specifically water vapor, will have a high emissivity (0.80.9) when condensed. This means that theemissivity of the first stage will rapidly increase in applications where water vapor is present, which means mostapplications! In order to minimize this effect, the diameter of the first stage is designed to be only slightly maller thanthat of the pumpbody, so that only a minimal amount of water vapor will condense on the outside of the first-stage canand its emissivity will not be affected significantly while pumping for long periods.

Radiation from the first-stage to the second-stage does not play a significant role. For a 200-mm-diameter pump, thesurface area of the second stage is approximately 0.07 m2. Assuming an average first-stage temperature of 60 K and a

second-stage temperature of 12 K, from Eq. (5.53) it can be calculated that the maximum heat load imposed is less than1 W. In many cryopumps the inside of the first stage is purposely made to have high emissivity. This will adsorb strayradiation entering through the frontal array, minimizing the heat load on the second stage.

As mentioned above, convection heat loads can be ignored when the pump is operating at a pressure below 101 Pa. Thepump then will be operating in the molecular flow regime, and the mean free path of the molecules will be significantlygreater than the distance between the second stage array and the pump body. A high degree of insulation between thepumpbody and the arrays is provided under these conditions. Convection loads become significant at pressures above101 Pa, certainly if a major constituent of the gas pumped is hydrogen.

Also, the heat load imposed by condensation is negligible at high-vacuum operation. For instance, the enthalpy fornitrogen is 15,580 J/gmol at room temperature and 134 J/gmol at 20 K. A total amount of 15,446 J must be removedby the refrigerator when condensing 1 mol of nitrogen. Some of this heat will be removed when the nitrogen strikes the

first stage. Ignoring this effect and assuming a worst case where all heat is removed by the second stage, calculationsshow that a second stage in a pump operating at 104 Pa with a nitrogen speed of 1500 liters1 will adsorb a heat load ofapproximately 1 mW. The obverse is that the heat of condensation becomes significant when the pump operates atpressures above 101 Pa (for example, when it is used in physical vapor deposition applications).

Pumps used in physical vapor deposition applications have a different first-stage can and frontal array design than thoseused in high-vacuum applications (see Fig. 5.77). In the first place, the pumpbody and first-stage can are longer thanthose of a high-vacuum pump, increasing the distance between first and second stage. This allows more gas, typicallyargon, to be accumulated before capacity is reached. In the second place, a barrier made of an insulating material isplaced between the pumpbody and first-stage can at the pump entrance. This minimizes the amount of gas entering thespace between the first stage and the pumpbody. In addition,


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Fig. 5.77

Schematic of cryopump used in physical vapor deposition.

openings are made in the can in the area where it attaches to the refrigerator, so that any gas entering the space will bepumped away. This geometry results in a pressure differential over the barrier. This differential is such that when thepressure above the pump is on the order of 1 Pa, the pressure between body and can below the barrier averages apressure below 101 Pa, effectively removing the convection heat load.

Finally, the conductance to the second stage is deliberately reduced by changing the geometry of the frontal array.Typically, the frontal array consists of a disk. The disk has a series of openings, sized so that the total conductance tothe second stage is on the order of several hundred liters per second. The conductance is calculated to result in theproper process gas flow at the required operating pressure. The reduction in gas flow through the use of a smallconductance results in a smaller heat load imposed on the second stage and in longer times to reach capacity. Full water-vapor pumping speed is maintained as the frontal array operates at temperatures below 100 K. So pumping performance

during pumpdown is not significantly affected.

One of the key parameters for a cryopump is the heat load that the pump can adsorb at the moment when the valveisolating the pump from the chamber is opened during pumpdown. The load imposed is called the impulsive heatload. Ifthe amount of gas impinging on the pump is too large, the arrays will warm up to the point where previouslyaccumulated gas will evolve from the arrays to the extent that a runaway condition occurs. The pressure will become sohigh that the resulting convection load will overwhelm the refrigerator. Hydrogen is the most critical gas in this respectin that it evolves at lower temperatures than other gases and causes a higher convection load at a given pressure. It isrecommended that the impulsive load imposed on the pump be limited to the amount that will allow the second stage toremain at or below


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20 K. At this temperature the hydrogen capacity is approximately 50% of the capacity existing when the sorbent is at 12K, and the amount of hydrogen desorbing will be limited. If no hydrogen has been accumulated, the pump will stillrecover when the second-stage array temperature is raised to approximately 30 K.

5.18Regeneration

Cryopump operation entails regenerating the pump periodically. Proper regeneration is key to optimum performance ofthe pump. If not done correctly, pump performance can be significantly degraded.

When the pump is operated at high vacuum, hydrogen usually will be the first gas for which capacity is reached. Whenthis happens, pumpdown times will become longer and the ultimate pressure will become higher. When the pump isused in sputtering applications, argon capacity will be reached first. The pump will apparently behave normally whenargon is flowing; but when argon flow is halted, large pressure bursts will be seen during pumpdown. This is due to thefact that the amount of argon accumulated on the second stage has become so large that the sorbate surface is no longer

adequately shielded from radiation by the first stage. Argon will sublimate irratically as it is exposed to roomtemperature radiation.

The need for regeneration is best determined experimentally. First, measure the time that the pump can operate in agiven application before behavior as described above appears. Regeneration should then be performed at approximately60% of this time limit to ensure optimum performance.

Essentially, regeneration is a simple process. The pump is warmed to room temperature. During warm-up, gas willescape through the safety pressure relief valve attached to the pumpbody. The pump is then roughed to a pressurebetween 5 and 10 Pa in order to establish an insulating vacuum inside the pump. Then the refrigerator is turned on andthe pump cooled to operating temperature.

There are several important considerations to be observed. Large amounts of sorbate might be accumulated on thesecond stage when regeneration is started. The second stage is shielded from convection and radiation loads. It can takeseveral hours before it warms up to a temperature at which significant amounts of gas evolve and the insulating vacuumis broken. Therefore, one of the first steps usually taken when regenerating the pump is to raise the internal pressure toatmospheric by purging it with a dry inert gas, typically nitrogen. The purge gas is usually heated to shorten the timeneeded for the arrays to reach room temperature.

The second, most important consideration is to ensure that accumulated water vapor will be adequately removed duringregeneration. Purging with dry nitrogen during pump warm-up will assist in sublimating water vapor from the firststage. Once a temperature of 273 K has been reached, the residual water vapor will liquefy. This means it can be easilytransferred to the second-stage sorbent. The capacity of the sorbent for pumping hydrogen will be decreased if all watervapor is not removed before subsequent pump chilldown. It is therefore necessary to determine that this residual watervapor has been removed before operating the pump. This is done empirically by purging the pump for an extended time

with dry gas and then performing a test to determine the amount of water evolving from the sorbent by measuring theincrease in pressure with time at the end of roughing. Because it is


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uncertain how much water has been accumulated since the previous regeneration, this method can lead to errors. Amore accurate method is to attach a hygrometer to the pressure relief valve and measure the dewpoint of the escapinggas during the purge. Dewpoint levels that do not affect hydrogen pumping at subsequent operation can be measuredduring each regeneration, and more repeatable performance can be attained.

5.19Partial Regeneration

Regenerating the pump to remove all gas as outlined above is calledfull regeneration. The pump has to be warmed toroom temperature, roughed, and chilled to operating temperatures. Another regeneration method, calledpartialregeneration, has been developed. In this process the arrays are only warmed to temperatures between 120 K and 180K, so that only gases accumulated in the second stage are removed. This process can be used in applications whereeither hydrogen or argon capacity will be reached long before enough water vapor has accumulated to affectperformance. The process can be accomplished in approximately 45 min, instead of several hours as with fullregeneration (see Fig. 5.78).

There are potential difficulties with partial regeneration. To complete the process in 4560 min, the accumulated gas hasto be removed in less than 15 min. In many applications, especially physical vapor deposition, large amounts of gas caneasily be accumulated in the second stage. The rate of gas removal needs to be high in order to remove it rapidly. Thismeans that the pressure in the pump will be high, essentially atmospheric pressure or higher. Conditions of viscous flowwill exist for several minutes. Large amounts of condensable gas (argon, nitrogen, oxygen, etc.) will be able to reach thesorbent and will be partially condensed and/or sorbed. The amount of gas accumulating on the sorbent will depend onthe type of gas, its pressure, and the temperature of the sorbent.

For optimum results, after array warm-up and gas removal at atmospheric pressures through a one-way valve, the pumpneeds to be roughed to a low pressure while the sorbent is at a relatively high temperature (> 120 K). This will removeany amount of condensable gas (nitrogen, argon) remaining on the sorbent before the array is subsequently cooleddown. Also, first- and/or second-stage array temperature should not exceed 180 K during the evaporation and roughing

process. This will exclude the possibility of water vapor sublimating off the first stage and reaching the sorbent.

The efficacy of partial regeneration on a pump can be checked by measuring the hydrogen pumping speed as a functionof the amount of hydrogen accumulated after a partial regeneration has been performed. That data can then be comparedto hydrogen pumping performance after a full regeneration, because full regeneration is the standard method by whichcryopumps are restored to their original performance.

5.20Sorption Roughing Pumps

Sorption roughing pumps or sorption pumps are used for pumping systems from atmospheric pressure to a pressure ofapproximately 101 Pa. They rely on the


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Fig. 5.78(a) Full-regeneration timetemperature cycle (200 mm diameter pump).

(Courtesy of Ebara Technologies, Inc.) (b) Partial-regeneration timetemperature cycle(200 mm diameter pump). (Courtesy of Ebara Technologies, Inc.)

dispersion forces existing between a gas and a surface to bind gas molecules on chilled surfaces inside the pump. Inother words, they pump by cryosorption.

Sorption pumps typically consist of a cylindrical canister that is filled with an absorbent (see Fig. 5.79). The adsorbentis usually molecular sieve material, or zeolite, which consists of pellets made of a calcium or a sodium aluminosilicatecrystalline matrix [183]. The canister is placed in a dewar cooled by liquid nitrogen. Zeolite is a poor heat conductor, so

an array of aluminum fins inside the pump is used to improve thermal contact with the sieve material.

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Sorption pumps need liquid nitrogen to operate; and, as with any capture pump, they have to be periodicallyregenerated. Therefore in present-day high-throughput applications, they have been replaced by dry mechanicalroughing pumps. However,


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Fig. 5.79Cross section of sorption roughing pump(Courtesy of Varian Vacuum Products).

sorption pumps are very clean noncontaminating roughing pumps and are mostly used in low-throughput applicationswhere this feature is of prime concern. They are used in conjunction with getter pumps, ion pumps, or mechanicalcryopumps.

In a sorption pump, molecules are held on the adsorbent surface by physisorption. The number of molecules that can beheld on an adsorbent is dependent on the temperature of both gas and surface, the chemical nature of gas and surface,the microscopic roughness of the surface, and the incident flux of molecules. There is a constant interchange betweenmolecules residing on the surface and molecules arriving from the gas phase. The key is to have equilibrium conditionssuch that practical amounts of gas can be captured at the desired pressures.

For nitrogen, the major gas load when air is pumped, the dwell time of a molecule on a surface at room temperature isapproximately 5 1011 s [183]. At a pressure of 102 Pa, about 2 106 molecules per square centimeter can beadsorbed. Considering that a monolayer of gas on a surface consists of 10141015 molecules per square centimeter, itfollows that a negligible amount of nitrogen will be pumped.

At liquid nitrogen temperatures, the dwell time will have increased to 8 103 s and the amount of nitrogen residing onthe surface at a pressure of 102 Pa is approximately 3 1014 molecules per square centimeter or half a monolayer. Byproviding large surface areas, practical amounts of nitrogen can be pumped (see Fig. 5.80).

As coverage increases to above half a monolayer, the effect of the surface is rapidly lost and the equilibrium pressurewill quickly rise to the saturation vapor pressure, which for nitrogen by definition is atmospheric pressure.

A cross section of a sorption pump is shown in Fig. 5.79. Key elements of the pump are the aluminum body, the array offins removing heat from the zeolite, and the pressure relief valve. The adsorbent used is usually Linde 5A molecularsieve. This material, with an internal pore diameter of 11 nm [189], has a high affinity for nitrogen and oxygen.

Zeolite also has a very high affinity for water vapor. Water vapor accumulated when repeatedly pumping down achamber filled with ambient air will eventually


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Fig. 5.80Adsorption isotherms for nitrogen, hydrogen, neon, andhelium for a liquid-nitrogen-cooled sorption roughing

pump with a 1.35-kg zeolite charge.

saturate the sieve material, eliminating its capacity for adsorbing nitrogen and oxygen. The pump must then be bakedout to 250C or higher to remove the water. The sorption pump therefore usually comes equipped with a bake-outheater. Normally, during operation of the pump, the heater is also immersed in liquid nitrogen.

Figure 5.80 shows the adsorption isotherms for nitrogen, hydrogen, neon, and helium for a pump as shown in Fig. 5.79.This pump has a charge of 1.35 kg of molecular sieve, which can pump approximately 107 Paliters of nitrogen at a

pressure of 101 Pa. Figure 5.80 shows that noble gases such as neon and helium are pumped poorly. If, for instance,neon is pumped together with air, its capacity will be less than that shown in Fig. 5.80 because the neon will be replacedby the active air gases, starting at pressures below 103 Pa. For this reason, sorption pumps are quite often staged. Whentwo pumps are staged, one pump is used to achieve a pressure of 103 Pa and is then valved off. The second pump isthen valved in and the pressure is further reduced. By this method, 99% of the air is removed by the first pump, andnoble gases are also swept into this pump. They cannot backstream into the system when pressure is further reduced.Figure 5.81 shows a pumpdown curve for a 200-liter chamber being pumped by three-staged sorption pumps.

It is not useful to characterize sorption pumps by their pumping speed due to their batch nature [190]. However, it canbe shown (Fig. 5.81) that by using recommended sequencing and sizing, speeds approaching 300 liters per minute canbe obtained.


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Fig. 5.81Pumpdown for staged sorption pumps.

A key safety element of the sorption pump is the pressure relief valve. When the pump is saturated with air and allowedto warm up to room temperature, very high pressures can be built up. The operation of this valve should never be

obstructed.

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References

1. W. A. Steel, The Interaction of Gases with Solid Surfaces. Pergamon, Oxford, 1974.

2. R. J. Madix, ed., Surface Reactions. Springer-Verlag, Berlin and New York, 1994.


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3. C. Kittel,Introduction to Solid State Physics. Wiley, New York, 1967.

4. B. Chalmers and R. King, eds.,Progress in Metal Physics. Vols. 1 and 2. Pergamon, Oxford, 1958.

5. C. O. Smith, The Science of Engineering Materials. Prentice-Hall, Englewood Cliffs, NJ, 1969.

6. J. D. Fast,Interaction of Metals and Gases, Vol. 1. Macmillan, New York, 1965.

7. J. D. Verhoeven,Fundamentals of Physical Metallurgy, Wiley, New York, 1975.

8. J. H. DeBoer, The Dynamical Character of Adsorption. Oxford University Press, Oxford, 1953.

9. D. M. Young and A. D. Crowell,Physical Adsorption of Gases. Butterworth, London, 1962.

10. D. O. Hayward and B. H. W. Trapnel, Chemisorption. Butterworth, London, 1964.

11. V. Ponec and Z. Knor,Adsorption on Solids. Butterworth, London, 1974.

12. Z. Knor, Chem. Listy 59, 277 (1965).

13. B. M. W. Trapnel,Proc. R. Soc. London, Ser. A 218, 566 (1953).

14. E. Myazaki,J. Catal. 65, 84 (1980).

15. I. J. Langmuir,J. Am. Chem. Soc. 40, 1361 (1918).

16. H. Freundlich, Colloid and Capillary Chemistry. London, 1926.

17. S. Brunauer, K. S. Love, and R. G. Keenan,J. Am. Chem. Soc. 64. 751 (1942).

18. J. R. Anderson, Structure of Metallic Catalists. Academic Press, New York, 1975.

19. G. C. Bond, Catalysis by Metals. Academic Press, New York, 1962.

20. J. Crank, The Mathematics of Diffusion. Oxford University Press, Oxford, 1957.

21. W. Jost,Diffusion in Solid, Liquids, Gases. Academic Press, New York, 1960.

22. J. D. Fast,Interaction of Metals and Gases, Vol. 2. Macmillan, New York, 1971.

23. A. Sieverts,Z. Metallkd. 21, 37 (1929).

24.Handbook of Chemistry and Physics, 67th ed. CRC Press, Boca Raton, FL, 1987.

25. R. E. Honig and H. O. Hook,RCA Rev. 21, 360, 567 (1960).

26. P. della Porta et al.,J. Vac. Sci. Technol. 6(1), 40 (1969).

27. P. della Porta, Vac. Conf., 13th, Int. Conf. Solid Surf., 9th, Yokohama (1995).

28. J. H. N. van Vucht,Proc. Int. Vac. Congr., Como, p. 170 (1959).

29. E. A. Lederer and D. H. Wamsley,RCA Rev. 2, 117 (1937).

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