As you may have observed, water condenses on cold water mains or windows and ice forms on the evaporator unit in your refrigerator. This effect of condensation of gases and vapors on cold surfaces - water vapor in particular - as it is known in everyday life, occurs not only at atmospheric pressure but also in vacuum.
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This effect has been utilized for a long time in condensers mainly in connection with chemical processes; previously the baffle on diffusion pumps used to be cooled with refrigerating machines. Also in a sealed space (vacuum chamber) the formation of condensate on a cold surface means that a large number of gas molecules are removed from the volume: they remain located on the cold surface and do not take part any longer in the hectic gas atmosphere within the vacuum chamber. We then say that the particles have been pumped and talk of cryopumps when the “pumping effect” is attained by means of cold surfaces.
Cryo engineering differs from refrigeration engineering in that the temperatures involved in cryo engineering are in the range below 120 K (< -243.4°F / -153°C). Here we are dealing with two questions:
a) What cooling principle is used in cryo engineering or in cryopumps and how is the thermal load of the cold surface lead away or reduced?
b) What are the operating principles of the cryopumps?
In continuous flow cryopumps the cold surface is designed to operate as a heat exchanger. Liquid helium in sufficient quantity is pumped by an auxiliary pump from a reservoir into the evaporator in order to attain a sufficiently low temperature at the cold surface (cryopanel).
The liquid helium evaporates in the heat exchanger and thus cools down the cryopanel. The waste gas which is generated (He) is used in a second heat exchanger to cool the baffle of a thermal radiation shield which protects the system from thermal radiation coming from the outside. The cold helium exhaust gas ejected by the helium pump is supplied to a helium recovery unit. The temperature at the cryopanels can be controlled by controlling the helium flow.
Today refrigerator cryopumps are being used almost exclusively (cold upon demand). These pumps operate basically much in the same way as a common household refrigerator, whereby the following thermodynamic cycles using helium as the refrigerant may be employed:
The Gifford-McMahon process is mostly used today and this process is that which has been developed furthest. It offers the possibility of separating the locations for the large compressor unit and the expansion unit in which the refrigeration process takes place. Thus, a compact and low vibration cold source can be designed. The cryopumps series manufactured by Leybold operate with two-stage cold heads according to the Gifford-McMahon process which is discussed in detail in the following.
The entire scope of a refrigerator cryopump is shown in Fig. 2.65 and consists of the compressor unit (1) which is linked via flexible pressure lines (2) – and thus vibration-free – to the cryopump (3). The cryopump itself consists of the pump casing and the cold head within. Helium is used as the refrigerant which circulates in a closed cycle with the aid of the compressor.
Phase 1:
The displacer is at the left dead center; V2 where the cold is produced has its minimum size. Valve N remains closed, H is opened. Gas at the pressure pH flows through the regenerator into V2. There the gas warms up by the pressure increase in V1.
Phase 2:
Valve H remains open, valve N closed: the displacer moves to the right and ejects the gas from V1 through the regenerator to V2 where it cools down at the cold regenerator.; V2 has its maximum volume.
Phase 3:
Valve H is closed and the valve N to the low pressure reservoir is opened. The gas expands from pH to pN and thereby cools down. This removes heat from the vicinity and it is transported with the expanding gas to the compressor.
Phase 4:
With valve N open the displacer moves to the left; the gas from V2,max flows through the regenerator, cooling it down and then flows into the volume V1 and into the low pressure reservoir. This completes the cycle.
The series manufactured refrigerator cryopumps from Leybold use a two-stage cold head operating according to the Gifford-McMahon principle (see Fig. 2.67). In two series connected stages the temperature of the helium is reduced to about 30 K in the first stage and further to about 10 K in the second stage. The attainable low temperatures depend among other things on the type of regenerator. Commonly copperbronze is used in the regenerator of the first stage and lead in the second stage. Other materials are available as regenerators for special applications like cryostats for extremely low temperatures (T < 10 K). The design of a two-stage cold head is shown schematically in Fig. 2.67. By means of a control mechanism with a motor driven control valve (18) with control disk (17) and control holes first the pressure in the control volume (16) is changed which causes the displacers (6) of the first stage and the second stage (11) to move; immediately thereafter the pressure in the entire volume of the cylinder is equalized by the control mechanism. The cold head is linked via flexible pressure lines to the compressor.
Fig. 2.68 shows the design of a cryopump. It is cooled by a two-stage cold head. The thermal radiation shield (5) with the baffle (6) is closely linked thermally to the first stage (9) of the cold head. For pressures below 10-3 mbar the thermal load is caused mostly by thermal radiation. For this reason the second stage (7) with the condensation and cryosorption panels (8) is surrounded by the thermal radiation shield (5) which is black on the inside and polished as well as nickel plated on the outside. Under no-load conditions the baffle and the thermal radiation shield (first stage) attain a temperature ranging between 50 to 80 K at the cryopanels and about 10 K at the second stage. The surface temperatures of these cryopanels are decisive to the actual pumping process. These surface temperatures depend on the refrigerating power supplied by the cold head, and the thermal conduction properties in the direction of the pump’s casing. During operation of the cryopump, loading caused by the gas and the heat of condensation results in further warming of the cryopanels. The surface temperature does not only depend on the temperature of the cryopanel, but also on the temperature of the gas which has already been frozen on to the cryopanel. The cryopanels (8) attached to the second stage (7) of the cold head are coated with activated charcoal on the inside in order to be able to pump gases which do not easily condense and which can only be pumped by cryosorption (see below).
The thermal conductivity of the condensed (solid) gases depends very much on their structure and thus on the way in which the condensate is produced. Variations in thermal conductivity over several orders of magnitude are possible! As the condensate increases in thickness, thermal resistance and thus the surface temperature increase subsequently reducing the pumping speed. The maximum pumping speed of a newly regenerated pump is stated as its nominal pumping speed. The bonding process for the various gases in the cryopump is performed in three steps: first the mixture of different gases and vapors meets the baffle which is at a temperature of about 80 K. Here mostly H2O and CO2 are condensed. The remaining gases penetrate the baffle and impinge in the outside of the cryopanel of the second stage which is cooled to about 10 K. Here gases like N2, O2 or Ar will condense. Only H2, He and Ne will remain. These gases can not be pumped by the cryopanels and these pass after several impacts with the thermal radiation shield to the inside of these panels which are coated with an adsorbent (cryosorption panels) where they are bonded by cryosorption. Thus, for the purpose of considering a cryopump, the gases are divided into three groups depending at which temperatures within the cryopump their partial pressure drops below 10-9 mbar:
Cryocondensation is the physical and reversible bonding of gas molecules through Van der Waals forces on sufficiently cold surfaces of the same material. The bond energy is equal to the energy of vaporization of the solid gas bonded to the surface and thus decreases as the thickness of the condensate increases as does the vapor pressure. Cryosorption is the physical and reversible bonding of gas molecules through Van der Waals forces on sufficiently cold surfaces of other materials. The bond energy is equal to the heat of adsorption which is greater than the heat of vaporization. As soon as a monolayer has been formed, the following molecules impinge on a surface of the same kind (sorbent) and the process transforms into cryocondensation. The higher bond energy for cryocondensation prevents the further growth of the condensate layer thereby restricting the capacity for the adsorbed gases. However, the adsorbents used, like activated charcoal, silica gel, alumina gel and molecular sieve, have a porous structure with very large specific surface areas of about 106m2/kg. Cryotrapping is understood as the inclusion of a low boiling point gas which is difficult to pump such as hydrogen, in the matrix of a gas having a higher boiling point and which can be pumped easily such as Ar, CH4 or CO2. At the same temperature the condensate mixture has a saturation vapor pressure which is by several orders of magnitude lower than the pure condensate of the gas with the lower boiling point.
The gas molecules entering the pump produce the area related theoretical pumping speed according the equation 2.29a with T = 293 K. The different pumping speeds have been combined for three representative gases H2, N2 and H20 taken from each of the aforementioned groups. Since water vapor is pumped on the entire entry area of the cryopump, the pumping speed measured for water vapor corresponds almost exactly to the theoretical pumping speed calculated for the intake flange of the cryopump. N2 on the other hand must first overcome the baffle before it can be bonded on to the cryocondensation panel. Depending on the design of the baffle, 30 to 50 percent of all N2 molecules are reflected.
H2 arrives at the cryosorption panels after further collisions and thus cooling of the gas. In the case of optimally designed cryopanels and a good contact with the active charcoal up to 50 percent of the H2 which has overcome the baffle can be bonded. Due to the restrictions regarding access to the pumping surfaces and cooling of the gas by collisions with the walls inside the pump before the gas reaches the pumping surface, the measured pumping speed for these two gases amounts only to a fraction of the theoretical pumping speed. The part which is not pumped is reflected chiefly by the baffle. Moreover, the adsorption probability for H2 differs between the various adsorbents and is < 1, whereas the probabilities for the condensation of water vapor and N2 ≈ 1.
Three differing capacities of a pump for the gases which can be pumped result from the size of the three surfaces (baffle, condensation surface at the outside of the second stage and sorption surface at the inside of the second stage). In the design of a cryopump, a mean gas composition (air) is assumed which naturally does not apply to all vacuum processes (sputtering processes, for example. See “Partial Regeneration,” below.)
The crossover value is a characteristic quantity of an already cold refrigerator cryopump. It is of significance when the pump is connected to a vacuum chamber via an HV / UHV valve. The crossover value is that quantity of gas with respect to Tn=293 K which the vacuum chamber may maximally contain so that the temperature of the cryopanels does not increase above 20 K due to the gas burst when opening the valve. The crossover value is usually stated as a pV value in in mbar · l.
The crossover value and the chamber volume V result in the crossover pressure pc to which the vacuum chamber must be evacuated first before opening the valve leading to the cryopump. The following may serve as a guide:
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The capacity of a cryopump for a certain gas is that quantity of gas (pV value at Tn = 293 K) which can be bonded by the cryopanels before the pumping speed for this type of gas G drops to below 50 % of its initial value.
The capacity for gases which are pumped by means of cryosorption depends on the quantity and properties of the sorption agent; it is pressure dependent and generally by several orders of magnitude lower compared to the pressure independent capacity for gases which are pumped by means of cryocondensation.
The refrigerating power of a refrigeration source at a temperature T gives the amount of heat that can be extracted by the refrigerating source whilst still main taining this temperature. In the case of refrigerators it has been agreed to state for single-stage cold heads the refrigerating power at 80 K and for two-stage cold heads the refrigerating power for the first stage at 80 K and for the second stage at 20 K when simultaneously loading both stages thermally. During the measurement of refrigerating power the thermal load is generated by electric heaters. The refrigerating power is greatest at room temperature and is lowest (Zero) at ultimate temperature.
Since the limitation in the service life of a cryopump depends in most applications on the capacity limit for the gases nitrogen, argon and hydrogen pumped by the second stage, it will often be required to regenerate only this stage. Water vapor is retained during partial regeneration by the baffle. For this, the temperature of the first stage must be maintained below 140 K or otherwise the partial pressure of the water vapor would become so high that water molecules would contaminate the adsorbent on the second stage.
In , Leybold was the first manufacturer of cryopumps to develop a method permitting such a partial regeneration. This fast regeneration process is microprocessor controlled and permits a partial regeneration of the cryopump in about 40 minutes compared to 6 hours needed for a total regeneration based on the purge gas method. A comparison between the typical cycles for total and partial regeneration is shown in Fig. 2.70. The time saved by the Fast Regeneration System is apparent. In a production environment for typical sputtering processes one will have to expect one total regeneration after 24 partial regenerations.
The throughput of a cryopump for a certain gas depends on the pV flow of the gas G through the intake opening of the pump:
QG = qpV,G; the following equation applies
QG = pG · SG with
pG = intake pressure,
SG = pumping capacity for the gas G
The maximum pV flow at which the cryopanels are warmed up to T ≈ 20 K in the case of continuous operation, depends on the net refrigerating power of the pump at this temperature and the type of gas. For refrigerator cryopumps and condensable gases the following may be taken as a guide:
TG - Gas temperature in K
M - Molar mass
Given in Table 2.7 is the surface area related pumping speed SA in l · s-1 · cm-2 for some gases at two different gas temperatures TG in K determined according to equation 2.29a. The values stated in the Table are limit values. In practice the condition of an almost undisturbed equilibrium (small cryopanels compared to a large wall surface) is often not true, because large cryopanels are required to attain short pumpdown times and a good end vacuum. Deviations also result when the cryopanels are surrounded by a cooled baffle at which the velocity of the penetrating molecules is already reduced by cooling.
Basically, it is possible to start a cryopump at atmospheric pressure. However, this is not desirable for several reasons. As long as the mean free path of the gas molecules is smaller than the dimensions of the vacuum chamber (p > 10-3 mbar), thermal conductivity of the gas is so high that an unacceptably large amount of heat is transferred to the cryopanels. Further, a relatively thick layer of condensate would form on the cryopanel during starting. This would markedly reduce the capacity of the cryopump available to the actual operating phase. Gas (usually air) would be bonded to the adsorbent, since the bonding energy for this is lower than that for the condensation surfaces. This would further reduce the already limited capacity for hydrogen. It is recommended that cryopumps in the high vacuum or ultrahigh vacuum range are started with the aid of a backing pump at pressures of p < 5 · 10-2 mbar. As soon as the starting pressure has been attained the backing pump may be switched off.
1.What is a cryopump?
A cryopump is vacuum pump that traps gases and vapors by condensing them on a cold surface. For efficient evacuation under ultra-high vacuum, the vapor pressure for condensation, or the equilibrium pressure for adsorption must be less than 10-8Pa. Figure 1 shows vapor pressures of different gases. According to this figure, if the cryocooled surface such as cryo-surface and cryopanel is cooled below 20K, the vapor pressure of the gas becomes below 10-8Pa, provided the vapor pressure is lower than that of nitrogen. The lightest gases such as hydrogen, helium, and neon are not condensed at 20K, therefore instead of relying on condensation alone, adsorbent made of special porous materials are provided to adsorb them. By cooling down the adsorbent below 20K, those gases are adsorbed efficiently, and thereby a cryopump can achieve ultra-high vacuum.
There are two ways to “cryocool” the surface of the cryopump. One is a use of coolant such as liquid nitrogen(LN2, 77K) or liquid helium(LHe, 4.2K), and the other is a small closed cycle helium refrigerator.
Figure 1:Vapor Pressures of Different Gases
A compact, closed cycle helium refrigerator can perform a long period stable operation without refilling of coolant. Also it can achieve clean and ultra-high vacuum with simple and straightforward operation.
2.Basic Principle and Structure
Let’s take a look at CRYO-U8H as an example. The refrigerator for CRYO-U cryopumps normally has two-stages. While the 1st stage has a large refrigerating capacity down to below 80K, the refrigerating capacity of the 2nd stage is small but cools down to 10 to 12K. Both 15K cryopanel(1) and 15K cryopanel(2) are mounted on the 2nd stage of the refrigerator, and shielded from the temperature radiation by the 80K shield and 80K baffle mounted on the 1st stage.
Figure2.CRYO-U8H
There are three types of gases pumped by cryopumps as follows.
(1)Air(N2、O2) ：The residual gases after rough pumping the vacuum chamber. (2)Outgas 1 H2O ：Adsorbed to the chamber wall.(The largest component in general vacuum system.)Major component of outgas from glass, plastic, and ceramic. 2 H2 ：Diffuses from inside of the metallic wall of the vacuum chamber.（Concerns in ultra-high or extreme high vacuum.）Outgas from high-temperature melted metal (especially Al) in deposition or sputtering process. 3 CO、CO2、As you see in Fig.1, the vapor pressure of water vapor becomes below 10-8 Pa at temperature of below 130K, and thus water vapor is pumped as a result of condensation on the 80K baffle and 80K shield. The next group of gases, nitrogen, oxygen, argon, and other gases of similar molecular weight are pumped at the exposed surface of 15K cryopanel(1) at the second stage, which is kept below 20K. The third group, mainly hydrogen, helium, and neon will not condense at 20K and will not be pumped efficiently by the metal surface because the equilibrium vapor pressure for cryosorption will be too high. To improve this situation, cryopumps have adsorbent of porous materials such as charcoal on the 2nd stage cryoarrays. The adsorbent is bonded to the inner surface of the 15K cryopanel(1) to prevent it from being covered with condensable gases.
Outside surface of 80K shield, 80K baffle, and 15K cryopanel(1) are specular finished in order to reflect radiant heat from room temperature. The inner surface of 80K shield is blackened to reduce the radiation heat transfer to the 15K cryopanel attached to the 2nd stage. In order that a cryopump to operate properly, both 80K shield and 80K baffle should be kept below 130K and 15K cryopanel to be kept below 20K.
In order to monitor those temperature, K(CA) thermocouple for 80K shield, and hydrogen vapor pressure gauge(H2VP) or CRYO METER MBS for 15K cryopanel are installed to the cryopumps.
(The standard for the electromotive force at 130K of K(CA) thermocouple is –5.5mV.)
3.Regeneration and Pressure Relief Valve
Cryopumps are not continuous throughout pumps such as oil diffusion pumps and turbo molecular pumps. As a cryopump keeps gases inside on 15K cryopanels by condensation and adsorption, it needs to be degassed and regenerated on a regular basis. During this regeneration process, the cryopump is warmed up, and condensed or adsorbed gases are turned into gas again. If large amount of gases are pumped, there is a risk of explosion. In order to prevent the explosion danger, all cryopumps feature a pressure relief valve.
The operating pressure of the pressure relief valve has been set at 20kPa(gage).
For safety reasons, DO NOT BLOCK the pressure relief valve, and DO NOT MODIFY it for other purposes. Also never use it as a purge valve in a regeneration process because refuse in purge gas may stick to the sheet of the pressure relief valve and may cause a leakage.
4.Cryopump System
Cryopump system consists of
《1》Cryopump Unit（incl. Cold Head）
《2》Compressor Unit
《3》Flexible Hose（2）
The connection of the cryopump system is shown in Figure 3.
In addition, rough pump (customer-supplied) is necessary to operate and regenerate cryopumps. (Cryopumps cannot start from the atmospheric pressure.)
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