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.
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?
Depending on the cooling principle a difference is made between
In the case of bath cryostats – in the most simple case a cold trap filled with LN2 (liquid nitrogen) – the pumping surface is cooled by direct contact with a liquefied gas. On a surface cooled with LN2 (T ≈ 77 K) H2O and CO2 are able to condense. On a surface cooled to ≈ 10 K all gases except He and Ne may be pumped by way of condensation. A surface cooled with liquid helium (T ≈ 4.2 K) is capable of condensing all gases except helium.
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.
Within the cold head, a cylinder is divided into two working spaces V1 and V2 by a displacer. During operation the right space V1 is warm and the left space V2 is cold. At a displacer frequency f the refrigerating power W of the refrigerator is: (2.26)
The displacer is moved to and fro pneumatically so that the gas is forced through the displacer and thus through the regenerator located inside the displacer. The regenerator is a heat accumulator having a large heat exchanging surface and capacity, which operates as a heat exchanger within the cycle. Outlined in Fig. 2.66 are the four phases of refrigeration in a single-stage refrigerator cold head operating according to the Gifford-McMahon principle.
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.
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.
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.
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:
A difference is made between the different bonding mechanisms as follows:
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.
Considering the position of the cryopanels in the cryopump, the conductance from the vacuum flange to this surface and also the subtractive pumping sequence (what has already condensed at the baffle cannot arrive at the second stage and consume capacity there), the situation arises as shown in Fig. 2.69.
Hydrogen - Water vapor - Nitrogen
Area related conductance of the intake flange in l / s · cm2:
43.9 - 14.7 - 11.8
Area related pumping speed of the cryopump in l / s · cm2:
13.2 - 14.6 - 7.1
Ratio between pumping speed and conductance:
30 % - 99 % - 60 %
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 characteristic quantities of a cryopump are as follows (in no particular order):
The cooldown time of cryopumps is the time span from start-up until the pumping effect sets in. In the case of refrigerator cryopumps the cooldown time is stated as the time it takes for the second stage of the cold head to cool down from 293 K to 20 K.
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:
V = Volume of the vacuum chamber (l),
Q2(20K) = Net refrigerating capacity in Watts, available at the second stage of the cold head at 20 K.
For the case of cryocondensation (see “Bonding of gases to cold surfaces,” above) the ultimate pressure can be calculated by:
pS is the saturation vapor pressure of the gas or gases which are to be pumped at the temperature TK of the cryopanel and TG is the gas temperature (wall temperature in the vicinity of the cryopanel).
Example: With the aid of the vapor pressure curves in Fig. 9.15 for H2 and N2 the ultimate pressures summarized in Table 2.6 at TG = 300 K result.
The table shows that for hydrogen at temperatures T < 3 K at a gas temperature of TG= 300 K (i.e. when the cryopanel is exposed to the thermal radiation of the wall) sufficiently low ultimate pressures can be attained. Due to a number of interfering factors like desorption from the wall and leaks, the theoretical ultimate pressures are not attained in practice.
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.
In the case of refrigerator cryopumps the net refrigerating power available at the usual operating temperatures (T1 < 80 K, T2 < 20 K) substantially defines the throughput and the crossover value. The net. refrigerating power is – depending on the configuration of the pump – much lower than the refrigerating power of the cold head used without the pump.
See page on Types of Flow
As a gas trapping device, the cryopump must be regenerated after a certain period of operation. Regeneration involves the removal of condensed and adsorbed gases from the cryopanels by heating. The regeneration can be run fully or only partially and mainly differs by the way in which the cryopanels are heated.
In the case of total regeneration a difference is made between:
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 1992, 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:
Q.2 (20 K) is the net refrigerating power in Watts available at the second stage of the cold heat at 20 K. In the case of intermittent operation, a higher pV flow is permissible (see crossover value).
The following applies to the (theoretical) pumping speed of a cryopump:
AK - Size of the cryopanels
SA - Surface area related pumping speed (area related impact rate according to equations 1.17 and 1.20, proportional to the mean velocity of the gas molecules in the direction of the cryopanel).
α - Probability of condensation (pumping)
pend - Ultimate pressure (see above)
p - Pressure in the vacuum chamber
The equation (2.29) applies to a cryopanel built into the vacuum chamber, the surface area of which is small compared to the surface of the vacuum chamber. At sufficiently low temperatures α = 1 for all gases. The equation (2.29) shows that for p >> pend the expression in brackets approaches 1 so that in the oversaturated case p >> pend > Ps so that:
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.
The duration of operation of the cryopump for a particular gas depends on the equation:
CG = Capacity of the cryopump for the gas G
QG(t) = Throughput of the cryopump for the gas at the point of time t
If the constant mean over time for the throughput QG is known, the following applies:
After the period of operation top,G has elapsed the cryopump must be regenerated with respect to the type of gas G.
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.
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