Cryopumps offer several advantages compared to other high-vacuum pumps. For instance, their pumping speed for water vapour is up to 4x higher than any other vacuum pump with the same inlet diameter. Furthermore, unlike gas transfer pumps, i.e. turbomolecular pumps or oil diffusion pumps, cryopumps condense all the gasses within them. The goal of this blog is to explain to you how they operate and where their capabilities are beneficial to the vacuum process.
How do cryopumps work?
Water vapour condenses on a cold mirror in the bathroom after showering – or on eyeglasses when the wearer comes from the cold outside into a warm and humid room. Ice forms on the windshield of a car after a night with temperatures below freezing temperatures.
So we can easily understand the physics behind cryopumps. Each gas has a (saturated) vapour pressure that is a function of temperature – the higher the temperature the higher the vapour pressure. Condensation on a cold surface will reduce the vapour pressure of the surrounding gas. If the temperature is low enough, the vapour will change to the solid phase and condense. So, for instance, the windshield of the car will 'pump' water vapour from the moisture in the air.
For pumping gases to high-vacuum pressures (e.g. 10-7 mbar) we have to provide much lower temperatures. Diagram 1 shows the saturation vapour pressures as functions of temperature for most gasses present in vacuum chambers.
Vapour pressure of various gasses as function of temperature
In the diagram, we can see that at temperatures below 20K (-253C) all these gases will condense to pressures below 10-10 mbar, which means pure ultra-high vacuum! Such temperatures can be easily maintained with commercial cryocoolers (these are operated by helium gas supplied in a closed cycle remote compressor).
The cryocoolers (or: 'coldheads' or 'cryorefrigerators') cools gases in 2 stages; the first stage cools the gas to well below 77K (temperature of liquid nitrogen) with a high cooling capacity, while the second stage cools it to below 20K at a lower cooling capacity.
To reduce the thermal radiation from the outer walls (a consequence of the pump being at room temperature), a radiation shield is connected to the 80K-stage. This also holds a gas inlet baffle which pre-cools all gases entering the inner of the pump. The outer sides are coated with highly reflective material (similar to thermos flasks) to reflect the heat radiation.
In the picture below, we can see that the water vapour is already condensing. This means the complete inlet surface of the pump is pumping water vapour at practically 100% efficiency. For comparison, a TMP preferentially pumps at the outer circumference of the rotor and at a much lower efficiency.
Schematic of a refrigerator-cooled cryopump
Most other gases condense at the inner cold panels mounted to the 20K-stage of the cryocooler. Helium and Hydrogen cannot be pumped here. Instead, they are adsorbed by the inner side of the cold panels through the activated charcoal (these frozen condensate layers can become up to a cm thick). If the condensation becomes too thick, the upper layer may not be cold enough due to the poor heat conductance of the ice. The Inuit know this effect very well – which is precisely why an igloo stays warm inside.
In cases where the condensate gets too thick, the cryopump reduces its performance and needs to be regenerated. This means switching the pump off, warming it up and releasing all gases previously pumped. These gases are either pumped out by an external pump, such as a rotary vane pump, dry screw pump or multistage roots pump. To accelerate this regeneration process, either electrical heaters or a constant purge gas flow through the pump can be used. After cooling down, the pump is ready again.
Watch this process in the video below:
To start the cryopump, a pressure of approximately 0.01 mbar is necessary. The roughing pump is not needed during operation, reducing both cost and noise.
Today, the complete operation – including regeneration – is done using modern pumps which are managed via an integrated control. This enables direct communication with a customer's system and shows the pump status on a display. Depending on the size of the pump, how much gas was condensed and the cooldown period, this process takes 2 - 6 hours.
Cryopump 10.000 l/s. Leybold COOLVAC 10000 iCL
Cryopump 1.500 l/s. Edwards CTI CT 8
A view of a cryopump after condensing Argon to its maximum capacity.
What are the outstanding benefits of cryopumps?
Fastest evacuation time of vacuum chambers: Among all high-vacuum pumps cryopumps deliver the highest pumping speed – 4x higher than TMPs for H2O, double for H2 and still 40% higher for N2 (air).
- Clean vacuum without risk of hydrocarbon contamination: Cryopumps are hydrocarbon-free, there are no moving parts in vacuum so no lubricants are required.
- Powerful alternative to oil diffusion pumps in large vacuum vessels: Available with giant pumping speeds up to 60,000 l/s for N2 and up to 180,000 l/s for H2O.
- Mounting on top or at the side of vacuum vessel without a 90o elbow possible, reducing conductance losses: cryopumps can be mounted in any orientation.
- Reduce costs and noise: roughing pump is only required for initial start and regeneration.
No risk to damage or burn the oil in diffusion pump
Insensitive to air inrush (sudden leaks or ventings); no backstreaming.
Pump can be located closer to vacuum system reducing conductance losses.
Less sensitive to ionising radiation or magnetic field than TMPs
Reduction of operation cost: lower energy and water consumption compared to oil diffusion pumps.
No risk to destroy pump, no inlet screen like in TMPs necessary: Insensitive to particles or dust from vacuum chamber.
Typical applications of cryopumps
Cryopumps are used for applications that require fast evacuation, especially where large surfaces and water vapour contamination are concerned. Some examples include:
- Space simulation chambers: large volumes, large degassing surfaces due to thermal shrouds inside the chamber and oil-free vacuum required to avoid risk to contaminate sensitive optics.
Large cryopump 60,000 l/s for air. For space simulation chambers, flange size 1250 mm.
Coating devices: faster evacuation, especially of large coating devices for higher substrate throughput.
Sputtering systems in semiconductor production: higher pumping speed allows faster evacuation of sputter chamber and higher wafer throughput.
- Electron beam welding: faster evacuation of welding chamber for higher workpiece throughput and hydrocarbon-free vacuum where sensitive workpieces such as titanium (used for example, in aeronautics) is required.
- High-vacuum furnaces for brazing/soldering: for shorter evacuation times.
Brazing furnace with cryopump 10.000 l/s replacing an oil diffusion pump
Note that in this application the cryopump cannot 'look' into the furnace directly due to the strong direct heat load.
MBE devices (Molecular Beam Epitaxy): require good and hydrocarbon-free ultrahigh vacuum < 10-09 mbar.
Special R&D applications like beamline experiments in synchrotrons or storage rings: hydrocarbon-free ultrahigh vacuum with large pumping speed for H2O and H2.
These examples show applications profiting from the advantages of cryopumps over oil diffusion and TMPs.
Practical notes when applying cryopumps
The typical vacuum system schematic using cryopumps looks like this:
It is quite similar to those using oil diffusion pumps or TMPs with a bypass line. The difference is that the roughing pump is not required during operation of the cryopump. Also, each cryopump must have a fixed safety valve to avoid overpressure in case of power failure or during regeration if the roughing pump should fail.
Suppliers provide technical data with the pump including: pumping speed for different gases, cooldown time, weight, recommended compressor etc. But two more values are given: capacity and crossover pressure.
What do they mean for the user?
The capacity C of a cryopump is how much gas can the pump condense before it reduces its performance. The value C in bar*l is, for example, for a 10,000 l/s pump; 5,500 bar*l for air. With this we can calculate the regeneration time t depending on gas flow according to:
At pressures of 10-5 mbar, we come to a time of 636 days – or almost 2 years. At lower pressures, the interval is longer and at higher pressures it is correspondingly shorter.
So when it comes to space simulation chambers, evaporation coating or HV furnaces, the capacity is not a restriction. There is, however, one exception: in sputter devices, the cryopump has to pump the high gas load during the sputtering. There the regeneration interval can be reduced to a week.
Crossover pressure pc is the pressure in the vacuum chamber when the high vacuum valve to a cold cryopump can be opened without the sudden gas flow warming it up above 25K.
A value of C = 800 mbar*l would tell us that we can open the valve of a 1000 l chamber at 0,8 mbar. In relatively small vacuum chambers, this saves a lot of pre-evacuation time.
It is essential to follow the instructions in the manuals of the suppliers concerning hazardous gases! Cryopumps condense gases and during warm-up or regeneration these can be mixed, so explosive mixtures like H2 and O2 can occur. Also, corrosive gases can cause chemical reactions when warm. Modern automatic pump controllers will avoid such conditions but – as in all vacuum systems – you, the user, must know what gases you’re pumping and what can happen in the vacuum system, including the roughing pump and exhaust line!
Last but not least: cryopumps are free of hydrocarbons and lubricants. However, if your process or vacuum chamber introduces oil vapour into the cryopump, the reflective coating of the cold panels or the surface of the activated charcoal can be affected, reducing the performance.
While cryopumps have several advantages in many vacuum processes, many potential users still feel that because they are gas-binding pumps they are difficult to use.
Hopefully this blog has shown you the distinct (and unique) advantages cryopumps have over other types of high vacuum pumps, as well as the fact they can be deployed readily with the right know-how.
In today’s world they are an indispensable part of vacuum devices and used for a variety of applications: space simulation chambers, coating and sputtering tools, MBE devices, and high-energy physics research. Please consider applying them whenever you require large pumping speeds for the gases mentioned in this blog and benefit from pure, hydrocarbon-free pumping performance.
To find out more about the different vacuum pump technologies, including benefits, limitations and how to evaluate which pump to use for your operation, download our free eBook, “Vacuum Pump Technologies Explained”: