How it works: Cryopump technology

August 26, 2020

5 MIN READ

Temperature affects the way water vapor behaves

Imagine the condensation on your bathroom mirror after a shower, the icy windshield that greets you after a freezing night, or how your eyeglasses fog up when you go inside after spending time in cold weather. 

This same principle applies to cryopump technology. 

Each gas has a saturated vapor pressure that is a function of temperature — the higher the temperature, the higher the vapor pressure. Condensation on cold surfaces reduces the vapor pressure of the surrounding gas. 

If the temperature is low enough, the vapor will change to the solid phase and condense. In the icy windshield example, the windshield is essentially "pumping" water vapor from the moisture in the air.

Cryocoolers

When it comes to pumping gases to high-vacuum (HV) pressures, we need much lower temperatures. For example, at temperatures below 423 degrees Fahrenheit (20K), most vapors will condense to ultra-high vacuum (UHV) pressures. 

Commercial helium-supplied cryocoolers, like  those in our product range, can easily maintain temperatures in this very low range. The cryocoolers, also called cold heads or cryo-refrigerators, cool the temperature of gases in two stages. 

First, the cryocooler cools the gas well below the temperature of liquid nitrogen (77K) using a high cooling capacity. Then, the cryocooler cools the gas to below 20K using a lower cooling capacity. 

Radiation shielding

As a cryopump is housed at room temperature, steps must be taken to reduce the thermal radiation from the outer walls. To this end, a radiation shield is connected to the 80K stage. 

The shield has a gas inlet baffle that pre-cools gases entering the inner pumping area. The outer sides are coated with highly reflective material — much like a Thermos flask — to reflect heat. 

Condensation considerations

If condensation becomes too thick, the upper layer, similar to an igloo, may not be cold enough due to the reduced heat conductance of ice. 

Should this happen, the cryopump’s performance will be reduced. It will need to be cleared of gas and regenerated at this point using modern pumping systems with integrated controls. Once the pump is back online, efficiency and UHV speeds will return.

Cryopump benefits

Cryopumps are more efficient than other pumps used for similar applications. Unlike gas transfer pumps (like  turbomolecular pumps  or oil diffusion pumps), cryopumps condense all gasses within them. 

Installers can mount cryopumps in any orientation. For example, when mounted on top or at the side of a vacuum chamber without a 90-degree elbow, the system experiences fewer conductance losses. 

Cryopumps are also quieter. Only initial starts and the regeneration process require rough pumping. 

Cryopumps versus Turbomolecular Pumps

Considering all HV pumps, cryopumps provide the fastest possible evacuation time out of all vacuum chambers. Versus a TMP, cryopumps can pump:

H2O four times higher
H2 twice as high
N2 40 percent higher

Cryopumps are also less sensitive to ionizing radiation and magnetic fields than TMPs. 

Cryopump versus oil diffusion pumps

Cryopumps are much more powerful than oil diffusion pumps in large vacuum vessels. They are available with giant pumping speeds of up to:

60,000 l/s for N₂ 
180,000 l/s for H₂O

Because they are hydrocarbon-free, cryopumps supply a clean vacuum without the risk of hydrocarbon contamination. Since there are no moving parts, they also do not need lubrication. 

Unlike oil diffusion pumps, oil-free cryopumps are not at risk of damage or burning oil. They require lower energy and water consumption, reducing overall operating costs.

Cryopump applications

Cryopumps are appropriate for fast evacuation applications, especially where large surfaces and water vapor contamination are a concern. Uses include:

Space simulation chambers
Coating devices
Sputtering systems in semiconductor production
R & D
HV furnaces used for brazing and soldering

Key processes also benefit from the advantages of cryopumps over oil diffusion pumps and TMPs. Among them are:

Molecular beam epitaxy (MBE), which requires good, hydrocarbon-free UHV < 10-09 mbar.
Special R&D applications like beamline experiments in synchrotrons or storage rings, which require hydrocarbon-free UHVs with high H2O and H2 pumping speeds.

Elevate your processes with modern cryopump technology

While cryopumps have several clear advantages in many vacuum processes, you may still have reservations about upgrading because they seem difficult to use.

The good news is that with the right know-how, cryopumps can be readily deployed. It is essential to partner with a provider who will work to make sure you're comfortable with the technology after deployment.  

Cryopumps can save you time, frustration, and money. They are an improvement over TMPs and oil diffusion pumps anywhere you can benefit from pure, hydrocarbon-free pumping of H₂O, H₂s and N₂ at high speeds.