The goal of cryopumping is to remove all gas molecules from a space. Cryopumping is a vacuum pump which captures gas molecules as surfaces are cooled. The gas molecules freeze at various temperatures depending on the elements in the molecules. For instance, helium freezes at a very low temperature. In the Cryostation, charcoal adsorbers are used to trap the molecules, maintaining a hard vacuum and clean the sample space. Many studies of various materials have been conducted to determine the most ideal substance to be used in cryotrapping. Activated charcoal is the most suitable material because of its amorphous regions within well-ordered microcrystalline structures. Since molecules are trapped by the activated charcoal, surface area is also a very important aspect of the cryopumping. The Cryostation has six charcoal adsorbers in the system with more than enough charcoal in each stack to successfully hold vacuum. There are two charcoal adsorbers in the sample space that sit on stage 1, as shown in Figure 1. There are also two stacks of charcoal on each stage of the cryo-cooler directly. Even with a small leak the charcoal adsorbers should be able to maintain vacuum and temperature of the Cryostation. The Nanoscale Workstation has four charcoal adsorbers in the sample space since there is more volume in the Nanoscale Workstation than in the Cryostation.
Figure 1: Charcoal Absorbers on 1st stage of sample space
At Montana Instruments we achieve vacuum within the cryostat in the following way:
- A roughing pump is used to bring the pressure in the system down to 2 Torr.
- Once the system reaches 2 Torr the compressor turns on.
- The pump continues to operate until pressure reaches 400mTorr, then turns off.
- As the Cryostation cools, stage 1 drops in temperature faster than stage 2. See Figure 2, in which the black line represents stage 1 temperature and the green line represents stage 2. With stage 1 cooling quicker than stage 2, this serves to evacuate all particles out of the stage 2/sample area first – leaving no remaining molecules in the sample space. The molecules in the warmer stage 2 escape to the colder stage 1 and become trapped in the charcoal adsorbers on stage 1.
Figure 2: Cool down profile
Why Cryopumping and Not Turbo Pumping
There are two main reasons that Montana Instruments has chosen to use cryopumping over a turbo pump.
- Cryopumps eliminate unnecessary equipment while preserving functionality.
- With cryopumping vacuum levels achieve 10-6 Torr outside of the radiation shield and on the order of 10-7 Torr in the sample space, this is better than turbo pumps can achieve.
For users who need to use a turbo pump please see: How to Use a Turbo Pump with the Cryostation