Practical Electron Microscopy and Database

Cryogenic pumps (cryo-pumps) use liquid N₂ (LN2) to cool air molecules with large surface areas, and thus the cold surface can efficiently absorb the air molecules from ambient pressure down to ~10^-4 Pa. These pumps are oil-free so that they are also applied to back ion pumps and prevent accidental contamination via back-streaming.

Cold surfaces are also used to enhance vacuums in the specimen stage of most non-UHV (ultra-high vacuum) TEMs. As an example, Figure 4319a shows the location of cold traps in a TEM system.The cold traps, in the gun and specimen areas, condense residual components in the vacuum and then can reach a high and ultra-high vacuum of < 10^-6 Pa.

Figure 4319a. Schematic illustration of pumping system in TEMs.

A cryopump contains cryroarrays working at temperatures between 10 and 80 K. The gasses from the vacuum chamber freeze out and become trapped in the pump. A cryopump is composed of two main parts:
         i) Gas compressor. The principle of the compressor and the cold head in a cryopump is similar to that of a refrigerator.
         ii) Pump module. The pump module consists of four major elements: the expander, the first- and second-stage cryoarrays, and the pump body.
As shown in Figure 4319b, high pressure helium (He) gas, supplied by the gas compressor, expands in the two stages, resulting in near cryogenic temperatures. The expander provides the expanded He gas that cools the cryroarrays. The cryoarrays are the surfaces with activated, porous charcoal, on which the gases are condensed and absorbed. Table 4319 lists the operation temperatures and condensed gases in the two stages. However, the arrays do not reach temperatures low enough to condense He, H, and Ne gases. These gases are pumped by the use of cryosorption. The pump body connects the pump to the vacuum chamber.

The cryopumps can evacuate a large gas-load in the high vacuum region and create a clean vacuum. Therefore, it can be used for thin film coaters such as sputtering instruments and vacuum evaporators. However, there are still some disadvantages with these cryopumps:
         i) These pump emits noise and vibration from the moving piston which compresses the He gas. Due to the vibration, the cryopump is not a good choice for some high spatial resolution instruments such as electron microscopes and scanning tunneling electron microscopes.
         ii) The maintenance expenses of cryopumps are relatively high. For instance, cryopumps may finally become saturated and then lose pumping speed, so that the most common maintenance for cryopumps is the regeneration of the pump, which releases the trapped gases in the charcoal and pumps them out through the backing (roughing) pump. The regeneration of the cryopumps is done by heating the pump up to room temperature. During this process, the temperature in the cryopump must be kept below 290-295 K so that the indium gasket in the cryostat will not be destroyed.

Figure 4319b. Typical structure of a cryopump.

Table 4319. Operation temperatures and condensed gases in the two stages.

Both ion getter pumps and cryo-pumps are often employed in applications requiring extremely clean, ultrahigh vacuums, e.g. in LaB₆ guns and FEGs.

Figure 4319c shows a cold trap is used in the specimen TEM chamber to prevent contamination of the TEM specimen on a FEI TEM system.

Figure 4319c. Cold trap to prevent contamination of the TEM specimen.

Anti-Contamination Device (ACD) is a “cold trap” or “cold finger” (in Figure 4319d) for improving vacuum around the sample. Liquid nitrogen (liquid N₂, also called LN2) is added to the decontaminator device (trap), which cools metal plates located just above or below the specimen, resulting in condensation of water and organic vapors and providing a low partial pressure of these components in the immediate vicinity of the TEM specimen. The liquid N₂ dewar is visible on the side of the TEM column. The anti-contamination fin installed in the specimen chamber is also considered to be a kind of cryo-pump.

Figure 4319d. Anti-Contamination Device (ACD) on a JEOL TEM system.

However, when the cryogenic pumps, cold traps or ACD are warmed up, then the vacuum will be degraded. Therefore, the warming-up process should only be performed when we are not using the microscope, and when the diffusion or mechanical pumps are still on in order to remove the released contaminants.