Vacuum
Like all electron microscopes, transmission electron microscopes operate in a vacuum environment, which removes air and its contaminants from the electron column. The vacuum environment in a TEM precludes the direct examination of living things, liquids or anything that contains liquids. Specimens must be dried before use in the system. If air or contaminants are present in the column or on the specimen, the high-energy electron beam will turn the hydrocarbon and water molecules into carbon that is deposited onto the specimen and other parts of the column, ultimately degrading the image. For this reason, all specimens and their holders must be kept clean and should be handled with gloves.
The LaB6 tip can become contaminated by gas particles in the system, deteriorating the quality of the electron beam. A good vacuum provides the needed insulation to prevent high voltage discharges in the electron gun. The gun itself is under extremely high vacuum conditions, requiring a pressure as low as 10-6 Pa or 10-10 torr. (The gun of a conventional SEM operates at 10-5 torr.) The column vacuum pressure is maintained at 10-5 Pa.
Commonly used vacuum systems in SEMs consist of an oil filled roughing pump to produce a vacuum level sufficient to operate the oil diffusion pump (DP) for high vacuum. The roughing pump evacuates the microscope column from atmosphere to low vacuum. The DP then creates the higher vacuum needed for the SEM to operate under standard conditions, which is why both pumps are needed to produce the operating vacuum. Maintaining a vacuumed environment in the TEM is difficult, as any small hole in the system results in a vacuum leak resulting in a system shutdown. Additionally, the DP requires a cold water source for cooling. TEM vacuum systems (and therefore cold water systems) are operated continuously. Keeping up a dependable 24/7/365 vacuum and maintaining the water source is time-consuming.
The vacuum level required to operate a TEM gun is greater than a DP can produce. Ion (or ion-getter) pumps are used in the TEM. Residual gases in the gun area are ionized in an electric field and adsorbed to a metal surface that traps them, creating a pumping action. Like the DP, a sufficient vacuum level must be present before activation of the ion pump. Two ion pumps are responsible for maintaining the correct vacuum level: one for the gun and one for the column.
The TEM is also equipped with two anti-contamination devices (ACDs). These components use liquid nitrogen to cool special plates positioned near the specimen. If contamination is emitted from the specimen, these plates attract the contamination, thus maintaining the desired conditions in the area of the specimen.
