Practical Electron Microscopy and Database

Table 4463a. Units conversion of vacuum levels.

Vacuum in electron microscopes (EMs) is needed because of a couple of reasons:
        i) Electron propagation is only possible through vacuum because strong scattering occurs in gases, so all EMs (electron microscopes) operate under vacuum. Therefore, an high (or even ultra-high) vacuum system is essential for EMs in order to avoid the strong scattering of the electron beam in the electron guns and other components.
        ii) Avoiding contamination and oxidation on the specimen surfaces. In general, the better the vacuum, the less the contamination and oxidation.
        iii) Avoiding electrical discharge between the anode and cathode (filament/shield). The discharging is a common reason of causing filament failure.

The vacuum level varies in the different areas of the EMs. However, in order to efficiently evacuate a vacuum system, different types of pumps should be used. The highest vacuum level in the range of 10^-9 mbar to 10^-7 Pa is required in the gun area. Table 4463b lists the typical pressure ranges of EM parts, and more numbers about vacuum levels are listed in page2351.

Table 4463b. Typical pressure ranges of EM parts.

Table 4463c more accurately lists the vacuum levels of different parts in EMs.

Table 4463c. Vacuum level of different parts in EMs.

To avoid specimen contamination, the specimen area also requires a high vacuum level especially for chemical analysis in an analytical TEM when the electron beam is resting for a longer time in the small area.

Note that the incident electrons still interact with the residual air molecules in the vacuum. For instance, the mean free path of 200 keV incident electrons in a vacuum of 10^-4 Torr (10¹⁰⁰ air molecules/cm³) is about 50 meters.

In general, the vacuum systems of standard EMs are composed of mechanical pump(s) (rotary pumps, rough pumps, or forepumps), diffusion pump(s), vacuum gauges, switching valves, and a network of connecting pipes. The operating vacuum is provided by a cascade system with different pumps because each pump only works in a limited vacuum range. The operational step of evacuation is performed by following special pumping sequences of the different pumps combined in the complex vacuum system. In general, however, the users do not need to understand or even be aware of the pumping arrangement since the pumping sequence is automatic and safety devices ensure that a specific pump and the microscope cannot operate until an appropriate vacuum is obtained. However, there are various pumps for TEMs and SEMs, and one often has a choice and needs to make a decision when purchasing an instrument. Note that a clean UHV (ultra-high vacuum) system is very expensive.

The specimens are always inserted through an air lock to prevent loss of vacuum in the TEM column or SEM chamber.