Practical Electron Microscopy and Database

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.

Table 2351. Typical working pressure ranges and accuracies of various vacuum gauges.

The design best-known methods (BKMs) for maintaining vacuum in Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) involve creating an environment where electron beams can travel without interference from air molecules, therefore optimizing image resolution and minimizing contamination. 

SEM Vacuum Design Best-Known Methods:

• Chamber Segmentation: SEMs often utilize a multi-chamber design with separate high-vacuum regions for the electron gun and intermediate vacuum regions for the sample. This improves vacuum efficiency while allowing quicker sample exchange.

• Turbo-Molecular Pumps: These pumps create the high vacuum necessary for SEM operation by rapidly removing air molecules, reducing pressure to around 10⁻³ to 10⁻⁶ Pa. They are highly effective at maintaining the necessary vacuum levels for electron beam stability.

• Ion Getter Pumps: Often used in conjunction with turbo-molecular pumps, ion getter pumps help maintain ultra-high vacuum in the electron gun chamber. This helps prevent contamination of the electron source and ensures beam quality.

• Low-Vacuum SEM Operation: Some SEMs operate in a low-vacuum mode to allow imaging of non-conductive or biological samples. This design uses differential pumping, maintaining higher pressure near the sample while preserving high vacuum in the electron gun.

• Environmental Control: To prevent contamination, SEMs often include cold traps or water-cooled baffles near the electron source to trap contaminants before they reach sensitive components.

• Sealed Gun Design: In field emission SEMs (FE-SEMs), a sealed gun design ensures a separate ultra-high vacuum environment around the electron gun, extending its lifespan by preventing contamination.

• Ultra-High Vacuum System: TEMs require ultra-high vacuum (typically 10⁻⁶ to 10⁻⁷ Pa) in both the electron gun and the sample chamber. This minimizes electron scattering and contamination from residual gases.

• Multiple Stage Pumping: TEMs typically use a combination of roughing pumps, turbo-molecular pumps, and ion pumps. The roughing pump handles initial pressure reduction, and the turbo and ion pumps bring the system to ultra-high vacuum.

• Cryogenic Traps: To capture gas molecules that could contaminate the electron beam or sample, cryogenic traps cooled with liquid nitrogen are often placed near critical areas in the TEM vacuum system.

• Differential Pumping for the Sample: High-resolution TEMs (HRTEMs) utilize differential pumping to allow small amounts of gas to be introduced in the sample area while maintaining ultra-high vacuum in other parts of the microscope.
• Hydrocarbon-Free Environment: To avoid contamination, especially during high-resolution imaging, TEMs often use hydrocarbon-free pumps and materials. Contamination from hydrocarbons can interfere with sample quality, particularly in electron energy loss spectroscopy (EELS) or atomic-resolution imaging.
• Vacuum Compatibility for Specimen Holders: The design of specimen holders in TEMs considers the ability to maintain vacuum during operation. Specimen holders are typically well-sealed, and vacuum interlocks prevent contamination when exchanging samples.