Vacuum Levels and UHV (Ultra-High Vacuum) in EMs
- Practical Electron Microscopy and Database -
- An Online Book -

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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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In most of electron microscopes, sputter ion pumps and turbo molecular pumps are used to maintain UHV (ultra high vacuum) conditions in the gun chamber. In UHV–TEMs, those pumps are also applied to maintain UHV conditions in the column of the microscopes, for instance, the vacuum in TEM holder area can be very high (e.g. 3 x 10-8 Pa) in order to avoid contaminating the specimens. However, it is very common that the vacuum in the specimen chambers of TEMs, e.g. JEM-2010 and JEM-ARM1000 TEMs made by JEOL Co., Ltd., is on the order of 10−5 to 10−6 Pa.

Table 4269. Operating parameters and characteristics of electron sources

Type of source
Tungsten
thermionic
LaB6
thermionic
Schottky
emission
Cold field
emission
Material
W
LaB6
ZrO/W
W
ds (µm)
≈40
≈10
≈0.02
≈0.01
ΔE (eV)
1.5
1.0
0.5
0.3
φ (eV)
4.5
2.7
2.8
4.5
T (K)
2700
1800
1800
300
E (V/m)
Low
Low
≈108
>109
Je (A/m2)
≈104
≈106
≈107
≈109
β (Am-2/sr-1)
≈109
≈1010
≈1011
≈1012
Vacuum (Pa)
<10-2
<10-4
<10-7
≈10-8
Lifetime (hours)
100
1000
104
104

** ds -- Effective (or virtual) source diameter; ΔE -- Energy spread of the electron beam; φ -- Work function; T -- Operating temperature; E -- Electric field; Je -- current density of electron beam; β -- Electron-optical brightness at the cathode.

As shown in Figure 4269 (a), in most modern TEMs, the electron gun, top lenses, and specimen chamber are maintained at ultra-high vacuum by an ion pump, while the viewing screens and photographic chamber are maintained at a lower vacuum, which is referred to as high vacuum, by either a diffusion pump or a turbomolecular pump. This vacuum level is backed by a mechanical (rotary) pump. However, some TEMs have lower vacuum in the specimen chamber as shown in Figure 4269 (b).

Vacuum in TEMs: (a) Modern TEMs, and (b) Some TEMs.

Figure 4269. Vacuum in TEMs: (a) Modern TEMs, and (b) Some TEMs.


 

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