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