Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 270
LaB6 and field emission guns can also be used at these low pressures for high quality
imaging [6]. Due to the presence of water (wet-mode), or a gas (4) (auxiliary-mode), in the
sample chamber, the conventional Everhart-Thornley detector mentioned earlier, cannot
be used to detect SEs in the ESEM. The environmental SE detector was developed to
work at pressures in excess of 13 mbar (1.3 kPa) [6].
Scattering of the electron beam between the gun and sample must be considered, due
to the large number of atoms / molecules present in the sample chamber. High resolution
can be achieved of the distance between the final PLA and sample is kept relatively small
(≈ 50 cm) and the pressure low (≈ 16 mbar ≡ 1.6 kPa) [6]. With these conditions, a 20
keV electron beam will result in a well defined probe on the sample surface, with a weak
background scatter extending tens of microns. A resolution of 5 nm can be obtained using
a LaB6 filament operating at low enough pressure [6].
SEs and BSEs emitted from the sample surface collide with atoms / molecules in the
chamber. This invariably will lead to ionization of the gas (particularly with SEs) [6].
Each ionization results in the emission of a daughter electron, which in turn ionizes
another gas molecule, causing a cascade effect in the gap between the sample and the
detector [6]. Fig. (7.4) illustrates this.
(4) typically nitrogen