This paper describes the various areas of analytical and high resolution
microscopy which can be greatly improved by the use of a high-brightness
field emission gun (FEG). The instrument used was a Philips EM400T equipped
with a FEG, 6585 STEM unit, EDAX EDS detector and Kevex 5100 spectrometer.
The <111> oriented W tip was supplied by the manufacturer.
The brightness β (current density per unit solid angle) normalized to
the ac-celeratiang voltage, V0 is defined by β =
4I/πd2α2jV0,
where I is the current in a probe of diameter d and divergence
αi. Results are presented in Table 1 for three
typical operating conditions. Probe currents >10−7 A
have been obtained in the TEM mode which is sufficient for work at medium
magnifications (20 to 100 K). Probe currents were measured from the screen
current/exposure time system which had been calibrated with a purpose built
Faraday cup. Probe diameters in TEM were measured from high magnification
TEM images and in STEM by imaging the STEM raster in the TEM mode. This is
possible because of the symmetric objective lens which can operate at the
same excitation in TEM and STEM. An example is shown in Fig. 1. The values
in Table 1 should be compared to conventional W hairpin sources for which β
≅ 1.
