Abstract
A standalone 100 kV field emission gun (FEG) has been developed that can be installed and operated on a standard transmission electron microscopy electron optical column or custom designed high voltage electron optical columns. The FEG comprises a thermally assisted field emission cathode and an asymmetric electrostatic lens that can operate from 20 to 100 kV in an ultrahigh vacuum (UHV) chamber. In its current configuration, the FEG has spherical and chromatic aberration coefficients (Cs and Cc, respectively) in the range of Cs = 607–670 mm and Cc = 60–87 mm at 100 keV over a range of working distances of 50–206 mm from the exit plane of the FEG unit. A dedicated high voltage supply unit with voltage ripples of less than 1 ppm at 100 kV has also been developed. The FEG is transported under UHV and does not require the use of SF6 gas during operation, as is customary in high voltage FEG TEMs. Preliminary results of operating the FEG on a Philips Tecnai 12 and a JEOL JEM-1400HR TEM show the resolution of gold (111) crystal planes at 0.235 nm and (200) planes at 0.202 nm.
Highlights
The use of high brightness electron sources such as cold field electron emitters (CFE) and the thermally assisted field emission cathodes (TFE) has allowed lower beam voltages to be used in both scanning electron microscopy (SEM) as well as in transmission electron microscopy (TEM) with optimum resolution
The results reported avs.scitation.org/journal/jvb here further demonstrate the possibility of upgrading a TEM that has a thermionic source that is inadequate to use for the type of samples imaged in to a field emission gun (FEG) featuring higher brightness and spatial coherence; both of which are essential in this application
For a TFE cathode operated with an angular current intensity of 300 μA/str, the estimated electron beam current, which has been confirmed by measurement, is of the order of 15 nA
Summary
The use of high brightness electron sources such as cold field electron emitters (CFE) and the thermally assisted field emission cathodes (TFE) has allowed lower beam voltages to be used in both scanning electron microscopy (SEM) as well as in transmission electron microscopy (TEM) with optimum (high) resolution. Egerton has extensively reviewed the suitability of the incident electron beam energy in transmission electron microscopy depending on the type of sample to be imaged and its thickness He found that the electron beam energy for TEM covers a range spanning 50–300 keV. In spite of such different electron beam energy requirements, most FEG based TEM instruments available in the market continue to be designed and optimized in terms of the highest spatial resolution for 200–300 kV operation Such instruments are always possible to operate at lower than their maximum voltages to suit the specimen being studied but such lower beam voltages may not represent the optimum electron optical characteristics of the instrument in question and certainly may not be as cost-effective. The results reported avs.scitation.org/journal/jvb here further demonstrate the possibility of upgrading a TEM that has a thermionic source that is inadequate to use for the type of samples imaged in to a FEG featuring higher brightness and spatial coherence; both of which are essential in this application
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