Practical Electron Microscopy and Database

The diode capacitors in photo-diode array (PDA) detectors lose charge not only through irradiation but also as a result of their thermal leakage current, which is slightly different for each element of the array. In order to obtain values which are proportional to spectral intensity, a leakage or bias spectrum must be subtracted. This bias spectrum is recorded while electrons are excluded from the array (e.g., TEM screen lowered to block the electron beam) and will remain the same provided the integration time and array temperature do not vary. To minimize the noise content of recorded data and allow longer integration times (without total discharge by thermal leakage), the photodiode array is cooled to −20 °C by a thermoelectric device.

Spectrum drift and energy drift in EELS measurements may be induced by various instabilities. It was proposed that the energy drift from low-frequency instabilities can be corrected by software techniques [1, 2], while the energy drift from high-frequency energy instabilities can be corrected by high speed acquisition technique [3].

In general, the requirements of TEM specimen thickness for EELS and EFTEM measurements are:
         i) The specimens should be sufficiently thin to prevent any multiple inelastic scattering, but the degree of single inelastic scattering should be relatively high. For most materials, the optimized specimen thickness is in the range of 25-100 nm, depending on the average atomic number and the beam energy.
         ii) To avoid surface effects, the specimen thickness cannot be less than 25 nm if low-loss EELS spectra are measured.

The collection angle for an energy loss can simply be optimized to an angle slightly larger than the relevant characteristic inelastic scattering angle (θ_E), for instance, for 100-keV incident electrons, θ_E has a value of 1 mrad for a 200 eV energy loss, while 10 mrad for a 2 keV energy loss (see details at effects of entrance aperture/collection angle on EELS and optimization).

[1] Kimoto, K., Matsui, Y., 2002. Software techniques for EELS to realize about 0.3 eV energy resolution using 300 kV FEG-TEM. J. Microsc. 208, 224–228.
[2] Kimoto, K., Ishizuka, K., Mizoguchi, T., Tanaka, I., Matsui, Y., 2003. The study of Al-L23 ELNES with resolution-enhancement software and firstprinciples calculation. J. Electron Microsc. 52, 299–303.
[3] Koji Kimoto, Kazuo Ishizuka, Toru Asaka, Takuro Nagai, Yoshio Matsui, 0.23 eV energy resolution obtained using a cold field-emission gun and a streak imaging technique, Micron 36 (2005) 465–469.