By using quadrupole lenses Gatan introduced the Model 666 spectrometer to obtain spectra onto a YAG transmission screen and photodiode array in 1986. On the other hand, by using a two-dimensional CCD array as the detector, Gatan imaging filters (GIFs) used both quadrupole and sextupole lenses to correct spectrometer aberrations. After that, the company has developed many versions of EEL spectrometers and GIF cameras. Figure 3942a shows the schematic illustration of a GIF (Gatan Imaging Filter) camera. The quadrupole lens used to finely tune the focus in the GIF system is also indicated in the figure.
Figure 3942a. Schematic illustration of a GIF (Gatan Imaging Filter) camera.
For the most common EFTEM unit as shown in Figure 3942b, the detection system consists of a slow scan CCD array detector rather than a single line of diodes used in PEELS detector. This system uses six quadrupoles. However, in PEELS systems, four quadrupoles are normally used to control both the dispersion and the width of the spectrum at the detector.
Figure 3942b. Gatan imaging filter using CCD detector.
A EELS spectrometer consists of a couple of lenses. For instance, Enﬁna ER has 2 hexapoles and 5 electromagnetic round lenses, 7 dipoles for alignment, and 1 quadrupole and 1 additional hexapole for astigmatism correction.
Figure 3942c shows an example on how to correct the achromaticity in EFTEM mode. In this process, an energy offset is applied to the electron beam by changing the accelerating voltage at the electron gun of the TEM. The differences between the current positions of the hole centers of the mask and those measured before the change in beam energy are determined. The net average displacement is taken as a measure of the departure of the energy filtering system from the true achromatic imaging. If the net average displacement is not within the operator-specified tolerance, the effect on the average displacement to the changes in the current of a chromatic adjustment quadrupole lens of post-slit electron optics of the energy filtering system is assessed. These operations are iteratively performed if more precise achromatic adjustment is required or until the measured average displacement is within the specified tolerance.
Figure 3942c. An example on how to correct the achromaticity.