Minimising the Change of Collection Angle with Energy Loss in
STEM‐EELS
Authors: Craven Alan J. , MacLaren Ian , McFadzean Sam , Sawada Hidetaka 
Published: December 2016

STEM‐EELS is a powerful tool for investigating materials systems. With aberration corrected columns, the probe is large, requiring a large collection angle and hence a small camera length. The probe angle and the collection angles are normally measured using electrons that have lost no energy. For the electrons that lose energy in the specimen, each lens after the specimen increases in strength. Thus the camera length, the pattern distortion and the position of the final cross‐over change.

With the four projector lenses in a modern TEM, many lens configurations can give a specific camera length. Some configurations lead to zero 3
rd
order distortions, and Craven and Buggy (1981) showed some configurations give no chromatic change of camera length. A lens can either create a real image of the probe after the lens (α mode) or a virtual image before it (β mode). These have opposite chromatic effects on the camera length and so can be used to null the change. If the four lenses are used in a
β, α, β, α
arrangement, the camera length and the final axial cross‐over position (the object for the spectrometer) can be set while maintaining no chromatic change of camera length.

Lens calculations were used to identify a series of configurations with the desired camera length and crossover position. These were then investigated experimentally and the two giving the best performance were identified for further electron optical investigation. This paper compares one, (S‐9), with the original best configuration (S‐1), which was based on a β
, α, α, α
arrangement.

Figure 1 shows the post‐spectrometer camera output when using a Si specimen in a JEOL ARM 200F equipped with a Gatan QUANTUM DualEELS system, operated at 200kV. The width corresponds to an energy loss range of 2048eV. The upper four parts are for S‐1 and have offsets of 0, 1000, 2000 and 3000eV. The lower four parts are the equivalent results for the S‐9. The narrowing of the intensity band with energy loss seen with S‐1 is no longer present with S‐9.
The upper part of Figure 2 shows the Si K‐edge recorded with the two configurations while the lower part shows the peak from stray scattering within the gun. In both cases, the features are sharper with S‐9.

To investigate the chromatic change of camera length, the microscope EHT was reduced in steps of 500V while leaving the lens excitations unchanged. The STEM image was refocused after each change using the condenser mini‐lens. In this way, the electrons in the lower column behave as though they had lost the equivalent energy. Diffraction patterns from a low index pole of Si were recorded at each offset using the pre‐spectrometer camera. The chromatic change of camera length was measured using the width of the Kikuchi bands. The first order Laue zone was used to determine the 3
rd
order radial distortion. The diameter of the pattern cut‐off was also measured.

The upper part of Figure 3 shows the corresponding change in the acceptance angle of the 2.5mm spectrometer aperture. This confirms the reduced chromatic change of camera length for S‐9. Quantification based on the 0V acceptance angle would give significant errors when using S‐1, particularly for higher losses.

S‐1 had zero 3
rd
order radial distortion at 0V offset but it became steadily more negative with increasing loss. S‐9 had approximately constant positive 3
rd
order radial distortion.

The lower part of Figure 3 shows the cut‐off diameters for the two configurations in camera pixels. Both shrink steadily with energy loss but the S‐9 diameters are much bigger. At 3000V, the cut‐off of S‐1 is close to the 5mm spectrometer aperture diameter. To determine the scattering angles corresponding to these cut‐offs needs a more detailed understanding of the higher order aberrations which cause them.
In conclusion, careful choice of the configuration of the projector lenses can minimise the chromatic effects on the collection angle in EELS and so avoid errors in quantification, particularly for higher energy losses.