70
axis orientation as possible using the goniometer tilt
controls, rather than tilting the beam. This will not in
itself completely eliminate the spherical abberation
effect since diffracted beams from the higher order zones
must enter the lens at large angles anyway. It is thus
more accurate to use the HOLZ line in the transmitted disc
than its counterpart in the HOLZ ring. The subsequent
image forming lenses of the instrument will also impart
radial and spiral distortion to the diffraction pattern.
These distortions are minimized on the optical axis of the
instrument.
Fifth, the ubiquitous presence of carbonaceous matter
both in the microscope and on the sample surface can lead
to contamination spikes at the specimen/beam interface
with a consequent attenuation of the beam current, a loss
in spatial resolution due to scattering, and the
introduction of astigmatism into the beam due to
charging. All of these are undesirable. Methods for
reducing contamination have been reviewed elsewhere (Hren,
1979). Contamination and its effects were minimized in
this study by using the minimum practical time to focus
the diffraction pattern, to tilt to the proper
orientation, to determine the exposure time for each
pattern, and to record the image.
In consideration of the above effects, the following
procedure for obtaining HOLZ patterns is recommended: