Extinction distances for two beam conditions are readily available through many sources, but under more complex zone axis conditions extinction distances are not well known. The use of thickness fringes as an approximate thickness estimate under zone axis conditions, such as in high resolution Annular Dark Field imaging, can be a useful tool. We have measured thickness vs. fringe number under zone axis conditions using low loss Electron Energy Loss Spectroscopy (EELS). Work was done on the Cornell UHV HB501 STEM.

Several orientations of silicon were glued together and thinned in cross section by the wedge technique. Additional thinning was done by low angle ion milling. Surface oxide was removed with an HF etch before loading into the microscope.Each piece of silicon was tilted to the zone axis condition and low loss EELS spectra was taken at selected points. The thickness fringes were numbered with the following convention: the center of the first black fringes close to the edge is 1 and the center of the second black fringe is 2, etc.