Cross-sectional TEM study of subsurface damage in SPDT machining of germanium optics.

In addition to surface roughness and shape precision, the subsurface damage (SSD) generated by single point diamond turning (SPDT) of Ge and Si crystal optics is of increasing importance with decreasing wavelength from infrared through visible, UV, and x-ray. There are various components of SSD, e.g., microcracks, dislocations, strain, and a near-surface amorphous layer, and there are also several techniques to evaluate various components of SSD. Cross-sectional transmission electron microscopy (XTEM) is expensive and not often directly used in the optics laboratory. However, because of its very high sensitivity to SSD and down to atomic resolution, it is often used as an external service for developing SPDT technology and other surface processing techniques. It is shown in the paper that improper sample preparation can generate near-surface amorphization. Measures to avoid this artifact and a test of reliability of XTEM sample preparation are proposed.