Comparison of the smoothing and shaping of optics by plasma-assisted chemical etching and ion milling using the surface evolution theory.

Plasma-assisted chemical etching (PACE) and ion milling (IM), originally developed for microelectronic fabrication, can now be used to shape or figure and smooth or polish optical and other surfaces with the unique advantage of doing so without mechanical contact. Using a recent theory of the temporal evolution of surfaces that is due to arbitrary additive or subtractive microscopic processes, the author critically compares the predicted and observed smoothing of PACE and IM in this paper to provide insight into their smoothing and other behavior. PACE removes > 10 mm(3)/min and smooths surfaces of considerable roughness (> 10-µm rms). It has produced smoothed surfaces to < 1.0-nm rms without generating electronic subsurface damage. The IM removal rate is > 10(3) times slower, is used primarily for making corrections of less than a micrometer, intrinsically generates electronic subsurface damage, and furthermore requires starting with a smooth surface (≈ 1.0 nm) to maintain smoothness. Both PACE and IM remove mechanical subsurface damage. In agreement with observation, it is shown theoretically that the central distinction between PACE and IM stems from substantial differences in their intrinsic smoothing.