Destructive crater depth

Interferometric methods operate by splitting a monochromatic coherent beam (all waves in phase) of hght into two parts. One part is directed at the surface from which it then reflects and the other part directed at some reference (optically smooth mirror) from which it reflects. When the two parts are recombined, their waves will either interfere in a constractive manner (waves in phase) or destructive manner (waves out of phase). As constructive interference enhances the amplitude of the combined beam, whereas destructive interference suppresses the amplitude, an interference pattern will be prodnced that is dependent on the crater depth. This interference pattern can also be nsed to derive the surface topography of the sputtered crater in both of the spatial dimensions, with the result being an AFM-like three-dimensional topographic image. 

Figure 7.44. Molecular dynamics simulation of the ion bombardment of a Ag crystal surface with a 15 keV Ga+ beam, and a 15 keV Cgo atomic cluster beattL The Cgo beam results in a larger crater and more material removed from the surface. In contrast, the Ga+ beam results in a destructive effect at greater sample depths, without successful sputtering of the sample. Reproduced with permission from Winograd, N. Anal. Chem. 2005, 77, 142A. Copyright 2005 American Chemical Society.

Ball cratering is related to angle lapping,in which the sample is polished at a slight bevel. Similar measurements can also be made on the wall of a sputter crater, but without an easy conversion from a lateral distance to depth. Although these methods are destructive, they have the advantage over ion bombardment that the measurement along the crater wall can be repeated. 

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