It has long been an axiom of mine that the little things are infinitely the most important. - Sherlock Holmes in A Case of Identity
Chapters 2 - 4 and 8 concentrated on the relationship between scatter and smooth-surface topography. However, another extremely useful application of light-scatter metrology is the detection and mapping of component defects that do not meet the smooth, clean, reflective conditions of mirror surfaces. Examples of such defects are surface contaminants, particulates, scratches, digs, coating globs, and residues. If a smooth surface is contaminated with very many defects, the combined scatter of the defects can dominate the surface BRDF as shown by Young (1976a, 1976b) in his study of particulate-contaminated mirrors. Nahm and Wolf (1986, 1987) also studied the contamination problem, using a modified Mie theory. In measurement situations where scatter is being used to detect defects, surface scatter is considered background noise, and the defect scatter is signal. Although defects often scatter more light per unit area than the surrounding surface topography, they can sometimes scatter considerably less total light because they have a cross-sectional area much smaller than the illuminated spot, or because they are buried just beneath a reflective surface. In such cases, a low signal-to-noise ratio results. If it can be established that nontopographic defects scatter light differently from the way surface topography scatters light, then these differences can be exploited to improve signal-to-noise ratio and map the defects, using the raster techniques described in Section 7.12. This chapter discusses the differences in topographic and defect scatter and outlines techniques that have been used to enhance defect detection.
Perhaps the most common application exploiting defect scatter is the use of particle scanners in the semiconductor industry. Because this is covered in Section 11.1 and further in Chapter 12, it is only mentioned here.