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Chapter 2:
Quantifying Surface Roughness

'Smooth as a baby's bottom' just isn't enough information. - Unknown

The optics industry has concerned itself with the measurement and characterization of roughness on (relatively) smooth surfaces (sometimes called microroughness) for many years. In the 1990s, as smaller defects became important to newer products, there was increasing concern in semiconductor-related industries (wafer processing, flat panel displays, computer disks, etc.) about the ability to measure and communicate roughness values. Light scatter proved to be an ideal solution for many of these measurement and process-control problems because it is fast, noncontact, and performs well on very smooth surfaces; however, even in the optics industry, there has been considerable confusion about which roughness parameters should be calculated and how they should be reported. This chapter addresses the issue of how roughness can be quantified.

Everyone knows what is meant by surface roughness, or topography, and it is generally recognized that when even the smoothest surfaces are viewed in enough detail, they will exhibit some form of texture. But describing surface topography in measurable, quantitative terms is more difficult. Even the simple surfaces of Section 1.3 are not easily compared for relative roughness. Are any of these surfaces inherently rougher than the others, or are they just different? How should those differences be reported? This chapter reviews some of the common methods of roughness measurement and presents definitions of common terms such as rms roughness, PSD, autocorrelation length, etc., used to quantify surface topography. It is left to the following chapters to develop the relationship of these statistical parameters to the associated scatter patterns.

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