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 there has been increasing concern in the semiconductor industries (wafer processing, flat panel displays, computer disks) about the ability to measure and communicate roughness values. Light scatter is proving 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 what roughness parameters should be calculated and how they should be reported. This chapter addresses those issues.
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 Sec. 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 the terms [root mean square (rms) roughness, power spectral density, autocorrelation length, etc.] used to quantify surface topography. It is left to following chapters to develop the relationship of these statistical parameters to the associated scatter patterns.
2.1 Profile Characterization
A real three-dimensional surface, described by height z over an x,y plane requires a huge amount of information to completely describe it. Given two such complete descriptions, how does one decide which is rougher? What sort of measurable, and easily reportable, quantities should be specified to characterize surface texture?
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