The NIST (National Institute of Standards and Technology) virtual/physical surface roughness calibration standard consists of physical specimens whose surfaces are manufactured by a numerically controlled diamond-turning process using digitized profiles. These standards are designed for checking the characteristics and algorithms of surface measuring systems, and for sensing the amount of distortion of the surface information flow though different measuring systems. The digitized profiles can also be used for remote instrument calibration and surface measurement unification. The design, manufacture, test results, and potential uses of the NIST prototype specimens are discussed.
Optical profiling instruments utilizing phase-shifting interferometry offer one the opportunity to obtain a large amount of surface texture data quickly without damaging the measured surface. Unfortunately, the presence of dissimilar surface materials or even spatial variations in the subsurface morphology can result in spurious optical measurements. This paper presents equations that can be used to calculate the reflection of electromagnetic radiation from thin film structures. These equations are utilized to determine what minimum metal overcoat thickness will ensure accurate optical step height measurements. Interferometric and stylus- based measurements of three thin film steps are presented and discussed. An opaque metal overcoat is found to be essential to the accurate optical measurement of step heights.
The accurate measurement of surface finish requires standard specimens to calibrate and check the operation of the measuring instruments. This is true both for profiling techniques such as the stylus and area averaging techniques such as light scattering. For profiling stylus instruments, the international standard ISO 5436 has enumerated four types of standard specimens which may be classified as step height specimens, periodic roughness specimens, random roughness specimens, and specimens for checking probe tip resolution. A draft of a related U.S. standard uses the same taxonomy. For light scattering instruments, the use of standard specimens are discussed in the ASTM standard for total integrated scatter. In addition, several ad hoc standard specimens have been used in an interlaboratory comparison of instruments that measure the bidirectional reflectance distribution function (BRDF). Our group has developed a series of specimens, known as standard reference materials (SRMs), for calibrating stylus profiling instruments. We are also in the process of developing a series of profile specimens for calibrating the linearity of BRDF instruments. Both types of specimens have sinusoidal profiles and are discussed in turn.
Angle-resolved light scattering (ARLS) was used to investigate the roughness of
coatings on glossy paper. Angular spectra were measured for laser light scattered
from several glossy paper samples and from uncoated paper. These spectra are compared
to those calculated using the Beckmann model of a random surface that is isotropic
and rough in two dimensions. Such a surface is characterized by its rms
roughness and autocorrelation function, which are determined from surface profiles
measured with a stylus instrument. There is very good agreement between the measured
and the computed ARLS spectra. The surfaces are too rough to produce a specular
beam large enough to provide an accurate value of the rms roughness, but ARLS
provides information about the coating roughness when the measured spectra are cornpared
to computed ones.
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