Abstract
This chapter discusses the origins of systematic errors in optical linewidth-measurement systems. It discusses the advances in modeling the linewidth-measurement process, including imaging in the optical microscope, a primary linewidth-measurement system in use at the National Bureau of Standards (NBS). The photomultiplier system is referred to as an optical image-scanning system, whereas a semiautomatic TV-microscope system, which determines linewidth from analysis of the video image profile, is referred to as a video image-scanning system. The accuracy of optical imaging methods is dependent upon the characteristics of the optical image formed by the microscope and is affected by diffraction and aberrations in the optical system as well as the partial coherence of the illumination. To determine linewidth to accuracy better than this, one must use an optical threshold that corresponds to the true line edge location. The threshold used for edge detection varies considerably with the type of measuring instrument used. In summary, optical image-scanning linewidth-measurement systems are found less frequently in production environments than image-shearing systems.
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