Electron detection modes and their relation to linewidth measurement in the Scanning Electron Microscope

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The basic premise underlying the use of the scanning electron microscope (SEM) for linewidth measurement for semiconductor research and production applications is that the video image acquired, displayed, and ultimately measured reflects accurately the structure of interest. It should be understood that not all the secondary electrons detected originate at the point of impact with the primary electron beam. Those that do are referred to as Type I electrons. Some of the signal is contributed by re-emergent backscattered electrons creating secondary electrons at the surface of the sample (Type II electrons) and at the final lens polepiece (Type III electrons). Other signal contributions include line-of-sight backscattered electrons and other sources particular to each instrument (Type IV electrons). The effects of these four types of contributions to the actual image or linewidth measurement have not been fully evaluated. In measurement applications, error due to the actual location of signal origination will not affect pitch measurements as the errors cancel. However, in linewidth measurement, the errors are additive and thus give twice the edge detection error to the measured width. The basic intent of this work is to demonstrate the magnitude of these errors relative to the mode of signal detection at a variety of beam acceleration voltages.