20 December 1989 Step Height Measurement Range Extended For An Interference Microscope Utilizing The Obliquity Effect
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Step heights measured with an interference microscope in monochromatic light are generally limited to steps not exceeding 1/4 of the illuminating wavelength due to the difficulty in following the fringe order across the step. There are several ways to overcome this ambiguity. One method tracks the fringe contrast across a step to help determine the absolute phase across the step height.1 Another method uses two measurements at differing wavelengths (i.e., two-wavelength interferometry) to create a synthetic wavelength larger than either single wavelength.2 The quarter-wave limit of the synthetic wavelength then being significantly larger than either single wavelength measurement. The method I propose utilizes the fact that the fringe spacing in interference microscopes with extended source illumination is greater than half a wavelength3 (i.e., 1 fringe width = λ/2•obliquity factor). The obliquity factor varies as a function of the illumination numerical aperture and the distribution of the intensity of the illumination across the pupil of the microscope objective. Changing the aperture stop setting in the illumination arm of the interference microscope between successive measurements changes the magnitude of the obliquity factor, thereby creating two single effective wavelengths. Two-wavelength interferometry can be accomplished by the creation of a larger equivalent wavelength from the two single effective wavelengths; that is, from one wavelength multiplied by two different obliquity factors.
© (1989) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
James F. Biegen, James F. Biegen, } "Step Height Measurement Range Extended For An Interference Microscope Utilizing The Obliquity Effect", Proc. SPIE 1164, Surface Characterization and Testing II, (20 December 1989); doi: 10.1117/12.962810; https://doi.org/10.1117/12.962810

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