An optical monitor (108) includes a body (110) having a first plurality of parallel, substantially opaque, spaced apart lines (112) thereon, and the second plurality of parallel, substantially opaque, spaced apart lines (122) thereon, with a relatively small angle between the first and second pluralities of lines (112, 122). As an image of the lines of the first plurality (112) thereof is provided on the semiconductor body (34), upon relative movement of the monitor (108) toward and away from the semiconductor body (34), the line images move relative to the semiconductor body (34). The images of the lines of the second plurality thereof (122) provided on the semiconductor body (34) move in a different manner upon relative movement of the monitor (108) toward and way from the semiconductor body (34). The moiré fringe formed on the semiconductor body (34) from images of the first and second plurality of lines (112, 122) during such movement is analyzed in order to achieve proper focus of the image on the semiconductor body (34).
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