WAFER MEASURING INSTRUMENT

PURPOSE:To more accurately and more efficiently measure patterns with a variety of widths by providing a slit having an opening with a width changeable within a predetermined range, changing the width within the predetermined range and using integral and differential measurement together. CONSTITUTION:A gradational image on the surface of a wafer, the image being determined by an objective lens, is further stopped down with a field of view through slit plates 30X and 30Y and focused on a linear image sensor. The plates 30X and 30Y are provided with base plates 31, movable plates 43 slidably mounted thereon and piezoelectric elements 44 and 45 provided on both sides of the movable plates 43 in the longitudinal directions thereof. The plates 31 have openings 46. The elements 44 and 45 are expanded or shrunk in accordance with the voltage supplied from a driving circuit. Therefore, the plates 43 are moved relative to the openings 46 of the plates 31 and the ranges of the openings 46 covered by the plates 43 are changed. Since the widths of the openings can be arbitrarily set in accordance with the voltage applied to the elements 44 and 45, a detecting field of view can be freely set in accordance with the width, kind and the like of a pattern to be measured.