An aligner for aligning a mask and a wafer during photolithography of a semiconductor chip uses detection of the differential capacitance between two sets of conductive fingers on the mask and ridges on the wafer. An A.C. signal is applied to the ridges and the phase or amplitude of the signals coupled...http://www.google.com/patents/US4607213?utm_source=gb-gplus-sharePatent US4607213 - Shielded capacitive mask aligner
