MOIRE TYPE MASK ALIGNING METHOD USING PHOTOTHERMOELASTIC EFFECT

摘要

PURPOSE:To align an optical mask with high accuracy by detecting elastic wave vibration generated in a wafer according to photothermoelastic effect generated as diffraction gratings provided on the mask and wafer opposite each other are displaced relatively. CONSTITUTION:The 2nd diffraction grating 3 on the optical mask is irradiated with a laser light beam which is intensity-modulated with an acoustic frequency - an ultrasonic wave frequency and the beam is projected to the wafer 4a made of an elastic vibration material so that the projection image on the grating 3 is in the same direction at the same pitch with the 1st diffraction grating cut in the wafer 4a. Then the elastic wave vibration generated in the wafer 4a by the photothermoelastic effect of the irradiation with the intensity-modulated laser light beam through the grating 3 is detected and converted into an electric signal. Then this electric signal is detected synchronously as to the reference phase of the acoustic frequency - ultrasonic wave frequency. Then the relative position of the optical mask to the wafer 4a in the same direction is set according variation in the output intensity of the synchronous detection proportional to variation in the intensity of a moire image formed owing to the position shift between the projection image of the grating 3 and the grating 4.