Gap control apparatus and method utilizing heterodyne signal phase

Abstract

Two diffraction gratings having the same grate pitch are respectively attached to a semiconductor wafer and a mask to be used in lithography in production of a semiconductor, such that the diffraction gratings are parallel to each other. When first and second frequency lights respectively having different frequencies, are incident, through a light path adjusting system, upon the mask- and wafer-side diffraction gratings in the directions of the first order diffraction angles symmetric with respect to normal-line directions of the diffraction gratings, the frequency lights first orderly diffracted by the diffraction gratings, interfere with each other, thus forming mask- and wafer-side interference lights. An initial phase difference .DELTA..phi.0 between the interference lights, is detected. Then, the light path adjusting system is adjusted to shift the interference lights by the same angle .theta.1 in the same direction, and a later phase difference .DELTA..phi.1 between the interference lights thus shifted, is detected. Based on the relation of .DELTA..phi.1-.DELTA..phi.0=(2.pi.Gtan .theta.1)/(P/2), a mask-wafer gap G is detected with high precision according to the change in phase difference (.DELTA..phi. 1-.DELTA..phi.0). This improves the precision of setting the gap G in a semiconductor production.