Novel mask-wafer gap measurement scheme with nanometer-level detectivity

We describe a means of measuring the gap between mask and substrate in an x -ray lithography system. The method does nor require that the gap be scanne d. The method encodes the gap in the spatial phase, spatial frequency, and separation of sets of interference fringes. The fringes result from the dif fraction from a checkerboard on the mask, with constant period in one direc tion and varying period in the transverse direction. The separation of frin ge sets gives an unambiguous measure of gap when the mask is approaching th e substrate, from 400 to 30 mu m. At the smaller gaps used for exposure, ch eckerboards with different chirp periods are utilized to indicate the gap w ithout ambiguity. The phases of the fringes as a function of gap were calib rated with a Fabry-Perot interferometer. The repeatability of the phases be tween consecutive scans of gap was found to have a 5 nm standard deviation. This method of measuring gap may prove useful in a variety of applications that require a controlled gap between two plates. (C) 1999 American Vacuum Society. [S0734-211X(99)12306-0].