Visual Determination of Optimal Focus in a Wafer Stepper
Original Publication Date: 1986-Jul-01
Included in the Prior Art Database: 2005-Mar-09
A test method for adjusting an optical imaging system to optimal focus uses an image sequence generated from an optical grating in a saturable photosensitive medium by systematically varying the optical distance and the exposure time such that a clearly visible grating image is obtained only at optimal focus. The proposed method is based on the modulation transfer function (MTF) of the optical image which decreases as the distance from the optimal focus increases. The image sequence is arranged in matrix fashion, with the optical distance increasing along the vertical axis in steps of 0.5 mm, starting at a reference point not too far from the optimal focus, and with the exposure time increasing from right to left in steps of, say, 0.1 sec.