Overlay can be improved by self-calibrated XY measuring instrument: a lattice perspective

摘要

Abstract: Although the traditional method of O/L measurement (relative comparison between two levels) has proved to be a practical and cost effective way of measuring overlays, in the future this method will have to be supplemented by other means that require measurement of feature position in an absolute coordinate system and then comparing the output with the database rather than with some other level the accuracy of which remains to be established. The use of well calibrated Coordinate Measuring Instruments (CMI's) is one way to achieve the desired accuracy. But calibrating CMI's is a chicken-or-egg dilemma; you can't calibrate one without an accurately measured artifact, and you can't make the artifact without a well-calibrated instrument. Or so it seems. Positional self-calibration methods were invented to solve this problem and show great promise. But still there are many subtleties that must be resolved before such methods can be trusted. This paper explains the geometric basis for lattice methods of self-calibration and concludes with a theorem that demonstrates one of the striking difficulties that must be faced when relying on self- calibration algorithms.!11