A Laser Measurement System for Fast and Precise Positioning

Abstract

Recently, there has been an increase in the positioning accuracy required in the production processes for ULSI. For instance, in a wafer stepper for a 4-Mbit DRAM, the required positioning accuracy is said to be more than 0.04μm. There is also a requirement to reduce the positioning times in such processes. To meet these needs, a new laser measurement system which can accurately measure high speed displacement has been developed. When the target's speed is low, its displacement is measured by comparing the phase change of light reflected by the object with the phase modulated by an electro-optic crystal. When the target's speed is high, its displacement is measured using a fringe counting technique. The system achieves an accuracy of ±4nm and a maximum allowab1e measurement speed of 1OOO mm/s.