Focusing Method for Projection Exposure Systems
Original Publication Date: 1988-Jan-01
Included in the Prior Art Database: 2005-Feb-14
Periodic checking and adjustment for best focus of a projection printer is performed by using alignment optics modified to permit bright field viewing and adjusting the projection lens for best focus of a test surface, e.g., an etched wafer. Referring to the figure, housing 2 contains the projection lens and has a means 4 for focusing, i.e., a means to adjust the distance from the projection lens to the surface of sample 6. Sample 6 is a photoresist-coated semiconductor wafer when the system is in use for alignment of a mask image or exposure to actinic light transmitted by a mask. Non-actinic light 8 is transmitted through element 10 which is a dark field mirror (a mirror with a hole in its center for transmission of light beam 8) for alignment of a mask to a wafer.