Laser Step Alignment For A Wafer Stepper

Abstract

The Laser Step Alignment (LSA) is a through-the-lens, field-by-field alignment system developed for an optical step-and-repeat system with a reduction projection lens (wafer stepper) intended for one micron or submicron lithography. The laser beam used for LSA is focused on a wafer through the projection lens. The alignment marks on the wafer are in the shape of diffraction gratings. The mark detection is performed by moving the wafer stage so that the alignment mark travels across the laser beam. The passage of the align-ment mark produces diffracted and scattered light, which is photo-electrically detected as the alignment signal. The position of the alignment mark is accurately measured by the laser interferomter attached to the wafer stage. The LSA system achieves overlay accuracy (20) less than 0.15 Pm for processed wafers. LSA uses a 633 nm He-Ne laser, so the photoresist is not sensitive to it, and LSA is well applicable to multilayer resist processes. The alignment time is about 0-6 seconds per shot in field-by-field mode. This LSA is attached to the Nikon wafer stepper (NSR), so field-by-field alignment, several types of alignment, and other measurement for compensation are performed. An appropriate alignment may be selected to meet the accuracy and throughput required in each case.