Aerial image formation with a KrF excimer laser stepper

Abstract

AbstractWe have studied the imaging characteristics of an excimer laser stepper as a function of laser wavelength. Focus, magnification, distortion, CD process window, and laser performance were all examined, and an operating wavelength was selected. The self‐metrology capabilities of both the laser and stepper were tested to insure that the system would set itself to the correct wavelength upon initialization, and maintain that setting during volume production activity. The use of an on‐line stepper image sensor to evaluate the quality of the aerial image, coupled with the built‐in wavelength tracking system as part of the laser optics, was found to provide the required closed loop system for wavelength and focus control. Based on these experiments, we have determined that a quick aerial image check before each production lot should suffice to insure focus control of better than 0.25 microns and magnification errors less than 15 nm during the entire time required to expose the lot.