An innovative design of wafer height and tilt sensor for lithography systems

Abstract

Since lithography systems have limited depth of focus, precise and real-time measurement of wafer height is important for the image quality control. The traditional grazing incidence height sensor for lithography system consists of two basic components: a light source to illuminate the wafer surface and a detection surface to receive the reflected light beam from the wafer. Mathematical study of this type of height sensor shows that they will fail to differentiate the height difference of the wafer from the tilt of the wafer. In order to obtain the tilt information, height sensors with more complicated designs are used, and examples of these designs are briefly reviewed. A new design for a wafer height sensor is proposed here. By adding an independent detection surface, the new design is able to obtain wafer height and tilt information without using a complicated optical design, which results in easy alignment and calibration.