New Optical Metrology Method for Measuring Shape of a Lithography Photo Mask

On product overlay (OPO), with its continually shrinking budget, remains a constraint in increasing device yield(1). The OPO performance consists of both scanner and process-related contributors. Both groups need to be addressed and optimized to minimize the overlay in order to keep up with Moore's law. Examples of process-related overlay contributors are wafer distortion due to patterned stressed thin films and/or etch. Masks can never be made identical since they represent different layers of the device.(2) It has been shown that shape measurements of the wafer can help to correct for most process-induced wafer distortions up to the 3(rd) order(3). However, another contributor to overlay challenges is related to photomask flatness. Wafer overlay errors due to non-flatness and thickness variations of a mask need to be minimized. Overlay metrology capability lags the need for improved overlay control, especially for multi-patterning applications(1). In this paper, we present a new metrology method that generates a very high-resolution shape map of an entire optical photomask optimized for DUV lithography. The technique is measuring the wave front phase change of the reflected light from both the front and backside of a quartz photomask. In this paper we introduce Wave Front Phase Imaging (WFPI), a new method for measuring flatness of an optical photomask that generates a shape map based on local slope. It collects 810 thousand (K) data points on an 86.4mmx 86.4mm area with a spatial resolution of 96 mu m.