New wafer shape measurement technique for 300mm blank vertically held silicon wafer

Abstract

Wave Front Phase Imaging (WFPI) is a new technique for measuring the free shape of a silicon wafer. To avoid the effects of gravity affecting the wafer shape, the silicon wafer is held vertically while measured using a custom made three-point wafer holder. The wave front phase is measuring using a non-coherent light source that is collimated and then reflected off the silicon wafer surface. The wave front phase is measured using a unique new method that only needs to record the intensity of the reflected light at two or more distances along the optical path. Since only intensity images are used to generate the phase, commercially available CMOS sensors with very high pixel count are used, which enables very high number of data points to be collected at the time required by the cameras shutter speed when using a dual camera setup with simultaneous image acquisition. In the current lab system, a single camera on a linear translation stage is used that acquires 16.3 million data points in 12 seconds, including the stage motion, on a full 300mm wafer providing lateral pixel resolution of 65μm. The flatness of the silicon wafers used to manufacture integrated circuits (IC) is controlled to tight tolerances to help ensure that the full wafer is sufficiently flat for lithographic processing. Advanced lithographic patterning processes require a detailed map of the free, non-gravitational wafer shape, to avoid overlay errors caused by depth-of-focus issues. We present WFPI as a new technique for measuring the free shape of a silicon wafer with high resolution and high data count acquired at very high-speed using a system where the wafer is held vertically without the effects of gravity.