New technique for measuring free-form wafer shape for feed-forward overlay corrections

On product overlay (OPO) is one of the most critical parameters for continued scaling according to Moore's law. Besides the lithography scanner, also non-lithography processes contribute to the OPO performance. For example, processes like etching and thin film deposition can introduce stress, or stress changes, in the thin films on top of the silicon wafers. In general, the scanner Higher Order Wafer Alignment model up to 3rd order (HOWA3) has proven to be adequate to correct for most process-induced wafer distortions. This model is typically used with 28 wafer alignment marks placed across the wafer to correct for more global stress-induced distortions. It is evident that if the stress variation manifests itself on shorter length scales, either more alignment marks are needed in combination with a more sophisticated wafer alignment model, or an alternative measurement of the wafer distortion is required. A viable alternative to characterize local wafer deformations is by measuring the free-form wafer-shape change due to processing. In case the wafer-shape change can be translated into a wafer distortion map, it can be complementary to what is already captured by the scanner wafer alignment model. In this paper, we would like to explore this functionality that is based on a new method to measure the free-form wafer shape. Wave Front Phase Imaging (WFPI) generates the wafer shape by registering the intensity of the light reflected off the patterned or blank silicon wafer surface at two different locations along the optical path. The wafer is held vertically to allow for the free-form wafer shape to be measured without being affected by gravity. We show data acquired on specialty made silicon wafers using a WFPI lab tool that acquired 16.3 million data points on a 300mm wafer with 65μm spatial resolution. The obtained free-form wafer-shape measurements are fed into existing prediction models and the resulting wafer distortion maps are compared with scanner measurements.