The use of CGS technology in a 3D manufacturing environment

As semiconductor processes shift toward the 3rd dimension, lithography overlay and wafer topography control is becoming increasingly difficult, particularly at the wafer edge. Overlay is measured after lithography, thus cannot account for an increasing proportion of wafer-to-wafer displacement variability. For defocus, edge roll up/down contributes to regions where the wafer topography exceeds the scanner depth of focus. In thick film stacks such as those used in BiCS 3D flash fabrication, both displacement and defocus control are particularly challenging, requiring an improved patterned wafer inspection solution. A novel technique for measuring both is Coherent Gradient Sensing (CGS) interferometry, which is capable of generating a high-density data set of topography, displacement, and stress on the full wafer. This paper is the outcome of a collaborative effort between Toshiba Corp. and Ultratech Inc. to evaluate the suitability of the CGS interferometry technology in a manufacturing environment. Currently the system is proven for quality control, process monitoring, and Displacement Feed Forward (DFF), which calculates displacement ahead of the lithography process and feeds it forward to the scanner as coefficients of correctable terms to be combined with conventional exposure corrections. Also, new analysis algorithm improvements have achieved a steady state throughput of 126 WPH showing the technology is capable of obtaining data within a time frame suitable for a high-volume manufacturing environment. Future collaborative work will be outlined including warpage control, planarization, edge yield, and inspection roadmap requirements for 3D processes.