A simple method of source optimization for advanced NAND FLASH process

We have developed a very simple source optimization (SO) method for L/S and C/H critical layers patterning of advanced NAND FLASH. Starting from the strong off-axis illumination shape which is optimized for the finest structure of the mask pattern, a systematic procedure is performed to extract the optimum parameters of additional assist sources to balance the imaging performance (DOF, contrast and optical proximity effect, etc.) of dense/sparse/rough patterns. Performance equations (linear optimization) with performance map (sensitivity) are utilized to search the best combination of intensity for each assist source. For C/H pattern, the optimization procedure is modified to solve the non-linearity and non-continuity problems on the relationship between assist source intensity and each imaging performance. Finally, optimized source shapes have been successfully demonstrated and verified on 40 nm node NAND FLASH L/S and C/H critical patterns despite the simplicity of the optimization method, without utilizing SO dedicated software.