A simple method of source optimization for advanced NAND FLASH process
2011
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.
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