Abstract
The impact of reticle imperfections on resist critical dimension (CD) variation has greatly increased as design rules shrink to smaller than exposure wavelength. Resolution enhancement techniques, such as optical proximity correction (OPC) and phase-shift mask (PSM), add a great deal of complexity to the mask manufacturing process. Stringent requirements in wafer processing make reticles a crucial factor in CD control. However, making a perfect reticle is a significant challenge for mask manufacturers. In this paper, the strong correlation between reticle and resist CD variation is reported. Multiple sources of hidden CD errors due to imperfect reticles are discussed. Examples include butting errors, grid snapping, OPC model incompatible among reticles with same design rule but with different mask processes, phase-angle and transmission variations in PSM, undetectable reticle defects from reticle inspection, and small reticle defects that are often classified as 'false' defects. Root causes are analyzed and procedures to minimize hidden CD errors are proposed.
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