70nm DRAM Intra-field CDU Improvement by Dose Modulation on Mask Transmittance

DRAM intra-field CD uniformity (CDU) demand becomes more crucial with pattern size shrink and wafer die or memory size expanding. Intra-field CDU error mainly comes from mask CD error, scanner exposure and wafer process. This study makes use of a method to extract systematic CDU error from multi-field CDU results. Based on the information of the systematic CDU error, localized mask transmittance modulation is implemented to compensate the intra-field systematic CDU error on wafer. A focused ultrafast laser beam forms shading elements in mask quartz substrate. Mask transmittance modulation is controlled by the shading element density variation. This study will demonstrate the intra-field CDU improvement result, CD modulation calibration validity, CD proximity variation result and mask inspection result etc.