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It is now generally accepted, that optical lithography will be the mainstream approach to manufacture the I Gbit DRAM device generation with minimum features between I 80 nm (first working samples) and I 50 nm (projected fabrication). This development demands a considerable tightening of mask specifications. The printing of 180 nm I 150 nm features on a 193 nm-, 0.6 NA- tool is a highly non-linear pattern transfer process (ki = 0.56 I 0.47). Therefore, mask irregularities (defects, linewidth variations) will print easier than at larger dimensions. This paper presents results of full lithography simulations for the printability of mask linewidth variations and mask defects. Assuming, that the wafer linewidth error budget is shared between the lithography process and the mask as it is currently done (75% for the process, 25% for the mask), the linewidth uniformity on a mask has to be better than 12 nm for 180 nm designrules and better than 8 nm for 150 nm designrules. Masks have to have no defects larger than 100 nm resp. 70 nm (all those numbers are given at the 4x mask!). These numbers are almost a factor of 2 tighter than assumed in published mask specification roadmaps. Besides tightening the current specifications, the mask roadmap has to include other important parameters: We show, that the butting error of the mask writing tool, corner rounding, and the quality of the mask repair have substantial impact on the linewidth error on the wafer. Since mask defects print proportional to the squareroot of their area, the definition of mask repair quality has to be revised. Current mask repair processes cannot provide adequate repair quality. Since masks with these specifications are - in our point of view - not easy to make, we propose to reevaluate carefully the sharing of the wafer Iinewidth error budget. In other words, 193 nm lithography must dedicate a significantly larger portion of its (actually not so large) process window to the mask.
Keywords: Optical Lithography, Mask Specifications, Mask Defect Printability, Mask Repair, Linewidth Error Budget
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