Lithography process simplification for reduced cost of ownership
Cost of ownership plays an important role in lithography process materials and methods decisions. Process simplifications brought about by layer-to-layer synergy drive significant cost of ownership advantages for multilayer lithography systems such as the Brewer Science® OptiStack® system. Savings in mask engineering and manufacture are the greatest cost difference. Optical proximity correction (OPC) algorithms need only be determined once for all layers, rather than individually for each layer, which results in fewer mask corrections. Advanced devices with smaller critical dimensions benefit most from this system in that there are more layers at smaller critical dimensions, requiring greater mask design and production costs.
For the purpose of providing a cost of ownership estimate, we compared a process using the OptiStack® system on eight layers to a typical dyed resist process. Note that the OptiStack® system process would use the same OPC algorithm on all layers. More layers utilizing the same system will result in greater savings. We assumed that 2500 wafers would be printed from each mask; this is said to be a typical usage - ASIC masks may be imaged on as few as 500 wafers. Mask design and mask production are the significant cost components in any analysis of lithography process cost of ownership. For a high-performance mask set with 90-nm design rules, individual binary chrome-on-glass masks can cost $100,000 to produce, and phase shift masks can cost as much as $124,000. As a conservative estimate, the average mask cost was taken to be $70,000 because some layers can be printed using DUV tools and masks. Accounting for these and other costs, the OptiStack® system saves about $674 per wafer. The cost comparison is shown in Table I. “Other Optistack system costs” are caused by additional materials and processing.
Using this type of multilayer system is likely to bring even more benefits to the overall fab cost and efficiency. Other areas to be studied in future work are the impact of simplified inventory arising from the need for fewer custom materials by layer or device.
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2. Vivek Bakshi, Ed., EUV Lithography, SPIE Press, Bellingham, WA, 2009.
3. B. Grenon and S. Hector, “Mask costs, a new look,” Proceedings of SPIE, vol. 6281, 2006, 62810H.