MICRO: MEMS Lithography

Michael S. Cassady, Ultratech Stepper A case study demonstrates that the use of 1 steppers can result in higher microlithography throughput and MEMS device yields than the use of 1 wafer steppers and scanners have been used for microlithography applications since the early and mid-1980s. However, because of their higher resolution capability and superior stage and alignment performance, 1 steppers have continuously replaced older-generation 1 scanning projection aligners. Cost and yield factors have prompted many fabs to decide in favor of stepper technology. This article, based on a study conduced at a West Coast IC facility with the assistance of Ultratech Stepper (San Jose), compares stepper and scanner performance. The study revealed that the manufacturer could achieve performance, accuracy, productivity, defectivity, yield, and—ultimately—return-on-investment advantages by converting to 1 stepper technology or even by implementing a mix-and-match strategy. The technology transformation enabled the chipmaker to increase throughput by 12,000 wafer starts in a given time period by lowering mask costs, boosting mask accuracy, improving total overlay, and upping speed binning. Although the semiconductor industry is advancing toward sub-100-nm design rules, many manufacturing facilities fabricate 0.8-µm and larger devices. Facilities producing 2-µm and larger devices have traditionally used 1 projection, proximity printers, or even contact printers. Manufacturers realized early on that contact printing creates large yield losses because that method causes masks to come into direct physical contact with the wafer, allowing residues from the lithography process to be transferred to wafers, generating killer defects. To avoid that problem, the industry introduced proximity and projection aligners. However, proximity aligners require accurate gapping, an added complexity that translates into maintenance and productivity issues. projection scanning systems, and especially wafer step-and-repeat systems, allowed for the complete physical decoupling of the wafer and mask or reticle in the printing process, greatly improving productivity and yield. Interestingly,