Driven by the growing demand of manufacturing modern integrated circuits with ever shrinking sizes, advanced lithography technologies are being extensively developed such that next generation lithography tools, e.g., extreme ultra-violet (EUV) lithography systems, are able to provide higher resolution, higher throughput, and more robust Performance. In order to accurately print complex patterns with nanometre-scale critical dimensions (CDs) onto a semiconductor wafer, e.g., by EUV lithography, positioning of wafer as well as reticle stages is critical. Hence, stage position must be tightly monitored and precisely controlled so as to ensure stage positioning errors are well below wafer CDs. Being able to offer sub-nm resolution, laser interferometry is a well-established metrology and has been widely employed in lithography and inspection tools for measuring and locating stage positions.
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