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To realize improved process control at high productivity, the TWINSCAN platform has been developed. This dual stage lithography system combines high throughput TWINSCAN technology with excellent dynamical performance and imaging capabilities required by sub-80nm lithography. The added value of a dual stage system is increased utilization efficiency by performing wafer measurements and other overhead in parallel with exposures thus increasing the net wafers per hour. And additionally these wafer measurements can be done more extensively and create performance advantages of a dual stage system above a single stage system through its predictive and compensation inherent capability. In the measurement position, the wafer surface height is fully mapped using a high spatial frequency measuring level sensor allowing a complete 3-dimensional wafer map to be generated. This allows the wafer surface to be placed optimally in the focal plane of the lens, minimizing the defocus and therefore delivering optimal CD control. In this paper the leveling performance advantage of a dual stage system, proven by several test cases using wafers with known/designed wafer topography, is outlined. One of these cases will address the leveling performance advantage on high topography wafers, which is presented by means of defocus and CD uniformity results. The near ideal leveling performance will be shown by comparison between experimental defocus results and theoretically best achievable defocus given the intrinsic wafer flatness and finite slit size on both inner and edge fields. In contrast to on-the-fly leveling of single stage systems, the separate measurement position eliminates the critical timing relation between the wafer height measurement and actual exposure leveling performance, independent of the exposure scan speed. Besides leveling performance advantages, the dual stage systems full wafer map capability allows in-situ metrology which results in added value, like detailed focus spot monitoring functionality covering all areas to be exposed. Measuring the wafer height before exposure also offers flexibility in the method of deriving the desired stage positioning for exposure and the possibility to extract detailed information for real time wafer flatness monitoring.
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