22 March 2011 Focus drilling for increased process latitude in high-NA immersion lithography
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Abstract
In this paper we discuss a laser focus drilling technique which has recently been developed for advanced immersion lithography scanners to increase the depth of focus and therefore reduce process variability of contact-hole patterns. Focus drilling is enabled by operating the lithography light-source at an increased spectral bandwidth, and has been made possible by new actuators, metrology and control in advanced dual-chamber light-sources. We report wafer experimental and simulation results, which demonstrate a process window enhancement for targeted device patterns. The depth of focus can be increased by 50% or more in certain cases with only a modest reduction in exposure latitude, or contrast, at best focus. Given this tradeoff, the optimum laser focus drilling setting needs to be carefully selected to achieve the target depth of focus gain at an acceptable contrast, mask error factor and optical proximity behavior over the range of critical patterning geometries. In this paper, we also discuss metrology and control requirements for the light-source spectrum in focus drilling mode required for stable imaging and report initial trend monitoring results over several weeks on a production exposure tool. We additionally simulate the effects of higher-order chromatic aberration and show that cross-field and pattern-dependent image placement and critical dimension variation are minimally impacted for a range of focus drilling laser spectra. Finally, we demonstrate the practical process window benefits and tradeoffs required to select the target focus drilling laser bandwidth set-point and increase effectiveness of the sourcemask solution for contact patterning.
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Ivan Lalovic, Jason Lee, Nakgeuon Seong, Nigel Farrar, Michiel Kupers, Hans van der Laan, Tom van der Hoeff, Carsten Kohler, "Focus drilling for increased process latitude in high-NA immersion lithography", Proc. SPIE 7973, Optical Microlithography XXIV, 797328 (22 March 2011); doi: 10.1117/12.882391; https://doi.org/10.1117/12.882391
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