Extensive testing of the 193 nm laser damage behavior of fused silica has been performed over the past few years by several researchers. The results have shown that compaction and rarefaction / expansion of the material can occur. The actually observed process depends on the studied fused silica type, the used energy density, and laser pulse number at constant pulse length. In order to check the influence of the different laser parameters in more detail, an experimental set up has been constructed that allows us to investigate not only the influence of the energy density and laser pulse number but also the effect of the integrated square pulse width on the laser damage behavior. An optical delay line is used to create a longer integrated pulse width than the natural laser pulse width. To make these tests relevant to the microlithography community, the integrated energy densities chosen for these tests span the range typically found in the projection optics of a 193 nm excimer laser-based microlithography tool. The samples are exposed to several billions of pulses with wavefront measurements made periodically.
Excimer laser light sources for photolithography are subject to a cycle of ever-tightening precision requirements, dictated by the design-rule shrinks planned into the industry roadmap. But pulse-to-pulse stability of the center wavelength of the emitted light is limited by the presence of vibration in key components and structures. This paper covers the application of Active Vibration Control (AVC) technology to an excimer laser to mitigate the effects unwanted vibration, and enable compliance with anticipated future stability specifications. The laser system is described, from a structural-dynamics point of view. A systematic approach to vibration diagnostics is presented, with experimental results to support key conclusions regarding the types and sources of vibrations. Next, analytical assessment of active control performance is discussed, followed by breadboard-type implementation results showing reductions of >30% in a key stability performance metric.
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