Polarization camera based fringe locking system for scanning beam interference lithography

A fringe displacement during the exposure process in scanning beam interference lithography (SBIL) systems leads to wavefront errors in linear gratings. The exact orientation of the fringe pattern along the scan direction is also essential for the production of high-quality gratings. In this paper, we report on the development of a fringe stabilization system with a separate measuring beam, which can precisely determine the fringe movement during the exposure of the photoresist and simultaneously record the orientation of the fringe pattern. This is specifically designed to minimize low-frequency phase errors originating in the lithography writing head itself, which can be a dominant source of instability independent of the stage positioning. Using polarization camera based phase-shifting interferometry, we determine the phase offset between the two interfering beams in real time, and thus reduce the fringe movement to single-digit nanometers by moving a piezo-actuated mirror. In addition, we can use the area-based phase measurement to align the setup and track the rotation of the fringe spot during exposure. In combination with a nano positioning and measuring machine, a fringe observation system was implemented that can precisely record the fringe displacement in the substrate plane. This allows the performance of the fringe stabilization system to be quantified.