Overlay accuracy of a synchrotron radiation stepper evaluated by two-mask double exposure

Abstract

The overlay accuracy of a synchrotron radiation stepper has been evaluated when different masks are used as in actual exposure. Using several masks requires both high repeatability and accurate offset controllability to perform the proper alignment of the stepper. To this end, a two-mask double-exposure method that considers mask error has been devised. It yields a 3σ deviation of 25 nm for the overlay repeatability of our SS-1 stepper. This is almost the same as in the one-mask double-exposure method, which means that the stepper effectively has an alignment repeatability of less than 25 nm. In addition, a mask stage is developed to improve the offset controllability and enlarge the stroke. The resolution and stroke of the stage are 20 nm and ±1000 μm, respectively, for x and y directions, and 0.5 and ±800 μrad, respectively, for theta. This stepper facilitates the reduction of alignment offset deviations to less than ±10 nm in the x and y directions and less than ±1.0 μrad for rotation.