Hyper high numerical aperature achromatic interferometerfor immersion lithography at 193 nm

An apparatus for immersion interferometric lithography is described here where the interfering beams are created by illuminating a first diffraction grating followed by a second diffraction grating recombining the diffracted beams onto the photoresist plane. The main advantage of this system is to be achromatic: thus it is possible to use a basic commercial ArF excimer laser as the exposure source. We present here the calculations made to evaluate the different parameters that can influence the depth of focus in the immersion configuration. As the setup is mainly based on the two diffraction gratings, it matters to properly design it. The purpose of this article is to show the optimization made on the diffraction gratings in taking into account their fabrication process since they are fabricated using the capabilities of the silicon line available in our laboratory. On one hand, calculations have been done to determine the second grating period as a function of the first grating period and the "immersion numerical aperature." By simply adding a fluid to a "dry" system, we will indeed be able to improve the depth of focus but not the resolution. In playing with the diffraction grating periods, we are able to benefit from the introduction of the immersion fluid. We have performed simulations in order to optimize the grating diffraction efficiency as a function of the etch depth and the fractional linewidth. Finally, we report on the results obtained with the achromatic immersion interferometer. The apparatus was used with a 193 nm GAM excimer laser to print resist patterns having a period of 100 nm with excellent contrast.