Resist performance in soft x-ray projection lithography

An important issue for resists used in soft x-ray projection lithography (SXPL) is high absorption. Absorption coefficients of both organic and inorganic formulations range from 2-6 micrometers -1, requiring the use of very thin (50-100 nm) layers to avoid excessive absorption and resulting pattern sidewall degradation. Such thin films are typically used as the imaging layer in a bi-or tri-level processing scheme to achieve the required pattern depth. In this work, we report experimental and theoretical studies of imaging performance in the resists PMMA and SAL 601 at an SXPL exposure wavelength of 13.9 nm. Absorption coefficients ((alpha)) have been measured for, and SXPL images recorded in both of these resists. A Mo/Si multilayer-coated Schwarzschild objective having known aberrations and illuminated by a laser plasma soft x-ray source has been used to produce images. Calculated aberrated aerial images are used in conjunction with the measured values of (alpha) to model the expected resist profiles and these are compared to experiment. Imaging performance as a function of resist absorption and estimates of resist exposure latitude are also discussed.