Material Removal And Surface Evolution Of Optical Surface During Ion Beam Machining

Deep ultraviolet (193nm) lithography and next-generation Extreme ultraviolet (13.5nm) lithography are the pivotal equipment of semiconductor industry and ultra large scale integration strategy of our country. Therein, the projection lithography lens is the key component in the lithography system, and a sub-nanometer level surface precision over the entire range from low-spatial frequency surface figure to high-spatial frequency surface roughness has to be guaranteed in fabrication. To achieve this aim, ion beam machining technology based on physical sputter has increasingly been utilized to correct the figure error of optical surfaces, by virtue of its atomic accuracy, non-contact processing and gentle interactions between ion beam and surface. Targeting the ultra-precision large optical lens (> 100mm), we have studied the material removal mechanism, surface atoms behaviors on the optical surface, and the essential ion beam processing technology, etc., including the following four parts:1. From interactions between energetic ions and solid atoms, a linear removal model, including the sputtering-related mechanisms (primary sputtering, secondary sputtering, surface reflection, geometrical shadowing, redeposition, etc.) and the diffusion-related mechanisms (thermally-activated surface self-diffusion, effective ion induced surface diffusion, ion enhanced viscous flow, ballistic drift diffusion, Ehrlich-Schwoebel effect, etc.), is founded to investigate roughening and smoothing characteristics of these diverse atomic mechanisms of surface mid-to-high spatial frequency roughness.2. Based on lattice kinetics and Monte Carlo strategy, we establish a mathematic probability model of ion beam processing. This model embodies diverse sputtering mechanisms and diverse diffusion mechanisms, and further embodies the evolution of surface roughness and surface topography, by means of diverse atomistic behaviors during ion beam sputtering. The study shows that, dominated by different atomistic surface processes, the surface always exhibits a two-stage evolution with time-from smoothing to roughening, and in the late roughening stage surface, roughness always follows the regular power law. These results were consistent with experimental observations.3. Aiming at the preferential sputtering on multi-component material surface during ion beam machining, we have founded a multi-element two-field coupling model. Compared with kinetics lattice Monte Carlo model and experiments using fused silica, a surface concentration reorganization phenomenon is confirmed on the surface due to preferential sputtering:For silica material, surface stoichiometry changes from Si 33% to Si 40%-42%, and thus surface reflectivity increases by 35% according to optical detection. Surface concentration reorganization induced by preferential sputtering indeed affects the optical performance.4. To figure the large surface, we model the CCOS processing equation. Combining sputtering experiments, continuum model and KMC model we have obtained and studied the machining removal function, which is proven to be accurate and efficient in time, energy and incident angle parameters. Good time-stability and good gradient-stability lay a firm foundation for ion beam figuring the optical surface. Therefore, after discretization and transformation, we solve the dwell time array using non-blind inverse filtering deconvolution algorithm according to the obtained removal function via pre-experiments. We have figured a plane mirror of 170mm in diameter from 7.367 nm RMS to 1.175 nm RMS in shape and smooth it to 0.56nm RMS in roughness; we have figured a concave spherical mirror of 130mm in diameter to 1.08 nm RMS in shape and have smoothed it to 0.4nm RMS in the central region and 0.6-0.7nm RMS in the edge region.