Smart Masks for Next Generation Lithography

摘要

Next generation lithography tools, such as x-ray, ion or electron beam use thin film membrane windows for the exposure masks. Systematic aberrations due to heat distortions during exposure and other influences are inevitable. The possibility of integrating an active correction system by mechanical stretching with precision actuators is investigated. The basic idea is to measure the placement error of the mask at a finite number of precision points and use these error measurements to stretch the membrane with in-plane actuators back to their proper locations. The actuation could be implemented by micro-actuators that are monolithically attached directly to the mask membrane. In order to implement such a system, the following are required: 1. A metrology system that precisely measures the displacement of a set of precision points; 2. Micro-actuators that can be directly attached to the mask and offer sufficient force and displacement to strain the mask; and 3.A control algorithm and system that correlates 1. with 2. The design of this control system requires selecting good locations for the actuators and precision points. A quantitative assessment of the performance of the system can be obtained from the condition number of the influence matrix [A]. Experiments and numerical calculations indicate that it is likely that the matrix condition number can be an effective tool in evaluating various actuator and precision point configuration designs. As a proof of concept, experiments were conducted on a 2000X large-scale model. The mask model was made of a thin sheet of polycarbonate, which has excellent elastic, as well as photoelastic properties. The results obtained by mask stretching were in excellent agreement with the predicted precision point displacements and were confirmed by a finite element analysis.