Design and optical performance evaluation of a variable attenuator in a lithography illumination system

The variable attenuator controls the lithography illumination system’s transmittance and is an important optical component in the dose control process. The coatings of the variable attenuator are critical to its function. In this study, we propose a design approach for a nonpolarizing optical coating whose transmittance varies with the incident angle. The designed variable attenuator achieved a wide transmittance range of 92.4% to 1.1% and excellent linearity within its adjustment range, satisfying the requirements for ArF lithography. Simulation results show that its optical performance is robust when the wavelength and polarization of the incident light change. Furthermore, a method for evaluating the optical performance of the variable attenuator is presented, and a test system is established for experimental validation. The experimental results confirm the conclusions of the simulation, and the developed test system demonstrates superior accuracy compared to the spectrophotometer.