Efficient source polarization optimization for robust optical lithography

Source optimization (SO) has become increasing important to improve the process window (PW) of optical lithography systems. Most of current SO approaches modify the source intensity distribution, but fix the polarization state thus limiting the degrees of optimization freedom. In addition, these SO methods simultaneously optimize the imaging performance on focal and defocal planes to extend the depth of focus (DOF) at the cost of increasing the computational complexity. To overcome these above limitations, this paper develops a pixelated gradient-based polarization optimization (PO) method to effectively extend the PW by seeking for the optimal polarization angle for each point source. In order to accelerate the optimization process, the proposed method tries to optimize a compact cost function incorporating the integral imaging performance over a certain defocus range, rather than taking into account the separate performance metrics on different imaging planes. A gradient-based algorithm is exploited to iteratively modulate the polarization angles to keep reducing the cost function. Finally, a post-processing method is applied to reduce the complexity of the optimized polarization angle pattern for improving its manufacturability. Simulations show that the proposed PO algorithm will achieve approximate two-fold speedup compared to the method using a traditional cost function. The proposed PO algorithm is potential to be applied independently or associated with source and mask optimizations to further improve the lithographic performance.