Enhanced model for the efficient 2D and 3D simulation of defective EUV masks

Predictive simulation of EUV (extreme ultraviolet) lithography is important for a better understanding of the technology because the capabilities for experimental studies are still limited. One of the most critical and important issues in EUV lithography modeling is the description of the mask including multilayer defects. Such defects can have a significant impact on the lithographic process. An earlier presented model for the simulation of defective EUV masks, developed at Fraunhofer IISB, is the basis for the new enhanced simulation model. The basic model is a combination of a rigorous electromagnetic field (EMF) computation, based on a finite-difference time-domain (FDTD) algorithm, and an analytical thin film computation. This approach leads to significant reduced computation time and memory requirements compared to other methods, such as fully rigorous EMF simulations. The model can be applied to two and three dimensional defects and masks. This contribution repor ts on the enhancement of the basic model, including new simulation results. The higher efficiency of the new simulation model is based on the assumption that only certain discrete mask illumination angles, the multiples of the discrete diffraction orders of the mask near field, are allowed. This approach allows for an decrease of computation time and memory requirements in half, compared to the computations of the predecessor-model allowing arbitrary angles. Comparisons of the two models, based on the reflected mask near fields, and the aerial images of typical mask structures in combination with a defective multilayer and various defect parameters, are presented. Furthermore, the alternative wave guide method, also applied to the computation of EUV mask near fields, is presented. First comparisons of the enhanced simulation model and the wave guide method are performed.