Compensation methods using a new model for buried defects in extreme ultraviolet lithography masks

A new method for predicting the reflection from an extreme ultraviolet (EUV) multilayer is described which when implemented into the new Defect Printability Simulator (DPS) can calculate the image produced by an EUV mask with a buried defect several orders of magnitude faster than the finite difference time domain (FDTD). A new buried defect compensation method is also demonstrated to correct the in focus image of a line space pattern containing a buried defect. The new multilayer model accounts for the disruption of the magnitude and phase of the reflected field from an EUV multilayer defect. It does this by sampling the multilayer on a non-uniform grid and calculating the analytic complex local reflection coefficient at each point. After this step, the effect of the optical path difference due to the surface defect profile is added to the total reflected field to accurately predict the reflected magnitude and phase at all points on the multilayer surface. The accuracy of the new multilayer model and the full DPS simulator is verified by comparisons to FDTD simulations. The largest difference between the two methods was 0.8nm for predicting the CD change due to a buried defect through focus. This small difference is within the margin of error for FDTD simulations of EUV multilayers. The runtime of DPS is compared to extrapolated FDTD runtimes for many simulation domain sizes and DPS is 4-5 orders of magnitude faster for all cases. For example, DPS can calculate the reflected image from a 1 mu m x 1 mu m mask area in less than 30 seconds on a single processor. FDTD would take a month on four processors. The new compensation strategy demonstrated in this work is able to remove all CD error in the simulated image due to a buried defect in a 22nm dense line space pattern. The method is iterative and a full DPS simulation is run for every iteration. After each simulation, the absorber pattern is adjusted based on the difference of the thresholded target image and thresholded defective image. This method is very simple and does not attempt to compensate for the defect through focus, but it does demonstrate the usefulness of a fast simulator for compensation.