Modeling methodologies and defect printability maps for buried defects in EUV mask blanks

摘要

A detailed analysis of FDTD simulations of EUV multilayers is performed for off-axis illumination angles. The reflections from the top half of the multilayer escape very easily, leading to fast ramp in the reflected field, however, convergence lulls can occur as the multitude of reflections within the bottom of the structure gather coherence before finally escaping and adding to the final reflection. FDTD simulations may need to be run 2-3X longer to ensure proper convergence when simulating EUV multilayers. Additionally, very small wavelength changes of 0.2% caused by numerical dispersion inside of FDTD can shift the Bragg reflection conditions in the multilayer to produce erroneous reflection results for angles >10°. Defect printability maps are generated with a ray tracing methodology for both 2D and 3D defects coated with both a standard and a smoothing deposition process. Defect volume is found to be critical in determining the printability of defects. Finally, FDTD and the ray tracing method are used to simulate defects located inside of the multilayer where a particle may fall on a partially coated multilayer during the deposition process. The ray tracing methodology was found to accurately predict defect printability when compared to FDTD results for defects residing below the 20th bilayer. The maximal printability impact for defects within the multilayer occurs when the defect is placed on the middle bilayer (20th) of the stack. Above this location, the defect impact is lessened since the multilayer is split into two sections and the bottom section is able to retain enough unperturbed multilayers to produce higher reflectivities.