Modification of boundaries conditions in the FDTD algorithm for EUV masks modeling

Rigorous modeling of diffraction from the mask is one of the most critical points in the extension of lithography simulation from its traditional spectral range between 150 and 500 nm into the area of extreme ultraviolet (EUV) between 10 and 15 nm. A typical EUV mask is made of a reflective multilayer (Mo/Si or Mo/Be for example) deposited on a substrate. Above the multilayer, a buffer layer acts as an etch stopper, and an absorber is used for the mask pattern. If we limit our scope to layers without defect, most of the mask parts (i.e. the buffer layer, the reflective multilayer and the substrate) can actually be described by analytical methods such as transfer matrices. Therefore we decided to split the mask into two parts : the first part includes the absorber and the buffer layer and it will be studied using a finite- difference time-domain (FDTD) algorithm, the second part includes the reflective multilayer and the substrate and it will be simply described by transfer matrices.