Phase-Modulation Optical Spatial Filters Applicable to Inspection Systems for Patterned Silicon Wafers

We have studied a method to realize an optical spatial filter applicable to inspection systems for patterned silicon wafers by means of a phase-modulation filter. A simulated-annealing method can be used to design a one-dimensional binary phase spatial filter which effectively levels a periodic pattern corresponding to memory cells and so on. The optimum cost function is the maximum optical amplitude on the observation plane of the inspection system. The suitable binary phase ranges from 0.7π to 1.3π. Furthermore, leveling of a nonperiodic pattern corresponding to particles or defects can be avoided when the cells of the spatial filter which make no contribution to the leveling of the periodic pattern are set to have the same phase as that of the cell at the lowest spatial frequency. As a result, the nonperiodic patterns can be emphasized more than periodic patterns. To verify the applicability of the simulation, we fabricated the designed optical spatial filter on a quartz substrate by photolithography and reactive ion etching, and measured its characteristics. As a result, we have confirmed that the fabricated optical spatial filter has the same effect as that of the designed one.