SPATIAL-FREQUENCY DOUBLING METHOD FOR SUB-0.15-MU-M OPTICAL LITHOGRAPHY

A new method for fabricating fine periodic patterns with a large depth of focus is proposed. It is based on flattening the 1st-order intensi ty distribution at the image plane and extracting the 2nd-order intens ity distribution which has double the spatial frequency of the mask pa ttern. The resolution is improved up to the incoherent. cutoff frequen cy (2NA/lambda) and the image can be created independently of the defo cus and the pattern direction. This method is achieved by superimposin g optical images with two focal planes under coherent illumination, Us ing this method, a 0.14-mu m lines and spaces pattern can be fabricate d by KrF lithography with a large depth of focus of 2.0 mu m.