26 May 1995 New spatial frequency doubling method for sub-0.15-um optical lithography
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A new method for fabricating fine periodic patterns with a large depth of focus is proposed. It is based on flattening the 1st-order intensity distribution and extracting the 2nd-order intensity distribution which has a frequency twice that of the mask pattern. The resolution limit is improved up to the cutoff frequency of the projection system and the image can be created independently of the defocus and the pattern direction. This method is achieved by superposing the images at two focal planes under coherent illumination and it was demonstrated by KrF lithography with a conventional binary mask. A 0.14 micrometers lines-and- spaces pattern was fabricated with a large depth of focus of 2.0 micrometers . A contrast enhancement technique and pattern edge correction method needed for actual application of this method in device pattern fabrication are also discussed.
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Akihiro Otaka, Akihiro Otaka, Yoshio Kawai, Yoshio Kawai, Akinobu Tanaka, Akinobu Tanaka, Tadahito Matsuda, Tadahito Matsuda, } "New spatial frequency doubling method for sub-0.15-um optical lithography", Proc. SPIE 2440, Optical/Laser Microlithography VIII, (26 May 1995); doi: 10.1117/12.209307; https://doi.org/10.1117/12.209307

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