Optimizing photon sieves to approach Fresnel diffraction limit via pixel-based inverse lithography

Abstract

This article presents a novel, rigorous method to construct photonic sieves whose imaging resolution approaches the Fresnel diffraction limit. Photon sieves (PS), which use holes in place of Fresnel Zone Plate (FZP) as diffractive elements, offer many advantages, such as low fabrication cost, higher contrast, narrower main lobe and smaller side lobes. Particularly in EUV imaging, the minimum feature size for fabricating a FZP limits the achievable NA or, equivalently, outermost zone width. And PS allows for much higher NA than FZP, because its hole size can be much larger than the outermost zone width in a FZP without losing resolution. Thus it is of great interest to apply PS in EUV imaging or maskless lithography (ML2). When used in imaging, the Point Spread Function (PSF) on the focal plane should approximate a δ-function. To explore the optimal design of PS, we presented a technique from inverse lithography that treats the PS as a mask, uses Fresnel diffraction as the transfer function, and seeks to minimize the difference between its coherent PSF and δ-function. The performances of optimized PS, common PS and FZP are presented. Optimized PS shows slightly smaller focused spot size than common PS (50 nm vs. 53 nm FWHM with NA 0.12), indicating the simple version of PS is close to optimum in low NA. However, when comparing the imaging results of 40–60 nm half-pitch gratings using these PS as objective lens, the optimized PS shows contrast over 100% higher than common PS. This is attributed to the first sidelobe of the PSF, which has 180° phase shift from the main lobe and reduces image blurring. So the slight reduction in spot size and favorable sidelobe of the optimized PS lead to significantly better image quality when the feature size to be imaged is near the resolution limit. The optimized PS also shows better tolerance of fabrication errors. Both PS are much superior to FZP, indicating great potential of PS in EUV microscopy. In ML2 with UV light, however, PS does not offer advantages over FZP.