Effect of Near-Field Diffraction in Photolithography of Hexagonal Arrays for Dichroic Filters

Dichroic filters with the hexagonal lattice of circular holes are used as fundamental devices for frequency selection from the infrared to the terahertz (THz) region. In the fabrication of such a high-density periodic array by the contact-mode photolithography, the depth of focus (DOF) turns out to be tremendously shrunk for ideal pattern contrast. In this paper, near-field diffraction of the hexagonal lattice was analytically treated, and the tolerant DOF was estimated in terms of the Talbot distance in photolithography. Affine transformation was applied to the rectangular lattice to derive the near-field diffraction intensity, and a process factor k
₂
was introduced to predetermine the practical DOF in photolithography. Numerical calculations according to the analytic predication agree well with the preliminary experiments, where the intensity evolved drastically to suppress the limited DOF zone. It was also revealed that the practical DOF can be improved by eliminating the high-order diffraction or using the photoresist with higher threshold energy to enhance the process factor. The analytic and experimental results can be used to optimize the high-contrast patterning by optical lithography.