Fabrication of 1-μm-width metal gratings with undercut-assisted lithography for infrared wire grid polarizers

Abstract A wire grid polarizer consists of periodically arranged metal wires, and the pitch between these wires is a critical parameter that determines the wavelength range in which polarization occurs. In this study, we introduced and further developed an undercut-assisted lithography strategy, through which highly uniform metal gratings with a line width of 1 μm were successfully fabricated. To fundamentally eliminate problems that could arise during the lift-off process of the grating structures, a controlled wet-etching step was proposed to intentionally form an undercut in the layer beneath the photoresist. This prevented conformal sidewall deposition of the metal, thereby enabling the achievement of a highly uniform sub-micrometer-scale pattern. The proposed method exhibited versatility across diverse metals (e.g., Au and Ti) and substrates (e.g., quartz and Si), underscoring its robustness and scalability. Moreover, the fabricated metal gratings demonstrated polarization characteristics in the infrared (IR) region (>10 μm), suggesting their potential application as IR polarizers. Distinct differences in the transmittance profiles were observed between the parallel and perpendicular stacking orientations beyond the specific onset wavelengths, confirming the polarization characteristics of the metal gratings. A clear linear correlation was evident between the grating pitch and the polarization onset wavelength, which was consistent with optical theory. Additionally, no such effect was observed for the nonmetal gratings, confirming that the polarization originated from the free-electron response in the metal wires. The uniform metal patterning within periodically structured SiO 2 frameworks fabricated via undercut-assisted lithography, will provide a practical and robust route for various application fields.