Rapid prototyping of infrared bandpass filters using aperture array lithography

We demonstrate the prototyping of infrared bandpass filters, which consist of cross-shaped openings in a thin gold film, using ion beam aperture array lithography. In the lithography process, a stencil mask containing a periodic array of square apertures is irradiated by a broad beam of helium ions. The ions that pass through the openings expose the resist on a substrate that is placed in close proximity and cross-shaped filter structures are printed by moving the stage underneath the substrate, thus allowing for rapid formation of periodic patterns. We have fabricated filter patterns with peak transmittance ranging from 53% to 67% at wavelengths between 1.2 and 1.3 μm that exhibit high reflectance for longer wavelength radiation. The prototyping throughput for masks with 2 μm pitch patterns was about 2 cm²/h. The spectral performance of the prototyped filters was measured. Large-area, second-generation masks with 667 nm pitch had a lithography throughput of 300 cm²/h and were used to print filter patterns of similar quality.