Development of Silicon Micromachined Waveguide Filter-Banks for On-Chip Spectrometers

TL;DR

This work tackles the challenge of compact, high-throughput LIM-capable spectrometers by presenting a metalized silicon micromachined waveguide filter-bank in a split-block architecture that is compatible with MKID detectors. The authors validate a single-channel Au-plated filter at 260 GHz, achieving a measured resolving power of $R=263$ and a loss quality factor of $Q_{loss}=1116$, with a room-temperature spectral efficiency of $η≈0.384$ in good agreement with HFSS simulations. The fabrication flow leverages DRIE on 150 mm SOI wafers and electroplating to ensure complete sidewall metallization, addressing prior sputtering limitations. A scalable path is outlined for an 80-channel bank operating from 200–300 GHz with ~74% band efficiency, which could be stacked into large, dense detector mosaics (e.g., 400-pixel, 32,000-detector MKID arrays) for wide-field LIM. Overall, the approach offers a compact, high-density, low-loss platform that could significantly accelerate on-chip spectrometry and LIM-era cosmology research.

Abstract

Development of high-speed, spatial-mapping spectrometers in the millimeter and far-infrared frequencies would enable entirely new research avenues in astronomy and cosmology. An "on-chip" spectrometer is one such technology that could enable Line Intensity Mapping. Recent work has shown the promise of high-speed imaging; however, a limiting factor is that many of these devices suffer from low optical efficiency. Here we present the fabrication of a metalized, Si waveguide filter-bank fabricated using deep reactive ion etching for use in millimeter spectroscopy. Our design simultaneously provides high-density pixel packing, high optical efficiency, high spectral resolution, and is readily compatible with simple and multiplexable MKID arrays. Gold plated test waveguide and filter show excellent match to simulations with a measured resolving power of 263 and a loss quality factor of 1116 at room temperature. The results show promise for extending the measurements to larger, multi-wavelength designs.

Figures (6)

• Figure 1: Filter-bank operation principle. Signal is coupled into a transmission line via an antenna. The power is coupled to resonant filters and then each filter couples to a detector. The terminator absorbs any power not absorbed or reflected by the filters, preventing any standing waves on the main transmission line.
• Figure 2: The 3D realization of a single channel filter-bank as a high-frequency structure simulator (HFSS, Ansys, see Section \ref{['footnote']}) model. The waveguide transmission line couples to a half-wave resonant filter, which in turn couples to a waveguide with an H-plane bend terminating into an MKID on a separate wafer. The inset shows the detector response with and without the filter. With the filter nearly perfect, 50% coupling can be achieved. This detector is on a 10 $\mu$m SOI membrane with a separate deep-etched metalized wafer serving as a quarter-wave backshort.
• Figure 3: Designed and fabricated test parts of a single filter waveguide test part. a) shows the full split-block design where the purple traces show an etch that stops on the SiO2 layer and the green is etched completely through the wafer. Each part of the split-block is almost a perfect mirror, but the feedhorn, terminator, and detector ports are not symmetric between the two halves. b) shows the general shape of the filter. The wavelength is determined by the length of the half-wave cavity and quality factors are set by the position and width of the coupling slots. c) shows the fabricated device after an initial Ti/Cu seed layer. d) shows the assembled, Au-plated waveguide with attached commercial VNA coupling parts.
• Figure 4: Experimental and simulated results for the Au-plated waveguide filter in the log (a) and linear (b) scales. The Au-plated simulation (blue) matches the measured data (orange). The lossless waveguide was simulated (green) to show a maximum expected efficiency for this design. (c) measured power loss of the filter when it is terminated resulting in a fit of $R=263$. (d) measured power loss when the filter is shorted resulting in a fit of $Q_{loss}=1116$.
• Figure 5: SEM images of DRIE etched waveguide parts, sputtered with Al. For a process that results in 380 nm of Al deposited on the top surface (a) only a fraction can coat the side walls within the etch cavity (b). The upward facing surface has $\sim$100 nm of Al while the downward, shadowed surface has minimal Al coverage. Resultant devices metallized in this manner showed excessive loss.