Developing a new generation of integrated micro-spec far-infrared spectrometers for the experiment for cryogenic large-aperture intensity mapping (EXCLAIM)

The current state of far-infrared astronomy drives the need to develop compact, sensitive spectrometers for future space and ground-based instruments. Here we present details of the $\rm \mu$-Spec spectrometers currently in development for the far-infrared balloon mission EXCLAIM. The spectrometers are designed to cover the $\rm 555 - 714\ \mu$m range with a resolution of $\rm R\ =\ \lambda / \Delta\lambda\ =\ 512$ at the $\rm 638\ \mu$m band center. The spectrometer design incorporates a Rowland grating spectrometer implemented in a parallel plate waveguide on a low-loss single-crystal Si chip, employing Nb microstrip planar transmission lines and thin-film Al kinetic inductance detectors (KIDs). The EXCLAIM $\rm \mu$-Spec design is an advancement upon a successful $\rm R = 64\ \mu$-Spec prototype, and can be considered a sub-mm superconducting photonic integrated circuit (PIC) that combines spectral dispersion and detection. The design operates in a single $M{=}2$ grating order, allowing one spectrometer to cover the full EXCLAIM band without requiring a multi-order focal plane. The EXCLAIM instrument will fly six spectrometers, which are fabricated on a single 150 mm diameter Si wafer. Fabrication involves a flip-wafer-bonding process with patterning of the superconducting layers on both sides of the Si dielectric. The spectrometers are designed to operate at 100 mK, and will include 355 Al KID detectors targeting a goal of NEP ${\sim}8\times10^{-19}$ $\rm W/\sqrt{Hz}$. We summarize the design, fabrication, and ongoing development of these $\rm \mu$-Spec spectrometers for EXCLAIM.