  Line Intensity Mapping (LIM) is a new observational technique that uses
low-resolution observations of line emission to efficiently trace the
large-scale structure of the Universe out to high redshift. Common mm/sub-mm
emission lines are accessible from ground-based observatories, and the
requirements on the detectors for LIM at mm-wavelengths are well matched to the
capabilities of large-format arrays of superconducting sensors. We describe the
development of an R = 300 on-chip superconducting filter-bank spectrometer
covering the 120--180 GHz band optimized for future mm-LIM experiments,
focusing on SPT-SLIM, a pathfinder LIM instrument for the South Pole Telescope.
Radiation is coupled from the telescope optical system to the spectrometer chip
via an array of feedhorn-coupled orthomode transducers. Superconducting
microstrip transmission lines then carry the signal to an array of channelizing
half-wavelength resonators, and the output of each spectral channel is sensed
by a lumped element kinetic inductance detector (leKID). Key areas of
development include incorporating new low-loss dielectrics to improve both the
achievable spectral resolution and optical efficiency and development of a
robust fabrication process to create a galvanic connection between ultra-pure
superconducting thin-films to realize multi-material (hybrid) leKIDs. We
provide an overview of the spectrometer design, fabrication process, and
prototype devices.
