  We are developing lumped-element kinetic inductance detectors (LEKIDs)
designed to achieve background-limited sensitivity for far-infrared (FIR)
spectroscopy on a stratospheric balloon. The Spectroscopic Terahertz Airborne
Receiver for Far-InfraRed Exploration (STARFIRE) will study the evolution of
dusty galaxies with observations of the [CII] 158 $\mu$m and other atomic
fine-structure transitions at $z=0.5-1.5$, both through direct observations of
individual luminous infrared galaxies, and in blind surveys using the technique
of line intensity mapping. The spectrometer will require large format
($\sim$1800 detectors) arrays of dual-polarization sensitive detectors with
NEPs of $1 \times 10^{-17}$ W Hz$^{-1/2}$. The low-volume LEKIDs are fabricated
with a single layer of aluminum (20 nm thick) deposited on a crystalline
silicon wafer, with resonance frequencies of $100-250$ MHz. The inductor is a
single meander with a linewidth of 0.4 $\mu$m, patterned in a grid to absorb
optical power in both polarizations. The meander is coupled to a circular
waveguide, fed by a conical feedhorn. Initial testing of a small array
prototype has demonstrated good yield, and a median NEP of $4 \times 10^{-18}$
W Hz$^{-1/2}$.
