Design and characterization of the Cosmology Large Angular Scale
  Surveyor (CLASS) 93 GHz focal plane
  The Cosmology Large Angular Scale Surveyor (CLASS) aims to detect and
characterize the primordial B-mode signal and make a sample-variance-limited
measurement of the optical depth to reionization. CLASS is a ground-based,
multi-frequency microwave polarimeter that surveys 70% of the microwave sky
every day from the Atacama Desert. The focal plane detector arrays of all CLASS
telescopes contain smooth-walled feedhorns that couple to transition-edge
sensor (TES) bolometers through symmetric planar orthomode transducer (OMT)
antennas. These low noise polarization-sensitive detector arrays are fabricated
on mono-crystalline silicon wafers to maintain TES uniformity and optimize
optical efficiency throughout the wafer. In this paper, we discuss the design
and characterization of the first CLASS 93 GHz detector array. We measure the
dark parameters, bandpass, and noise spectra of the detectors and report that
the detectors are photon-noise limited. With current array yield of 82%, we
estimate the total array noise-equivalent power (NEP) to be 2.1
aW$\sqrt[]{\mathrm{s}}$.
