  BICEP Array is a degree-scale Cosmic Microwave Background (CMB) experiment
that will search for primordial B-mode polarization while constraining Galactic
foregrounds. BICEP Array will be comprised of four receivers to cover a broad
frequency range with channels at 30/40, 95, 150 and 220/270 GHz. The first
low-frequency receiver will map synchrotron emission at 30 and 40 GHz and will
deploy to the South Pole at the end of 2019. In this paper, we give an overview
of the BICEP Array science and instrument, with a focus on the detector module.
We designed corrugations in the metal frame of the module to suppress unwanted
interactions with the antenna-coupled detectors that would otherwise deform the
beams of edge pixels. This design reduces the residual beam systematics and
temperature-to-polarization leakage due to beam steering and shape mismatch
between polarized beam pairs. We report on the simulated performance of single-
and wide-band corrugations designed to minimize these effects. Our optimized
design alleviates beam differential ellipticity caused by the metal frame to
about 7% over 57% bandwidth (25 to 45 GHz), which is close to the level due the
bare antenna itself without a metal frame. Initial laboratory measurements are
also presented.
