BICEP Array: 150 GHz detector module development
  The BICEP/Keck Collaboration is currently leading the quest to the highest
sensitivity measurements of the polarized CMB anisotropies on degree scale with
a series of cryogenic telescopes, of which BICEP Array is the latest Stage-3
upgrade with a total of $\sim32,000$ detectors. The instrument comprises 4
receivers spanning 30 to 270 GHz, with the low-frequency 30/40 GHz deployed to
the South Pole Station in late 2019. The full complement of receivers is
forecast to set the most stringent constraints on the tensor to scalar ratio
$r$. Building on these advances, the overarching small-aperture telescope
concept is already being used as the reference for further Stage-4 experiment
design.
  In this paper I will present the development of the BICEP Array 150 GHz
detector module and its fabrication requirements, with highlights on the
high-density time division multiplexing (TDM) design of the cryogenic circuit
boards. The low-impedance wiring required between the detectors and the
first-stage SQUID amplifiers is crucial to maintain a stiff voltage bias on the
detectors. A novel multi-layer FR4 Printed Circuit Board (PCB) with
superconducting traces, capable of reading out up to 648 detectors, is
presented along with its validation tests.
  I will also describe an ultra-high density TDM detector module we developed
for a CMB-S4-like experiment that allows up to 1,920 detectors to be read out.
TDM has been chosen as the detector readout technology for the Cosmic Microwave
Background Stage-4 (CMB-S4) experiment based on its proven low-noise
performance, predictable costs and overall maturity of the architecture. The
heritage for TDM is rooted in mm- and submm-wave experiments dating back 20
years and has since evolved to support a multiplexing factor of 64x in Stage-3
experiments.
