The Universal Focal-plane Module for Simons Observatory

Abstract

The earliest observable light in our universe is the cosmic microwave background (CMB), a thermal relic of the big bang. The nearly uniform millimeter-wavelength signal can be used to make predictions across many physical and temporal scales. Cryogenic superconducting detectors have achieved near-fundamental limits in sensitivity, such that precise measurement of the CMB is limited by the number of photons in the faint cosmic signal. The best path forward for improving CMB maps, therefore, is to overcome obstacles in increasing the number of detectors in observation. The Simons Observatory (SO) will deploy over 60,000 transition-edge sensor (TES) detectors to collect millimeter-wavelength temperature and polarization data across a wide range of angular scales. The SO detector modules, called universal focal-plane modules (UFMs) for their universal compatibility with all the SO instruments, package a TES array wafer with a novel readout architecture: O(1000) microwave-SQUID multiplexing, a frequency-domain multiplexing architecture in which 896 TESes can be read out with a single wide-bandwidth amplification chain, an order of magnitude increase compared to previously fielded instruments. Building an O(1000) multiplexer for TES detectors required developing a controlled microwave environment for microwave resonators, which are modulated by incoming radiation as it couples to the detectors. The multiplexer for SO, called the universal multiplexing module (UMM), consists of 29 silicon components that must be assembled with micron precision and electrically connected with nearly 10,000 microscopic wires. The UMM is then coupled to the detector array and additional optical components, forming the UFM. The UFMs were developed to be robust to handling, cryocycling, and integration into the telescope receivers. In total, 49 UFMs will be deployed to achieve the requisite sensitivity for SO, with fabrication of an additional 15 planned for contingency and spares. The scale of the detector array production, which is also an order of magnitude increase on previous experiments, requires designing the modules and the assembly procedures to be highly repeatable and reliable. This thesis describes the development, final design, and assembly procedures of both the UMMs and UFMs. We also report on the performance, which supports their usability for the Simons Observatory surveys.