The recent advent of low temperature detectors has enabled incredibly precise measurements ofthe sub-millimeter sky that have revolutionized our understanding of the origin, structure, and formation history of our Universe. One such detector technology is the transition edge sensor (TES) bolometer, a photon-noise-limited detector that takes advantage of the sharp change in resistance with temperature of a superconducting thin film biased into its phase transition. The sensitivity of TES bolometers increases with lower bath temperatures and lower thermal conductance to the bath temperature.

My PhD work has aimed to push TES sensitivity limits further on two fronts. First, I have studied and measured the thermal conductance of the membrane and microstrip wiring thin films that constitute the TES legs. I have found that in our case of very thin membranes, the thermal conductance of the often-neglected microstrip significantly dominates that of the membrane. Second, I have contributed to the development of integrating normal metal-insulator-superconductor junction refrigerators onto TES bolometers to cool the local TES bath temperature from 300 mK to below 150 mK. These devices have demonstrated TES cooling from 300 mK to 187 mK.