Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Joel N. Ullom

Second Advisor

Charles T. Rogers

Third Advisor

Nils W. Halverson

Fourth Advisor

John C. Price

Abstract

In superconducting transition-edge sensors (TES) and superconducting tunnel junction refrigerators, thermal conductance of the silicon nitride membrane is a key design character. Thermal conductance engineering in silicon nitride membrane is still challenging, especially not introducing extra noise to sensors and diminish its mechanical strength.

In this thesis, we introduce our new generation of macroscopic general-purpose refrigerator using superconducting tunnel junction. Using the product of the cooled mass and the temperature reduction as a performance metric, this work is a more than tenfold advance over previous efforts. Based on our model, a better understanding of phonon thermal conductance of silicon nitride membrane and a method to reduce it is needed to get a better phonon cooling performance. We present a theory that quantitative and qualitative agree with our measurements of the thermal conductance G of thin silicon nitride beams in various geometries and with various metal patterns as a function of temperature over the range 0.075 to 0.5 K. We demonstrate that the method of deposit metal patterns on top of silicon nitride manipulates the thermal conductance of freestanding SiN membranes without compromising the robustness of the membrane or the noise of a co-located TES sensor. We deposit and pattern an additional metal layer in different geometries on the SiN membrane thereby achieving a maximum of 56% reduction in G and the change in G is precisely controlled by the details of the patterning.

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