Thermal Conductance of Microstrip Wiring Layers on Cryogenic Microbolometers

The transition-edge sensor (TES) is a photon-noise-limited detector technology used widely for observations across the electromagnetic spectrum. This high sensitivity is enabled in part by thermally isolating the sensor on a micromachined silicon nitride island via long, thin legs. A microstrip must run along these legs to carry the TES bias and the signals of interest and consists

of superconducting wiring and dielectric insulating layers. The thermal conductance of these ~100-nm-thick microstrip layers is often assumed to be dominated by the mechanical substrate layer.

Precise engineering of TES thermal conductance is critical for optimizing detector sensitivity for signals on the order of a picoWatt and requires a thorough characterization of the contributions

from all TES leg layers.

We present a study of the thermal conductance contributions of the wiring, insulation, and substrate leg layers of TES bolometers designed for mm-wavelength observations. We have fabricated TES bolometers with different film stack leg geometries and model their total measured thermal conductance as the summed contributions of the individual layers. In sufficiently thin films at low temperatures, boundary effects will suppress thermal transport and alter how thermal conductance scales with film geometry. This model accommodates boundary effects in phonon transport within the diffusive and ballistic regimes. 

We find that for our TES bolometers, the thermal conductance of the combined Nb and SiNx microstrip layers dominate that of the SiOx-SiNx mechanical substrate at the TES critical temperature of 170 mK. We find evidence of disparate thermal transport mechanisms between the individual layers. With these measurements, the resulting model accurately predicts GTES of ~80 other CMB TES bolometers with significantly thicker and wider legs than the bolometers under test.