Fabrication of phononic filter structures for far-IR/sub-mm detector applications

TL;DR

This paper reports on the fabrication of phononic filter structures integrated into bolometers to enhance thermal isolation and reduce noise for far-IR/sub-mm space telescopes, using nanoscale patterning to control phonon transport.

Contribution

It introduces a novel fabrication process for phononic filters in bolometers, enabling improved thermal isolation and tunable phonon stop-bands for sensitive space-based detectors.

Findings

Successful fabrication of phononic filters with nanoscale features.

Enhanced thermal isolation in bolometers using phononic structures.

Tunable phononic stop-bands through quasi-periodic patterning.

Abstract

A photon noise limited sub-mm/far-IR cold telescope in space will require detectors with noise equivalent power (NEP) less than 1x10-19 W/Hz1/2 for imaging applications and at least an order of magnitude lower for spectroscopic studies. The detector NEP can be reduced by lowering the operation temperature and improving the thermal isolation between the bolometer and a heat bath. We report on the fabrication of membrane isolated transition edge sensor bolometers incorporating compact (<50 {\mu}m) thermal isolation beams based on phononic filters. Phononic filters are created by etching quasi-periodic nanoscale structures into supporting thermo-mechanical beams. The cross-sectional dimensions of the etched features are less than the thermal wavelength at the operating temperature, enabling coherent phonon transport to take place in one dimension. The phonon stop-band can be tuned by adjusting the scale of the quasi-periodic structures. Cascading multiple filter stages can increase bandwidth and provide improved thermal isolation similar to the function of a multi-stage electrical filter. We describe the fabrication of AlMn based transition edge sensor bolometers on silicon and silicon nitride membranes isolated by one- and two-dimensional phononic filters. The phononic filters are patterned through electron beam lithography and isolated with deep reactive ion etching.