  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.
