THz Waveguide Mixers With NbTiN HEBs on Silicon Nitride Membranes

We describe the design and performance of waveguide mixers at 1.4 THz and 1.9 THz based on NbTiN phonon-cooled hot electron bolometers (HEBs) fabricated on a 2-
$mu$
m thick Si
$_3hboxN_4$
membrane. The membrane is bonded to a silicon frame in the mixer block using a flip chip process. Simulated RF coupling is compared with experimental results, showing good agreement. Receiver noise temperature measurements show uncorrected values of 1600 K at 1.4 THz and 2100 K at 1.9 THz, both at 1.5GHz intermediate frequency. Device cooling on the membrane seems not to be problematic. The mixers are used in receivers for the Stratospheric Observatory for Infrared Astronomy (SOFIA) [German REceiver At THz frequencies (GREAT)] and the Atacama path finder experiment (APEX) [CO, N
$^+$
, deuterium observations receiver (CONDOR)].