Stability of Al-Mn Transition Edge Sensors for Frequency Domain Multiplexing

We are developing arrays of 150 GHz transition edge sensor (TES) polarimeters for the South Pole Telescope polarimeter (SPTpol). Prototype devices use an aluminum manganese (Al-Mn) alloy TES with a normal resistance R
_n
suited to frequency domain multiplexing (fMUX) used in SPTpol. Using the fMUX readout, the devices exhibit noise performance consistent with expectations when R >; 0.8R
_n
. Below 0.8R
_n
, the detectors have high loopgain and become unstable, which is predicted by use of a compound TES model. We address this issue in a recent fabrication with increased TES heat capacity and normal metal structures on the TES to tune the temperature sensitivity.