Transition Measurements of a MicronSized Transitio n Edge HotElectron Microbolometer

We are developing a Transition$Edge Hot$electron M(THM) for use in large detector array applications in millimeter$wave astronomy. This bol ometer detector consists of a superconducting bilayer TES with an overlapping thin$film semi$metal absorber. The dete ctor is deposited directly on the substrate and thermal isolation of the bolometer is controlled by electron$phonon scat tering within the small volume of the detector. We present measurements characterizing the transition behavior of several micron$sized THM test devices which are optimized for photon$noise$limited observations of the Cosmic Mic rowave Background (CMB). We interpret these measurements in terms of a lateral proximity effect between the TES and the superconducting Nb TES leads. We discuss possible modifications to the THM design to compensate for this effect while retaining the small detector volume necessary to obtain the desired value of electron$phonon thermal conductivity.