Speed measurements of diffusion-cooled tantalum bolometers

Thin tantalum films were deposited on silicon wafers using a niobium seed layer to promote alpha-phase growth. These films were patterned into submicrometer size diffusion-cooled bolometers with superconducting transition temperatures of up to 2.35 K and a transition width of about 200 mK. The thermal relaxation times of the devices were determined by measuring the device impedance as a function of frequency and by fitting a theoretical model to the data. Measured relaxation times at low bias voltages range from 0.75 GHz for a 400-nm long device to 6 GHz for a 100-nm device, excluding electrothermal feedback. This should allow sufficiently high instantaneous bandwidths for most low-noise mixer applications in astrophysics.