Characterization and Modeling of a Large Area Spiderweb Bolometer for CMB Observation

Abstract

We present the characterization of a prototype TES bolometer suitable for Cosmic Microwave Background (CMB) measurements. This detector consists of a double layer (Ti-Au) superconducting film thermally coupled to a large area (8 mm diameter) spiderweb absorber, fabricated at INFN Genova. Thermo-electrical properties of the superconducting film are measured by means of four-wires and two-wire setups. These include Resistance vs Temperature curve, which allows us to determine the critical temperature, other parameters of the transition, and the critical current. From the measurement of the critical retrapping current, we can estimate the thermal conductance ($G$) of the device. TES properties are also measured by reading it out through a SQUID amplifier. This allows us to measure the current-voltage curve (I-V) and saturation power ($P_{sat}$). A second, independent, estimate of $G$ is obtained by measuring $P_{sat}$ at different thermal bath temperatures, which agrees with the first one. Furthermore, a red LED, installed in the cryogenic environment, illuminates the detector absorber, while operating the bolometer in negative electrothermal feedback. We investigate the bolometric and calorimetric regime of the device using a slow modulated LED signal for the former and a fast (compared to the expected detector time constant) pulsed signal for the latter. The bolometer time constant ($\tau$) is extracted by using this method. To study the behavior of the device in the presence of cosmic rays background, and to achieve a further estimation of $\tau$, we expose the bolometer to a $^{214}$Am alpha source.