Design and characterization of a 90 GHz CMB TES bolometer

The transition-edge sensor (TES) as a type of low-temperature superconducting detector offers superior sensitivity due to its low thermal noise. In this work, we present a prototype TES bolometer designed for cosmic microwave background (CMB) polarization measurements. This TES is made of aluminum doped with a low concentration of manganese (2000 ppm by atomic percentage), and is deposited on the SiNx membrane which connects to the silicon substrate via narrow legs. In order to calculate its electrothermal parameters we have performed dark characterizations, which include measuring voltage-current (IV) curve at different bath temperatures, square-wave time response at various bias voltages, and noise level. This TES bolometer shows a noise equivalent power (NEP) of about 5× 10^-17 W/ √(Hz) , which meets the requirement of CMB observation. However, its saturation power is smaller and time constant is larger than what are expected. We have analyzed the reasons and will make corresponding improvements in our future work.