Fabrication of Ultralow-Noise Phononically Isolated TES Bolometers for Far-Infrared Astronomical Applications

Far-infrared space-borne instruments will require high sensitivity focal-plane arrays of densely packed low temperature detectors with noise equivalent power (NEP)$\sim 1\times 10^{-19}$W/$\sqrt{\text{Hz}}$for imaging applications and one or more orders of magnitude lower than that for spectrometry. Transition-edge sensor (TES) bolometers, a mature and well-understood detector architecture, have demonstrated background limited performance in both ground-based and sub-orbital astronomical instruments. The fundamental noise source of a bolometer is thermal fluctuation noise, which scales as the square root of the thermal conductance. In order to make a more sensitive detector, thermal conductance must decrease. Aiming to advance TES bolometers to the next level of noise performance for space applications, we have developed a direct-absorber-coupled sensor that utilizes phononically engineered thermally isolated legs to achieve low enough thermal conductance for background limited operation in the 30$\mu$m to 300$\mu$m waveband. In practice, simultaneously achieving the required low thermal conductance yet high mechanical yield while maintaining a compact footprint is extremely challenging. We present the fabrication process of thin silicon nitride membrane released sub-arrays of densely packed TES bolometers with short ($\sim$50$\mu$m) legs built with phononic filter structures. The phononic filters are created by etching periodic or semi-periodic high- and low-impedance structures into the silicon nitride legs. The direct-absorption TES bolometer sub-arrays have been fabricated with pixel sizes of 600$\mu$m, 800$\mu$m, and 1000$\mu$m and multiple phononic filter leg designs, all defined with standard optical photolithographic processes and isolated with a deep reactive ion etch process. High mechanical yield has been achieved, allowing for initial dark measurements which show that noise performance$\sim 3\times 10^{-19}$W/$\sqrt{\text{Hz}}$can be achieved with$T_{c}$$\sim$120 mK, a notable result given that the thermally isolating bolometer legs are only 50$\mu$m long. We report on the fabrication details of the TES bolometer detector sub-arrays and discuss future work.