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) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim 1\times 10^{-19}$</tex-math></inline-formula> W/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt{\text{Hz}}$</tex-math></inline-formula> 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m to 300 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m, 800 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m, and 1000 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim 3\times 10^{-19}$</tex-math></inline-formula> W/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt{\text{Hz}}$</tex-math></inline-formula> can be achieved with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_{c}$</tex-math></inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>120 mK, a notable result given that the thermally isolating bolometer legs are only 50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m long. We report on the fabrication details of the TES bolometer detector sub-arrays and discuss future work.