Lithographic Integration of TES Microcalorimeters With SQUID Multiplexer Circuits for Large Format Spectrometers

Arrays of hundreds or thousands of low-temperature detectors have been deployed for many experiments, both bolometers for long-wavelength applications and calorimeters for shorter wavelength applications. One challenge that is common to many of these arrays is the efficient use of the focal plane area to achieve a large fill fraction of absorbers coupled to detectors. We are developing an integrated fabrication of soft X-ray transition-edge sensors (TES) and microwave superconducting quantum interface device (SQUID) multiplexers (<inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>MUX) with the goal of maximizing the fill fraction of the focal plane area on a scale of many thousand pixel detectors. We will utilize lithographically defined high-density interconnects to circumvent limitations in existing solutions that use wirebonds or flip-chip bonds. Here, we report the first demonstration of combining TES and <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>MUX processes into a single TES-system-on-a-chip (TES-SoC) fabrication on a silicon wafer. The <inline-formula><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>MUX SQUIDs and TES electrothermal feedback circuits are microfabricated first and protected with passivating SiO<inline-formula><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula>, then the TES devices and TES-to-SQUID interconnects are fabricated, and finally the protective layer is removed before the fabrication of the microwave resonators. We show that the microwave SQUIDs are functional and have reasonable yield, and that we are able to read out the transition temperature of the connected TESs using those SQUIDs.