Department of Physics and Astronomy
Cryogenic Electronics Group
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Icy ICs
Integration of Superconducting Electrical and Thermal Circuits for Microscale Cooling
Superconducting detectors and bolometers are the enabling technology for a wide range of activities because they offer unprecedented resolution and efficiency. Applications include imaging of solar and cosmic X-ray sources, measurement of the microwave background radiation, mass spectrometry of biopolymers, and ultrahigh resolution X-ray analysis of materials. Exploitation of this remarkable technology has been limited because conventional cryostats use liquid cryogens which must be handled by technical specialists. A major research goal of the Cryogenic Electronics Group (CEG) at SFSU is to make doubly-integrated circuits in which microscale refrigerators “cool where it counts” by removing heat from critical features of superconducting devices fabricated on micromachined thermal isolation structures. We call these chips “Icy ICs”.
Past research in the CEG focused on the development of photolithographic techniques for building advanced detectors by coupling energy absorbers to Normal-Insulator-Superconductor (NIS) tunnel junctions, which are extremely sensitive thermometers for the electrons in the normal metal electrode. That research led to the development of microfabrication and cryogenic facilities and produced a full-wafer process for making high-quality NIS devices with predictable tunneling properties and very few defects. The Icy IC project will exploit another feature of NIS junctions: a carefully designed and properly biased NIS junction will self-cool because the tunneling current removes the more energetic electrons from the normal electrode. Our first Icy IC will integrate NIS tunnel junction coolers with TES (Transition Edge Sensor) pixels of an X-ray imaging array as a demonstration of a compact, low mass, nonmagnetic solution for producing the ultralow operating temperature required by the new generation of superconducting sensors.
Progress in developing a new type of integrated circuit is made most efficiently when a microfabrication facility is dedicated to the production of that particular type of device. Rapid feedback from a cryogenic test facility is another essential feature of a successful superconducting device development program. Several generations of SFSU students have worked with Professor Neuhauser to build the Thin Film Laboratory and the Cryogenic Device Test Facility for the purpose of developing superconducting sensors and amplifiers rapidly and efficiently.
SFSU students have greatly valued the opportunity to work in the Cryogenic Electronics Group. Our research program has provided unique opportunities for them to work with scientists from many of the nation’s leading research institutions including Stanford, Berkeley, LLNL, and Fermilab. Furthermore, it has enabled SFSU students to learn integrated circuit technology which is highly marketable in Silicon Valley. Many of our graduates do not continue in Ph.D. programs but rather go on to face the challenges of industrial R & D positions; these people may well look back on this program not only as their professional apprenticeship but also as their finest hour as physicists.