In-Situ Superconductor Temperature Sensor for Cryogenic Integrated Circuits

Abstract

Cryogenic circuits, such as those based on single flux quantum (SFQ) logic, function at extremely low temperatures. Therefore, the designs target the utilization of liquid helium (LHe) temperatures, maintaining them at 4.2 K. These specialized circuits can be subjected to measurement either within liquid helium (LHe) baths or enclosed within closed-cycle cryocoolers. However, when utilizing LHe in cryocooler systems, inherent weak thermal contact can lead to temperature gradients between the circuit chip and the cold head, where conventional temperature sensors are typically placed. To address this challenge, this study introduces an innovative on-chip temperature sensing approach that capitalizes on the temperature dependence of the Josephson junction's (JJ) critical current (IC ). The relationship between JJ's I C and temperature (IC vs. T) is meticulously derived and calibrated within the cryocooler system. Subsequently, this I C vs. T profile is mathematically fitted to a polynomial function and integrated into an embedded microcontroller. This microcontroller facilitates the supply and measurement of bias currents and voltages, effectively establishing an autonomous temperature monitoring system. Implementing this sensor mitigates temperature disparities on the chip's surface compared to the intended target temperature and eradicates measurement errors that might arise due to such discrepancies.