Cryogenic wafer probing below one Kelvin: Characterization of normal-metal Coulomb blockade thermometers at wafer scale
Coulomb blockade thermometers (CBTs) have attracted more interest in recent years, as the demand for sub-one Kelvin thermometry has increased, especially due to the prevalence of dilution refrigerators in research and applications in quantum technology. CBTs can be operated both as a primary thermometer, requiring no prior calibration, or as a simple resistance thermometer in the secondary mode after calibration. As new scalable fabrication processes for quantum devices and cryogenic electronics are being developed, cryogenic wafer characterization methods must also scale up to provide statistical data on device parameters. Currently, characterization throughput of cryogenic devices is limited by the turnover time and sample capacity of traditional cryostats, where a measurement cycle for only a few devices can take several days. In this work, we demonstrate wafer-scale cryogenic characterization of a recently developed TiW/Al-AlOx/TiW normal-metal tunnel junction technology using CBTs. Measurements performed in a 300 mm cryogenic wafer prober (CWP) show on-chip electron temperatures below 700 mK across a full 150 mm wafer, as determined by primary thermometry. These results establish wafer-level testing below 1 K as a viable approach for large-scale cryogenic characterization of electrical devices, opening a pathway toward high-throughput screening of quantum devices and direct wafer-scale characterization of aluminum-based superconducting circuits.