Zero-field Coulomb blockade thermometers*

Coulomb Blockade Thermometers (CBTs) exploit the temperature dependence of conductance in normal metal tunnel junction arrays for primary thermometry at cryogenic temperatures. Traditionally, tunnel junctions at millikelvin temperatures utilize superconducting materials, leading to the use of external magnetic field to suppress the superconductivity. To this end, we describe our latest efforts to realize CBTs at zero magnetic field. Fabrication process relying on the side wall passivation by spacer isolation (SWAPS) [1] was used to manufacture tunnel junctions, followed by electroplating of copper with thicknesses up to tens of micrometers on thermalization islands, ensuring good scalability of the process. The large Copper islands enhance the electron-phonon coupling and enable the usage of nuclear demagnetization refrigeration techniques while pursuing temperatures in the microkelvin regime [2]. We discuss CBT designs for different temperature ranges and show measurement results that verify their operation at the lowest temperatures.   

[1] L. Grönberg et al,” Side-wall spacer passivated sub-µm Josephson junction fabrication process”, Supercond. Sci. Technol. 30, 125016 (2017). 

[2] Samani, M et al. ”Microkelvin electronics on a pulse-tube cryostat with a gate Coulomb-blockade thermometer”. Physical Review Research, 4(3), 033225 (2022).