Since more than 15 years, Nb/AlOx/Nb-based direct current Superconducting QUantum Interference Device (dc SQUID) current sensors have been developed at PTB [1] for a range of applications, such as biomagnetic measurements in ultra-low field environments, readout of superconducting low-temperature detectors of radiation and particles or noise thermometry at millikelvin temperatures. The sensor circuit concept is based on a second-order gradiometric dc-SQUID combined with a first-order gradiometric double transformer input circuit to couple the signal currents to be measured to the SQUID. This configuration allows in a flexible manner to cover a range of input inductances from about 25 nanohenry to 2 microhenry and reduces the sensitivity of the devices to external magnetic interference. The sensors achieve coupled energy sensitivities below 150 h (Planck's constant) at 4.2 K that reduce to <50 h when operated at 0.3 K. Additional functionalities, e. g., input current limiters, two feedback coil circuits and rf-filters are integrated on-chip. So far, photolithography has been used for the fabrication of the sensors and limits the minimum lateral dimensions of circuit structures to about 2.5 micrometers. Further improvements in their noise performance and functionality requires the dimensions of sensors circuit elements, namely Josephson junction sizes as well as input coil line widths and distances to be reduced to < 1 micrometer. To this end, we are developing a new fabrication process employing electron beam lithography performance. Josephson junctions with lateral lengths as low as 0.7 micrometer and Nb signal input coils with line widths / pitch dimensions as low as 0.15 / 0.3 micrometers can be fabricated this way, a reduction of more than an order of magnitude compared to the photolithography-based patterning. The “fine-pitch” layout of the signal input coils is matched to the existing sensor designs and allows to significantly extend the range of input inductances and increased signal-to-SQUID inductive coupling while improving the overall compactness of the sensors. The presentation discusses design aspects, details of the new sensor fabrication process and results of the characterization of the sub-micrometer circuit elements.

[1] D. Drung, C. Aßmann, J. Beyer, A. Kirste, M. Peters, F. Ruede, Th. Schurig, “Highly sensitive and easy-to-use SQUID sensors,” IEEE Trans. Appl. Supercond. vol. 17, no. 2, pp. 699-704, June 2007.