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IEEE Transactions on Applied Superconductivity

Development of Submicrometer Circuit Elements for Ultrasensitive DC SQUIDs

Mauro Esattore, Oliver Kieler, Michael Paulsen, Rainer Körber, Patryk Krzysteczko, Constantin Schuster, Mark Bieler, J. Beyer

Published Jun 3, 2026• 0 citations
Publisher

Abstract

The direct current superconducting quantum in terference device (dc SQUID) sensor represents one of the most established applications of superconductor technology due to its sensitivity to magnetic flux. In this work, we present superconducting “fine-pitch” input coils with sub-micrometer parameters fabricated with electron beam lithography (EBL) that will be integrated into existing Nb/Al-AlOx/Nb-based SQUID sensor designs. The aim is to reduce inductive losses of the signal to-SQUID coupling without compromising the overall device layout. In a SQUID current sensor, maximization of the inductive coupling constant k between the signal input coil and the SQUID loop results in a low coupled energy sensitivity ϵc = ϵ/k2where ϵ is the intrinsic energy sensitivity. The fine-pitch coils are expected to increase the inductive coupling constant k as well as extend the range of input coil inductance for our existing devices. The SQUID energy sensitivity ϵ ∼ √CJJcan be lowered by reducing the Josephson junction (JJ) capacitance CJJ. We are developing a process, also based on EBL, to fabricate window-type JJs with sub-micrometer lateral size to obtain reduced JJ capacitances. Details concerning design aspects of the components, their fabrication and characterization results are provided in this paper.

DOI
10.1109/tasc.2026.3699943

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IEEE Transactions on Applied Superconductivity•Jun 1, 2009•6 citations
The direct current superconducting quantum in terference device (dc SQUID) sensor represents one of the most established applications of superconductor technology due to its sensitivity to magnetic flux. In this work, we present superconducting “fine-pitch” input coils with sub-micrometer parameters fabricated with electron beam lithography (EBL) that will be integrated into existing Nb/Al-AlOx/Nb-based SQUID sensor designs. The aim is to reduce inductive losses of the signal to-SQUID coupling without compromising the overall device layout. In a SQUID current sensor, maximization of the inductive coupling constant k between the signal input coil and the SQUID loop results in a low coupled energy sensitivity ϵc = ϵ/k2where ϵ is the intrinsic energy sensitivity. The fine-pitch coils are expected to increase the inductive coupling constant k as well as extend the range of input coil inductance for our existing devices. The SQUID energy sensitivity ϵ ∼ √CJJcan be lowered by reducing the Josephson junction (JJ) capacitance CJJ. We are developing a process, also based on EBL, to fabricate window-type JJs with sub-micrometer lateral size to obtain reduced JJ capacitances. Details concerning design aspects of the components, their fabrication and characterization results are provided in this paper.
The dc Superconducting Quantum Interference Device (dc-SQUID) is one of the most sensitive magnetic field sensors available. In this thesis we concentrate on its application as an amplifier. In this configuration, an input circuit of interest can be connected by means of a coupling coil. The intended application of our developed low-Tc SQUID amplifiers is the readout of the first spherical gravitational wave antenna MiniGRAIL. Using small-signal analysis as well as numerical simulations, we study the achievable signal-to-noise ratio of such sensors in practical measurements. Published theories and studies could be extended. Some interesting points treated are the influence of the often used negative feedback (flux-locked loop), an altered input impedance of the SQUID amplifier and the influence of back-action noise, which directly affects the object of interest. The altered operation of the SQUID due to the presence of the input circuit is studied in numerical experiments. Another important issue, the influence of integrated input coils on the dynamics of SQUIDs, is investigated theoretically and experimentally. The performance of our developed sensors is compared to numerical simulations on detailed models. The results give insights into the behavior, design and usage of dc-SQUIDs with integrated coils. For a SQUID with input inductances of 1.5 μH we reached a good coupled energy sensitivity of about 170 ¯h in a dilution refrigerator. Furthermore, we investigated the hot-electron effect. This effect typically limits the sensitivity of superconducting electronics at sub-Kelvin bath temperatures due to a weakened electron-phonon coupling. We performed heating experiments on thin-film resistors which are similar to the shunt resistors of the Josephson junctions used in our sensors. The suppression of the hot-electron effect via electronic thermal transport to attached cooling fins is investigated both experimentally, theoretically and numerically. The numerical technique turns out to be a useful tool for the thermal design of superconducting electronics.
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The authors have designed, fabricated, and measured a number of Nb-AlO/sub x/-Nb octagonal washer DC superconducting quantum interference devices (SQUIDs) and miniature DC SQUID susceptometers having minimum feature size down to 0.5 mu m. With SQUID inductance values on the order of 100 pH, typical noise performance is better than 1 mu Phi /sub 0// square root Hz. The small minimum feature size of input coils and pickup loop structures will facilitate tight coupling to a wide variety of systems ranging from submicron particles and structures to conventional approximately mu H input circuits. A single-washer SQUID with an 80-turn 0.5- mu m-linewidth, 630-nH input coil has user-friendly V- Phi (voltage-flux) curves and a coupled energy sensitivity of 20 h at 4.2 K. Susceptometers with pickup loops ranging from 20- mu m to 0.8- mu m across have very user-friendly V- Phi curves and a resolution of order 100 mu /sub B// square root Hz at 4.2 K for devices with the smallest loops.>
We present the results of an investigation of thin-film dc super conducting quantum interference device (SQUID’s) with very high energy resolution. The SQUID’s were fabricated using integrated-circuit techniques appropriate for superconducting devices. A weakly coupled SQUID with very low inductance (1 pH) had an intrinsic energy sensitivity 6×10−34 J/Hz = 0.9 h. A well-coupled SQUID (coupling constant α = 0.43) had an energy sensitivity referred to the input coil of 4.7×10−32 J/Hz = 71 h.
An integrated magnetic nanosensor based on a niobium dc SQUID (superconducting quantum interference device) for nanoscale applications is presented. The sensor, having a washer shape with a hole of 200 nm and two Josephson-Dayem nanobridges of 80 nm × 100 nm, consists of a Nb(30 nm)/Al(30 nm) bilayer patterned by electron beam lithography (EBL) and shaped by lift-off and reactive ion etch (RIE) processes. The presence of the niobium coils, integrated on-chip and tightly coupled to the SQUID, allows us to easily excite the sensor in order to get the voltage-flux characteristics and to flux bias the SQUID at its optimal point. The measurements were performed at liquid helium temperature. A voltage swing of 75 µV and a maximum voltage-flux transfer coefficient (responsivity) as high as 1 mV/Φ(0) were directly measured from the voltage-flux characteristic. The noise measurements were performed in open loop mode, biasing the SQUID with a dc magnetic flux at its maximum responsivity point and using direct-coupled low-noise readout electronics. A white magnetic flux noise spectral density as low as 2.5 μΦ(0) Hz(-1/2) was achieved, corresponding to a magnetization or spin sensitivity in units of the Bohr magneton of 100 spin Hz(-1/2). Possible applications of this nanosensor can be envisaged in magnetic detection of nanoparticles and small clusters of atoms and molecules, in the measurement of nanoobject magnetization, and in quantum computing.
We optimized the design and operation of a low-Tc direct current superconducting quantum interference device (dc SQUID) with an integrated coupling coil of 1.5 muH inductance taking into account typical effects observed for similar devices. Numerical simulations were performed on a model including the capacitance of the Josephson junctions, thermal noise of the integrated shunt- and damping- resistors as well as a complex frequency dependent impedance of the SQUID loop originating from the integrated coils. The experimentally and numerically determined characteristics and sensitivity are in good agreement. A minimum additional coupled energy resolution of 700 (h/2p) and 250 (h/2p) was measured at a temperature of 4.2 K and 1.5 K, respectively.