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IEEE Transactions on Magnetics

Design and optimization of DC SQUIDs fabricated using a simplified four-level process

R. Cantor, Tapani Ryhänen, D. Drung, H. Koch, Heikki Seppä

Published Mar 1, 1991• 35 citations
Publisher

Abstract

An optimization procedure is presented which is used to design two types of ultralow-noise DC superconducting quantum interference devices (SQUIDs) with integrated flux coupling circuits, a magnetometer, and a gradiometer. Essential to this design approach is the reduction of the parasitic capacitance, the choice of a layout which moves the input coil resonances well away from the desired operating frequency of the SQUID, and the proper damping of these resonances. It is shown that both the microwave and LC resonances can be damped in a nearly noise-free manner by placing all RC shunt in parallel with the input coil. This leads to smooth voltage-flux modulation characteristics and significantly improved noise performance. A simple, four-level Nb-Si/sub x/N/sub y/-Nb Josephson junction technology for device fabrication is described. White flux noise levels of 5*10/sup -7/ Phi /sub 0// square root Hz for the magnetometer and 7.9*10/sup -7/ Phi /sub 0// square root Hz for the gradiometer were measured. The corresponding uncoupled energy resolutions are 100 h and 130 h, respectively. The 1/f noise of the magnetometer at 1 Hz is less than 4*10/sup -6/ Phi /sub 0// square root Hz.

DOI
10.1109/20.133822

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An optimization procedure is presented which is used to design two types of ultralow-noise DC superconducting quantum interference devices (SQUIDs) with integrated flux coupling circuits, a magnetometer, and a gradiometer. Essential to this design approach is the reduction of the parasitic capacitance, the choice of a layout which moves the input coil resonances well away from the desired operating frequency of the SQUID, and the proper damping of these resonances. It is shown that both the microwave and LC resonances can be damped in a nearly noise-free manner by placing all RC shunt in parallel with the input coil. This leads to smooth voltage-flux modulation characteristics and significantly improved noise performance. A simple, four-level Nb-Si/sub x/N/sub y/-Nb Josephson junction technology for device fabrication is described. White flux noise levels of 510/sup -7/ Phi /sub 0// square root Hz for the magnetometer and 7.910/sup -7/ Phi /sub 0// square root Hz for the gradiometer were measured. The corresponding uncoupled energy resolutions are 100 h and 130 h, respectively. The 1/f noise of the magnetometer at 1 Hz is less than 4*10/sup -6/ Phi /sub 0// square root Hz.
A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.>
We have fabricated dc superconducting quantum interference devices (SQUIDs) incorporating Nb/Al-oxide/Nb Josephson junctions in both stripline and washer geometries. Low-frequency noise in excess of that predicted by the resistively-shunted junction model is present in both geometries and is demonstrated to be flux noise. This flux noise is not environmental. Improvements in fabrication processing over the past four years have reduced the level of this flux noise. SQUIDs are now fabricated with PdAu resistors, Nb wiring layers, and SiO/sub 2/ interlayer dielectric. In our best well-coupled SQUIDs, the white-noise energy sensitivity is 5/spl times/10/sup -31/ J/spl middot/s, with a 1/f knee below 0.1 Hz. We believe further reduction in the flux noise might be obtained with the use of on-chip flux shielding and/or trapping structures.
We have fabricated and characterized the noise performance of well-coupled Nb-Pb alloy edge junction dc superconducting quantum interference devices (SQUIDs) with a resistively shunted inductance. The extra damping resistor removes the resonances in the current-voltage characteristics resulting in a very low value of the extrinsic energy sensitivity in flux-locked-loop operation. Numerical simulations, with the junction capacitance included, indicate that the damping resistance does not degrade noise performance until it is significantly less than the junction-shunting resistance. We also tested different SQUID designs and demonstrated that a large SQUID inductance, proposed earlier to be desirable when resistively shunted, can seriously affect the low-frequency flux noise.
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.
We investigated, at temperature 4.2 K, electric transport, flux noise, and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi barriers. The SQUIDs are either of the magnetometer-type or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise SΦ1/2=250nΦ0/Hz1/2, corresponding to a spin sensitivity Sμ1/2≥29μB/Hz1/2. For the gradiometer we find SΦ1/2=300nΦ0/Hz1/2 and Sμ1/2≥44μB/Hz1/2. The devices can still be optimized with respect to flux noise and coupling between a magnetic particle and the SQUID, leaving room for further improvement towards single spin resolution.
We have fabricated superconducting quantum interference devices (SQUIDs) with an inductance of 140 pH by using sub- µm Nb/Al–AlO x –Al/Nb edge junctions. The small capacitance of the junction resulted in the very large voltage swing of 210 µV and gradient dV/ dΦ of 940 µV/Φ0. Consequently, the very low flux noise of 0.61 µΦ0/ Hz0.5 corresponding to the energy sensitivity ε of 8.6h at the white noise level was obtained by using a conventional flux-locked loop configuration. The low-frequency noise was also very low, 2.7 µΦ0/ Hz0.5 corresponding to ε= 170 h at 1 Hz. We could obtain very low flux noise for the SQUID which had the practical inductance using only a conventional readout circuit.