SQUID Design Research Articles

Low noise DC SQUIDs fabricated in Nb-Al/sub 2/O/sub 3/-Nb trilayer technology

The authors have designed, fabricated and tested all-refractory DC SQUIDs in Nb-Al/sub 2/O/sub 3/-Nb trilayer technology that have noise performance comparable to the best previously reported for any technology. A variety of SQUID designs were incorporated as part of a trilayer process development test vehicle. SQUID inductance, junction area, and resistive shunt geometry were varied in matrix fashion to give SQUIDs with near-optimum parameter values for a factor of five range in Josephson current density and shunt sheet resistance. The devices were fabricated using a selective niobium anodization with a minimum feature size of 2 mu m. The base electrode and Nb wiring were patterned with dry etching, and the junction areas were defined by anodization: the Ti resistors were patterned with a lift-off process. Current density on different wafers was varied from 400 to 1000 A/cm/sup 2/ with typical junction V/sub m/'s of 60 mV. The shunt sheet resistance was varied in the 1-5- Omega / Square Operator range. The noise was measured with an RF SQUID direct small-signal readout scheme. A 50-pH SQUID with 3- mu m/sup 2/, 16- mu A junctions and 14- Omega shunt resistors was shown to have an ideally low white noise of 1*10/sup -13/ Phi /sub 0//sup 2//Hz, a white to 1/f crossover frequency at 7 Hz, and a noise level less than 6*10/sup -12/ Phi /sub 0//sup 2//Hz at 0.1 Hz.

Design, optimization, and construction of a dc SQUID with complete flux transformer circuits

The design of a complete dc SQUID with a flux transformer input circuit is discussed. The flux coupling circuits introduce a substantial capacitance across the SQUID and give rise to many resonances which may couple strongly to the SQUID dynamics. Both effects lead to multiple modes in the SQUID dynamics and consequently to excess noise. For a low-noise SQUID with smooth characteristics, our analysis and practical considerations suggest signal coupling via an intermediary transformer. This method allows simultaneous optimization of the SQUID parameters, minimizing the parasitic capacitance, control over the resonances, and good inductance matching to practical magnetometer coils. A model is developed to optimize the structure: it describes the whole circuit with the help of a suitably modified autonomous SQUID, provided that the system is free from multiple modes due to resonances or large parasitic capacitance. Following these design principles, we have built a dc SQUID, primarily for use in biomagnetic research, but also well suited for other applications. The fabrication of the SQUID and the high-quality electronics especially suitable for multiple-SQUID devices is presented. The SQUIDs showed smooth characteristics, and the lowest measured noise of our complete SQUID is % MathType!MTEF!2!1!+-% feaafeart1ev1aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaGymaiaac6% cacaaIZaGaey41aqRaaGymaiaaicdadaahaaWcbeqaaiabgkHiTiaa% iAdaaaGccqqHMoGrdaWgaaWcbaGaaGimaiaac+caaeqaaOWaaOaaae% aacaWGibGaamOEaaWcbeaaaaa!428B!$1.3 \times 10^{ - 6} \Phi _{0/} \sqrt {Hz} $, indicating the success of the design.

Design of improved integrated thin-film planar dc SQUID gradiometers

Key issues in the design of improved first and second derivative, thin-film, planar dc SQUID gradiometers are discussed. The introduction of a planar coupling scheme to optimally couple the planar dc SQUID to the gradiometer pickup loops leads to significantly increased sensitivity as well as elimination of the sensitivity differences between series and parallel gradiometer loop configurations. Two-hole and figure-8 SQUID designs are presented which are consistent with intrinsic gradiometer balance ≲10−4 against uniform field changes. Straightforward calculations together with data from existing low-noise SQUIDs suggest improvements in gradient sensitivity on the order of 102 over existing planar gradiometers.

Hybrid DC SQUIDs containing all refractory thin film Josephson junctions

During the past five years Sperry designed, fabricated, and tested DC SQUIDs and feedback transformer assemblies (FTA) for sensor applications. The SQUID design included a silicon chip with deposited, all refractory, thin-film Josephson junctions contained in a bulk niobium toroidal cavity housing. Features include (1) minimum SQUID loop inductance provided by a cone-shaped structure extending from the chip containing the two Josephson junctions to the toroidal cavity where the input coil and concentric modulation coil are housed, and (2) a removable cap that allows easy access to the chip for repair or replacement. This paper presents (1) the design details of the junction chip, niobium housing, and FTA and (2) the flux noise results in the 0.01 to 1,000 Hz frequency range.

DC SQUIDs 1980: The state of the art

The four primary areas of concern in the development of high performance dc SQUIDs are white noise, 1/f noise, readout schemes, and coupling. For an optimized dc SQUID the intrinsic energy sensitivity in the white noise region is given by e w =γ 1 hk B T(eI 0 R)-1+γ 2 h, where I o is the critical current per junction and R is the shunt resistance per junction. γ 1 , which multiplies the thermal noise contribution, and γ 2 , which multiplies the shot noise/zero point fluctuation contribution, are numerical factors of order unity. Values of e w approaching h have recently been measured for several members of a new generation of low noise dc SQUIDs. The intrinsic energy sensitivity in the 1/f noise region, e f , is predicted to scale as (I 0 R)2for tunnel junctions. This may impose significant low frequency limitations on SQUIDs with very low values of e w . Readout schemes for high sensitivity dc SQUIDs will require further development. At the moment primarily small signal amplifier readout schemes are being used to evaluate the new generation of low noise SQUIDs. Planar thin-film coupling schemes are about to have a big impact on dc SQUID design. Such schemes can achieve tight coupling between SQUID and input coil. and can be implemented using the same fabrication techniques that produce SQUIDs with low values of e w .

Analog-to-digital conversion with unlatched SQUID's

Digital Josephson circuits that do not latch into the voltage state provide an opportunity for high-speed serial processing. These fast logic gates must ultimately generate and record small fast pulses generated by single fluxoid events. This is demonstrated in a concept for a high-speed analog-to-digital (A/D) converter. Low β-value unlatched SQUID's are used throughout for ultimate speed, sensitivity, and simplicity. The analog signal is quantized by a single-junction SQUID and converted into a pulse train resulting from fluxoid transitions. These are counted in a chain of bistable two-junction SQUID's which successively scale the pulse train by a factor 2. These SQUID's perform both logic and memory functions at the single fluxoid level. The bit count stored in the array is gated out each sampling interval to provide the digitized signal. The performance of this device has been simulated and criteria established for the unlatched SQUID design. These include dimensionality, impedance, damping, intra- and inter-SQUID delays and β factor. Large analog dynamic range is available with the high intrinsic linearity of the SQUID quantizer and a long chain of scalers.

Coupling considerations for SQUID devices

Most applications of superconducting quantum interference devices (SQUID’s) involve use of a coupling coil. There are two classes of such applications: magnetometry (magnetic field, field gradient) and voltage or current measurements. We have analyzed the factors which determine the ultimate sensitivity, and present a guide for optimization of signal/noise in each of the two cases. It is found that a single figure of merit, involving both the equivalent input noise of the SQUID and the effectiveness of the coupling to the coil, is applicable to either type of measurement. We propose that this figure of merit be used in evaluating the performance of new SQUID designs.