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IEEE Transactions on Magnetics

Design considerations for DC SQUIDs fabricated in deep sub-micron technology

M. B. Ketchen

Published Mar 1, 1991• 64 citations
Publisher

Abstract

Practical SQUIDs with well-coupled input coils of usable inductance have generally been fabricated at the 2-5 mu m level of lithography. Other technologies, silicon in particular, are now routinely practised at the 0.5 mu m level of lithography with impressive demonstrations at the 0.1-0.25 mu m level not uncommon. The implications of applying such a fabrication capability to advance DC SQUID technology are explored. In particular, the issues of scaling practical DC SQUIDs down to the 0.1-0.25 mu m regime are examined, using as a prototype design the basic washer SQUID with a spiral input coil. A technical path is mapped out that leads to a practical SQUID less than 0.05 mm/sup 2/ in area with a tightly coupled 2- mu H input coil, user-friendly voltage-flux characteristics, minimal flux creep related hysteresis, and coupled energy sensitivity approaching the quantum limit at 4.2 K.

DOI
10.1109/20.133820

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IEEE Transactions on Magnetics•Mar 1, 1991•24 citations
Practical SQUIDs with well-coupled input coils of usable inductance have generally been fabricated at the 2-5 mu m level of lithography. Other technologies, silicon in particular, are now routinely practised at the 0.5 mu m level of lithography with impressive demonstrations at the 0.1-0.25 mu m level not uncommon. The implications of applying such a fabrication capability to advance DC SQUID technology are explored. In particular, the issues of scaling practical DC SQUIDs down to the 0.1-0.25 mu m regime are examined, using as a prototype design the basic washer SQUID with a spiral input coil. A technical path is mapped out that leads to a practical SQUID less than 0.05 mm/sup 2/ in area with a tightly coupled 2- mu H input coil, user-friendly voltage-flux characteristics, minimal flux creep related hysteresis, and coupled energy sensitivity approaching the quantum limit at 4.2 K.
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.>
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.
We have, for the first time, demonstrated integrated dc superconducting quantum interference devices (SQUIDs) with input coils of linewidth down to 0.5 μm. The SQUID inductance L consists of a single octagonal washer or two or four such washers configured in parallel. The input coil of inductance Li, which couples to L with a mutual inductance of Mi, consists of fine-line octagonal spiral(s) fabricated in close proximity to the washer(s). For a two-washer SQUID with twin 80-turn, 0.5 μm spirals, Li=2.5 μH, Mi=12.7 nH, L=109 pH, and the coupling constant k2=0.85. This entire device occupies an area of less than 0.2 mm2.
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.
We present a new superconducting quantum interference device (SQUID) that combines ultra-low-noise performance with tight coupling to a useable input coil. The inductive loop of the SQUID consists of a superconducting square washer that is slit in a low inductance fashion from its central hole to an outside edge where two Josephson tunnel junctions are incorporated. The inductance of the SQUID is about 90 pH, and each junction has a critical current of 15 μA and a resistive shunt of 4.5 Ω. The 190-nH input coil is a planar spiral of 50 turns situated immediately above the washer and coupled to the SQUID with a coupling constant k2 of 0.86. The SQUID has a minimum coupled energy sensitivity of 19h at 4.2 K and 8h at 1.5 K, in good agreement with theory. The crossover between white and low frequency noise occurs at ∼250 Hz.
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.