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Journal of Applied Physics

Proximity effect bridge and superconducting microcircuitry

Harris Notarys, James E. Mercereau

Published Apr 1, 1973• 69 citations
Publisher

Abstract

Techniques have been developed with which it is possible to fabricate superconducting thin-film structures with a wide variety of electrical and superconducting parameters which show Josephson-like phenomena. These bridges-based on the proximity effect-are made in layered thin-film ``substrates'' using a large variety of superconducting and nonsuperconducting materials. In this paper we detail the fabrication techniques and the electrical and superconducting characteristics for these proximity effect bridges including a simple low-frequency (<10 GHz) equivalent circuit based on analysis and experiment which closely describes the quantum phase coherence operation of the bridges in their voltage sustaining state. Extension of these fabrication techniques to entire thin-film electrical circuits including resistors, capacitors, and inductors with these bridges and leading to an entire superconducting microcircuit technology is discussed.

DOI
10.1063/1.1662457

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Journal of Vacuum Science and Technology•Jan 1, 1974•67 citations
Techniques have been developed with which it is possible to fabricate superconducting thin-film structures with a wide variety of electrical and superconducting parameters which show Josephson-like phenomena. These bridges-based on the proximity effect-are made in layered thin-film ``substrates'' using a large variety of superconducting and nonsuperconducting materials. In this paper we detail the fabrication techniques and the electrical and superconducting characteristics for these proximity effect bridges including a simple low-frequency (<10 GHz) equivalent circuit based on analysis and experiment which closely describes the quantum phase coherence operation of the bridges in their voltage sustaining state. Extension of these fabrication techniques to entire thin-film electrical circuits including resistors, capacitors, and inductors with these bridges and leading to an entire superconducting microcircuit technology is discussed.
Standard microelectronic fabrication techniques have been utilized to produce batch quantities of superconducting quantum electronic devices and circuits. The overall goal is a fabrication technology yielding circuits that are rugged and stable and capable of being fabricated controllably and reproducibly in sizeable quantities. Our progress toward this goal is presented, with primary emphasis on the most recent work, which includes the use of electron-beam lithography and techniques of hybrid microelectronics. Several prototype microcircuits have been successfully fabricated. These microcircuits are formed in a thin-film parent material consisting of layers of superconducting and normal metals, and use proximity-effect structures as the active circuit elements.
Experimental investigations on the proximity effect bridge (a Josephson device) at zero voltage and at finite voltages in the µV range are reported. The phase-super current relation at zero voltage was measured using an asymmetric superconducting quantum interferormeter circuit. The data are in agreement with the Josephson supercurrent-phase relation I_S=I_C sinδ with deviation less than 5% of the critical current I_c. The supercurrent density in the measured bridges reached as high as 50-100 µA/µm^2. Using microcircuitry techniques, proximity effect bridges were strongly coupled to superconducting microstrip resonators. Selfinduced steps in the I-V characteristics of bridges coupled to resonators were observed in the GHz region at voltages (frequencies) corresponding to the expected modes of the resonators. Two types of steps were seen depending on whether the resonator impedance on resonance was much higher or much smaller than the bridge resistance. A simple two fluid model of the bridge-resonator circuit was developed and the size and shape of self-induced steps were calculated for a generalized Josephson oscillator relation I_S = I_c(l-q + q sin∫ 2e/hV dt) where q = 1 corresponds to the original Josephson relation and q = 1/2 represents the phase slip regime. At low critical currents (I_c < 10 µA) and low voltages (V < 3µV) the size and shape of experimentally observed self-induced steps agree with the q = 1 model. At higher voltages and/or critical currents the step size increasingly deviates from the q = 1 model towards q = 1/2. These observations are interpreted to indicate a progressive reduction of the amplitude of the oscillating Josephson supercurrent in proximity effect bridges from I_c towards I_c /2 as the critical current and/or voltage are increased.
Weakly superconducting circuits have been developed - based on the proximity effect in hard superconductors - which show Josephson-like phenomena over a wide range of electrical and superconducting parameters. Characteristics of these circuits will be discussed and a simple physical model will be presented to account for a modification of the Josephson phenomena at high current density in these devices. Response of the devices at high frequency will be emphasized with special emphasis on detection processes and the Dayem-Wyatt effect.
In this paper, we present our latest results on the properties of superconducting weak links prepared in Mo films by ion-implantation and in Nb films by electron-beam lithography. In the case of the ion-implanted weak links, which structurally resemble proximity-effect bridges, we find that the static properties are quite well described by a recent theory of Likharev and Yakobson. We further find that the rf response of these weak links is in excellent agreement with quasistatic resistively-shunted-junction model calculations based on a sinusoidal current-phase relation at those temperatures for which such a current-phase relation is predicted by the static theory of Likharev and Yakob-son. For the e-beam fabricated Nb bridges, which have the Dayem bridge geometry, our sample preparation techniques have allowed us to produce bridges and two-junction interferometers in which the link dimensions are as small as about 2000 Angstroms. We find that Josephson effects are observable in these bridges even when the bridge dimensions exceed the temperature-dependent coherence length by about an order of magnitude, but are still less than the effective thin-film penetration depth.
A multilevel thin-film process has been developed for fabricating experimental circuits containing Josephson tunneling gates, insulated crossings, superconducting contacts, and terminating resistors. The process utilizes a Nb film for the ground plane and Pb alloy films for overlying superconducting layers. Insulation is achieved through the use of vacuum-deposited SiO films and anodization of the Nb ground plane. An oxide formed by rf sputter-etching in an oxygen plasma is used for forming the tunneling barriers. Photoprocessing techniques are used for patterning. Through use of the process it has been possible to fabricate experimental memory and logic circuits which survive thermal cycling to 4.2 K and intervening storage at room temperature without significant changes in electrical characteristics.