JP4595069B2 - Quasi-planar waveguide type Josephson junction array structure, digital-analog converter using the same, programmable array for Josephson voltage standard, chip for Josephson voltage standard, Josephson voltage generator - Google Patents

Description

  The present invention relates to a Josephson junction array in which a large number of quasi-planar waveguide type Josephson junctions that generate a constant voltage by applying a bias current and a microwave are connected in series, a voltage standard device constituted by the Josephson junction array, and the like.

The Josephson voltage standard is based on the physical law of AC Josephson effect that can accurately convert frequency to voltage.
Josephson junction devices can generally be considered voltage-frequency converters. When a DC voltage V is applied to the junction, the Josephson current oscillates at a frequency f = K _J V (K _J = 483597.9 GHz / V; Josephson constant). However, the electromagnetic radiation output from one junction is relatively small. Since the phase of each junction can be made in-phase by a high-frequency coupling circuit (such as a shunt resistor), an output proportional to N ² can be taken out. Here, N is the number of junctions in the laminated structure. On the contrary, a voltage can be output by changing the frequency.
Further, a Josephson D / A converter can be configured by connecting an arbitrary number of Josephson junctions in series. For example, when a digital input bias current (binary number 000... 001010) is applied, an analog output voltage V _a-out (0 + 2 × V ₀ + 0 + 8 × V ₀ + 0 = 10V ₀ ) can be obtained. The element corresponding to each bit is a stack of junctions having a specific number of junctions.
By the way, a microwave transmission line is used to supply a microwave to a Josephson junction on a chip made of silicon or the like. A quasi-planar waveguide (CPW) has been used as a microwave transmission line (see Non-Patent Document 1 below).

FIG. 5 is a cross-sectional view of a conventional quasi-planar waveguide.
FIG. 5 shows a quasi-planar waveguide having a wiring structure in which the signal conductor wiring 2 and the ground conductor (external conductor) 3 are provided on one surface of the dielectric substrate 1. Since the signal wiring has a structure sandwiched between ground conductors and signal transmission is possible using only one surface of the dielectric, circuit characteristics evaluation and connection with other circuits are easy.

FIG. 6 is a plan view of a conventional Josephson junction array.
The Josephson junction array used in the voltage standard of FIG. 6 is a Josephson junction composed of a superconductor lower electrode 4, a normal conductor intermediate layer 5 and a superconductor upper electrode 6. A plurality of lower wirings 4 and upper wirings 7 are also connected in series.

  As shown in the plan view of FIG. (6) and the cross-sectional view of FIG. (7), in order to uniformly apply microwaves to all the junctions, the Josephson junction array is also provided with ground conductors 3 on both sides of the junction, A device in which a quasi-planar transmission line is formed is also known (see Non-Patent Document 2).

In order to prevent the superconductivity of the lower electrode 4 or the upper electrode 6 from being lost due to the heat generated in the Josephson junction, the lower electrode 9 having a larger width as shown in FIG. Improvements have been made to release heat to the substrate 1. (See Non-Patent Document 3)
C. P. Wen: IEEE Trans on Microway Theory and Techniques MIT-17 (2) December 1969, pp. 15-27. 1087-1090 “Coplanar Waveguide: A Surface Strip Transmission Line Sustainable for Non-reciprocal Gyromagnetic Devices Applications” S. P. Benz: Appl. Phys. Lett. 67 (18), 30 October 1995, pp. 2714-2716 "Superconductor -normal -superconductor junctions for programmable voltage standards" Y. Chong, P.A. D. Dresselhaus, and S.M. P. Benz: Appl. Phys. Lett. 83 (9), 1 September 2003, pp. 1794-1796 "Thermal transport in stacked superconductor-normal metal-superconductor Josephson junctions" Hiroaki Kogure: High-frequency world to learn with an electromagnetic simulator, CQ Publisher, 1999, Chapter 1, page 20 K. C. Gupta: “Microstrip Lines and Slotlines”, (Artech House, Boston / London, 1996) Chapter 7, page 376 C. A. Hamilton, C.I. J. et al. Burroughs, R.A. L. Kautz: IEEE Trans on Appl. Supercond. , 44 (2) April 1995, pp. 223-225 “Josephson D / A Converter with Fundamental Accuracy”

ここで、ωは角周波数、μ_０は真空中の透磁率、λはロンドン侵入長、σは複素導電率、ｄは膜厚である。従って、超伝導材料を電極および配線として用いる場合、膜厚ｄがロンドン侵入長λより小さいと表面インピーダンスが増加し減衰が急激に大きくなる。ジョセフソン接合アレーの配線で減衰があると、大きなマイクロ波を入力しなければならないという問題だけでなく、電圧標準用ジョセフソン接合アレーにおいて接合に印加されるマイクロ波電力がすべての接合において均一である必要があるにもかかわらず、減衰によりマイクロ波の電力にばらつきが生じ動作マージンを低下させるという深刻な問題も発生する。したがって、下部電極兼下部配線９及び上部配線７はロンドン侵入長より十分厚くなければならない。しかしここで問題となるのは、超伝導材料としてチッ化ニオブのようなロンドン侵入長が長い材料を用いた場合、膜厚を非常に厚くしなければならいことである。チッ化ニオブは段差が大きいと結晶の粒界が発生し流せる電流が急激に減少してしまうという問題点がある。図（７）に示すように下部電極４の端（エッジ）の上を上部配線７が横切ると、上部配線７には下部電極４の膜厚に相当する段差が発生し、上部配線７に流せる電流が著しく減少してしまう。したがって、上部配線７の臨界電流値が著しく減少してしまうという問題があり、下部電極４を厚くするには限界がある。
本発明の目的は、上記問題点に鑑み、表面インピーダンスの増加によるマイクロ波の減衰を防止するようにした準平面導波路型ジョセフソン接合アレー構造体、それを用いたデジタル−アナログ変換器、プログラマブルジョセフソン電圧標準用接合アレー、ジョセフソン電圧標準用チップ、ジョセフソン電圧発生装置を提供することにある。 The surface impedance Z _s of the superconducting thin film is given by the following equation (1). (See Non-Patent Document 4)

Here, ω is the angular frequency, μ ₀ is the magnetic permeability in vacuum, λ is the London penetration length, σ is the complex conductivity, and d is the film thickness. Therefore, when using a superconducting material as an electrode and wiring, if the film thickness d is smaller than the London penetration length λ, the surface impedance increases and the attenuation increases rapidly. Not only does a large microwave input have to be attenuated in the Josephson junction array wiring, but the microwave power applied to the junction in the voltage standard Josephson junction array is uniform across all junctions. In spite of the necessity, there is a serious problem that the microwave power varies due to attenuation and the operation margin is lowered. Therefore, the lower electrode / lower wiring 9 and the upper wiring 7 must be sufficiently thicker than the London penetration length. However, the problem here is that when a material having a long London penetration length such as niobium nitride is used as a superconductive material, the film thickness must be very large. Niobium nitride has a problem in that when the step is large, crystal grain boundaries are generated and the current that can be flowed rapidly decreases. As shown in FIG. 7, when the upper wiring 7 crosses the end (edge) of the lower electrode 4, a step corresponding to the film thickness of the lower electrode 4 is generated in the upper wiring 7 and can flow to the upper wiring 7. The current is significantly reduced. Therefore, there is a problem that the critical current value of the upper wiring 7 is remarkably reduced, and there is a limit to making the lower electrode 4 thick.
In view of the above problems, an object of the present invention is to provide a quasi-planar waveguide type Josephson junction array structure that prevents microwave attenuation due to an increase in surface impedance, a digital-analog converter using the same, and a programmable It is an object to provide a junction array for Josephson voltage standard, a chip for Josephson voltage standard, and a Josephson voltage generator.

The distribution of the current flowing in the line is not uniform, and a strong current flows in a small portion along both edges of the line. This is also called edge peculiarity or edge bias (see Non-Patent Document 5).
FIG. 1 is a plan view of an embodiment of the present invention. Also in the case of the quasi-planar waveguide type Josephson junction as shown in FIG. 1A, the electric field is concentrated in the gap between the ground conductor 3 and the lower electrode 9 (see Non-Patent Document 6). And flows in the vicinity of the gap between the lower electrode 9 in a concentrated manner. Therefore, if the thickness in the vicinity of the gap (10 portion in FIG. 1) is increased, the same effect can be obtained without increasing the entire thickness of the lower electrode 9, and the microwaves due to the increase in surface impedance can be obtained. It becomes possible to prevent attenuation.
In particular,
(1) Josephson junction array structure is
The quasi-planar waveguide type Josephson junction array structure has a plurality of lower electrodes arranged in series in a state of being separated from each other on a substrate,
Two intermediate layers are arranged on the lower electrode in series along the arrangement direction of the lower electrode in a state of being separated from each other, and are connected to the two intermediate layers adjacent to each other in the series direction. It has a number of Josephson junctions with an upper electrode.
A microwave wiring is provided on the lower electrode so as to be separated from both sides of the intermediate layer, and a ground conductor is provided outside the microwave wiring.
(2) In the Josephson junction array structure according to (1) above,
The substrate is formed of a dielectric or a semiconductor.
(3) In the Josephson junction array structure according to (1) above,
The outer conductor is made of a superconductor.
(4) In the Josephson junction array structure according to (1) above,
The insulating film is formed of a dielectric material.
(5) The digital-analog converter
The quasi-planar waveguide type Josephson junction according to any one of the above (1) to (4) is arranged in series between any number of junctions between a microwave input terminal and a microwave termination resistor. To do.
(6) The programmable Josephson voltage standard junction array is
The digital-analog converter described in the above (5) is used.
(7) Josephson voltage standard chip is
The programmable voltage standard junction array described in (6) above is provided on a substrate.
(8) The Josephson voltage generator is
A programmable voltage standard chip described in (7) above, a refrigerator for cooling the chip, a microwave source, and a bias current source are provided.

  As shown in FIG. 1 (1), the microwave-dedicated wiring 10 is disposed above the lower electrode / lower wiring 9 of the quasi-planar waveguide type Josephson junction array and in the vicinity of the gap with the ground conductor 3. The same effect can be obtained as when the entire film thickness of the electrode and lower wiring is increased. Since the lower electrode / lower wiring 9 itself can be made thin, it is possible to prevent the microwave electrode from being attenuated at the same time without causing an increase in the level difference of the circuit and the problem of a decrease in the critical current of the upper wiring 7. Preventing microwave attenuation not only reduces costs when it is no longer necessary to supply large microwaves, but also reduces the attenuation of microwaves in the Josephson junction array if the attenuation of the microwaves in the array decreases. The uniformity can be improved and the operation margin of the voltage standard circuit can be improved.

  The best mode for carrying out the invention will be described below.

In the quasi-planar waveguide type Josephson junction array structure of the present invention, the ground conductor 3 is disposed on both sides of the junction of the Josephson junction array in order to uniformly apply microwaves to all junctions. The transmission line is formed.
FIG. 2 is a plan view of the embodiment of the present invention. As shown in FIG. 2B, a lower electrode / lower wiring 9 made of superconducting material is formed on a substrate 1, and an intermediate layer (normal conductor) 5 and an upper electrode made of superconducting material are formed on the lower electrode / lower wiring 9. 6 are sequentially formed. Next, the upper electrode 6 and the intermediate layer 5 are pattern-etched to form a Josephson junction. An interlayer insulating film 8 is formed thereon to cover the whole. In order to connect the Josephson junctions in series, a hole is formed in the interlayer insulating film 8 above the upper electrode 6 to form a contact hole. At the same time, in order to connect the wiring for microwaves to the lower electrode / lower wiring 9, a hole is also formed in the interlayer insulating film 8 on the lower electrode / lower wiring 9 to form a contact hole 11. A superconducting film to be the upper wiring is formed on the entire surface, patterned and etched to form the upper wiring 7 and the microwave wiring 10 simultaneously. In order to facilitate the fabrication as described above, the upper wiring 7 and the microwave wiring 10 can use a conductor film of the same layer.

FIG. (3) is equivalent to the BB cross-sectional view of FIG. (1) and is an embodiment different from FIG. (2).
As shown in FIG. 3, a lower electrode / lower wiring 9, an intermediate layer (normal conductor) 5, and an upper electrode 6 are formed on the substrate 1. The upper electrode 6 and the intermediate layer 5 are etched to form a Josephson junction. At the same time, the dummy Josephson junction 12 is left. This dummy Josephson junction 12 becomes a path for microwaves, and the same effect can be obtained as when the lower wiring 9 is made thicker. Next, an interlayer insulating film 8 is formed to cover the whole. In order to connect the Josephson junctions in series, a hole is formed in the interlayer insulating film 8 above the upper electrode 6 to form a contact hole. An upper wiring 7 is formed by forming a superconducting film as an upper wiring on the whole, patterning and etching. Since the Josephson junction is used to form the microwave wiring, it can be easily realized without increasing the burden on the process.

FIG. 4 is a conceptual diagram of the digital-analog converter for programmable voltage standard according to the present invention.
By arranging m ^k junctions in series between the microwave input terminal 13 and the microwave termination resistor 14, the Josephson junctions constituting the quasi-planar waveguide as shown in FIG. A digital-analog converter having the minimum number of junctions of m and the number of bits of k bits is configured. This Josephson junction arranged in series with m ^k junctions is called a binary array. (For example, see Non-Patent Document 6) For example, when the resolution is 8 bits and the minimum number of junctions is 128 junctions, 128, 256, 512, 1024, 2048, 4096, 8192, 16384 An array including one Josephson junction may be arranged in series.
As shown in FIG. 4 (4), the microwave is supplied from one side of the Josephson junction array and transmitted to the microwave termination resistor 14 connected to the opposite end to be terminated. By turning on / off the DC bias current in the binary array 15, the digital input via the binary array 15 is converted into the analog output voltage 16.

  A binary-arranged Josephson junction array constitutes a digital-analog converter, which is fabricated on a substrate such as a Si wafer and cut into a chip, called a Josephson voltage standard chip, a refrigerator for cooling the chip, A set required to operate as a voltage standard such as a microwave source and a bias current source is called a Josephson voltage standard device. By constructing a voltage standard device using the quasi-planar Josephson junction array of the present invention, it becomes possible to use a niobium nitride thin film having a high critical temperature, so that the operating temperature is about 10K which is much higher than the conventional 4.2K. There are many advantages related to cost reduction and portability, such as miniaturization of the refrigerator, reduction of power consumption, and reduction of noise of the refrigerator. Further, since the attenuation of the microwave is reduced by the present invention, the microwave source can be made small, and the cost is similarly reduced. Further, preventing the attenuation of the microwave also makes the microwave supplied to the Josephson junction uniform, which increases the operation margin and improves the performance.

It is a top view of the Example of this invention. It is BB sectional drawing of FIG. 1 is a cross-sectional view taken along the line BB in FIG. It is a conceptual diagram of the digital-analog converter for programmable voltage standards. It is sectional drawing of the conventional quasi-planar waveguide. It is a top view of the conventional Josephson junction array. It is AA sectional drawing of FIG. It is sectional drawing of the conventional improved type Josephson junction array.

Explanation of symbols

1: Substrate 2: Signal wiring conductor 3: Ground conductor 4: Lower electrode and lower wiring 5: Intermediate layer (normal conductor)
6: Upper electrode 7: Upper wiring 8: Interlayer insulating film 9: Widened lower electrode / lower wiring 10: Microwave dedicated wiring 11: Contact hole 12: Microwave dummy junction 13: Microwave input terminal 14: Micro Wave termination resistor 15: Bias current input (digital input)
16: Output voltage 17: Josephson junction array

Claims (8)

A plurality of lower electrodes are arranged in series on the substrate in a state of being separated from each other,
Two intermediate layers are arranged on the lower electrode in series along the arrangement direction of the lower electrode in a state of being separated from each other, and are connected to the two intermediate layers adjacent to each other in the series direction. It has a number of Josephson junctions with an upper electrode.
A quasi-planar waveguide type Josephson junction array structure in which microwave wiring is provided on both sides of the intermediate layer on the lower electrode, and a ground conductor is provided outside the microwave wiring. body.   The Josephson junction array structure according to claim 1, wherein the substrate is made of a dielectric or a semiconductor.   2. The Josephson junction array structure according to claim 1, wherein the outer conductor is made of a superconductor.   The Josephson junction array structure according to claim 1, wherein the insulating film is formed of a dielectric.   5. A digital-characteristic circuit comprising: a quasi-planar waveguide type Josephson junction according to any one of claims 1 to 4 arranged in series between any number of junctions between a microwave input terminal and a microwave termination resistor. Analog converter.   6. A programmable Josephson voltage standard junction array comprising the digital-analog converter according to claim 5.   7. A Josephson voltage standard chip, comprising the programmable voltage standard junction array according to claim 6 provided on a substrate. A Josephson voltage generator comprising the programmable voltage standard chip according to claim 7, a refrigerator for cooling the chip, a microwave source, and a bias current source.

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