JPH05291633A - Nonlinear resistive element by superconductor - Google Patents

Abstract

PURPOSE:To set the critical current value of a bridge to several mu A to several mA and respond to electromagnetic waves from several 10Hz to several MHz by setting the sectional area of a superconductive wire to a specified value or less and the length to a specified value or more. CONSTITUTION:A superconductor 11 and a superconductor 11 are coupled by a fine superconductive wire 0.1mum<2> or less in sectional area and 10mum or more in length. The maximum value of a current flowing in the superconductive wire, that is, the critical current value increases together with the sectional area of the superconductive wire. Accordingly, a critical current from several muA to several mA can be gotten by setting the sectional area of a bridge 12 to 0.1mum<2> or less. Moreover, the Josephson effect can be eliminated by enlarging the length of the bridge 12 to more than 10mum above the length of coherence. Furthermore, since the quasiparticles inside the bridge 12 resonate with the electromagnetic waves applied from outside and work to decrease the superconductive current, the resistance property of the bridge 12 can be changed by the low electromagnetic waves from several 10Hz to several MHz.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-linear resistance element using a superconductor, and more particularly to a non-linear resistance element that can be used in various measuring circuits such as biomagnetic measurement and electromagnetic wave detection.

[0002]

2. Description of the Related Art Conventionally, Anderson et al.
ys. Rev. Lett. , 13, PP195-197
In (1964), in a bridge structure obtained by coupling two superconducting wires through a fine superconducting wire, when this is irradiated with microwaves, a constant voltage step structure called Shapiro step appears in the current-voltage characteristics. , The Josephson effect was first reported, and many reports have been made regarding this. The structure shown by Anderson has a structure in which two wide superconductor electrodes 1 are connected on a Si substrate 3 by a fine superconducting wire 2 as a bridge, as shown in FIG. At that time, in order to obtain the Josephson effect, the length of the bridge, that is, the length or width of the superconducting wire 2 is set to be shorter or at least about the same depending on the coherence length of the superconductor. The Shapiro step is a step in which the state of resistance change changes stepwise as shown in FIG. 8, and there is a step indicating the existence of the Josephson effect.

There have been many reports on the structure of the above-mentioned bridge. For example, the thickness of the constant-thickness bridge or the thickness of the coupling region of the superconductor is reduced, or the thickness-variable bridge in which both the thickness and the width are combined. Are known. For example, as the latter film thickness modification type bridge, Sandell R.A.
D. , G. J. Dolan, and J.M. E. Luke
ns, IC-SQUID76, PP93-100 (19
76) shows that indium is used as the superconductor, the thickness of the superconductor electrodes at both ends is 200 to 300 nm, and the thickness of the bridge is as thin as 50 nm.

As a method of manufacturing the above-mentioned bridge, a method of using a glass fiber as a mask, photolithography, electron beam lithography and the like are known. Also,
With respect to the bridge-type junction shown above, K. K. by Ricarel was used. K. Likeharev, Reviews of
Modern physics, Vol. 51, PP1
01-159 (1979) gives a review. On the other hand, regarding the bridge having a long length such as several μm to several tens of μm, R. D. Parks. J. M. Mo
chel. and L.D. V. Surgent, Jr. ，
phys. Rev. Letters, Vol. 13, p
p331-333 (1964).
According to this report, the width of the bridge is several μm, which is wider than the coherent length, and unlike the Josephson effect, the vortex flowing in the bridge causes the critical current, that is, the resistance characteristic to change with the applied magnetic field strength. Is configured.

[0005]

In the conventional bridge type junction shown above, the purpose is to obtain a Josephson element, and the bridge length or bridge width is designed and manufactured so as to be shorter than the coherent length. Therefore, the influence of electromagnetic waves on the current-voltage characteristics is only the response in the microwave band extending from several 100 MHz to several 100 GHz, and from several 10 Hz to several 10 M.
There is a problem in that the current-voltage characteristics of the superconducting bridge are controlled by electromagnetic waves in the Hz band, and in particular, it is impossible to change the bridge from the superconducting state to the resistance state with respect to electromagnetic waves of a specific frequency. Furthermore, the microwave response above is
This is known as the Diem-Wyatt effect, and the critical current of the bridge is increased by microwave irradiation, but the superconducting current is not decreased.

In addition, the standard fabrication of the bridge has a standard coherence length as small as about 0.1 μm. Therefore, it is impossible to produce a bridge-type junction by optical photolithography, and therefore, electron beams or electron beams are used instead of light. Submicron patterning using X-ray lithography has been considered, but the apparatus therefor is expensive and complicated, and there is a problem that it cannot be easily manufactured. Further, when a submicron fine wire is formed, when a large transient current flows, the superconducting state is broken, heat is generated, and the wire is broken. On the other hand, in the case of Park, et al., In which the bridge length is longer than the coherence length, only the response to the applied magnetic field was obtained, and the frequency response to the electromagnetic wave was not obtained. Further, since the critical current has a large bridge width of several μm, there is a problem that it cannot be controlled with a current value of several μA to several mA which is practical for a superconducting device.
Therefore, an object of the present invention is to set the critical current value of the bridge to a few μA to a few mA, and to set a critical current value of several 10 H.
To provide a non-linear resistance element made of a superconductor capable of responding to an electromagnetic wave of several tens of MHz from z, protecting a bridge from an excessive current, and capable of forming a submicron superconducting wire by optical lithography. It is in.

[0007]

In order to achieve the above object, a non-linear resistance element made of a superconductor according to the present invention has a cross-sectional area of 0.1 μm ² or less between the superconductors. Is to be connected by a fine superconducting wire of 10 μm or more. Further, in the nonlinear resistance element according to the present invention, by adjusting the cross-sectional area and the length of the nonlinear resistance element, the region of resistance change caused by the interference effect of the nonlinear resistance element with respect to electromagnetic waves in an arbitrary band is made variable. It is a thing. Further, the non-linear resistance element according to the present invention is one in which the resistance of the normal conductor is connected in parallel with the superconducting wire between the superconductors. Further, the non-linear resistance element according to the present invention forms an eave-shaped step on the substrate with an insulating film, forms a thin film of a superconductor to cover the step, and then removes it by dry etching to form a microscopic portion under the eaves. It is designed to leave the superconductor.

[0008]

The maximum current value flowing in the superconducting wire, that is, the critical current value increases with the cross-sectional area of the superconducting wire. Therefore,
By setting the cross-sectional area of the bridge to 0.1 μm ² or less, a critical current value of several μA to several mA can be obtained. Further, the Josephson effect can be eliminated by making the length of the bridge larger than 10 μm, which is equal to or longer than the coherence length.

Further, at the same time, since the bridge length is very large as compared with the sub-micron thinness, a large number of quasi-particles can be present in the bridge. Since it has a capacitive component, the quasi-particles in the bridge resonate with the electromagnetic wave emitted from the outside and act to reduce the superconducting current. Therefore, the resistance characteristic of the bridge can be changed by a low electromagnetic wave of several tens Hz to several MHz. Such a resonance frequency can be freely changed because the inductance and the capacitance component can be changed according to the length of the bridge.

Further, by connecting a resistor in parallel with the bridge, even if an excessive current flows in the bridge to break the superconducting state and the high resistance state is reached, the current can be bypassed by the parallel resistor. Therefore, the heat generation of the bridge can be suppressed. Also, by forming an eave-shaped step on the substrate with an insulating film and then forming the superconductor in a thin film shape by sputtering, etc., the superconductor can be deposited even under the eaves, and then dry etching is performed to form a superconductor. Even if the conduction band is removed, the superconductor under the eaves remains in its place as it is. Therefore, a conventional photolithography technique can form a bridge having a length of several tens of μm or more and a thickness of submicron.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of a non-linear resistance element made of a superconductor of the present invention will be described with reference to the drawings. 1 is an external view showing a first embodiment of the present invention, and FIG. 2 is a broken line AA 'in FIG.
FIG. In FIG. 1, 11 is a pair of Nb
A superconductor consisting of, 12 is a width of 75 nm (0.075 μm) and a thickness of 250 nm for coupling between the two superconductors 11.
(0.25 μm), the cross-sectional area is 0.01875 μm ² ,
A superconductor microbridge made of Nb as in the case of the fine superconductor 11 having a length of 40 μm, 14 is a resistance body made of Mo, which is a normal conductor connected in parallel to the above microbridge 12, and 15 is this resistance. This is a contact hole used to connect the body 14 and the superconducting electrode 11.
Further, in FIG. 2, 16 is a Si substrate and 17 is SiO 2.
It is an insulating film consisting of ₂ .

Next, a method of manufacturing the above non-linear resistance element will be described. The non-linear resistance element is manufactured by a thin film forming method such as photolithography or sputtering used in manufacturing semiconductors and the like. That is, the Si substrate 1
A resistor 14 is laminated on the substrate 6 by RF (radio frequency) sputtering to a thickness of 300 nm (0.3 μm). Next, the pattern of the resistor 14 is dry-etched by RIE (reactive ion etching) using a photoresist. An insulating film 17 of 200 nm is formed on the patterned resistor 14.
(0.2 μm) is laminated. Here, in order to form an eaves-shaped step in the insulating film 17, sputtering with no bias is used, which has poor coverage. Next, Nb is laminated on the insulating film 17 by DC sputtering to form the superconducting electrode 11 and the microbridge 12. Furthermore, using a photoresist for the pattern of the superconducting electrode 11,
Dry etching is performed by RIE. In this way, the Nb etching residue is formed under the eaves simultaneously with the patterning of the superconducting conductive electrode 11. Therefore, the microbridge 12 having a submicron size should be formed by using a technique such as advanced electron beam or X-ray lithography. Can be formed without.

Next, a second embodiment in which the nonlinear resistance element of the present invention is applied to a measuring circuit will be described. FIG. 3 is a schematic circuit diagram showing this second embodiment. In the figure, 2
Reference numeral 1 is a non-linear resistance element according to the present invention, 22 is a constant current source for supplying a current to the non-linear resistance element 21, and 23 is a voltmeter for measuring the voltage across the non-linear resistance element 21.

The operation of the second embodiment is as follows.
First, since the non-linear resistance element 21 uses a superconductor, it has to be placed in a temperature environment exhibiting superconducting characteristics. Therefore, the non-linear resistance element 21 is arranged in the liquid He. When the non-linear resistance element 21 was irradiated with electromagnetic waves of various frequencies, the voltage value read by the voltmeter 23 was as shown in FIG. The critical current value when the non-linear resistance element 21 was not irradiated with electromagnetic waves was 220 μA. Therefore, the measurement was performed by setting the current value of the constant current source 22 to be smaller than the critical current value, for example, 160 μA here. As is clear from FIG. 4, there is a maximum resistance value at about 200 Hz. In addition, at a frequency of 40 KHz or less, there was no change in resistance and the state of superconductivity remained. In this way, a resistor having a non-linear resistance characteristic according to the frequency of the electromagnetic wave could be obtained. The non-linear resistance element of the present invention shown in this embodiment can be applied to detection of electromagnetic waves and control elements using electromagnetic waves.

Next, the non-linear resistance element of the present invention is applied to SQUI.
A third embodiment applied to D (superconducting quantum interferometer) will be described. FIG. 5 is a schematic circuit diagram showing the third embodiment. In the figure, 31 is a non-linear resistance element according to the present invention, 32 is a pickup coil made of a superconductor which is arranged in the vicinity of an object to be measured and is connected in series to the non-linear resistance element 31, 33 is a SQUID, and 34 is a SQUID3.
3 is an input coil for transmitting the magnetic flux captured by the pickup coil 32.

The embodiment of the present invention configured as described above is used for biomagnetism measurement such as magnetoencephalography and magnetocardiography. The pickup coil 32 and the input coil 34 are superconductingly connected. Here, the result of measuring the ratio of the magnetic flux Φp captured by the pickup coil 32 and the magnetic flux Φi transmitted to the input coil 34 with respect to the frequency is shown in FIG. As is clear from the figure,
Similar to the embodiment, the nonlinear resistance element 31 is about 200 KHz.
Shows the maximum resistance value at the frequency. Therefore, the transmission of the magnetic flux Φp near 200 KHz, which is higher than the band of the biomagnetic signal, can be blocked. Generally, in an environment where biomagnetism measurement is performed, electromagnetic noise is a problem because biomagnetism is weak. The biomagnetic signal has a low frequency of 100 KHz or less, and it is necessary to remove high frequencies higher than that.
Conventionally, since it was composed of only a pickup coil and an input coil, all magnetic signals were transmitted to the SQUID regardless of the frequency. Therefore, if a frequency component larger than the biomagnetism enters from the outside, a measurement error will occur, but by using the nonlinear resistance element of the present invention, such a measurement error can be eliminated. Thus, it becomes possible to measure a weak biomagnetic signal.

[0017]

As is apparent from the above, according to the present invention, a pair of superconductors are connected by a fine superconducting wire having a cross-sectional area of 0.1 μm ² or less and a length of 10 μm or more. By configuring a non-linear resistance element, several tens of Hz to several tens of MH
There is an effect that a non-linear resistance element capable of changing the resistance characteristic with respect to the electromagnetic wave of z can be implemented. Furthermore, since the response frequency of the non-linear resistance element of the present invention can be adjusted by changing its length or cross-sectional area, a non-linear resistance element that responds to electromagnetic waves in an arbitrary frequency band can be obtained. Further, by connecting the resistance of the normal conductor in parallel with the non-linear resistance element, there is an effect that an excessive current flows in the non-linear resistance element to protect it. Furthermore, a superconductor is laminated on an insulating film having a canopy-like step, and then dry etching is performed to form a sub-micron fine pattern, which is conventionally produced only by electron beam or X-ray lithography, by optical lithography. There is an effect that can be easily realized with dimensions.

[Brief description of drawings]

FIG. 1 is a front view showing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along the line A-A ′ of FIG.

FIG. 3 is a schematic circuit diagram showing a second embodiment.

FIG. 4 is a characteristic diagram showing voltage values when electromagnetic waves of various frequencies are irradiated in the second embodiment.

FIG. 5 is a schematic circuit diagram showing a third embodiment.

FIG. 6 is a characteristic diagram in which the ratio between the magnetic flux captured by the pickup coil and the magnetic flux transmitted to the input coil in the third embodiment is plotted against frequency.

FIG. 7 is an explanatory diagram showing a conventional superconductor bridge structure.

FIG. 8 is a characteristic diagram showing a Shapiro step in a superconducting phenomenon.

[Explanation of symbols]

11 Superconducting Electrode 12 Micro Bridge 14 Mo Resistor 17 SiO ₂ Insulating Film 21 Nonlinear Resistance Element 31 Nonlinear Resistance Element 32 Pickup Coil 33 SQUID 34 Input Coil

Claims (4)

[Claims] 1. A smooth substrate, a pair of superconductor electrodes formed on the substrate, and a fine superconductor formed on the substrate and connecting the pair of superconductor electrodes to each other. And a fine superconducting wire having a cross-sectional area of 0.1 μm ² or less and a length of 10 μm or more. 2. The non-linear resistance element according to claim 1, wherein a cross-sectional area and a length of the fine superconducting wire are variable within the range. 3. The superconductor according to claim 1, wherein a resistor made of a normal conductor is arranged and connected in parallel with the fine superconducting wire between the pair of superconductor electrodes. Non-linear resistance element made of conductor. 4. With respect to the fine superconductor, an appropriate resistor is patterned on the smooth substrate, and an insulator is laminated thereon to form a canopy-like step on the resistor. , After forming a thin film of superconductor to cover this step,
The non-linear resistance element according to claim 1, wherein the superconductor left under the eaves is removed by dry etching and used as the fine superconductor.