JPH0828537B2 - Superconducting inductor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting circuit that operates at cryogenic temperatures, and in particular to a superconducting device in which an opening is provided in a ground plane under wiring that forms self-inductance or mutual inductance. Regarding the structure of the inductor.

(Prior Art) A superconducting circuit that operates at a cryogenic temperature is expected as a switching element for a computer or the like because it operates at a high speed with a very small amount of power. The superconducting circuit is composed of Josephson elements, resistors, self-inductors, and mutual inductors.
The present invention is a self-inductor used in this superconducting circuit,
The present invention relates to the structure of a mutual inductor. In principle, the self-inductance is realized by superconducting wiring, and the mutual inductance is realized by two magnetically coupled superconducting wirings. Quantum interference circuits and quantum magnetic flux parametrons using Josephson devices use magnetic flux quantum Φo (2.07 × 10 ^-15 Wb) as a unit of information, so the inductance used in these circuits is as small as several tens to 0.1 pH. is there. In a superconducting integrated circuit, the inductance is realized by superconducting wiring placed on a superconducting film called a ground plane via an insulating layer. This ground plane applies the Meissner effect of the superconductor to block external geomagnetism, noise magnetism, and noise electromagnetic waves from the superconducting element, and also functions as a ground plane for returning the bias current of the superconducting circuit to the power supply. have. The width and length of the superconducting wire that realizes the inductor vary depending on the inductance value, the thickness of the insulating layer, the magnetic field penetration length of the superconducting film used for the ground plane and the superconducting wire. For example, if the insulating layer has a thickness of 0.2 μm and lead or niobium is selected as the ground plane and superconducting wiring metal, a self-inductor with a pH of 1 to 10 pH can be realized with a superconducting wiring with a width of 5 to several 10 μm and a length of several 10 μm to several 100 μm. To be done. The mutual inductor is realized by two superconducting wires magnetically coupled to each other on the ground plane. Usually, in order to strengthen magnetic coupling, two superconducting wires are stacked with an insulating layer in between.

In order to realize high-speed performance of the circuit, the size of the Josephson junction is minute and the size is 1 to 5 μm square or round. Compared with the size of this Josephson junction, the size of the superconducting wiring that realizes the above-mentioned inductor is extremely large. If the size of the superconducting wiring that realizes the inductor is large, the parasitic capacitance is increased, the circuit speed is reduced, and the degree of integration of the integrated circuit cannot be increased. The size of the inductor configured on the ground plane (dimension of the superconducting wire) is large because the magnetic flux generated by the current flowing in the superconducting wire is due to the thin film thickness of the insulating layer between the ground plane and the superconducting wire. This is because the amount of magnetic flux generated due to being trapped in

As a method of configuring the inductor in a more compact manner, an inductor structure has been proposed in which an opening is provided in the ground plane and wiring is provided in this opening (this opening may be referred to as an inductor opening hereinafter). With this structure, the magnetic flux generated by the current flowing in the superconducting wire does not get trapped in a narrow area because it passes through the opening,
A large amount of magnetic flux is generated. Therefore, an inductor having a structure in which an opening is provided in the ground plane and the superconducting wiring is provided in this opening can have a smaller size than inductors of other structures. The structure of this inductor is shown below in Figures 2A and 2B.
Let's explain using figures. FIG. 2A is a structural example of a conventional inductor. This inductor has a structure in which a superconducting wire 3 is passed through an opening 1 provided in a ground plane 2. Since the magnetic flux generated by the current flowing through the superconducting wiring 3 passes through the opening 1, a large inductance can be realized with a small structure.
FIG. 2B is another structural example of the conventional self-inductor. In this figure, a superconducting wire 3 having a folded structure is arranged above an opening 1 provided in a ground plane 2. Also in this structure, the magnetic flux generated by the current flowing through the superconducting wire 3 passes through the opening 1. 2A and 2B, a second superconducting wire is arranged above or below the superconducting wire 3, and the second superconducting wire and the superconducting wire 3 are respectively provided with primary windings,
A secondary inductor can form a mutual inductor. As described above, the inductor having the structure in which the opening is provided in the ground plane can reduce the element size and the parasitic capacitance, which is advantageous for a high-speed highly integrated circuit.

(Problems to be Solved by the Invention) On the other hand, since the inductors having the structures shown in FIGS. 2A and 2B are all composed of superconductors, there is a drawback that magnetic flux is trapped in the opening 1. The magnetic flux trapped in the opening 1 is the magnetic flux quantum Φo (2.07 × 10
^-15 Wb). In an element using a magnetic flux as a signal medium such as a magnetic flux quantum element, the trapped magnetic flux is superimposed on the signal magnetic flux to prevent normal operation. Therefore, in the element using the inductor shown in FIGS. 2A and 2B, the aperture 1
It is necessary to have a structure in which the magnetic flux is not captured or a structure in which the captured magnetic flux is eliminated (expulsed). The magnetic flux is trapped in order to operate the superconducting circuit at room temperature (300
It occurs in the process of cooling from K) to an extremely low temperature of liquid helium (4.2 K) for the following reasons a) and b).

a) The geomagnetism and noise magnetism on the earth are sufficient to trap the magnetic flux quantum in the aperture of several + μm square. Therefore,
Geomagnetism and noise magnetic flux are captured in the opening.

b) In the process of cooling the superconducting circuit, a thermoelectromotive force is generated due to a temperature difference between the integrated circuits and between the elements, and a current flows through the superconductor. The magnetic flux generated by this current is captured in the opening.

In order to avoid the trapping of magnetic flux due to these causes, in the prior art, a method mainly for a) is adopted in which the superconducting circuit is surrounded by a ferromagnetic material such as permalloy or a superconductor to shield it from earth magnetism and noise magnetism. By this method, the magnetic field from the outside can be made extremely weak, and the magnetic flux trapped in the opening of the inductor can be set to about the unit magnetic flux quantum. Also, in combination with magnetic shielding, regarding a) and b)
A groove is provided near the inductor on the ground plane called MOAT, and the magnetic flux is captured in this groove, and the structure in which the magnetic flux is hard to be captured is used in the opening of the inductor. This MOAT is, for example, S. Bermon, T. Gheewala, “MOAT-GUARDED
JOSEPHSONSQUIDS ", IEEE Transaction on MAGNETICS, VO
L.MAG-19, No. 3, pp. 1160-1164, MAY 1983. The MOAT has a structure in which the energy when the magnetic flux quantum is trapped in the MOAT is smaller than the energy when the magnetic flux quantum is trapped in the opening of the inductor. Therefore, in the process of cooling the superconducting circuit, the flux quanta are
If trapped in either of the ATs, the magnetic flux is more easily trapped in the MOAT than in the inductor aperture. In this way, MOAT avoids trapping magnetic flux in the inductor aperture. This prior art has a structure in which the MOAT placed in the vicinity of the inductor facilitates the trapping of the magnetic flux quantum, but whether or not the trap can actually be trapped depends on the probability. Therefore, it is difficult to prevent all the elements from catching the magnetic flux in the large-scale system using 1 million or more elements like a computer by the method of the prior art MOAT. Further, when the magnetic flux is actually captured by the inductor, there is no means for expelling the captured magnetic flux.

It is an object of the present invention to provide a structure in which an opening is provided in a ground plane below a wiring and a magnetic flux is hard to be captured in the opening. Yet another object is to provide a means for expelling the magnetic flux trapped in the opening of the inductor.

(Means for Solving the Problems) In order to achieve the above object, in the present invention, a second opening that is connected to the opening of the inductor through a channel is formed, and at the same time, an electric current that controls the flow of the magnetic flux in the channel is formed. By providing the conductor, the magnetic flux trapped in the opening of the inductor is expelled to the second opening.

(Examples) Hereinafter, the present invention will be described using examples. FIG. 1 shows an embodiment of the present invention. In FIG. 1, the ground plane 2
This is a structure in which the opening 1 of the inductor provided in 1 and the magnetic flux erasing opening 10 provided in the ground plane 2 are connected by a channel 11 dug in the ground plane. A conductor 12 is bridged to the channel 11. Both ends of the conductor 12 are ground planes 2
Is in contact with Therefore, when both the ground plane 2 and the electric conductor 12 are in the superconducting state, FIG. 1 is the same as having two openings in the ground plane 2, that is, the opening 1 of the inductor and the magnetic flux displacement opening 10. is there. Further, when the electric conductor 12 is in the normal electric conductor state, in FIG. 1, one opening in which the opening 1 of the inductor and the magnetic flux erasing opening 10 are connected by the channel 11 is opened in the ground plane 2. If magnetic shielding is performed, the magnetic field from the outside can be made extremely weak, and only the unit magnetic flux quantum can be trapped in the opening provided in the ground plane or not trapped at all. In the configuration of FIG.
Is in a superconducting state, the energy when the magnetic flux quantum is trapped in the magnetic flux displacing opening 10 is smaller than the energy when the magnetic flux quantum is trapped in the opening 1 of the inductor. Next, a method for expelling the trapped unit flux quantum to the flux expelling opening 10 in the structure of FIG. 1 will be described. It is assumed that the unit flux quantum is captured in the opening 1 of the inductor. In this state, the electric conductor 12 is brought into the normal conducting state. If the conductor 12 is in the normal conducting state, the unit magnetic flux quantum is trapped in one opening in which the opening 1 of the inductor provided in the ground plane 2 and the magnetic flux displacement opening 10 are connected by the channel 11. The ground plane is superconducting. When the electric conductor 12 is brought into a superconducting state in this state, the magnetic flux moves in units of magnetic flux quantum, so that the captured unit magnetic flux quantum moves (is trapped) to either the opening 1 or the magnetic flux displacement opening 10 of the inductor. Since the energy when the unit magnetic flux quantum is captured is smaller than that when it is captured in the magnetic flux displacement opening 10, the unit magnetic flux quantum moves to the magnetic flux displacement opening 10, and as a result, the magnetic flux captured in the opening 1 of the inductor. Is exiled.

Next, the energy and the structure of the opening when the unit magnetic flux quantum is trapped in the opening provided in the ground plane will be described. In general, the inductance L when a circular opening having a radius r is opened in the infinite plane is expressed by the equation (1).

L = 2rμ (1) where μ is the magnetic permeability. This equation is derived by M. Hosoya, EG
oto, T.Soma, “Extrapolated Boundary Element Method
for Three Dimensional Inductance Calculatin ", Techn
ical-Report 86-16, Department of Information Scienc
e, Faculty of Science, University of Tokyo. When the expression (1) is expressed with the area A of the opening as a variable, the expression (2) is obtained.

From equation (2), it can be seen that the inductor is proportional to the square root of the area of the opening. Expression (2) is a case of a circular opening, but it can be used as an approximate expression for a square opening, for example. The trapping energy E when the quantum magnetic flux Φo (2.07 × 10 ⁻¹⁵ Wb) is trapped in the inductance L is expressed by the equation (3).

E = Φo ₂ / 2L (3) From this equation, it can be seen that the larger the area A of the opening, the smaller the energy. Therefore, in order to remove the trapped magnetic flux described with reference to FIG. 1, the area of the magnetic flux expulsion opening 10 may be made larger than the area of the opening 1 of the inductor. One way to make the energy difference when capturing the unit magnetic flux quantum in the aperture is to change the area of the aperture, but another method is to fill the aperture with a substance with different magnetic permeability such as a ferromagnetic material. And so on. In any case, the unit flux quantum moves to the one with the smaller trapping energy in the two openings.

The inductor having the structure shown in FIG. 1 will be described more specifically. In FIG. 1, it is assumed that the temperature of the conductive film forming the ground plane 2 in the superconducting state is higher than the temperature of the conductor 12 in the superconducting state. Usually, various metals and metal oxides are used to create a superconducting circuit, but the temperature at which each substance enters the superconducting state is different. For example, lead is 7K, niobium is 9K, niobium nitride is 16K, and oxide superconductors are over 90K. These materials are selected in consideration of the transfer temperature. If the inductor shown in FIG. 1 is cooled in this state, first the ground plane 2 becomes superconducting. Here, even if the unit magnetic flux quantum is captured in the opening surrounded by the ground plane 2, that is, the single opening in which the opening 1 of the inductor and the magnetic flux erasing opening 10 are connected by the channel 11, it is further cooled and the electric conductor is cooled. When 12 is in the superconducting state, the trapped magnetic flux is expelled to the magnetic flux expulsion opening 10, and the magnetic flux is not trapped in the inductor opening 1. If the magnetic flux is not captured in the opening surrounded by the ground plane 2 in the first stage, that is, the opening 1 of the inductor and the magnetic flux expulsion opening 10 are connected by the channel 11, the next conductor 12 It is obvious that no change occurs even when the superconducting state is entered, and the magnetic flux is not captured in the opening 1 of the inductor. Also, if the magnetic flux is trapped in the inductor opening during circuit operation, raise the temperature so that only the conductor 12 is temporarily in the normal conducting state, and then cool it and repeat the cycle of returning the conductor 12 to the superconductor. ,
The trapped magnetic flux can be expelled to the opening 1 of the inductor.

FIG. 3 shows a configuration in which the openings 1a, 1b, 1c, 1d of a plurality of inductors are connected to one magnetic flux erasing opening 10 via channels 11a, 11b, 11c, 11d. Channels 11a, 11b, 11c, 11d
The conductors 12a, 12b, 12c, 12d are bridged to each other. In the configuration of FIG. 3, the unit magnetic flux quantum trapped in the inductor openings 1a, 1b, 1c, 1d can be expelled to the magnetic flux expulsion opening 10.
With this structure, openings 1a, ..., 1d of multiple inductors
Can share one magnetic flux expulsion opening 10.

FIG. 4 shows channels 11a, 11b, 11c and 11d in grooves 20 formed by digging the openings 1a, 1b, 1c and 1d of the inductor in the ground plane 2.
And the groove 20 is connected to the free space 22 via the end 21 of the ground plane 2. Channel 11
Conductors 12a, 12b, 12c, and 12d bridge a, 11b, 11c, and 11d. In the configuration shown in FIG. 4, the magnetic flux expulsion opening corresponds to the free space 22. With this configuration, the magnetic flux trapped in the openings 1a, 1b, 1c, 1d of the inductor can be expelled to the free space.

FIG. 5 is a modification of FIG. The example of FIG.
The structure is such that a groove 20 dug in the ground plane 2 in the figure is bridged with an electric conductor 12 'which becomes superconducting at a temperature lower than that of the electric conductor 12. In the structure of FIG. 5, the opening 1 of the inductor is
The magnetic flux trapped in a, 1b, 1c and 1d can be expelled sequentially to the groove 20 and the free space 22. In the structure of FIG. 5, the opening 1a, 1b, 1c, 1d of the inductor is isolated from the free space 22 by the closed groove 20 as compared with the structure of FIG. 4, so that the free space 22 has less influence on the inductor. .

FIG. 6 shows another embodiment of the present invention. In the embodiment of FIG. 6, the low antibody 15 is placed near the conductor 12 of the embodiment of FIG.
A current is supplied from the current source 18 to the resistor 15 via the wiring 16 and the switch 17. With this configuration, the electric conductor 12 can be controlled to either the normal conducting state or the superconducting state by causing a current to flow through the resistor 15 to generate heat. Therefore, the switch
By controlling 17, the trapped magnetic flux expulsion operation described in FIG. 1 can be performed. In the circuit of FIG. 6, the conductor
Since the trapped magnetic flux is expelled by partially heating 12, it is not necessary to heat the entire circuit and device to raise the temperature as in the embodiment of FIG. Therefore, there is an advantage that the trapped magnetic flux can be expelled while confirming the circuit operation.

FIG. 7 shows a structure in which the electric conductor 12 is heated in the embodiment shown in FIG. 6, but the channel 30 formed by a part of the ground plane is heated by the resistor 15. If the amount of heat generated by the resistor 15 is small, it is possible to limit the portion that changes to the normal conducting state by heating to the vicinity of the channel. Therefore,
The circuit of FIG. 7 can perform the same operation as the circuit of FIG.

In order to control the conductor 12 of FIG. 1 into the superconducting state and the normal conducting state, FIG.
Shows how to apply. Obviously, this method can also perform the same operation as the circuit shown in FIG.

FIG. 9 is an application example of the embodiment shown in FIG. Ninth
In the illustrated embodiment, the openings 1a and 1b of the inductor are the magnetic flux erasing openings.
Further, the magnetic flux displacing opening 100, which has a structure in which it is connected to the channel 100 through the channels 11a and 11b, is connected to the free space 22 through the channel 11c and the end 21 of the ground plane 2. Conductors 12a, 12b, 12c bridge the channels 11a, 11b, 11c, resistors 15a, 15b, 15c are placed near the conductors, and wirings 16a, 16b, 16c are respectively placed in these resistors. Currents are supplied from the power supplies 18a, 18b, 18c via the switches 17a, 17b, 17c. In the configuration of FIG. 9, resistors 15a, 15b, 15c
It is clear that the magnetic flux trapped in the openings 1a, 1b of the inductor can be expelled to the free space by heating the conductors and controlling the conductors 12a, 12b, 12c to normal conductors and superconductors as shown in FIG. Is.

(Effects of the Invention) As described above, according to the present invention, it is possible to provide means for expelling the magnetic flux trapped in the inductor having the opening in the ground plane. Therefore, it is possible to expel the magnetic flux trapped in the inductor of a cryogenic device that operates at high speed and low power consumption, such as a magnetic flux quantum device and a quantum interference device, and use these devices to build a highly integrated system. become able to. Therefore, the present invention is useful for realizing an ultra-high speed computer using a cryogenic element.

[Brief description of drawings]

FIG. 1 is a plan view of a first embodiment of the present invention, FIGS. 2A and 2B are plan views showing the structure of a conventional inductor, FIG. 3, FIG. 4, FIG. 5, FIG. FIG. 7, FIG. 8 and FIG. 9 show the second and third embodiments of the present invention, respectively.
It is a top view of a 4th example, a 5th example, a 6th example, a 7th example, and an 8th example. 1,1a, 1b, 1c, 1d …… Aperture, 2,2a, 2b, 2c, 2d …… Ground plane, 3 …… Superconducting wiring, 10,100 …… Magnetic flux expulsion opening,
11,11a, 11b, 11c, 11d, 30 …… Channel, 12,12a, 12b, 12c,
12d, 12 ′ …… Electric conductor, 15,15a, 15b, 15c …… Resistor, 16,1
6a, 16b, 16c …… wiring, 17,17a, 17b, 17c …… switch, 1
8,18a, 18b, 18c ... current source, 20 ... groove, 22 ... free space, 40 ... light.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-58-40875 (JP, A) JP-A-59-161884 (JP, A) JP-A-57-114294 (JP, A)

Claims (13)

[Claims] 1. A superconducting inductor in which an opening is provided in a ground plane below wiring for forming self-inductance or mutual inductance, i) a magnetic flux erasing space is connected to the opening via a channel, ii) The energy required to trap the magnetic flux quantum in the magnetic flux displacement space is smaller than the energy required to trap the magnetic flux quantum in the opening of the inductor, and iii) by switching from the normal conducting state to the superconducting state, A superconducting inductor, wherein an electric conductor for expelling the trapped magnetic flux to the magnetic flux expelling space is bridged to the channel. 2. The superconducting inductor according to claim 1, wherein the magnetic flux erasing space is an opening provided in the ground plane. 3. The superconducting inductor according to claim 1, wherein the magnetic flux erasing space is a free space outside the ground plane. 4. The superconducting inductor according to claim 1, wherein the temperature at which the conductor is in a superconducting state is lower than the temperature at which the ground plane is in a superconducting state. 5. The superconducting inductor according to claim 2, wherein an opening area of the magnetic flux expulsion space is larger than an opening area of the inductor. 6. The superconducting inductor according to claim 2, wherein the openings of the plurality of inductors are connected to one magnetic flux expulsion opening through a channel. 7. An opening of a plurality of the inductors is once connected to a groove provided in the ground plane via the channel, and is connected to the free space via the groove. The superconducting inductor according to claim (3). 8. The bridge is bridged to the groove by a second conductor which becomes superconducting at a temperature lower than a temperature at which the conductor becomes superconducting.
A superconducting inductor according to the item. 9. The superconducting inductor according to claim 1, further comprising means for locally heating the electric conductor. 10. The superconducting inductor according to claim 9, wherein the heating means comprises a resistor provided near the conductor and a means for supplying a current to the resistor. . 11. The heating means is a means for shining light on the electric conductor, as set forth in claim 9.
A superconducting inductor according to the item. 12. An electrical conductor bridged to the channel comprises:
The superconducting inductor according to claim (9), which is constituted by the ground plane itself. 13. The heating means is means for expelling the magnetic flux trapped in the opening of the inductor while sequentially changing the electric conductor from a normal conductor state to a superconducting state or vice versa. What is claimed is (9).
A superconducting inductor according to the item.