JP2760481B2 - Superconducting output buffer circuit element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting circuit element, and more particularly to a superconducting output buffer circuit element having a structure suitable for a superconducting logic circuit which determines a logical value "1" or "0" depending on whether the current flows in a positive or negative direction. .

[0002]

2. Description of the Related Art Conventionally, as a superconducting logic circuit in which the sign of the direction of current flow corresponds to a logical value of "1" or "0", IEE Transactions on Update Superconductivity has been proposed. Volume 2 199
Year 2 pp. 21 to 25 (IEEE Transactions on Ap
plied Superconductivity, vol.2, no.1 (1992), pp.21-
25) Then, as shown in Fig. 1, QFP (Qua
It is known as ntum Flux Parametron (Quantum Flux Parametron), and QFP is also known as one of the fluxoid type logic circuits. This conventional example is an example of a comparator composed of a QFP, but in a logic circuit for making the direction of current correspond to a logical value, as shown in FIG. 1 of the conventional example, a dc (direct current) SQUID (Supercondu
A cting Quantum Interference Device (superconducting quantum interference device) is used as an output buffer circuit element.
The operation result of the comparator is expressed by the direction of the current.
The signal of the direction of the current cannot be taken out. However, since a voltage signal can be extracted to the outside, a dc SQUID is used as a signal converter. The dc SQUID is composed of a superconducting closed loop and a control line, and the superconducting closed loop is composed of an inductor and two Josephson junctions. The control line functions as an input section, and an input signal current flows. The superconducting closed loop functions as an output unit and is magnetically coupled to a control line, and a voltage generated in the superconducting closed loop is taken out as an output signal. The superconducting closed loop and the control line constituting the dc SQUID are composed of two different superconducting layers, as shown in Fig. 2 of the above-mentioned conventional example, and these two superconducting layers are formed by insulating films. It has a laminated structure formed through the interposition. In the above conventional example, an Nb or Pb alloy is used as the superconductor. In a metal-based superconducting circuit made of an Nb or Pb alloy, a lamination process technology has been almost established, and it is possible to produce a laminated dc SQUID without significantly deteriorating the characteristics of a Josephson junction. An example in which dc SQUID was fabricated using an oxide superconductor is described in Applied Fixix.
Letters Vol. 59, 1991, pp. 3051 to 3
053 [Applied Physics Letters, vol.59, no.23,
(1991), pp. 3051-3053]. dc
Although the SQUID is a stacked type, the process of stacking oxide superconductors has various problems as described in the same document, page 3051, left half, line 14 to line 33,
It has not yet reached a technically established stage.

[0003]

In a superconducting logic circuit in which the logic values "1" and "0" are determined by the sign of the direction in which the current flows, dc is required to output the operation result to the outside.
The SQUID is used as an output buffer circuit element.
In the above prior art, the structure of the dc SQUID is:
Each of them is a laminated type in which the superconducting closed loop and the control line are constituted by two superconducting layers, and no consideration is given to a planar structure in which the superconducting closed loop and the control line are constituted by one superconducting layer. This is because the magnetic coupling between the control line and the superconducting closed loop is generally considered to be smaller in the planar type than in the stacked type. The magnitude of the magnetic coupling is roughly determined by the distance between the two superconducting wires forming the inductor, that is, in the above-described conventional example, the distance between the control line and the inductor portion of the superconducting closed loop. Since the thickness is equivalent to the thickness of the insulating film interposed therebetween, a short interval of 1 μm or less can be easily realized by reducing the thickness. On the other hand, in the case of the planar type, the above-mentioned interval depends on the etching accuracy of the single superconducting film. So far, it has been thought that it is difficult to achieve a sufficiently large magnetic coupling with a planar dc SQUID because it is difficult to realize a narrow interval of 1 μm or less in a normal manufacturing process. Accordingly, no study has been made on the optimum element structure of the planar type and whether or not it can be applied as a circuit element. In particular, detailed considerations regarding the line width and spacing of two superconducting wires have been made at all. Did not. In addition, since the lamination technology has been established especially for metal-based superconducting circuits, the lamination type is sufficient for the dc SQUID, and there was no need to consider a planar type structure. This is one of the reasons not considered. An object of the present invention is to provide a planar dcSQ which has not been considered in the prior art.
It is an object of the present invention to provide a high-performance superconducting output buffer circuit element having a simple planar structure and a magnetic coupling of a size applicable as a circuit element.

[0004]

Means for Solving the Problems In order to achieve the object of the present invention, the present invention is configured as described in the claims. That is, the present invention comprises an input unit including a first superconducting wire and an output unit including a second superconducting wire, wherein the first superconducting wire and the second superconducting wire are provided. lines are disposed on the same plane, out the
The force section has an inductor and two Josephson junctions.
A conductive closed loop, wherein the second superconducting wire is
A closed loop inductor, wherein the first superconducting wire is
A control line magnetically coupled to the second superconducting wire.
The line width of the first superconducting wire and the second superconducting wire is 0.5 μm or more
3.5 μm or less, and a first superconducting wire and a second superconducting wire
The distance between the first superconducting wire and the first superconducting wire is set to 0.5 μm or less.
The value of the mutual inductance of the second superconducting wire is 0.15.
(PH / square) or higher .

[0005]

The superconducting output buffer circuit element of the present invention comprises an input section and an output section, and the input section and the output section are respectively provided with a first
, And at least a second superconducting wire. A superconducting wire has its own inductance,
The degree of magnetic coupling between the first superconducting wire and the second superconducting wire, that is, the mutual inductance of both, determines the operating performance as a superconducting output buffer circuit element. Therefore, the above-mentioned mutual inductance is obtained for each of the stacked type and the planar type, and the usefulness thereof can be determined by comparing them. First, a case of a conventional laminated type will be described.
At a height h on the ground plane, there is a second superconducting wire having a line width w ₂ and a thickness t ₂ , and a line width w above the second superconducting line at an interval s.
_1, determine the mutual inductance in the presence of the first superconducting wire having a thickness of t _1. The line width of the superconducting wire is w ₂ = 2w ₁ in consideration of the lithography limit of the resist pattern, the superconductor uses an oxide superconductor, the magnetic field penetration length λ = 0.14 μm, and the film thickness of the superconducting wire is T ₁ = t ₂ = 0.2 μm, which is longer than the magnetic field penetration length and is as thin as possible in consideration of the formation of a fine pattern. At this time, the first
Self-inductance L ₁₁ of the superconducting wire, a second superconducting wire self-inductance L _22, of both the mutual inductance M ₁₂ of the dependence of the line width w ₂ of the second superconducting line in FIG. 3 (a), FIG. 3B shows the dependence of the distance s between the first and second superconducting wires. However, in FIG. 3A, the distance s = 1 μm between the first and second superconducting wires is set, and FIG.
Then, the line width w ₂ of the second superconducting wire was set to 10 μm, and the height h was set to 1 μm in each case. In addition, square is a unit having a line width w ₂ as one side.
In a fluxoid type logic circuit such as the above-described conventional QFP, it is generally desirable that the self inductance per square be small in terms of layout design. Typical values for this self-inductance, L ₂₂ = 0.5 to 1.0 [pH
(Pico Henry) / square]. At this time, as shown in FIG. 3A, the value of the mutual inductance is M ₁₂ = 0.2 to 0.2.
6 [pH / square]. Next, the flat type of the present invention will be described. The height h on the ground plane and the line width w
₁ , a first superconducting wire having a thickness of t ₁ exists on the same plane, that is, at the same height h, at a distance s between the first and second superconducting wires.
Then, a mutual inductance is obtained by using a second superconducting wire having a line width w ₂ and a thickness t ₂ . Line width w and thickness t of the superconducting wire is a first superconducting wire, and are both equal to the second superconducting wire w ₁ = w ₂ = w, t ₁ = t ₂ = t. Magnetic field penetration length λ,
Assuming that the film thickness t of the superconducting wire is equal to that of the above-mentioned laminated type, λ is 0.14 μm and t is 0.2 μm, respectively.
Also, the self-inductance of the first superconducting wire L _11, a second superconducting wire self inductance L _22, the mutual inductance of both the M _12. A self-inductance L ₁₁ of the first superconducting wire from the symmetry of the structure, the self-inductance L ₂₂ of the second superconducting line is equal (L ₁₁ = L _22).
When this relates self-inductance L ₁₁ and mutual inductance M _12, the dependence of the line width w of the first and second superconducting line in FIG. 2 (a), FIG dependence on the spacing s of the superconducting wire 2 It is shown in (b). However, in FIG.
In FIG. 2B, the line width w was 2 μm, and the height h was 1 μm. Further, the square is a unit of a square with one side being w. Accordingly, the line width w is 0.5 μm or more and 3.5 μm or less, and the interval s is 0.5 μm.
Mutual inductance M ₁₂ in the following cases 0.15 [p
H / square] or more, that is, about 0.2 [pH / squ
are]. Comparing the above-mentioned conventional laminated type with the planar type of the present invention, if the line width of the two superconducting wires is 0.5 μm to 3.5 μm and the interval is 0.5 μm or less, mutual The inductance value is equal to the minimum value of 0.2 [pH / square] in the case of the stacked type. Since the circuit element operation is obtained in the laminated structure, the magnitude of this magnetic coupling is in a range applicable as a circuit element, and thereby, the planar type superconducting output buffer circuit element of the present invention is used. And the object of the present invention is achieved. The planar superconducting output buffer circuit element of the present invention is shown in FIG.
It can be seen that there is a more optimal configuration range. That is, in FIG. 2 (a), the order self-inductance L ₁₁ is a monotonically increasing with respect to the line width w, the line width w in order to reduce the self-inductance will be as small as possible is desirable. However, on the other hand, the mutual inductance M
₁₂ has a maximum value with respect to the line width w and becomes a maximum when w = 1.5 μm. Therefore, in order to strengthen magnetic coupling and obtain a large mutual inductance, the vicinity of the line width w = 1.5 μm, for example, , 0.5 μm or more and 3.5 μm or less. Furthermore, the mutual inductance M ₁₂ is 0.
The optimum condition for obtaining a strong magnetic coupling of 2 [pH / square] or more is that the line width w is 0.8 μm or more and 2.8 μm or less. In particular, when a dc SQUID is used as a superconducting output buffer circuit element, it can be said that the value of the inductance is appropriate. That is, dc S
The QUID is composed of a superconducting closed loop and a control line, and the superconducting closed loop is composed of at least an inductor and two Josephson junctions.
The inductance L and the maximum superconducting current Im of the Josephson junction must satisfy the following two conditions. The first is a condition for obtaining a sufficiently large input sensitivity, LIm = 0.1 to 0.4 [pH · mA]. The second is a condition for obtaining an output signal having a magnitude exceeding the noise level, and Im ≧ 0.05 [mA]. Therefore, the inductance L must be suppressed to about 8 [pH] or less. By the way, if the line width of the two superconducting wires in the planar type is 0.5 μm or more and 3.5 μm or less and the interval is 0.5 μm or less, as shown in FIG.
From (b), the self-inductance can be suppressed to 1 [pH / square] or less. Further, since the superconducting closed loop can be formed with a minimum length of 8 [square], the inductance L becomes 8 [pH] or less, thereby realizing a superconducting output buffer circuit element having a planar structure applicable as a circuit element. be able to. The present invention comprises an input section including at least a first superconducting wire and an output section including at least a second superconducting wire, wherein the first superconducting wire and the second superconducting wire are provided. The wires are arranged on the same plane, and the degree of magnetic coupling between the first superconducting wire and the second superconducting wire is adjusted by adjusting the line width and interval of the wires, that is, the value of the mutual inductance between the two wires. The superconducting output buffer circuit element is configured by setting the range as applicable. With this configuration, for example, as shown in FIG. 2 (a), the mutual inductance M ₁₂ indicates a maximum value with respect to the line width w, w = 1.
Since it becomes a maximum at 5 μm, in order to strengthen magnetic coupling and obtain a large mutual inductance, for example, 0.5 μm
m and 3.5 μm or less, and by adjusting the line width w of the superconducting wire to 0.8 μm to 2.8 μm, the mutual inductance M ₁₂ becomes 0.2 [pH / square].
Since the strong magnetic coupling described above is obtained, there is an effect that a high-performance planar superconducting output buffer circuit element can be realized. Further, the present invention is as claimed in claim 1, the line width of the first superconducting line and the second superconducting wire 0.5μm or more, 3.5
μm or less and no, and can be applied to distance between the first superconducting line and the second superconducting wire by forming the first layer superconducting layer of a very simple pattern that is set to 0.5μm or less, as circuitry element A superconducting output buffer circuit element having strong magnetic coupling can be easily manufactured. Further, the present invention is as claimed in claim 1, the output portion including at least a second superconducting wire consists superconducting closed loop having an inductor and two Josephson junctions, the superconducting closed loop the second superconducting wire And an input unit including at least a first superconducting wire, a control line for magnetically coupling the first superconducting wire constituting the input unit to a second superconducting wire which is an inductor of the superconducting closed loop. For example, as shown in FIG. 1, the line width of each superconducting wire is 2 μm, the interval is 0.2 μm, and since they are close to each other, they have strong magnetic coupling with each other. Mutual inductance M ₁₂ = 0.5
[PH], and inductors 12, 13, 14, 1
5 have 1.6 [pH],
The values are 0.8 [pH], 0.8 [pH], and 3.2 [pH], and the total self-inductance of the superconducting closed loop 18 is the sum of these, so that Ltotal = 6.4 [pH]. The value of the mutual inductance and the total self-inductance is sufficiently applicable range as a dc SQUID, it is possible to obtain a very strong performance of the superconducting output buffer circuit element of the magnetic coupling easily with a simple structure Te because very .

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in more detail with reference to the drawings. FIGS. 1A and 1B are a schematic diagram showing an example of a configuration of a superconducting output buffer circuit element [dc SQUID] having a planar structure manufactured in this embodiment and an equivalent circuit diagram thereof. 3) to 3 (g) are process diagrams showing a process for manufacturing the superconducting output buffer circuit element. Note that a step-edge type junction was used as the Josephson junction. First, using FIGS. 4A to 4G,
A method for manufacturing a superconducting output buffer circuit element having a planar structure according to this embodiment will be described. On the first SrTiO ₃ substrate 41, a two-layer film of an Si (silicon) film 42 and an organic resist film 43 was formed to form steps (FIG. 4A). Next, by photolithography, the half-plane of the substrate was exposed and developed, and patterned so that the boundary between the region where the organic resist film 43a was present and the region where the organic resist film 43a was not formed was formed straight at the center of the substrate [ FIG.
(b)]. Next, using the organic resist film 43a as a mask,
Etching the Si film 42 using SF ₆ gas,
2a was formed [FIG. 4 (c)]. Next, the organic resist film 4
After removing 3a, this time, the first SrTiO ₃ substrate 41 was etched with Ar + O ₂ ions using the Si film 42a as a mask to form a step 44 having a height of 300 nm [FIG. 4 (d)]. Next, after removing the Si film 42a,
A YBa ₂ Cu ₃ O ₇ oxide superconducting thin film 45 was formed to a thickness of 200 nm by laser vapor deposition [FIG.
(e)]. Next, an electron beam resist is applied, and a resist pattern is formed using electron beam lithography.
Patterns of a control line (first superconducting wire) 11 and a superconducting closed loop (second superconducting wire) 18 which are superconducting wires made of the YBa ₂ Cu ₃ O ₇ oxide superconducting thin film 45 shown in FIG. 1 are formed by ion etching. did. Next, in order to form a ground plane, YBa ₂ Cu ₃ is formed on the second SrTiO ₃ substrate 46 by using the same laser deposition method as described above.
An O ₇ oxide superconducting thin film 47 was formed to a thickness of 300 nm (FIG. 4F). Next, a CaF ₂ film 48 was formed to a thickness of 1 μm on the YBa ₂ Cu ₃ O ₇ oxide superconducting thin film 47 [FIG. 4 (g)]. Further, the first SrTiO ₃ substrate 41 and the second SrTiO ₃ substrate 46 were bonded together so that the surfaces on which the films were formed were in contact with each other, and a dc SQUID having a ground plane using the CaF ₂ film 48 as an interlayer insulating film was produced. . In FIG. 1, a control line 11 corresponds to a first superconducting line and forms an input of a superconducting output buffer circuit element.
The superconducting closed loop 18 has a square hole with a side of 2 μm at the center, is composed of eight squares of 2 μm on a side, and has two Josephson junctions 16 and 17 and inductors 12, 13, 14 and 15. Inductor 12
Corresponds to the second superconducting wire, and the superconducting closed loop 18
The output of the superconducting output buffer circuit element is formed. The control line 11 and the inductor 12 exist on the same plane, as is clear from the above manufacturing process. Each of these line widths was 2 μm and the interval was 0.2 μm. Since they were close to each other, they were magnetically coupled to each other, and exhibited a mutual inductance M ₁₂ = 0.5 [pH]. Inductor 12, 1
The self-inductances of 3, 14, and 15 are 1.6 [pH], 0.8 [pH], 0.8 [pH], and 3.2, respectively.
[PH], and the total self-inductance of the superconducting closed loop 18 is the sum of these, and Ltotal = 6.4 [pH]. The magnitudes of the mutual inductance and the total self-inductance were operable values of the dc SQUID, and were applicable values as circuit elements. The manufacturing process of the superconducting output buffer circuit element does not include a lamination process. Therefore, the YBa ₂ Cu ₃ O ₇ oxide superconducting thin film 4 formed on the first SrTiO ₃ substrate 41
No. 5 superconducting critical current density 5 × 10 at superconducting critical temperatures 90K and 77K without any deterioration in superconducting characteristics
⁶ [A / cm ² ], showing good characteristics. This value was similar to that of the YBa ₂ Cu ₃ O ₇ oxide superconducting thin film 47 formed on the second SrTiO ₃ substrate 46. Regarding the characteristics of the Josephson junction, the current-voltage characteristic at 77K is in accordance with the RSJ (Resistively Shunted Junction) model, and the product of the maximum superconducting current Ic and the junction resistance Rn is equal to or less than 0.
It exhibited good characteristics of 1 mV. That is, the dc SQUID having good superconducting characteristics could be easily manufactured by using a single superconducting layer without including the lamination process.

[0007]

As described above, in the superconducting output buffer circuit element of the present invention, the input section and the output section each include at least two superconducting wires existing on the same plane. Line width is 0.5 μm or more and 3.5
Since the distance is not more than μm and the interval is not more than 0.5 μm, a sufficiently strong magnetic coupling can be obtained. Than this,
It is possible to easily realize a superconducting output buffer circuit element (dc SQUID) having a good superconducting characteristic, which is constituted by a simple single superconducting layer and has a magnetic coupling of a size applicable as a circuit element.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration and an equivalent circuit of a superconducting output buffer circuit element exemplified in an embodiment of the present invention.

FIG. 2 shows a relationship between a self-inductance and a mutual inductance when two superconducting wires of a superconducting output buffer circuit element exemplified in an embodiment of the present invention are arranged on the same plane, and a line width and a spacing of the superconducting wires. FIG.

FIG. 3 is a diagram showing the relationship between the self-inductance and mutual inductance of a conventional superconducting output buffer circuit element in which two superconducting wires are stacked and arranged, and the line width and spacing of the superconducting wires.

FIG. 4 is a process chart showing a manufacturing process of the superconducting output buffer circuit element exemplified in the embodiment of the present invention.

[Explanation of symbols]

11 control line (first superconducting wire) 12, 13, 14, 15 ... inductor 16, 17 ... Josephson junction 18 ... superconducting closed loop (second superconducting wire) 19 ... step edge 20 ... first superconducting wire 21 ... second superconducting wire 22 ... ground plane 41 ... first SrTiO ₃ substrate 42, 42a ... silicon (Si) film 43 and 43a ... organic resist film 44 ... step 45, 47 ... YBa ₂ Cu ₃ O ₇ oxide superconductive thin film 46 ... self-inductance M ₁₂ ... mutual inductance w ₁ ... first second SrTiO ₃ substrate 48 ... CaF ₂ film L ₁₁ ... first superconducting wire of self-inductance L ₂₂ ... second superconducting wire line width w ₂ ... thickness h ... ground of the second thickness t '... ground plane of the line width of the superconducting wire t ₁ ... the first of the thickness of the superconducting wire t ₂ ... the second of superconducting wire of superconducting wire Distance lambda ... penetration depth height above the lane s ... first and second superconducting wire

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Uki Kabazawa 1-280 Higashi Koikekubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. In the Central Research Laboratory (72) Inventor Kazumasa Takagi 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory Hitachi, Ltd. (56) References JP-A-1-302784 (JP, A) JP-A-3-198389 (JP, A) IEEE TRANSACTION ON APPLIED SUPERCONDUCTIVITY, VOL. 2, NO. 1, MARCH 1992, PP. 21−25 (58) Field surveyed (Int.Cl. ⁶ , DB name) H01J 39/22-39/24 ZAA G01R 33/035 ZAA H01L 39/00 ZAA

Claims (1)

(57) [Claims] 1. An input section including a first superconducting wire and an output section including a second superconducting wire, wherein the first superconducting wire and the second superconducting wire are disposed on the same plane. Out above
The force section has an inductor and two Josephson junctions.
A conductive closed loop, wherein the second superconducting wire is
A closed loop inductor, wherein the first superconducting wire is
A control line magnetically coupled to the second superconducting wire.
The line width of the first superconducting wire and the second superconducting wire is 0.5 μm or more
3.5 μm or less, and a first superconducting wire and a second superconducting wire
The distance between the first superconducting wire and the first superconducting wire is set to 0.5 μm or less.
The value of the mutual inductance of the second superconducting wire is 0.15.
(PH / square) or more .