JP5187559B2 - Superconducting quantum interference device - Google Patents

Description

The present invention relates to a superconducting quantum interference device (SQUID) using a Josephson junction.

Until now, there is a SIS type Josephson junction in which a superconductor layer (S) is sandwiched between both sides of an oxide film (I) as a superconducting tunnel junction. When this SIS type Josephson junction is manufactured, at least three film forming steps are required in order to sequentially stack a total of three layers of a superconductor layer, an insulating barrier layer, and a superconductor layer (for example, Non-Patent Document 1). The characteristics of the superconducting tunnel junction are characterized by the so-called I _{c RN} product, and the larger this value, the better the junction.

A single flux quantum device (hereinafter referred to as an SFQ element) is a logic circuit using a Josephson junction (see Non-Patent Document 2). In this SFQ element, a logic circuit is configured by using a pulsed voltage generated when one magnetic flux quantum crosses a Josephson junction. The switching speed in this case is inversely proportional to the I _{c RN} product. Therefore, the larger the I _{c RN} product, the faster the operation. I _c R _N product is approximately increases in proportion to the critical temperature of the superconductor (Tc) used for the Josephson junction.

Gomez Espinoza Luis Beltran, PhD thesis, University of Cincinnati, USA, May 30, 2003 Shinichi Tsuji, “Single Flux Quantum Device”, Solid State Physics, Vol. 40, No. 10, p. 807, 2005

According to the Josephson junction using the high-temperature superconductor layer, its Tc is larger than that of a conventional superconductor such as Nb, and therefore it is predicted that the I _{c RN} product will be large. However, as in the high-temperature superconductor of the current, I _c R _N product is obtained only Josephson junctions with a value of about 10% with respect to the theoretically expected value can not be higher I _c R _N product There are challenges.

  In particular, in a high-temperature superconductor, since it is very difficult to improve the controllability and reproducibility of the production of the insulating barrier layer, a high-quality superconducting SIS tunnel junction cannot be produced. Therefore, it has been difficult to produce a quantum magnetic flux interference element (SQUID) having stable characteristics by a simple method.

In view of the above problems, the present invention provides an insulating barrier layer formed in order to overcome the technical difficulty that a good insulating barrier layer cannot be obtained in a SIS-type Josephson junction using a superconductor, particularly a high-temperature superconductor. An object of the present invention is to provide a new superconducting quantum interference device that is unnecessary.

  In the present invention, in the structure in which the ferromagnetic layer is coated on the superconductor layer, the magnetic material forming the ferromagnetic layer suppresses the superconductivity of the lower superconductor layer, particularly for the high-temperature superconductor layer. It was made paying attention to the point which causes the transition from the superconductor (S) to the insulator (I).

In order to achieve the above object, a superconducting quantum interference device of the present invention includes a superconductor layer having a ring portion formed by connecting a pair of antenna portions by a pair of constricted portions, and one of the pair of constricted portions. And a ferromagnetic layer disposed so as to straddle.
The superconducting quantum interference device of the present invention is disposed so as to straddle the superconductor layer having a ring portion formed by connecting a pair of antenna portions by a pair of constricted portions and the pair of constricted portions. And a ferromagnetic layer.
Preferably, the superconductor layer and the ferromagnetic layer are disposed on a mesa formed by overetching the substrate.
Preferably, each of the pair of antenna portions has a rectangular shape with a vertical width of 0.1 to 1000 μm × a horizontal width of 0.1 to 1000 μm, the narrowed portion has a width on the order of μm, and the ferromagnetic layer has a thickness of 2 nm to The width is 1 μm.

  According to the superconducting quantum interference device of the present invention, since the ferromagnetic layer is arranged in a part of the loop portion of the superconductor layer, the superconductor layer covered with the ferromagnetic layer is super A transition from a conductor to an insulator occurs, thereby forming a SIS-type Josephson junction. Therefore, unlike a conventional structure in which a barrier layer is sandwiched between superconductor layers, a superconducting quantum interference device can be realized with a simple structure.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each figure, the same code | symbol is used for the same or corresponding member.
A first embodiment of a superconducting quantum interference device will be described.
1A and 1B show a superconducting quantum interference device according to the first embodiment, in which FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line X1-X1. The superconducting quantum interference device 10 includes a superconductor layer 12 having a ring portion 12A, and ferromagnetic layers 13 and 13 disposed across a part of the ring portion 12A of the superconductor layer 12. . In the first embodiment, the plurality of ferromagnetic layers 13 and 13 covers the ring portion 12A. Of the ring portion 12A of the superconductor layer 12, the regions 12B and 12B covered with the ferromagnetic layers 13 and 13 are transferred from the superconductor (S) to the insulator (I) by the ferromagnetic layer 13, and the barrier and Become. This structure forms a so-called DC-SQUID because so-called SIS-type Josephson junctions are formed at two locations.

  In the above structure, current terminals 12G and 12H are provided at both ends of the superconductor layer 12. When a current is passed between the current terminals 12G and 12H, a non-zero voltage is generated in the superconducting quantum interference device 10 when a certain threshold value is exceeded. This threshold value is called a critical current. By utilizing the fact that the critical current is a function of the magnetic flux, the magnetic flux can be detected with high sensitivity by measuring the critical current.

This structure is different from a conventional SIS type Josephson junction in which a tunnel layer is sandwiched between superconductor layers, and is composed of at least three layers of a superconductor layer, a tunnel layer such as an oxide film, and a superconductor layer. Since a film process is not necessary, it is not necessary to form a tunnel layer, and the number of processes can be reduced.
In particular, in the conventional structure, since the tunnel layer is laminated on the high-temperature superconductor made of copper oxide, the cleaning surface thus formed is always contaminated, and the characteristics of the Josephson junction are deteriorated. However, in the superconducting quantum interference device 10, the oxide film is not partially removed to form the tunnel layer, so that control and reproducibility of the electrical characteristics of the Josephson junction are improved, and superconducting quantum interference is achieved. Characteristics such as sensitivity of the element 10 are also greatly improved.

Further, the configuration of the superconducting quantum interference device 10 will be specifically described.
As shown in FIG. 1, the superconducting quantum interference device 10 includes a superconductor layer 12 and a ferromagnetic layer 13 disposed on a substrate 11. In FIG. 1A, the substrate 11 is omitted.
Examples of the substrate 11 include a magnesium oxide (MgO) substrate, a sapphire (Al ₂ O ₃ ) substrate, a compound composed of La, Al, and O (LaAlO ₃ ) substrate, and a compound composed of Sr, Al, and O (SrAlO ₃ ) substrate. Can be mentioned.

The superconductor layer 12 has a quadrangular, rectangular, annular or other ring portion 12A. In the superconductor layer 12, a pair of antenna portions 12C and 12D are connected by a pair of constricted portions 12E and 12F to form a loop. Each of the pair of antenna portions 12C and 12D has a rectangular shape with a width W ₁₁ , a width W ₁₂ , and a thickness D ₁₁ as shown in FIG. 1, for example. The pair of antenna portions are not shown, but may be trapezoidal, particularly isosceles trapezoidal. In this case, a pair of constricted portions 12E and 12F are connected to either one of the upper bottom portion and the lower bottom portion that are parallel to each other. The pair of constricted portions 12E and 12F are thin line portions having a width W ₂₁ , a length L ₂₁ , and a thickness D ₁₁ as shown in FIG.

Here, the horizontal width W ₁₁ and the vertical width W ₁₂ are both preferably 0.1 μm or more and 1000 μm or less, and particularly preferably in the following ranges.
0.1 μm ≦ W ₁₁ ≦ 10 μm
1 μm ≦ W ₁₂ ≦ 10 μm

W ₂₁ may be about 5 μm or less, but may be about the same as the coherence length of the high-temperature superconductor layer 12, for example, about 30 nm to 50 nm. The width W ₂₁ needs to be set so that the current distribution is uniform. This is because when the current distribution is not uniform, Josephson magnetic flux is generated, which hinders operation. Therefore, the width W ₂₁ in the high-temperature superconductor layer 12 must be in a condition that it is about Josephson penetration depth or less. Since the Josephson penetration depth is inversely proportional to the critical current, the Josephson junction should satisfy this condition by changing the temperature.

The only restriction of the thickness D ₁₁ of the high temperature superconductor layer 12 is that it has become more than necessary minimum level for superconductivity is exhibited. The specific value depends on what is used as the material of the superconductor. For example, in the case of a copper oxide superconductor of an example to be described later, there are examples in the past literature in which relatively good superconducting characteristics are expressed at several nm.

A pair of antenna portions 12C, 12D has a pair of narrowing portions 12E, it is arranged apart by a length _{L 21} of 12F. The length L ₂₁ is preferably in the following range.
0.1 μm ≦ L ₂₁ ≦ 10 μm
This is because when the above range is exceeded, it is extremely difficult to extract the motion of one magnetic flux quantum.

The superconductor layer 12 is made of a superconductor such as niobium, niobium nitride, aluminum, a compound containing lead, or a copper oxide superconductor. As a copper oxide superconductor, a compound composed of La, Sr, Cu and O (LSCO), a compound composed of Y, Ba, Cu and O (YBCO), a compound composed of Bi, Sr, Ca, Cu and O (BSCCO) ) And the like. In particular, the superconductor layer 12 having a large Tc increases the I _{c RN} product of the Josephson junction.

In the first embodiment, the ferromagnetic layer 13 is coated on the thin line portions as the pair of constricted portions 12E and 12F, respectively. As shown in the figure, the ferromagnetic layer 13 is covered with the narrowed portions 12E and 12F, and even though both ends thereof are extended on the substrate 11, the illustration is omitted, but the ferromagnetic layer 13 is formed on the narrowed portions 12E and 12F. It may be coated only. In the ferromagnetic layer 13, a region covered by the ferromagnetic layer 13 in the narrowed portions 12 </ b> E and 12 </ b> F is transferred from the superconductor (S) to the insulator (I) by the ferromagnetic layer 13 to form a barrier. Therefore, the width W ₃₁ of the ferromagnetic layer 13 is preferably in the following range.

The width W ₃₁ of the ferromagnetic layer 13 in the direction of the narrowed portions 12E and 12F may be a length that covers the narrowed portion of the high-temperature superconductor layer 12. For example, if the width W ₃₁ of the ferromagnetic layer 13 is theoretically not about the coherence length of the superconductor, the Josephson effect will not occur, but the Josephson effect will occur at a longer distance due to unexplained mechanisms. There is also a possibility to do. In this case, there are no dimensional restrictions. Specifically, the width W ₃₁ of the ferromagnetic layer 13 may be in the range of several nm to 1 μm, that is, 2 nm to 1 μm. The range of 2 nm to 0.1 μm is particularly preferable.
The thickness t in the laminating direction of the ferromagnetic layer 13 is only required to suppress the superconducting property of the high-temperature superconductor layer 12 below, so that the Josephson property appears. There are no restrictions. For example, it may be 10 nm or more (t ≧ 10 nm). If the thickness t is less than about 10 nm, it is not preferable because the barrier function to the constricted portion 12C in the high-temperature superconductor layer described later cannot be achieved.
The length L ₃₁ of the ferromagnetic layer 13 is about the same as the width W ₂₁ of the narrowed portions 12E and 12F and has a length sufficient to cover the narrowed portions 12E and 12F. , the constriction 12E, may be longer than the width _{W 21} of 12F.

The ferromagnetic layer 13 may be composed of various ferrites or other insulating ferromagnetic materials such as Fe ₃ O ₄ , but may be a simple substance or an alloy such as iron (Fe), nickel (Ni), manganese (Mn), or chromium (Cr). You may comprise with another electroconductive ferromagnetic material.

A second embodiment of the superconducting quantum interference device will be described.
2A and 2B show a superconducting quantum interference device 20 according to the second embodiment, in which FIG. 2A is a plan view and FIG. 2B is a cross-sectional view taken along line X2-X2. In addition, the same code | symbol is attached | subjected to the member which is the same as that of FIG. 1, or respond | corresponds.
Unlike the first embodiment, the superconducting quantum interference device 20 shown in FIG. 2 has a superconductor layer 12 having a ring portion 12A and a ferromagnetic layer covering a portion of the ring portion 12A of the superconductor layer 12. 13. The region 12B covered with the ferromagnetic layer 13 in the ring portion 12A of the superconductor layer 12 is transferred from the superconductor (S) to the insulator (I) by the ferromagnetic layer 13 and becomes a barrier. With this structure, a so-called SIS type Josephson junction is formed at one place, and thus an RF-SQUID is formed. Also in this RF-SQUID, the magnetic flux obtained by measuring the critical current can be detected with high sensitivity.

A third embodiment of the superconducting quantum interference device will be described.
3A and 3B show a superconducting quantum interference device 30 according to the third embodiment, in which FIG. 3A is a plan view and FIG. 3B is a cross-sectional view taken along line X3-X3. Unlike the first embodiment, the superconducting quantum interference device 30 shown in FIG. 3 is over-etched in the substrate 11 where the superconductor layer 12 and the ferromagnetic layer 13 are not formed. That is, the difference is that the superconductor layer 12 and the ferromagnetic layer 13 are formed on the mesa portion 31 A of the substrate 11. By over-etching the substrate 11, no wet etching residue remains on the step with the superconductor layer 12 or the ferromagnetic layer 13 on the substrate 11. Therefore, it does not cause deterioration of the characteristics of the Josephson junction.

A fourth embodiment of the superconducting quantum interference device will be described.
4A and 4B show a superconducting quantum interference device 40 according to the fourth embodiment, in which FIG. 4A is a plan view and FIG. 4B is a cross-sectional view taken along line X4-X4. Unlike the second embodiment, the superconducting quantum interference device 40 shown in FIG. 4 is over-etched in the substrate 11 where the superconductor layer 12 and the ferromagnetic layer 13 are not formed. That is, the difference is that the superconductor layer 12 and the ferromagnetic layer 13 are formed on the mesa portion 31 A of the substrate 11. By over-etching the substrate 11, no wet etching residue remains on the step with the superconductor layer 12 or the ferromagnetic layer 13 on the substrate 11. Therefore, it does not cause deterioration of the characteristics of the Josephson junction.

  Here, in the superconducting quantum interference devices 10, 20, 30, and 40 described above, the superconductor layer 12 is directly provided on the substrate 11, but in order to increase the crystallinity of the superconductor layer 12, A buffer layer may be provided between the conductor layer 12.

In the superconducting quantum interference devices 10, 20, 30, and 40, the ferromagnetic layer 13 is provided directly on the superconductor layer 12, but in the case of a conductive ferromagnetic layer, the superconductor is provided. An insulating film may be interposed between the layer 12 and the ferromagnetic layer 13. This insulating film does not act as a tunnel layer indispensable for a conventional SIS type Josephson junction. The insulating film to be interposed only needs to have insulating characteristics, and has a thickness of, for example, several nm to several tens of nm. The insulating film may be a natural oxide film formed on the surface of a simple substance such as iron or nickel or an alloy or other ferromagnetic material, which becomes the ferromagnetic layer 13 in the manufacturing process of the superconducting quantum interference device, and is intentionally formed Si ₃ N. An insulating film such as _four films or SiO ₂ film may be used.

Next, the manufacturing method of the superconducting quantum interference device of the present invention will be described taking the case of the first embodiment shown in FIG. 1 as an example. FIG. 5 is a diagram showing a manufacturing process of the superconducting quantum interference device of the present invention.
First, as shown in FIG. 5A, a film 51 to be the superconductor layer 12 is formed on the substrate 11. At that time, a buffer layer (not shown) may be formed on the substrate 11 in advance. For the film formation, an ablation method using a pulse laser, that is, a deposition method such as a PLD method is used, and deposition is performed until a predetermined thickness is reached.

  Next, electrodes connected to the current terminals 12G and 12H of the superconducting quantum interference device are formed. In that case, it is preferable to form the alignment marker simultaneously. Specifically, an electron beam exposure resist is applied on the film 51, the electron beam exposure resist is exposed, the electron beam exposure resist is developed, and a pattern in which regions serving as electrodes and markers are opened is formed. . An electrode and a marker can be simultaneously formed by a so-called lift-off method in which a metal layer serving as an electrode is deposited on this mask pattern and the resist is removed.

  Next, a lift-off mask for forming the ferromagnetic layer 13 is formed. Specifically, an electron beam exposure resist 52 is applied on the film 51 as shown in FIG. 5B, and an exposure 56 is applied to the electron beam exposure resist 52 as shown in FIG. 5C. Then, by developing the electron beam exposure resist 52, a resist pattern 53 having an opening in the region where the ferromagnetic layer is to be formed is formed by etching (see FIG. 5D).

  Next, a ferromagnetic material 54 is deposited on the entire surface of the substrate on which the resist pattern 53 is formed by vapor deposition using resistance heating or the like (see FIG. 5E), and the resist pattern is removed by etching (FIG. 5 ( F) See). Thus, the extra ferromagnetic film is removed by a so-called lift-off process, and the pattern of the ferromagnetic layer 13 is formed on the superconducting film 51.

  Next, the superconductor film 12 is formed by removing the excess superconductor film 51 by etching. Specifically, an electron beam exposure resist 55 is applied (see FIG. 5G), the electron beam exposure resist exposure 56 is performed, and the electron beam exposure resist is developed, whereby a superconductor layer is formed. A resist pattern 57 having an opening in a region other than that to be formed is formed by etching (see FIG. 5I).

  Thereafter, by removing the resist pattern 57 shown in FIG. 5J, the superconducting quantum interference device 10 shown in FIG. 1 can be manufactured (see FIG. 5K).

  Here, when the superconducting quantum interference device 20 shown in FIG. 2 is manufactured, it can be realized by changing the resist pattern in which the region where the ferromagnetic layer is arranged is opened in the above-described process.

  A method for manufacturing the superconducting quantum interference devices 30 and 40 shown in FIGS. 3 and 4 will be described. Since the superconducting quantum interference device 30 and the superconducting quantum interference device 40 only differ in the pattern of the ferromagnetic layer, the case of the superconducting quantum interference device 30 will be described in particular.

FIG. 6 is a diagram showing a part of the manufacturing process of the superconducting quantum interference device 30 shown in FIG.
When the superconducting quantum interference device 30 shown in FIG. 3 is manufactured, the same process as the manufacturing process of the superconducting quantum interference device 10 is performed as shown in FIG. Then, the state shown in FIG. 5J, that is, the state shown in FIG.
Next, as shown in FIG. 6B, ion milling or reactive ion etching (RIE) is performed with ions 58, so that a region of the substrate 11 where the superconductor layer and the ferromagnetic layer are not formed is formed. Over-etching is performed to form a mesa portion 31A (see FIG. 6C).
Thereafter, by removing the resist pattern 57 shown in FIG. 6C, the superconducting quantum interference device 30 shown in FIG. 3 can be manufactured (see FIG. 6D).

Thus, although the superconducting quantum interference devices 10, 20, 30, and 40 can be manufactured, it is preferable to form a final protective film in a region other than the electrodes as a final process.
As described above, in a method for manufacturing a superconducting quantum interference device including a superconductor layer having a ring portion and one or more ferromagnetic layers disposed on the ring portion, Forming a conductive film; forming a ferromagnetic film on the superconductive film; forming a ferromagnetic layer by patterning; and etching an unnecessary portion of the superconductive film to form a superconductor layer. And. After the superconducting film is etched to form a superconductor layer, the substrate may be further over-etched.
According to this, it is not necessary to form a conventional barrier layer, and it can be produced by arranging one or more ferromagnetic layers across a part of the loop portion of the superconductor layer. Thus, it is not necessary to go through a process of patterning with hydrofluoric acid or the like to form a barrier layer. Therefore, unlike the conventional case, there is no problem that the interface characteristics between the superconductor layer and the barrier layer deteriorate. Further, a manufacturing method with fewer process steps and easy manufacturing control can be provided. Thereby, a superconducting quantum interference device with good reproducibility and high sensitivity can be produced.

As a superconducting quantum interference device, a DC-SQUID shown in FIG. 1 was produced by the following process.
First, about 42 nm of La _2-x Sr _x CuO ₄ (LSCO) was deposited on the LaSrAlO ₄ substrate 11. A PLD method using a KrF excimer laser with a wavelength of 248 nm was used.
Next, a positive electron beam exposure resist (manufactured by Shipley, model number 1813) is applied on the film 51 to be the La _2-x Sr _x CuO ₄ superconductor layer, electron beam exposure is performed, and development is performed. Thus, a resist pattern having an opening in a region where the electrode is to be arranged was formed.
Gold serving as an electrode was deposited on the entire surface of the substrate 11 on which the resist pattern was formed by a vapor deposition method using resistance heating. The basic vacuum was about 5 × 10 ⁻⁶ Torr, and the film was deposited at 250 nm at a deposition rate of 1 nm / second. Then, excess gold was removed by a so-called lift-off process in which the resist pattern 53 was removed by etching, and electrodes and alignment markers were formed on the film 51.

Next, a positive type electron beam exposure resist 52 (manufactured by Shipley, Model No. 1813) is applied on the film 51 to be the La _2-x Sr _x CuO ₄ superconductor layer, electron beam exposure is performed, and development is performed. As a result, a resist pattern 53 having an opening in a region where the ferromagnetic layer 13 is to be disposed was formed (see FIG. 5D).
As shown in FIG. 5E, iron to be the ferromagnetic layer 13 was deposited on the entire surface of the substrate 11 on which the resist pattern 53 was formed by a vapor deposition method using resistance heating. The basic degree of vacuum was about 5 × 10 ⁻⁷ Torr, and 60 to 100 nm was deposited at a deposition rate of 1 nm / second.
Then, excess iron was removed by a so-called lift-off process in which the resist pattern 53 was removed by etching to form a pattern of the ferromagnetic layer 13 on the superconductor layer 12 (see FIG. 5F). At that time, the width W ₃₁ of the ferromagnetic layer 13 was 100 to 500 nm, and the length L ₃₁ was 2 μm to 10 μm.

  Thereafter, for etching the LSCO thin film, a resist 55 for electron beam exposure is applied (see FIG. 5G), the resist for electron beam exposure is exposed, and the resist for electron beam exposure is developed, thereby superconducting. A resist pattern 57 having an opening other than the body layer was formed by etching. Then, the portion of the superconductor film not covered with the mask was etched with 0.01 to 0.05% hydrochloric acid, and then the mask 57 was removed.

  FIG. 7 is a view showing an SEM image of the manufactured superconducting quantum interference device 10. As shown in FIG. 7, as the superconductor layer 12, current terminals 12G and 12H are formed on both sides of a pair of rectangular antenna portions 12C and 12D, and a pair is formed between the pair of rectangular antenna portions 12C and 12D. It can be seen that the narrow wire portions as the narrow portions 12E and 12F are connected to form a substantially square loop. Moreover, it turns out that the ferromagnetic layer 13 is each covered by the thin wire | line part as a pair of constriction parts 12E and 12F ranging over bridge shape.

FIG. 8 is a diagram showing the temperature dependence of the resistance of the La _2-x Sr _x CuO ₄ (x = 0.15) film deposited by the pulse laser ablation method in the example. The vertical axis in FIG. 8 is resistance (Ω), and the horizontal axis is absolute temperature (K). As is clear from FIG. 8, it was found that the critical temperature of the superconductor layer made of La _{2 -x} Sr _x CuO ₄ (x = 0.15) film was about 30K.

  FIG. 9 is a diagram showing current-voltage characteristics of the manufactured superconducting quantum interference device 10. The vertical axis in FIG. 9 is current (μA), and the horizontal axis is voltage (μV). The measurement temperature was 8K. From FIG. 9, it can be seen that the superconducting quantum interference device 10 exhibits good current-voltage characteristics.

  FIG. 10 is a diagram showing the voltage-magnetic flux characteristics of the manufactured superconducting quantum interference device 10. The vertical axis in FIG. 10 is the generated voltage (μV), and the horizontal axis is the external magnetic flux (G). The measurement temperature was 18K, and a constant current of 530 μA was passed between the current terminals. From FIG. 10, it was found that the voltage oscillates according to the change in the magnetic flux from the outside, and the characteristic peculiar to DC-SQUID is obtained.

(Comparative example)
As a comparative example, the material of the ferromagnetic layer was changed from iron (Fe) to copper (Cu) of the nonmagnetic layer, and elements having the same dimensions and the same structure were produced.
FIG. 11 is a diagram showing current-voltage characteristics in Examples and Comparative Examples. In the figure, the vertical axis represents current (mA), and the horizontal axis represents voltage (mV). The results of the examples are indicated by solid marks (■, etc.), and the results of the comparative examples are indicated by hollow marks (□, etc.). The measurement temperature was 8, 10, 12, 14, 16, 18, 20K.
From FIG. 11, it was found that the critical current is small in the example, whereas the critical current is large in the comparative example. Further, the current-voltage characteristic curve is flux flow in the comparative example, suggesting that no Josephson junction is formed.

FIG. 12 is a diagram showing the magnetic field dependence of the critical current, where the vertical axis represents the critical current Ic (μA) and the horizontal axis represents the magnetic flux density B (G). The plot of the mark (●) is the result of the example, and the plot of the mark (■) is the result of the comparative example.
From FIG. 12, when a magnetic layer made of iron is placed on a constricted portion to form a bridge structure, as shown by the ● plot, a Franchophor-type magnetic field dependence peculiar to Josephson junction was observed. Such a change was not observed when the layer consisting of was placed on the constriction.

FIG. 13 is a diagram showing the results of measuring current-voltage characteristics and differential resistance when a 2 GHz microwave is irradiated in the Josephson junction used in the superconducting quantum interference device of the example. The horizontal axis in the figure is the applied voltage (μV) between the pair of antenna units, the left vertical axis indicates the current (μA), and the right vertical axis indicates the differential resistance (dV / DI [Ω]). The measurement temperature is 35K. In the figure, the ■ plot shows the measurement result of the current-voltage characteristic when the microwave is not irradiated, the ● plot shows the measurement result of the current-voltage characteristic when the microwave is irradiated by 800 μW, and the ○ plot is The measurement result of differential resistance when 800 μW of microwaves is irradiated is shown.
As can be seen from FIG. 13, in the example, the current flowing through the Josephson junction showed a gentle kink in the differential resistance every 4 μV or so.

The Shapiro step, which is an interference effect exhibited by the Josephson junction, is expressed by the following formula (1).
V = hf / (2e) = Φf (1)
Where V is the interval voltage (V) of the Shapiro step, h is the Planck constant (6.626 × 10 ⁻³⁴ J · s), e is the unit charge of electrons (1.602 × 10 ⁻¹⁹ C), Φ = h / ( 2e) is the magnetic flux quantum.
The interval of the Shapiro step when the 2 GHz microwave is irradiated is calculated as about 4 μV from the equation (1). Therefore, it was found that the kink step interval observed in the example satisfied the equation (1), and that the Shapiro step due to the Josephson junction interference effect was observed.

FIG. 14 is a diagram showing results of measuring current-voltage characteristics and differential resistance when a 2 GHz microwave is irradiated in the element of the comparative example. In the figure, the horizontal axis represents the applied voltage (μV) between the pair of antenna units, and the vertical axis represents the current (μA). The measurement temperature is 9K. In the figure, the ▪ plot shows the measurement result of the current-voltage characteristic when the microwave is not irradiated, and the ● plot shows the measurement result of the current-voltage characteristic when the microwave is irradiated by 800 μW.
As is clear from FIG. 14, in the comparative example, unlike FIG. 13, only the critical current changes, and no interference structure is observed.

  From the results of the above examples and comparative examples, it was found that a Josephson junction was formed in the element in which the ferromagnetic layer made of iron was placed on the pair of constricted portions.

  In the above embodiment, it was found that the superconducting quantum interference devices 10 to 40 having a so-called bridge structure Josephson junction in which the ferromagnetic layer 13 is arranged in a part of the superconductor layer 12 can be realized by a simple manufacturing process. .

The present invention is not limited to the structure such as the shape shown in the drawings, and various modifications are possible within the scope of the invention. For example, the dimensions of the superconductor layer 12 and the ferromagnetic layer 13 can be appropriately designed so as to obtain a desired I _{c RN} product, and it goes without saying that these are also included in the scope of the present invention.

The superconducting quantum interference element concerning a 1st embodiment is shown, (A) is a top view and (B) is a sectional view which meets an X1-X1 line. The superconducting quantum interference element concerning a 2nd embodiment is shown, (A) is a top view and (B) is a sectional view which meets a X2-X2 line. The superconducting quantum interference element concerning a 3rd embodiment is shown, (A) is a top view and (B) is a sectional view which meets a X3-X3 line. The superconducting quantum interference element concerning a 4th embodiment is shown, (A) is a top view and (B) is a sectional view which meets a X4-X4 line. It is a figure which shows the manufacturing process of the superconducting quantum interference element of this invention. It is a figure which shows a part of manufacturing process of the superconducting quantum interference element shown in FIG. It is a figure which shows the SEM image of the produced superconducting quantum interference element. It is a diagram showing temperature dependence of the resistance of the _{La 2-x Sr x CuO 4} (x = 0.15) film deposited by the pulse laser ablation in the examples. It is a figure which shows the current-voltage characteristic of the produced superconducting quantum interference element. It is a figure which shows the voltage-magnetic flux characteristic of the produced superconducting quantum interference element. It is a figure which shows the current-voltage characteristic in an Example and a comparative example. It is a figure which shows the magnetic field dependence of a critical current. It is a figure which shows the result of having measured the current voltage characteristic and differential resistance when a 2 GHz microwave is irradiated in the Josephson junction used with the superconducting quantum interference element of an Example. It is a figure which shows the result of having measured the current-voltage characteristic and differential resistance when a 2 GHz microwave is irradiated in the element of a comparative example.

Explanation of symbols

10, 20, 30, 40 Superconducting quantum interference device 11 Substrate 12 Superconductor layer 12A Ring portion 12B Region 12C, 12D Antenna portion 12E, 12F Constriction portion 12G, 12H Current terminal 13 Ferromagnetic layer 31A Mesa portion 51 Superconducting film 52 Electron Beam Exposure Resist 53 Resist Pattern 54 Ferromagnetic Material 55 Electron Beam Exposure Resist 56 Exposure 57 Resist Pattern 58 Ion Flow

Claims (5)

A superconductor layer having a ring portion formed by connecting a pair of antenna portions with a pair of constricted portions;
And a ferromagnetic layer disposed so as to straddle one of the pair of constricted portions. A superconductor layer having a ring portion formed by connecting a pair of antenna portions with a pair of constricted portions;
And a ferromagnetic layer disposed so as to straddle each of the pair of constricted portions. It said superconductor layer and said ferromagnetic layer is disposed on the mesa portion formed by over-etching the substrate, the superconducting quantum interference device according to claim 1 or 2. Each of the pair of antenna portions, a rectangular shape of vertical width 0.1 to 1000 [mu] m × width 0.1 to 1000 [mu] m, a width of the narrowed portion is of the order of [mu] m, according to claim 1 or 2 Superconducting quantum interference device. The superconducting quantum interference device according to claim 1, wherein the ferromagnetic layer has a width of 2 nm to 1 μm.