Josephson junction element and its manufacturing method

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a method for manufacturing a Josephson junction element using a superconductor, and more particularly to a production method for reducing variations in the characteristics of a Josephson junction element using an oxide high-temperature superconductor.

【0002】

[Conventional technology]

Superconductors have properties that other substances do not have, such as 1) zero electrical resistance, 2) complete anti-magnetism, and 3) Josephson effect. It is expected to be widely applied to confinement, magnetic levitation trains, magnetic shields, high-speed computers, etc. Superconducting transition temperature T of about 30K by Bednorz and Muller in 1986_cCopper oxide superconductor with (La)_1-xBa_x)₂CuO₄Was discovered. After that YBa₂Cu₃O_7-y(Tc = 90K), Bi₂Sr₂Ca₂Cu₃O_y(Tc = 110K), Tl₂Ba₂Ca₂Cu₃O_y(Tc = 125K), HgBa₂Ca₂Cu₃O_ySuperconducting transitions at high temperatures were reported one after another at (Tc = 135K) and the like. Currently, much research is being conducted on the production methods, physical properties, applications, etc. of these substances. Above all, YBa₂Cu₃O_ySuperconductors do not contain harmful elements such as Tl and Hg, and are relatively small anisotropy superconductors, so they are the most promising practical materials for electronic devices and wire rods.

【0003】

Application to electronic devices using the Josephson effect requires Josephson junction fabrication technology using thin film technology. As a commonly used film forming method, a raw material is excited in a vacuum vessel such as a sputtering method, a laser ablation method, a vacuum vapor deposition method, or a molecular beam epitaxy method to be scattered in a gas phase and deposited on a substrate. , The so-called physical vapor deposition method is used. Various element structures such as a bicrystal type, a biepitaxial type, a step edge type, a lamp edge type, and a laminated type have been proposed as Josephson junctions of cuprate superconductors. (Susumu Takada, Applied Physics 62 (1993) p443) Among them, the lamp edge type junction is promising because it has a large driving ability at the time of switching and the critical current can be changed by controlling the thickness of the tunnel barrier layer (Hidaka). Mutsuo et al., Applied Physics 67 (1998) 1167).

【0004】

I as one of the indicators of Josephson junction performance_cR_nThere is a product, but the larger this value is, the faster the operation becomes possible. I_cR_nThe product is the critical current I at a certain temperature._cThat is, it is a value obtained by normalizing the product of the maximum current value that can flow in the superconducting state and the resistivity when the superconducting state is broken and becomes the normal conducting state by the size of the junction. Qualitatively, it is an index showing the magnitude of the signal at the time of switching. Larger I in lamp edge type junction than other element structures_cR_nThe product has been obtained. YBa as the upper and lower superconducting electrodes₂Cu₃O_yIs often used. On the other hand, research on laminated joints, which is advantageous for large-scale integration in the future, is also ongoing. PrBa on the barrier layer, which is the joint between both₂Cu₃O_yLayer, Nb-doped SrTiO₃Layers, process damage layers, etc. are used.

【0005】

In recent years, Josephson junctions (called IEJ (Interface-Engineered Junction)) that utilize the damaged layer during the process to form the junction have been actively studied (BH Moeckly et al., Appl. Phys. Lett. 71 (1997) 2526). The formation of an ultrathin layer of about 1 to 2 nm has been confirmed by transmission electron microscopy in the lamp edge type junction, and it is thought that this functions as a Josephson junction, but the detailed structure has not yet been elucidated. Not (JG Wen et al., Advances in Superconductivity XII-Proc. ISS'99 (1999 10 / 17-19, Morioka) p.984 and Y. Soutome et al., Advances in Superconductivity XII- Proc. ISS'99 (1999 10 / 17-19, Morioka) p.990). If the parameters at the time of fabrication are inappropriate and the thickness of the damaged layer becomes extremely thin and it becomes difficult to control the thickness, the joint may be short-circuited in some places. The current-voltage dependence (IV characteristic) through the joint changes depending on the thickness of the joint. If it is too thick, superconducting current cannot flow. If the thickness is appropriate, the critical current value I_cThe superconducting current can tunnel the junction without generating a voltage within the range of. In this case I_cWhen a current exceeding the above is applied, a voltage is suddenly generated. The IV characteristic when a voltage is generated gradually approaches a straight line passing through the origin. The IV characteristic characteristic of such Josephson junctions is called the RSJ (Resistively Shunted Junction) characteristic. If it is too thin and short-circuited, I_cA voltage is gradually generated by the above current. Since the voltage is induced by the movement of the magnetic flux, it is called the FF (Flux Flow) -like IV characteristic.

【0006】

In order to realize a superconducting electronic device using Josephson junction, specifically, the above RSJ characteristics are shown, and an appropriate I_cAnd I_cR_nMany Josephson junctions with product values need to be made. Especially I_cThe value is sensitive to the bonding structure and manufacturing process, and the immediate task is to establish a technique for suppressing the variation in this value. It has been estimated that it is necessary to suppress the characteristic variation 1σ to 10% or less in order to operate a circuit with 100 or more junctions (J. Talvacchio et al., IEEE Trans. Appl. Supercond. 7 (1997) 2051). Recently, a good 1σ satisfying this condition has been reported in the IEJ lamp edge type Josephson junction. Satoh et al. YBa on superconducting electrodes₂Cu₃O_7-yTo the insulating layer (La_0.3Sr_0.7) (Al_0.65Ta_0.35) O_yAchieved 1σ = 8% at 4.2K at 100 junctions (T. Satoh et al., IEEE Trans. Appl. Supercond. 9 (1999) 3141). Their inventions are disclosed in Japanese Patent Application Laid-Open No. 2000-150974. It is said that the formation of a homogeneous barrier layer with a thickness of 2 nm or less between the two superconducting electrodes and the mixing of La from the insulating layer at the interface during etching give good bonding characteristics. However, the amount of La is extremely small, and it cannot be confirmed even by means using an analytical transmission electron microscope that analyzes using characteristic X-rays generated by irradiating an electron beam with a beam diameter of about 1 nm (JG Wen et al). ., Appl. Phys. Lett. 75 (1999) 2470). May women YBa on superconducting electrodes₂Cu₃O_7-yTo the insulating layer, CeO₂Achieved 1σ = 7.9% at 4.2K with 100 junctions (Mayonna et al., 62nd JSAP Academic Lecture 14a-G-7 (2001, 9 / 11-14, Toyota City) Proceedings No. 1, p.195). They do not use any La-containing material and achieve this value with a structure and process that does not allow La to mix between the two superconducting electrodes.

【0007】

[Problems to be Solved by the Invention]

When considering the industrial application of Josephson joints, a technique for producing a plurality of joints operating with appropriate characteristics with good reproducibility is indispensable. For large-scale integration in the future, it is necessary to fabricate a large number of Josephson junctions with small variations in characteristics.

【0008】

However, research on Josephson junctions using high-temperature oxide superconductors with high Tc has been conducted with the aim of realizing superconducting electronic devices with high operating temperatures. Demonstrations were limited to very small scales, and large-scale integrated practical circuits could not be realized. As mentioned above, it has been reported that 1σ is about 8% for 100 joints, but there is a long-awaited method for producing Josephson joints that can realize even smaller characteristic variations with a larger number of joints.

【0009】

Further, regarding the second superconducting layer formed on the barrier layer forming the Josephson junction, a manufacturing method for suppressing variation in the characteristics of the device has not been sufficiently studied so far.

【0010】

Therefore, the present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a Josephson junction device having less variation in characteristics, stable characteristics, and high reliability, and a method for manufacturing the same. It is to be. In particular, to provide a method for manufacturing a Josephson junction element, in which the layer formed at the initial stage of the second superconducting layer on the barrier layer is uniformly formed over the entire substrate to further suppress the variation in characteristics. That is.

【0011】

[Means for solving problems]

As described in claim 1, the above objectives are a step of forming a first superconducting layer on a substrate, a step of forming an insulating layer on the first superconducting layer, and a second superconducting layer on the insulating layer. A method for manufacturing a Josephson junction element including a step of forming the second superconducting layer, wherein the step of forming the second superconducting layer includes a step of removing the insulating layer at at least a predetermined position and the second superconducting layer. Is achieved by including a first step of forming the second superconducting layer and a second step of forming the second superconducting layer again on the second superconducting layer formed in the first step.

【0012】

In the case of Josephson junction using cuprate superconductor, there is a peculiar problem that it is particularly difficult to form a reliable barrier layer with good reproducibility due to the short coherent length. .. As a result of the research of the inventor of the present application, it was found that the characteristics of the barrier layer are not only determined by the process of forming the non-superconducting layer, but are also affected by the subsequent deposition of the second superconductor. In particular, the film formation conditions of the second superconductor portion near the barrier layer greatly affect the characteristics of the Josephson junction. In order to prepare a large number of Josephson junctions on a substrate with uniform characteristics, not only the homogeneity of the non-superconductors of the individual barrier layers but also the film forming conditions of the superconductors on the barrier layers must be matched. Moreover, the optimum deposition conditions for bond formation do not always match the film formation conditions for high-quality superconductors. In the present invention, the film formation of the superconductor located on the barrier layer is performed in at least two stages. It is important that the film forming conditions in the first step are uniform conditions for individual bonding on the substrate, rather than whether good quality superconductors can be obtained. In the subsequent stages, the film is formed under the conditions that a good quality superconductor can be obtained.

【0013】

According to the present invention, by performing the film formation in the first step, which is the initial stage of the second superconductor forming step, under conditions suitable for Josephson junction formation, the characteristics are less likely to vary and the characteristics are stable. And, a highly reliable Josephson junction element can be manufactured.

【0014】

Further, as described in claim 2, in the method for manufacturing a Josephson junction element according to claim 1, the step of removing the insulating layer is configured to remove a part of the first superconducting layer. be able to.

【0015】

According to the invention of claim 2, by removing the insulating layer and removing a part of the first superconducting layer, the altered layer formed at the time of removal is used for forming the barrier layer. It is possible to form a uniform barrier layer over a wide range of the substrate, and it is possible to suppress variations in Josephson junction elements.

【0016】

Further, as described in claim 3, in the method for manufacturing a Josephson junction element according to claim 1 or 2, the first step is formed by a place on a substrate with respect to the second step. This is achieved by reducing the difference in conditions.

【0017】

Further, as described in claim 4, in the method for manufacturing a Josephson junction element according to any one of claims 1 to 3, the first step is physical vapor deposition using a plurality of vapor deposition sources at the same time. Achieved by doing by law.

【0018】

Further, as described in claim 5, in the method for manufacturing a Josephson junction element according to any one of claims 1 to 4, the first step is an obstacle between the vapor deposition source and the substrate. Is achieved by providing a physical vapor deposition method.

【0019】

Further, as described in claim 6, in the method for manufacturing a Josephson junction element according to any one of claims 1 to 5, the first and second steps are performed by a physical vapor deposition method, and the first step is performed. The first step is achieved by increasing the distance between the vapor deposition source and the substrate with respect to the second step.

【0020】

Further, as described in claim 7, in the method for manufacturing a Josephson junction element according to any one of claims 1 to 5, the first and second steps are performed by a physical vapor deposition method, and the first step is performed. The first step is achieved by lowering the excitation energy of the vapor deposition source with respect to the second step.

【0021】

Further, as described in claim 8, in the method for manufacturing a Josephson junction element according to any one of claims 1 to 5, the first and second steps are performed by a physical vapor deposition method, and the first step is performed. The first step is achieved by increasing the atmospheric pressure with respect to the second step.

【0022】

Further, as described in claim 9, in the method for manufacturing a Josephson junction device according to any one of claims 1 to 8, the first step is a sputtering method, a vacuum vapor deposition method, or a molecular beam. It is performed by the epitaxy method, and the first step is achieved by performing the laser ablation method.

【0023】

It is a physical vapor deposition method that excites raw materials in a vacuum vessel, scatters them in the gas phase, and deposits them on a substrate. In order to deposit them under exactly the same conditions on any part of the substrate, it is uniform from the vapor deposition source on the surface. Ideally, the raw material particles that jump out to the surface are deposited on the substrate facing the surface. However, it is difficult to assemble such equipment in reality. In many cases, deviation of the film formation conditions on the substrate is unavoidable. However, it is possible to reduce the deviation of the film forming conditions.

【0024】

In the physical vapor deposition method, there are the following two methods for equalizing the energy of the particles deposited on the substrate. 1) Make multiple excited parts of the raw material or high-density parts of high-energy particles. 2) Keep the excited part of the raw material or the dense part of high-energy particles away from the substrate. Here, the excitation source of the raw material is the portion on the target irradiated with the laser in the laser ablation method, and the portion having a high density of high-energy particles is the plume that appears during laser irradiation. In the case of the sputtering method, the portion of the target where the plasma hits (called erosion) and the plasma, respectively. In the vacuum vapor deposition method and the molecular beam epitaxy method, it is the evaporation source and its vicinity. It is also effective to change the film forming means at the initial stage of film formation and the subsequent steps. Generally, when the uniformity of the energy of the particles deposited on the substrate is arranged in descending order, the order is vacuum deposition method / molecular beam epitaxy method> sputtering method> laser ablation method. The purpose can be achieved by forming the initial stage with the higher film and then performing the subsequent film formation with the lower film.

【0025】

According to the present invention, in the first step of forming the second superconducting layer, the Josephson junction element is formed by forming a film under the condition that the energy of the vapor-deposited molecules or particles becomes uniform over a wide range of the substrate. Characteristics, especially I_cIt is possible to suppress the variation of the above over a wide range of the substrate, and it is possible to manufacture a Josephson junction element having stable characteristics and high reliability.

【0026】

Further, as described in claim 10, the Josephson junction element manufactured by the method for manufacturing a Josephson junction element according to any one of claims 1 to 9;_cIt is achieved when the variation 1σ of is 8% or less.

【0027】

According to the present invention, in the first step of forming the second superconducting layer, the Josephson junction element is formed by forming a film under the condition that the energy of the vapor-deposited molecules or particles becomes uniform over a wide range of the substrate. Characteristics, especially I_cVariation can be suppressed over a wide range of substrates, and 100-junction I_cThe variation of 1σ can be suppressed to 8% or less, and a Josephson junction element with stable characteristics and high reliability can be realized.

【0028】

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, a stable and highly reliable Josephson junction can be manufactured with good reproducibility. Specifically, when a large number of Josephson junctions are prepared, I_cThe variation can be made smaller than ever.

【0029】

In the case of Josephson junction using cuprate superconductor, there is a peculiar problem that it is particularly difficult to form a reliable barrier layer with good reproducibility due to the short coherent length. .. Previous studies have focused on what to use for non-superconductors in the barrier layer and how to stack them. The present inventor has also repeated a lot of trial and error on this subject. As a result, it was clarified that the characteristics of the barrier layer are not only determined by the process of forming the non-superconducting layer, but are also affected not only by the subsequent deposition of superconductors. In particular, the film formation conditions near the barrier layer greatly affect the characteristics of the Josephson junction. In order to prepare a large number of Josephson junctions on a substrate with uniform characteristics, not only the homogeneity of the non-superconductors of the individual barrier layers but also the film forming conditions of the superconductors on the barrier layers must be matched. However, the optimum deposition conditions for barrier layer formation do not always match the film formation conditions for high-quality superconductors. In the present invention, the film formation of the superconductor located on the barrier layer is performed in at least two stages. It is important that the initial film formation conditions are uniform for each bond on the substrate, rather than whether a good quality superconductor can be obtained. In the subsequent stages, the film is formed under the conditions that a good quality superconductor can be obtained.

【0030】

It is a physical vapor deposition method that excites raw materials in a vacuum vessel, scatters them in the gas phase, and deposits them on a substrate. In order to deposit them under exactly the same conditions on any part of the substrate, it is uniform from the vapor deposition source on the surface. Ideally, the raw material particles that jump out to the surface are deposited on the substrate facing the surface. However, it is difficult to assemble such equipment in reality. In many cases, deviation of the film formation conditions on the substrate is unavoidable. However, it is possible to reduce the deviation of the film forming conditions.

【0031】

In the physical vapor deposition method, there are the following two methods for equalizing the energy of the particles deposited on the substrate. 1) Make multiple excited parts of the raw material or high-density parts of high-energy particles. 2) Keep the excited part of the raw material or the dense part of high-energy particles away from the substrate. Here, the excitation source of the raw material is the portion on the target irradiated with the laser in the laser ablation method, and the portion having a high density of high-energy particles is the plume that appears during laser irradiation. In the case of the sputtering method, the portion of the target where the plasma hits (called erosion) and the plasma, respectively. In the vacuum vapor deposition method and the molecular beam epitaxy method, it is the evaporation source and its vicinity. It is also effective to change the film forming means at the initial stage of film formation and the subsequent steps. Generally, when the uniformity of the energy of the particles deposited on the substrate is arranged in descending order, the order is vacuum deposition method / molecular beam epitaxy method> sputtering method> laser ablation method. The purpose can be achieved by forming the initial stage with the higher film and then performing the subsequent film formation with the lower film.

【0032】

Hereinafter, examples of the present invention will be described with reference to the drawings. (1st Example) Y in the 1st superconductor_0.9Ba_1.9La_0.2Cu₃O_yAnd Yb on the second superconductor_0.9Ba_1.9La_0.2Cu₃O_yWill be used to describe a method for manufacturing a lamp-edge Josephson junction element that forms a junction by the IEJ method. In this embodiment, in the first step, two laser beams are simultaneously irradiated on the target surface and the laser ablation method is provided with two vapor deposition sources.

【0033】

FIG. 1 is a diagram showing a manufacturing process of the Josephson junction element 10 according to the first embodiment of the present invention.

【0034】

To refer to Fig. 1 (A), first, in a square with a size of 10 mm × 10 mm, (La_0.3Sr_0.7) (Al_0.65Ta_0.35) O₃The first superconductor 12 having a thickness of 200 nm is Y on the substrate 11 using (hereinafter referred to as LSAT) or the like by the off-axis RF sputtering method._0.9Ba_1.9La_0.2Cu₃O_yOn top of this, an interlayer insulating layer 13 having a thickness of 200 nm is formed using LSAT by the off-axis RF sputtering method. Next, referring to FIG. 1 (B), after patterning by the photolithography method, a reflow process is performed so that the end portion of the photoresist has a gentle inclination to form a resist 14 which is a mask for ion etching. .. Next, referring to FIG. 1 (C), the interlayer insulating layer 13 and the first superconductor 12 are etched by irradiating Ar ions with an acceleration voltage of 400 V and an ion current of 50 mA using this resist 14. Ar ion irradiation is performed while the substrate is rotated with the incident direction tilted by 30 ° with respect to the substrate surface. As a result, a gentle slope of the interlayer insulating layer 13 and the first superconductor 12 is formed, and the surface of the slope is covered with a layer damaged by ion irradiation. Next, with reference to FIG. 1 (D), the resist 14 is removed by an ashing device. The laminated substrate 50 immediately before the formation of the second superconductor 15 is a substrate 11 on which the first superconductor 12 and the interlayer insulating layer 13 are laminated.

【0035】

Next, the second superconductor 15 was formed on this by the laser ablation method. FIG. 2 is a diagram showing the arrangement of the substrate 100, the target 101, and the like by the laser ablation method. With reference to FIG. 2A, the first step is performed so that the laser beam is split into two in the middle so that the two laser beams simultaneously irradiate the surface of the target 101. Yb with a diameter of 30 mm, each with a laser beam of 300 mJ_0.9Ba_1.9La_0.2Cu₃O_yIrradiate the sintered target 101 5 times per second. The energy density on the surface of this target 101 is 1.0 mJ / cm, respectively.²Is. The substrate temperature is 660 ° C, the atmospheric pressure is oxygen 26.6 Pa, and the distance between the substrate 100 and the target 101 is 60 mm. The substrate 50 and the target 101 were rotated at 3 rpm and 8 rpm, respectively, in order to achieve the uniformity of the film quality, and the laser beam moved the mirror to scan on the target 101. The particles excited on the target 101 have kinetic energy and are released into the gas phase, collide with oxygen in the gas phase and accumulate on the substrate 100 while losing energy.

【0036】

In the case of a single laser beam that is normally used, as shown in FIG. 2 (C), particles scattered from the target generate a light emitting portion called a plume 103. In the case of this embodiment, the plume 103 becomes as shown in FIG. 2 (A) by using two laser beams. The change width of the energy of the particles reaching the substrate depending on the location becomes small.

【0037】

After the film formation in the first step is performed for 10 minutes, the branching of the laser beam is stopped and the film formation is performed with one laser beam as shown in FIG. 2 (C). The energy of the laser beam is 600 mJ, and the film is formed for 30 minutes.

【0038】

As a result, as shown in FIG. 1 (E), a Josephson junction element 10 having a Josephson junction in which the barrier layer 16 is sandwiched between the first superconductor 12 and the second superconductor 15 is formed. Further, a plurality of Josephson junction elements 10 are manufactured by depositing gold electrodes and performing appropriate patterning. On the substrate 11, 10 joints and 100 joints of the series are prepared so as to be evenly distributed in a square area of 6 mm × 6 mm.

【0039】

All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 2.1 ~ 2.6mV, I_cWas about 0.9mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 6.2%. (1st Comparative Example) A lamp edge type Josephson junction was produced in the same manner as in the 1st Example. However, in the film formation of the second superconductor, the first step of irradiating the target with two laser beams at the same time was omitted. All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 1.9 ~ 2.7mV, I_cWas about 0.9mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 12.5%. (Second Comparative Example) A lamp edge type Josephson junction was produced in the same manner as in the first embodiment. However, the difference from the first embodiment is that in the film formation of the second superconductor, the first step of irradiating the target with two laser beams at the same time is continued as it is, and the film formation is performed with one laser beam. The second step of the first embodiment to be performed was not provided. Of the 10 junctions made on the substrate, 8 exhibited RSJ-type current-voltage characteristics, while the remaining 2 exhibited FF-type characteristics. When the surface of the second superconductor was observed with an optical microscope after the film formation, more granular precipitation was observed as compared with the case of the first example. It is presumed that the junction that did not show the RSJ type just hit the place where the deposit was present, and the junction was short-circuited.

【0040】

Based on the above, according to the first embodiment, in the formation of the second superconductor, the target is simultaneously irradiated with two laser beams in the first step, and particles excited from the two vapor deposition sources are deposited. By I_cThe variation of 1σ can be suppressed to 6.2%, and I_cR_nA Josephson junction element with a high product of 2.1 to 2.6 mV and good characteristics can be manufactured. Further, in the formation of the second superconductor, in the second step, it is preferable to form the second superconductor with a single high-power laser beam. (Second Example) In this example, a method for manufacturing a lamp edge type Josephson junction element that forms a junction by the IEJ method using the same materials of the first and second superconductors as in the first embodiment. In the first step of forming the second superconductor, an obstacle is provided between the substrate and the target to form a film.

【0041】

Hereinafter, a method for manufacturing the Josephson junction element of this embodiment will be described.

【0042】

The steps up to immediately before the step of forming the second superconductor are carried out in the same manner as in the first embodiment. The second superconductor is formed by the laser ablation method. At the time of film formation in the first step of the second superconductor, an Inconel square bar-shaped obstacle 104 is installed between the substrate 100 and the target 101 as shown in FIG. 2 (B). The film formation is performed by irradiating the target 101 with a laser beam of 600 mJ 5 times per second and depositing the particles excited from the target 101 on the substrate 100. The particles excited on the target 101 are released into the gas phase with kinetic energy, collide with oxygen in the gas phase, lose kinetic energy, and are deposited on the substrate 100. In the normal method where there is no obstacle between the substrate 100 and the target 101, when the laser beam is irradiated as shown in FIG. 2 (C), the scattered particles generate a light emitting portion called a plume 103. In the case of this example, the plume 105 was deformed by an obstacle as shown in FIG. 2 (B). The number of particles deposited by the obstacle 104 is reduced, the film formation rate is reduced, but the width of the kinetic energy difference of the particles reaching the substrate 100 is reduced.

【0043】

All of the 10 junctions formed on the substrate 11 showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 2.0 ~ 2.6mV, I_cWas about 0.9mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 6.6%.

【0044】

From the above, according to the present embodiment, in the first step, an obstacle is installed between the substrate and the target, the portion of the particle having high kinetic energy is suppressed by the obstacle, and the movement of the particle reaching the substrate is suppressed. By reducing the width of the energy difference, I_cThe variation of 1σ can be suppressed to 6.6%, and I_cR_nA Josephson junction element with a high product of 2.0 to 2.6 mV and good characteristics can be manufactured. (Third Example) In this example, the same materials of the first and second superconductors as in the first embodiment are used, and a method for manufacturing a lamp edge type Josephson junction element that forms a junction by the IEJ method. In the first step of forming the second superconductor, instead of the laser ablation method, the film is formed by the off-axis RF sputtering method in which two sputtering cathodes are arranged facing each other. ..

【0045】

Hereinafter, a method for manufacturing the Josephson junction element of this embodiment will be described.

【0046】

The steps up to immediately before the step of forming the second superconductor are carried out in the same manner as in the first embodiment. The second superconductor is formed by the off-axis RF sputtering method. FIG. 3 is a diagram showing the arrangement of the substrate 100, the target 107, and the like by the off-axis RF sputtering method. Figures 3 (A) and 3 (B) show the kinetic energy distribution of the sputtered molecules. The dark part corresponds to the part with high kinetic energy, and the light part corresponds to the part with low kinetic energy. A sputtered cathode is attached to the target 107, but the sputtered cathode is omitted in FIG.

【0047】

As shown in FIG. 3 (A), at the time of film formation in the first step, another target 107B installed opposite to one target 107A is also used to form a film using two targets 107A and 107B. The kinetic energy of the particles reaching the substrate 100 is higher in the case shown in FIG. 3 (A) using two targets 107A and 107B than in the case shown in FIG. 3 (B) using one target 107A. The range of change depending on the location becomes small. After the film formation in the first step is performed for 15 minutes using two targets 107A and 107B, the film formation in the second step is performed using one sputtered cathode 107A.

【0048】

All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 1.9 ~ 2.6mV, I_cWas about 1.0 mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 6.9%.

【0049】

According to this embodiment, by performing the film formation in the first step by the off-axis RF sputtering method using two targets installed facing each other, I_cThe variation of 1σ can be suppressed to 6.9%, and I_cR_nA Josephson junction element with a high product of 1.9 to 2.6 mV and good characteristics can be manufactured.

【0050】

It is possible to continue the film formation in the second step while using the two sputtered cathodes, but in general, a plurality of cathodes are used to suppress the erosion of these targets to a certain degree of variation. It is difficult to maintain a constant quality and continue production. The film formation by the sputtering method largely depends on the characteristics of the cathode used and the quality of the target. Increasing the number of cathodes is nothing but increasing the items that must be managed to maintain quality. Therefore, it is preferable to use a small number of cathodes for the film formation in the second step of the second superconductor. On the other hand, in the film formation in the first step, the uniformity of the deposition conditions on the substrate is the most important. Specifically, it is most important that the energies of the particles deposited on the substrate are uniform. Therefore, the two-step film formation shown in this example is effective. (Fourth Example) In this example, a method for manufacturing a lamp edge type Josephson junction element that forms a junction by the IEJ method using the same materials of the first and second superconductors as in the first embodiment. In the first step of forming the second superconductor, the distance between the substrate and the target is made longer than in the second step, and the film is formed by the laser ablation method.

【0051】

Hereinafter, a method for manufacturing the Josephson junction element of this embodiment will be described.

【0052】

The steps up to immediately before the step of forming the second superconductor are carried out in the same manner as in the first embodiment. The second superconductor is formed by the laser ablation method. A 600 mJ laser beam was applied onto the target 101 5 times per second. The energy density on the surface of the target 101 is 1.0 mJ / cm, respectively.²Is. The substrate temperature is 660 ° C, the atmospheric pressure is oxygen 26.6 Pa, the substrate 100 and the target 101 rotate at 3 rpm and 8 rpm, respectively, to achieve uniformity of film quality, and the mirror is moved so that the laser beam scans on the target 101. .. As shown in FIG. 4A, the film formation in the first step is performed by increasing the distance between the substrate 100 and the target 101, and the film formation in the second step is performed by increasing the distance. Specifically, the first step was 80 mm and the second step was 60 mm. In the first step, the substrate is located at a distance clearly away from the plume, and the kinetic energy of the particles reaching the substrate is low, so the conditions are not suitable for producing a good quality superconductor layer. Is. However, by increasing the distance between the substrate 100 and the target 101, the width of the kinetic energy difference of the particles reaching on the substrate becomes smaller.

【0053】

All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 2.0 ~ 2.4mV, I_cWas about 0.8mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 6.0%.

【0054】

From the above, according to the present embodiment, in the first step, the distance between the substrate and the target is increased with respect to the second step, and the width of the kinetic energy difference of the particles reaching on the substrate is reduced. By_cVariation 1σ can be suppressed to 6.0%, I_cR_nA Josephson junction element with a high product of 2.0 to 2.4 mV and good characteristics can be manufactured. (Fifth Example) In this example, the same materials as in the first and second superconductors are used, and a method for manufacturing a lamp edge type Josephson junction element that forms a junction by the IEJ method. This is an example in which the laser energy of the laser ablation method is reduced to 300 mJ in the first step of forming the second superconductor to form a film.

【0055】

Hereinafter, a method for manufacturing the Josephson junction element of this embodiment will be described.

【0056】

The steps up to immediately before the step of forming the second superconductor are carried out in the same manner as in the first embodiment. The second superconductor is formed by the laser ablation method. In the first step, a 300 mJ laser beam was applied onto the target 5 times per second. The distance between the substrate and the target is 60 mm, and other conditions are the same as in the fourth embodiment. FIG. 5 is a diagram showing the arrangement of the substrate 100, the target 101, and the like by the laser ablation method according to this embodiment. In the first step, as shown in FIG. 5, the size of the plume is reduced by reducing the energy of the laser beam to 300 mJ as compared with FIG. 4 (B). The substrate is located clearly away from the laser plume, and the kinetic energy of the particles reaching the substrate is low, which is not suitable for producing a high-quality superconductor layer. However, by increasing the distance between the substrate and the target relatively, the width of the energy difference of the particles reaching on the substrate becomes smaller.

【0057】

All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 1.9 ~ 2.4mV, I_cWas about 0.8mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 7.0%.

【0058】

From the above, according to the present embodiment, in the first step, the energy of the laser beam is reduced to reduce the width of the kinetic energy difference of the particles reaching the substrate._cVariation 1σ can be suppressed to 7.0%, I_cR_nA Josephson junction element with a high product of 1.9 to 2.4 mV and good characteristics can be manufactured. (6th Example) In this example, the same materials as those in the 1st example of the 1st and 2nd superconductors are used, and a method for manufacturing a lamp edge type Josephson junction element in which a junction is formed by the IEJ method. In the first step of forming the second superconductor, the oxygen gas pressure in the vapor deposition atmosphere is raised to 53.2 Pa, and the film is formed by the laser ablation method.

【0059】

Hereinafter, a method for manufacturing the Josephson junction element of this embodiment will be described.

【0060】

The steps up to immediately before the step of forming the second superconductor are carried out in the same manner as in the first embodiment. The second superconductor is formed by the laser ablation method. In the first step, the distance between the substrate 100 and the target 101 is 60 mm, and the oxygen gas pressure in the vapor deposition atmosphere is raised to 53.2 Pa. Other conditions are the same as in the fourth embodiment.

【0061】

Increasing the oxygen gas pressure increases the number of times particles released from the target collide with oxygen molecules in the gas. Therefore, energy was lost quickly, and the size of the plume became smaller as in the plume of the fifth embodiment. In this first step, the substrate is located clearly away from the plume, and the kinetic energy of the particles reaching the substrate is low, which is not suitable for producing a high-quality superconductor layer. .. However, the width of the energy difference of the particles reaching on the substrate becomes small.

【0062】

All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nThe product is 2.1 ~ 2.7mV, I_cWas about 1.0 mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 7.1%.

【0063】

From the above, according to the present embodiment, in the first step, the oxygen gas pressure in the vapor deposition atmosphere is increased, the kinetic energy of the particles reaching the substrate is decreased, and the particles are deposited._cThe variation of 1σ can be suppressed to 7.1%, and I_cR_nA Josephson junction element with a high product of 2.1 to 2.7 mV and good characteristics can be manufactured. (7th Example) In this example, Y is used for the first superconductor._0.9Ba_1.9La_0.2Cu₃O_yAnd Yb on the second superconductor_0.9Ba_1.9La_0.2Cu₃O_yPrBa for the barrier layer₂Cu₃O_yThis is an example of a method for manufacturing a lamp-edge type Josephson junction element in which a junction is formed by an artificial barrier method using the above.

【0064】

Hereinafter, a method for manufacturing the Josephson junction element 20 of this embodiment will be described.

【0065】

FIG. 6 is a diagram showing a manufacturing process of the Josephson junction element 20 according to this embodiment.

【0066】

With reference to FIG. 6 (A), first, a first superconductor 22 having a thickness of 200 nm is formed on a substrate 21 having a size of 10 mm × 10 mm and using MgO or the like by the off-axis RF sputtering method._0.9Ba_1.9La_0.2Cu₃O_yOn top of that, an interlayer insulating layer 23 with a thickness of 200 nm was formed by CeO by the off-axis RF sputtering method.₂Is formed using. Next, referring to FIG. 6 (B), after patterning by the photolithography method, a reflow process is performed so that the end portion of the photoresist has a gentle inclination to form a resist 24 which is a mask for ion etching. .. Next, referring to FIG. 6C, the interlayer insulating layer 23 and the first superconductor 22 are etched by irradiating Ar ions with an acceleration voltage of 400 V and an ion current of 50 mA using this resist 24. Ar ion irradiation is performed while the substrate 21 is rotated with the incident direction tilted by 30 ° with respect to the substrate surface. As a result, a gentle slope of the interlayer insulating layer 23 and the first superconductor 22 is formed. The damaged layer on the slope is then removed with dilute phosphoric acid. Next, referring to FIG. 6 (D), the resist 24 is removed by an ashing device. Next, referring to FIG. 6 (E), on top of that, the barrier layer 26 was added to PrBa by the off-axis RF sputtering method.₂Cu₃O_y5 nm deposit using. Next, the first step of the second superconductor 25 is performed by the off-axis RF sputtering method. Yb_0.9Ba_1.9La_0.2Cu₃O_yIs deposited at 10 nm, and the second step is Yb by the laser ablation method._0.9Ba_1.9La_0.2Cu₃O_yThe second superconductor 25 is formed again using.

【0067】

As a result, as shown in FIG. 6 (E), a Josephson junction element 20 having a Josephson junction in which the barrier layer 26 is sandwiched between the first superconductor 22 and the second superconductor 25 is formed. Further, a plurality of Josephson junction elements 20 are manufactured by depositing gold electrodes and performing appropriate patterning. On the substrate 21, 10 joints and 100 joints of the series are prepared so as to be evenly distributed in a square area of 6 mm × 6 mm.

【0068】

All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 1.8 ~ 2.4mV, I_cWas about 0.8mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 7.2%. (Third Comparative Example) In this comparative example not based on the present invention, the film formation by the off-axis RF sputtering method in the first step was omitted, and the film formation was performed in one step by the laser ablation method in the second step. Other conditions are the same as in the seventh embodiment.

【0069】

All of the 10 junctions made on the substrate showed RSJ-type current-voltage characteristics. I at 4.2K_cR_nProduct is 1.8 ~ 2.7mV, I_cWas about 0.8mA. About 100 joints I_cWhen the variation of 1σ was measured, it was 10.2%.

【0070】

Based on the above, according to the seventh embodiment, in the production of the lamp edge type Josephson junction element in which the junction is formed by the artificial barrier method, the process of forming the second superconductor is divided into the first and second steps. In the first step, the kinetic energy of the particles reaching the substrate is low, and the film is formed by the off-axis RF sputtering method having a small kinetic energy width._cThe variation of 1σ can be suppressed to 7.2%, and I_cR_nA Josephson junction element with a high product of 1.8 to 2.4 mV and good characteristics can be manufactured. In this embodiment, the off-axis RF sputtering method was used in the first step, but other sputtering methods, vacuum vapor deposition methods, and molecular epitaxy methods may be used.

【0071】

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiment, and various modifications and modifications are made within the scope of the gist of the present invention described in the claims. It can be changed. For example, in the present embodiment, only the lamp edge type Josephson junction element has been described, but the present invention is also applicable to the laminated Josephson junction element, and further, the Josephson junction of the laminated Josephson junction element. However, it can be applied to either the case where the joint is formed by the IEJ or the artificial barrier method.

【0072】

[Effect of the invention]

As is clear from the details described above, according to the invention of claim 1, the conditions suitable for the formation of the Josephson junction in the first step, which is the initial step of the second superconductor forming step, are formed. By doing so, it is possible to manufacture a Josephson junction element having less variation in characteristics, stable characteristics, and high reliability.

【0073】

Further, according to the invention of claim 2, by removing the insulating layer and removing a part of the first superconducting layer, a barrier layer is formed on the altered layer very close to the surface formed at the time of removal. It is possible to form a uniform barrier layer over a wide range of the substrate, and it is possible to suppress variations in Josephson junction elements.

【0074】

Further, according to the inventions of claims 3 to 9, in the first step of forming the second superconducting layer, a film is formed over a wide range of the substrate under the condition that the energy of the vapor-deposited molecules or particles becomes uniform. By doing so, the characteristics of the Josephson junction element, especially the critical current I_cIt is possible to suppress the variation of the above over a wide range of the substrate, and it is possible to manufacture a Josephson junction element having stable characteristics and high reliability.

【0075】

Further, according to the invention of claim 10, in the first step of forming the second superconducting layer, a film is formed over a wide range of the substrate under the condition that the energy of the vapor-deposited molecules or particles becomes uniform. , Josephson junction element characteristics, especially I_cVariation can be suppressed over a wide range of substrates, and 100-junction I_cThe variation of 1σ can be suppressed to 8% or less, and a Josephson junction element with stable characteristics and high reliability can be realized.

[Simple explanation of drawings]

[Figure 1]

It is a figure which shows the manufacturing process of the Josephson junction element of 1st Example.

[Figure 2]

It is a figure which shows the arrangement of a substrate, a target, etc. by a laser ablation method.

[Fig. 3]

It is a figure which shows the arrangement of the substrate, the target, etc. by the off-axis RF sputtering method.

[Fig. 4]

It is a figure which shows the arrangement of the substrate, the target, etc. by the laser ablation method which concerns on 5th Example.

[Fig. 5]

It is a figure which shows the arrangement of the substrate, the target, etc. by the laser ablation method which concerns on 6th Example.

[Fig. 6]

It is a figure which shows the manufacturing process of the Josephson junction element of 7th Example.

[Explanation of symbols]

10, 20 Josephson junction element 11, 21 boards 12, 22 First superconductor 13, 23 Interlayer insulation layer 14, 24 resist 15, 25 Second superconductor 16, 26 barrier layer 100 boards 101, 107 target 102 Deposition source 103, 105 plume 104 Obstacles