JPH0680845B2 - How to make a Josephson device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a method for producing a Josephson device, and in particular,
The present invention relates to an improvement in a cutting method in which individual minute Josephson elements are cut out from one large Josephson junction structure in accordance with a predetermined pattern.

<Prior Art> As is well known, the Josephson element is generally made of silicon Si.
On a suitable substrate, such as a lower electrode (base electrode), a tunnel barrier, and an upper electrode (counter electrode).
Electrodes) are laminated in order, but the manufacturing method can be roughly divided into two streams.

One is to rely on the lift-off method, which has a relatively long history in the production of semiconductor integrated circuits.

However, according to this method, in the photo resist exposure and development process for each layer formation, the residual resist removal process at the time of lift-off, etc., the layer portion that has already been formed under the layer is contaminated. However, even if a cleaning step is continuously added, it is impossible to perform cleaning in the atomic layer order, so that the physical and electrical properties of the individual tunnel barrier layers of each Josephson device are dispersed. And deterioration had occurred.

On the other hand, as a second method, the method of making Josephson devices, in which the manufacturing process of each layer can be performed continuously without breaking the vacuum and, in principle, a fairly good tunnel barrier can be obtained, There is a so-called "cutting method" which the applicant disclosed in Japanese Patent Application No. 57-60571.

The details are described in Japanese Patent Application Laid-Open No. 58-17683, which relates to the application, but this document will be briefly described with reference to FIG.

First, as shown in FIG. 3 (A), a planar lower electrode layer 12 made of a suitable superconductor material such as a lead alloy system or niobium system is formed on a large area of the silicon substrate 11, generally on the entire surface. To do.

Then, a planar tunnel barrier layer 13 and a planar upper electrode layer 14 are successively formed thereon in this order, and as shown in FIG. 3 (B), a structure 15 that can be regarded as one large Josephson junction is formed. I will make it.

In particular, at this time, the apparatus used for forming the planar lower electrode layer 12 first, such as a vacuum vapor deposition apparatus or a high frequency sputtering apparatus, is similarly used in the subsequent formation of each layer, and each step is semi-finished. Without releasing the product from the inside of the device, in other words, without breaking the vacuum inside the device,
The process up to the diagram (B) is designed to be a series of continuous processes.

Therefore, the tunnel barrier layer 13 inside the structure 15 has no room for exposure to problems such as contamination in the manufacturing process, and is extremely uniform and favorable.

In this conventional method, the integrity of the tunnel barrier layer 13 is ensured in this way, and then individual minute Josephson elements are cut out from the structure 15 shown in FIG. 3 (B). It was However, even if it says cutting, it is not by physical cutting, but by photolithography.

In other words, an appropriate photo film is formed on the entire surface of the planar upper electrode layer 14.
After applying the resist 16, the photo resist film 16 is formed according to a predetermined pattern so as to leave the resist portion 16 corresponding to the area required for the lower electrode of each Josephson element after completion.
The resist 16 is exposed and developed.

In FIG. 3 (C) showing the result, only one remaining resist portion 16 is shown, but in general, the substrate
Since a large number of Josephson elements are integrated on the layer 11, the residual resist portion 16 is also left in a plurality of places according to the plan layout pattern planned for these large numbers of Josephson element groups.

After this, preferably, dry etching is performed to remove unnecessary portions of the planar upper and lower electrode layers and the tunnel barrier layer to obtain the structure shown in FIG. 3 (D). Of the remaining portions 12 ', 13', 14 'of each layer remaining at this point, that of the lower electrode 12' is equivalent to the area of the lower electrode necessary for the device to be finally formed. Become.

However, the remaining portion 14 'of the upper electrode and the corresponding remaining portion 13' of the tunnel barrier layer 13 are further reduced in area as necessary.

In that case, the residual resist portion 16 in the previous step is removed, the newly applied resist is patterned, and as shown in FIG. 3 (E), the resist portion 17 having an area necessary for the upper electrode is formed. Of the upper electrode remaining portion 14 'and the remaining portion of the tunnel barrier layer 13' are removed to obtain the structure shown in FIG. Then, as shown in FIG. 3 (G), the Josephson device 18 having the required structure having the tunnel barrier layer 13 ″ having a predetermined area and the upper electrode 14 ″ having a predetermined area is formed.
To get

<Problems to be Solved by the Invention> As described above, according to the method for producing the Josephson element shown in FIG. 3, each step of the large Josephson junction structure 15 before cutting out each Josephson element 18 according to a predetermined pattern. Then
Indeed, it is possible to obtain a good and uniform planar tunnel barrier layer 13, which is in fact the relevant Josephson junction structure.
It has been confirmed from 15 voltage-current characteristics.

Further, since the continuous process is adopted and the cleaning process in the intermediate process can be omitted, this conventional method has an advantage of improving productivity.

However, when each Josephson element cut out from this structure 15 was closely inspected, it was good depending on the material and thickness of each layer forming the structure 15 and especially on the lower electrode. What appeared to be the result that seemed to have caused some problems with the supposed tunnel barrier 13 ″.

For example, take a look at FIG. FIG. 4 (A) shows the upper and lower planar electrode layers at the stage of FIG. 3 (B) with a layer thickness of about 2000.
Å Niobium Nb, aluminum oxide Al
_The current-voltage characteristics (100) of the composition 15 composed of ₂ O ₃ formed in series by the above-mentioned conventional cutting method on the same silicon substrate, each having 2.5 μm mouth size Josephson devices. The IV characteristic) is an accurate trace of the oscilloscope photograph.

As a matter of fact, in the characteristics obtained in this element array, the hysteresis characteristic peculiar to this kind of Josephson element as it should be is not seen.

For example, there is no concept of the critical current value Io, and the transition from the zero voltage state (the existence of itself is not clear) to the voltage state that should be defined by the gap voltage Vg follows a gradual ohmic curve. There is.

Also, the gap voltage Vg is about 2.8 mV per unit in the Josephson element of the above material, so it is multiplied by 100 to be about 280 mV, but it is actually read accurately. It is impossible to do so, and it must be seen that the point at which the voltage state generally starts to return to the zero voltage state or the origin O, which is generally called a Knee point, does not exist in this characteristic.

FIG. 4 (B) uses the same thickness, material, and manufacturing method as the Josephson element having the characteristics shown in FIG. 4 (A), except that the plane dimension is changed to 10 μm per piece. Is also the current-voltage characteristic obtained by creating 100 in series on the same silicon substrate.

With this characteristic, a hysteresis characteristic shape peculiar to this kind of Josephson element has been created, the critical current value Io can be read as about 560 μA, and the gap voltage Vg is also clear to some extent 280
It can be read as about mV, and the existence of the knee point Pk can be recognized.

However, it is by no means desirable. It is generally considered that the knee point Pk is desired to be sharply bent. However, the characteristic shown in the figure still indicates a gentle arc, and a gentle arc is formed. The corners are also relatively large. This suggests that the tunnel barrier contains ohmic properties that should not be inherent.

Thus, according to the conventional method as shown in FIG. 3, a very good and uniform Josephson tunnel barrier layer is obtained at the stage of the structure 15 as one large Josephson junction. And then, cutting out individual Josephson elements, it is said that those advantages may be lost from the completed Josephson elements.
In some cases, strange results can occur.

The present invention recognizes this fact and provides a solution therefor.

In other words, the present invention resides in the method of making Josephson shown in FIG. 3 which is excellent in principle, and is intended to correct the drawbacks thereof and to extend the advantages thereof.

<Means for Solving Problems> In order to achieve the above-mentioned object, the present inventor first sought from various viewpoints the cause of the characteristic deterioration due to the cutting.

As a result, it was found that the deterioration of the characteristics, which appears when the individual Josephson elements 18 are cut out by photolithography from the large Josephson junction structure 15 as described above, seems to be caused when the structure 15 is formed. The mechanical stress accumulated in the structure in relation to the silicon substrate 11, that is, the internal stress may be one of the causes.

For example, when the large Josephson junction structure 15 is formed on the entire surface of the substrate 11 up to the stage of FIG. 3 (B),
In some cases, the substrate may be warped to the extent that it can be visually observed. This clearly indicates that there is a physical working relationship between the substrate and the planar lower electrode layer directly on it, resulting in the accumulation of extremely large stresses within the structure. There is.

Therefore, if this hypothesis is correct, then
When the individual Josephson elements are cut out by applying lithography, the area of the individual Josephson elements is extremely small compared to the area of the substrate. As each layer left by the etching is physically released, the internal stress released as energy at once becomes considerably large, and the strain is serious in the tunnel barrier layer. It can be understood as having a significant physical and electrical effect.

After thinking up to this point, when we actually examined a number of Josephson devices from this point of view, we obtained the result that this hypothesis has considerable credibility. Rather, it can be asserted that this internal stress release phenomenon is, if not all, at least one of the major factors that cause deterioration of the characteristics of the created Josephson device. On the contrary, according to this, it is possible to well explain that the deterioration of the characteristics occurred even when the individual Josephson elements were cut out from one large Josephson junction structure.

From this point of view, the simplest measure for reducing the stress is to reduce the thickness of the lower superconductor layer formed on the substrate. That is, the idea is that the thinner the layer, the smaller the stress accumulated inside.

However, for example, in the Nb-based Josephson element, if the thickness of the lower electrode is set to 1000 Å or less, the transition temperature of the superconductor
The phenomenon that Tc decreases may occur. Such a phenomenon is a problem that can occur not only in this material, although the specific thickness value changes. In other words, the lower limit is often set for the thickness of the lower electrode.

Therefore, as described above, it is practically impossible to arbitrarily thin the lower electrode only for the purpose of reducing the internal stress.

The present inventor proposes a method for producing a Josephson device by the following steps as an improvement over the conventional cutting method, based on the knowledge from various directions.

(Step 1) On the substrate, at each predetermined location where a plurality of Josephson elements should be formed in the future, a plane dimension that is at least not smaller than the plane dimension required for each Josephson element. And having a suitable thickness and forming part of the lower electrode of each of the Josephson elements,
A pedestal that is dedicated to each of the Josephson elements and that is independent from the other Josephson elements is formed.

(Step 2) As described above, the lower superconducting layer, the tunnel barrier forming insulating layer, and the upper superconducting layer are successively formed on the global area portion on the substrate on which the plurality of independent pedestals are formed. And stacked to form one large Josephson junction structure.

(Step 3) Each of these Josephson junction structures is etched according to a desired pattern, and one on each of the above-mentioned pedestals, each Josephson element including a lower electrode, a tunnel barrier, and an upper electrode each having a predetermined plane size. To create.

<Operation> According to the present invention by the above process groups 1 to 3, each pedestal formed on the substrate is a part of the bottom electrode of each finally formed Josephson device. Therefore, if the individual pedestals dedicated to each Josephson element are made sufficiently thick in this way, the lower superconducting layer formed thereon can be made much thinner at the element forming portion. .

However, even though each pedestal forms a part of the lower electrode of each dedicated Josephson element, it does so only in the portion where the individual Josephson element is formed. It is not used as a wiring layer while maintaining its thickness. If so, as a result, a plurality of Josephson elements are formed on one pedestal, and such a structure is not included in the scope of the present invention.

In any case, according to the present invention, since the lower superconducting layer formed on each pedestal can be made thin, one large Josephson without causing the disadvantage of lowering the superconducting transition temperature Tc of the completed Josephson device. It is possible to avoid the problem of internal stress when forming the Son-junction structure.

That is, in order to form a thick pedestal, a stress accumulation relationship is created between the pedestal forming material layer and the substrate, and as a result, when each pedestal is individually cut out by etching or the like, stress is not applied to each pedestal. Even if the strain occurs due to the opening of the above, it does not reach the superconducting layer for the lower electrode formed on each of these pedestals since this effect has already been completed.

The lower superconducting layer for the lower electrode may be sufficiently thin as described above, so that the global area portion on the substrate,
For example, even if it is formed in a series on the entire surface of the substrate, the stress accumulated inside becomes small due to the physical working relationship with the substrate, and then the element formation Therefore, even if the stress is released by reducing the cutting process such as etching, no significant strain is generated.

In addition, even if internal stress is accumulated to some extent in the superconductor layer for the lower electrode, the pedestal already formed as a member having sufficient physical strength is subjected to this stress. Acts as a general resistance object.

Therefore, even when individual minute Josephson elements are cut out from a large Josephson junction structure by an appropriate etching means, the effect of releasing the internal stress does not occur, or at least is minimized. The tunnel barrier layer in each individual Josephson device is not adversely affected.

<Embodiment> FIG. 1 shows a preferred embodiment of a method for producing a Josephson device according to the present invention.

First, as shown in FIG. 1 (A), the physical area of a Josephson device that will be created in the future will be formed on a large area (generally the entire surface of the substrate) of a suitable substrate 20, which is usually represented by silicon Si. In order to form the pedestal 22 that also serves as a support base and a part of the lower electrode, the pedestal forming material layer 21 is formed as a starting layer by an appropriate method such as vapor deposition or high frequency sputtering.

In the example here, niobium Nb is used as the material of the upper and lower electrodes, and aluminum oxide Al ₂ O _{3 is used as} the material of the tunnel barrier.
The description will be made assuming that the Josephson device selected is created.

The Josephson element of this combination is easily affected by the release of internal stress due to the physical action relationship with the silicon substrate, particularly when it is produced by the conventional method shown in FIG.

Therefore, the material of the pedestal forming material layer 21 is
It is preferable to select the same Nb as the electrode material. However, another material may be used as long as it is electrically and mechanically familiar to the lower electrode formed thereon by the procedure described later.

Then, this pedestal forming material layer 21 is etched by a suitable etching method, preferably dry etching, according to the plane arrangement pattern on the substrate of each Josephson element, and at a predetermined location where each Josephson element will be formed in the future, Form the pedestal 22.

Although only one pedestal 22 is shown in FIG. 1 (B) showing the result, of course, since a large number of Josephson devices are generally formed on the substrate 21, this pedestal 22 is formed.
22 will be formed by that few minutes.

The plane dimension Wb and the thickness tb of the pedestal 22 will be explained again when the Josephson element is finally completed on it.If the thickness tb is thick, stress release between this pedestal and the substrate will occur. Depending on the relationship, the pedestal may be physically distorted.

However, even in such a case, this effect does not affect the respective layer structures sequentially formed by the steps described later. This is because, when the pedestal 22 is cut out, this stress relief becomes a problem that has already been completed.

As shown in FIG. 1 (B), if a pedestal 22 is formed at a predetermined position on the surface of the substrate 21, then, FIG.
As shown in FIG. 2, preferably, the vacuum of the apparatus is not broken, and the lower superconductor layer 31 and the tunnel barrier forming insulating layer 32 are continuously formed by the same vapor deposition apparatus or high frequency sputtering apparatus.
The upper superconductor layer 33 is sequentially formed on the large area portion of the substrate 21, generally on the entire surface.

For example, if Nb is sputtered to form the lower superconducting layer 31, an aluminum Al film is sputtered on the lower superconducting layer 31 without breaking the vacuum of the sputtering apparatus, and then oxygen is introduced into the apparatus. By introducing and oxidizing the Al film, an Al ₂ O ₃ film is formed, and this is used as a tunnel barrier forming insulating layer 32 as a starting layer to be a tunnel barrier of each Josephson device in the future.

After exhausting oxygen, Nb is further sputtered on the Al ₂ O ₃ film by the same device to form the upper superconducting layer 33 to obtain the Josephson junction structure 34 shown in FIG. 1 (C). .

That is, the structure 34 configured up to this stage,
It can be thought of as one large Josephson junction on substrate 21.

In the steps up to this point, except that the pedestal 22 is formed, the same method as the conventional Josephson device manufacturing method shown in FIG. The insulating layer 32 as a tunnel barrier layer in the Josephson junction structure 34 can exhibit extremely uniform and good electrical and physical properties.

However, up to the conventional process, in the case of the present invention, the pedestal
Since 22 will form a part of the lower electrode of the Josephson element created in the future, the thickness of the lower superconducting layer 31 formed on the pedestal 22 should be sufficiently thinner than the conventional one. You can

For example, if the lower electrode has conventionally required a thickness of about 2000 Å, when the present invention is applied, the thickness tb of the pedestal 22 is tb.
Is about 1500Å, and the thickness of the lower superconducting layer 31 is 500Å
You can make it as thin as possible. In other words, it is possible to design the thicknesses of the both to be approximately equal to the thickness of the lower electrode in the conventional element.

As a result, even if the lower superconducting layer 31 is in direct contact with the silicon substrate 20 in the portion where the pedestal 22 is not present, if the lower superconducting layer 31 can be made sufficiently thin in the first place, it is large inside. There is little risk of stress buildup. Even if it is accumulated, it will be small.

On the other hand, the thickness of the tunnel barrier forming insulating layer 32 may be about the same as the conventional one, for example, about 30 Å, and the thickness of the upper superconductor layer 33 may be about 2000 Å, which is about the same as the conventional one. . This is because it is considered that a stress relationship that causes a problem does not occur between themselves.

Once one large Josephson junction structure 34 has been constructed as described above, a predetermined Josephson junction structure 34 is formed so that each Josephson element is formed on each pedestal 22. Cut out according to the pattern.

To this end, if general photolithography is appropriately used, after applying an appropriate photoresist layer on the upper surface of the upper superconductor layer 33, as shown by phantom lines in FIG. 1 (C), In this case, the patterned residual resist portion 40 is left only in the area of approximately the same size of the pedestal 22, and the other portions of the Josephson junction structure 34 are preferably dry-etched using this as an etching mask. Figure (D)
As shown in FIG. 5, a Josephson element 60 including a lower electrode 51, a tunnel barrier 52, and an upper electrode 53, which are sequentially stacked on the pedestal 22, is obtained.

If it is necessary to further reduce the area of the tunnel barrier 52 and the upper electrode 53, the patterned residual resist portion 41 is formed again as shown by a virtual line in FIG. 1 (D). Then, using this as an etching mask, unnecessary portions of the tunnel barrier 52 and the upper electrode 53 may be removed to obtain a Josephson element 60 having a cross-sectional configuration as shown in FIG. 1 (E).

As for the cutting out, that is, the etching in the process from the step shown in FIG. 1 (C) to FIG. 1 (D), FIG. 1 (E), as long as the method of the present invention is used, the internal stress causes
The obtained tunnel barrier 52 is not adversely affected. For the reasons described above, the lower superconducting layer for forming the lower electrode
This is because the physical working relationship between the substrate 31 and the substrate 20 is not such that a stress that causes a problem is generated.

The effectiveness of the method of the present invention will be demonstrated with specific examples of characteristics.

For comparison, except that the pedestal 22 is provided, a Josephson element group having the same size, same material, and same number as the Josephson element corresponding to the fourth illustrated characteristic previously described in the description of the conventional example is formed. I made it by the invention.

That is, FIG. 2 (A) shows a pedestal thickness of 1500Å, a lower electrode thickness of 500Å, and thus a lower electrode thickness of a Josephson device having the characteristics shown in FIG. 2.5 μm Nb / Al ₂ O ₃ / Nb with a lower electrode structure of the same thickness
The current-voltage characteristics obtained by forming 100 Josephson devices in series on the same silicon substrate are shown. As with the characteristic example of the conventional example, we also made an effort to trace the oscilloscope photograph as accurately as possible.

As is clear from a comparison between this figure and FIG. 4 (A), in the Josephson element example to which the present invention is applied, a remarkable improvement effect is recognized.

The characteristic shown in FIG. 4 (A), which is far from the desired hysteresis characteristic only by the conventional method, is
The characteristic shown in FIG. 2A as a result of applying the present invention appears with clear hysteresis, and the knee point Pk can also be clearly identified. Moreover, it has a fairly excellent sharpness.

By the way, in FIG. 2 (A), the critical current Io can be read as about 20 μA, and the gap voltage Vg is about the theoretical value.
It can be clearly read as 280 mV.

Similarly, FIG. 2 (B) shows an example of characteristics obtained under exactly the same conditions as above except that only the plane dimension is changed to 10 μm.

This characteristic example is also sufficiently satisfactory. About 80
The transition region from the critical current Io of 0 μA to the voltage state is regarded as a phenomenon with almost no ohmic component, and the knee point Pk is sharp enough to be seen as a “point” which is almost literally. . The return path from the knee point Pk to the origin O is also sufficiently close to the voltage axis. The gap voltage Vg is of course the theoretical value of about 280 mV.

Further, the extremely good quality of the tunnel barrier can be understood from the fact that the absolute value of the critical current Io is considerably improved as compared with that shown in FIG. 4 (B).

In the above embodiment, the Josephson device to be created
The type is Nb / Al ₂ O ₃ / Nb, but of course, it is not limited to this. As apparent from the principle of the present invention, as a result of the physical interaction relationship with the substrate, stress is generated between the lower electrode and the lower superconductor layer for forming the lower electrode, and this is released. The pedestal structure according to the invention can be used without hesitation whenever the Josephson element thus cut out may have problems with its properties.

Further, in the figure, the pedestal 22 is shown to be removed from the component part of the Josephson element 60, but as already mentioned, since this pedestal forms a part of the lower electrode 51, the pedestal is shown. Of course, it may be considered that one Josephson element is included including up to 22.

Further, as for the protective layer and the wiring layer generally arranged for the Josephson element 60, the existing method may be adopted as they are, and therefore, these are not shown in FIG.

Finally, for reference, Nb / A in the above embodiment
When the total thickness of the lower electrode structure of the l ₂ O ₃ / Nb type Josephson element is about 2000Å, the pedestal thickness Wb is less than the thickness of the lower superconducting layer for forming the lower electrode. However, the effect of providing a pedestal did not appear to be significant unless it was more than doubled. From this, it is understood that, on the contrary, an optimum design value can be selected for the pedestal thickness depending on the thickness and the thickness of the material used.

<Effects of the Invention> According to the present invention, it is possible to eliminate the drawbacks of the Josephson fabrication method by the cutting method and to extend only its advantages.

That is, it is possible to overcome the problem of strain generation due to release of internal stress while maintaining the principle that an extremely good tunnel barrier can be obtained.

Moreover, the great effect as described above can be obtained by the simple method of simply adding the pedestal structure to the existing cutting method.

[Brief description of drawings]

FIG. 1 is a process diagram of a preferred embodiment of the Josephson device manufacturing method of the present invention, FIG. 2 is a characteristic diagram relating to two examples of Josephson device manufactured by the method of the present invention, and FIG. 3 is a conventional cutting method. Process diagram of Josephson fabrication method, 4th
The figure is a characteristic diagram concerning two examples of the Josephson device produced by the conventional method shown in FIG. In the figure, 20 is a substrate, 21 is a pedestal forming material layer, 22 is a pedestal, 31
Is a lower superconductor layer, 32 is an insulating layer for forming a tunnel barrier, 33
Is an upper superconducting layer, 34 is a Josephson junction structure, 50 is a lower electrode, 51 is a tunnel barrier, 52 is an upper electrode, and 60 is a completed Josephson device.

Claims (1)

[Claims] 1. A flat surface on a substrate at which a plurality of Josephson elements are to be formed in the future, at least not smaller than a plane dimension required for each of the Josephson elements. Having a size and having an appropriate thickness, forming a part of the lower electrode of each of the Josephson devices, dedicated to each of the Josephson devices, and shared with other Josephson devices. A pedestal independent of each other; a lower superconducting layer, an insulating layer for forming a tunnel barrier, and an upper portion on the global area portion of the substrate on which the plurality of independent pedestals are formed. A step of successively superposing superconducting layers to form one large Josephson junction structure; etching the Josephson junction structure according to a desired pattern, and forming each of the above-mentioned pedestals. One by one, each lower electrode of said predetermined planar dimensions, tunnel barrier, and a step of creating the respective Josephson elements consisting of an upper electrode; method of creating Josephson device characterized by comprising a.