JP2707531B2 - How to make Josephson junctions - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention makes it possible to recognize the occurrence of defective products before completion of the fabrication of Josephson junctions,
More desirably, it is possible to suppress the characteristic deterioration of the fine Josephson junction to be manufactured, thereby improving the yield.
About the method.

[0002]

2. Description of the Related Art A Josephson junction device, which is formed by sandwiching a thin tunnel barrier layer between a lower superconductor layer and an upper superconductor layer and is used as an active device in a cryogenic environment, typically includes the following devices. It is manufactured through such steps.

In each of the following drawings, the left side shows a sectional view up to that step, and the right side shows a plan view up to that step,
First, as shown in FIG. 2A, a lower superconducting layer 21, a barrier layer 22 serving as a tunnel barrier layer of a Josephson junction, and an upper superconducting layer 23 are sequentially laminated on an appropriate substrate 20 to form a laminated structure ( 21 + 22 + 23). Although a conductive ground plane may be formed between the substrate 20 and the lower superconductor layer 21, it is omitted because it is not involved in the present invention. Next, as shown in FIG. 2B, in order to cut out the lower superconductor layer 21 into a predetermined planar shape, a resist layer 24 patterned according to the predetermined planar shape is formed on the upper superconductor layer 23. Then, using this as an etching mask, dry etching (generally, anisotropic dry etching with high dimensional accuracy) is performed, and first, as shown in FIG. 2C, a laminated structure (21 + 22 +
Cut out the whole of 23) into a predetermined plane shape. After a series of insulating films 25 are formed on this structure as shown in FIG.
By immersing in an organic solvent, the so-called lift-off method is used to remove the resist layer 24 together with the insulating film 25 formed thereon,
As shown in FIG. 2E, the surface of the upper superconductor layer 23 is exposed again.

Next, as shown in FIG. 3A, a bonding area defining resist 27 having a predetermined plane area, for example, one of two orthogonal sides W1 and the other W2 is placed on the upper superconductor layer. 23 is formed by patterning. At this time, if the predetermined planar shape is a square, naturally, W1 = W2. When the upper superconducting layer 23 is dry-etched to the surface of the barrier layer 22 using the resist 27 as an etching mask (generally, also anisotropic dry etching which can obtain a high precision) with respect to the laminated structure as described above, FIG. As shown in B), it is possible to obtain a laminated structure (21 + 22 + 23) in which the upper superconductor layer 23 is cut into a predetermined planar shape. The area of the barrier layer 22 and the area of the lower superconductor layer 21 thereunder are much larger than the area of the cut-out upper superconductor layer 23, but the effective area as a Josephson junction is these three layers 21, 22, Since 23 is defined by the overlapping area, if only the upper superconductor layer 23 is cut into a predetermined area as described above, the area becomes the effective area of the Josephson junction to be finally manufactured. .

However, the structure shown in FIG.
That is, the laminated structure after dry etching (21
+ 22 + 23) is given a special reference numeral 10 for the sake of convenience when describing the drawbacks of the conventional manufacturing method described later here, or also when describing the present invention. Name it “etched sample”. Also, in the following, simply referring to “sample 10” also means that FIG.
This indicates the laminated structure (21 + 22 + 23) with the patterned resist 27 in the state shown in FIG.

After obtaining the etched sample 10, FIG.
As shown in (A), a series of oxide films 28 are deposited while leaving the bonding area defining resist 27, and then immersed in an organic solvent to remove the resist 27 by a so-called lift-off method. As shown in FIG. 4 (B), the surface of the upper superconductor layer 23 cut into a predetermined plane shape is exposed again.

Although the process of fabricating a Josephson junction may be considered substantially up to this point, in general, a completion of a Josephson junction device is seen through a wiring layer forming process following this. That is, for the structure of FIG.
After etching to an appropriate depth for surface cleaning as required, a wiring-forming superconductor layer 29 for the upper superconductor layer 23 is formed as shown in FIG. 5A.
Next, in order to finally cut out the wiring configuration superconductor layer 29 into a desired predetermined shape or wiring pattern, FIG.
As shown in FIG. 5C, a patterning resist 30 having a corresponding shape is formed, and the superconducting layer 29 is dry-etched using this as an etching mask. As shown in FIG. 5C, a corresponding planar shape or a corresponding wiring pattern is obtained. Can be obtained. Thereafter, if the patterned resist 30 is removed, a Josephson junction element having a fine Josephson junction is finally completed. However, in the process of FIG. 5, a lift-off method can be adopted instead of the etching.

[0008]

However, when the electrical characteristics of many of the fine Josephson junctions manufactured as described above are taken, even if they have undergone the same manufacturing process, there are variations in the characteristics. In some cases, the current-voltage characteristics are greatly degraded in the so-called subgap voltage region from zero V to the gap voltage, and this phenomenon is reduced to a junction area of 1 μm square or less. It became more noticeable. Then, when the cause was examined, it was found that the cause was as shown in FIG.

That is, as described above, FIG.
3B, the effective area of the Josephson junction to be finally formed is defined by the dry etching process of the upper superconductor layer 23 from FIG. 3B to FIG. 3B. The etched sample 10 shown in FIG.
), The upper superconductor layer 23
If dry etching is not sufficiently performed on the portion that is in direct contact with the barrier layer 22 and the upper superconductor layer 23 as schematically shown in an enlarged portion surrounded by a virtual line circle in FIG. Residue 31 that cannot be completely etched may be formed at a portion in contact with the surface of the tunnel barrier layer along the periphery of the lower edge. In this case, the residue 31 is extremely thin and acts as an amorphous upper superconductor layer.Therefore, it is as if the upper superconductor layer 23 itself was formed into a moderate shape from the beginning, and a part thereof was made too thin. It is the same as the result of having lost it, and the desired design expectation cannot be obtained.

[0010] However, conventionally, even if such a consciousness was present, there was no concrete means itself for recognizing the problem before the present invention. In fact, the inventor came to think about the cause of the above-mentioned characteristic deterioration, but did not actually confirm the residue 31 visually at first, but
This is because the longer the process time of the dry etching process from (A) to FIG. 3 (B) was, the less the characteristic deterioration was. In other words, it was considered that the longer the length, the higher the probability that the residue 31 was completely removed.

The present invention has been made in view of such a fact, and even if the area of a Josephson junction is reduced from 1 μm square to a submicron square or less, it is necessary to preliminarily prepare the intermediate stage before the completion of the fabrication process. It is possible to recognize the occurrence of defective products, avoid the inconvenience of repeating the subsequent steps, and more desirably, more positively remove the cause of joints that may result in defective products. it is possible to prevent deterioration of the characteristics of the Josephson junctions are fabricated, as can improve the manufacturing yield as a result, concrete residue observation means together, Josephson junction manufacturing method may make a countermeasure against dry etching residue Is to be provided .

[0012]

According to the present invention, in order to attain the above object, a laminated structure is formed by sequentially laminating a lower superconductor layer, a tunnel barrier layer, and an upper superconductor layer. Is etched by anisotropic dry etching (hereinafter, simply referred to as "dry etching") which can achieve high dimensional accuracy, and cuts out the upper superconductor layer into a predetermined planar shape. A method of manufacturing a Josephson junction including a step of, after completion of the dry etching, using a scanning electron microscope to dry-etch the portion of the upper superconductor layer that contacts the surface of the tunnel barrier layer along the periphery of the lower edge. The present invention proposes a method for manufacturing a Josephson junction including an observation step of observing whether or not there is a residue as a result of the above.

The present invention further proposes, in addition to the above basic structure, a method of: i) performing dry etching again when a residue is confirmed in the above-mentioned observation step using a scanning electron microscope. ii) A method is also proposed in which after dry etching, the dry-etched laminated structure is transported to a region observed by a scanning electron microscope without breaking vacuum.

The present invention also proposes that the tunnel barrier layer in the above-mentioned laminated structure is made of a material having resistance to dry etching and functioning as an etching stop layer. In this regard, when the upper superconductor layer to be dry-etched is made of a niobium or niobium nitride-based material, a material for a tunnel barrier layer that also functions as an appropriate etching stop layer when dry-etching such a material is used. As magnesium oxide or aluminum oxide.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an example of manufacturing a Josephson junction according to the present invention will be described with reference to FIG. However, the present invention relates to the addition of a new elementary process, rather than a modification to each elementary step itself in a series of steps in a known existing Josephson junction manufacturing method that has already been adopted. Therefore, if we also make effective use of the description of the conventional example,
The bonding area defining resist 27 in the state shown in FIG.
Until a laminated structure (21 + 22 + 23) is obtained, FIG.
The series of steps shown in (E) may be followed, and these steps are not particularly modified in the present invention. Therefore, a series of steps up to this point are denoted by reference numerals in FIG.
As indicated by 100, they are collectively handled as “pre-process”.

[0016] However, in the "pre-step" for defining the planar area of the finally obtained should Josephson junction, the junction area defined resist area = W1 × W2 on the upper superconducting layer 23
27 was patterned, after obtaining the laminated structure (21 + 22 + 23) shown in FIG. 3 (A), Dora which is shown in FIG. 1 (A)
By the etching apparatus 11 , the laminated structure (21 + 22 + 23)
The upper superconductor layer 23 is dry-etched to a depth reaching the barrier layer 22, which is also anisotropic dry-etching with high dimensional accuracy. This step is shown simply as “dry etching” step 101 in FIG.
The apparatus in FIG. 1A also shows a high-frequency power supply 14 attached to this type of anisotropic dry etching apparatus.

[0017] As a result of after the dry etching process 101, but, etched sample 10 already shown in FIG. 3 as described (B) is obtained, in the present invention after this step, FIG. 1
As shown in (B), the observation step 10 by the scanning electron microscope 12
Enter 2. To this end, as shown by the linking line A from the step 101 to the step 102 and the corresponding solid line A in FIG. Must be transported to the observation area.

[0018] Therefore, Nozomu as is shown in FIG. 1 (A)
More preferably, a dry etching apparatus 11 and a scanning electron microscope
As a member for forming a transfer passage that can transfer the sample 10 without breaking vacuum between the two devices 1 and 2 by the vacuum layer 13
1 and 12 are combined. In this case, the etching apparatus 11
After taking out the sample 10 and putting it in the observation area to be placed in the vacuum environment in the scanning electron microscope 12, the trouble of evacuating the observation area again becomes unnecessary, including the subsequent steps. Besides shortening the time and improving the efficiency of all processes,
The sample 10 can be protected from contamination, and the quality of the produced Josephson junction can be prevented.

[0019] Samples 10, housed in the scanning electron microscope 12 in the observation area, by a scanning electron microscope 12, the peripheral portion of the upper superconductor layer 23 is observed. As is well known and as schematically shown in FIG. 1A, a scanning electron microscope irradiates an observation sample while scanning an electron beam 16 to emit secondary electrons from the observation sample. If the upper superconducting layer 23 of the sample 10 is about 1 μm square, the upper superconducting layer is observed by observing the sample 10 at a magnification of 5,000 to 10,000 times. If there is a dry etching residue 31 as shown in FIG. 6 around the lower edge of the upper superconductor layer at the portion where the body layer 23 and the barrier layer 22 are in direct contact, the residue 31 is obtained by the scanning electron microscope 12. The image is clearly displayed as a "dark portion" in the obtained two-dimensional image.

[0020] That is, the surface of such residue 31 is not obtained becomes the parallel clean surface on the substrate surface as generally shown in FIG. 6, it at an angle, so more easily also becomes irregular surface, release The resulting secondary electrons are scattered, and as a result, are captured as dark areas in the scanning electron microscope image.

[0021] On the other hand, it does not go well in the other observation means. For example, in an optical microscope, the maximum magnification is at most about 1000 times. In this case, even if a fine upper superconductor layer 23 ranging from about 1 μm square to less than 1 μm square is magnified to about 1 mm square at the maximum magnification, even the finer residue 31 remaining around it can be visually recognized. Impossible. Further, even with an existing etching monitor using plasma spectroscopy, such a minute residue 31 cannot be observed. The scanning electron microscope 12 is limited as the observation means in the present invention for such practical reasons. In addition, the scanning electron microscope 12
Then, the maximum magnification still has a sufficient margin, so even if the dimensions of the upper superconductor layer 23 and thus the dimensions of the Josephson junction are further reduced from the order of submicron squares, this will be dealt with. be able to.

In this way, as a result of observation by the scanning electron microscope 12, if there is no dry etching residue 31, as shown in FIG. 1B, after the observation step 102,
For example, the “post-process” as described above with reference to FIG.
You can move on to 103. On the other hand, if the residue 31 is confirmed in the image of the scanning electron microscope 12, for example, the sample 10 may be determined as a defective product to stop the addition of the post-process at that time, or the next production After the lot, it is possible to take measures such as making the dry etching time by the dry etching apparatus 11 slightly longer. Even with such an effect alone, the generation of the residue is overlooked as in the related art, and the post-processes shown in FIG. 4 and subsequent figures are unnecessarily continued to produce a defective product. This is sufficiently significant in view of the inconvenience of continuing dry etching for an extremely long time.

[0023] However, such a residue 31 is by no means impossible to remove defective factors. Therefore, the scanning electron microscope 12
When the residue 31 was observed as a result of observation by
As shown by a process link line B in FIG. 1B and a virtual arrow B in FIG.
It is preferable that the dry etching apparatus 11 is returned to the dry etching apparatus 11 and dry etching is performed again for a suitable time. afterwards,
If necessary, the re-dry-etched sample 10 may be observed again by the scanning electron microscope 12 to observe whether the residue 31 has been removed. Of course, such a process can be repeated until residue 31 is gone,
The process may be shifted to the post-process 103 after it is confirmed that there is no 31.

[0024] However, among which repeated the above observations with respect to many samples, depending on the amount and degree of residue 31, how much time, if Hodokose additional dry etching, such residue 31 can be taken in Ippen Can be learned empirically.

[0025] However, this way, according to the present invention given that might repeated dry etching, it must strictly not control the dry etching time for each dry etching per each time it is at the manufacturing site It becomes a big burden. Therefore, in order to alleviate such inconvenience, the barrier layer 22 located under the upper superconducting layer 23 to be dry-etched is resistant to dry etching of the upper superconducting layer 23. Desirably, it is composed of a material that can function as a stop layer. If so, the dry etching time can be set somewhat longer, with the emphasis on the certainty for removing the residue 31 without the risk of the thickness of the barrier layer 22 deviating significantly from the design dimensions. .

[0026] For example, when the upper superconducting layer 23 is niobium-based material typified by niobium (Nb) or niobium nitride (NbN) (which is the same material also lower superconducting layer 21 generally at this time), Usually, the upper superconductor layer 23 is processed by anisotropic plasma dry etching using a carbon fluoride gas, that is, carbon fluoride (CF ₄ ) gas or CF ₄ + O ₂ gas in which oxygen is mixed with carbon fluoride. However, as a material that functions well as an etching stop layer under the dry etching conditions at this time, and that completely functions as the tunnel barrier layer 22,
Magnesium oxide (MgO) can be mentioned.

[0027] That is, looking like a concrete dry etching parameters example, FIG. 1 13.56 MHz the frequency of the high frequency power source 14 in the dry etching apparatus 11 (A), the high-frequency power output density 0.1 W / cm ^2, device When the internal pressure is set to 200 mTorr and the dry etching of the Nb upper superconducting layer 23 is attempted by the CF ₄ gas introduced into the apparatus 11, the etching rate of the Nb layer 23 is about 300Å / min, The etching rate of the barrier layer 22 is at most 1-22 / mi
Only about n. Therefore, it is understood that the MgO layer 22 has a sufficient function as an etching stop layer.

Further , such an MgO layer a) does not adversely affect the upper and lower superconductor layers sandwiching the MgO layer electrically and physically. B)
Even if it is an extremely thin film, it has a sufficient insulation function, has few so-called pinholes, and has good coverage and adhesion. C) It has good thermal cycle characteristics, and it has an extremely low temperature and etching process temperature (generally 100 to 200 ° C). D) can be easily etched if necessary, and in particular, can be easily etched even with the same carbon fluoride gas as described above by changing the conditions. E) Sputtering The film can be easily formed by a vapor deposition method, and a non-special ordinary method such as a lift-off method can be used for removal.
The tunnel barrier layer sandwiched between the upper and lower superconductor layers 21 and 23 of the Josephson junction because it has various excellent features
22 is the best.

[0029] However, this is not limiting, the beginning of aluminum oxide (Al ₂ O _3), according to the dry etching conditions in the outer, barrier layer materials that can function as a suitable etch stop layer, each selection can do.

In the above embodiment, the upper superconductor layer to be dry-etched by the dry-etching apparatus 11 is used.
On the upper surface 23, a resist 27 for defining a junction area defining a rectangular pattern having an area dimension W1 × W2 is formed in accordance with a planar area shape to be finally obtained.In the presence of the resist 27, the upper superconductor layer 23 is formed. Was cut out uniformly into the corresponding rectangular planar shape, but this can be changed to a two-step process. That is,
First, a stripe-shaped resist having a width W1 is formed, and the upper superconductor layer 23 is dry-etched into a stripe having a width W1,
Thereafter, a second stripe-shaped resist having a width dimension W2 is formed in a direction orthogonal to the width direction, and again, the upper superconductor layer 23 having a stripe shape is dry-etched, thereby obtaining an area dimension W1 × W2. The upper superconductor layer 23 having a rectangular pattern defined by the following equation may be obtained. At this time, an observation step using the scanning electron microscope 12 can be added according to the present invention after completion of each dry etching step. If a residue 31 is found, additional dry etching can be performed again. As described above, the method of cutting out the upper superconductor layer 23 of the rectangular pattern as a result using the respective striped resists is a method suitable for obtaining a fine rectangular pattern and a pattern having sharp corners.

[0031]

According to the present invention, during the process of manufacturing a Josephson junction, after the upper superconductor layer of the junction is cut into a predetermined planar shape by dry etching, the generation of dry etching residues that cause defects is generated. Can be known in advance. Therefore, the inconvenience of adding a post-process needlessly can be avoided, and significant information can be obtained regarding the dry etching time required for dry-etching the upper superconductor layer during the next manufacturing process.

In addition , according to a specific embodiment of the present invention, the residue remaining around the lower edge of the upper superconductor layer can be removed by dry etching again.
Deterioration of the characteristics of the manufactured Josephson junction can be prevented in advance (residual defects are repaired), and as a result, the production yield can be improved.

Furthermore, according to still another aspect of the present invention,
Even if an observation step using a scanning electron microscope is added according to the basic requirements of the present invention, a sample can be transported without breaking vacuum between an observation area in the scanning electron microscope and the dry etching apparatus. Therefore, it is possible to avoid the problem of sample contamination without greatly deteriorating the efficiency of the manufacturing process.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram relating to a characteristic portion of a method of the present invention.

FIG. 2 is an explanatory diagram of an initial stage of a manufacturing process of a Josephson junction.

FIGS. 3A and 3B are explanatory views of a state before and after cutting an upper superconductor layer of a Josephson junction into a predetermined planar shape by dry etching.

FIG. 4 is an explanatory diagram of a process leading to completion of a single Josephson junction.

FIG. 5 is an explanatory diagram of a step of attaching a wiring layer to an upper superconductor of a Josephson junction.

FIG. 6 is an explanatory diagram of a residue that can be generated as a result of dry etching.

[Explanation of symbols]

10 Etched sample, 11 Dry etching equipment, 12
Scanning electron microscope, 13 Vacuum chamber for forming sample transport passage,
21 Lower superconductor layer, 22 Tunnel barrier tank, 23 Upper superconductor layer, 27 Resist for defining junction area.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masaaki Maezawa 1-1-4 Umezono, Tsukuba, Ibaraki Pref. Inside the Research Institute of Electronics and Technology (72) Inventor Susumu Takada 1-4-1 Umezono, Tsukuba, Ibaraki Pref. (56) References JP-A-62-165379 (JP, A) JP-A-1-188685 (JP, A) JP-A 64-32629 (JP, A)

Claims (5)

(57) [Claims] After forming a laminated structure by sequentially laminating a lower superconductor layer, a tunnel barrier layer, and an upper superconductor layer, the upper superconductor layer is anisotropically extended to a depth up to the tunnel barrier layer. A method for producing a Josephson junction including a step of conducting anisotropic dry etching and cutting out the upper superconductor layer into a predetermined planar shape; after completion of the anisotropic dry etching, using a scanning electron microscope, A step of observing whether or not there is a residue resulting from the anisotropic dry etching left in a portion in contact with the surface of the tunnel barrier layer along a periphery of a lower edge of the tunnel barrier layer; How to make a son junction. 2. The method according to claim 1, wherein when the residue is confirmed in the observation step, the anisotropic dry etching is performed again. 3. The method according to claim 1, wherein after the anisotropic dry etching, the laminated structure is anisotropically dry-etched toward the region to be observed by the scanning electron microscope. Transporting the vacuum without breaking vacuum. 4. The method according to claim 1, wherein the tunnel barrier layer is made of a material having resistance to the anisotropic dry etching and functioning as an etching stop layer. Features method. 5. The method according to claim 4, wherein the upper superconductor layer to be subjected to anisotropic dry etching is made of a niobium or niobium nitride-based material; and the tunnel barrier layer is made of magnesium oxide or aluminum oxide. A method characterized by: