JPS63224273A - Josephson junction element and its manufacture - Google Patents

Abstract

PURPOSE:To make it possible to form a plurality of junctions with different current density on the same substrate, by applying Ta which is hard to oxidize in the air to a material for forming a tunnel barrier. CONSTITUTION:As a material to form a tunnel barrier, tantalum Ta and the like are used which are rather difficult to oxidize in the air at a normal temperature. A region for forming a junction in a Ta layer 13 formed on a lower electrode 12 is selectively masked by a resist 15, and the exposed part is anodized to form a comparatively thick oxide film 14. The region forming a junction is oxidized by thermal oxidation or an oxidation method using oxygen plasma, and a comparatively thin oxide film 16 as a tunnel barrier is formed. By repeating selectively the oxidation of the desired region for forming a junction, a plurality of junctions having a tunnel barrier of different thickness can be formed on the same substrate.

Description

[Detailed Description of the Invention] [1 Already Required] Materials that are relatively difficult to oxidize in the atmosphere at room temperature, such as tantalum (Ta), can be used instead of conventional aluminum (AI) as a material for forming the tunnel barrier. Use. In the TQFJ formed on the lower electrode, a region where a junction is to be formed is selectively masked with a resist, and the exposed portion is oxidized to form a relatively thick oxide film. The junction formation region is oxidized by thermal oxidation or an oxidation method using oxygen plasma to form a relatively thin insulating film as a tunnel barrier. By further selectively repeating oxidation of a desired junction formation region, a plurality of junctions having tunnel barriers with different thicknesses (that is, current densities) can be fabricated on the same substrate.

[Industrial application field]

The present invention relates to Josephson junctions, and in particular to a method of fabricating Josephson junctions with different current densities on the same substrate, which is considered to be a necessary technology for the practical application of integrated circuits using Josephson elements.

[Conventional technology]

As a typical example of conventional technology, niobium (Nb) is used as an electrode material.
),) Aluminum oxide (AlOx) is used as a tunnel barrier material.
) is used to fabricate a Josephson junction with an Nb/AlOx/Nb structure.

FIGS. 8(a) to 8(c) are cross-sectional views in the manufacturing process of a Josephson junction having the above structure. 8th
As shown in Figure (a), for example, silicon (St) &
On the plate 31, a first NbN layer for forming the lower electrode
32, A10xlJ3 for forming tunnel barrier
3. A second Nb layer 34 forming the upper electrode,
+l [lt stacking without breaking the vacuum. Here, A10'xF133 is formed by thermally oxidizing (at room temperature) an Al film produced on NbJi332 in low pressure oxygen. NbFJ32 and 34 and AlOxlOx layer type 3
The Al film for this purpose may be formed using a well-known technique such as sputtering or vapor deposition.

By the way, the density of the current flowing through the Josephson junction is A10xJ'!, which forms the tunnel barrier. Determined by the thickness of i33. In order to obtain the current density required for the circuit, an AlOx layer must be formed as thin as about 10 layers. However, the Δl film is extremely susceptible to oxidation, and the Δl film formed when exposed to air at room temperature for several seconds
Even the lOx layer is too thick. For this purpose, the Al film formed on the Nb eyebrow 32 is oxidized at room temperature by low pressure oxygen gas introduced into the vacuum chamber after the film is formed. The thickness of the oxide film produced at this time is controlled by the pressure of oxygen gas and the oxidation time. After oxidation treatment is completed,
Immediately add Nb to the surface to form the upper electrode.
[It will be covered by 34.

Next, as shown in FIG. 8(b), the second Nb eyebrow 34
After masking the surface corresponding to the region on which the bond is to be formed with a resist film 35, the exposed Nb layer 3 is removed.
4, for example, a mixed gas of carbon tetrafluoride and oxygen (CF4
Reactive ion etching (RI
B) for selective removal. In this case, Al0x
Because ffi33 does not react with CF4 gas, the above RI
Examples of the etching gas and applied power in the above RIE, which also acts as an etching stopper in E, are 7 Pa and 50 W, respectively. In this way, the upper electrode 341 having an area equal to the bonding area is formed.

Subsequently, the Nb layer 32 is processed into a lower electrode 321 having a shape as shown in FIG. 8(C). That is, the upper electrode 34
1 and a predetermined area around it, the surface corresponding to the lower electrode 321 is selectively masked with a resist, and the exposed AlOx layer 33 and the Nb[32
is selectively removed, for example, by sputtering mode etching using argon (Ar) ions. After processing the lower electrode 321, the resist is removed and an insulating WJ3G made of silicon oxide (Si02), for example, is formed over the entire surface of the substrate 31. The formation of the insulating layer 36 is as follows:
For example, a known high frequency sputtering method may be used.

After this, as shown in FIG. 8(d), on the insulating layer 36,
A contact hole 37 for connecting a wiring layer (not shown) formed on the insulating Ji 36 and the upper electrode 341 is provided.
Formed using standard photoringraph technology. Subsequently, Nb[
is deposited and a wiring pattern is formed using normal photolithography techniques. In FIG. 8(0), code 38
is the distribution layer A connected to the upper electrode 341.

[Problem that the invention seeks to solve]

As can be easily inferred from the above, the process for determining the bonding area shown in FIG. 8(b) is particularly important. However, the irg of the Nb layer 34 (see FIG. 8(a))
, and variations in the conditions in RIE for selectively removing Nb[34, etc., the Nb layer 34 remains without being completely removed in locations other than the bonding formation region, making it impossible to obtain a predetermined bonding area. No, or Al
Since 0xFf33 is not formed uniformly in the first place, its function as an etching stopper in selectively removing the Nb layer 34 is incomplete, and there is an inconvenience that the part of the Nb layer 32 that should become the lower electrode 321 is etched. arise.

As a result, there is a problem in that the characteristics of a large number of Josephson junctions formed on the silicon substrate 31 vary greatly, making it difficult to obtain integrated circuit chips of desired quality at a high yield.

In the above-mentioned conventional technology, a particularly important problem is that, as mentioned above, the AI film for forming the tunnel barrier is easily oxidized, so the Nb/Al0X/Nb structure is
The formation must be continued in a vacuum chamber without breaking the vacuum. For this, on one substrate, cells of different thicknesses and therefore of different current densities,
It was not possible to form multiple tunnel barriers. Solving this problem has been one of the important challenges in realizing integrated circuits using Josephson junctions.

[Means for solving problems]

The problems with the above-mentioned conventional Josephson junction are the process of forming four layers to form the lower electrode on the substrate, the process of forming a tantalum (Ta) layer on the conductor layer, and the process of forming the TaJW. forming a resist pattern for selectively masking each region corresponding to a plurality of junctions to be formed; a step of anodizing a part of the forming conductor layer; a step of oxidizing the Ta layer in the region where the bond is to be formed; If necessary, after TarrI in a plurality of regions where junctions are to be formed is oxidized, the Ta layer in a predetermined region is further selectively oxidized. This problem is solved by the method for manufacturing a Josephson junction of the present invention, which is characterized by being oxidized to

[Effect]

Since the periphery of the bonding region determined in the initial resist patterning step is covered with a relatively thick oxide film, the positions and bonding areas of the plurality of bondings on the substrate do not cause errors in subsequent steps.

Furthermore, since the tunnel barrier is formed using a material such as Ta that is not easily oxidized in the atmosphere at room temperature, it is possible to form junctions with different current densities.

〔Example〕

1(a) to g) show embodiments of the present invention,
FIG. 3 is a cross-sectional view of a Josephson junction manufacturing process. As shown in FIG. 1(a), for example, an Nb layer 12 and a TaJi layer 13 are formed in this order on a silicon substrate 11. As shown in FIG. The film formation method in this case may be either sputtering or vapor deposition, but in this example, both layers were formed using DC magnetron sputtering. Further, both layers do not necessarily need to be formed successively without breaking the vacuum.

The above magnetron sputtering conditions are as follows: Nb layer 1
For 2, applied voltage cuff, 5 W/cd, deposition rate 2
00 nm/win, while for TaJi)i13, the applied power was 0.4 W/cJ, and the deposition rate was 10 nm/win.
min % and the gas and its pressure are in each case Ar and 1.3 Pa. In addition, NJy has Nb112 of 200 nm.

The Ta layer 13 is 1.0 to lo, Ona+.

Next, as shown in FIG. 1(b), a region above the Ta layer 13 corresponding to the junction to be formed is covered with a resist pattern 15.
, and the T exposed around this junction formation area.
oxidize by a known anodic oxidation method. (Appl.
eid Physics Letters 48+25
4 (1986)) In the same figure, 14 is an oxide film formed by anodic oxidation, and its thickness is 20 to 10.
0 na+. Therefore, 7 in the area
afm is oxidized throughout the layer thickness. Furthermore, a portion of the Nb layer 12 below may be oxidized.

After removing the resist pattern 15, as shown in FIG. 1C, the surface of the Ta layer remaining in the junction formation region is oxidized to form a relatively thin oxide film 16 constituting a tunnel barrier. As the oxidation method used in this case, thermal oxidation and high frequency oxidation using oxygen plasma are suitable. Thermal oxidation treatment conditions include, for example, io in an oxygen atmosphere.
o~300'C15~60 minutes, and the thickness of the oxide film 16 is controlled by the above temperature and oxygen pressure in the atmosphere. Further, the high frequency oxidation treatment conditions are, for example, 0 applied power.
．． 5 W/C! ! , oxygen pressure 10 Pa, processing time 5 W
The thickness of the oxide film 16 is controlled by the above-mentioned applied power and oxygen pressure.

In addition, by reversing the steps in FIG. 1(b) and (C), Ta
After forming the oxide film 16 on the entire surface of the Ji 13, it may be selectively masked with the resist pattern 15, and the exposed portions around it may be anodized to form the anodic oxide film 14.

Next, NbJi is deposited on the entire surface of the silicon substrate 11. The thickness of this Nb layer is about 100 to 200 na+, and the deposition method and conditions are the same as those for forming the NbFJ 12 for the lower electrode. This Nb layer is shown in Figure 1(d).
As shown in FIG. 3, processing is performed so that the upper electrode 17 covering the bonding region determined by the oxide film 16 remains. In this process, the portion of the surface of the Nb layer deposited as described above corresponding to the upper electrode 17 is masked with a resist, and the exposed portion is removed, for example, by RIB using a mixed gas consisting of CF4 + 5%02. It is done by doing. The size and shape of the upper electrode 17 to be formed are as follows:
Provided in the insulating layer above the upper electrode 17 in a later step,
It is determined by considering the position of the contact hole.

Subsequently, the anodic oxide film 14 and N
bJi 12 is selectively removed to form a lower electrode 121 as shown in FIG. 1(6). In this case, processing is performed on the upper electrode 17 and the lower electrode 1 in a predetermined area around it.
The surface corresponding to 21 is masked with a resist, and the exposed portion is removed, for example, by etching in sputtering mode using argon (Ar) ions.

After that, as shown in FIG. 1(f), the silicon substrate 1
For example, the entire surface of 1 is made of 5i02 and has a thickness of 40O.
An absolute U layer 18 of about n+++ is deposited, and an insulating layer 18 is further formed.
A contact hole 181 is formed in iiflB. In this case, the processing method includes, for example, resist mask and 12
A known dry etching method using 1E is used.

The size and position of contact hole 181 are not substantially affected by the size and position of the bonding area. In particular, even if the junction region determined by the oxide film 16 is as small as 1 μm or less, the contact hole 181 can be set to any size as long as it falls within the range occupied by the upper electrode 17.

Further, the position of the contact hole 181 does not need to be directly above the bonding region, and can be placed at any position within the range occupied by the upper electrode 17. FIG. 4 shows an example of a contact hole 182 formed at a position away from directly above the bonding region.Other reference numerals in the same figure are shown in FIGS.
is the same as

Next, Nb1li is deposited on the entire surface of the silicon substrate 11. The thickness of this NbJi is about 500 to 600 na+, and the deposition method and conditions are the same as those for forming the Nb layer 12 for the lower electrode. This Nb[ is shown in Figure 1 (g
) is processed into a wiring layer 19 as shown in FIG. This processing method and conditions may be the same as those for processing the upper electrode 17.

In this way, a Josephson junction is formed on the silicon substrate 11. Further, by the above method, a plurality of Josephson junctions having the same tunnel barrier thickness, that is, the same current density, can be fabricated in parallel on the silicon substrate 11.

FIGS. 2(a) to 2(a) are cross-sectional views in the process of producing Josephson junctions with different current densities on the same substrate. In the same manner as in FIGS. 1(a) to 1(c), an Nb layer and a Ta layer are sequentially deposited on the silicon substrate 21, the region where the junction is to be formed is masked with a resist, and the exposed TaJi is anodized. . Thereafter, the resist mask is removed, and the Tarf4a surface in the junction formation region is oxidized to form a thin oxide film. In this way, Figure 2 (a
), an oxide film 26 is formed on the silicon substrate 21.
Accordingly, a plurality of (two in the figure) bonding areas are determined.

In FIG. 2(a), ζ, reference numeral 22 is an Nb layer for forming a lower electrode, and 24 is a Ta anodic oxide film around the junction region.

In order to fabricate junctions with different current densities, one of the oxide films 26, for example, the oxide film 261 shown in FIG.
Increase the thickness of. For this purpose, as shown in the figure,
The surface of the silicon 3Fi 21 other than the oxide film 261 is masked with a resist 25.

In this state, the TaJi existing under the oxide film 261 in the exposed portion is oxidized. As the oxidation method in this case, the above-mentioned thermal oxidation or high frequency oxidation may be used.

Thereafter, the resist 25 is removed and NbTll1 is deposited on the entire surface of the silicon substrate 21. This Nb layer was processed by the same method as the process shown in FIG. 1(d), and the process shown in FIG. 2(C)
As shown in FIG.
form. The subsequent steps, such as processing the Nb layer 22 to form the lower electrode, forming an insulating layer and contact holes, and depositing and patterning Nbi for wiring, are as follows:
This is similar to FIGS. 1(e) to (g).

Note that in the steps leading up to obtaining the structure shown in FIG. 2(a), after the Ta layer for forming the tunnel barrier is oxidized, the Ta around the junction formation region may be anodized. That is, as shown in FIG. 5(a), Ta
A relatively thin oxide film 56 is formed by oxidizing the entire surface of )
As shown in FIG. 2, a relatively thick anodic oxide film 54 is formed around the junction region determined by the oxide film 5G. Thereafter, in the same manner as in the step of FIG. 2(b), oxidation of the oxide film 561 in one junction region is proceeded to form a thick tunnel barrier. In addition, in FIGS. 5(a) and 5(b), numerals 51 and 52 represent the silicon substrate and NbJ, respectively.
It is g.

In the above embodiments, the upper electrode and the lower electrode are not limited to Nb, and one or both of them may be made of other relatively high melting point superconductor material, such as niobium nitride (
NbN). Further, the conductive material for forming the tunnel barrier is not limited to Ta, and may be formed from other materials that are relatively difficult to oxidize in the atmosphere.

〔Effect of the invention〕

According to the present invention, by using Ta, which is difficult to oxidize in the atmosphere, as the material for forming the tunnel barrier, the anodic oxidation for determining the junction area and the oxidation for forming the tunnel barrier are performed to form a Ta layer. After that, the oxidation can be carried out by taking it out into the atmosphere, making it possible to change the degree of Ta oxidation depending on the location within the same substrate. the result,
The present invention has the effect of making it possible to form a plurality of junctions with different current densities on the same substrate. Further, according to the present invention, since the area of the upper electrode is no longer involved as a determining factor in the bonding area, it is possible to reduce variations in bonding characteristics caused by processing accuracy of the upper electrode, and to improve manufacturing yield. There is. Furthermore, in the Josephson junction structure of the present invention, the size and position of the contact hole for connection to the upper electrode can be determined for the upper electrode having an area larger than the junction area, and it depends on the size and position of the junction. Restrictions are greatly reduced, resulting in greater freedom in circuit design and improved manufacturing yield.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIGS. 1(a) to g) show embodiments of the present invention,
Cross-sectional views in the process of manufacturing Josephson junctions FIGS. 2(a) to c) show other embodiments of the present invention, and are cross-sectional views in the process of manufacturing Josephson junctions with different current densities on the same substrate. , Figure 8(a) or e
4 is a sectional view showing another embodiment of the present invention, and FIG. 5 is a sectional view showing still another embodiment of the present invention. In the figure, 11, 21, 31 and 51 are silicon substrates, 12, 22, 32, 34 and 52 are Nb layers, 13, 23 and 53 are Ta layers, 14, 24 and 54 are anodized films, 15 and 25 are 35 is resist, 16, 26, 56, 261 and 561 are oxide films, 17
and 27 and 341 are upper electrodes, 1 and 36 are insulating layers, 19 and 38 are wiring layers, and 33 is A10xF3. 37, 181 and 182 are contact holes, 121 and 32
1 is a lower electrode. ! 771 7 Fufan ep4
Ko Shidarizohi J's job) '/n insertion. Mouse 3 1'B

Claims (1)

[Scope of Claims] (1) A tunnel barrier made of an insulating layer formed by passivating the surface of a layer made of a conductive material that is not substantially oxidized by air at room temperature to a predetermined thickness; and an insulating layer formed by passivating the surrounding conductive material layer. (2) The Josephson junction device according to claim 1, wherein the conductive material is tantalum (Ta). (3) The Josephson junction device according to claim 2, characterized in that it has an upper electrode and a lower electrode, at least one of which is made of niobium (Nb). (4) The Josephson junction device according to claim 2, characterized in that it has an upper electrode and a lower electrode, at least one of which is made of niobium nitride (NbN). (5) The Josephson junction device according to claim 2, wherein the tunnel barrier is tantalum oxide formed by oxidizing the conductive material layer of Ta. (6) The Josephson junction element according to claim 2, wherein the insulating layer surrounding the tunnel barrier is tantalum oxide formed by oxidizing the conductive material layer made of Ta. . (7) The Josephson junction device according to claim 1, further comprising an upper electrode that includes the tunnel barrier and extends over the insulating layer surrounding the tunnel barrier. (8) forming a conductor layer for forming a lower electrode on the substrate; forming a layer made of tantalum (Ta) on the conductor layer; and forming a layer made of tantalum (Ta) on the Ta layer. forming a resist pattern for selectively masking each region corresponding to a plurality of junctions to be formed, and forming a resist pattern for forming the Ta layer in a portion not masked by the resist pattern and the lower electrode immediately below the Ta layer. a step of anodizing a part of the conductor layer; a step of oxidizing the Ta layer in the region where the junction is formed; and an upper surface of the oxidized Ta layer surrounding the region where the junction is formed. 1. A method for manufacturing a Josephson junction element, comprising the step of forming a conductor layer for forming an electrode. (9) After the Ta layer in a plurality of regions where the junctions are formed is oxidized, the Ta layer in a predetermined region is further selectively oxidized. The method for manufacturing the Josephson junction device described in Section 1. (10) The method for manufacturing a Josephson junction element according to claim 8, wherein at least one of the conductor layers forming the upper electrode and the lower electrode is made of Nb or NbN.