JPH0530310B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for patterning an Nb-based Josephson junction element that operates at extremely low temperatures, and particularly to a method for patterning a three-layer film constituting the junction.

[Conventional technology]

Conventional Nb-based Josephson junction elements are disclosed in Japanese Patent Application Laid-open No. 58
− As described in Publication No. 176983, the lower electrode
Nb film, tunnel barrier layer AlO _x film, upper electrode Nb
A method has been used in which a three-layer film is continuously formed without breaking the vacuum, and then a desired bonding and wiring pattern is formed by dry etching using a resist as a mask. However, in conventional three-layer etching, the upper and lower electrodes are
The Nb film was processed by reactive ion etching using CF ₄ gas, and the AlO _x film of the tunnel barrier layer was processed by ion etching using Ar gas. For this reason, the AlO _x film of the tunnel barrier layer serves as a mask at the pattern edge of the lower electrode, resulting in a structure in which undercuts are likely to be formed. Furthermore, when attempting to form a bonding pattern with the Nb film of the upper electrode after this, the pattern edge of the lower electrode and the underlying insulating film are exposed before etching, so undercutting progresses further. The thickness of the insulating film also decreased due to etching. For this reason, it has been difficult to completely cover the pattern edge of the lower electrode when backfilling the etched portion with an insulating film after processing the bonding pattern. As a result, short circuits often occur between the lower electrode and the electrode connection wiring, which hinders circuit operation.

[Problems that the invention attempts to solve]

FIG. 2 shows a pattern forming process of a Nb/AlO _x /Nb three-layer film formed by a conventional method.

That is, in FIG. 2a, on the substrate 21
Lower electrode 22 made of Nb film, tunnel barrier layer
AlO _x 23, Nb/Nb of the upper electrode 24 made of Nb film
A three-layer film of AlO _x /Nb was deposited by sputtering. Next, in FIG. 2b, AZ1350J resist (trade name manufactured by Hoechst, USA)
A resist pattern 25 including a bonding portion and wiring was formed on the upper electrode 24 using a method. Then the second
In Figure c, first, the upper electrode 24 is made of AlO _x 23
Process by reactive ion etching using CF ₄ gas until the surface is exposed.

Next, the tunnel barrier layer AlO _x 23 is processed by ion etching using Ar gas, and when the surface of the lower electrode 22 is exposed, the lower electrode 22 is etched again by reactive ion etching using CF ₄ gas.
Processing was continued until the Nb film was completely removed. FIG. 2d is a cross-sectional view of the three-layer film pattern after resist removal. As is clear from the figure, within the dotted circle, the AlO _x 23 serves as a mask and the lower electrode 22 forms an undercut. Also, undercuts are observed in the upper electrode 24 due to the etching of the lower electrode 22. Therefore, with such a cross-sectional shape, the pattern edges are likely to be incompletely covered by the insulating film, which significantly reduces the reliability of the interlayer insulating film. For this reason, in patterning a three-layer film, there has been a strong demand for a patterning method that has a structure that prevents the formation of undercuts.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for forming a pattern of a Josephson junction element having a structure in which a direct short circuit cannot occur between a lower electrode and an upper electrode wiring via an interlayer insulating film.

[Means for solving problems]

The above purpose is to deposit only the Nb film of the lower electrode,
This is then achieved by processing the pattern including the wiring and the bonding portion by taper etching. After that, the Nb pattern surface of the lower electrode was coated with Ar
It is cleaned by sputter cleaning with gas, and the AlO _x film for the tunnel barrier layer and the Nb film for the upper electrode are continuously deposited without breaking the vacuum. Next, a resist mask for defining the junction area is formed on the Nb film of the upper electrode, and etching is performed using CF ₄ gas to obtain the desired Josephson junction element.

[Effect]

In the present invention, since the AlO _x film is deposited on the entire surface of the wafer, there is no risk of over-etching, and there is an advantage that the process can have a wide margin. That is, since the lower electrode pattern is tapered, the coverage of the AlO _x film is sufficient, making it possible to completely eliminate undercuts in the lower electrode pattern.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.

Figure 1 shows Nb/AlO _x /
This figure shows the pattern formation process for a Nb-based three-layer film.

First, in FIG. 1a, a lower electrode 12 made of a Nb film was formed on a substrate 11 by sputtering. Next, in FIG. 1b, a resist pattern 13 including a bonding portion and wiring was formed on the lower electrode 12 using AZ1470 resist (trade name, manufactured by Hoechst, USA). Then, in Figure 1c, CF ₄ +10
Taper etching of the lower electrode 12 was performed by reactive ion etching using a mixed gas of % O ₂ . At this time, the taper angle tends to become smaller by increasing the amount of O ₂ gas relative to CF ₄ gas. Next, in FIG. 1d, the resist was removed using acetone to form a pattern for the lower electrode 12. Etching conditions were set so that the taper angle at this time was 30 to 40 degrees. Next, in FIG. 1e, the surface of the Nb pattern of the lower electrode 12 is coated with Ar.
After completely removing the contaminant layer and oxide layer by sputter etching using gas, the AlO _x film 14 of the tunnel barrier layer is deposited, and then the Nb film of the upper electrode 15 is continuously deposited without breaking the vacuum. A three-layer film constituting a Josephson junction was then formed. As is clear from the figure, within the dotted circle, the lower electrode 12
Because the Nb pattern is tapered,
It can be seen that the AlO _x film 14 and the Nb film of the upper electrode 15 are continuous films. Therefore, this
If the three-layer film structure is Nb/AlO _x /Nb, the AlO _x film will serve as a gas stopper material when processing the next bonding pattern, and the lower electrode 12 can be prevented from being over-etched. That is, the problems of undercutting of the Nb pattern of the lower electrode 12 and overetching of the underlying substrate 11 are solved. This method solves the conventional problem of insufficient coverage of the interlayer insulating film and completely eliminates short circuits between the connection wiring of the lower electrode and the upper electrode.

FIG. 3 shows the manufacturing process of the Nb/AlO _x /Nb type Josephson junction device formed according to the present invention.

First, in Figure 3a, the board has a diameter of 50mmφ.
A Si substrate 31 was used. Note that an interlayer insulating film of SiO is deposited on the Si substrate 31 to a thickness of 200 nm.

This Si substrate 31 was inserted into a sputtering device, and a 200 nm thick Nb film, which would become the lower electrode 32, was deposited by DC magnetron sputtering. The adhesion conditions are
The Ar pressure was 0.6 pa and the deposition rate was 3 nm/sec.

Next, after taking out the Si substrate 31 from the sputtering apparatus, first, a resist pattern including a bonding portion and wiring was formed under the following conditions. AZ1470 resist (trade name, Hoechst, USA) was formed by spin coating to a thickness of 1.6 μm. Next, after prebaking at 90℃ for 20 minutes, UV light with a light intensity of 6mW/ ^cm2 was used for 12 hours.
Pattern transfer was performed in seconds. Next, at a liquid temperature of 24°C at a composition ratio of AZ developer: water = 1:1.
A developing process was performed for 60 seconds. Next, this Si substrate 31 was inserted into an etching apparatus for etching, and after the pressure was reduced, the lower electrode 32 was tapered by reactive ion etching using CF ₄ gas. Processing conditions are CF ₄ +20%
The pattern taper angle of the lower electrode 32 at this time, which was performed using a mixed gas of O ₂ at a pressure of 26 pa and a power of 100 W, was approximately 35 mm, taking into account the wiring pattern (2.5 μm).
The conditions were set so that the It was taken out of the etching apparatus again and the resist was removed with acetone. Next, it was inserted into the sputtering device again in order to form a three-layer film constituting a Josephson junction. Next, in order to completely remove the contamination layer and oxide layer on the Nb pattern surface of the lower electrode 32,
Cleaning treatment was performed by sputter etching using Ar gas. The conditions at this time were Ar pressure 0.8 pa, power 70 W, and processing time 20 minutes. Next, in order to form a tunnel barrier layer, the Al layer was moved to a layer directly below the Al target in the same sputtering device to form an Al film with a thickness of 5 nm.
It was covered. The Al deposition rate was 0.2 nm/sec. Al
After film formation, 100 pa of O ₂ gas was introduced into the sputtering apparatus and natural oxidation was performed for 40 minutes at room temperature (24 to 26° C.) to form an AlO _x film 33, which is a surface oxide film of Al.
After evacuating the inside of the sputtering device again, the Si substrate 31 was moved directly below the Nb target, and a 100 nm thick Nb film was continuously deposited as the electrode 34 using the DC magnetron sputtering method to form a three-layer film. . After taking out the Si substrate 31 from the sputtering apparatus, a resist pattern defining a bonding area was formed on the upper electrode 34 under the following conditions. AZ1470 resist was formed by spin coating to a thickness of 1.2 μm. Then,
After pre-baking at 90℃ for 20 minutes, 12
Pattern transfer was performed in seconds. Joint area is 1.8
× ^1.8μm2 . Next, in FIG. 3b, after inserting into the etching apparatus again and reducing the pressure,
The Nb film of the upper electrode 34 was patterned by reactive ion etching using CF ₄ gas under conditions of CF ₄ gas pressure of 26 pa and power of 100 W. At this time, even if over-etching is performed, the Nb pattern of the lower electrode 32 and the Si substrate 31 will not be damaged by etching since the AlO _x 33 is coated over the entire surface and serves as a stopper material. Next, in FIG. 3c, in order to backfill the etched portion with an insulating film using the resist on the bonding pattern as a mask, the film is inserted into the vacuum device again and the pressure is reduced. Backfilling was performed with an insulating film 36 made of Si and about 20% thick. In FIG. 3D, the film was taken out of the vacuum apparatus again and lifted off with acetone to completely backfill the etched portion to form an interlayer insulating film 36. Next, in FIG. 3e, the substrate was inserted into the sputtering apparatus again, and the surface of the bonding pattern of the upper electrode 34 was subjected to sputter etching under the same conditions as the surface cleaning of the lower electrode described above. Next, in order to perform connection wiring with the upper electrode 34.
A 300 nm thick Nb film was deposited to form a wiring electrode film.
The Nb film was deposited by DC magnetron sputtering under the same conditions as for the lower electrode 32 described above. again,
After taking it out from the sputtering apparatus, a resist pattern was formed under the same conditions as for the lower electrode 32 described above. Then, after inserting it into the etching apparatus again and reducing the pressure, the Nb film other than the resist pattern was etched away by reactive ion etching using CF ₄ gas under the same conditions as described above. Then again,
After removing it from the etching equipment, remove the resist on the pattern with acetone and remove the upper electrode 3.
A wiring electrode 37 connected to 4 was formed.

Through the above manufacturing steps, an Nb/AlO _x /Nb film Josephson junction device was manufactured. Note that in this example, Si was used as the insulating film for backfilling.
Similar effects can be obtained using SiO, SiO ₂ , Al ₂ O ₃ , MgO, Ge, MgF, etc. Further, although Nb is used as the superconducting thin film, the present invention is not limited to this, and similar effects can be obtained by using NbN, MoN, TaN, TiN, etc.

For example, 256 Josephson junctions of 1.7×1.7 μm ² connected in series can be formed with variations in superconducting critical current (Ic) within ±5 to 6%. For this reason,
It has also become possible to significantly improve the operating margin of the circuit.
Furthermore, in the present invention, most of the internal stress generated in the Nb film of the lower electrode is relaxed because the patterning is performed before the formation of the three-layer film, so that no slight deterioration of the bonding properties was observed.

Furthermore, even in a cruciform junction of 1.5 × 1.5 μm ²
The variation width of 256 ICs connected in series is ±3 to 4
%, there were no short-circuit failures between the electrodes, and results of bonding characteristics with good reproducibility were obtained.

〔Effect of the invention〕

According to the present invention, the short-circuit failure that occurs between the lower electrode and the upper electrode wiring via the interlayer insulating film, which has been a problem in the past, is completely eliminated by forming a taper in the lower electrode Nb pattern. This and the perfect coverage of the interlayer insulating film made it possible to form highly reliable Nb/AlO _x /Nb-based Josephson junction elements with good reproducibility.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the pattern formation process of the Nb/ _AlO
A diagram showing the pattern formation process of Nb _/ AlO _x /Nb-based three-layer film, FIG.
FIG. 3 is a diagram showing a manufacturing process of an Nb-based Josephson junction device. 11, 21, 31...Si substrate, 12, 22, 3
2... Lower electrode, 13, 25, 35... Resist pattern, 14, 23, 33... Tunnel barrier layer, 15, 24, 34... Upper electrode, 36... Insulating film, 37... Wiring electrode.

Claims (1)

[Claims] 1 (1) After forming the Nb film of the lower electrode on the substrate,
Step of forming a resist pattern including wiring and bonding on this Nb film, (2) Step of patterning the above Nb film into a tapered shape by dry etching, (3) Cleaning the surface of the above Nb film pattern with Ar sputtering. (4) Step of sequentially forming an AlO _x film as a tunnel barrier layer and Nb as an upper electrode on the Nb pattern and the substrate to partially form a three-layer film; (5) (6) forming a pattern on the upper electrode Nb film by dry etching; (7) using the resist remaining on the Nb film as a mask; A method for forming a pattern of a Josephson junction element, comprising the steps of: backfilling the etched portion with an insulating film; and (8) forming a Nb wiring electrode for connection with the upper electrode Nb film. 2. The method for forming a pattern of a Josephson junction element according to claim 1, wherein the taper angle of the tapered shape is in the range of 15 to 75 degrees.