JPH0228908B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of forming a Josephson junction.

[Prior art]

Usually, as shown in FIG. 1, in a tunnel type Josephson junction, after forming a lower electrode 2 on a substrate 1,
This surface is oxidized to form an oxide film 3 serving as a tunnel barrier, and then the upper all-poles 4 are formed. Here, oxide film 3
When using a high frequency plasma oxidation method for the lower electrode 2, the substrate 1 is formed as shown in FIG.
is mounted on the cathode 5 and discharged in a vacuum chamber 6 for high frequency plasma oxidation to oxidize the surface of the lower electrode, followed by vapor deposition of an upper electrode (not shown) in the same vacuum chamber 6.

The cathode 5 is an upper cathode 7 and a lower cathode 8
It consists of A lower cathode 8 accommodating a substrate 1 having a lower electrode 2 and an aluminum spacer 9 placed on the substrate 1 is attached to the upper cathode 7 with mounting screws 10 .

The high-frequency plasma oxidation vacuum chamber 6 is connected via a gate valve 11 to an evaporation vacuum chamber 12 equipped with an electron beam evaporation mechanism. Pump is installed.

The gate valve 11 is closed during high frequency plasma oxidation, but is opened during formation of the upper electrode. In addition,
In FIG. 2, reference numeral 13 indicates a high frequency power source, and reference numeral 14 indicates a resist covering a part of the lower electrode 2. It is known that in the high frequency plasma oxidation method, oxidation caused by oxygen plasma and reverse spatter caused by oxygen ions proceed simultaneously. Such a reverse sputter effect acts not only on the surface of the lower electrode on which the oxide film is to be formed on the sample wafer, but also on the surface of the cathode on which the sample wafer is mounted. JMBaker and othersIBM.
As shown in J.RES.DEVELOP. Vol. 24, p. 223 (1980), when high-frequency plasma oxidation is performed using a cathode with a lead alloy attached to its surface, tunnels are formed on the junction oxide film by reverse sputtering from the cathode surface. It is known that lead oxide, which has a high barrier, adheres and reduces the current density of the junction. RF Broom et al.
As shown in IBM.J.RES.DEVELOP Vol. 24, p. 206 (1980), such reverse sputtering materials with a high tunnel barrier make the oxide film very thin when attempting to form a junction with a high current density. Therefore, there was a problem of increased junction leakage current. Conventionally, in order to prevent a decrease in current density, In was evaporated on the cathode surface and the material that adhered to the junction oxide film was reduced to a material with a low tunnel barrier.
A method of using In ₂ O ₃ has been used.

However, in the method of vapor-depositing In on the cathode surface, when the upper electrode lead alloy is vapor-deposited to form a bond, the In and lead alloy react on the cathode surface and form an alloy. When attaching another substrate to the cathode after taking it out from the cathode to form a bond, it was necessary to remove the lead-In alloy on the cathode surface with acid, etc., and then use a cathode with newly vapor-deposited In. . Therefore, there was a drawback that the process was complicated and time consuming.

[Problems to be solved by the present invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for forming a Josephson junction that can reduce variations in current density in a junction and reduce leakage current in the junction without employing complicated steps.

[Means for solving problems]

The method for forming a Josephson junction of the present invention is characterized by forming an oxide film serving as a tunnel barrier by subjecting the surface of the lower electrode to high-frequency plasma oxidation using a cathode whose surface is coated with an organic resist.

〔Example〕

An embodiment of the present invention will be described in detail below with reference to the drawings. The tunnel-type Josephson junction obtained in this embodiment is manufactured in the same manner as described in the prior art with reference to FIG. Further, an oxide film which becomes a tunnel barrier by oxidizing the surface of the lower electrode is formed by high frequency plasma oxidation of the lower electrode in the same manner as described in the prior art with reference to FIG. Here, to form a bond, an alloy of Pd84, In12, Au4, wt% was used as the lower electrode material, and prior to high-frequency plasma oxidation at an oxygen pressure of 1 × 10 ^-2 Torr, 30 W, and 10 minutes, Ar pressure was applied. 5×10 ^-3
The surface of the lower electrode was spatter cleaned under the conditions of Torr, 100W, and 3 minutes. The upper electrode is Pd71, Bi29,
wt% alloy was deposited in the order of Pd and Bi. Experimental example 1 in the conventional case where the oxidation conditions were fixed to the above conditions and a cathode with Pd71, Bi29, wt% alloy deposited on the surface as shown in Figure 3; AZ1470 resist is 2μ
Experimental example 2 in the case of the present invention using a cathode coated with m thickness;
After forming a bond using the cathode in (2), a new 2 μm thick layer was added on the attached Pd71, Bi29, wt% alloy.
Experimental example 3 in the case of the present invention using a cathode coated with AZ1470 resist; As shown in FIG. , acetic acid 60,
The bonding characteristics of Experimental Example 4 in the case of the present invention in which a bond was formed by setting a wafer on the cathode which had been removed with an etching solution consisting of 139 parts of water and 1 part of hydrogen peroxide and returned to the same shape as shown in FIG. The comparison yields the following two results.

In FIGS. 3 to 5, 5 indicates a cathode, 15 indicates a resist, and 16 indicates a Pd-Bi alloy deposited during formation of the upper electrode.

(i) The current density of the junction is, in the case of Experimental Example 1,
While it was 500 A/cm ^{2 ,} in the methods of the present invention shown in Experimental Examples 2, 3, and 4, it was as large as 2 kA/cm ² .

(ii) While the variation in current density of the junction was ±15% in Experimental Example 1, it was reduced to ±10% in all of the methods of the present invention shown in Experimental Examples 2, 3, and 4.

It is known that junction leakage current increases in proportion to current density. Therefore, in order to compare the leakage currents of junctions with the same current density, we changed only the power during high-frequency plasma oxidation to 22W in the case of Experimental Example 1 above, and compared the junction of Experimental Example 5 with a current density of 2kA/ ^cm2 . Created.

As shown in Figure 6, the current-voltage characteristics of the Josephson junction are as follows: resistance value R _NN (4
(defined in mV) and the resistance value R _SG (defined in 2 mV) in the sub-gap region B. The leakage current of a junction is evaluated by the ratio of R _SG /R _NN , and it can be said that the larger the R _SG /R _NN value is, the smaller the leakage current is, and the higher the quality of the junction. In Fig. 6, 2, 3, and 4 are the above experimental examples 2, 3, and 4.
5 indicates Experimental Example 5. Same as 2, 3, 4
Comparing 5 with the R _NN value, 5 has a smaller resistance value in the sub-gap region (B) than 2, 3, and 4. Therefore, the R _SG /R _NN ratio is 12 of 2, 3, and 4. It was confirmed that it was as small as 8.

Note that the step of coating the cathode surface with resist,
In the step of newly coating the cathode surface with a new resist after forming the external electrode, or in the step of re-exposing the resist on the cathode surface, the cathode is removed from the vacuum chamber for high-frequency plasma oxidation when the substrate is removed after the external electrode is formed, and the cathode is removed from the vacuum chamber. new with
A 2 μm thick AZ1470 resist may be applied by spraying or spin coating, and heated for 15 minutes in a baking oven at 90° C., for example.

〔effect〕

As explained above, according to the method of the present invention, when a Josephson junction is formed by high-frequency plasma oxidation using a cathode whose surface is coated with an organic resist, reverse spatter from the cathode can be prevented. This has the advantage of reducing density variations and reducing junction leakage current.

Also, to prevent reverse spatter,
The conventional technology that uses an In-coated cathode has the disadvantage that the process is complicated and time-consuming because it requires a process to evaporate new In on the cathode surface after a process to remove the Pb-In alloy produced when forming the external electrode. It was hot.

In contrast, in the present invention, since it is sufficient to coat the cathode surface with an organic resist, it is sufficient to simply coat the Pb that adheres to the resist surface when forming the external electrode with a new resist, or
The Pb may be removed by etching to expose the underlying resist and reused. In any case, conventional 2
The process can be simplified by requiring only one process, either a new resist coating process or an etching process, and both processes can be performed in an extremely short time compared to In vapor deposition. , which also leads to a reduction in process time. Furthermore, the resist that covers the cathode can be the same resist that masks part of the lower electrode when forming an oxide film on the lower electrode, so there is no risk that this resist will adversely affect the device characteristics. .

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the structure of a tunnel-type Josephson junction element, Figure 2 is an explanatory diagram of an apparatus used for high-frequency plasma oxidation, Figure 3 is a cross-sectional view of a conventional cathode structure, and Figures 4 and 5. is a sectional view of the cathode structure of the present invention, and FIG. 6 is a characteristic diagram when using the conventional cathode structure and the cathode structure of the present invention.

Claims (1)

[Claims] 1. A method for forming a Josephson junction, which comprises forming an oxide film serving as a tunnel barrier by high-frequency plasma oxidation of the surface of a lower electrode using a cathode whose surface is coated with an organic resist.