JPH04298084A - Josephson junction element and manufacture thereof - Google Patents

Abstract

PURPOSE:To provide a Josephson junction type element with superior heat resistance and manufacturing thereof. CONSTITUTION:A lower Nb electrode 14 with film thickness of about 200nm is formed on an Si substrate 12. On this lower Nb electrode 14, an upper Nb electrode 20 with film thickness of 200nm is provided via an Hf over-layer layer 16 with film thickness of about 200nm and an HfOx junction barrier layer 18 to form an Nb/Hf-HfOx/Nb junction that constitutes an over-layer structure.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Josephson junction device and a method of manufacturing the same. Josephson junctions are characterized by excellent high speed performance and low power consumption, so high-speed processing that takes advantage of these features becomes possible with Josephson integrated circuits.

0002

[Prior Art] Conventionally, bonding materials for Josephson integrated circuits include Nb (niobium) as an electrode, Al (aluminum) as an overlayer layer, and Al as a bonding barrier.
Nb/Al-AlOx/Nb junctions made of Ox exhibit good characteristics and are therefore widely used.

0003

[Problems to be Solved by the Invention] However, in a Josephson junction element using a conventional Nb/Al-AlOx/Nb junction, the current-voltage characteristic immediately after fabrication shown in FIG. By the heat treatment of Fig. 6(b)
) Deterioration of characteristics occurs as shown in (). The reason for this is that Nb is a high melting point metal with a melting point of 2468°C, while Al
This is thought to be due to the fact that it is a metal with a low melting point of 660°C. That is, this is because Al diffuses through the grain boundaries of the Nb thin film at a relatively low temperature. Nb/ shown in Figure 7(a)
When the Al-AlOx/Nb junction was heat treated at 200° C. for 1 hour and examined by Auger analysis, it was found that the Nb/Al interface was sagging, as is clear from FIG. 7(b). The deterioration of the characteristics shown in FIG. 1 is caused by this sag at the bonding interface.

[0004] Therefore, improving the resistance of Josephson junctions to heat treatment has become a major issue in the pursuit of future large-scale integrated circuits. Therefore, the present invention
An object of the present invention is to provide a Josephson junction element with excellent heat resistance and a method for manufacturing the same.

[0005]

[Means for Solving the Problems] In order to improve the resistance of Josephson junctions to heat treatment, the present inventors have investigated materials to replace Al. That is, in order to obtain good device characteristics when bonding an overlayer structure, it is important that the bonding interface is clear, but before that, (1
) The overlayer layer has a sufficiently thin film thickness and has excellent surface coverage that covers the Nb electrode surface well, (2) The oxide film of the overlayer layer has thermal stability as a bonding barrier layer. It is required to satisfy both electrical conductivity and electrical conductivity.

[0006] Therefore, the Miedema theory (A.R. Mied
ema et al. , Physica. 100B (1
980) 1p. (Reference) was used to examine the surface coverage of Nb electrodes. The results are shown in Figure 1. As is clear from this figure, it was found that there are two groups of metals with good coating properties. One is Al
A group of low melting point metals including (Be, Al, Si, Z
n, Ga, Ge, Cd, In, Sn, Pd, Bi), and the other is a group of high melting point metals including Nb (V, Zr
, Nb, Mo, Hf, Ta, W, Re, Os, Ir).

[0007] The low-melting point metal group was excluded from consideration as a new bonding material because low-temperature diffusion is likely to occur. Therefore, we considered the thermal stability and electrical conductivity of oxide films for a group of high-melting point metals. First, we investigated the oxide formation energy per oxygen as an indicator of thermal stability. As a result, Hf
>Zr>Nb>Ta>W>Mo>V>Re>Ir>Os
It turns out that it can be ordered.

Next, the electrical conductivity was investigated. It is considered desirable that the junction barrier layer has insulation properties. Oxides can be roughly classified into three types based on their electrical properties: (a) insulators, (b) semiconductors, and (c) metal conductors. Therefore, when investigating the above metals, (a) insulators; HfO2, ZrO2, Nb2 O5, Ta2
O5 (b) Semiconductor; V2 O5 (c) Metal conductor; WO2, MoO2, ReO2
, IrO2, and OsO4.

To summarize the above results, by using Hf (hafnium) as an overlayer layer and HfO2, which is an oxide of Hf, as a junction barrier layer,
It was predicted that this would improve heat resistance. According to the inventor's experiments, this Hf overlayer layer and HfO
2. It was confirmed that the Josephson junction device using the junction barrier layer has good device characteristics and heat resistance.

[0010] Therefore, the above problem is solved by a lower electrode made of Nb or NbN, an Hf overlayer layer formed on the lower electrode, an HfOx junction barrier layer formed on the Hf overlayer layer, and a HfOx junction barrier layer formed on the Hf overlayer layer. This is achieved by a Josephson junction element characterized by having an upper electrode made of Nb or NbN formed on a junction barrier layer.

[0011] Furthermore, the above-mentioned problem is solved by the presence of Nb or N on the substrate.
a step of forming a lower electrode made of bN; a step of forming an Hf overlayer layer on the lower electrode;
forming an HfOx junction barrier layer on the f overlayer layer; and forming a HfOx junction barrier layer on the HfOx junction barrier layer;
Manufacturing a Josephson junction element, comprising the steps of forming an upper electrode made of bN, and selectively etching the upper electrode using a mask patterned in the junction area to form a junction. achieved by the method.

[0012] Furthermore, in the above method for manufacturing a Josephson junction device, HfOx is formed on the Hf overlayer layer.
The step of forming the bonding barrier layer is characterized in that the step of forming the HfOx bonding barrier layer by oxidizing the surface of the Hf overlayer layer by thermal oxidation or high frequency discharge oxidation in an oxidizing atmosphere. This is achieved by a method of manufacturing a Josephson junction device.

[0013] Furthermore, in the above method for manufacturing a Josephson junction element, the step of selectively etching the upper electrode to form a junction is performed using a CF4-based material that does not react with the Hf overlayer layer and the HfOx junction barrier layer. This is achieved by a method for manufacturing a Josephson junction element, which comprises the step of selectively etching the upper electrode by reactive etching using a method.

[0014]

[Function] By using Hf for the overlayer layer and HfO2, which is an oxide of Hf, for the bonding barrier layer, the present invention has excellent surface coverage of the lower electrode made of Nb or NbN as the overlayer layer, and the bonding barrier layer is made of Hf. It has excellent thermal stability and electrical conductivity as a barrier layer, and can improve heat resistance compared to the case where conventional Al is used.

In addition, in the manufacturing process, an HfOx junction barrier layer can be easily formed by oxidizing the surface of the Hf overlayer layer, and the Hf overlayer layer and HfOx junction barrier layer can be easily formed.
Nb as the fOx junction barrier layer as an etching stop layer
Alternatively, since the upper electrode made of NbN can be selectively etched, a sophisticated and complicated process is not required compared to the conventional case of using Al.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below based on illustrative embodiments. FIG. 2 is a cross-sectional view of a Josephson junction device according to an embodiment of the present invention. A film thickness of 20 mm on the Si substrate 12
A Nb lower electrode 14 with a thickness of about 0 nm is formed. Then, on this Nb lower electrode 14, a Hf film with a thickness of 1 to 10 nm is applied.
An Nb upper electrode 20 with a thickness of approximately 200 nm is provided via the overlayer layer 16 and the HfOx junction barrier layer 18, forming an overlayer structure of Nb/Hf-HfOx.
/Nb junction is formed.

Furthermore, these Si substrate 12, Nb lower electrode 14, Hf overlayer layer 16, HfOx junction barrier layer 18, and Nb upper electrode 20 are made of SiO with a thickness of 400 nm.
2 Covered with an insulating film 22. Then, the Nb upper electrode 20 and Nb
Nb wiring layer 2 with a thickness of 600 nm connected to the lower electrode 14
8a and 28b are formed.

As described above, in this embodiment, the Nb lower electrode 14
The feature is that an Hf layer is used as the upper overlayer layer, and an HfOx layer is used as the junction barrier layer. When the current-voltage characteristics of the Josephson junction device according to this example were subjected to heat treatment at 300°C for 1 hour, it was found that
As shown in FIG. 3, good device characteristics were obtained as predicted logically, and no characteristic deterioration was observed. Therefore, it has been demonstrated that the heat resistance of this Nb/Hf-HfOx/Nb junction is significantly improved compared to the conventional Nb/Al-AlOx/Nb junction.

Note that an NbN upper electrode and a NbN lower electrode may be used instead of the Nb upper electrode 20 and Nb lower electrode 14 used in this embodiment. Next, a method for manufacturing the Josephson junction element shown in FIG. 2 will be described with reference to FIGS. 4 and 5. A Nb/Hf-HfOx/Nb structure is created within the same vacuum device without breaking the vacuum. That is, the Nb lower electrode 14 having a thickness of about 200 nm is formed on the Si substrate 12 using the DC magnetron sputtering method. This film forming condition is Ar pressure 1.3 Pa (Pascal)
, the applied voltage was 300 V, and the applied current was 2.0 A, and the deposition rate at this time was 200 nm/min. In addition, Si
Other substrate materials may be used instead of the substrate 12, and RF magnetron sputtering may be used instead of DC magnetron sputtering to form the Nb film.

An Hf overlayer layer 16 having a thickness of 1 to 10 nm is deposited continuously in the same apparatus using the DC magnetron sputtering method under the same Ar pressure, applied voltage of 240 V, and applied current of 0.2 A. At this time, the deposition rate was 12 nm.
/min. In addition, O2
(Oxygen) gas is introduced to oxidize the surface of the Hf overlayer layer 16 by thermal oxidation to form an HfOx junction barrier layer 18. The thickness of the HfOx junction barrier layer 18 at this time can be controlled to a desired thickness using oxidation time, pressure, and temperature as parameters. Furthermore, in the same apparatus, a film with a thickness of 200 mm was continuously deposited under the same conditions as for forming the Nb lower electrode 14.
A Nb upper electrode 20 with a thickness of approximately nm is formed. In this way, N
b lower electrode 14, Hf overlayer layer 16, HfOx
The junction barrier layer 18 and the Nb upper electrode 20 are successively formed (see FIG. 4(a)).

Next, using a resist patterned in the bonding area by a photoresist process as a mask,
Perform reactive etching. CF4 + as reaction gas
The test is carried out using O2 under the conditions of a pressure of 13.3 Pa and an applied power of 50 W. The etching rate of the Nb upper electrode 20 at this time was 100 nm/min, and CF4 was
f overlayer layer 16 and HfOx junction barrier layer 18
Therefore, only the Nb upper electrode can be selectively etched and the etching stops at the surface of the HfOx junction barrier layer 18. That is, the Hf overlayer layer 16 and the HfOx junction barrier layer 18 function as an etching stopper layer. Therefore, the manufacturing method according to this embodiment also has the advantage of being able to improve the throughput in the process.

Next, the exposed HfOx junction barrier layer 18 is etched using an ion beam etching method.
and removing the Hf overlayer layer 16. The conditions at this time were an applied power of 500 W, a pressure of 1.3×10 −2 Pa, and an etching rate of 30 nm/min. Then, the resist is removed (see FIG. 4(b)). Then,
Using a resist patterned into a predetermined shape by a photoresist process as a mask, the Nb lower electrode 14 is processed by reactive etching using CF4 + O2 gas. The conditions at this time are the same as those for etching the Nb upper electrode 20. Then, the resist is removed (see FIG. 4(c)).

Next, a SiO2 insulating film 22 with a thickness of 400 nm is deposited on the entire surface using RF magnetron sputtering. This sputtering film forming condition is an applied power of 900W.
, Ar pressure is 1.3 Pa. Note that since the heat resistance of the bonding has been improved, the film may be formed using a CVD method at about 300° C. instead of the sputtering method. Subsequently, contact holes 24 and 26 are opened on the Nb upper electrode 20 and the Nb lower electrode 14 by reactive etching using a resist patterned into a predetermined shape by a resist process as a mask. The etching conditions were as follows: CHF3 was used as the reaction gas, the applied power was 100 W, and the pressure was 5.
It is Pa. Then, the resist is removed (see FIG. 5(a)). Note that as the insulating film, an SiN insulating film having excellent moisture resistance may be used instead of the SiO2 insulating film 22.

Finally, after depositing an Nb film with a thickness of 600 nm, using a resist patterned into a predetermined shape by a photoresist process as a mask, CF4+O2
Wiring is processed by reactive etching using gas,
Nb wiring layers 28a and 28b are formed. Then, the resist is removed (see FIG. 5(b)). Although the HfOx junction barrier layer 18 was formed by thermally oxidizing the surface of the Hf overlayer layer 16, it may also be formed by oxidizing the surface of the Hf overlayer layer 16 by RF oxidation.

Although the bonding area was determined by resist patterning using a photoresist process, the bonding area may also be determined by anodic oxidation. Furthermore, an NbN upper electrode and a NbN lower electrode may be used instead of the Nb upper electrode 20 and the Nb lower electrode 14.

[0026]

As described above, according to the present invention, in a Josephson junction element having an overlayer structure using Nb or NbN electrodes, Hf derived from theoretical studies is used as an overlayer layer, and the surface oxide layer HfOx By using this as a junction barrier layer, it is possible to realize a Josephson junction element that maintains good element characteristics and has significantly superior heat resistance.

This allows the provision of Josephson junctions for future large scale integrated circuits.

[Brief explanation of drawings]

FIG. 1 is a periodic table used for theoretical investigation of the principle of the present invention.

FIG. 2: Nb/Hf-HfOx according to an embodiment of the present invention
FIG. 3 is a cross-sectional view showing a Josephson junction element having a /Nb junction structure.

FIG. 3 is a graph showing the current-voltage characteristics of the Josephson junction device of FIG. 2 after heat treatment.

4 is a process diagram (part 1) for explaining the method for manufacturing the Josephson junction element shown in FIG. 2; FIG.

5 is a process diagram (part 2) for explaining the method for manufacturing the Josephson junction element of FIG. 2; FIG.

FIG. 6 is a graph showing deterioration of current-voltage characteristics due to heat treatment of a Josephson junction element having a conventional Nb/Al-AlOx/Nb junction structure.

FIG. 7 is a diagram showing changes in Al concentration distribution at the Nb/Al-AlOx/Nb junction interface due to heat treatment.

[Explanation of symbols]

12...Si substrate 14...Nb lower electrode 16...Hf overlayer layer 18...HfOx junction barrier layer 20...Nb upper electrode 22...SiO2 insulating film 24, 26...Contact hole 28a, 28b...Nb wiring layer

Claims (4)

[Claims] Claim 1: A lower electrode made of Nb or NbN;
an Hf overlayer layer formed on the lower electrode;
A Josephson junction element comprising: an HfOx junction barrier layer formed on the Hf overlayer layer; and an upper electrode made of Nb or NbN formed on the HfOx junction barrier layer. 2. Forming a lower electrode made of Nb or NbN on a substrate, forming an Hf overlayer layer on the lower electrode, and forming an HfOx junction barrier layer on the Hf overlayer layer. forming an upper electrode made of Nb or NbN on the HfOx junction barrier layer; selectively etching the upper electrode using a mask patterned in the junction area;
1. A method of manufacturing a Josephson junction element, comprising the step of forming a junction. 3. The method of manufacturing a Josephson junction device according to claim 2, wherein Hf is formed on the Hf overlayer layer.
The step of forming the fOx junction barrier layer is a step of oxidizing the surface of the Hf overlayer layer by thermal oxidation or high frequency discharge oxidation in an oxidizing atmosphere to form the HfOx junction barrier layer. A method for manufacturing a Son junction element. 4. The method of manufacturing a Josephson junction device according to claim 2 or 3, wherein the step of selectively etching the upper electrode to form a junction includes etching the Hf overlayer layer and the HfOx junction barrier layer. A method for manufacturing a Josephson junction device, characterized in that the step includes selectively etching the upper electrode by reactive etching using a CF4-based material that does not react with the CF4.