JPS6258677B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for making a tunnel-type Josephson junction device, and in particular to a method for forming a stable tunnel barrier layer. [Technical background of the invention and its problems] Tunnel-type Josephson junction devices basically have a base electrode (superconductor) on a substrate (Si, sapphire, etc.), and an electrically insulating tunnel barrier layer on top of the base electrode (superconductor). There is an upper electrode (superconductor) on top of that.
It has a certain structure. This tunnel barrier layer
It is an extremely thin insulating film of 20 to 50 Å and greatly influences device characteristics. For example, if the thickness of the tunnel barrier layer changes by 10%, the Josephson current flowing through the tunnel barrier layer changes by one order of magnitude. Furthermore, the material and quality of the tunnel barrier have a large effect on device characteristics.
Therefore, developing a method for forming a high-quality barrier layer with good control is important from the viewpoint of improving device reliability and yield, and forming a tunnel barrier layer is the most important step in the device manufacturing process. It becomes a process. In general, tunnel-type Josephson devices can be roughly divided into forming a superconducting thin film as a base electrode, forming a pattern of this thin film, forming a tunnel barrier layer, and forming a counter electrode. There are two methods for forming the tunnel barrier layer. One method is to directly oxidize (natural oxidation or plasma oxidation) the underlying electrode after pattern formation to form a predetermined barrier layer, and the other method is to form a predetermined barrier layer on the underlying electrode (Al, Si, Bi, Te, etc.). ) is formed into an extremely thin film (several 10 Å thick) by vapor deposition or CVD to form a barrier, or it is oxidized to form a tunnel barrier layer. In the former case, it is easy to control the thickness of the tunnel barrier layer, but only a predetermined barrier material can be obtained depending on the type of underlying electrode. On the other hand, the latter is a different type of Al ₂ O ₃ from the base electrode,
Although this method forms a barrier with a low dielectric constant such as SiO ₂ , Bi, Te, etc., it is not easy to control the thickness of an extremely thin film. As described above, both barrier layer formation methods have advantages and disadvantages. When focusing on controllability of the barrier layer, the underlying electrode is directly oxidized to form the barrier layer.
The following problems exist when this method is applied to Nb. The first problem is that if spatter cleaning is performed with Ar gas before forming the tunnel barrier layer, the yield of spatter is small, so it is necessary to increase the discharge voltage, but the energy is high.
The surface of the underlying electrode is damaged by the collision of Ar ions, and metallic Nb--O compounds other than Nb ₂ O ₅ are likely to be formed during subsequent oxidation. 2
The second problem is that Nb is very easily oxidized, and even if only 0.01 Torr of Ar-O ₂ mixed gas is introduced for plasma oxidation, an oxide film of several angstroms will grow before discharge. Film thickness control lacks controllability. As above
Although sputter cleaning with Ar cleans the surface, defects are introduced into the Nb surface, and direct oxidation thereafter has the drawback of poor controllability of the oxide film. [Objective and Summary of the Invention] An object of the present invention is to provide a method for manufacturing a tunnel-type Josephson device that can form a high-quality tunnel barrier layer with good reproducibility. According to one embodiment of the present invention, the surface of a base electrode made of a Nb superconducting thin film is heated using Ar-CF ₄ gas or Ar-
A method for manufacturing a tunnel-type Josephson junction device is provided, which comprises performing RF sputtering in CF ₄ --O ₂ gas and then oxidizing to form a tunnel barrier layer. According to another aspect of the present invention, the surface of the base electrode made of a Nb superconducting thin film is treated with Ar-CF ₄ gas or Ar-
A method for fabricating a tunnel-type Josephson junction device is provided, which comprises depositing a foreign material after RF sputtering in a CF ₄ --O ₂ gas, and then oxidizing the foreign material to form a tunnel barrier layer. In the above method of the present invention, preferably Ar-
The _CF4 concentration of _CF4 - _O2 gas is 0.5% to 20%,
The _O2 concentration is below 20% and the RF sputtering is carried out at a gas pressure of 0.003-0.1 Torr. When fabricating a tunnel-type Josephson device using a photo process, when patterning the base electrode,
When the Nb surface is contaminated with resist, acid, or alkali, or because it adsorbs oxygen from the atmosphere, the base electrode surface is covered with an oxide film. Therefore, before forming the tunnel barrier layer, a sputter etching process is performed in Ar gas, and then an oxide barrier is formed by RF plasma oxidation in Ar--O ₂ mixed gas. Although this method has been conventionally used for Pb and Nb devices, it is not always possible to form an excellent barrier in Nb devices. Sputter etching with Ar gas uses the physical effect of knocking out particles from the surface by collision with Ar ions, but the sputter yield is small for Nb, so contamination of about 100 Å near the surface of the underlying electrode Self-biasing is required to remove the layer in a relatively short time (〓60 minutes).
It is necessary to raise the voltage to about 400V, and due to surface damage, metals other than Nb ₂ O ₅ will be removed during subsequent oxidation.
Nb--O compounds are also formed and the barrier quality deteriorates. Furthermore, if sputter etching is performed with the self-bias lowered to about 200 V, damage to the surface can be avoided, but it takes a long time (more than 8 hours) to remove the contaminant layer, resulting in a lack of speed in the device fabrication process. In addition, the clean surface of Nb is very easily oxidized, so when Ar-O ₂ mixed gas is introduced after sputter etching,
Before starting RF discharge, an oxide layer of several angstroms grows and this becomes the initial value of the oxide film thickness, so it is not possible to obtain a predetermined oxide film thickness by controlling only the discharge time. As described above, the sputter etching treatment using Ar gas has the drawback of impairing controllability as far as Nb elements are concerned as a pretreatment for RF plasma oxidation.
Therefore, in order to form a high-quality Nb ₂ O ₅ barrier in the Nb thin film with good reproducibility and to improve the efficiency of the process, it is necessary to reduce the voltage during sputtering to avoid damage to the surface while maintaining the speed of etching. Immediately after completion, it is necessary to cover the clean Nb surface with a sublimable protective film to prevent deterioration during gas exchange. CF ₄ to satisfy these conditions
The chemical action of the gas can be used. The CF ₄ gas plasma not only removes Nb by fluoridizing it, but also removes the oxygen in the surface oxide as CO ₂ gas.
Furthermore, under oxygen-deficient conditions, sublimable carbon is deposited on the surface. If Ar is used to stabilize the discharge and RF sputtering is performed in a gas mixed with a specified CF ₄ gas and a specified amount of O ₂ to improve the etching rate and control the amount of carbon deposited, damage to the Nb surface can be done at low voltage. Not only can the contaminated film be quickly removed without causing damage, but the clean surface of the Nb is protected by sublimable amorphous carbon with a thickness of several tens of angstroms, and the thickness of the carbon film can be controlled.
After carbon is first removed as CO ₂ in plasma oxidation in Ar + O ₂ , the thickness of the Nb ₂ O ₅ -only barrier can be controlled by controlling the discharge time. Table 1 shows the etching rate of Nb with Ar and _CF4 . As is clear from this table, the etching rate of Ar is significantly lower than that of _CF4 .

[Embodiments of the invention]

Example 1 A Si substrate with a thermal oxide film (SiO ₂ film thickness 1000
A Nb thin film of 2000 Å for the base electrode was formed on the substrate at a substrate temperature of 400°C in a vacuum of 2 × 10 ^-8 Torr. The superconducting critical temperature of this thin film was 9.2K. This thin film substrate was coated with a resist, exposed, developed, and etched to form a base electrode with a pattern width of 20 μm.
Next, tunnel-shaped elements were fabricated using these thin film substrates. That is, SiO stencils with a film thickness of 2500 Å were formed on two patterned base substrates by the lift-off method, and one was sputtered on the base electrode surface in Ar gas (20 mTorr, V _CSB = 300 V, 20
The other one was sputtered in Ar-10% CF ₄ gas (20 mTorr, V _CSB =200 V, 20 minutes). After sputtering, exchange to Ar-10% O ₂ gas and plasma oxidation (30 mTorr, V _CSB = 100 V, 5 minutes), form Nb for the upper electrode on a water-cooled substrate by electron beam evaporation (4000 Å), expose, develop, and etch. Patterned by The characteristics of the obtained device were that the one sputtered with Ar had bridge-type characteristics, but
The material sputtered with -10% CF ₄ had quasi-particle tunneling characteristics with little leakage, as shown in Figure 3. The gap voltage at this time is 4.2K.
It was 3.0mV. Example 2 Four substrates on _which base electrodes and SiO stencils were formed in _{Example 1} were subjected to RF sputtering (V _CSB = 200 V, 20 After plasma oxidation ( _{V CSB =} 100V,
2 minutes), and then a Nb upper electrode was formed to fabricate a device. Table 2 shows the characteristics of these four types of elements. It is thought that when the CF ₄ concentration was high, the amount of carbon deposited increased and the carbon could not be removed by subsequent oxidation.

【table】

[Table] Example 3 Four substrates on which base electrodes and _SiO stencils were formed in Example ₁ were subjected to RF sputtering (V _{CSB =} 250V, 20 minutes), then plasma oxidation ( _{V CSB =} 80V, 4
min) and then cooled the substrate with Nb for the upper electrode.
The device was fabricated by vapor deposition and patterning. Table 3 shows the characteristics of the device obtained under each condition.

[Table] When the oxygen concentration is high like this, oxidation progresses at the same time as etching during sputtering in Ar-CF ₄ -O ₂ gas, so the oxide film becomes thicker due to subsequent oxidation. Example 4 Four substrates on which base electrodes and _SiO stencils _were formed in Example 1 were subjected to RF sputtering (V _CSB =
200V, 20 minutes), then plasma oxidation ( _{V CSB =} 70V, 3
minutes) and then deformed the Nb upper electrode to fabricate the device. Table 4 shows the characteristics of each element.

〔Effect of the invention〕

As explained above, the Nb film surface is exposed to Ar―CF ₄ ―O ₂
By selecting the mixing ratio of the mixed gas and performing RF sputtering, it is possible to obtain a clean Nb surface in a short time even at low voltage without damaging the Nb surface, and the clean surface is protected by carbon with a thickness of more than 10 Å. This prevents surface deterioration during gas exchange. If a barrier is then formed by a direct oxidation method or a foreign material deposition method, there is an advantage that all the deterioration of Nb caused by the photo process can be recovered.

[Brief explanation of the drawing]

Figure 1 is a graph showing the thickness of carbon deposited on the Nb surface when sputtering is performed by changing the O ₂ concentration of the Ar-CF _{4 -O 2} mixed gas.
A graph showing the thickness of carbon deposited on the Nb surface when sputtered with varying self-bias in an O ₂ mixed gas. Figure 3 is a graph showing the V-characteristics of quasiparticle tunneling with little leakage. Figure 4 The figure is a graph showing the V-characteristics of a tunnel-type Josephson junction, and FIG. 5 is a graph showing the V-characteristics of a SiO ₂ barrier device formed by oxidizing vapor-deposited Si.

Claims (1)

[Claims] 1. The surface of the base electrode made of Nb superconducting thin film is
A method for producing a tunnel-type Josephson junction element, which comprises performing RF sputtering in _{-CF 4} gas or Ar-CF ₄ -O ₂ gas and then oxidizing to form a tunnel barrier layer. 2 The surface of the base electrode made of Nb superconducting thin film was
A tunnel-type Josephson junction device characterized in that after RF sputtering in -CF ₄ gas or Ar-CF ₄ -O ₂ gas, a foreign material is deposited, and then this foreign material is oxidized to form a tunnel barrier layer. How to make