JP2958054B2 - Surface treatment method and surface treatment device for oxide superconducting thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a surface treatment method and a surface treatment apparatus for an oxide superconducting thin film, which are indispensable for producing wiring and electrode contacts of an electronic device using a superconductor. It is about.

[Related Art] An electronic device using a superconducting phenomenon can be applied to a wide range of applications, for example, as a high-speed switching element, a high-sensitivity electromagnetic wave detection element, and a high-sensitivity magnetometer. In addition, superconductors are characterized by having zero DC electric resistance, and are therefore expected to be applied as wiring for integrated circuits. Therefore, in order to form a device or a wiring layer having high practical value, a thin film made of an oxide-based superconductor having a high superconducting transition temperature (Tc) (over 77 K) is used.

Depending on these applications and purposes, superconducting thin films are laminated,
It is used after being patterned. At this time, since the superconducting thin film is exposed to the atmosphere, moisture, and chemicals, its surface is contaminated, and a surface deterioration layer whose superconductivity is extremely weakened is formed on the surface of the thin film. Therefore, when manufacturing various superconducting devices and wiring layers, it is essential to perform a surface treatment for removing the surface deterioration layer without damaging the thin film.

As a conventional surface treatment method, there is a method of performing surface treatment by high-frequency discharge (reverse sputtering) in an atmosphere containing an inert gas. It is known that the use of this method effectively removes a surface-deteriorated layer in a metal-based superconductor. For example, a typical superconducting device such as a tunnel-type Josephson junction or an electrode contact Is realized by this method.

However, oxide superconductors have a very short superconductor coherence length (ξ) (about 2 to 50 A) and are susceptible to damage compared to conventional metal-based superconductors. It is very difficult to perform effective surface treatment depending on the method.

That is, in the case of the oxide superconducting thin film, even if the surface treatment is performed by this method, the surface deterioration layer is hardly removed, and conversely, adverse effects such as oxygen desorption and surface damage from the oxide superconducting thin film are caused. It was found to happen.

As a variation of this method, it is conceivable to take measures such as mixing oxygen or a halogen gas in a discharge atmosphere or reducing a discharge voltage, but none of these methods is effective surface treatment. For this reason, a tunnel-type Josephson junction using an oxide superconductor has not yet been realized that exhibits sufficient characteristics.

Also, when a noble metal electrode such as Au, Ag, etc. is formed on an oxide superconducting thin film, sufficient contact characteristics cannot be obtained with this surface treatment alone. It has become.

As described above, it is very difficult to remove the surface degradation layer from the oxide superconducting thin film by the conventional method.
Conventionally, there has been no effective surface treatment technology for an oxide superconducting thin film.

[Problem to be Solved by the Invention] An object of the present invention is to form a surface deterioration layer of an oxide superconducting thin film by:
Provided are a surface treatment method and a surface treatment apparatus for an oxide superconducting thin film that can be removed without causing surface damage and that can easily form wiring, electrode contacts, and the like of a high-performance superconducting device. It is in.

[Means for Solving the Problems] In the surface treatment method for an oxide superconducting thin film of the present invention, a substrate on which an oxide superconducting thin film is formed is placed in a vacuum chamber evacuated to a vacuum, And irradiating an ion beam having an energy in the range of 10 to 200 eV generated from an ion source provided in the vacuum chamber.

The surface treatment apparatus for an oxide superconducting thin film of the present invention has at least a vacuum chamber and an exhaust system; a holding / cooling mechanism for holding and cooling the substrate on which the oxide superconducting thin film is formed in the vacuum chamber; At least two or more electron cyclotron resonance ion sources for generating an ion beam and irradiating the substrate surface with the ion beam; and after irradiating the ion beam, a metal layer and / or an insulating layer on the substrate surface. And a thin film deposition mechanism for continuously forming.

[Operation] The operation of the present invention will be described below together with the detailed configuration.

According to the present invention, an ion beam having a predetermined energy is applied to a surface deterioration layer.

Here, an inert gas element (for example, He, Ne, Ar, Kr, Xe, or the like) can be used as a source of the ion beam, and efficient surface treatment can be performed by the physical etching action. In this ion beam irradiation, setting of ion energy is important. This energy
If it exceeds 200 eV, the crystallinity of the oxide thin film surface will be damaged, and if it is less than 10 eV, the irradiation effect (ie, the effect of removing the surface deteriorated layer) cannot be expected. Therefore, the energy is set in the range of 10 to 200 eV.

In order to further reduce surface damage, for example, a gas containing a halogen element such as F, Cl, or Br as a constituent element (for example, CnF _{2n + 2} , CCl ₄ , CBr ₄ , CCl ₂ F ₂ , CBrF ₃ ) Is preferably mixed with the inert gas ion beam. In this case, since the chemical etching action is added in addition to the physical etching action, the irradiation effect becomes very remarkable.

As a first method for performing this, there is a method of simultaneously irradiating an ion beam of an inert gas and an ion beam of a gas containing a halogen element as a constituent element.

On the other hand, as a second method, there is a method of irradiating an ion beam generated using a mixed gas of a gas containing a halogen element as a constituent element and an inert gas as a source.

In the second method, the mixing ratio between the inert gas and the halogen gas is set as follows. If the amount of the gas containing a halogen element as a constituent element is too large, a large amount of an oxide halogenated layer is formed on the surface of the oxide thin film and effective etching does not proceed. Is set to 10% or less by volume.

The degree of vacuum in the vacuum chamber at the time of ion beam irradiation is, for example, preferably 10 ^-4 Torr or less, and more preferably 10 ^-6 or less.

As a source of such an ion beam, a Kaufman-type ion source can be considered first. In the case of a Kaufman-type ion source, a filament made of tungsten or the like is used for emitting thermionic electrons for generating plasma, so when the source gas contains an active halogen gas,
The life of the ion source due to filament disconnection is short, about several hours, and the filament gradually deteriorates until the disconnection occurs.
It is difficult to control well. Therefore, it is preferable to use an ECR (Electron Cyclotron Resonance) ion source in which plasma is generated by microwaves as a source of the ion beam. In the case of an ECR type ion source, since a filament is not used, an ion beam containing a reactive gas can be stably generated, and a low energy ion beam can be easily generated.

Therefore, in the surface treatment apparatus of the present invention, an ECR type ion source is used as an ion source, and at least two or more of them are provided, and a metal layer or an insulating layer is formed continuously after removing the surface deterioration layer. Provide a thin film deposition mechanism.

Note that this surface treatment apparatus is suitable for using both an ion beam of an inert gas and an ion beam of a gas containing a halogen element as a constituent element. Can also be used.
In this case, all but one of the two or more ECR ion sources may be stopped.

On the other hand, examples of the thin film deposition mechanism include those having functions such as vapor deposition, sputtering, ion plating, and chemical vapor deposition.

In the device of the present invention, since the metal layer or the insulating layer can be continuously formed after the removal of the surface deterioration layer, the surface of the clean oxide conductive thin film exposed after the removal of the surface deterioration layer can be kept clean. A tunnel type Josephson junction having excellent characteristics, in which a metal layer or an insulating layer made of a noble metal or the like can be formed on the clean surface thereof,
An electrode contact or the like can be formed.

As described above, in the present invention, since the physical and chemical etching action by the low energy ion beam is used, the point that the surface damage to the oxide superconducting thin film can be minimized as compared with the prior art. different.

EXAMPLES Hereinafter, the present invention will be described in more detail based on examples.

Example 1 In this example, the surface treatment apparatus shown in FIG. 1 was used.

This apparatus has a vacuum chamber 8 and an exhaust system 7, a holding / cooling mechanism 1 for holding and cooling the substrate 2 on which the oxide superconducting thin film 9 is formed in the vacuum chamber 8, and generating an ion beam. ,
And a first for irradiating the surface of the substrate 2 with an ion beam.
An ECR ion source 3, a second ECR ion source 4, a metal layer deposition mechanism 5 for continuously forming a metal layer on the surface of the substrate 2 after the irradiation of the ion beam, and an insulating layer are continuously formed. For depositing an insulating layer.

In this example, the following surface treatment was performed using this apparatus.

 The details will be described below.

First, on a SrTiO ₃ substrate, a Y-Ba-Cu-O thin film (film thickness: 2000
A) formed. The thin film was analyzed by X-ray diffraction. As a result, the lattice constant of the c-axis was 11.7A, and the superconducting transition temperature Tc = 90K
And the crystal orientation (c-axis direction) of the crystal was very high.

Next, the substrate on which the thin film is formed is subjected to high-speed reflection electron beam diffraction (RHEED), Auger electron spectroscopy (AES), an ECR ion beam irradiation device, and a reverse sputtering mechanism (all not shown).
Was fixed to the substrate holding / cooling mechanism 1 in the vacuum chamber 8 equipped with At this time, the substrate 1 was exposed to the atmosphere.

On the surface of the thin film before surface treatment, a pattern in which weak spots and rings were mixed in the RHEED pattern was seen.As a result of surface composition analysis by AES, in addition to Y, Ba, Cu, O Approximately several tens of percent of C and N were detected. That is, it was found that a contaminant layer having low crystallinity was formed on the surface by taking out into the atmosphere.

As described above, after the measurement of the characteristics before the surface treatment was completed, the surface treatment was performed according to the following procedure.

First, the inside of the vacuum chamber 8 is evacuated to about 10 ⁻⁶ Torr using the exhaust system 7.
Exhausted.

Next, the surface treatment was performed by irradiating the thin film surface with an Ar ion beam of 100 eV (beam current density: 0.5 mA / cm ² ) for 10 minutes using the ECR ion beam source 4.

After the surface treatment, the RHEED pattern of this sample was examined. As a result, a state in which spots having extremely strong RHEED patterns were regularly arranged was observed. Therefore, it was found that the surface had excellent crystallinity.

Further, as a result of analyzing the surface composition of this sample by AES, elements other than Y, Ba, Cu, and O, which are the constituent elements of the thin film, were not detected at AES detection sensitivity (0.5%) or less. Therefore, it was found that the contaminant layer existing on the surface was removed.

In addition, an experiment was performed in the same manner as in the above example using an ECR ion beam in which He, Ne, Kr, and Xe were used as ion species instead of Ar, but the same effect as in the case of the Ar ion beam was obtained. I found it to have.

As described above, according to this example, it was confirmed that the surface-deteriorated layer of the oxide superconducting thin film could be removed without causing surface damage.

Comparative Example In this example, an ion beam having an energy of less than 10 eV and an ion beam having an energy of more than 200 eV were irradiated.

 Other conditions were the same as in Example 1.

After the irradiation was completed, the crystallinity and surface composition were analyzed by RHEED and AES.If it was less than 10 eV, the removal of the surface deteriorated layer was hardly performed, and the irradiation effect could not be expected. In this case, it was confirmed that crystallinity was affected.

(Conventional Example 1) Next, another sample was introduced into the same apparatus, and the conventional method was used.
That is, reverse sputtering (bias voltage V
_(PP = 800V).

On the surface of this thin film, a pattern in which weak spots and rings were mixed in the RHEED pattern was observed, and C and N were detected in the AES at about several tens of percents in addition to the constituent elements of the thin film, Y, Ba, Cu, and O. That is, it was found that the contaminant layer having low crystallinity formed on the surface was hardly removed and remained as it was.

In the conventional method, the surface treatment was performed by changing the Ar gas pressure in the range of 1 to 50 Pa and the bias voltage in the range of 100 to 1000 V, but the same result was obtained for both RHEED and AES, and the surface contamination was obtained. It was found that the layer was not removed.

Table 1 summarizes the results of Example 1 and Conventional Example 1 described above.

(Example 2, Conventional Example 2) Using a Bi-Sr-Ca-Cu-O superconducting thin film formed on a MgO substrate, the difference in electrode contact resistance between the present invention and the conventional surface treatment was compared.

Bi-Sr-Ca-Cu-O thin film thickness is 3000A, superconducting transition temperature Tc
_zero was 105K.

The thin film was patterned into a shape for four-terminal electrical resistance measurement by a photo process (resist coating, exposure, development) and ion beam etching.

Then, a lift-off stencil for the Au electrode was formed by a photo process (resist coating, exposure, development). Here, the area of the electrode part is 200 μm × 200 μm, and the number of the electrode part is
There were ten.

Two such samples were prepared, and surface treatment was performed using the present invention and the conventional method. After that, a 500A thick Au thin film is deposited, and lift-off is performed in acetone, so that Au
A contact electrode pattern was formed.

Here, in the present invention, the surface treatment is (Ar + 5%
ECR ion beam using a mixed gas of CF ₄ ) as a source gas (ion beam energy: 50 eV, beam current density: 0.3 A)
/ cm ² ) for 5 minutes. On the other hand, in the conventional method, RF sputter cleaning (V _PP = 500 V, 5 min) was performed in a mixed gas of (Ar + 10% CF ₄ ) (10 Pa).

The contact resistance between the superconducting thin film and the Au thin film was evaluated using the sample thus manufactured. In the sample prepared according to the present invention, the contact resistance is 1 × 10 ⁻⁵ to 1 × 10 ⁻⁶ Ωcm.
^A very low value of ² was obtained, indicating that a good contact was formed. On the other hand, in the sample manufactured by the conventional method, the contact resistance was in the range of 1 × 10 ^{−3 to} 1 Ωcm ² , the variation was large, and the value was large as a whole, and it was found that the contact formation was insufficient. Was.

Further, when CF ₄ in the mixed gas as the source gas is generally CnF _{2n + 2,} and the mixed gas is (Ar
+ 2% CCl ₂ ), (Ar + 6% CCl ₂ F ₂ ), (Ar + 8% CBr ₄ ), (Ar + 10% CBrF ₃ )
The same experiment was carried out for each of the cases where the mixed gas was used, but the same results as above were obtained.

In addition, Ar in these mixed gases is converted into another inert gas (He, N
e, Kr, Xe), the same result was obtained. Here, the mixing ratio of the gas containing a halogen element as a constituent element needs to be 10% or less, and if it is more than this, a large amount of a halogenated layer of oxide is formed on the surface of the oxide thin film, and effective etching is performed. It was also confirmed that it did not progress. further,
Also in this case, it was confirmed that the ion energy of the mixed gas had to be kept within the range of 10 to 200 eV as in the first embodiment.

Example 3 A 1500 A thick Eu-Ba-Cu-O superconducting thin film was deposited on a LaAlO ₃ substrate by sputtering. This thin film was patterned into a lower electrode shape by a photo process (resist coating, exposure, development) and ion beam etching. Then, a lift-off stencil for the upper electrode was formed by a photo process (resist coating, exposure, development). Here, the area of the joint was 50 μm × 50 μm. Two such samples were prepared, and a tunnel-type Josephson junction was manufactured using the present invention and the conventional method.

Here, the surface treatment apparatus according to the present invention shown in FIG. 1 was used. In this example, the electron gun evaporation source is used as the metal layer deposition mechanism.
The vapor deposition device 5 having 5a was used. In addition, an AC magnetron sputtering device 6 was used as an insulating layer deposition mechanism. Note that an MgO target was used as the target 6a.

A sample is placed on the substrate holding / cooling mechanism in the vacuum chamber of this device, and an Ar ion beam (energy: 120
eV, ion current density 0.2 A / cm ² ) and ion beam of gas containing halogen element (source gas: CCl ₄ , energy: 20 eV, ion current density 0.1 A / cm ² ) for 10 minutes Then, 20 A of MgO was deposited as a barrier layer using an AC magnetron sputter cathode, and finally an upper electrode Nb was formed to 1200 A using an electron gun evaporation source to form a tunnel-type Josephson junction.

On the other hand, according to the conventional method, that is, RF sputter cleaning (V _PP = 500) in a mixed gas of (Ar + 10% O ₂ ) (6 Pa).
V, 15 min), and then use MgO as a barrier layer in the same manner.
Was deposited at 20 A, and finally an upper electrode Nb was formed at 1200 A to form a tunnel-type Josephson junction.

The current-voltage characteristics of the tunnel-type Josephson junction prepared as described above were measured at a liquid temperature of 4.2K. The junction fabricated according to the present invention exhibited nonlinearity peculiar to a tunnel junction, and a Josephson current and a superconducting gap structure were observed. On the other hand, the junction fabricated by the conventional method did not exhibit nonlinearity, and no Josephson current and superconducting gap structure were observed.

When He, Ne, Kr, Xe is used instead of Ar as an inert gas, and as a gas containing a halogen element as a constituent element, CCl _{4 is used} instead of CrF _{2n + 2} , CBr ₄ , CCl ₂ F ₂ , And CBrF ₃ were used, and experiments were performed. The same results as above were obtained. In this case also, it was confirmed that the irradiation of each ion beam was required to be performed within the energy range of 10 to 200 eV.

[Effects of the Invention] As described above, according to the present invention, for example, the surface degradation layer and the contaminant layer of the oxide superconducting thin film caused by the process such as exposure to the air and the photo process are reduced by 10 to 20%.
By irradiating an ion beam having an energy of 0 eV, the surface layer is efficiently etched without leaving any damage, and the surface is recovered and cleaned. There is an advantage that good interface characteristics with the barrier layer and the metal layer can be realized. Therefore,
Tunnel junctions and electrode contacts can be formed using oxide superconductors.

In particular, when an ion beam of an inert gas and an ion beam of a gas containing a halogen element as constituent elements are irradiated, the above-mentioned effect becomes even more remarkable (claims 2 and 3).

[Brief description of the drawings]

 FIG. 1 is a cross-sectional view showing a configuration of a surface treatment apparatus of the present invention. (Explanation of symbols) 1... Substrate holding / cooling mechanism 2... Substrate (substrate) 3... First ECR ion source 4... Second ECR ion source 5. ... Electron gun evaporation source 6 ... Insulating layer deposition mechanism (sputtering apparatus) 6a ... Target 7 ... Vacuum exhaust system 8 ... Vacuum tank 9 ... Oxide superconducting thin film

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-252504 (JP, A) JP-A-2-162780 (JP, A) JP-A-64-50581 (JP, A) JP-A-1- 236663 (JP, A) JP-A-1-157579 (JP, A) JP-A-64-87516 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C01G 3/00 C01G 29 / 00 C01G 1/00 H01L 39/24

Claims (4)

(57) [Claims] 1. A substrate on which an oxide superconducting thin film is formed is placed in a vacuum chamber evacuated to a vacuum, and the surface of the thin film is generated from an ion source provided in the vacuum chamber.
A surface treatment method for an oxide superconducting thin film, comprising irradiating an ion beam having an energy in the range of 10 to 200 eV. 2. An ion source comprising two ion sources, wherein an inert gas ion beam generated from a first ion source and a halogen element generated from a second ion source are used. The surface treatment method for an oxide superconducting thin film according to claim 1, wherein the surface of the thin film is simultaneously irradiated with an ion beam of a gas as a constituent element. 3. A mixed gas of an inert gas and a gas containing a halogen element as a constituent element, wherein the mixed gas having a volume fraction of a gas containing a halogen element as a constituent element of 10% or less is generated as a source. 2. The surface treatment method for an oxide superconducting thin film according to claim 1, wherein the surface of the thin film is irradiated with the ion beam. 4. A holding / cooling mechanism having at least a vacuum chamber and an exhaust system; holding and cooling a substrate having an oxide superconducting thin film formed therein in the vacuum chamber; and generating an ion beam; At least two or more electron cyclotron resonance ion sources for irradiating the substrate surface with an ion beam; and a thin film for continuously forming a metal layer and / or an insulating layer on the substrate surface after the ion beam irradiation. A surface treatment apparatus for an oxide superconducting thin film, comprising: a deposition mechanism.