JP3105014B2 - Superconducting thin film manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a superconducting thin film applicable to a superconducting device having a laminated structure.

[0002]

2. Description of the Related Art In recent years, progress in superconducting electronics in Japan has been remarkable, and the transition temperature Tc
Bismuth-based superconducting materials and yttrium-based superconducting materials having a high content have been proposed. When a superconducting device is manufactured using the above-described superconducting substance, for example, a tunnel junction having a constant operating voltage and excellent circuit operation stability is used. This tunnel junction has a sandwich structure composed of SIS (S is a superconducting thin film and I is an insulating layer). In this case, if the bismuth-based superconducting material or the like is used for the S layer, the coherent length is short. For this reason, it is necessary to set the thickness of the I layer to 1 to 1.5 nm or less.

[0003] Therefore, as a superconducting material, Ba _0.6 K
It has been proposed such as using long superconducting substance coherent length having the composition of _0.4 BiO _X. When this superconducting material is used, the transition temperature Tc is slightly lowered (Tc = about 30 K), but the thickness of the I layer may be set to about 4 nm or less, so that the superconducting device can be easily manufactured. The Ba _0.6 K _0.4 as an example of a method for manufacturing a superconducting material having the composition of BiO _X, a method of producing by performing a heat treatment and magnetron sputtering has been proposed under the following conditions (NATURE · VOL338 · 16MARCH 1989 p
241-p243). Sputtering conditions sputtering target: Ba _1.4 K _1.4 BiO _X introduced gas: argon gas + oxygen gas (about 1%) substrate temperature: 400 ° C. apparatus pressure: several Pa heat treatment conditions used gas: Oxygen gas temperature: 500 ° C.

[0004]

However, the above method has the following problems. Due to the reason that the amount of oxygen gas during sputtering is small, the sputtered thin film is not in a superconducting state at the time of film formation. Therefore, after the completion of the sputtering as described above, heat treatment is performed to bring the superconducting state. However, when the superconducting state is obtained by such a method, the surface of the superconducting thin film becomes uneven. Therefore, a high-quality superconducting thin film cannot be obtained. As a result, the performance of the superconducting device having the SIS structure in which the insulating layer and the superconducting thin film are formed on the superconducting thin film is reduced. Since the gas pressure in the apparatus is low, the film forming speed is increased, and it is difficult to control the film thickness. For this reason,
Poor productivity. For example, the mean free path of the ions in the plasma increases due to the reason that the gas pressure in the apparatus decreases, and resputtering occurs on the substrate. Therefore, the composition of the sputtering target greatly fluctuates, and a sputtering target having a stoichiometric ratio cannot be used. In addition, since the sputtering thin film is damaged, a high quality superconducting thin film cannot be obtained. Since the heat treatment step is indispensable as described above, the step becomes complicated and the productivity is also poor.

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a method of manufacturing a superconducting thin film capable of obtaining a high-quality superconducting thin film while improving productivity.

[0006]

According to the present invention, in order to achieve the above object, Ba _{1 -x} K _x BiO ₃ (0.2 <X <) after introducing argon gas and oxygen gas into the apparatus. the superconducting thin film having the composition of 0.5), use the sputtering method
In the method of manufacturing a superconducting thin film to be created on a substrate,
With the amount of the oxygen gas during creation superconducting thin film, it becomes 20-80% on the total gas volume, and device gas pressure during both the gas introduction is set to be a 40～120Pa, superconducting thin film The substrate temperature at the time of fabrication is 300 ° C to 500
The method is characterized in that a superconducting thin film is formed by a sputtering method under a set condition of ℃ .

[0007]

The following operations are realized by the above-described method. Because the amount of oxygen gas during sputtering is large,
A superconducting thin film is formed during sputtering. Therefore, a heat treatment step after the end of sputtering is not required, and the surface of the superconducting thin film becomes smooth. As a result, a smooth I layer can be formed on the superconducting thin film.

When the amount of oxygen gas is too small, the above effect cannot be obtained. On the other hand, when the amount of oxygen gas is too large, it becomes difficult to form plasma. Therefore, it is necessary to set the amount of oxygen gas to be 20 to 80% of the total gas amount as described above. Since the gas pressure in the apparatus is high, the film forming speed is slow, and the film thickness can be easily controlled.

If the gas pressure in the apparatus is too low, the above effect cannot be obtained, and the transition temperature Tc decreases and the composition of the sputtering target fluctuates. Is significantly slower. Therefore, the gas pressure in the apparatus is 40 to 120 Pa as described above.
It is necessary to set so that. Due to reasons such as an increase in the gas pressure in the apparatus, the mean free path of ions in the plasma is reduced, and re-sputtering on the substrate can be suppressed. Accordingly, the variation in the composition of the sputtering target is small, and a sputtering target having a close stoichiometric ratio can be used. In addition, by suppressing re-sputtering, damage to the sputtered thin film is reduced. Since a heat treatment step is not required, the step is simplified. Since a powder target can be used, the components of the sputtering target can be strictly adjusted.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. Figure 1 is a sectional view showing an example of a superconducting device using a superconducting thin film produced by the production method of the present invention, the substrate 1 made of SrTiO _3, SI
It has a structure in which a main body portion 2 of a sandwich structure made of S (S is a superconducting thin film, I is an insulating layer) is formed. Each of the S layers 3a and 3b has a thickness of about 1000 ° and a thickness of Ba _0.6
It is composed of K _0.4 BiO ₃ . On the other hand, the I layer 4 has a thickness of about 4 nm and is made of MgO.

Here, a superconducting device having the above structure was manufactured as follows. First, the substrate 1 is ultrasonically cleaned in ethyl alcohol, and then boiled and dried. next,
After the washed substrate 1 is mounted in an rf-magnetron sputtering apparatus equipped with a sputtering target, vacuum evacuation is performed until the internal pressure of the apparatus becomes 10 ^-4 to 10 ^-6 Pa. Next, Ar gas and O ₂ gas are introduced into the apparatus. At this time, the ratio of Ar gas to O ₂ gas is 50:
It was set to 50 and the gas pressure in the apparatus was set to 80 Pa. Thereafter, plasma is generated in the device by applying a discharge power of 50 to 150 W between the positive and negative electrodes in the device, and then the substrate 1 is heated to 300 ° C. by a heating device in the device.
Heat to 500 ° C. Then, after performing pre-sputtering for 0.5 to 2 hours, the shutter in the apparatus is opened to start main sputtering. Thus, the formation of the S layer 3a is started. At this time, since the film formation rate is 500 ° / hr, the main sputtering is performed for about 2 hours. After the main sputtering, the shutter is closed and the plasma is turned off. Thereafter, the introduction of the Ar gas is stopped, and the introduction state of the O ₂ gas is maintained. Thus, the substrate 1 is cooled in the O ₂ gas atmosphere.

Then, an MgO film (4 nm thick) is formed on the S layer 3a by sputtering or vapor deposition to form an I layer 4, and then the I layer 3 is formed in the same manner as described above.
b is formed. The sputtering target is
Barium compounds [for example, BaCO ₃ , BaO, Ba
(NO) ₂ ], a potassium compound [eg, K ₂ CO ₃ ,
KNO ₃ ] and a bismuth compound [for example, Bi ₂ O ₃ ] are mixed at a predetermined ratio, and then mixed in a nitrogen gas atmosphere (600 to
700 ° C.) for 2 to 5 hours in an oxygen gas atmosphere (400 to 5
(00 ° C.) for 2 to 5 hours, and then pressed at a pressure of 1 to 3 tons. [Experiment 1] Since the S layer 3 was examined by the X-ray diffraction method,
The result is shown in FIG.

As is clear from FIG. 2, it was confirmed that the S layer 3 was oriented almost completely in the (110) direction. The width of the peak is similar to that of a substrate having a single crystal structure. This indicates that the film quality of the S layer 3 is good.
Further, from the photograph obtained by the RHEED method, it was confirmed that the S layer 3 was epitaxially grown. [Experiment 2] The relationship between the temperature and the resistance of the S layer 3 was examined, and the results are shown in FIG.

As is apparent from FIG. 3, the S layer 3 has a thickness of 16K.
It is confirmed below that the material is in a superconducting state. [Experiment 3] The relationship between the gas pressure in the apparatus and the transition temperature Tc was examined, and the results are shown in FIG. As is clear from FIG. 4, when the gas pressure in the apparatus becomes 40 Pa or less, the transition temperature Tc
Was 10K or less. Therefore, the gas pressure in the apparatus needs to be set to 40 Pa or more. [Experiment 4] The relationship between the gas pressure in the apparatus and the change in the composition of the sputtering target was examined, and the results are shown in FIG. The black circles in FIG. 5 indicate the composition of the superconducting thin film,
Open circles indicate the composition of the sputtering target when the gas pressure in the apparatus is predetermined.

As is apparent from FIG. 5, it was confirmed that when the gas pressure in the apparatus became 40 Pa or less, the composition of the sputtering target and the composition of the superconducting thin film were significantly different from each other.
Therefore, the gas pressure in the apparatus needs to be set to 40 Pa or more. [Experiment 5] The relationship between the gas pressure in the apparatus and the film formation rate was examined, and the results are shown in FIG.

As is apparent from FIG. 6, when the gas pressure in the apparatus becomes 120 Pa or more, the film forming rate becomes about 0.5 nm / mi.
It was confirmed that the value was n or less. Therefore, the gas pressure in the apparatus needs to be set to 120 Pa or less. [Summary of Experiments 3 to 5] From the above Experiments 3 and 4, when considering the transition temperature Tc and the composition of the sputtering target, the gas pressure in the apparatus needs to be set to 40 Pa or more. On the other hand, from the above Experiment 5, it is necessary to set the gas pressure in the apparatus to 120 Pa or less from the viewpoint of the deposition rate. Therefore, taking these facts into consideration, the gas pressure in the apparatus is 40P
It is necessary to set the pressure to not less than a and not more than 120 Pa. [Experiment 6] The relationship between the O ₂ gas concentration and the transition temperature Tc of the superconducting thin film was examined, and the results are shown in FIG.

As is apparent from FIG. 7, it was confirmed that when the O ₂ gas concentration was less than 20%, the transition temperature Tc of the superconducting thin film was lowered. Therefore, the O ₂ gas concentration is 20
% Must be set. [Experiment 7] The relationship between the O ₂ gas concentration and the degree of plasma generation was examined, and the results are shown in FIG.

As is apparent from FIG. 8, it was confirmed that no plasma was generated when the O ₂ gas concentration exceeded 80%. Therefore, the O ₂ gas concentration needs to be set to 80% or less. [Summary of Experiments 6 and 7] From the above Experiment 6, when considering the transition temperature Tc of the superconducting thin film, the O ₂ gas concentration needs to be set to 20% or more. Considering the degree of plasma generation, the O ₂ gas concentration needs to be set to 80% or less. Therefore, in consideration of both, it is necessary to set the O ₂ gas concentration to 20% or more and 80% or less.

[0019]

As described above, according to the present invention, no heat treatment is required after the end of sputtering, so that the surface of the superconducting thin film becomes smooth, and a good quality superconducting thin film can be obtained. Since the process is simplified, productivity is improved. Further, the film formation rate is reduced, the film thickness can be easily controlled, and a sputtering target having a close stoichiometric ratio can be used, so that productivity is improved.

In addition, by suppressing re-sputtering, damage to the sputtered thin film is reduced, and
By using the powder target, the components of the sputtering target can be strictly adjusted. As a result, a high-quality superconducting thin film can be obtained. From the above, it is possible to obtain a high-quality superconducting thin film while improving the productivity, so that the performance can be dramatically improved while reducing the cost of the superconducting device having the SIS structure. Play.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing an example of a superconducting device using a superconducting thin film (S layer) produced by a manufacturing method of the present invention.

FIG. 2 is a graph showing an X-ray diffraction pattern of an S layer.

FIG. 3 is a graph showing a relationship between temperature and resistance of an S layer.

FIG. 4 is a graph showing a relationship between a gas pressure in the apparatus and a transition temperature Tc.

FIG. 5 is a graph showing a relationship between a gas pressure in the apparatus and a composition change of a sputtering target.

FIG. 6 is a graph showing the relationship between the gas pressure in the apparatus and the film forming speed.

FIG. 7 is a graph showing a relationship between an O ₂ gas concentration and a transition temperature Tc of a superconducting thin film.

FIG. 8 is a graph showing the relationship between the O ₂ gas concentration and the degree of plasma generation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Main part 3a S layer 3b S layer 4 I layer

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification code FI // H01B 12/06 ZAA H01B 12/06 ZAA (72) Inventor Kazuhiko Takahashi 2-18-18 Keihanhondori Moriguchi City SANYO Electric Co., Ltd. (72) Inventor Junnobu Yoshizato 2-18-18 Keihanhondori, Moriguchi-shi Sanyo Electric Co., Ltd. (56) References JP-A-1-164727 (JP, A) JP-A-2-92808 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C01G 29/00 ZAA C01G 1/00 ZAA C04B 41/87 C23C 14/08 ZAA H01B 13/00 565 H01B 12/06 ZAA

Claims (1)

(57) [Claims] After introducing an argon gas and an oxygen gas into an apparatus, Ba _{1 -x} K _x BiO ₃ (0.2 <X <0.5)
A superconducting thin film consisting of
In the method of manufacturing a superconducting thin film formed on a substrate by using the method, the amount of the oxygen gas at the time of forming the superconducting thin film is 20 to 80% of the total gas amount, and the gas pressure in the apparatus when both gases are introduced. Is set to be 40 to 120 Pa, and the substrate temperature at the time of forming the superconducting thin film is 300 ° C. to 500 ° C.
A method for producing a superconducting thin film, comprising forming a superconducting thin film by a sputtering method under a set condition of ℃ .