JP2590142B2 - Superconductor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconductor having a superconducting material thin film,
In particular, the present invention relates to a superconductor in which an oxide-based superconducting thin film having a high superconducting critical temperature is formed on Si, which is useful in the semiconductor field.

[Related Art] Recently, it has been revealed that an oxide material having a perovskite structure, such as a Y-Ba-Cu oxide, has a high superconducting critical temperature (Tc) higher than a liquefaction point of nitrogen (77 K). In order to apply this material to a Josephson element or a conductive film of a semiconductor circuit, it is necessary to form a thin film on a substrate, and attempts have been made to form a thin film of these oxide materials by a sputtering method or a vacuum evaporation method. . Examples of such technologies include the Japanese Journal of Applied Physics, Vol. 26, No. 5,
(1987) pp. L709-L710 (Japanese Journal of Applie)
d Physics Vol 26, No. 5 (1987) pp. L709-L710).

In order to apply these superconducting oxide materials to Si devices, which are widely used in the semiconductor field, it is necessary to develop a technology to stably form oxide thin films with good superconducting properties on Si substrates. . But show superconductivity, for example When a film of (0δ0.5) is formed on a Si substrate by a method such as a sputtering method, a reaction occurs with Si during the film formation or a heat treatment process after the film formation, and the superconductivity is deteriorated or lost. It turned out that the problem occurred.

[Problems to be Solved by the Invention] The present invention suppresses the reaction between Si and an oxide thin film having superconductivity, and stably forms a superconducting thin film having good superconductivity on a Si substrate. The purpose is to provide.

[Means for Solving the Problems] In order to achieve the above object, a Si substrate and M _I -M _II -Cu-
A method of providing a barrier layer between the O-based superconducting oxide thin films (M _I : at least one rare earth metal, M _II : at least one alkaline earth metal) for suppressing the reaction between both,
The structure and material of the barrier layer were studied.

As a result, as a barrier layer, a silicon oxide film (first layer film) on the substrate side, a group of BeO and MgO on the side far from the base side, TiO ₂ , Zr
At least one selected from the group consisting of O ₂ and HfO ₂ , or Sc ₂ O ₃ , Y ₂ O ₃ and R ₂ O ₃ (R: rare earth metal)
It has been found that the above object can be achieved by using a barrier layer having a two-layer structure, which is a kind of oxide film (second layer film).

[Operation] FIG. 1 shows a cross-sectional structure of a sample having a superconducting oxide thin film formed on a Si substrate according to the present invention. Forming a superconducting oxide thin film 4 on a reaction barrier layer composed of two layers, a first layer film 2 made of silicon oxide on a Si substrate 1 and a second layer film 3 made of an oxide material described below. It is. The first-layer silicon oxide film 2 has an effect of increasing the adhesion strength between Si of the substrate 1 and the second-layer oxide film 3 and relieving stress and strain between Si and the second-layer oxide film. To fulfill. The second oxide film 3 hardly reacts with the superconducting oxide thin film 4 and has an effect of not deteriorating superconducting characteristics (critical temperature Tc, critical current density Jc). The second layer oxide film 3 assists the growth of the film in a state where the superconducting oxide film is formed thereon with a dense and good surface flatness, and in some cases, the perovskite The superconducting oxide film with the structure (00
1) It has a positive effect of promoting growth as an alignment film. The direction parallel to the (001) plane is a direction in which the superconducting current can easily pass, and the (001) oriented film is practically desirable because Tc and Jc can be improved.

The material used for the second oxide film 3 is a group of BeO, MgO, a group of TiO ₂ , ZrO ₂ , HfO ₂ , or Sc ₂ O ₃ , Y ₂ O ₃ , Re ₂ O
₃ (Re: rare earth metals La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, T
At least one oxide selected from the group consisting of m, Yb, and Lu) is preferred. Here, the meaning of at least one kind is, for example, TiO in the group of TiO ₂ , ZrO ₂ , and HfO _2.
Even if any of O ₂ , ZrO ₂ , and HfO ₂ is used alone, TiO ₂ + ZrO ₂ , ZrO ₂
That is, two kinds of oxides such as + HfO ₂ , HfO ₂ + TiO ₂ or a mixture of three kinds of oxides such as TiO ₂ + ZrO ₂ + HfO ₂ may be used.

The first layer of silicon oxide 2 is suitably amorphous. The oxide 3 of the second layer may be polycrystalline, but is more preferably amorphous. In the case of polycrystal, microscopic undulations occur due to the distribution of crystal grain boundaries on the surface, and when a superconducting oxide film is formed thereon, the undulations deteriorate the flatness of the film surface.
Furthermore, the growth direction of the crystal grains constituting the superconducting oxide film 4 is often diversified under the influence of the axial orientation distribution of the irregular crystal grains exposed on the surface of the second oxide film 3. In this case, the superconducting oxide film 4 becomes polycrystalline, which causes a problem that Jc cannot be increased. On the other hand, when the oxide film 3 of the second layer is made amorphous, the surface of the film tends to be smooth, and in this case, it is easy to form a superconducting oxide film having good surface flatness. Further, when the crystal grains constituting the superconducting oxide 4 do not have a large interaction with the substrate 1, (0000)
1) Since there is a strong tendency to nucleate with the surface parallel to the substrate,
On the amorphous second layer oxide 3, a superconducting oxide thin film 4 having (001) orientation is easily obtained, and a desirable effect of improving Tc and Jc occurs.

The first layer of the silicon oxide film 2 may be formed by oxidizing the Si substrate 1 by a method such as thermal oxidation or plasma oxidation, or by a sputtering method, a vapor deposition method, a vapor deposition method (CVD method), or the like. May be used to form a silicon oxide film. An appropriate film thickness is 0.5 nm to 2 μm. If it is less than 0.5 nm, the above-mentioned role of the first layer does not play any role. If it exceeds 2 μm, it takes a long time to form a film, which is not practical. In practice, a particularly desirable range of the film thickness is 1 to 500 nm. An oxide of the same type as that of the second oxide film 3 may be formed by a physical vapor deposition method such as a sputtering method or a vacuum vapor deposition method, a CVD method, or a film of a metal element constituting the oxide. Or a method of forming an oxide film by introducing oxygen while oxidizing a metal element by a physical vapor deposition method. In order to obtain an amorphous dense and highly flat film, it is preferable to use a sputtering method such as a high frequency sputtering method or a magnetron sputtering method. The thickness of the second oxide film 3 is
A range of 2 nm to 2 μm is appropriate. If the thickness is less than 2 nm, the frequency of the superconducting oxide film reacting with the first silicon oxide film 2 through defects such as pinholes increases, which is not desirable. Becomes less practical because the tendency of the surface flatness to deteriorate is increased. A practically particularly desirable thickness range is 5 to 500 nm.

[Examples] Hereinafter, the present invention will be described in detail with reference to examples.

Example 1 An oxide superconducting thin film was formed on a Si substrate by the following procedure using a high-frequency sputtering apparatus capable of mounting three types of target materials. 3 inch diameter Si (100) as substrate
A wafer was used. One of the targets is SiO ₂ and the second target is BeO, MgO, TiO ₂ , ZrO ₂ , HfO ₂ , Sc ₂ O ₃ , and a rare earth oxide R ₂ O ₃ (R = La, Ce, Pr, Nd , Sm, Eu, Gd, Tb, Dy, H
o, Er, Tm, Yb, Lu), SiO ₂ , Al ₂ O ₃ , Ta ₂ O ₃ , SiC, TiC, Si ₃ N ₄ , BN,
MgO ・ SiO ₂ , BeO ・ MgO, Ti _0.6 Zr _0.4 O ₂ , (Ti _0.3 Zr _0.5 H
f _0.2 ) O ₂ , (Y _0.8 La _0.2 ) ₂ O ₃ One of each of 34 types was selected and used. YBa ₂ Cu _4.6 O _X as the third target
A sintered body having the following composition was mounted.

Prior to film formation, the Si substrate was etched with Ar ions by reverse sputtering to clean the surface. While covering half of the Si substrate with a shutter, a 25-nm SiO ₂ film was deposited by sputtering. The substrate temperature was 250 ° C., the sputtering atmosphere was 3 mTorrAr, and the film formation speed was 2 nm / min. Next, the shutter was opened to form a second layer film having a thickness of 300 nm. Raise the substrate temperature to 700 ℃ and 3mTorr
It was sputter deposited YBa ₂ Cu _4. O _X target Ar + 10% O ₂ atmosphere, to form the YBa-Cu-O film of 500nm thickness. Subsequently, the sample after film formation was kept at 820 ° C. for 1 hour in an oxygen atmosphere at 1 atm, and then cooled to room temperature at a rate of 100 ° C./h. The Y-Ba-Cu-O film of the obtained sample was evaluated for defects (peeling and cracking), surface smoothness, and Tc and Jc of superconductivity. The results are summarized in Table 1. O:
No peeling or cracking was observed. Δ: Peeling or cracking was observed in an area of 5% or less of the total area. ×: Peeling or cracking was present in an area exceeding 5%. . Surface smoothness: ○: mirror-like,
Δ: No undulation was observed when inspected with a 100-fold optical microscope, ×: Clear clouding or undulation was visually observed. Jc is indicated by the value of the critical current density at 70K. The-mark indicates that Tc did not reach 70K and measurement was impossible.

As is clear from Table 1, the first layer made of SiO ₂ and BeO, MgO, TiO ₂ , ZrO ₂ , Hf were placed between the Si substrate and the oxide superconducting electric film.
o ₂ , Sc ₂ O ₃ , Y ₂ O ₃ , R ₂ O ₃ (R = La, Ce, Pr, Nd, Sm, En, Gd, Tb, D
y, Ho, Er, Tm, Yb, Lu), BeO ・ MgO, Ti _0.6 Zr _0.4 O ₂ , (Ti _0.3 Zr
By providing a barrier layer having a two-layer structure of a second layer made of any one of oxides of _0.5 Hf _0.2 ) O ₂ and (Y _0.8 La _0.2 ) ₂ O ₃ , surface smoothness is good and defects are small, In addition, an oxide superconducting thin film exhibiting good superconducting properties can be obtained. On the other hand, when the SiO ₂ film as the first layer is omitted, an oxide superconducting thin film having a Tc higher than the liquefaction point of nitrogen (77 K) may be obtained, but Jc at 70 K is 10 ⁵ A / cm ² or less.
The effect of using two barrier layers is apparent. Further, BeO other than Si oxide on a Si substrate, MgO, that defects such as cracks and peeling in the film to form and TiO ₂ is liable to occur in Table 1,
It is clearer than the result when the first layer is omitted, and it is obvious that the first layer requires an oxide of Si.

Example 2 Using the same apparatus as in Example 1, under the same conditions and under the same conditions, a 50 nm thick SiO ₂ film as the first layer and a 200 nm thick ZrO ₂ film as the _second layer were formed, and then Y-Ba-Cu- The Y position of O is Nd, Sm, Eu, G
An oxide in which one or more elements of d, Dy, Ho, Er, Tm, or Yb were replaced, or an oxide in which part of Ba was replaced with Sr or Ca was formed. Examination of the state of the thin film after heat treatment and the superconducting properties revealed that there was no peeling or cracking, the surface was mirror-like, Tc was 77K or more, and Jc was 10 ⁵ A / cm.
Values of ² or more were obtained. The method of the present invention is characterized in that M _I -M _II -Cu-
O-based oxide thin film (M _I : at least one rare earth metal, M
_II : at least one alkaline earth metal) was confirmed to be effective on the Si substrate.

In addition, it was confirmed that the same effect was obtained even when the type of the Si substrate was changed to (110) or (111), and that the Si substrate was hardly dependent on the orientation of the Si substrate.

Example 3 An SiO ₂ film having a thickness of 100 nm was formed on a (100) Si substrate by a thermal oxidation method. This substrate was divided into two, and one was mounted on a vacuum vapor deposition apparatus and heated to about 600 ° C., and ZrO ₂ was vapor-deposited to a thickness of 300 nm by electron beam impact heating. Later analysis revealed that this ZrO ₂ film was polycrystalline and the crystal grain size was distributed between 10 and 150 nm. The other substrate was mounted on a magnetron sputtering apparatus, and a 300 nm original ZrO ₂ film was formed at a rate of 50 nm / min while heating to 600 ° C. in the same manner. This ZrO ₂ film was confirmed to be amorphous by a later analysis. These two types of samples were mounted on a high-frequency sputtering apparatus, and while the samples were heated to 600 ° C., an Eu—Ba—Cu—O film was originally formed at 600 nm, and then heated to a certain temperature of 650 to 820 ° C. for 2 hours in an oxygen atmosphere. After keeping the temperature, the mixture was gradually cooled to form a Eu-Ba-Cu-O-based superconducting thin film. The surfaces of the two types of samples were both mirror-like. Of these samples
The structure of the Eu-Ba-Cu-O-based superconducting thin film is shown in Table 2 by X-ray diffractometry using an electron microscope and the Tc and Jc measured at 70K.

As is clear from Table 2, the Eu-Ba-Cu-O film formed on the amorphous ZrO ₂ film has a value of Jc which is about the same as that formed on the polycrystalline ZrO ₂ film. 10 ² times have improved. This can be interpreted as that the structure of the superconducting oxide film is (001) oriented and the direction in which the superconducting current easily flows is aligned in a direction parallel to the film surface. In another experiment for investigating the growth process of the Eu-Ba-Cu-O film, a Eu-Ba-Cu having a perovskite structure was maintained on the amorphous ZrO ₂ while keeping the (001) plane parallel to the substrate. It was found that -O crystals easily formed nuclei. This tendency was also observed on oxide films other than ZrO _{2 such} as MgO, BeOY ₂ O ₃ and HfO ₂ . It is obvious from the consideration of the thin film growth mechanism that the same effect can be expected in the case of other superconducting oxides having a common perovskite structure as in the case of Eu-Ba-Cu-O.

Comparative Example 1 A sample similar to that of Example 2 was produced except that the thickness of the SiO ₂ film as the first layer of the barrier layer was changed to 0.4 nm. As a result of repeating the same experiment, defects such as cracks and peeling were observed in the oxide superconducting thin film at a rate of about 20%.

Comparative Example 2 A sample similar to that of Example 2 was produced except that the thickness of the oxide film as the second layer of the barrier layer was changed to 1.8 nm. As a result of repeating the same experiment, it was found that the Tc of the oxide superconducting thin film tended to decrease to 20K or less at a rate of about 40%.

[Effects of the Invention] As described above, according to the present invention, an oxide-based superconducting thin film having a high critical temperature and a high critical current density can be formed on a Si substrate useful in the semiconductor field with good reproducibility. Such a technique expands the use of an oxide-based superconducting material having excellent characteristics and is industrially effective.

[Brief description of the drawings]

FIG. 1 is a sectional view of a sample having an oxide superconducting thin film formed on a Si substrate according to the present invention. 1 ... Si substrate, 2 ... First layer film made of silicon oxide,
3... Second layer film made of oxide material, 4... Superconducting oxide thin film.

Claims (3)

(57) [Claims] An Si substrate, a silicon oxide film formed on the Si substrate, and BeO, MgO formed on the silicon oxide film.
Group, TiO ₂ , ZrO ₂ , HfO ₂ group, or Sc ₂ O ₃ , Y ₂ O ₃ , R ₂ O ₃
An oxide film made of at least one oxide selected from the group consisting of (R: rare earth metal), and M _I -M _II -Cu- formed on the oxide film. An O-based superconducting oxide thin film (M _I : at least one rare earth metal, M _II : at least one alkaline earth metal), wherein the oxide film is either polycrystalline or amorphous A superconductor characterized by the following. 2. The superconductor according to claim 1, wherein said silicon oxide film has a thickness in a range of 1 to 500 nm. 3. The superconductor according to claim 1, wherein said oxide film has a thickness in a range of 5 to 500 nm.