JPH0343214B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an amorphous lithium tantalate/lithium niobate thin film and a method for producing the same. In recent years, interest in lithium ion conductive solids has been increasing. In particular, lithium ion conductors are being put into practical use as solid electrolytes for lithium batteries and as electrolytes for electrochromic displays. The use of the former is rapidly expanding in recent years, as there has been an increasing demand for thinner batteries as electronic devices become smaller and thinner. The latter method uses the solid electrolyte described above to make an electrochromic display element completely solid, thereby stabilizing the characteristics of the element. What these two methods have in common is the development of solid electrolytes that have high ionic conductivity and are stable in normal atmospheres. Currently, due to the large resistance associated with ionic conduction of solid electrolytes,
The above device has not yet been put into practical use. One possible method for increasing the ionic conductivity of the above device is to reduce the resistance geometrically by making the solid electrolyte thinner. However, with conventional ceramic technology, it is difficult to create a thin film of 10 μm or less. In recent years, due to advances in ultra-quenching methods,
Although ionically conductive amorphous materials with a film thickness of about 15 μm have been reported, this method has the drawback that only a flake-like thin film of about several mm square can be obtained.
It is very difficult to realize the above device using this flaky electrolyte. Therefore, 10μm
There is a strong desire for a thin film ion conductive material with a film thickness below and a relatively large area, and a technology for forming the same. However, even when using conventional film-forming techniques such as CVD and sputtering, thin films with high ionic conductivity have not been obtained. The causes of this are that the synthesis temperature of the oxide-based solid electrolyte is high, and that composition control is difficult because the alkali metal oxide is lost through evaporation or the like during thin film formation. An object of the present invention is to provide a lithium tantalate/lithium niobate-based amorphous thin film with excellent ionic conductivity and a method for producing the same, which solves the problems of the prior art as described above. The characteristics of the amorphous lithium tantalate/lithium niobate thin film of the present invention are that Li ₂ O,
In the ternary composition diagram with the three components of Ta ₂ O ₅ and Nb ₂ O ₅ as the vertices, sputtering with the composition indicated by the point in the region where Li ₂ O is 40% or more and 67% or less It is made of a thin film, and its manufacturing method is characterized by the general formula (1-x) LiTaO ₃ xLiNbO ₃ (where x is
a lithium tantalate-lithium niobate composition having a value in the range of 0x1.0, or a mixture forming the lithium tantalate-lithium niobate composition by sputtering, and by sputtering Li ₂ O. By sputtering a mixture of lithium compounds that produce Li ₂ O, Ta ₂ O ₅ ,
Amorphous lithium tantalate/lithium having the composition indicated by the point in the region where Li ₂ O is 40% or more and 67% or less in the ternary composition diagram with the three components of Nb ₂ O ₅ at the top. To produce a thin film of niobate. Figure 1 is a three-component system composition diagram with the three components Li ₂ O, Ta ₂ O ₅ , and Nb ₂ O ₅ as the vertices, and AD in the diagram.
is a line segment where Li ₂ O is 67%, BC
is a line segment connecting points where Li ₂ O is 40%, and the composition indicated by the points within the range surrounded by the rectangle ABCD is the range of the lithium tantalate/lithium niobate composition of the present invention. It is. In addition, Li ₂ O is removed by sputtering, which is used in the method for producing an amorphous lithium tantalate/lithium niobate thin film according to the present invention.
Lithium compounds that produce include Li ₂ O,
LiOH, _Li2CO3 can _be mentioned. In the above _, the composition of _the amorphous lithium tantalate/lithium _{niobate thin} film in the present invention is expressed as _{Li 2} Those with the composition indicated by the dots in the region where O is 40% or more and 67% or less are those with the composition indicated by the dots in the region where Li ₂ O is less than 40%. This is because those having the composition shown by the points in the region greater than 67% are hygroscopic and are not suitable for practical use in terms of moisture resistance. The present invention will be explained in more detail below with reference to test examples. Test Examples 1 to 6 Test Examples 1 and 6 are comparative examples, and Test Examples 2,
3, 4, and 5 are examples. The following operations were performed in common to each test example. The raw material powders of Ta ₂ O ₅ and Li ₂ CO ₃ were weighed, mixed, and reacted at 1100° C. for 3 hours so that the resultant product after reaction became a composition represented by the general formula LiTaO ₃ .
The above composition was obtained. This powder was used as it was, or a sintered body obtained by firing it again was used as a sputtering target. When a stoichiometric target is used, as in the conventional sputtering method, Li ₂ O is lost during sputtering, and the formed thin film exhibits almost no ionic conductivity. Therefore, in this test example, Li ₂ O pellets were mixed into the LiTaO ₃ target described above and used as a target. In each test example, Li ₂ O on the target surface and
The area ratio occupied by LiTaO ₃ was set as shown in Table 1 below.

[Table] Using the above six types of targets, thin films were formed by sputtering. To prevent the substrate temperature from rising, the vacuum level is 2×10 ^-2 mm while cooling the substrate with water.
A thin film with a thickness of about 2 μm was formed on a quartz glass substrate under sputtering conditions of Hg, discharge gas (Ar/O ₂ =60/40), plate voltage of 2 kV, and film formation rate: 0.5 μm/h. The chemical analysis results are shown on the ternary composition diagram in FIG. Points 1 and 6 in the ternary composition diagram of FIG. 1 are the compositions corresponding to the above test examples, respectively, and therefore the area of Li ₂ O and LiTaO ₃ on the surface of the target shown in Table 1 is This is the case when using something with a ratio. Note that these thin films were confirmed to be amorphous as a result of electron beam diffraction and X-ray diffraction. The ionic conductivities at room temperature measured by the alternating current method using the blocking electrodes of these thin films were as shown in Table 2. Further, FIG. 2 shows the temperature change in ionic conductivity measured by a similar method. In FIG. 2, lines 1, ′ to 6′ correspond to the thin films of Test Examples 1 and 6, respectively.

【table】

[Table] As you can see by comparing Figure 1 and Table 2,
The thin film of Test Example 1 has a composition in which Li ₂ O is smaller than the range of the present invention and is not of the present invention, and its conductivity is extremely low as seen from Table 2. On the other hand, the thin film of Test Example 6 has a composition in which Li ₂ O is larger than the range of the present invention and is not of the present invention, and although its conductivity is good, its moisture resistance is poor and it cannot be used as a device. It is an inconvenient material. As can be seen from FIG. 1, the thin films of Test Examples 2 and 5 other than Test Examples 1 and 6 above have the composition of the present invention, and while crystalline LiTaO ₃ is insulating, As shown in Table 2 and FIG. 2, it can be seen that it exhibits very good ionic conductivity. Furthermore, it exhibits ionic conductivity equivalent to or better than LiTaO ₃ amorphous material obtained by ultra-quenching method that can only be obtained in flake form. Test Examples 7-13 Test Example 10 is a comparative example, and Test Examples 7-9, 11-
13 is an example. Table 3 was obtained by the same operation as in Test Examples 1 to 6.
A thin film was formed using the (1-x) LiTaO ₃ xLiNbO ₃ composition shown in (1-x) and a mixture target having a surface area ratio of Li ₂ O and the composition.
An amorphous film having a similar thickness to that in Example 6 was formed.

[Table] The compositions of the thin films formed in each of Test Examples 7 to 13 are as indicated by the numerical symbols corresponding to the test example numbers in the ternary composition diagram in Figure 1. The ionic conductivity (room temperature) measured in the same manner as in Examples 1 to 6 was as shown in Table 4.

[Table] As can be seen from the composition diagram in Fig. 1 and the ionic conductivity values in Table 4, Test Examples 7 to 7 with the composition of the present invention
Samples Nos. 9 and 11 to 13 exhibited good ionic conductivity, whereas Test Example 10, which was outside the composition range of the present invention, was shown to have poor ionic conductivity. As can be seen from the above explanation, according to the present invention, a thin film with extremely high ionic conductivity can be obtained even in a thin film with a thickness of 5 μm or less,
Further, according to the method of the present invention, a highly ionic conductive amorphous thin film of any size can be obtained, whereas the ultra-quenching method can only obtain small fragmented amorphous materials. The composition of a thin film that is practical and exhibits high ionic conductivity is within the composition range of the present invention, which is the quadrilateral in the ternary composition diagram in Figure 1.
It can be seen that the points are within the range of ABCD. Furthermore, since the thickness of the thin film is three orders of magnitude smaller than that of the bulk material, the resistance of the device due to ion conduction is three orders of magnitude smaller. This can be expected to have significant effects when the solid electrolyte is applied to various devices, and is considered to be very promising as a material for various electronic devices.

[Brief explanation of drawings]

FIG. 1 is a ternary composition diagram of Li ₂ O.Ta ₂ O _{5.Nb 2} O ₅ showing the composition range of the highly ionic conductive thin film of the present invention. FIG. 2 is a graph showing changes in ionic conductivity of thin films of various compositions in test examples of the present invention.

Claims (1)

[Claims] 1. In a ternary system composition diagram with the three components Li ₂ O, Ta ₂ O ₅ , and Nb ₂ O ₅ as vertices, Li ₂ O is 40.0% or more.
An amorphous lithium tantalate/lithium niobate thin film comprising a sputtered thin film having a composition indicated by a dot in the region of 67% or less. 2 A lithium tantalate/lithium niobate composition having the general formula (1-x) LiTaO ₃ xLiNbO ₃ (where x is a value in the range of 0x1.0), or the lithium tantalate by sputtering. - By sputtering and a mixture to produce a lithium niobate composition
By sputtering _a mixture _of lithium compounds that produce _{Li 2 O} as a target _{, Li 2} O is over 40%
A method for producing an amorphous lithium tantalate/lithium niobate thin film, comprising producing an amorphous lithium tantalate/lithium niobate thin film having a composition indicated by a point in a region of 67% or less.