JP3168299B2 - Dielectric thin film and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dielectric thin film having a lead titanate-lanthanum perovskite structure formed on a substrate and usable for a capacitor, a memory element, a sensor and the like, and a method of manufacturing the same. About.

[0002]

2. Description of the Related Art The dielectric constant of lead titanate, which is one of the typical ferroelectric materials, is about 200.

In addition, lanthanum oxide is added to lead titanate in an amount of 15 mo.
A solid solution of 1% shows a dielectric constant of several thousand at around normal temperature.
Further, a solid solution of the same lanthanum oxide has paraelectric properties and excellent temperature characteristics. Therefore, by thinning these elements, thin and short capacitors,
Although it can be considered to be used as a sensor or the like, a thin film in which a large amount of lanthanum oxide is dissolved in lead titanate has not been obtained.

[0004] Conventionally, as a method for producing a dielectric thin film of lead, titanium-based oxide or the like, a sputtering method, a vacuum deposition method, or the like has been used. In these methods, since a vacuum system is used in the apparatus, the production cost is large, it is difficult to form a uniform film on a large-area substrate, and lead and titanium-based oxides usually contain a plurality of metal elements. Therefore, it has been extremely difficult to strictly control the composition of these composite oxides in the thickness and plane direction by using a sputtering method or a vacuum evaporation method.

On the other hand, there is a method for producing a thin film generally called a sol-gel method. In this method, a desired thin film or powder is formed from a uniform solution. It is characterized in that not only a uniform film without fluctuation can be obtained, but also a thin film having a large area can be easily and inexpensively manufactured.

However, when a thin film is obtained by using this sol-gel method, it is difficult to obtain a solution (coating solution) in which lanthanum oxide is dissolved in lead titanate at a high concentration, and usually, 5 mol% of lanthanum oxide is used. A thin film composed of a single-phase crystalline phase having a more perovskite structure has not been obtained.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to obtain a dielectric thin film from a solution of an oxide obtained by dissolving lanthanum oxide in a high concentration in lead titanate and represented by the following formula. is there.

(PbTiO ₃ ) _1-X · (La ₂ O ₃ )
_X (where 5 <X ≦ 30) A solid solution having such a composition is represented by L
As the concentration of a ₂ O ₃ increases, the Curie temperature shifts to a lower temperature, and shows a high dielectric constant near room temperature. In addition, L
When a ₂ O ₃ is dissolved in solid solution, it becomes a paraelectric material, and a dielectric material having a small dielectric loss and excellent temperature characteristics can be obtained. However, a thin film in which lanthanum oxide is dissolved in a high concentration region has not been obtained.

That is, an object of the present invention is to prepare a transparent coating solution in which a dielectric thin film having a high dielectric constant or excellent temperature characteristics is further dissolved in lanthanum oxide to a high concentration, and apply the coating solution. Another object of the present invention is to provide a method for producing a dielectric thin film having a high dielectric constant or excellent temperature characteristics by firing.

[0010]

Means for Solving the Problems The present inventors have intensively studied for solving the above-mentioned problems, and have reached the present invention. That is, the present invention provides (PbTiO ₃ ) _1-X · (La ₂
A dielectric thin film (abbreviated as PLT thin film) formed by forming a single crystal phase having a perovskite structure having a composition of O ₃ ) X (where 5 <X ≦ 30);
Furthermore, it has been found that the object of the present invention can be achieved by the production in the following steps. That is, the method of the present invention comprises the steps of (PbTiO ₃ ) _{1 -X} · (La ₂ O ₃ )
_X (where 5 <X ≦ 30) A mixed solution of an organometallic compound containing lead, titanium, and lanthanum is formed so as to have a composition ratio represented by 5 <X ≦ 30, and the mixed solution is heated to remove water from the solution if water exists. After removing and / or removing formed alcohol, a solution obtained by heating in a temperature range of 100 to 130 ° C. is applied on a substrate, dried, and fired. It is a manufacturing method.

Hereinafter, the present invention will be described in more detail.

In the present invention, an organic metal compound comprising lead and lanthanum is weighed in a predetermined amount (so as to have a composition ratio represented by the above formula) and mixed in a solvent. The solvent used here is usually alcohol, but a solvent having a relatively high boiling point such as methoxyethanol is preferred. It is not preferable to use a solvent having a low boiling point because the solvent evaporates and evaporates in the subsequent dehydration step. The organometallic compound consisting of lead and lanthanum used here is an acetate of lead and lanthanum or a compound having 2 carbon atoms.
Alkoxides having the above alkyl groups are preferred. When a hydrate is used as a raw material, it is desirable to dehydrate by heating in the air or under reduced pressure. At this time, in order to sufficiently perform dehydration, it is preferable to heat to 100 to 120 ° C. when performing at atmospheric pressure. If the temperature is too high, decomposition of the raw materials and evaporation of the solvent occur, which is not preferable. The organic titanium compound is added to the sufficiently dehydrated mixed solution so as to have a composition represented by the above formula. Although the organic titanium compound used here is not particularly limited, it is usually Ti (OR) ₄ (where R = C _n H _{2n + 1).}
n = 2 to 5). When the organic titanium compound is added to the solution, a precipitate is observed in the mixed solution. By heating and refluxing the precipitate in the temperature range of 100 to 130 ° C. with the time shown below as a guide, a complexation reaction (polycondensation reaction) occurs between the respective compounds of lead, titanium and lanthanum, and no precipitate is formed. A clear solution is obtained. The heating reflux time is as follows.

5 <X ≦ 10 30 minutes or more 10 <X ≦ 15 1 hour or more 15 <X ≦ 20 2 hours or more 20 <X ≦ 304 4 hours or more (where X is represented by the above formula) In order to allow a sufficient complexing reaction to proceed by such a heating and refluxing operation, for example, when methoxyethanol is used as a solvent, an alcohol formed by a reaction between an alkoxide of titanium and methoxyethanol before the heating and refluxing operation is used. -Remove the rule.

When the alcohol is not sufficiently removed, the complexing reaction does not proceed sufficiently and a transparent solution cannot be obtained. The concentration of the solution of the composite obtained in this manner is 0.1 to
It is desirably 1 mol%. If the concentration is too low, the required number of coatings when applying the same to the substrate increases, while if the concentration is too high, the thickness of one application increases, which causes cracks in the film.

The atmosphere in the mixed solution preparation step of the present invention is preferably carried out in a stream of an inert gas such as oxygen gas, argon or the like, or a nitrogen gas stream, but it is also possible to carry out the treatment in the atmosphere.

The thus obtained mixed solution is applied onto a substrate and further baked to obtain a thin film having the above composition.

The substrate used at this time is a single crystal or polycrystal of alumina, glass, silicon, MgO or the like, or an electrode formed of Pt, Au, Ni or the like on them. The method of applying the mixed solution to the substrate is not particularly limited, and examples thereof include a spin coating method, a dipping method in which the substrate is immersed in the mixed solution and pulled up, or a coating method by spraying.

The film applied on the substrate is heated, if necessary, to remove the solvent. This solvent removal step is preferably performed as soon as possible. Usually 150-4
It is preferable to carry out the heating by heating to 50 ° C. at a rate of 100 ° C./min or more, and this operation is preferably carried out on a hot plate heated to a predetermined temperature.

If the drying temperature is higher than 450 ° C., it may cause cracks in the film, and if it is lower than 150 ° C., the solvent may not be sufficiently removed. The PLT thin film having a perovskite structure can be obtained by firing the film from which the solvent has been removed at 500 ° C. or higher for several minutes to several hours. Here, when the firing temperature is lower than 500 ° C., the coating film does not easily have a perovskite structure, and the obtained film does not show high dielectric properties. Further, carbon generated by thermal decomposition of film components and the like remains in the film, which causes an increase in dielectric loss, which is not preferable. Conversely, if the firing temperature is higher than necessary, atoms in the film evaporate, causing a composition deviation, which is not preferable. Usually, this calcination is preferably performed at 500 to 1000 ° C.

On the other hand, the thickness of the film to be subjected to one firing is 0.1 mm.
It is preferably not more than 5 μm, and if it is more than 5 μm, it is not preferable because it may cause cracks. In general, the steps of coating, drying and baking are repeated several times to obtain a required film thickness. However, if the film thickness is too small, the material does not exhibit its original characteristics due to interaction with a substrate or an electrode. For this reason, the film thickness is usually 0.1 μm, preferably 0.3 μm or more. In order to obtain a crack-free film in the PLT film of the present invention, the thickness is preferably 2 μm or less.

[0021]

As described above, the present invention is a high dielectric thin film containing a high concentration of lanthanum, using a coating liquid containing a lanthanum metal organic compound which is hardly dissolved in a solvent, Easy and inexpensive (PbTiO ₃ )
It is possible to manufacture a dielectric constant thin film having a composition of _1-X · (La ₂ O ₃ ) _X (where 5 <X ≦ 30).

[0022]

EXAMPLES 15.173 g of lead acetate trihydrate and lanthanum acetate 1.
6.858 g of pentahydrate was added to 300 ml of methoxyethanol.
After the addition, the mixture was heated to about 120 ° C. to remove water of crystallization. After removal, the mixture is cooled to about 90 ° C. or less,
11.369 g of titanium isopropoxide were added. After the addition, the mixture was heated to 120 ° C. again to remove isopropanol produced by the reaction between titanium propoxide and methoxyethanol. At this point, a precipitate was observed, but the mixture was heated to 130 ° C. and refluxed for 2 hours or more to form a transparent (PbTiO ₃ ) ₈₀ (La).
₂ O ₃ ) ₂₀ coating liquid was obtained. This coating solution was spin-coated on a Pt-coated silicon substrate, dried at 250 ° C., and baked at 600 ° C. for 1 hour. This process is further repeated five times to obtain about 1 μm of (PbTiO ₃ ) ₈₀ (L
a ₂ O ₃ ) ₂₀ thin film was obtained. When the obtained thin film was subjected to X-ray diffraction (Cu-Ka), it showed a perovskite structure as shown in FIG. Further, Table 1 shows examples having different compositions. Table 1 also shows the electrical characteristics of the obtained PLT thin film.

[0023]

[Table 1]

[Brief description of the drawings]

FIG. 1 is a view showing an X-ray diffraction pattern of a PLT thin film obtained in the example.

FIG.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-126116 (JP, A) JP-A-62-283620 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/42-35/49 CA (STN) REGISTRY (STN)

Claims (3)

(57) [Claims] 1. The method according to claim 1, wherein (PbTiO ₃ ) _{1 -X} · (La ₂
O ₃ ) A dielectric thin film formed by forming a single crystal phase having a perovskite structure having a composition of _X (where 0.2 ≦ X ≦ 0.3 ). 2. The dielectric thin film according to claim 1, wherein the thickness of the crystal phase is 0.1 to 2 μm. 3. (PbTiO ₃ ) _{1 -X} · (La ₂ O ₃ )
_X (however, a mixed solution of an organometallic compound containing lead, titanium, and lanthanum is formed so as to have a composition ratio represented by 0.05 <X ≦ 0.3 ), and the mixed solution is heated to remove water from the solution. In this case, after removing water and / or alcohol produced, a solution obtained by heating in a temperature range of 100 to 130 ° C. is applied to a substrate, dried and fired. Manufacturing method of thin film.