JPH11171590A - Method of forming functional ceramic thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To unnecessitate a heat treatment at a high temp. by alternately laminating a seed layer capable of forming the same crystal structure as the crystal structure of a multiple oxide consisting of >=2 kinds of metal elements and oxygen at a temp. lower than the crystallizing temp. of the multiple oxide and a layer having a specific metal element in a quantity than larger that in the seed layer and treating by heating to integrate. SOLUTION: The laminated body is obtained by alternately laminating the seed layer, which is capable of forming the same crystal structure as the crystal structure of the multiple oxide consisting of >=2 kinds of the metal elements and oxygen at a temp. lower than the crystallizing temp. of the multiple oxide and has 10-40 μm thickness, and the layer, which has a quantity of the metal element selected from zirconium and lanthanum larger than that in the seed layer and has 20-80 μm thickness. The laminate is heated at 450-600 deg.C under an oxygen-containing atmosphere to be integrated. As a result, the excellent multiple oxide is obtained at a low temp. to enable to use an inexpensive substrate. The selection width of an annealing device is enlarged and the production cost is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for producing a thin layer made of a composite oxide of a plurality of metals, particularly a functional ceramic thin film such as a ferroelectric thin film such as PZT or PLZT.

[0002]

2. Description of the Related Art A ceramic of perovskite type Pb (ZrxTi1-x) 03, which is a composite oxide, and its thin film have ferroelectricity, are excellent in piezoelectric effect and pyroelectricity, and are suitable for various uses such as sensor elements. Used. Of these, not only PZT, which is a composite oxide of lead, zirconium and titanium, but also ferroelectrics represented by PLZT, which is a composite oxide of lead, lanthanum, zirconium and titanium, have recently attracted attention.

[0003] When these functional ceramics are used for sensors and the like, they are often used in the form of thin films. Here, these thin films are usually produced by providing a thin layer made of lead, zirconium and titanium (or their oxides) using sputtering, CVD, or the like, and forming crystals under high-temperature conditions. At this time, a heat-resistant substrate such as a silicon wafer or alumina has been used as the substrate. However, these are expensive to use as a substrate, and it has been required that a cheaper one can be applied. However, inexpensive materials such as aluminum and glass have low softening points and cannot be used as substrates for these composite oxide thin films.

Further, electrodes made of platinum or the like provided on these substrates in order to apply these thin films to sensors and the like have increased the price of the entire substrate in terms of its material and workability. At the same time, since the heat treatment is performed at a high temperature, an expensive processing furnace having a high temperature specification must be used, which is disadvantageous in terms of equipment costs.

Recently, the so-called sol-gel method has been considered as a very promising method. According to this method, the composition can be strictly controlled at a relatively low temperature and at the molecular level. That is, the present inventors have already used a thin layer made of lead and titanium (or a compound thereof) formed by the sol-gel method as a seed layer (seeding layer), and put a thin layer on this seed layer by the sol-gel method. It has been shown that a ferroelectric PZT thin film can be obtained by laminating lead, zirconium and titanium (or a compound thereof) in which the amount of zirconium formed is relatively large, by treating at a relatively low temperature (this method is described below). "Single seeding method"). However, in order to form a thin film having a sufficient function even by the single seeding method, a heat treatment at 600 ° C. or more was required, and the temperature was not sufficiently low.

[0006]

SUMMARY OF THE INVENTION The present invention provides a method for forming a functional ceramic thin film which solves the above-mentioned disadvantages of the prior art, that is, a method for forming a functional ceramic thin film which does not require heat treatment at a high temperature. With the goal.

[0007]

According to the present invention, there is provided a functional ceramic thin layer having a composite oxide crystal comprising at least two kinds of metal elements and oxygen, according to the present invention. A seed layer capable of forming the same crystal structure as the complex oxide at a temperature lower than the crystallization temperature of the complex oxide, and a layer having a specific metal element more than the seed layer. Are alternately laminated, and then heat-treated to form a functional ceramic thin film.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, it is preferable that the amount of a specific metal element in the seed layer is 0, because a predetermined composite oxide thin layer can be obtained by a lower temperature treatment.

The present invention is, for example, a composite oxide of lead, zirconium and titanium, a thin layer having a perovskite-type crystal structure, or a composite oxide of lead, lanthanum, zirconium and titanium, and similarly perovskite. It is an optimal method for forming a thin layer having a type crystal structure. In these examples, the specific metal elements are zirconium and lanthanum.

In the present invention, the seed layer has the same crystal structure as that of the composite oxide in the target functional ceramic thin film, but forms the crystal structure at a temperature lower than the crystallization temperature of the composite oxide. I do. In the crystal structure of this seed layer,
It is thought that this specific metal element is supplied from a layer having more specific metal elements than the seed layer, and that the crystal grows using this seed layer as a crystal seed to form a uniform crystal structure as a whole. By alternately laminating these layers, a good composite oxide thin layer can be obtained even at a low temperature (the alternate lamination of such two layers is hereinafter referred to as a "multi-seeding method"). That is, the conventional seed layer and a layer having a specific metal element more than the seed layer are laminated one by one by a multi-seeding method, and heat treatment is performed to perform a functional ceramic thin layer having composite oxide crystals. It is possible to obtain a composite oxide thin layer having a high dielectric constant at a low temperature, which cannot be achieved by the single seeding method for producing a thin film.

In the present invention, the thickness of the seed layer is 10
It is desirable that the thickness be equal to or more than 40 nm. That is, if it is less than 10 nm, it is difficult to obtain a uniform and good seed layer, and if it exceeds 40 nm, it becomes difficult to obtain uniform crystals unless the heat treatment temperature is increased. On the other hand, the thickness of the layer having a specific metal element more than the seed layer is preferably 20 nm or more and 80 nm or less. Outside this range, it is difficult to obtain a uniform functional ceramic thin film. In these layers, since the thickness of each layer can be easily controlled precisely, a CSD (Chemical) is used.
It is desirable to produce by a Solution Deposition method. Here, in the CSD method, when a substrate is put into an alcohol solution of an alkoxide of a required element such as lead, titanium, lanthanum, zirconium and the like and gradually pulled up,
A method using a film formed on the substrate surface (dipping method), a method using a film formed by dropping these alkoxide / alcohol solutions on a rotated substrate (spin method), and the like. The thickness of the film is determined by viscosity, pulling speed (dipping method), rotation speed (spin method),
It can be adjusted by the solution concentration or the like, and the alkoxide of the film thus formed is hydrolyzed in air to form an oxide. In addition, a laminated body can be obtained by repeating these film formation.

In the present invention, a heat treatment (annealing) temperature of 450 ° C. or more is sufficient in an oxygen-containing atmosphere. At this temperature, a composite oxide thin layer having a sufficient dielectric constant can be obtained. Although a heat treatment at a higher temperature is also possible, from the viewpoint of the present invention, 4 ° C.
Processing at about 50 ° C. to 600 ° C. is possible. When a lead compound is contained as a composite oxide material, it is necessary to perform the reaction in a temperature range where the evaporation is not large. Note that the atmosphere for this treatment may be an atmosphere having oxygen enough to form a composite oxide, and this heat treatment is usually performed in air. In the present invention, any substrate having a heat resistance of 450 ° C. or more can be used as the substrate. It should be noted that a glass substrate is inexpensive for the purpose of the present invention, which is desirable. When a transparent electrode such as ITO is provided on this substrate, the formed composite oxide thin film can be easily applied to a sensor or the like. In addition, a general platinum electrode used when transparency is not required is extremely expensive in its material, and takes time and effort to make the electrode.
Although this raises the cost of the substrate, in the case of the present invention, it is possible to use an aluminum plate as the substrate because annealing at a particularly low temperature is possible. In this case, since the conductivity of the substrate itself can be used, a separate electrode is used. Formation can be omitted, and an extremely low-cost element can be formed.

[0013]

Embodiments of the present invention will be described below. The experiment and various measurements were performed as follows. As raw materials, lead acetate Pb (CH3COO) 2, zirconium-n-propoxide Zr (OC3H7) 4, and titanium isopropoxide Ti [(CH3) 2CHO] 4 were used. Lead acetate trihydrate was dehydrated and refluxed with anhydrous ethanol in NH3 to form an alkoxide, thereby obtaining a lead precursor solution that suppresses volatilization of lead oxide PbO during annealing described later.

Thereafter, the above-mentioned alkoxytitanium and / or alkoxyzirconium is added to this lead precursor solution to form a PT precursor solution or a PZT precursor solution (where PT is lead (Pb) and titanium (Ti)). And PZT means zirconium (Z
r) is included). Acetylacetone was added for stabilizing these precursor solutions. The above method is
H. Suzuki, MBOthman, K. Murakami, K. Kaneko and T. Hayas
Hi, Jpn. J. Appl. Phys. 35B (1996) 4896.

FIG. 1 shows an outline of a method for forming a functional ceramic thin film according to the present invention. PT seed layer and PZ
The T layer was formed on a silica glass substrate having a platinum electrode or a silicon wafer by repeating the dipping process by the CSD method. After forming these layers, a drying treatment at 110 ° C. for 5 minutes, and then 350 ° C.
The organic component is removed from the thin film by the thermal decomposition treatment of Thus, the PT layer is formed between the PZT layers,
Functions as a layer. After these treatments, 410 to 5 in air
Annealing was performed at 00 ° C. for 2 hours. In the case of PZT, x is equal to 0 in the formula Pb (ZrxTi1-x) 03.
53 samples were obtained. This value coincides with the morphotropic phase boundary, and is a value obtained by examination by the conventional single seeding method.

The crystal phase obtained by the above annealing was identified by X-ray diffraction (XRD). A thin silicon layer coated with titanium and platinum (Pt / Ti / SiO2 /
The relative dielectric constant of the thin layer formed on Si) was determined to be HP-4284.
It measured using the A impedance analyzer. In addition, the microstructure of the obtained thin layer was analyzed by a field emission scanning electron microscope (FE).
-SEM).

Hereinafter, experimental results and considerations will be specifically described. The choice of the seed layer is critical in the low temperature process of the PZT thin layer of the present invention. The growth of epitaxial thin layers is strongly influenced by the matching of the thin layers, seed layers and lattices in the substrate. In the present invention, a thin seed layer is provided in order to suppress a decrease in electrical properties of the obtained multilayer thin film. FIG. 2 shows the relationship between the thickness of the thin film and the number of superposed thin PT layers and thin PZT layers. In the present invention, PT
Is a thin seed layer (40 nm) obtained by performing a dip coating process by a CSD method at a pulling rate of 10 cm / min, and a PZT layer having a thickness of about 60 nm by one coating process.
Met. FIG. 3 shows an X-ray diffraction pattern of a PZT thin film formed on a silica glass substrate under different conditions. These thin layers have been subjected to a heat treatment at 450 ° C. for 2 hours, and crystallization has not progressed much in a PZT thin layer without a PT seed layer (FIG. 3A). Further, a perovskite structure was obtained by the single seeding process, but it was not complete (FIG. 3B). Here, it can be seen that according to the multi-seeding method of the present invention, a complete and uniform perovskite structure can be obtained even at a low temperature of 450 ° C. (FIG. 3).
(C)). From this result, according to the multi-seeding method of the present invention, even on a glass substrate having low heat resistance, or an aluminum substrate which does not need an electrode which is a high cost factor such as platinum due to its conductivity, ferroelectricity can be obtained. It is understood that PZT having a perovskite structure, which is a body, can be formed.

The multi-seeding PZT thin film is rich in titanium. According to FIG. 2, the thickness of Pb (ZrxT
In i1-x) 03, it is calculated that X = 0.31. Shu et al. Found that the temperature of single phase PZT perovskite formation decreased with increasing titanium content, with X = 0.2
Reports that the temperature can be lowered to 550 ° C. From this, it can be seen that a single-phase PZT perovskite thin film could be obtained even at a temperature as low as 450 ° C. by the configuration of the present invention (multi-seeding method).

The layer formed in the thin film is greatly affected by the crystal symmetry and lattice constant of the substrate used. FIG. 4 shows the crystallization behavior of the PZT film obtained by the multi-seeding method. In this case, crystallization starts at 410 ° C., and at 430 ° C., a considerable amount of the pyrolysis phase is converted to the perovskite phase, and at 450 ° C., a homogeneous perovskite phase is obtained. This crystallization behavior is exactly the same for the PT seed layer. From this, the PT seed layer is PZT
It can be seen that the crystallization behavior of the layer is controlled and the processing temperature is reduced.

Next, the results of an investigation on the dielectric properties will be described. The effect of the thickness of the obtained thin layer on the relative permittivity is shown in FIG. 5 together with the result of the sample by the single seeding method. The relative permittivity of the thin film obtained according to FIG. 5 is the same as in the case of the single seeding method.
It can be seen that it increases with increasing thickness. Further, it can be seen that the relative dielectric constant of the sample that has been annealed at 450 ° C. among those manufactured by the multi-seeding method is not significantly affected by the film thickness. The low dielectric constant of a thin film is derived from the amorphous phase formed at the interface between the electrode and the film by a chemical reaction during annealing. Conceivable. In the case of a PZT thin film formed by the single seeding method, its composition is in the range of the morphotropic phase boundary (MPB), which exhibits excellent electrical properties.

On the other hand, the PZT thin film obtained by the multi-seeding method has two types of PZT having the above-mentioned MPB composition.
Layer and a PT layer. Here, the PT layer is PZ
Assuming that it does not melt into the T layer, the relative dielectric constant of the obtained multilayer thin film can be considered as a model of a capacitor connected in series. As a result, the capacitance of the multilayer thin film is expressed by the following equation.

C = (ε0S) (εPZTεPT) / (d
PTεPZT + dPZTεPT)

C is the capacitance of the multilayer thin film, S is the electrode area, dPT and dPT are the thickness of the PT thin film and the PZT thin film,
εPT and εPZT are PTs each having an MPB composition.
The relative dielectric constant between the thin film and the PZT thin film, and ε0 is the dielectric constant in a vacuum.

It has been reported by Remeika et al. That the relative permittivity of PT ceramic is about 300. The PZT-PT thin film comprising a multilayer obtained by a multi-seeding method and obtained by annealing at 450 ° C. P
The relative permittivity of the ZT layer should be about 395,
As shown in FIG. 5, the relative dielectric constant of a multilayer thin film having a thickness of about 1.9 μm is 350. This estimate is the same thickness of PZ obtained by the single seeding method.
Close to T thin layer. This suggests that at 450 ° C., the PT seed layer hardly forms a solid solution with the PZT layer. On the other hand, the relative dielectric constant of the PZT-PT multilayer thin film annealed at 500 ° C. shows almost the same value as that of the PZT thin layer manufactured by the single seeding method. By the same calculation as above, the PZT of the PZT-PT multilayer thin film
The estimated permittivity of the layer was calculated to be approximately 900. This value is a very high value for a PZT thin film obtained by the CSD method.

According to FE-SEM observation, the PZT-PT multilayer thin film annealed at 500 ° C. forms a titanium-rich (X = 0.31) solid solution. However, the relative permittivity of the PZT thin film decreases as the titanium content increases.
Therefore, the dielectric constant of the PZT thin layer obtained by the multi-seeding / 500 ° C. annealing is a value superior to that of the PZT thin film obtained by the single seeding method having the same composition.

Next, these microstructures were examined. The electrical properties of a ferroelectric having a perovskite structure largely depend on the microstructure and orientation of the obtained thin layer.
The high orientation of the perovskite structure shows excellent electrical properties. In the above, PZ annealed at 500 ° C.
It was inferred that the PZT layer of the T-PT multilayer thin film had a high dielectric constant. Then, the fine structure of the thin layer produced by different methods and different annealing treatments was observed by a field emission scanning electron microscope. The results are shown in FIGS. FIGS. 6A and 6B show 450 ° C. annealing, and FIGS. 7A and 7B show field emission near the boundary between the PZT thin film and the electrode obtained by performing 500 ° C. annealing. It is a type | mold scanning electron microscope photograph, (a) of each figure is a photograph of the sample obtained by the single seeding method, and (b) of each figure is a photograph of the sample obtained by the multi-seeding method.

FIG. 6 shows that the fine structure ((a) and (b)) of the thin film annealed at 450 ° C. is not affected by the seeding method. In addition, electrodes and PZ
It can be seen that the microstructure of the boundary layer between the T thin films consists of very finely combined fine particles. This microstructure is very typically found in thin films made by the sol-gel method. On the other hand, the thin film annealed at 500 ° C. (FIGS. 7A and 7B) shows a very different microstructure. 450 ° C for PZT thin film obtained by single seeding method, except for particle size
Shows a microstructure similar to that annealed in
The PZT film obtained by the multi-seeding method shows a columnar structure.

This columnar microstructure is considered to be due to the laminated structure of the film obtained by the multi-seeding method. However, it is very difficult to know the formation mechanism of the columnar structure, and further investigation is needed to clarify this. Fine particles are observed at the bottom of the film, near the boundary layer between the electrode and the film. Even with current multi-seeding methods, the interaction between electrodes and thin films cannot be avoided. First PT
The seed layer can be a buffer layer to prevent interaction between the electrode and the film. In addition, this structure shows a reaction between the PT thin film and the PZT thin film that forms a solid solution by annealing at 500 ° C. Therefore, the obtained multilayer thin film has a titanium-rich composition. Despite the titanium-rich composition, the resulting film has a relatively higher dielectric constant than the thin film formed by the single seeding method due to its cylindrical microstructure.

FIG. 8 shows PZ annealed at 500 ° C.
The X-ray diffraction pattern of the T-PT multilayer thin film was shown by a JCPDS card together with the diffraction pattern of a PZT thin film having X of 0.52. Although the orientation of the obtained thin film is not perfect, the X-ray diffraction pattern shows that the thin film has a high orientation.

In the above description, examples of functional ceramics made of lead, zirconium and titanium have been described.
In the same manner as above, except that lanthanum ethoxide is used in combination and the PT layer is used as a seed layer and alternately laminated with a PLZT layer (lead, zirconium, lanthanum, and titanium), annealed, and examined to produce a PLZT functional ceramic thin film. However, a uniform perovskite structure at a low temperature (450 ° C.) was obtained as in the case of the PZT functional ceramic thin film.

[0031]

According to the present invention, an extremely excellent composite oxide can be obtained at a low temperature, and an inexpensive substrate can be used. In addition, the range of choice of annealing treatment equipment is expanded,
Furthermore, since the energy is saved, the manufacturing cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a view showing an outline of a method for forming a functional ceramic thin film of the present invention.

FIG. 2 is a diagram showing the relationship between the thickness of a thin film and the number of superposed thin PT layers and thin PZT layers.

FIG. 3 shows P formed on a silica glass substrate under different conditions.
FIG. 3 is a view showing an X-ray diffraction pattern of a ZT thin film.

FIG. 4 is a view showing a crystallization behavior of a PZT film obtained by a multi-seeding method.

FIG. 5 is a diagram showing the influence of the thickness on the relative permittivity of a thin layer obtained by a multi-seeding method, together with the results of a sample by a single seeding method.

FIG. 6A is a field emission scanning electron micrograph of a cross section near a boundary between a PZT thin film and an electrode obtained by a single seeding method / annealing treatment at 450 ° C. (B) A field-emission scanning electron micrograph of a cross section near a boundary between a PZT thin film and an electrode obtained by annealing at 450 ° C. by a multi-seeding method.

FIG. 7 (a) is a field emission scanning electron micrograph of a cross section near the boundary between a PZT thin film and an electrode obtained by a single seeding method / annealing treatment at 500 ° C. (B) A field-emission scanning electron micrograph of a cross section near the boundary between a PZT thin film and an electrode obtained by a multi-seeding method at 500 ° C. annealing.

FIG. 8 is a view showing an X-ray diffraction pattern of a PZT-PT multilayer thin film annealed at 500 ° C. together with a diffraction pattern of a PZT thin film having X of 0.52 by a JCPDS card.

Claims (7)

[Claims] 1. A method for forming a thin functional ceramic layer having crystals of a composite oxide comprising two or more metal elements and oxygen, wherein the composite oxide has the same crystal structure as that of the composite oxide. A layer formed by alternately laminating a seed layer that can be formed at a temperature lower than the crystallization temperature and a layer having a specific metal element more than the seed layer, and then performing a heat treatment to integrate them. Method of forming conductive ceramic thin film. 2. The method for forming a functional ceramic thin film according to claim 1, wherein the content of the specific metal element in the seed layer is 0. 3. The method for forming a functional ceramic thin film according to claim 1, wherein the laminate is formed by a CSD method. 4. The functional material according to claim 1, wherein the functional ceramic thin layer is made of a composite oxide of lead, zirconium and titanium having a perovskite type crystal structure. A method for forming a ceramic thin film. 5. The method according to claim 4, wherein the specific metal element is zirconium. 6. The method according to claim 1, wherein the functional ceramic thin layer is made of a composite oxide of lead, lanthanum, zirconium and titanium having a perovskite type crystal structure. A method for forming a functional ceramic thin film. 7. The method according to claim 6, wherein the specific metal element is zirconium and lanthanum.