JPH0967193A - Method of manufacturing ferroelectric thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a thin ferroelectric film with 001 axes oriented in a direction perpendicular to the surface of a substrate independently of the kind of the substrate by forming a buffer layer made of a specified thin perovskite film on the substrate and then bringing a thin ferroelectric film into crystal growth. SOLUTION: When a thin perovskite film is formed as a buffer layer on a substrate having 1.0×10<-5> -2.1×10<-5> / deg.C coefft. of linear thermal expansion, a thin oxide film having an amorphous structure and/or a pyrochlore structure as the crystal structure is formed in 0.05-0.3μm thickness on the substrate and the structure is turned into a perovskite structure by heating the film at 550-800 deg.C to form a buffer layer made of a thin perovskite film having 100 orientation at the Curie temp. or above on the substrate. The oxide is perovskite represented by the general formula ABO3 and the molar ratio of A to B is 0.8-1.2. A thin ferroelectric film having a perovskite structure and having tetragonal crystals at room temp. is then brought into epitaxial crystal growth on the thin perovskite film to form a perovskite structure having 001 orientation at room temp.

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 ferroelectric thin film useful for pyroelectric infrared detecting elements, actuators, nonvolatile and nondestructive memories and the like.

[0002]

BACKGROUND OF THE INVENTION A ferroelectric, electric field also exists spontaneous polarization (P _S) is not a substance that its direction can be reversed by an external electric field, a pyroelectric infrared detecting element,
Widely applied to actuators, non-volatile and non-destructive memory devices, etc.

[0003] pyroelectric infrared element is one that utilizes the temperature change of the spontaneous polarization, it is possible to maximize the output when the direction of P _S in the material are aligned in one direction.
However, most of the infrared detectors currently in practical use are made of polycrystalline ceramics,
Since the crystal axes are not aligned, P _S
Can not be perfectly aligned.

In recent years, there has been an increasing demand for miniaturization of electronic parts, and in order to achieve this, the use of a ferroelectric material in the form of a thin film is drawing attention. Also in that case, the spontaneous polarization P
_A ferroelectric thin film in which the _S directions are aligned in one direction is considered effective. PbTi whose crystal system is tetragonal at room temperature
O ₃ , Pb (Zr, Ti) O ₃ , (Pb, La) TiO
_{Since a} typical ferroelectric substance having a perovskite structure such as ₃ has a spontaneous polarization direction in the 001 axis direction of the crystal, if the 001 axis of the crystal orientation is aligned as perpendicular to the substrate surface as possible, it will be caused by the spontaneous polarization. The characteristics can be maximized. As a conventional example for producing such a film,
As described in JP-A-58-186105, {1
The ferroelectric thin film having the above composition, which is produced by epitaxial growth on the surface of the base substrate of the MgO single crystal substrate cleaved along the {00} plane, can be mentioned. The lattice on the {100} plane of the MgO single crystal is {1} of the ferroelectric substance having the above composition.
Since the matching with the lattice on the {00} plane is good,
An epitaxial film can be formed by sputtering or CVD film formation while heating to 650 ° C. Since the Curie temperature of the above ferroelectric substance is 500 ° C. or lower,
It is cubic at a temperature of 0 to 650 ° C. and takes 100 orientation during epitaxial growth on a MgO single crystal substrate. In the process of cooling to room temperature after film formation, the ferroelectric substance undergoes a tetragonal phase transition, but the thermal expansion coefficient of MgO is 1.5 × 10 ⁻⁵ (° C. ⁻¹ ) and Since the coefficient of linear thermal expansion of ferroelectric substance is larger than 0.9 × 10 ^-5 (℃ ^-1 ), compressive stress acts on the film from the substrate to the substrate surface during cooling, and the film is perpendicular to the substrate after phase transition. The tetragonal 001 axis is predominantly formed in the direction. Therefore, it exhibits excellent characteristics when used as a pyroelectric sensor that utilizes the temperature change of spontaneous polarization.

[0005]

However, the MgO single crystal substrate used in this method is expensive. Therefore, there is a drawback that the ferroelectric thin film element manufactured using this and the electronic parts manufactured from this become expensive. The present invention solves this problem, and provides a method for manufacturing a ferroelectric thin film in which the direction of the 001 axis is aligned perpendicular to the substrate surface regardless of the type of substrate.

[0006]

The method for producing a ferroelectric thin film of the present invention for solving the above problems is as follows. First, in the first step, the coefficient of linear thermal expansion is 1.5 × 10.
A perovskite thin film having 100 orientation in a crystal system (cubic system) having a Curie temperature or higher is formed as a buffer layer on a substrate having a temperature of ⁻⁵ (° C. ⁻¹ ) or higher. 0.05 ~ on the substrate
It has a thickness of 0.3 μm and has a component having an A / B molar ratio of 0.8 to 1.2 when the general formula of the perovskite thin film of the buffer layer is ABO _3, and has an amorphous structure and And / or an oxide thin film having a pyrochlore structure is prepared. The oxide thin film has, for example, 25 to 520 ° C.
It can be formed by the sputtering method or the vacuum deposition method at the substrate temperature of. The oxide thin film is 550 to 800 ° C.
When the oxide thin film has the above-mentioned film thickness, composition and crystal structure, the thin film changes its lattice matching with the substrate when it changes to the perovskite structure. The orientation is such that the film itself is easily formed regardless of that, and 100 orientation is easily formed in a crystal system (cubic crystal) having a Curie temperature or higher.
Therefore, if this property is utilized, 100 orientation can be formed on any substrate at the Curie temperature.

Next, in a second step, the perovskite thin film is used as a buffer layer, and a ferroelectric thin film having a perovskite structure is epitaxially grown on the buffer layer. For epitaxial crystal growth, the substrate temperature during sputtering or chemical vapor deposition should be 5
It may be heated to 50 to 650 ° C. At such a temperature, the ferroelectric thin film is a cubic crystal, and the crystal grows while maintaining 100 orientation. When the film is cooled after the film formation, the linear thermal expansion coefficient of the substrate is larger than that of the film. Therefore, compressive stress acts in the in-plane direction of the film from the substrate, and the 001 axis is predominant in the direction perpendicular to the substrate surface. It is formed.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION With a film thickness of 0.05 to 0.3 μm,
A thin film having a component having an A / B molar ratio of 0.8 to 1.2 and having an amorphous structure and / or a pyrochlore type structure is heat-treated at a temperature of 550 to 800 ° C. to be converted into a perovskite structure. And, at a Curie temperature or higher, it tends to have 100 orientation. Since such orientation is not formed by lattice matching with the substrate, it can be realized on any substrate. It goes without saying that if this thin film is used as a buffer layer and a ferroelectric thin film having a perovskite structure is epitaxially grown thereon, this also takes 100 orientation at the Curie temperature or higher. If this is performed on a substrate having a linear thermal expansion coefficient of 1.5 × 10 ⁻⁵ (° C. ⁻¹ ) or higher, the film will be subjected to compressive stress in the in-plane direction from the substrate during cooling after film formation. The 001 axis is predominantly formed in the direction perpendicular to the substrate surface. That is, a 001-oriented perovskite ferroelectric thin film can be produced.

In the production of the buffer layer, the composition of the amorphous or pyrochlore structure film before heat treatment preferably has a composition A / B molar ratio of 0.8 to 1.2. If it is less than 8, the A ion deficiency is too large to form the perovskite structure after the heat treatment, and if it exceeds 1.2, the film having the perovskite structure after the heat treatment tends to have the 111 orientation.

The reason why the film thickness of the amorphous or pyrochlore structure film before heat treatment is preferably 0.05 to 0.3 μm is as follows. If the thickness is less than 0.05 μm, the perovskite thin film formed after the heat treatment is formed in stripes on the substrate and does not completely cover the substrate, so that a good quality thin film cannot be formed on the buffer layer of the perovskite thin film. A film thickness of 0.05 μm or more is required to form a buffer layer that completely covers the surface of the substrate.
On the other hand, if it is thicker than 0.3 μm, it cannot be used as a buffer layer because it takes a random orientation instead of the 100 orientation when the perovskite is formed during the heat treatment. Therefore, in order to form the perovskite buffer layer in the first step, the film thickness is 0.05 to 0.3 μm.
It is desirable that it is m.

Further, the heat treatment temperature for forming the buffer layer is preferably 550 to 800 ° C., 550 ° C.
If it is less than 1, the single-phase film having the perovskite structure is not formed, and the film having the mixed pyrochlore structure or the single-phase film having the pyrochlore structure is not used as the buffer layer. Further, at a temperature higher than this temperature range, the component of PbO has a very large tendency to volatilize from the film, so that it does not change to a perovskite structure and cannot be used as a buffer layer.

In the first step, various sputtering methods or vacuum vapor deposition methods can be used to form an amorphous thin film and / or a pyrochlore type oxide thin film having the above-mentioned conditions on a substrate. For example, a high frequency sputtering method using an oxide powder containing components A and B as a target may be used, or a reactive sputtering method or oxygen vapor deposition in which oxygen is contained in an atmosphere gas using an alloy or a composite metal containing components A and B as a target. It may be the law. In the second step, when the perovskite ferroelectric thin film is epitaxially formed on the buffer layer, the substrate is 50
The components may be deposited and grown on the buffer layer by heating at 0 to 700 ° C. by a sputtering method or a chemical vapor deposition method.

Further, the coefficient of linear thermal expansion is 1.0 × 10 ⁻⁵ or more.
If a film is formed under the above conditions using a substrate of 2.1 × 10 ⁻⁵ ° C. ⁻¹ , the linear thermal expansion coefficient (0.9 × 1
Since it is higher than 0 ⁻⁵ ° C. ⁻¹ ), compressive stress is exerted on the film in the substrate surface direction during cooling after the main film formation, and the film is likely to be 001-oriented. When the substrate has a coefficient of linear thermal expansion of less than 1.0 × 10 ^-5 , compressive stress does not work effectively in the substrate surface direction during cooling, and a film with a high 001 orientation cannot be obtained, or tensile stress is generated in the substrate surface direction. Will work, resulting in a 100-oriented film. When a substrate having a coefficient of linear thermal expansion larger than 2.1 × 10 ^-5 ° C ^-1 is used, the compressive stress that the film receives from the substrate is too large because the difference in the coefficient of linear thermal expansion between the film and the substrate is large. ,
The film peels off.

Based on the above principle, even if an expensive MgO single crystal substrate is not used, a ferroelectric having 001 orientation on any substrate having a linear thermal expansion coefficient of 1.5 × 10 ^-5 ° C ^-1 or more. Body thin films such as PbTiO ₃ , Pb (Zr, Ti) O ₃ ,
A (Pb, La) TiO ₃ thin film can be produced.

[0015]

EXAMPLES Hereinafter, a method for manufacturing a ferroelectric thin film according to the present invention will be specifically described based on examples. The film formation was performed with a high frequency magnetron sputtering device capable of mounting two 6-inch targets. A heater is installed in the substrate holder to heat up to 700 ° C., and the target-substrate distance can be freely set within a range of 50 to 160 mm. Further, a target tray made of oxygen-free copper is arranged on each of the two targets, and the target powder used for the formation of the buffer layer and the target powder used for the main deposition can be filled therein. An example for producing a 001-oriented ferroelectric thin film PbTiO ₃ is shown below.

(First Step) First, a buffer layer was formed on the substrate by the following procedure. A powder having a stoichiometric composition of PbTiO ₃ was used as a target material, and this powder was spread on a target dish made of oxygen-free copper and pressed while flattening the surface to obtain a target. Various mirror-polished metal plates were used as the substrate, which were fixed to a substrate holder, the target-substrate distance was set to 60 mm, and an amorphous film was formed under the film forming conditions shown in Table 1. It is known by ICP emission spectrometry that the composition of the amorphous film produced under such conditions has a Pb / Ti molar ratio of 1.1.
Next, the amorphous film was heat-treated at 575 ° C. for 5 minutes in the chamber with a heater installed in the substrate holder. It has been confirmed by XRD that a crystalline film having a perovskite structure having a composition of PbTiO ₃ is formed on the amorphous film on the substrate by this heat treatment, and that it is strongly 001-oriented at room temperature. In other words, if the cubic crystal is above the Curie temperature (490 ° C), the buffer layer
This means that the 00 orientation is formed.

[0017]

[Table 1]

(Second step) Next, the distance between the target and the substrate was set to 100 mm, and the target layer on the buffer layer was formed as shown in Table 2.
The main film formation was performed under the conditions of. The target of this film formation is PbO
Was excessively contained so that excessive PbO was supplied during the film formation. The substrate temperature during film formation was set at 620 ° C.,
Epitaxial growth was performed on the buffer layer.

[0019]

[Table 2]

A thin film having a thickness of 2.5 μm thus obtained was prepared. From the X-ray diffraction pattern, it was found that a PbTiO ₃ film having a strong 001 orientation was prepared, and the composition was examined by ICP emission spectrometry.
It was found to be TiO ₃ . Observation of the structure of the film with a scanning electron microscope revealed that a dense film was formed. The 001 orientation rate α calculated from the X-ray diffraction peak intensity using the following formula is shown below. α (%) = {I ₀₀₁ / (I ₀₀₁ + I ₁₀₀ + I ₁₀₁ + I ₁₁₀ + I ₁₁₁ )} × 100 (1)

[0021]

[Table 3]

A gold upper electrode is formed on the surface of this thin film,
The substrate was used as the bottom electrode to determine the pyroelectric coefficient of the film. The temperature was raised from -50 ° C at a rate of 3 ° C / min, and the pyrocurrent was measured with an electrometer. As a result, the pyroelectric current was measured without polarization treatment. The PbTiO ₃ film produced on the Ni substrate has a pyroelectric coefficient of 1.0 × 10 ⁻⁸ C /
It was cm ² ° C., and exhibited extremely excellent pyroelectric properties as compared with the PbTiO ₃ system sintered body.

The results of other examples of the present invention which were tried by the same procedure are shown below. From the table below, it can be seen that a 001-oriented ferroelectric thin film can be obtained according to the conditions of the claims of the present invention.

[0024]

[Table 4]

[0025]

[Table 5]

[0026]

As described above in detail, according to the method for manufacturing a ferroelectric thin film element of the present invention, expensive Mg as a base substrate is used.
Even if another inexpensive substrate is used without using the O single crystal substrate,
A ferroelectric thin film element having 01 orientation can be manufactured. Therefore, the method for manufacturing a ferroelectric thin film element of the present invention can be used in a wider range in the field of electronic parts,
It is extremely effective in practical use.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C23C 16/40 C23C 16/40 16/44 16/44 C30B 23/00 C30B 23/00 29/22 29/22 Z H01B 3/00 H01B 3/00 F

Claims (2)

[Claims] 1. The linear thermal expansion coefficient is 1.0 × in the first step.
In forming a perovskite thin film as a buffer layer on a substrate of 10 ^{−5 to} 2.1 × 10 ⁻⁵ (° C. ⁻¹ ), 0.
It has a film thickness of 05 to 0.3 μm and a film component of 0.8 to A / B molar ratio with respect to the general formula ABO _{3 of} perovskite.
1.2, forming an oxide thin film having a crystalline structure of an amorphous structure and / or a pyrochlore type structure on a substrate, and then heating the oxide thin film at 550 to 800 ° C. to change it to a perovskite type structure. To form a buffer layer of a 100-oriented perovskite thin film on the substrate at a Curie temperature or higher, and as a second step, form a tetragonal crystal at room temperature of the perovskite structure on the substrate epitaxially with respect to the perovskite thin film. A method of manufacturing a ferroelectric thin film having a perovskite structure having 001 orientation at room temperature, which comprises crystallizing the ferroelectric thin film. 2. A method for producing an oxide thin film having a crystalline structure of an amorphous structure and / or a pyrochlore type structure on the substrate in the first step is a sputtering method or a vacuum evaporation method, and the ferroelectric film in the second step is used. The method for producing a ferroelectric thin film according to claim 1, wherein the method for crystal-growing the body thin film is a sputtering method or a chemical vapor deposition method.