JPH04264317A - Ferrodielectric substance - Google Patents

Abstract

PURPOSE:To prevent dielectric breakdown due to polarizing treatment and improve a yield by molding a ferrodielectric substance of a substrate with cyclic irregularities on the surface formed by a Sol-Gel technique and a ferrodielectric substance crystal orientation film thereon. CONSTITUTION:A ferrodielectric is molded of a substrate 1 which is formed by a Sol-Gel technique with cyclic irregularities on the surface and a ferrodielectric substance crystal orientation film 2 formed thereon. The film 2 has atoms arrayed along the irregularities on the substrate 1. To obtain film quality, crystal property and orientation approximated to those of a single crystal, the sizes of the irregularities are approximated to an atomic level to make L1, L2 as small as possible. As a change in size affects the film quality of the film 2, uniform irregularities are required all over the substrate. That is why a Sol-Gel technique is applied. No polarizing treatment is required at all and natural polarization is provided, so that the rate of crystal orientation is 50% or more to improve a yield of a high performance ferrodielectric substance.

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD The present invention relates to ferroelectric materials. The ferroelectric material of the present invention has various applications such as piezoelectric elements, infrared detection elements, optical waveguides, spatial modulation elements, electro-optical elements, memory elements, and displays.

0002

[Prior Art] Applications of ferroelectric materials in the electronics and optoelectronics fields include piezoelectric elements,
There are various types such as infrared detection elements, optical waveguides, spatial modulation elements, electro-optical elements, memory elements, and displays. By the way, in an element that extracts a change in the spontaneous polarization Ps of a ferroelectric substance as an output, such as a piezoelectric element or a pyroelectric infrared detection element, the largest output can be obtained when the spontaneous polarization Ps of the ferroelectric substance is aligned in one direction. . By the way, the ferroelectric materials used in these devices are polycrystalline.
There is no directionality in the arrangement of the crystal axes, and therefore the spontaneous polarization Ps is also arranged randomly. Therefore, when these ferroelectric materials are used in the devices described above, it is necessary to apply a high electric field (~
100 kV/cm) is required to align the directions of Ps. Further, although there are many reports regarding the production of films of PLZT, PbTiO3, etc., the method of producing films in which the ferroelectric phase region of these films is oriented along the c-axis, which is the polarization axis, has not been clarified. Moreover, the method for manufacturing ferroelectric materials in which even spontaneous polarization is oriented in one direction has not been elucidated at all.

0003

[Purpose] The following problem occurs in the method of aligning Ps by applying a high electric field to a ferroelectric material. (1) Dielectric breakdown may occur due to polarization treatment.
Yield decreases. (2) In devices such as high-resolution array devices in which many microelements are arranged at high density, it is difficult to polarize them uniformly. (3) In integrated devices in which a thin film of ferroelectric material is formed on a semiconductor device, polarization itself may not be possible. (4) The thickness of the thin film of ferroelectric material used in each element is about several microns, and if the thermal expansion coefficients of the substrate and film are different, cracks and peeling occur. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a ferroelectric material having a thin film of ferroelectric material that eliminates these drawbacks.

0004

[Structure] The first aspect of the present invention is a ferroelectric device comprising a substrate formed by the Sol-Gel method and having periodic irregularities on the surface, and a crystal oriented film of a ferroelectric substance formed on the substrate. Regarding the body. A second aspect of the present invention relates to the ferroelectric material, wherein the substrate contains dopant atoms that change the coefficient of thermal expansion. A third aspect of the present invention relates to the ferroelectric material, wherein the crystal orientation film of the ferroelectric material has an orientation rate of 50% or more. A fourth aspect of the present invention relates to the ferroelectric material having an electrode layer between the substrate and the crystal orientation film of the ferroelectric material.

Since the atoms of ferroelectric thin films are arranged along the irregularities of the substrate, they have good film quality, crystallinity, and
In order to obtain orientation, it is desirable to bring the size of the unevenness close to the atomic level and to make L1 and L2 as small as possible. Since variations in these sizes greatly affect the quality of the upper ferroelectric thin film, it is necessary to form extremely uniform irregularities over the entire surface of the substrate. In order to solve this requirement, in the present invention, a substrate is formed by the Sol-Gel method. The advantages of producing a substrate using the Sol-Gel method are as described below. The present inventors separately conducted ``Cr sputter deposition, photolithography, etching, and after forming a desired Cr pattern, R using the Cr pattern as a mask.
The authors proposed an invention related to a ferroelectric material consisting of a laminate of a ferroelectric alignment film and a textured substrate, which includes a step of performing IE (reactive ion etching) to form depressions and depressions on the substrate and removing Cr. While the method requires a complicated process, the method according to the present invention is extremely simple. For the effect of graphioepitaxy, it is preferable that the surface unevenness be submicron, and as the substrate area increases, there are concerns about whether photolithography and etching techniques can process uniformly or not, including reproducibility. However, Sol-
In the Gel method, after firing, the initial pattern pitch is 5
Since 0% reduction occurs, the stamper design (currently 1
A pattern of 0.5 μm can be easily formed. Another advantage is that it allows doping of the substrate (glass) to control the coefficient of linear expansion. As the substrate forming material, quartz, spinel, metal oxide, etc. can be used. FIG. 1 is a cross-sectional view of a ferroelectric material produced by the Sol-Gel method. Usually, the irregularities formed on the substrate of the present invention are on the order of microns. L in Figure 1
1 and L2 are both about 0.1 μm to 1 μm, and the depth d of the unevenness is about 0.01 μm to 2 μm.

[0006] Regarding the method of manufacturing a substrate by the Sol-Gel method, reference can be made to the examples and the section regarding the method of manufacturing a ferroelectric thin film. Sol-Gel as described below
A substrate is prepared by the method, and the thermal expansion coefficients of the obtained substrate and the ferroelectric thin film formed on the substrate are made approximately equal,
In order to avoid disadvantages such as film peeling, it is preferable to adjust the coefficient of thermal expansion by incorporating a dopant into the substrate. Therefore, titanium, boron, lithium, sodium, potassium, magnesium, calcium,
It is preferable to add atoms such as aluminum as a dopant. For example, when adding titanium atoms, titanium atoms need only exist as titanium oxide when the substrate is glass, so in order to supply titanium oxide, titanium isopropoxide is added to ethyl silicate. Thereafter, it is hydrolyzed, aged, and annealed to form a glass body. FIG. 4 shows the change in thermal expansion coefficient with respect to the titanium oxide concentration at this time. The vertical axis is the coefficient of thermal expansion (d×1/1
07°C), and the horizontal axis is the TiO2 content (wt%).

Next, a method for producing a ferroelectric thin film using the Sol-Gel method will be explained. This Sol-Gel method grows metal-oxygen-metal bonds by hydrolyzing and polycondensing metal organic compounds such as metal alkoxides in a solution system.
This is a method for producing an inorganic oxide that is completed by final sintering. A feature of the Sol-Gel method is that a uniform large-area film can be obtained at a low substrate temperature. Furthermore, since the film is formed from a solution, it has excellent adhesion to the substrate. Specifically, a solution containing a metal organic compound is applied onto a substrate, a film made of an inorganic oxide precursor is laminated, and then sintering is performed. At this time, it may be applied to both sides of the adhesive surface, that is, the substrate side and the thick film side. The metal-organic compounds used include methoxide, ethoxide, propoxide of metals constituting the inorganic oxide,
Examples include alkoxides such as butoxide and acetate compounds. Inorganic metal salts such as nitrates, oxalates, and perchlorates may also be used. When using these metal salts as raw materials, it is necessary to dissolve them in polar organic solvents such as alcohols, polyhydric alcohols, and carbinols, and use them as reaction intermediates in a controlled manner so that they have metal-oxygen bonds. . Furthermore, for ferroelectric materials containing alkali metals and alkaline earth metals, alcoholates obtained by dissolving these metals in alcohols can also be used. In order to produce inorganic oxides from these compounds, it is necessary to proceed with hydrolysis and polycondensation reactions, so it is necessary to add water to the coating solution. The amount added varies depending on the system, but if it is too large, the reaction proceeds quickly, resulting in a non-uniform film quality, and it is difficult to control the reaction rate. Even if the amount of water added is too small, it is difficult to control the reaction, so there is an appropriate amount. Generally, it is preferred to use an equivalent mole to 5 times the equivalent mole relative to the number of bonds to be hydrolyzed. Furthermore, addition of a hydrolysis catalyst allows control of reaction rate and reaction form. Common acids and bases are used as catalysts. It is said that acidic catalysts facilitate the production of linear polymers, and basic catalysts facilitate the production of three-dimensional polymers, but this cannot be said unconditionally, depending on the overall concentration and pH of the solution. In the case of the present invention, a structure intermediate between the two is desirable. As the solvent for addition, it is desirable that the above-mentioned material does not precipitate, that is, one that has excellent solubility. Although the solution concentration depends on the coating method, in the case of a spin coating method, it is preferably adjusted so that the solution viscosity is from several cP to more than ten cP. Furthermore, a chelating agent or the like may be added. The obtained precursor film is completed by sintering. Although the sintering temperature varies depending on the material, it can be produced at a temperature lower than that required for firing ordinary metal oxide powder. Since many substrate materials react or undergo compositional or structural changes at high temperatures, the use of this method, which allows film formation at low temperatures, expands the possibilities for use of the substrate. The substrate may be in the form of a plate or a film, and its materials include (a) an insulating substrate, (b) a semiconductor substrate,
(c) Any conductive substrate may be used. Examples of (a) are SiO2, Al2O3, MgO, SrTiO
Examples of (b) include Si, GaAs, I
Examples of (c) include nP, ITO/glass, etc.
There are Pt, stainless steel (SUS), etc. Conventionally, there have been reports that C-axis orientation, a-axis orientation, and random orientation occur depending on the type of substrate. There is no unified opinion in that report, and I believe that this will be clarified in future research. However, in the present invention, to put it simply, it is important that no matter what kind of substrate is used, once the unevenness is designed on the surface, the orientation will be along the C axis. Therefore, there is no relationship between substrate type and film quality. However, although the Sol-Gel film crystallizes perovskite at a temperature of several hundred degrees,
It is undesirable if a reaction progresses at the substrate interface due to this heat, so Pt (platinum) is used as the barrier layer, and this metal is sometimes used as the lower electrode.

As an example of a ferroelectric material, a case where PLZT is used will be described. The chemical composition of PLZT is the following general formula:

It is a ceramic composite oxide represented by the following formula, and is obtained by sintering after going through the processes of mixing metal organic compounds, hydrolysis, and polycondensation. In other words, in the case of PLZT, lead acetate,
A ceramic composite oxide precursor is prepared by dissolving lanthanum acetate, zirconium alkoxide, and titanium alkoxide in an organic solvent. The lead metal element can be provided by dissolving lead nitrate in ethylene glycol. The same can be done for lanthanum, zirconium, and titanium. What is important here is that the organic compound in which a metal atom has a chemical bond with an oxygen atom (organic substances with such a bond are called a metal-organic compound) is uniformly dissolved in the solvent. Further, even if the starting material is not a metal organic compound, it is sufficient if a metal-oxygen bond is provided as a reaction intermediate. Although PLZT is shown as an example of a ferroelectric material, other similar materials (characteristics) have a crystal structure belonging to the perovskite system, and any complex oxide having a perovskite structure may be used, and there are no particular limitations. This crystal system includes KNbO3, LiNbO3, BaTiO3, SrTiO
3, PbTiO3, Pb(Zr,Ti)O3, etc. In addition, there is a tungsten bronze type crystal system that has similar characteristics, and all of these are considered to belong to the name composite oxide. Then, the target PL is produced by firing the precursor thus obtained.
A bonded alignment film of ZT can be obtained.

[0009]

Examples Example 1 Ethyl silicate is hydrolyzed by adding water under acidic conditions, and silica powder having a particle size of about 1 μm is added. After this, pour the liquid raw material into a suitable container and it will become soft So.
Since l becomes gel with time, a stamper having a desired uneven pattern to be transferred is pressed at this time, and the pattern is transferred to the liquid undergoing Sol-Gel transition. Thereafter, it was removed from the stamper and dried in air at 60°C for 7 days. Next, to prevent cracks from forming, the material is gradually heated from room temperature to approximately 1200.degree. C. and fired for 2 hours. At this time 3
Dimensional shrinkage occurs and a fused silica substrate 1 is obtained which has shrunk by 50% from the stamper pattern. The pattern obtained is 1/2 the size of the stamper. This means that when photolithography and etching techniques are used to form a stamper, half of the minimum pattern that can be obtained using those techniques can be obtained. After producing the base substrate in this way, Sol-Gel is applied on it.
A ferroelectric thin film 2 is formed by a method. Lead acetate and lanthanum acetate are dissolved in methoxyethanol, zirconium normal propoxide and titanium isopropoxide are added to the solution, and after aging for more than 5 hours, a small amount of water is added to cause partial hydrolysis. was used as the coating liquid. This coating solution is applied onto the fused quartz substrate having micron-order unevenness as described above, prebaked at 160°C for 5 minutes or more, sintered at 400°C for 30 minutes or more, and then annealed at 500°C or more and 1200°C or less. After 60 minutes or more, a PLZT ceramic thin film is obtained. The PLZT obtained by this method has a film thickness of 500 Å or more, and by repeating the steps from coating to annealing, a film of several hundred microns can be obtained on the substrate. Figure 3 shows a typical thin film (thickness: 1
(μm) X-ray diffraction pattern is shown. Only the (001) and (100) reflections of the perovskite structure and their higher-order reflections are observed. Also, the intensity of (001) reflection is (100)
Since it is significantly larger than that of , it can be seen that it is a c-axis oriented film. The c-axis orientation rate α is defined by the following equation.

[Math. 1] α=I(001)/{I(001)+I(10
0)} Here, I(001) and I(100) represent the diffraction intensities of (001) and (100) reflections, respectively. In the sample of the base substrate processed according to the present invention, when compared with the case where the base substrate is not processed, the case where the base substrate is not processed shows almost no orientation although it varies somewhat depending on the annealing conditions, whereas the base substrate of this embodiment shows almost no orientation. In the processed sample, an orientation ratio as defined above of 78% or more was obtained.

Example 2 After producing the quartz substrate 1 as described in Example 1, a Pt thin film having a thickness of 0.3 μm was formed as the lower electrode 3 by sputtering. Sputtering gas is Ar or Ar
+O2 mixed gas. This base electrode may be made of a metal other than platinum or an oxide conductor (ZnO:Al), as long as it is sufficiently heat resistant to the Sol-Gel process temperature. In this manner, a thin film 2 of ferroelectric material made of PLZT was formed on at least one layer of the lower electrode by the Sol-Gel method. When the orientation ratio α was similarly measured, a value of 76% was obtained. Next, the upper electrode 4 was formed as shown in FIG. 2, and the pyroelectric coefficient was measured. The pyroelectric coefficient increases in proportion to the orientation of the spontaneous polarization Ps. The pyroelectric coefficient γ of the above ferroelectric is 1.7×1/108C/cm2
This value is for a conventional sample made with a fused silica substrate and subjected to polarization treatment at 200℃, 100KV/cm, and 10 minutes (γ=1.6×1/108C/cm2K
) was obtained.

Example 3 A base substrate was prepared in the same manner as in Example 1, except that silicate ethyl ester titanium isopropoxide was hydrolyzed by adding water under acidic conditions. Then, in the same manner as in Example 1, a ferroelectric thin film was formed thereon. The X-ray diffraction pattern and orientation rate of this thin film were both the same as those of Example 1.

Example 4 After producing the quartz substrate described in Example 3, a Pt thin film having a thickness of 0.3 μm was formed as a lower electrode by sputtering. The sputtering gas is Ar or a mixed gas of Ar+O2. On this lower electrode, Sol-Gel similar to Example 1 was applied.
When PLZT was formed by the method and the orientation ratio α was measured in the same manner, a value of 76% was obtained. Next, an upper electrode was formed as shown in FIG. 4, and the pyroelectric coefficient was measured. The pyroelectric coefficient γ is 1.7 × 1/108C/cm2K, and this value is the same for a conventional sample made with a fused silica substrate at 200°C.
Polarized at 100KV/cm for 10 minutes (γ
= 1.6 x 1/108C/cm2K) was obtained.

[0013]

[Effects] (1) Since the atoms are arranged along the irregularities of the substrate, the thin film of ferroelectric material formed thereon can have high quality film quality, crystallinity, and orientation closer to those of a single crystal. (2) If a thin film of ferroelectric material is formed by the Sol-Gel method, there is no need to perform any polarization treatment, and it will be polarized naturally and its crystal orientation will be over 50%, making it a high-performance ferroelectric material. can be obtained with good yield. (3) If the thermal expansion coefficients of the substrate and the ferroelectric thin film are adjusted to the same level, cracks and peeling will not occur.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a ferroelectric material according to Example 1 of the present invention.

FIG. 2 is a cross-sectional view of a ferroelectric material according to Example 2 of the present invention.

FIG. 3: Ferroelectric thin film PbZrx obtained in Example 1
The X-ray diffraction pattern of TiyO3 (x=0.2, y=0.8) is shown. The vertical axis shows intensity, and the horizontal axis shows 2θ (°).

FIG. 4 is a graph showing the influence of TiO2 content on the thermal expansion coefficient of a fused silica substrate.

[Explanation of symbols]

1 Board 2 Crystal orientation film (thin film) of ferroelectric material 3 Lower electrode 4 Upper electrode

Claims (4)

[Claims] 1. A ferroelectric material comprising a substrate formed by the Sol-Gel method and having periodic irregularities on its surface, and a crystal oriented film of a ferroelectric material formed on the substrate. 2. The ferroelectric material of claim 1, wherein the substrate contains dopant atoms that change the coefficient of thermal expansion. 3. The ferroelectric material according to claim 1, wherein the crystal orientation film of the ferroelectric material has an orientation ratio of 50% or more. 4. The ferroelectric material according to claim 1, further comprising an electrode layer between the substrate and the crystal orientation film of the ferroelectric material.