JP2001135876A - Manufacturing method of piezoelectric or ferrodielectric thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resolve the controlling difficulty of the crystalline orientation of a piezoelectric thin film, when forming it on a substrate by a sol-gel method. SOLUTION: In the forming process of the precursor of a ceramic thin film, its heating is stopped simultaneously with the elimination of its organic components.

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 piezoelectric thin film or a ferroelectric thin film, and manufactures a thin film element utilizing physical properties derived from crystallinity, that is, piezoelectricity and ferroelectricity, by a sol-gel method. About the method.

[0002]

2. Description of the Related Art Along with the progress of thin film technology, there have been many attempts to make ferroelectric materials into thin films and to make use of their ferroelectricity and piezoelectricity. In particular, among oxides having a perovskite-type crystal structure represented by the chemical formula of ABO3, there are many materials exhibiting excellent characteristics, and application to thin film elements is expected.

[0003] There are roughly two types of methods for producing a thin film. One is an epitaxial growth method, and the other is to form an amorphous film, which is a precursor of a desired crystalline thin film, and then apply the energy from the outside by firing or light irradiation to form the crystalline thin film. How to get. The latter is a film forming method represented by a so-called post-annealing method.

A typical example of the post-annealing method is a sol-gel method. This method has much better composition controllability than other film forming methods. It is extremely effective for adding trace elements, and it can be said that it is an indispensable technique when the characteristics are controlled by a delicate composition ratio. In addition, from the viewpoint of film forming properties, it is possible to form a uniform film on a large substrate, and from an economic viewpoint, large-scale vacuum equipment is not required. Has excellent features.

[0005]

However, it is still difficult to say that the produced thin film has sufficient characteristics. In order to reach the expected level from the bulk value, further improvement in characteristics is desired.

[0006] As one of the main factors influencing the characteristics of a thin film, the crystallinity of the thin film is often discussed. Piezoelectricity and ferroelectricity are closely related to the spontaneous polarization behavior of the crystal itself, and it is important to control the direction of the polarization axis in order to obtain these characteristics more effectively. For example, considering the above-mentioned perovskite-type crystal, it is considered that obtaining a thin film having a c-axis orientation in a tetragonal crystal is effective for maximizing the film performance.

However, it is extremely difficult to control the crystal orientation of the sol-gel method as compared with other film forming methods. This is because, when the amorphous precursor film is crystallized, crystal nuclei are randomly generated in the film. In order to obtain a desired orientation, it is necessary to control the generation of crystal nuclei, and it is necessary to take some measures from the stage of forming the precursor film.

For example, Japanese Patent Application Laid-Open No. 4-259380 discloses that the heat treatment temperature of a sol applied on a substrate causes a difference in the orientation of a subsequently crystallized thin film. By taking advantage of this fact, a heat treatment temperature at which a desired orientation can be obtained is set, and a thin film oriented in a specific axis is obtained. However, the heat treatment temperature itself is a value empirically determined. When the sol components are different, the state of the film is considered to be completely different even at the same heat treatment temperature. If the sols are different, there is a problem that the relationship between the heat treatment temperature and the orientation cannot be reproduced.

In Japanese Patent Application Laid-Open No. Hei 6-305714, the orientation of a finally laminated thin film is controlled by reducing the thickness of one layer after firing to a certain value or less. However, in this method, it is necessary to perform a multilayer coating of the sol in order to obtain a desired film thickness. There has been a problem that restrictions are imposed on productivity.

An object of the present invention is to control the orientation of a thin film with good reproducibility without reducing productivity by the sol-gel method.

[0011]

According to a first aspect of the present invention, there is provided a method of manufacturing a piezoelectric thin film or a ferroelectric thin film, comprising: applying a sol to an electrode formed on a substrate; and heating and drying the sol. A step of forming an amorphous precursor film, and a method of manufacturing a piezoelectric thin film or a ferroelectric thin film, comprising heating the substrate by lamp annealing and crystallizing the amorphous precursor film. The timing for stopping the heating and drying of the sol in the step of heating the substrate may be simultaneous with the desorption of the organic component contained in the sol.

According to the above method, since the generation of random crystal nuclei in the drying stage is suppressed, the precursor film has an effect of being oriented and grown reflecting the structure of the lower electrode.

According to a second aspect of the present invention, there is provided a method for producing a piezoelectric thin film or a ferroelectric thin film, wherein a sol is coated on an electrode formed on a substrate, and the sol is heated and dried to form an amorphous precursor. The step of forming a film, the step of heating the substrate by lamp annealing, and the step of crystallizing the amorphous precursor film are sequentially repeated to form a piezoelectric thin film or a ferroelectric thin film in which a crystalline thin film is laminated. In the production method, the timing at which the heating and drying of the sol is stopped may be simultaneous with the elimination of the organic component contained in the sol.

According to the above-mentioned method, there is an effect that all the laminated thin films exhibit an orientation reflecting the structure of the lower electrode.

According to a third aspect of the present invention, there is provided a method for producing a piezoelectric thin film or a ferroelectric thin film, wherein the sol comprises 2-n-butoxyethanol as a main solvent and each metal element is a carboxylate, acetylacetonate or alkoxide. Characterized in that it is produced by dissolving in any one of the following forms.

According to the above method, there is an effect that the structure of the lower electrode is easily reflected on the orientation of the thin film.

According to a fourth aspect of the present invention, there is provided a method of manufacturing a piezoelectric thin film or a ferroelectric thin film, wherein platinum is used as the electrode, and an incident angle of X-rays on the platinum surface is θ, and 0 ° ≦ 2θ
In the range of ≦ 90 °, all the diffraction peak intensities of platinum were I (hkl), and the diffraction peak intensity of the platinum (002) plane was I (00).
When defined as 2), it is characterized in that I (002) / ΣI (hkl) = 0.9.

According to the above method, there is an effect that the piezoelectric thin film or the ferroelectric thin film is oriented (001).

According to a fifth aspect of the present invention, there is provided a method of manufacturing a piezoelectric thin film or a ferroelectric thin film, wherein a conductive oxide represented by a chemical formula of ABO3 is epitaxially formed on the platinum.

According to the above method, the piezoelectric thin film or the ferroelectric thin film has an effect of exhibiting further excellent crystal orientation.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS When a ceramic thin film is produced by a sol-gel method, first, a sol is coated on a substrate and dried to form a precursor film. In order to obtain a desired film thickness, a precursor film is laminated by repeating the sol coating and drying steps. In such a process, the drying temperature of the sol has a great effect on the film forming property and film quality, and setting this temperature is important. For example, when the drying temperature is too low, the organic component and the solvent component coordinated to the metal in the sol do not evaporate and remain in the film. Even if a sol is further applied on the semi-dried state, the remaining organic components are dissolved again in the sol. Therefore, even if the application and drying of the sol are repeated, the film thickness cannot be increased, and at the same time, the film quality becomes uneven. On the other hand, if the drying temperature of the sol is too high, bumping of the organic component occurs, leaving a hollow structure corresponding to the volume. Although the degreasing itself is completely performed, the film quality is significantly deteriorated. When crystallization also occurs at the same time, cracks occur in the thin film due to rapid film shrinkage. In order to prevent the above-mentioned phenomena, sufficient attention is paid to the drying temperature after the application of the sol, and an optimum temperature is set in advance according to the sol component.

On the other hand, the drying time of the sol has not received much attention until now. This is because the main purpose of drying is considered to be the evaporation of organic components in the film and the formation of a precursor film accompanying the evaporation. Therefore, even if the drying temperature is determined, the drying time tends to be set longer for the purpose of providing a margin. Up to now, no quantitative discussion focusing on the drying time has been conducted, and the correlation with the film properties has not been sufficiently investigated.

By examining the drying process of the sol and the state of the precursor film by paying attention to the degreasing time, it was confirmed that the characteristics of the thin film changed depending on the drying time. The shorter the heating of the substrate after the evaporation of the organic components, the better the properties of the fired thin film were obtained. A detailed examination of the film structure revealed that crystallization partially started at the stage of the precursor film, even though the drying temperature was almost the same as the evaporation temperature of the organic components in the sol. However, crystallization did not start simultaneously with the heating, and no generation of crystal nuclei was observed during the evaporation of the organic component. As the substrate temperature rises, first, the solvent components evaporate, and the organic molecules coordinated to each metal dissociate and decompose. When all the organic components contained in the sol are finally eliminated, it is considered that the applied thermal energy starts to contribute to crystallization. This indicates that the precursor film is kept in a completely amorphous state when there is no extra substrate heating after the elimination of the organic component.

Finally, when lamp annealing is used to heat and bake the precursor film, the radiation energy is most efficiently absorbed by the substrate first. Since the substrate temperature rises first and reaches the maximum temperature, the temperature of the precursor film is highest in the region in contact with the substrate. Therefore, crystal nuclei are generated on the substrate in the initial stage of firing. Then, as the crystallization temperature reaches the film surface, these crystal nuclei continue to grow, reflecting the structure of the substrate, and eventually the entire region of the film is crystallized. By utilizing this fact, a crystalline thin film having a desired orientation can be obtained by using a substrate having good matching with the target thin film in advance. By controlling the polarization axis to a specific orientation with respect to the substrate,
Piezoelectricity and ferroelectricity are exhibited more effectively. Since the thin film sufficiently exhibits its original characteristics, its performance is dramatically improved when it is incorporated as a device.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Example 1) First, lead acetate, zirconium acetylacetonate and titanium tetraisopropoxide were dissolved in appropriate amounts in 2-n-butoxyethanol as a solvent and adjusted to a desired concentration. This solution is hereinafter referred to as a raw material sol liquid.

Next, platinum was formed as a lower electrode by sputtering on the SrTiO3 (001) wafer. This is hereinafter referred to as a substrate. X-ray (Cu-
Kα) A diffraction pattern was obtained. Assuming that the X-ray incident angle with respect to the platinum surface is θ, all the diffraction peak intensities of platinum in the range of 0 ° ≦ 2θ ≦ 90 ° are defined as I (hkl), and the diffraction peak intensity of the platinum (002) surface as I (002). When you do, I (002) /
ΔI (hkl) = 0.9, confirming that platinum has a strong (001) orientation.

The above raw material sol solution was applied on the substrate by spin coating, and dried in an oven at 450 ° C. First, in order to determine the time required for the organic components in the sol to completely desorb, the substrate was taken out of the oven after a certain time had elapsed, and the weight of the substrate was recorded.
After the measurement, the drying of the sol was continued in the oven again, and after a lapse of a predetermined time, the weight of the substrate was measured again as described above. Thereafter, by repeating this process, the weight change accompanying the drying of the sol was tracked, and it was considered that the elimination of the organic component was completed when the change disappeared. The cumulative time the substrate has been in the oven was 1 minute. The obtained precursor film is designated as Sample 1.

On the other hand, for comparison, a precursor film was formed by a conventional method. That is, the time required for the sol drying step was set longer. Here, the precursor film was formed by placing the substrate in an oven at 450 ° C. for 10 minutes (sample 2).

For both samples, X-ray (C
u-Kα) diffraction pattern was obtained. The results are shown in FIG. Sample 2 has a broad peak. It does not reflect a particular crystal structure, but suggests that long-range order is beginning to form. Further, a clear peak slightly observed in this broad peak is considered to be a diffraction peak caused by lead zirconate titanate (hereinafter referred to as PZT). It was suggested that crystallization had already begun in some of the precursors. On the other hand, Sample 1 does not show such a phenomenon. The film is expected to be in a completely amorphous state. Further observation by a transmission electron microscope was performed to examine the film structure in detail. In Sample 2, initial crystal growth was observed throughout the film. It was found that crystal nuclei were randomly generated. On the other hand, in Sample 1, no crystal growth was observed inside the film. Finally, in both Sample 1 and Sample 2, the substrate was heated to 900 ° C. using a rapid temperature rising lamp annealing apparatus (hereinafter referred to as RTA) to obtain a crystalline thin film. The crystal orientation of both samples was examined from the X-ray diffraction pattern by the wide angle method (FIG. 2).

In sample 1, the diffraction peak derived from PZT is (00
1) There are only sharp peaks from the plane. This is sample 1
Indicates that the film is a (001) -oriented PZT thin film.
On the other hand, no specific strong diffraction peak was observed in Sample 2,
Multiple peaks derived from PZT appear. PZT of this sample
The films were shown to be randomly oriented. Both samples are
Despite the same manufacturing conditions except for the drying time of the sol, it became clear that the orientation of PZT was significantly different.

As described above, even when the drying temperature of the sol was the same, the state of the precursor film was greatly different depending on the drying time. The drying temperature in the present embodiment is a temperature sufficiently high to remove the organic components in the sol. That is, the evaporation of the solvent itself and the dissociation of the organic molecules coordinated to the metal were surely performed. When the substrate is heated at such a drying temperature even after the organic component is separated from the precursor film, the applied thermal energy starts to contribute to crystallization. As a result, in Sample 2, crystal nuclei were randomly generated in the film at the stage of forming the precursor film. In the firing step, the crystallization temperature is gradually reached from the region of the precursor film in contact with the substrate, and accordingly, these crystal nuclei grow. Originally, these crystal nuclei are oriented in arbitrary directions, and thus the grown crystal grains have different orientations from each other. Therefore, the thin film crystallized finally to the film surface became non-oriented. On the other hand, in Sample 1, the precursor film was kept in a completely amorphous state because extra substrate heating was not performed after desorption of the organic component. Crystal nuclei are generated on the substrate (lower electrode) for the first time in the firing step, and have crystal orientations reflecting the structure of the lower electrode (platinum). Crystallization proceeds toward the film surface based on these factors, and eventually, the entire region of the film grows to reflect the structure of the lower electrode. In this example, a (001) -oriented PZT thin film could be obtained reflecting the orientation of platinum.

Example 2 First, a suitable amount of lead acetate, zirconium acetylacetonate and titanium tetraisopropoxide was dissolved in 2-n-butoxyethanol as a solvent and adjusted to a desired concentration. This solution is hereinafter referred to as a raw material sol liquid.

Next, platinum was formed as a lower electrode on the SrTiO3 (001) wafer by sputtering. This is hereinafter referred to as a substrate. X-ray (Cu-
Kα) A diffraction pattern was obtained. Assuming that the X-ray incident angle with respect to the platinum surface is θ, all the diffraction peak intensities of platinum in the range of 0 ° ≦ 2θ ≦ 90 ° are defined as I (hkl), and the diffraction peak intensity of the platinum (002) surface as I (002). When you do, I (002) /
ΔI (hkl) = 0.9, confirming that platinum has a strong (001) orientation.

The above raw material sol solution was applied on this substrate by spin coating, and dried in an oven at 450 ° C. Here, the drying time was limited to one minute. Continue RTA
The substrate was heated to 900 ° C. to crystallize the precursor film. The sol was applied again on the obtained crystalline thin film, and drying and heating of the substrate by RTA were performed under the same conditions as the first layer. Thereafter, by repeating the same process several times, a crystalline thin film was laminated to obtain a desired film thickness. This is designated as Sample 3. On the other hand, a sample was prepared by a conventional method for comparison. That is, the time required for the sol drying step was set to 10 minutes. A crystalline thin film was laminated under the same conditions as in Sample 3 except for the drying time (Sample 4).

In order to examine the crystal orientation of both samples, an X-ray (Cu-Kα) diffraction pattern was obtained using a wide angle method. The results are shown in FIG.

In sample 3, the diffraction peak derived from PZT was (00
1) There are only sharp peaks from the plane. This indicates that Sample 3 is a (001) -oriented PZT thin film.
On the other hand, no specific strong diffraction peak was observed in Sample 4,
Multiple peaks derived from PZT appear. PZT of this sample
The films were shown to be randomly oriented.

The drying temperature in this embodiment is a temperature high enough to remove organic components in the sol. That is, the evaporation of the solvent itself and the dissociation of the organic molecules coordinated to the metal were surely performed. When the substrate is heated at such a drying temperature even after the organic component is separated from the precursor film, the applied thermal energy starts to contribute to crystallization. As a result, in Sample 4, crystal nuclei were generated randomly in the precursor film formation stage. In the firing step, the crystallization temperature is gradually reached from the substrate side of the precursor film, and accordingly, these crystal nuclei grow. Originally, these crystal nuclei are oriented in arbitrary directions, and thus the grown crystals have different orientations from each other. That is, the crystalline thin films to be laminated were all randomly oriented. On the other hand, in Sample 3, since the substrate was not heated excessively after desorption of the organic component, the precursor film to be laminated was kept in a completely amorphous state. First, when the first precursor film is fired, crystal nuclei are generated on the substrate (lower electrode) having the highest temperature in the initial stage. Subsequently, they begin to grow with a crystal orientation reflecting the lower electrode structure. Since crystallization proceeds uniformly toward the film surface based on such microcrystals, the entire region of the film eventually has an orientation reflecting the structure of the lower electrode. During the lamination of the second and subsequent layers, crystal growth always reflects the orientation of the crystalline thin film corresponding to the base. As a result, the orientation does not change even if the number of layers is increased in order to increase the film thickness. In the present embodiment, reflecting the orientation of platinum (0
01) An oriented PZT thin film was obtained.

(Example 3) First, lead acetate, zirconium acetylacetonate and titanium tetraisopropoxide were dissolved in 2-n-butoxyethanol as a solvent in an appropriate amount to adjust the concentration to a desired concentration. This solution is hereinafter referred to as a raw material sol liquid.

Next, a platinum film is formed on the SrTiO3 (001) wafer by sputtering, and SrRuO3 (hereinafter referred to as SRO) is formed on the film.
Was similarly formed by sputtering. X-ray (Cu-Kα) diffraction patterns were obtained using the wide-angle method to determine the crystal orientation of SRO. When the X-ray incident angle is θ, 0 ° ≦ 2θ ≦ 9
The intensity of all diffraction peaks of the SRO in the range of 0 ° is ΔI
(hkl), when the diffraction peak intensity of the SRO (040) plane was defined as I (040), it was found that I (040) / ΣI (hkl) = 0.9.

The above raw material sol solution was applied on the substrate by spin coating, and dried in an oven at 450 ° C. Here, the drying time was limited to one minute. Continue RTA
The substrate was heated to 900 ° C. to crystallize the precursor film (Sample 5).

In order to examine the crystal orientation of the obtained thin film, an X-ray (Cu-Kα) diffraction pattern was obtained by a wide angle method.
FIG. 4 shows the results. For comparison, the diffraction pattern for Sample 1 (described in Example 1) is also shown. As is clear from the figure, the PZT thin film of Sample 5 was also made of the lower electrode (S
It was found that the (001) orientation was shown reflecting the orientation of (RO). Further, a rocking curve of X-ray diffraction was obtained for the PZT (001) plane. The incident angle of the X-ray with respect to the sample surface is represented by θ, and the deviation from θ is represented by Δω (Δθ). Δ
The diffraction intensity from the (001) plane was measured within the range of ω (Δθ) = ± 10 °. FIG. 5 shows the results. Comparing the half-widths around Δω = 0 °, Sample 5 is clearly smaller. That is, it was found that the PZT thin film of Sample 5 had better crystal orientation.

In sample 5, an SRO is used as a lower electrode. Since the oxide has the same crystal structure as PZT, the interface consistency is even better than platinum. Therefore, it is considered that the crystal at the initial stage of growth had a stable composition and structure from the stage of generation of the crystal nucleus. By reducing the number of defects, it can be said that lattice disorder during the subsequent growth process was suppressed, and more excellent crystal orientation was realized.

[0044]

As described above, the generation of random crystal nuclei can be suppressed by considering the drying time of the sol,
An almost completely amorphous precursor film can be formed. Such a precursor film is capable of crystal growth reflecting the structure of the lower electrode in the firing step, and the crystalline thin film finally obtained has a desired orientation. Since the original characteristics of the thin film are effectively exhibited, the piezoelectricity and ferroelectricity are dramatically improved.

[Brief description of the drawings]

FIG. 1 is an X-ray diffraction pattern of a sample 1 and a sample 2 using a wide angle method.

FIG. 2 shows the X of the fired samples 1 and 2 using the wide angle method.
Line diffraction pattern.

FIG. 3 is an X-ray diffraction pattern of Sample 3 and Sample 4 using a wide angle method.

FIG. 4 is an X-ray diffraction pattern of Sample 5 using a wide angle method.

FIG. 5 is a rocking curve of the PZT (001) plane of Samples 5 and 1.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H01B 3/12 301 H01L 41/22 A // B01J 19/00 C04B 35/49 Y F term (reference) 4G031 AA11 AA12 AA32 AA39 BA09 BA10 CA03 CA08 GA07 4G059 EA07 EB07 GA02 GA04 GA14 4G075 AA24 BB02 BC02 BC05 BD16 CA02 CA32 CA57 ED01 4G077 AA03 AA07 AB02 AB08 BC43 CG01 GA03 GA07 HA11 JA03 5G303 AA05 AB05 AB20 BA03 CA01 CB25

Claims (5)

[Claims] A step of applying a sol on an electrode formed on a substrate, a step of heating and drying the sol to form an amorphous precursor film, and heating the substrate by lamp annealing. In the method for manufacturing a piezoelectric thin film or a ferroelectric thin film, which comprises a step of crystallizing the amorphous precursor film, a timing at which heating and drying of the sol in the step of stopping the substrate is included in the sol. A method for producing a piezoelectric thin film or a ferroelectric thin film, which is simultaneous with desorption of an organic component. 2. A step of applying a sol on an electrode formed on a substrate, a step of heating and drying the sol to form an amorphous precursor film, and heating the substrate by lamp annealing. In the method for manufacturing a piezoelectric thin film or a ferroelectric thin film in which a crystalline thin film is laminated by sequentially repeating the step of crystallizing the amorphous precursor film, the timing for stopping the heating and drying of the sol is as follows. A method for producing a piezoelectric thin film or a ferroelectric thin film, wherein the organic component contained in the sol is simultaneously desorbed. 3. The sol is prepared by dissolving each metal element in 2-n-butoxyethanol as a main solvent in the form of a carboxylate, acetylacetonate or alkoxide. The method for producing a piezoelectric thin film or a ferroelectric thin film according to claim 1 or 2. 4. The method according to claim 1, wherein platinum is used as the electrode, and an angle of incidence of X-rays on the platinum surface is θ, and 0 ° ≦ 2θ ≦ 90.
The diffraction intensity of all the diffraction peaks of platinum in the range of °
4) The piezoelectric body according to claim 3, wherein when the diffraction peak intensity of the platinum (002) plane is defined as I (002), I (002) / ΣI (hkl) = 0.9. Manufacturing method of thin film or ferroelectric thin film. 5. The method for producing a piezoelectric thin film or a ferroelectric thin film according to claim 4, wherein a conductive oxide represented by the chemical formula of ABO3 is epitaxially formed on the platinum.