JP2008042069A - Piezoelectric element and manufacturing method thereof - Google Patents

Abstract

An object of the present invention is to provide a piezoelectric element that improves crystal orientation and suppresses cracking of a film, and a method for manufacturing the same.
By forming an orientation control layer 5 on the lower electrode layer 4 by vapor phase epitaxy, element diffusion between the lower electrode layer 4 and the piezoelectric layer 6 and a deviation in composition due to the element are suppressed. The piezoelectric layer 6 having high orientation can be obtained, and by applying a compressive stress to the orientation control layer 5, the generation of cracks due to the tensile stress generated when the piezoelectric layer 6 is formed by the liquid phase growth method can be achieved. Can be suppressed.
[Selection] Figure 1

Description

  The present invention relates to a piezoelectric element having an electromechanical conversion function and a method for manufacturing the same.

An oxide dielectric thin film having a perovskite structure is represented by the general formula ABO ₃ and exhibits excellent ferroelectricity, piezoelectricity, pyroelectricity, and electro-optical properties, and is an effective material for a wide range of devices such as various sensors and actuators. The range of use is expected to expand rapidly in the future.

Lead zirconate titanate perovskite oxide (PZT: general formula _{Pb (Zr x Ti 1-x} ) O 3 (0 <x <1)) based thin film, since it has a high piezoelectric properties, Ya piezoelectric sensor It is used as a piezoelectric element such as a piezoelectric actuator. The piezoelectric sensor uses a ferroelectric piezoelectric effect. A ferroelectric has spontaneous polarization inside and generates positive and negative charges on its surface. In the steady state in the atmosphere, it is in a neutral state combined with the charge of the molecules in the atmosphere. When an external pressure is applied to the piezoelectric body, an electrical signal corresponding to the amount of pressure can be extracted from the piezoelectric body. The piezoelectric actuator uses the same principle. When a voltage is applied to the piezoelectric body, the piezoelectric body expands and contracts in accordance with the voltage, and displacement can be generated in the expansion / contraction direction or a direction orthogonal to the direction.

  The PZT-based thin film is formed by a vapor deposition method typified by a vapor deposition method, a sputtering method (sputtering method), a chemical vapor deposition method (CVD method), or a chemical solution method (CSD method: Chemical Solution Deposition method), a hydrothermal synthesis method. Fabrication has been attempted using a liquid phase growth method represented by the above. Among them, the CSD method is characterized by easy composition control, easy production of a thin film with good reproducibility, and low cost required for manufacturing equipment and mass production. A method for manufacturing a piezoelectric element using a PZT thin film by the CSD method is shown below.

  FIG. 6 is a cross-sectional view showing the structure of the piezoelectric element.

The substrate 11 is a SiO ₂ / Si substrate in which (100) -oriented single crystal silicon (Si) is used and thermally oxidized to form a silicon oxide (SiO ₂ ) layer 12 on the surface. A titanium (Ti) thin film 13 is formed as an adhesion layer between platinum (Pt) serving as an electrode and SiO ₂ on the laminated film composed of the SiO ₂ / Si substrate, and a Pt thin film 14 is formed thereon. Sputtering is used to form the Ti thin film 13 and the Pt thin film 14.

As starting materials for the CSD method, lead acetate trihydrate, titanium isopropoxide, and zirconium propoxide are used, and 2-methoxyethanol is used as a solvent. First, lead acetate trihydrate is dissolved and distilled in 2-methoxyethanol to discharge crystal water. Next, Zr and Ti alkoxides weighed so as to have a predetermined composition (in this case Pb / Ti / Zr = 100/47/53) are added to the dehydrated lead acetate solution and refluxed to obtain Pb, A composite alkoxide solution containing Ti and Zr is prepared. This solution is applied onto the substrate by a spin coating method (rotational speed 3000 rpm). Next, after drying this, organic substances are thermally decomposed at 400 to 450 ° C., and crystallization annealing is performed in an O ₂ atmosphere at 700 ° C. for 1 minute at a heating rate of 30 ° C./sec.

  Then, the PZT thin film 15 composed of a plurality of PZT layers can be produced by repeating the application of the composite alkoxide solution on the substrate to the crystallization annealing. The PZT thin film 15 thus formed can be formed to a thickness of 2 μm or more required for a piezoelectric device. The crystal orientation of the PZT thin film 15 depends on the crystal orientation of the first layer because the second and subsequent layers use the first PZT layer as the nucleation site.

  FIG. 7 shows the X-ray diffraction result of the crystal structure of the first PZT thin film produced by this method.

As prior art document information relating to the invention of this application, for example, Patent Document 1 is known.
JP-A-11-220185

  However, the PZT thin film 15 formed on the Pt thin film 14 by the CSD method needs to have a very high temperature of 700 ° C. when crystallization annealing is performed, and diffusion of Pb and Ti of the film composition during the annealing, Due to the formation of the Pb—Pt compound and the diffusion of Ti in the adhesion layer, the desired composition cannot be obtained, and the first PZT layer becomes a film having a low crystal orientation.

  This conventional example is an invention intended to be oriented in the PZT (100) direction, but from FIG. 7, it is oriented not only in the PZT (100) direction but also in the (110) and (111) directions. I understand.

Here, the orientation degree (α (100)) of the (100) plane is expressed as α (100) = I (100) / ΣI (hkl)
In other words, α (100) = 88%, which is a low value.

  ΣI (hkl) is the total sum of diffraction peak intensities from each crystal plane in PZT of a perovskite crystal structure with 2θ of 10 to 70 ° when using Cu-Kα rays in the X-ray diffraction method. Note that the (002) plane and the (200) plane are equivalent to the (001) plane and the (100) plane, and thus are not included in ΣI (hkl).

  Further, the gel thin film formed by drying the composite alkoxide solution in the course of crystallization annealing is densified and crystallized, and tensile stress is generated in the PZT thin film 15 due to shrinkage of the film at that time. In the process of repeating the annealing step to obtain a desired film thickness, there is a problem that cracks are generated in the PZT thin film 15 due to this tensile stress.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve such problems and to provide a piezoelectric element that improves the crystal orientation of a piezoelectric layer such as a PZT thin film and suppresses cracks, and a method for manufacturing the same.

  In order to achieve the above object, the present invention has the following configuration.

  The present invention particularly relates to a substrate, a lower electrode layer formed on the substrate, an alignment control layer formed on the lower electrode layer by a vapor deposition method, and a liquid phase growth method on the alignment control layer. And a top electrode layer formed on the piezoelectric layer.

  According to the present invention, by forming the orientation control layer on the lower electrode layer at a low temperature by the vapor phase growth method, it is possible to suppress element diffusion between the lower electrode layer and the piezoelectric layer and a deviation of the composition thereby, By forming the piezoelectric layer using the orientation control layer as a nucleation site, low-temperature growth of a film with high crystal orientation is achieved by liquid phase growth, and by applying compressive stress to the orientation control layer, the liquid phase It is possible to suppress the generation of cracks in the piezoelectric layer due to the tensile stress generated when the piezoelectric layer is formed by the growth method.

(Embodiment 1)
The first embodiment will be described below with reference to the drawings.

  FIG. 1 is a cross-sectional view showing the structure of a piezoelectric element according to Embodiment 1 of the present invention.

In FIG. 1, the piezoelectric element of the first embodiment is formed on a Si substrate 1 for forming a piezoelectric element, a SiO ₂ layer 2 formed on the Si substrate 1, and a SiO ₂ layer 2. Formed on the adhesion layer 3, the lower electrode layer 4 formed on the adhesion layer 3, the orientation control layer 5 formed on the lower electrode layer 4 by sputtering, and the CSD method on the orientation control layer 5. The piezoelectric layer 6 and the upper electrode layer 7 formed on the piezoelectric layer 6 are configured.

The Si substrate 1 is a single crystal substrate oriented in the (100) plane. The lower electrode layer 4 is made of a material containing platinum (Pt) as a main component. Since Pt and Si may form a compound during film formation, the SiO ₂ layer 2 is provided between them. However, since Pt and SiO ₂ have a problem in adhesion, titanium (Ti) or titanium oxide (TiO ₂ ) was disposed as an adhesion layer. In the present embodiment, the thickness of the adhesion layer 3 is about 0.02 μm, but the film thickness may be in the range of 0.005 to 1 μm. The SiO ₂ layer 2 also has a role of a barrier layer that prevents mutual diffusion of Si and Ti. In the orientation control layer 5, the content of lead titanate (PT) or Zr formed by sputtering is 0 or more and 20 mol% or less, and the content of Pb exceeds 0 compared with the stoichiometric composition. Lead lanthanum zirconate titanate (PLZT) in excess of 30 mol% or less, or PLZT added with at least one of magnesium (Mg) and manganese (Mn) was used.

  Here, the lower electrode layer 4 not only has a role as an electrode, but also, for example, by adding 1.6 mol% of Ti, the orientation control layer 5 is preferentially oriented in the (100) plane or the (001) plane. Take a role. Such additives include Ti, aluminum (Al), iron (Fe), cobalt (Co), nickel (Ni), manganese (Mn), copper (Cu), magnesium (Mg), calcium (Ca), What is necessary is just to be at least 1 sort (s) chosen from the group of strontium (Sr), barium (Ba), and these oxides. The film thickness may be in the range of 0.05 to 2 μm, and is 0.4 μm in the present embodiment. The addition amount of the additive is preferably more than 0 and 30 mol% or less. In addition, the main component of the lower electrode layer 4 is not limited to Pt, and any material containing at least one noble metal selected from the group of iridium (Ir), palladium (Pd), and ruthenium (Ru) can be used. good.

  By using PT or PLZT (including lead lanthanum titanate (PLT) when the content of Zr is 0), the orientation control layer 5 is more easily oriented on the (100) plane or the (001) plane. As a result, the crystallinity and orientation of the piezoelectric layer 6 can be improved. In addition, when the Zr content is 20 mol% or less in PLZT, a low crystallinity layer composed of a Zr oxide is hardly formed at the initial stage of crystal growth, and the Pb content is 0 compared with the stoichiometric composition. By exceeding 30 mol% and exceeding 30 mol%, it is possible to reliably suppress a decrease in crystallinity of the orientation control layer 5. Therefore, the crystallinity and orientation of the piezoelectric layer 6 can be reliably improved, and the piezoelectric characteristics and the withstand voltage characteristics of the piezoelectric element can be further improved. The film thickness of the orientation control layer 5 may be in the range of 0.01 to 0.2 μm, and is 0.04 μm in the present embodiment.

  In addition, the La content in the PLZT film is more than 0 and 25 mol% or less, and the total addition amount when adding at least one of Mg and Mn to the PLZT film is more than 0 and 10 mol% or less. Further, it is possible to more effectively suppress a decrease in crystallinity of the orientation control layer 5.

  Since the PT film, the PLT film, or the PLZT film is formed by a sputtering method, it can be formed at a lower temperature as compared with a case where it is formed by a liquid phase growth method such as a CSD method. Specifically, the CSD method requires heating at a high temperature of 600 ° C. or higher, whereas the sputtering method can be formed by heating at about 450 ° C. This is presumably because the sputtered particles are smaller, that is, have a larger specific surface area than the composite alkoxide raw material gelled by the CSD method, and thus the reactivity is high. Therefore, by using the orientation control layer 5 formed by the sputtering method, the element diffusion of Pb and Pt on the surface of the Pt lower electrode layer 4 is suppressed as compared with the case of forming the film by the CSD method. A film having a small compositional deviation and a high crystal orientation can be obtained, and as a result, the piezoelectric layer 6 having a higher crystal orientation can be obtained.

  The piezoelectric layer 6 is a rhombohedral or tetragonal (001) oriented PZT thin film formed by the CSD method. By forming on the orientation control layer 5, it is possible to obtain remarkably high crystal orientation as compared with the case of forming a film on a normal Pt electrode.

  The composition of PZT is a composition (Zr / Ti = 53/47) near the boundary between the tetragonal system and rhombohedral system (morphotropic phase boundary). The Zr / Ti composition in the piezoelectric layer 6 is not limited to Zr / Ti = 53/47, but may be Zr / Ti = 30/70 to 70/30. The constituent material of the piezoelectric layer 6 may be a piezoelectric material mainly composed of PZT, such as PZT containing additives such as Sr, Nb, Al, etc., and may be PMN or PZN. Also good. Furthermore, the film thickness should just be the range of 0.5-5.0 micrometers.

  Further, by using the orientation control layer 5 to which a compressive stress is applied, which is obtained by applying a negative bias to the substrate 1 at the time of sputtering film formation, the occurrence of cracks in the piezoelectric layer 6 can be suppressed. it can. This is because the tensile stress applied to the piezoelectric layer 6 during the formation by the CSD method is canceled by the compressive stress applied to the orientation control layer 5.

  In this embodiment, a film formed by sputtering is used for the orientation control layer 5. However, the present invention is not limited to this, and various films can be formed at a low temperature of about 450 ° C. and compressive stress can be applied to the formed film. For example, a CVD method, a pulse laser deposition method (PLD method), an aerosol deposition method (AD method), or the like can be used.

  In the present embodiment, the film formed by the CSD method is used for the piezoelectric layer 6, but the present invention is not limited to this, and various liquid phase growth methods such as a hydrothermal synthesis method and an oxidation thermal decomposition method can be used. .

  A cantilever obtained by cutting the piezoelectric element shown in the present embodiment into 20 mm × 2 mm by dicing was manufactured, a 0.2 μm thick gold (Au) upper electrode layer 7 was formed, and the piezoelectric constant d31 was measured. However, a very high value of d31 = −115 pC / N was obtained. The material of the upper electrode layer 7 is not limited to Au but may be any conductive material, and the film thickness may be in the range of 0.1 to 0.4 μm.

  Since the piezoelectric element has a high piezoelectric constant, stress is canceled between the orientation control layer 5 and the piezoelectric layer 6, there is no crack on the film surface, and warping of the substrate 1 is suppressed. It is suitable for various sensor applications such as piezoelectric actuators used in laser beam scanners and angular velocity sensors.

  Next, a method for manufacturing the piezoelectric element will be described with reference to the drawings.

  FIG. 2 is a manufacturing process diagram of the piezoelectric element.

First, the SiO ₂ layer 2 is formed on the Si (100) substrate 1. The SiO ₂ layer 2 can be easily obtained by heating the Si substrate 1 to 700 ° C. or higher in an oxygen-containing gas or a water vapor-containing gas. Alternatively, the film may be formed by a reactive sputtering method in which sputtering is performed in an O ₂ atmosphere using a Si target, a high-frequency (PF) sputtering method in which a SiO ₂ target is used, or a CVD method using silane gas or TEOS gas. An adhesion layer 3 made of Ti or TiO _{2 is formed} on the SiO ₂ layer 2 produced by such a method by sputtering.

  Next, the lower electrode layer 4 containing Pt as a main component is formed by sputtering. On the lower electrode layer 4, an orientation control layer 5 made of PT, PLZT or PLT is formed by sputtering. When forming the orientation control layer 5, a negative bias is applied to the substrate. As a result, positive ions having a large mass such as Ar particles are actively driven into the film surface, and compressive stress is applied to the orientation control layer 5 by the nail effect (peening effect).

Further, when the orientation control layer 5 is formed, the substrate temperature is preferably set to 450 ° C. or higher and 750 ° C. or lower. This is because when the substrate temperature is lower than 450 ° C., the crystallinity of the orientation control layer 5 is lowered and a pyrochlore phase is easily generated. On the other hand, when the temperature is higher than 750 ° C., Pb contained in the film evaporates at the time of film formation, resulting in insufficient crystallinity. More preferably, the substrate temperature is set to 450 ° C. or higher and 550 ° C. or lower. This is to suppress element diffusion and compound formation between the lower electrode layer 4 and the orientation control layer 5. In this embodiment, the substrate temperature is 450 ° C. The sputtering gas was a mixed atmosphere of argon (Ar) and oxygen (O ₂ ), and the gas volume ratio was Ar: O ₂ = 19: 1. The degree of vacuum was 0.8 Pa. The O ₂ partial pressure in the mixed gas of Ar and O ₂ is preferably more than 0% and 10% or less. This is because, in the state where no O ₂ exists, the crystallinity of the orientation control layer 5 is lowered. On the other hand, when the O ₂ partial pressure exceeds 10%, the orientation of the (100) plane or the (001) plane is lowered. Because. The degree of vacuum is preferably 0.05 Pa or more and 5 Pa or less. This is because when the degree of vacuum is less than 0.05 Pa, the crystallinity of the orientation control layer 5 varies, while when it exceeds 5 Pa, the orientation of the (100) plane or the (001) plane decreases.

  Furthermore, by alternately repeating the step of depositing the orientation control layer 5 on the substrate while keeping the substrate temperature at 450 ° C., and the step of performing only the annealing of the film without deposition, the crystal orientation is further improved. High orientation control layer 5 can be obtained. In general, when a dielectric thin film is formed by sputtering, the thin film tends to form columnar growth and often has grain boundaries and defects. If the deposition rate of the thin film is reduced, a denser thin film tends to be formed, but the throughput is lowered and there is a limit to the reduction of the deposition rate. In addition, there are cases where the dielectric thin film formed to a predetermined film thickness can be improved from the sputtering apparatus and annealed at a high temperature to improve the crystal orientation and stability of the dielectric thin film. When the formed thin film is heat-treated in the next step, the effect of improving the characteristics is limited. Therefore, in the sputtering process, by intermittently introducing a process that does not deposit while keeping the substrate temperature around 450 ° C., the deposited thin film can be sequentially stabilized, and the quality of the dielectric thin film can be improved. It is.

  Next, a PZT precursor solution for forming the piezoelectric layer 6 is applied on the orientation control layer 5 by spin coating. The adjustment method of this PZT precursor solution is shown below. The ethanol used in this adjustment method is anhydrous ethanol that has been subjected to a dehydration treatment in advance in order to prevent hydrolysis of the metal alkoxide by the contained water.

First, lead (II) acetate trihydrate (Pb (OCOCH ₃ ) ₂ .3H ₂ O) is used as a starting material for adjusting the Pb precursor solution. This is taken into a separable flask and dried at 150 ° C. for 2 hours or more to remove the hydrate. Next, absolute ethanol is added and dissolved, and refluxed at 78 ° C. for 4 hours to prepare a Pb precursor solution. As a starting material for adjusting the Ti—Zr precursor solution, titanium isopropoxide (Ti (OCH (CH ₃ ) ₂ ) ₄ ) zircon normal propoxide (Zr (OCH ₂ CH ₂ CH ₃ ) ₄ ) is used. This is also taken in another separable flask, dissolved by adding absolute ethanol, and refluxed at 78 ° C. for 4 hours to prepare a Ti—Zr precursor solution. The Ti / Zr ratio was weighed so that the molar ratio was Ti / Zr = 47/53. This Ti—Zr precursor solution is mixed with the Pb precursor solution. At this time, Pb component was made 20 mol% excess with respect to the stoichiometric composition (Pb (Zr _0.53 , Ti _0.47 ) O ₃ ). This is to compensate for the shortage due to volatilization of the lead component during annealing. This mixed solution was refluxed at 78 ° C. for 4 hours, 0.5 mol equivalent of acetylacetone as a stabilizer relative to the total amount of metal cations was added, and further refluxed at 78 ° C. for 1 hour to prepare a PZT precursor solution. .

  The conditions for spin coating the PZT precursor solution on the orientation control layer 5 were 30 seconds at 2500 rpm. Next, the PZT precursor solution applied on the orientation control layer 5 was dried at 115 ° C. for 10 minutes, and then the residual organic component was thermally decomposed by heat treatment at 350 ° C. for 10 minutes. The drying step is intended to remove physically adsorbed moisture in the precursor solution, and the temperature is desirably higher than 100 ° C. and lower than 200 ° C. This is because decomposition of residual organic components in the precursor solution starts at 200 ° C. or higher, and prevents moisture from remaining in the produced film. The temperature in the pyrolysis step is preferably 200 ° C. or higher and lower than 500 ° C. This is because crystallization of the dried precursor solution proceeds greatly at 500 ° C. or higher, and the organic component remains in the produced film.

  The process from the step of applying this PZT precursor solution onto the orientation control layer 5 to the thermal decomposition is repeated a plurality of times, and when a desired film thickness is obtained, a rapid thermal annealing (RTA) is used. Then, crystallization is performed. The crystallization conditions were 550 ° C. for 5 minutes, and the temperature rising rate was 200 ° C./min. The crystallization temperature is preferably 500 ° C. or higher and 750 ° C. or lower. Usually, when a PZT thin film is formed on the lower electrode layer 4 by the CSD method, heating at 600 ° C. or more is required for making the perovskite single phase, but in this embodiment, the orientation with high crystal orientation Since the PZT thin film is formed on the control layer 5, the orientation control layer 5 serves as a nucleation site for crystal growth and promotes crystallization, so that the crystallization temperature can be lowered to 500 ° C. become. On the other hand, when the temperature is higher than 750 ° C., the Pb contained in the film evaporates at the time of film formation, resulting in insufficient crystallinity.

  By forming the piezoelectric layer 6 through the above steps, a highly oriented PZT thin film is obtained in the (001) plane direction.

  Finally, the upper electrode layer 7 made of Au is formed on the piezoelectric layer 6 by ion beam evaporation. The method for forming the upper electrode layer 7 is not limited to the ion beam evaporation method, and a resistance heating evaporation method, a sputtering method, or the like may be used.

  In the present embodiment, in the formation of the piezoelectric layer 6, crystallization was performed after repeated application to thermal decomposition a plurality of times in order to obtain a desired film thickness, but the steps from application to crystallization were repeated each time. May be.

(Embodiment 2)
Hereinafter, Embodiment 2 will be described with reference to the drawings.

  FIG. 3 is a cross-sectional view showing the structure of the piezoelectric element according to the second embodiment of the present invention. Here, the same components as those of the first embodiment are given the same reference numerals.

In FIG. 3, the piezoelectric element of the second embodiment is formed on the Si substrate 1 for forming the piezoelectric element, the SiO ₂ layer 2 formed on the Si substrate 1, and the SiO ₂ layer 2. Formed on the adhesion layer 3, the lower electrode layer 4 formed on the adhesion layer 3, the orientation control layer 8 formed on the lower electrode layer 4 by sputtering, and the CSD method on the orientation control layer 8. The piezoelectric layer 6 and the upper electrode layer 7 formed on the piezoelectric layer 6 are configured.

The Si substrate 1 is a single crystal substrate oriented in the (100) plane. A material containing Pt as a main component is used for the lower electrode layer 4. Since Pt and Si may form a compound during film formation, the SiO ₂ layer 2 is provided therebetween. However, since Pt and SiO ₂ have a problem in adhesion, Ti or TiO ₂ was disposed as the adhesion layer 3. In the present embodiment, the thickness of the adhesion layer 3 is about 0.02 μm, but the film thickness may be in the range of 0.005 to 1 μm. The SiO ₂ layer 2 also has a role of a barrier layer that prevents mutual diffusion of Si and Ti. The alignment control layer 8 includes a first alignment control layer 8a and a second alignment control layer 8b. The first orientation control layer 8a has a PT or Zr content of 0 to 20 mol% formed by sputtering and a Pb content exceeding 0 and exceeding 30 mol% compared to the stoichiometric composition. PLZT, or PLZT with at least one of Mg and Mn added thereto was used. A PZT thin film formed by sputtering is used for the second orientation control layer 8b.

  Here, the lower electrode layer 4 not only has a role as an electrode, but also, for example, by adding 1.6 mol% of Ti, the orientation control layer 8 is preferentially oriented in the (100) plane or the (001) plane. Take a role. Such an additive may be at least one selected from the group consisting of Ti, Al, Fe, Co, Ni, Mn, Cu, Mg, Ca, Sr, Ba, and oxides thereof. The film thickness may be in the range of 0.05 to 2 μm, and is 0.4 μm in the present embodiment. The addition amount of the additive is preferably more than 0 and 30 mol% or less. The main component of the lower electrode layer 4 is not limited to Pt, and any material containing at least one noble metal selected from the group of Ir, palladium (Pd), and ruthenium (Ru) may be used.

  By using PT or PLZT (including lanthanum lead titanate (PLT) when the content of Zr is 0), the first orientation control layer 8a is further oriented in the (100) plane or the (001) plane. As a result, the crystallinity and orientation of the second orientation control layer 8b can be improved. Moreover, in PLZT, when the Zr content is 20 mol% or less, a low crystallinity layer made of Zr oxide is difficult to form at the initial stage of crystal growth, and the Pb content is 0 compared with the stoichiometric composition. By exceeding 30 mol% and exceeding 30 mol%, it is possible to reliably suppress a decrease in crystallinity of the first orientation control layer 8a. Therefore, the crystallinity and orientation of the second orientation control layer 8b can be reliably improved, and further the crystallinity and orientation of the piezoelectric layer 6 can be improved. In addition, the withstand voltage characteristics can be further improved.

  In addition, the La content in the PLZT film is more than 0 and 25 mol% or less, and the total addition amount when adding at least one of Mg and Mn to the PLZT film is more than 0 and 10 mol% or less. Further, it is possible to more effectively suppress the decrease in crystallinity of the first orientation control layer 8a.

  Since the PT film, the PLT film, or the PLZT film used for the first orientation control layer 8a is formed by sputtering, the film is formed at a lower temperature than when formed by a liquid phase growth method such as the CSD method. be able to. Specifically, the CSD method requires heating at a high temperature of 600 ° C. or higher, whereas the sputtering method can be formed by heating at about 450 ° C. This is presumably because the sputtered particles are smaller, that is, have a larger specific surface area than the composite alkoxide raw material gelled by the CSD method, and thus the reactivity is high. Therefore, by using the first orientation control layer 8a formed by the sputtering method, the element diffusion of Pb and Pt on the surface of the Pt lower electrode layer 4 is suppressed as compared with the case where the film is formed by the CSD method. Thus, a film with small compositional deviation and high crystal orientation can be obtained.

  On the other hand, by forming the second orientation control layer 8b made of PZT formed by sputtering on the first orientation control layer 8a, the effect as a nucleation site when forming the piezoelectric layer 6 is further increased. Thus, the piezoelectric layer 6 with further improved crystal orientation can be obtained.

  The film thicknesses of the first alignment control layer 8a and the second alignment control layer 8b may be in the range of 0.01 to 0.2 μm, respectively, and are 0.04 μm in this embodiment.

  The piezoelectric layer 6 is a rhombohedral or tetragonal (001) oriented PZT thin film formed by the CSD method. By forming the film on the orientation control layer 8, a remarkably high crystal orientation can be obtained as compared with the case where the film is formed on a normal Pt electrode.

  The composition of PZT is a composition (Zr / Ti = 53/47) near the boundary between the tetragonal system and rhombohedral system (morphotropic phase boundary). The Zr / Ti composition in the piezoelectric layer 6 is not limited to Zr / Ti = 53/47, but may be Zr / Ti = 30/70 to 70/30. The constituent material of the piezoelectric layer 6 may be a piezoelectric material mainly composed of PZT, such as PZT containing additives such as Sr, Nb, Al, etc., and may be PMN or PZN. Also good. Furthermore, the film thickness should just be the range of 0.5-5.0 micrometers.

  Further, by using the orientation control layer 8 to which a compressive stress is applied, which is obtained by applying a negative bias to the substrate 1 at the time of sputtering film formation, the occurrence of cracks in the piezoelectric layer 6 can be suppressed. it can. This is because the tensile stress applied to the piezoelectric layer 6 during the formation by the CSD method is canceled by the compressive stress applied to the orientation control layer 8.

  In this embodiment, a film formed by sputtering is used for the orientation control layer 8. However, the present invention is not limited to this, and various films can be formed at a low temperature of about 450 ° C. and compressive stress can be applied to the formed film. For example, a CVD method, a pulsed laser deposition method (PLD method), an aerosol deposition method (AD method), or the like can be used.

  In the present embodiment, the film formed by the CSD method is used for the piezoelectric layer 6, but the present invention is not limited to this, and various liquid phase growth methods such as a hydrothermal synthesis method and an oxidation thermal decomposition method can be used. .

  A cantilever obtained by cutting the piezoelectric element shown in the present embodiment into 20 mm × 2 mm by dicing was manufactured, a 0.2 μm thick gold (Au) upper electrode layer 7 was formed, and the piezoelectric constant d31 was measured. However, a very high value of d31 = −120 pC / N was obtained. The material of the upper electrode layer 7 is not limited to Au but may be any conductive material, and the film thickness may be in the range of 0.1 to 0.4 μm.

  Since the piezoelectric element has a high piezoelectric constant, stress is canceled between the orientation control layer 8 and the piezoelectric layer 6, there is no crack on the film surface, and warping of the substrate 1 is suppressed. It is suitable for various sensor applications such as piezoelectric actuators used in laser beam scanners and angular velocity sensors.

  Hereinafter, a method for manufacturing the piezoelectric element will be described with reference to the drawings.

  FIG. 4 is a manufacturing process diagram of the piezoelectric element. This manufacturing process is an improvement of the formation process of the orientation control layer 8 in the manufacturing process of the first embodiment, and a description of the same parts as those of the first embodiment is omitted.

First, a first orientation control layer 8a made of PT, PLZT, or PLT is formed on the lower electrode layer 4 by sputtering. When forming the orientation control layer 8a, a negative bias is applied to the substrate. As a result, positive ions having a large mass such as Ar particles are actively implanted into the film surface, and compressive stress is applied to the orientation control layer 8a due to a nail effect (peening effect). Further, when forming the orientation control layer 8a, the substrate temperature is preferably set to 450 ° C. or higher and 750 ° C. or lower. This is because when the substrate temperature is lower than 450 ° C., the crystallinity of the orientation control layer 8a is lowered and a pyrochlore phase is easily generated. On the other hand, when the temperature is higher than 750 ° C., Pb contained in the film evaporates at the time of film formation, resulting in insufficient crystallinity. In this embodiment, the substrate temperature is 450 ° C. The sputtering gas was a mixed atmosphere of argon (Ar) and oxygen (O ₂ ), and the gas volume ratio was Ar: O ₂ = 19: 1. The degree of vacuum was 0.8 Pa. The O ₂ partial pressure in the mixed gas of Ar and O ₂ is preferably more than 0% and 10% or less. This is because, in the state where no O ₂ exists, the crystallinity of the orientation control layer 8a is lowered. On the other hand, when the O ₂ partial pressure exceeds 10%, the orientation of the (100) plane or the (001) plane is lowered. Because. The degree of vacuum is preferably 0.05 Pa or more and 5 Pa or less. This is because if the degree of vacuum is less than 0.05 Pa, the crystallinity of the orientation control layer 8a varies, whereas if it exceeds 5 Pa, the orientation of the (100) plane or the (001) plane decreases.

  Further, by alternately repeating the step of depositing the orientation control layer 8a on the substrate while keeping the substrate temperature at 450 ° C., and the step of performing annealing of the film without deposition, the crystal orientation is further improved. High orientation control layer 8a can be obtained. In general, when a dielectric thin film is formed by sputtering, the thin film tends to form columnar growth and often has grain boundaries and defects. If the deposition rate of the thin film is reduced, a denser thin film tends to be formed, but the throughput is lowered and there is a limit to the reduction of the deposition rate. In addition, there are cases where the dielectric thin film formed to a predetermined film thickness can be improved from the sputtering apparatus and annealed at a high temperature to improve the crystal orientation and stability of the dielectric thin film. When the formed thin film is heat-treated in the next step, the effect of improving the characteristics is limited. Therefore, in the sputtering process, by intermittently introducing a process that does not deposit while keeping the substrate temperature around 450 ° C., the deposited thin film can be sequentially stabilized, and the quality of the dielectric thin film can be improved. It is.

  Next, a second orientation control layer 8b made of PZT is formed on the first orientation control layer 8a by a sputtering method.

As a target, a sintered body of PZT (Zr / Ti = 53/47) was used. In a mixed atmosphere of Ar and O ₂ (gas volume ratio Ar: O ₂ = 19: 1) at a substrate temperature of 450 ° C., It is formed under the condition of a vacuum degree of 0.3 Pa. A negative bias is applied to the substrate. As a result, positive ions having a large mass such as Ar particles are positively implanted into the film surface, and compressive stress is applied to the orientation control layer 8b by the nail effect (peening effect).

The oxygen partial pressure in the mixed gas of Ar and O ₂ used when forming the second orientation control layer 8b by sputtering is preferably more than 0% and 30% or less. This is because in the state where no oxygen is present, the crystallinity of the second orientation control layer 8b is lowered, whereas when the oxygen partial pressure exceeds 30%, the (001) plane orientation degree is lowered. Moreover, it is preferable that a vacuum degree is 0.1 Pa or more and 1 Pa or less. This is because when the degree of vacuum is less than 0.1 Pa, the crystallinity and piezoelectric characteristics of the second orientation control layer 8b vary, while when it exceeds 1 Pa, the degree of (001) plane orientation decreases.

  The temperature of the substrate 1 when forming the second orientation control layer 8b by sputtering is preferably 450 ° C. or higher and 750 ° C. or lower. This is because when the temperature of the substrate 1 is lower than 450 ° C., the crystallinity of the second orientation control layer 8b is lowered and pyrochlore is easily generated. On the other hand, when the temperature is higher than 750 ° C., the film is formed during film formation. This is because Pb contained therein is insufficient due to evaporation and crystallinity is lowered.

  More preferably, the oxygen partial pressure is 1% or more and 10% or less, and the degree of vacuum is 0.15 Pa or more and 0.8 Pa or less.

  Furthermore, by alternately repeating the step of depositing the second orientation control layer 8b on the substrate and the step of annealing the film without deposition while the substrate temperature is maintained at 450 ° C. The orientation control layer 8b having high crystal orientation can be obtained. In general, when a dielectric thin film is formed by sputtering, the thin film tends to form columnar growth and often has grain boundaries and defects. If the deposition rate of the thin film is reduced, a denser thin film tends to be formed, but the throughput is lowered and there is a limit to the reduction of the deposition rate. In addition, there are cases where the dielectric thin film formed to a predetermined film thickness can be improved from the sputtering apparatus and annealed at a high temperature to improve the crystal orientation and stability of the dielectric thin film. When the formed thin film is heat-treated in the next step, the effect of improving the characteristics is limited. Therefore, in the sputtering process, by intermittently introducing a process that does not deposit while keeping the substrate temperature around 450 ° C., the deposited thin film can be sequentially stabilized, and the quality of the dielectric thin film can be improved. It is.

  If the second orientation control layer 8b is formed as described above, the second orientation control layer 8b has a surface on the second orientation control layer 8b side of the first orientation control layer 8a of (100) plane or Due to the (001) plane orientation, the (001) plane exhibits very high orientation. Further, since the crystal orientation of the second orientation control layer 8b is good, the crystal orientation of the piezoelectric layer 6 is also good.

FIG. 5 shows an X-ray diffraction result of the crystal structure of the PZT thin film of the second orientation control layer 8b produced according to this embodiment. FIG. 5 also shows that a film having a very high crystal orientation is obtained. The (001) plane orientation degree α (001) obtained from FIG. 5 is approximately 100%. Also, the X-ray diffraction peak intensity I _B (100) of the PZT thin film 15 shown in the background art and the X-ray diffraction peak intensity I _N (001) of the PZT thin film of the second orientation control layer 8b shown in the present embodiment. )
I _N (001) / I _B (100)> 15
Thus, it can be seen that the peak intensity of the orientation control layer 8b is extremely large. That is, compared with the PZT thin film 15 shown in the background art, a film having a very high degree of crystallinity is obtained. Therefore, the crystallinity of the piezoelectric layer 6 is also good.

  As described above, the piezoelectric element and the manufacturing method thereof according to the present invention can form a piezoelectric layer with high crystal orientation, and thus have excellent ferroelectricity, piezoelectricity, pyroelectricity, and electro-optical characteristics. Can be formed, various sensors such as angular velocity sensors and infrared sensors used in various electronic devices, various actuators such as piezoelectric actuators and ultrasonic motors, optical devices such as optical waveguides and optical switches, and ceramic capacitors and This is useful for applications such as ferroelectric memory.

Sectional drawing of the piezoelectric element in Embodiment 1 of this invention Manufacturing process diagram of the piezoelectric element Sectional drawing of the piezoelectric element in Embodiment 2 of this invention Manufacturing process diagram of the piezoelectric element X-ray diffraction graph of the crystal structure of the PZT thin film in the orientation control layer of the piezoelectric element Sectional view of a conventional piezoelectric element X-ray diffraction graph of crystal structure of conventional PZT thin film

Explanation of symbols

1 Si substrate 2 SiO ₂ layer 3 Adhesion layer 4 Lower electrode layer 5 Orientation control layer 6 Piezoelectric layer 7 Upper electrode layer 8 Orientation control layer 8a First orientation control layer 8b Second orientation control layer 11 Substrate 12 Silicon oxide (SiO ₂ ) layer 13 titanium (Ti) thin film 14 Pt thin film 15 PZT thin film

Claims (34)

A substrate, a lower electrode layer formed on the substrate, an orientation control layer formed on the lower electrode layer by a vapor deposition method, and a piezoelectric body formed on the orientation control layer by a liquid phase growth method A piezoelectric element comprising: a layer; and an upper electrode layer formed on the piezoelectric layer. 2. The piezoelectric element according to claim 1, wherein compressive stress is generated in the orientation control layer. The orientation control layer is a perovskite-type oxide dielectric preferentially oriented in a cubic or tetragonal (100) plane or (001) plane containing at least lead and titanium. The piezoelectric element as described. A perovskite oxide dielectric in which the orientation control layer is composed of two layers, and the first orientation control layer is preferentially oriented in a cubic or tetragonal (100) plane or (001) plane containing at least lead and titanium The second orientation control layer is a perovskite oxide ferroelectric that is preferentially oriented in a rhombohedral or tetragonal (001) plane mainly composed of lead zirconate titanate. The piezoelectric element according to claim 1. The piezoelectric layer is a perovskite oxide ferroelectric that is preferentially oriented in a rhombohedral or tetragonal (001) plane mainly composed of lead zirconate titanate. The piezoelectric element as described in 2. 2. The piezoelectric element according to claim 1, wherein an adhesion layer is disposed between the substrate and the lower electrode layer. 7. The piezoelectric element according to claim 6, wherein the lower electrode layer is made of a material mainly containing a noble metal, and the adhesion layer is made of a refractory metal or an oxide of a refractory metal. The piezoelectric element according to claim 7, wherein the lower electrode layer is mainly composed of platinum. 9. The piezoelectric element according to claim 8, wherein the lower electrode layer is mainly composed of platinum and contains titanium or titanium oxide. 8. The piezoelectric element according to claim 7, wherein the adhesion layer is made of titanium or titanium oxide. 2. The piezoelectric element according to claim 1, wherein the substrate is formed by forming a silicon oxide film on single crystal silicon. 2. The piezoelectric element according to claim 1, wherein the vapor phase growth method is a sputtering method. The piezoelectric element according to claim 1, wherein the liquid phase growth method is a chemical solution method. Forming a lower electrode layer on the substrate, forming an orientation control layer on the lower electrode layer by vapor deposition, and forming a piezoelectric layer on the orientation control layer by liquid phase growth And a method of forming an upper electrode layer on the piezoelectric layer, and a method for manufacturing a piezoelectric element. The orientation control layer is a perovskite type oxide dielectric preferentially oriented in a cubic or tetragonal (100) plane or (001) plane containing at least lead and titanium. The manufacturing method of the piezoelectric body element of description. A perovskite oxide dielectric in which the orientation control layer is composed of two layers, and the first orientation control layer is preferentially oriented in a cubic or tetragonal (100) plane or (001) plane containing at least lead and titanium The second orientation control layer is a perovskite oxide ferroelectric that is preferentially oriented in a rhombohedral or tetragonal (001) plane mainly composed of lead zirconate titanate. The method for manufacturing a piezoelectric element according to claim 14. 15. The method of manufacturing a piezoelectric element according to claim 14, wherein compressive stress is applied to the orientation control layer in the step of forming the orientation control layer by vapor deposition. The method of manufacturing a piezoelectric element according to claim 17, wherein a negative bias is applied to the substrate in the step of forming the orientation control layer by a vapor phase growth method. 19. The method for manufacturing a piezoelectric element according to claim 18, wherein the vapor phase growth method is a sputtering method. 20. The step of forming an orientation control layer by a sputtering method comprises alternately repeating a step of depositing a film on a substrate and a step of performing only annealing of the film without performing deposition. A method for manufacturing the piezoelectric element according to the above. 21. The method for manufacturing a piezoelectric element according to claim 20, wherein in the step of forming the orientation control layer by a sputtering method, the substrate temperature is kept at 450 ° C. or higher and 750 ° C. or lower. 15. The piezoelectric layer is a perovskite oxide ferroelectric that is preferentially oriented in a rhombohedral or tetragonal (001) plane mainly composed of lead zirconate titanate. A method for manufacturing the piezoelectric element according to the above. The method for manufacturing a piezoelectric element according to claim 14, wherein the liquid phase growth method is a chemical solution method. In the step of forming a piezoelectric layer by a chemical solution method, a precursor sol solution composed of at least an organic metal compound or an inorganic metal compound of lead, zirconium and titanium is applied on the orientation control layer formed on the substrate. The method for manufacturing a piezoelectric element according to claim 23, comprising a step and a firing step of firing the precursor sol solution. The firing step includes a drying step, a temporary firing step, and a crystallization step, and the temperature of the drying step is lower than the temperature at which the organic matter in the precursor sol solution starts thermal decomposition, and the temporary firing step is the precursor. Precursor sol in which the organic substance in the body sol solution is at a temperature equal to or higher than the temperature at which the organic substance starts thermal decomposition, and lower than the temperature at which the crystallization of the precursor sol solution in which the organic substance is thermally decomposed proceeds, 25. The method of manufacturing a piezoelectric element according to claim 24, wherein the temperature is higher than a temperature at which crystallization of the solution proceeds and lower than a temperature at which the crystallized precursor sol solution is decomposed. The temperature of the drying step is higher than 100 ° C. and lower than 200 ° C., the temperature of the preliminary baking step is 200 ° C. or higher and lower than 500 ° C., and the temperature of the crystallization step is 500 ° C. or higher and 750 ° C. or lower. A method for manufacturing a piezoelectric element according to claim 25. 27. The method of manufacturing a piezoelectric element according to claim 26, wherein a rapid heating furnace is used in the crystallization process. The method for manufacturing a piezoelectric element according to claim 23, wherein the step of forming the piezoelectric layer by a chemical solution method is repeated a plurality of times. The method for manufacturing a piezoelectric element according to claim 14, wherein an adhesion layer is disposed between the substrate and the lower electrode layer. 30. The method of manufacturing a piezoelectric element according to claim 29, wherein the lower electrode layer is made of a material mainly containing a noble metal, and the adhesion layer is made of a refractory metal or an oxide of a refractory metal. 31. The method for manufacturing a piezoelectric element according to claim 30, wherein the lower electrode layer is mainly composed of platinum. 31. The method of manufacturing a piezoelectric element according to claim 30, wherein the lower electrode layer is mainly composed of platinum and contains titanium or titanium oxide. 31. The method for manufacturing a piezoelectric element according to claim 30, wherein the adhesion layer is made of titanium or titanium oxide. 15. The method of manufacturing a piezoelectric element according to claim 14, wherein the substrate is formed by forming a silicon oxide film on single crystal silicon.

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