JP2019057699A - Piezoelectric element and method for manufacturing the same - Google Patents

Abstract

To provide a piezoelectric element having a ZnO based piezoelectric material film without substantially including alkali metal, such as lithium and excellent in piezoelectric properties, and a method for manufacturing the same.SOLUTION: The piezoelectric element comprises a Ca containing ZnO based piezoelectric material film and first and second electrodes sandwiching the ZnO based piezoelectric material film and facing each other. The ZnO based piezoelectric material film has a first region in which a content molar ratio R of a Ca content to the total of the Ca content and a Zn content increases in the thickness direction of the ZnO based piezoelectric material film toward the second electrode from the first electrode.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a piezoelectric element and a method for manufacturing the piezoelectric element.

There is an increasing demand for mechanical sensors that sense vibration, strain, angular velocity, and the like. In particular, the development of a piezoelectric element having a piezoelectric film and a pair of electrodes facing each other with the piezoelectric film interposed therebetween has been promoted because it is easy to downsize and monolithically integrate with an IC (integrated circuit). ing. In particular, studies are being made to improve piezoelectric characteristics of a piezoelectric element having a ZnO-based piezoelectric film mainly composed of ZnO used in an FBAR (Film Bulk Acoustic Resonator) device or the like.
As such a technique, Patent Document 1 discloses that “a wurtzite type crystal film formed on a substrate, in which two or more particles are filled and deposited in a direction perpendicular to the substrate surface, A wurtzite crystal film having a film structure including a bonding layer in which particles are bonded to each other, wherein the particles are doped with at least one element of an element group consisting of an alkali metal element and an alkaline earth metal element. Wherein the wurtzite crystal structure is oriented in the [000-1] direction or the [0001] direction in a direction perpendicular to the substrate surface, and [000-1] The wurtzite crystal film is characterized in that the polarity distribution ratio in the [0001] direction or the [0001] direction is 72% or more, and a piezoelectric film containing Li as an alkali metal element is disclosed. Yes.

JP 2013-107782 A

  The present inventors have found that a piezoelectric element having a ZnO-based piezoelectric film containing an alkali metal (specifically, Li) as disclosed in Patent Document 1 is difficult to apply to a semiconductor or an electric circuit. I know. This is because when the alkali metal is applied to a semiconductor or an electric circuit for the ZnO-based piezoelectric film, the alkali metal diffuses in the semiconductor or the electric circuit, and a defect may occur.

  Therefore, an object of the present invention is to provide a piezoelectric element having a ZnO-based piezoelectric film substantially free of alkali metal such as lithium and having excellent piezoelectric characteristics. Another object of the present invention is to provide a method for manufacturing a piezoelectric element.

  As a result of intensive studies to achieve the above-described problems, the present inventors have found that the above-described problems can be achieved with the following configuration.

[1] A ZnO-based piezoelectric film that includes a ZnO-based piezoelectric film containing Ca and a first electrode and a second electrode that face each other with the ZnO-based piezoelectric film interposed therebetween, and that is directed from the first electrode to the second electrode. A piezoelectric element in which a ZnO-based piezoelectric film has a first region in which a content molar ratio R of Ca content to a sum of Ca content and Zn content increases along the thickness direction of the body film.
[2] The ZnO-based piezoelectric film maintains the maximum content molar ratio R in the first region substantially constant in the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. The piezoelectric element according to [1], which has two regions.
[3] The piezoelectric element according to [1] or [2], wherein the ZnO-based piezoelectric film has an average content molar ratio R of 0.12 to 0.50.
[4] The piezoelectric element according to any one of [1] to [3], wherein the difference between the minimum value and the maximum value of the molar content ratio R in the first region is 0.06 to 0.50.
[5] A substrate is further provided on the surface opposite to the ZnO-based piezoelectric film side of the first electrode or on the surface opposite to the ZnO-based piezoelectric film side of the second electrode. ] The piezoelectric element in any one of [4].
[6] The method for manufacturing a piezoelectric element according to any one of [1] to [5], wherein a step A of forming a ZnO-based piezoelectric film on the first electrode and a ZnO-based piezoelectric film formed A step B of forming a second electrode, and the step A includes a Ca content and a Zn content along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. The manufacturing method of a piezoelectric element which has the process of forming the 1st area | region where the content molar ratio R of content of Ca with respect to the sum increases.
[7] The method for manufacturing a piezoelectric element according to [6], wherein in step A, the first region is formed by a sputtering method that increases an applied power to a target containing at least Ca.
[8] The second region in which step A maintains the maximum value of the content molar ratio R in the first region substantially constant along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. The method for manufacturing a piezoelectric element according to [6] or [7], including a forming step.
[9] The method for manufacturing a piezoelectric element according to [8], wherein the second region is formed by a sputtering method in which an applied power to a target containing at least Ca is substantially constant.
[10] Further, the method includes a step of forming a first electrode on the substrate before the step A, or after the step B, opposite to the ZnO-based piezoelectric film of the first electrode or the second electrode. The method for manufacturing a piezoelectric element according to any one of [6] to [9], including a step of attaching a substrate on the surface on the side.

  ADVANTAGE OF THE INVENTION According to this invention, the piezoelectric element which has the ZnO-type piezoelectric material film | membrane which does not contain alkali metals, such as lithium substantially, and has the outstanding piezoelectric characteristic can be provided. According to the present invention, a method for manufacturing a piezoelectric element can also be provided.

1 is a schematic cross-sectional view of a piezoelectric element according to an embodiment of the present invention. It is a conceptual diagram which shows the relationship between the content molar ratio R in the ZnO-type piezoelectric material film which the piezoelectric element which concerns on embodiment of this invention has, and the thickness of a ZnO-type piezoelectric material film.

Hereinafter, the present invention will be described in detail.
The description of the constituent elements described below may be made based on typical embodiments of the present invention, but the present invention is not limited to such embodiments.
In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.

[Piezoelectric element]
FIG. 1 is a schematic cross-sectional view of an example of a piezoelectric element (hereinafter also referred to as “the present piezoelectric element”) according to an embodiment of the present invention. The piezoelectric element 10 includes a substrate 11, a first electrode 12, a ZnO-based piezoelectric film 13, and a second electrode 14, and an electric field is applied to the ZnO-based piezoelectric film 13 by the first electrode 12 and the second electrode 14. it can. In addition, although this piezoelectric element has the board | substrate 11 in the surface on the opposite side to the ZnO-type piezoelectric material film 13 of the 1st electrode 12, the piezoelectric element which concerns on embodiment of this invention is not restrict | limited above. The piezoelectric element may have the substrate 11 on the surface opposite to the ZnO-based piezoelectric film 13 of the second electrode 14 or may not have the substrate.
In this specification, the ZnO-based piezoelectric film is a piezoelectric film made of a metal oxide containing Zn as a main component among metal atoms. The main component is intended to include at least 50 mol% of Zn with respect to all metal atoms.

[ZnO-based piezoelectric film]
A ZnO-based piezoelectric film (hereinafter also referred to as “the present piezoelectric film”) included in the piezoelectric element contains Ca, and in the present piezoelectric film, a ZnO-based piezoelectric body directed from the first electrode to the second electrode. A first region in which the content molar ratio R of the Ca content to the sum of the Ca content and the Zn content increases along the thickness direction of the film.
In the present specification, “increase” means the content molar ratio from the minimum value of the content molar ratio R in the ZnO-based piezoelectric film to the maximum value of the content molar ratio R in the ZnO-based piezoelectric film. It means that R changes. That is, the minimum value of the content molar ratio R in the first region corresponds to the minimum value of the content molar ratio R in the ZnO-based piezoelectric film, and the maximum value of the content molar ratio R in the first region is the ZnO-based piezoelectric material. This corresponds to the maximum value of the molar ratio R contained in the film.

  Moreover, in the 1st area | region, the content molar ratio R should just increase, and the area | region where the content molar ratio R becomes partially constant may exist. In addition, the first region may have a form in which the content molar ratio R does not decrease and gradually increases without maintaining a constant value. That is, the content molar ratio R may gradually increase.

Since the first region can be more easily formed, the first region has a stepped (stepwise) content molar ratio R along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. It is preferable to increase.
Here, the content molar ratio R is the content molar ratio R of the Ca content relative to the sum of the Ca content (M _Ca ) and the Zn content (M _Zn ) in the ZnO-based piezoelectric film (M _Ca / _(M Ca ₊ M _Zn) is calculated as. molar ratio R, in the case of forming the ZnO-based piezoelectric film by sputtering, which will be described later, typically, the applied power to the ZnO and / or CaO target The molar ratio R can be measured by the following method.

In other words, the content molar ratio R of the entire ZnO-based piezoelectric film can be measured by an energy dispersive X-ray fluorescence analysis. In addition to the above, measurement may be performed by high frequency inductively coupled plasma emission spectroscopy or wavelength dispersive X-ray fluorescence analysis.
In addition, the manner in which the content molar ratio R changes along the thickness direction of the ZnO-based piezoelectric film can be applied by any method that enables analysis in the thickness (depth) direction of the ZnO-based piezoelectric film.
For example, XPS (X-ray photoelectron spectroscopy), SIMS (secondary ion mass spectrometry), Auger electron spectroscopy, and GD-MS (glow discharge mass spectrometry) are methods for analysis while etching in the depth direction. ). Further, when the thickness of the ZnO-based piezoelectric film is about several hundred nm, RBS (Rutherford backscattering analysis) can be used.

  For example, in the case of an analysis method obtained only by signal intensity, such as SIMS (secondary ion mass spectrometry), the integral value in the film thickness direction of the signal chart matches the measured value obtained by measuring the entire film. The content molar ratio R can also be determined by determining the proportionality coefficient. Also, if there is a large divergence between the analysis value in the depth (ZnO-based piezoelectric film thickness) direction and the average value of the molar ratio R of the entire film, it is considered that there is an error in the analysis method. There may be differences in values. In this case, the value of the measurement method of the average value of the whole film is adopted, and the proportionality coefficient is determined and the analysis result is complemented and marked as described above.

  The inventors of the present invention have investigated how the piezoelectric characteristics of ZnO having a wurtzite crystal structure change when a part of Zn is substituted with various divalent cations. It has been found that when a certain amount of Zn is substituted with Ca, the piezoelectric characteristics are improved as compared with the case where Zn of ZnO is substituted with other alkaline earth metal (Mg, Sr, Ba, or Cd). In particular, a piezoelectric element having a ZnO-based piezoelectric film having a first region in which the content molar ratio R increases along the thickness direction of the ZnO-based piezoelectric film from the first electrode to the second electrode is a particularly excellent piezoelectric element. The present invention was completed by finding that it has characteristics.

  The reason why the piezoelectric element having the ZnO-based piezoelectric film has particularly excellent piezoelectric characteristics is not necessarily clear, but the present inventors presume as follows. In addition, the mechanism by which the effect of this invention is acquired by the following estimation is not limited.

FIG. 2 is a conceptual diagram showing a change in the content molar ratio R along the thickness direction L of the ZnO-based piezoelectric film from the first electrode 12 to the second electrode 14 of the ZnO-based piezoelectric film 13 in FIG. It is an example. 2, the horizontal axis X represents the thickness (μm) of the ZnO-based piezoelectric film, and the vertical axis Y represents the content molar ratio R. The positive direction of the horizontal axis X corresponds to the direction L in FIG.
As shown in FIG. 2, the ZnO-based piezoelectric film has a first region in which the content molar ratio R increases along the thickness direction L of the ZnO-based piezoelectric film from the first electrode 12 toward the second electrode 14 (FIG. 2). 2).
The first region preferably starts from the surface of the ZnO-based piezoelectric film on the first electrode 12 side, as shown in FIG. In other words, it is preferable that the surface of the ZnO-based piezoelectric film on the first electrode 12 side shows the minimum content molar ratio R, and the first region exists from the surface of the ZnO-based piezoelectric film on the first electrode 12 side. . However, the position where the first region exists is not limited to the mode shown in FIG. 2 and may exist inside the ZnO-based piezoelectric film.
Note that the ZnO-based piezoelectric film included in the piezoelectric element according to the embodiment of the present invention is not limited to the above, and the ZnO-based piezoelectric film extends along the thickness direction L of the ZnO-based piezoelectric film from the first electrode to the second electrode. The entire thickness direction of the piezoelectric film may be the first region.

  In the present piezoelectric film, Ca is typically at least partly taken into the crystal lattice of ZnO (part of Zn in ZnO is replaced by Ca). In such a case, the crystal after substitution is pure ZnO because at least a part of Zn is substituted by Ca or Ca (or CaO) is mixed between ZnO crystal lattices as compared with the crystal before substitution. It is estimated that the lattice constant and the like are different from those of the crystal. In addition, it is presumed that the lattice constant and the like differ depending on the Ca content for the above reason. When such crystals with different lattice constants are stacked in the thickness direction to form a ZnO-based piezoelectric film, distortion due to the difference in lattice constants and the like is the cause, and the orientation state of the crystals is likely to be disturbed. It is assumed that a piezoelectric element having desired piezoelectric characteristics may not be obtained.

  On the other hand, since this piezoelectric film has the first region, changes in the crystal lattice constant and the like are gentle in the thickness direction of the ZnO-based piezoelectric film. For this reason, it is presumed that a ZnO-based piezoelectric film having excellent orientation was obtained with less distortion in the film. As a result, this piezoelectric element is presumed to have excellent piezoelectric characteristics.

  The thickness of the first region is not particularly limited. It can be appropriately selected according to the change rate (increase rate or decrease rate) of the content molar ratio R with respect to the thickness direction of the ZnO-based piezoelectric film and the final content molar ratio R in the first region.

  The first region is preferably present in the piezoelectric film as a layered region substantially parallel to the surface of the piezoelectric film. If the first region is present in a layered manner in the piezoelectric film, the variation in the piezoelectric constant within the surface of the piezoelectric film tends to be smaller.

The piezoelectric film has a second region. The second region is a region in which the maximum value of the content molar ratio R in the first region is maintained substantially constant in the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. The second region is preferably present in the piezoelectric film as a layered region substantially parallel to the surface of the piezoelectric film. If the second region is present in a layered manner in the piezoelectric film, the variation in the piezoelectric constant within the surface of the piezoelectric film tends to be smaller.
The second region is preferably present adjacent to the first region. That is, it is preferable that the second region exists in the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode from the position where the maximum value of the first region is shown.

  The present piezoelectric film having the above-described region is easy to control the content molar ratio R (in other words, the average value of the content molar ratio R) as a whole within a desired range. That is, by adjusting the thickness of the second region, the average content molar ratio R of the ZnO-based piezoelectric film can be easily controlled within a desired range. Although the piezoelectric film has the second region, the piezoelectric element according to the embodiment of the present invention and the piezoelectric film may not have the second region.

In the second region, the maximum value of the content molar ratio R in the first region is maintained substantially constant.
“Substantially constant” means that the contained molar ratio R at an arbitrary point in the second region is 80 to 100% with respect to the maximum value R of the contained molar ratio R in the first region, and is 90 to 100%. It is preferable that

The thickness of the second region is not particularly limited, and can be appropriately selected according to the average value of the molar ratio R contained in the piezoelectric film and the thickness of the piezoelectric film.
In addition, it is preferable that a 2nd area | region exists from the position which shows the maximum value of a 1st area | region to the surface by the side of the 2nd electrode of a ZnO type piezoelectric material film from the point which the performance of a piezoelectric element is more excellent.
Further, the ratio of the thickness of the second region to the thickness of the first region (thickness of the second region / thickness of the first region) is not particularly limited, but is preferably 1.0 or more in terms of more excellent performance of the piezoelectric element. 5.0 or more is more preferable. Although an upper limit is not restrict | limited in particular, 100 or less is mentioned.
When the thickness of the second region / the thickness of the first region is 1.0 or more, the obtained piezoelectric element has more excellent piezoelectric characteristics, and when the thickness of the second region / the thickness of the first region is 100 or less, Since the thickness of the first region is sufficiently thick and the film stress generated in the ZnO-based piezoelectric film tends to be small, as a result, cracks and the like are less likely to occur in the ZnO-based piezoelectric film.

Although it does not restrict | limit especially as an average value of content molar ratio R in a piezoelectric film, 0.12-0.50 is preferable at the point from which the piezoelectric element which has the more excellent effect of this invention is obtained.
When the average content molar ratio R is 0.12 to 0.50, a piezoelectric element having excellent piezoelectric characteristics can be obtained. The average value of the content molar ratio R can be measured by energy dispersive X-ray fluorescence analysis, and details are as described in the examples.
Further, the difference between the minimum value and the maximum value of the content molar ratio R in the first region is not particularly limited, but is 0.06 to 0.50 in that a piezoelectric element having more excellent effects of the present invention can be obtained. Is preferable, and 0.12-0.50 is more preferable.

The piezoelectric film preferably does not substantially contain an alkali metal. Alkali metals are very diffusive and tend to cause defects when the piezoelectric film is applied to a semiconductor or an electric circuit. Note that “substantially no alkali metal” means that the content of alkali metal atoms is 0.1 mol% or less with respect to all metal atoms contained in the piezoelectric film, and 0 mol. % Is preferred.
The content of alkali metal contained in the piezoelectric film can be measured by energy dispersive X-ray fluorescence analysis. In addition to the above, measurement may be performed by high frequency inductively coupled plasma emission spectroscopy or wavelength dispersive X-ray fluorescence analysis.

The crystal structure of the present piezoelectric film is not particularly limited, but a large number of columnar crystals are formed in the thickness direction of the ZnO-based piezoelectric film (the L direction in FIG. 1) in that a piezoelectric element having more excellent piezoelectric characteristics can be obtained. It is preferable that the aggregate is oriented in the following manner. In the present specification, the above ZnO-based piezoelectric film is also referred to as a columnar crystal film.
In a ZnO-based piezoelectric film, the crystal often takes a wurtzite crystal, the wurtzite crystal has a hexagonal crystal structure, and the unit cell is an equilateral triangle with a lattice constant a on one side, c Six triangular prisms extending in the axial direction are gathered and formed. In the present ZnO-based piezoelectric film, the c-axis of the wurtzite crystal is preferably preferentially oriented in the thickness direction of the ZnO-based piezoelectric film (T direction in FIG. 1).

This piezoelectric film has superior piezoelectric characteristics, and in the X-ray diffraction spectrum by the out-of-plane method (2θ / ω) using CuKα rays, the (002) plane diffraction peak intensity is I (002 ), When the (100) plane diffraction peak intensity is I (100), the following formula (1) is satisfied, and in the X-ray diffraction spectrum by the in-plane method using CuKα rays of the piezoelectric film, It is preferable that both the (100) plane diffraction peak and the (110) plane diffraction peak are detected.
Formula (1) I (100) / I (002) <1.0 × 10 ⁻²
In addition, the said X-ray-diffraction spectrum is measured in the range of 30-60 degrees. That is, the (002) plane diffraction peak, the (100) plane diffraction peak, and the (110) plane diffraction peak described later are detected in the measurement range of 2θ: 30 to 60 °. Moreover, in this specification, the (XXX) plane diffraction peak intensity represents the maximum value of the diffraction intensity of the peak derived from the (XXX) plane.
In this specification, in the X-ray diffraction spectrum by the out-of-plane method and the in-plane method using CuKα rays, the (XXX) plane diffraction peak is defined as Powder Diffraction File number 36-1451 (PDF # 36-1451), intended for diffraction peaks corresponding to the (XXX) plane diffraction peaks of pure ZnO. The corresponding diffraction peak means a diffraction peak detected at substantially the same diffraction angle as each surface diffraction peak in PDF # 36-1451. Specifically, a peak detected at a position where 2θ is −3 to + 0.5 ° as compared with each surface diffraction peak in PDF # 36-1451 is intended. Compared to each surface diffraction peak in PDF # 36-1451, it is estimated that 2θ at the peak of the ZnO-based piezoelectric film shifts in the positive direction due to film stress or the like, and shifts in the negative direction. In the crystal lattice of ZnO, it is presumed to be caused by a change in lattice constant caused by substitution of a part of Zn with Ca.

First, in the X-ray diffraction spectrum by the out-of-plane method, when the (002) plane diffraction peak intensity is I (002) and the (100) plane diffraction peak intensity is I (100), the following formula The state in which (1) is established represents a state in which the c-axis of the crystal is preferentially oriented in the thickness direction of the piezoelectric film (T direction in FIG. 1).
In the above formula (1), I (100) / I (002) is calculated by two significant digits (the third digit is rounded off).
In the above formula (1), the lower limit of I (100) / I (002) is not particularly limited, but is generally preferably 0 or more. The case where I (100) / I (002) is 0 means that I (100) is not detected, that is, is below the detection limit.

Moreover, in the X-ray diffraction spectrum by the in-plane method using the CuKα ray of the ZnO-based piezoelectric film, the state where both the (100) plane diffraction peak and the (110) plane diffraction peak are detected is ZnO. This shows that the a-axis of the crystal is randomly oriented in the in-plane direction of the piezoelectric film. The state in which both the (100) plane diffraction peak and the (110) plane diffraction peak are detected means that the (100) plane diffraction peak intensity is I _in (100) and the (110) plane diffraction peak intensity is I. _{When the in} (110) and (002) plane diffraction peak intensities are I _in (002), it means that the following formulas (3) and (4) hold.
Formula (3) I _in (002) / I _in (100) <1.0 × 10 ⁻²
Formula (4) I _in (002) / I _in (110) <1.0 × 10 ⁻²

In the ZnO-based piezoelectric film according to the above-described embodiment, the volume of the crystal unit cell (unit cell) is preferably larger than the volume of the pure ZnO unit cell.
That is, it is preferable that the volume V _{1 of} the unit cell defined below and the volume V _{0 of the} unit cell defined below satisfy the following formula (2).
Equation _{(2) V 1 / V 0} > 1.02

Here, the volume V ₁ of the unit cell means “a-axis length (ZnO-based piezoelectric film)” and “c-axis length (ZnO-based piezoelectric film)” determined by the following method for the ZnO-based piezoelectric film. Represents the volume of the unit cell calculated from
C-axis length (ZnO-based piezoelectric film): c-axis length calculated from the (002) plane diffraction peak in the X-ray diffraction spectrum by the out-of-plane method using CuKα rays.
A-axis length (ZnO-based piezoelectric film): an a-axis length calculated from a (100) plane diffraction peak in an X-ray diffraction spectrum by an in-plane method using CuKα rays.

The unit cell volume V ₀ was calculated from the c-axis length calculated from the (002) plane diffraction peak and the (100) plane diffraction peak in the X-ray diffraction spectrum using CuKα rays of pure ZnO. The unit cell volume calculated using the a-axis length.

Here, the unit of the a-axis length and the c-axis length is angstrom (“に”, corresponding to 10 ⁻¹ nm), and the volume of the unit cell is Å ³ (10 ⁻³ nm ³ ). is there.
Further, in this specification, the X-ray diffraction spectrum using pure ZnO CuKα rays means the diffraction spectrum of PDF # 36-1451.

V ₁ / V ₀ is preferably more than 1.02, more preferably 1.04 or more. The upper limit value of V ₁ / V ₀ is not particularly limited, but is generally preferably 1.15 or less.

In addition, the state where V ₁ / V ₀ exceeds 1.02 represents that the size of the unit cell is increased by incorporating Ca atoms into the crystal lattice of ZnO.

[Electrodes (first electrode and second electrode)]
The present piezoelectric element has a first electrode and a second electrode that face each other with the present piezoelectric film interposed therebetween. The material for each electrode is not particularly limited, and known materials for electrodes can be used. In particular, examples of the material of the electrode with a substrate having a substrate on the main surface opposite to the piezoelectric film include Au, Pt, Ir, IrO ₂ , RuO ₂ , LaNiO ₃ , SrRuO ₃ , ITO (Indium Tin oxide). ) And a metal such as TiN (titanium nitride), a metal oxide, a transparent conductive material, a combination thereof, and the like can be preferably used, and it is particularly preferable to contain Ir.

On the other hand, the material of the electrode having no substrate on the main surface opposite to the piezoelectric film is not particularly limited, and a known material can be used. Examples of the material for the electrode include, for example, the materials described as the electrode material for the electrode with a substrate, the electrode materials generally used in semiconductor processes such as Al, Ta, Cr, and Cu, and these The combination of these is mentioned.
The thickness of each of the two electrodes is not particularly limited, but is generally preferably 50 to 500 nm.

〔substrate〕
The piezoelectric element according to this embodiment may have a substrate. The substrate is not particularly limited, and a known substrate can be used. Examples of the substrate include an organic substrate and an inorganic substrate.
A typical example of the organic substrate is a resin substrate, and examples of the material of the resin substrate include PET (Polyethylene terephthalate) and polyimide.
Examples of the inorganic substrate include a metal substrate. Examples of the material of the inorganic substrate include substrates such as silicon, glass, stainless steel, yttrium-stabilized zirconia (YSZ), SrTiO ₃ , alumina, sapphire, and silicon carbide.
Further, as the substrate, a laminated substrate such as an SOI (Silicon on Insulator) substrate in which a SiO ₂ film and a Si active layer are sequentially laminated on silicon may be used.

Although it does not restrict | limit especially as thickness of a ZnO type piezoelectric material film, 20-10000 nm is preferable and 100-2000 nm is more preferable at the point from which the ZnO type piezoelectric material film which has the more excellent effect of this invention is obtained.
When the ZnO-based piezoelectric film is 20 nm or more, the ZnO-based piezoelectric film is unlikely to be island-shaped and a continuous film is easily obtained. Similarly, a good continuous film is easily obtained for the electrode material used as the lower electrode. Further, if the piezoelectric film is sufficiently thick as compared with the thickness of the electrode, the ZnO-based piezoelectric film has more excellent piezoelectric performance.
If the ZnO-based piezoelectric film is 10000 nm or less, the film stress does not increase excessively, so that the ZnO-based piezoelectric film is difficult to peel from the electrode.

  The piezoelectric element according to this embodiment can be used for various applications. Typically, the piezoelectric element according to the present embodiment can be used as a sensor or an actuator, and specifically, can be used for a wearable device, a touch pad, a display, a controller, and the like.

[Piezoelectric element manufacturing method]
Although it does not restrict | limit especially as a manufacturing method of this piezoelectric element, Typically, it is preferable to have the following processes in this order.
-Step A: Step of forming a ZnO-based piezoelectric film on the first electrode-Step B: Step of forming a second electrode on the formed ZnO-based piezoelectric film Note that Step A is performed from the first electrode to the second. Forming a first region in which the content molar ratio R increases along the thickness direction of the ZnO-based piezoelectric film facing the electrode.
Furthermore, as a manufacturing method of the present piezoelectric element, the step of forming the first electrode on the substrate (step C) before the step A or the step B after the step B, the ZnO-based first electrode or the second electrode. You may have the process (process D) which affixes a board | substrate on the surface on the opposite side to a piezoelectric material film.
Below, the form is demonstrated for every process.

・ Process A
Step A is a step of forming a ZnO-based piezoelectric film on the first electrode. The method for forming the ZnO-based piezoelectric film on the first electrode is not particularly limited, and a known method can be used as long as the first region described later can be formed. As a method of forming a ZnO-based piezoelectric film, for example, a sputtering method, a plasma CVD (Chemical Vapor Deposition) method, a MOCVD (Metal Organic Chemical Vapor Deposition) method, a PLD (Pulsed Laser Deposition) method, or the like. A liquid phase method such as a sol-gel method and an organometallic decomposition method; an aerosol deposition method; and the like.
Among these, the vapor phase growth method is preferable because the formation conditions are more easily controlled. Moreover, according to the vapor phase growth method, it is possible to suppress the occurrence of lateral streaks in the ZnO-based piezoelectric film during formation, and a more durable ZnO-based piezoelectric film can be obtained.

  A method for forming a piezoelectric film or the like by vapor phase growth is not particularly limited, but typically, the substrate (or temporary substrate) and a target are opposed to each other on the substrate (or temporary substrate) using plasma. The method of forming the film | membrane containing the structural element of a target is mentioned.

Examples of the vapor phase growth method include a bipolar sputtering method, a tripolar sputtering method, a direct current sputtering method, a radio frequency sputtering method (RF: Radio Frequency Sputtering method), an ECR (Electron Cyclotron Resonance) sputtering method, a magnetron sputtering method, and a counter target. Examples thereof include a sputtering method, a pulse sputtering method, and an ion beam sputtering method.
Among these, as the vapor phase growth method, a sputtering method (especially a high frequency sputtering method is preferable), an ion plating method, or a plasma CVD method is preferable, and a sputtering method is preferable.
When the formation method of the ZnO-based piezoelectric film is a sputtering method, the obtained ZnO-based piezoelectric film tends to be a columnar crystal film. That is, in the ZnO-based piezoelectric film, it tends to be an aggregate of columnar crystals in which crystal grains are c-axis oriented in the thickness direction of the piezoelectric film (L direction in FIG. 1).
In the film formation by the sputtering method, since a columnar crystal film is easily formed, it is excellent in that it is not necessary to use an expensive substrate (for example, a sapphire substrate) used for a single crystal film.

The step A includes a step of forming a first region in which the content molar ratio R increases along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. As a method of forming the first region in the ZnO-based piezoelectric film, a sputtering method that increases the applied power to a target (Ca-containing target) containing at least Ca is preferable.
It is not particularly limited as containing CaO target, CaO, _{and, Zn 1-x Ca x O} ( where, 0 <X <1), and the like.
Typically, as a method for forming the ZnO-based piezoelectric film, there is a method in which CaO is used as a Ca-containing target and ZnO is used in combination as a target other than the above. When the first region is formed by the above method, the applied power to the CaO target may be increased, or the applied power to the ZnO target may be decreased (relatively increasing the applied voltage to the CaO target). In particular, it is preferable to increase the power applied to the CaO target in that the first region can be formed more efficiently.
The method of increasing the applied power is not particularly limited, but a method of increasing the applied power stepwise is preferable.
In this case, the increase rate of the applied power to the CaO target (the amount of change in the magnitude of the applied power with respect to the power application time) is not particularly limited, and the increase rate of the content molar ratio R with respect to the thickness of the ZnO-based piezoelectric film can be set as desired. What is necessary is just to adjust suitably so that it can control to the range.

  The above is an example of a method for forming a ZnO-based piezoelectric film having a first region, and the formation of a ZnO-based piezoelectric film having a first region is not limited to the above, and the conditions at the time of formation (target A method of appropriately adjusting or changing the distance between the substrate and the substrate and the pressure) can also be used.

The step A forms a second region that maintains the maximum content molar ratio R in the first region substantially constant along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. It is preferable to include a process.
The step of forming the second region is not particularly limited, and a known method can be used. For example, in the case of the sputtering method using both the CaO target and the ZnO target described above, for example, a method of maintaining the applied voltage to the CaO target at the end of the first region and making the applied voltage substantially constant is preferable.
In this case, the applied power to the CaO target and the ZnO target may be adjusted as appropriate so that the molar ratio R in the second region becomes a desired value.

・ Process B
Step B is a step of forming the second electrode on the formed ZnO-based piezoelectric film, and the method is not particularly limited, and a known method can be used. Examples of the step of forming the second electrode include vapor phase growth methods such as sputtering, plasma CVD, MOCVD, and PLD; liquid phase methods such as sol-gel method and organometallic decomposition method; aerosol deposition method The vapor phase growth method is preferable in that the second electrode can be easily formed.

・ Process C
Step C is a step of forming the first electrode on the substrate before Step A. The step of forming the first electrode on the substrate is not particularly limited, and the same method as that for forming the second electrode on the ZnO-based piezoelectric film can be used in Step B. In addition, before forming the first electrode on the substrate, a primer layer may be further formed on the substrate, and then the first electrode may be formed on the primer layer.

・ Process D
Step D is a step of attaching a substrate to the surface of the first electrode or the second electrode opposite to the ZnO-based piezoelectric film after Step B. When the manufacturing method of this piezoelectric element has the process D, before the process A, the process of forming a 1st electrode on a temporary board | substrate, and after the process B and before the process D, the 1st electrode A step of removing the temporary substrate from the substrate may be further included.
The temporary substrate is not particularly limited, and a known temporary substrate can be used. As a temporary board | substrate, what was already demonstrated as a board | substrate which a piezoelectric element has, for example can be used.
In addition, the piezoelectric element manufacturing method includes the step D, and the step of forming the first electrode on the temporary substrate before the step A, the trace of the step B, and from the first electrode before the step D. And a step of removing the temporary substrate, it is preferable to use a resin substrate as the substrate. Compared with the case where the first electrode is directly formed on the resin substrate by, for example, sputtering, deterioration of the resin substrate can be further suppressed.

  Hereinafter, the present invention will be described in more detail based on examples. The materials, amounts used, ratios, processing details, processing procedures, and the like shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the examples shown below.

Example 1
On a Si substrate (corresponding to the substrate) having a thermal oxide film (thickness: 300 nm) on the surface, a lower electrode (corresponding to the first electrode) of Ti (20 nm) / Ir (150 nm) using a sputtering apparatus To obtain a substrate with electrodes.

Next, on this substrate with electrodes, ZnO (φ100) and CaO (φ100) are used as targets, the applied power to ZnO is 250 W, and the applied power to CaO is sequentially increased from 0 to 400 W by 20 W, A first region was formed. Next, the power applied to the CaO target was kept at 400 W, and a ZnO-based piezoelectric film was formed until the film thickness reached 500 nm. At this time, a sputtering apparatus MPS-6000 manufactured by ULVAC was used for the formation of the ZnO-based piezoelectric film. O ₂ / (Ar + O ₂ ) = 16%, and the pressure in the chamber was 0.2 Pa (chamber pressure). An upper electrode (corresponding to the second electrode) was formed on the obtained ZnO-based piezoelectric film, and the piezoelectric element of Example 1 was obtained.

(Comparative Examples 1 and 2)
Except that the applied power to each of the ZnO target and the CaO target was made constant, and each applied power was adjusted so that the average value of the molar ratio R contained in the ZnO-based piezoelectric film was as shown in Table 1. In the same manner as in Example 1, the piezoelectric elements of Comparative Examples 1 and 2 were formed.

Energy dispersive X-ray fluorescence analysis was performed on the compositions of the ZnO-based piezoelectric films prepared in Examples and Comparative Examples. As a result, the molar ratio of the Ca content and the Zn content in the ZnO-based piezoelectric film to the sum of the molar contents The average value of the content molar ratio R ( _MCa / ( _MCa + _MZn )) of the reference Ca content was as described in Table 1.

  For the X-ray diffraction measurement, “Ultima III” manufactured by Rigaku Corporation having a 40 kV, 40 mA CuKα ray tube was used. The out-of-plane method was measured by a general 2θ / ω method. In the in-plane method, an incident angle ω = 0.2 ° and a reflection angle θ = 0.2 ° were set, and measurement was performed by general 2θχ / φ measurement.

  Each diffraction surface of the ZnO-based piezoelectric film was identified based on PDF # 36-1451 (X-ray diffraction spectrum of pure ZnO). In the measurement range of 30 to 60 °, in the case of pure ZnO, peaks of (100) plane, (002) plane, (101) plane, (102) plane, and (110) plane appear, depending on the presence or absence of orientation. Some peaks do not appear.

From the X-ray diffraction spectra of the out-of-plane method and the in-plane method of the examples and comparative examples, almost only the (002) plane diffraction peak appears, and the crystal is preferentially oriented on the c-axis. It was found to be a film. In addition, since the diffraction peaks of both the (100) plane and the (110) plane were observed from the measurement results of the in-plane method, the in-plane direction was not a single crystal film (epitaxial film) but a random crystal in the in-plane direction. It was shown to be an aggregate of grains.
In addition, when the cross sections of the ZnO-based piezoelectric films of Examples and Comparative Examples were observed with a transmission electron microscope, the ZnO-based piezoelectric film was an aggregate of columnar crystals (columnar crystal film), and isotropic particles In addition, a portion having a low pressure resistance and mechanical characteristics such as a bonding layer was not found.

  Further, the volume of the unit cell calculated from the a-axis length and the c-axis length obtained from the X-ray diffraction spectrum, and the volume of the unit cell obtained from PDF # 36-1451 (X-ray diffraction spectrum of pure ZnO) The ratio is shown in Table 1.

[Measurement of piezoelectric constant]
Next, an upper electrode was formed on the ZnO-based piezoelectric film by sputtering to obtain a piezoelectric element. Next, the piezoelectric element was cut into a 2 mm × 25 mm strip to produce a cantilever. Kanno et. al. Sensor and Actuator A 107 (2003) 68. The piezoelectric constant was measured according to the method described in 1. The applied voltage is 1 Vpp, 2 Vpp, 4 Vpp, and a 2 kHz sine wave is applied. In any voltage measurement, the maximum and minimum values of each measurement result, the arithmetic average of the measured values, and the difference between them are the arithmetic average of the measured values. The average value was taken as the measurement result. In addition, the measurement result was shown by the ratio by setting the measurement result of the piezoelectric element of Comparative Example 2 to 1.0. A larger numerical value indicates a piezoelectric element having more excellent piezoelectric characteristics.

  From the results shown in Table 1, the piezoelectric element of Example 1 had the effect of the present invention. On the other hand, the piezoelectric elements of Comparative Example 1 and Comparative Example 2 did not have the effect of the present invention.

DESCRIPTION OF SYMBOLS 10 Piezoelectric element 11 Board | substrate 12 1st electrode 13 ZnO type piezoelectric material film | membrane 14 2nd electrode

Claims (10)

A ZnO-based piezoelectric film containing Ca;
A first electrode and a second electrode facing each other with the ZnO-based piezoelectric film interposed therebetween,
The content molar ratio R of the Ca content to the sum of the Ca content and the Zn content increases along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. A piezoelectric element, wherein the ZnO-based piezoelectric film has one region.   The ZnO-based piezoelectric film maintains the maximum value of the contained molar ratio R in the first region substantially constant in the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. The piezoelectric element according to claim 1, further comprising a second region.   3. The piezoelectric element according to claim 1, wherein in the ZnO-based piezoelectric film, an average value of the content molar ratio R is 0.12 to 0.50.   The piezoelectric element according to any one of claims 1 to 3, wherein a difference between a minimum value and a maximum value of the content molar ratio R in the first region is 0.06 to 0.50.   A substrate is further provided on the surface of the first electrode opposite to the ZnO-based piezoelectric film side or on the surface of the second electrode opposite to the ZnO-based piezoelectric film side. Item 5. The piezoelectric element according to any one of Items 1 to 4. A method of manufacturing a piezoelectric element according to any one of claims 1 to 5,
A step A of forming a ZnO-based piezoelectric film on the first electrode, and a step B of forming a second electrode on the formed ZnO-based piezoelectric film,
In step A, along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode, the molar ratio R of the Ca content to the sum of the Ca content and the Zn content is R. The manufacturing method of a piezoelectric element which has the process of forming the 1st area | region which increases.   The method for manufacturing a piezoelectric element according to claim 6, wherein in the step A, the first region is formed by a sputtering method for increasing an applied power to a target containing at least Ca.   In the step A, the maximum value of the content molar ratio R in the first region is maintained substantially constant along the thickness direction of the ZnO-based piezoelectric film from the first electrode toward the second electrode. The method for manufacturing a piezoelectric element according to claim 6, comprising a step of forming two regions.   The method for manufacturing a piezoelectric element according to claim 8, wherein the second region is formed by a sputtering method in which an applied power to a target containing at least Ca is substantially constant. Further, the step of forming the first electrode on the substrate before the step A, or the step of forming the ZnO-based piezoelectric film of the first electrode or the second electrode after the step B. The manufacturing method of the piezoelectric element as described in any one of Claims 6-9 which has the process of sticking a board | substrate on the surface on the opposite side to.