JP7345411B2 - Piezoelectric bodies and piezoelectric elements - Google Patents

Description

The present invention relates to a highly reliable piezoelectric material and a piezoelectric element using the piezoelectric material.

In recent years, demand for electronic components using piezoelectric materials has been increasing. An example of such an electronic component is a SAW filter using surface acoustic waves (hereinafter sometimes referred to as SAW).

In order to achieve stable electrical characteristics in such electronic components, it is necessary to stabilize various electrical characteristics of the piezoelectric body. Patent Document 1 focuses on the pyroelectricity of a piezoelectric material, and proposes a technique for keeping the volume resistivity constant at a desired value in order to suppress sparks and the like due to the pyroelectricity.

Japanese Patent Application Publication No. 2008-201640

There is a need to provide a piezoelectric material that can realize more stable electrical characteristics and a piezoelectric element using the piezoelectric material.

The present invention was devised based on the above-mentioned circumstances, and its purpose is to provide a piezoelectric material that can realize stable electrical characteristics and a piezoelectric element using the same.

A piezoelectric body according to one embodiment of the present disclosure is made of a LiTaO ₃ single crystal, and satisfies the following formula, where the Curie temperature is X (° C.) and the Ir content is Y (ng/g).

0<Y<10^(-0.10034×X+63.67) and Y≦400ng/g
A piezoelectric element according to an aspect of the present disclosure includes an IDT electrode on the piezoelectric body, and the piezoelectric body has a thickness that is twice or less the pitch of the IDT electrode.

The piezoelectric body according to one aspect of the present disclosure described above suppresses changes in electrical characteristics due to pyroelectricity. Moreover, a piezoelectric element including such a piezoelectric body has excellent electrical characteristics.

1 is a cross-sectional view showing an embodiment of a piezoelectric element according to the present disclosure. 2 is a top view of the piezoelectric element shown in FIG. 1. FIG. FIG. 7 is a diagram showing frequency characteristics of a piezoelectric element according to a comparative example. 4(a) is an AFM image of the piezoelectric element shown in FIG. 3, and FIG. 4(b) is a piezoelectric response force microscope image in the region shown in FIG. 4(a). FIG. 3 is a diagram showing the relationship between applied voltage and the difference (Δf) between the resonant frequency and the anti-resonant frequency. FIG. 3 is a diagram showing the relationship between the Curie temperature, Ir content, and ease of polarization reversal of a piezoelectric material. The relationship between the Curie temperature and the Δf value when a DC voltage of 50 V is applied is shown. FIG. 8(a) is a diagram showing the relationship between the Ir content and the Δf value when a DC voltage of 50V is applied, and FIG. 8(b) is a diagram showing the relationship between the Zr content and the Δf value when a DC voltage of 50V is applied. FIG. 3 is a diagram showing the relationship with the Δf value. 9(a) and 9(b) are cross-sectional views showing modified examples of the piezoelectric element shown in FIG. 1, respectively. 2 is a sectional view showing a modification of the piezoelectric element shown in FIG. 1. FIG.

Embodiments of the piezoelectric body and piezoelectric element of the present disclosure will be described in detail below with reference to the drawings. Note that the drawings used in the following explanation are schematic, and the dimensional ratios, etc. in the drawings do not necessarily match the actual ones.

Furthermore, in the description of modifications and the like, configurations that are the same as or similar to the configurations of the already described embodiments may be given the same reference numerals as those of the already described embodiments, and the description thereof may be omitted.

A piezoelectric body or a piezoelectric element may have any direction upward or downward; however, for convenience, below, for convenience, the D1 direction, D2 direction, and D3 direction, which are orthogonal to each other, are defined, and the positive direction of the D3 direction is defined. Terms such as upper surface, lower surface, etc. shall be used with the side facing upward. The orthogonal coordinate system defined by the D1 direction, D2 direction, and D3 direction mentioned above is defined based on the shape of the piezoelectric body and the piezoelectric element, and the crystal axis ( It does not refer to the X-axis, Y-axis, Z-axis). In addition, if the basic configuration is similar, the description of the first, second, etc. may be omitted and the description will be made without distinguishing between them.

<Piezoelectric element: elastic wave element 1>
Hereinafter, as an example of a piezoelectric element, an elastic wave element such as a SAW that excites elastic waves will be used. FIG. 1 is a schematic cross-sectional view of an acoustic wave element 1 as a piezoelectric element according to an embodiment of the present disclosure.

The acoustic wave element 1 includes a support substrate 3, an intermediate portion 5, a piezoelectric body 7, and an IDT electrode 9. The support substrate 3, the intermediate portion 5, and the piezoelectric body 7 are laminated in this order.

In this example, the support substrate 3 supports the intermediate portion 5 and the piezoelectric body 7 stacked thereon, and is not particularly limited as long as it has a certain strength. For example, when the piezoelectric body 7 is made of a material having a smaller coefficient of linear expansion than the piezoelectric body 7, by reducing the deformation of the piezoelectric body 7 due to temperature changes, it is possible to reduce characteristic changes due to temperature changes. Furthermore, if the material is selected so that the transverse sound velocity of the elastic waves propagating in the support substrate 3 is faster than the transverse sound velocity of the elastic waves propagating in the piezoelectric body 7, the elastic waves can be confined in the piezoelectric body 7. Therefore, it is possible to provide an acoustic wave element 1 with excellent frequency characteristics.

Examples of such materials include sapphire substrates and Si substrates. In this embodiment, an example will be described in which a Si substrate is used as the support substrate 3.

Note that the material of the support substrate 3 may be selected so as to reduce warping of the laminate of the support substrate 3, the intermediate portion 5, and the piezoelectric body 7. For example, when the coefficient of linear expansion of the intermediate portion 5 is smaller than that of the piezoelectric body 7, the support substrate 3 may be made of a material having a coefficient of linear expansion larger than that of the intermediate portion 5.

The intermediate portion 5 is made of an insulating material such as silicon oxide, silicon nitrogen, or aluminum oxide, and its crystallinity is not particularly limited. By providing the intermediate portion 5, it is possible to reduce the formation of an unnecessary potential or an unnecessary capacitance, so that the electrical characteristics of the acoustic wave element 1 can be improved. In this example, silicon oxide is used as the intermediate portion 5.

In particular, when a Si substrate, which is a semiconductor material, is used as the support substrate 3 as in this example, by providing an insulating intermediate portion 5 between the piezoelectric body 7 and the support substrate 3, the support substrate 3 can be The impact can be reduced. In order to ensure insulation and take advantage of the characteristics of the high-sonic material of the support substrate 3, the thickness of the intermediate portion 3 should be 0.01p or more 2p with respect to the pitch p defined by the IDT electrodes 9, which will be described later. The following may be used. In particular, when the value is 0.1p to 0.4p, the influence of the conductivity of the support substrate 3 (Si in this case) can be avoided.

Further, when silicon oxide is used as the intermediate portion 5, the transverse wave sonic velocity of the elastic wave propagating in the intermediate portion 5 is slower than the transverse wave sonic velocity of the elastic wave propagating in the piezoelectric body 7. In this case, robustness can be improved by the following mechanism. That is, when the piezoelectric body 7 becomes thinner than the expected thickness and the resonant frequency of the elastic wave element 1 becomes high, the amount of elastic wave distribution in the intermediate portion 5 increases, working to lower the resonant frequency. When the piezoelectric body 7 becomes thicker than a specified thickness and the resonant frequency of the elastic wave element 1 becomes lower, the amount of elastic wave distribution in the intermediate portion 5 decreases, working to increase the resonant frequency. In this way, even if the thickness of the piezoelectric body 7 changes in the acoustic wave element 1, changes in frequency characteristics can be reduced, so robustness can be improved. Furthermore, the effect of improving temperature characteristics can also be expected.

The piezoelectric body 7 includes a first surface 7a and a second surface 7b opposing the first surface 7a. For convenience, the direction from the second surface 7b to the first surface 7a (direction D3) may be referred to as upward. The intermediate portion 5 is joined to the second surface 7b of the piezoelectric body 7.

The piezoelectric body 7 is, for example, a piezoelectric single crystal substrate made of lithium tantalate (LiTaO ₃ :hereinafter referred to as LT) crystal, or a piezoelectric single crystal substrate made of lithium niobate (LiNbO ₃ :hereinafter referred to as LN) crystal. A crystal substrate, thin film, etc. can be used.

The thickness of the piezoelectric body 7 is 2p or less, that is, the thickness is less than the wavelength λ of the elastic wave. By setting the thickness to be equal to or less than the wavelength λ of the elastic wave, the elastic wave can be confined in the piezoelectric body 7, and the Q value of the elastic wave element 1 can be increased. Further, by selecting the Euler angle of the piezoelectric body 7, the resonance frequency can be increased.

Here, when using LT, the piezoelectric body 7 satisfies the following relational expression (1) when its Curie temperature is X (°C) and the Ir element content is Y (ng/g). .
0<Y<10^(-0.10034×X+63.67) ・・・(1)
The relationship between X and Y will be described in detail later.

As a pair of electrodes provided on the piezoelectric body 7, in this example, an IDT electrode 9 is provided. The IDT electrode 9 includes a pair of comb-teeth electrodes 90 and is located on the first surface 7a of the piezoelectric body 7. A base layer may be located between the IDT electrode 9 and the first surface 7a.

The IDT electrode 9 is made of a conductive material, and in this example is made of an Al--Cu alloy in which Cu is added to Al. The IDT electrode 9 can be made of various conductive materials such as Al, Cu, Pt, Mo, and Au, and may also be constructed by laminating a plurality of these layers. Also. In the case of a multilayer laminate, a base layer may be interposed at the interface between the laminates.

FIG. 2 shows the shape of the IDT electrode 9. FIG. 2 is a top view of the acoustic wave element 1. As shown in FIG. 2, the IDT electrode 9 includes two busbars 91 and a plurality of long electrode fingers 92 connected to either of the busbars 91 arranged in one direction. Electrode fingers 92 connected to one bus bar 91 and electrode fingers 92 connected to the other bus bar 91 are alternately arranged. Further, a dummy electrode 93 is provided which faces the tip of the electrode finger 92 connected to one bus bar 91 and is connected to the other bus bar 91 .

That is, one bus bar 91 and the electrode fingers 92 and dummy electrodes 93 connected thereto constitute one comb-teeth electrode 90, and the other bus bar 91 and the electrode fingers 92 and dummy electrodes 93 connected thereto constitute one comb-teeth electrode 90. The other comb-teeth electrode 90 is configured. In the figure, in order to distinguish between the configuration connected to one bus bar 91 (one comb-teeth electrode 90) and the configuration connected to the other bus bar 91 (other comb-teeth electrode 90), Diagonal lines are added.

When a high frequency signal is applied to such an IDT electrode 9, a standing wave whose center-to-center spacing p of the electrode fingers 92 is a half wavelength is excited.

Note that reflectors 11 are located on both sides of the IDT electrode 9 in the direction in which the electrode fingers 92 are arranged. Thereby, the IDT electrode 9 and the reflector 11 function as a one-port type resonator. Note that the acoustic wave element 1 of the present disclosure only needs to include such IDT electrodes 9, and the number, arrangement, etc. thereof are not particularly limited. For example, it is also possible to configure a ladder type filter including a plurality of such resonators, a vertically coupled filter, or the like.

According to the acoustic wave device 1 of the present disclosure, having the above-described configuration provides excellent electrical characteristics. The mechanism will be explained in detail below.

The inventor discovered that when producing an acoustic wave element, the frequency characteristics may deteriorate regardless of the design of the IDT electrode. Specifically, we discovered that ripples may occur near the resonant frequency and anti-resonant frequency.

FIG. 3 shows frequency characteristics near the passband of an acoustic wave element according to a reference example when a filter is configured with IDT electrodes. The horizontal axis represents frequency, and the vertical axis represents transmission characteristics. Line L1 indicates the value of comparative product 1 with good frequency characteristics, and line L2 indicates the value of comparative product 2 with confirmed deterioration of frequency characteristics. It can be seen that, compared to line L2, line L1 has ripples in the pass band as shown by the arrows in the figure, and its electrical characteristics have deteriorated.

Generally, in order to improve the pyroelectricity of the piezoelectric body 7, it is known to use an LT substrate or an LN substrate that has been previously subjected to a reduction treatment as the piezoelectric body. On the other hand, the inventor has found that when the piezoelectric material becomes thin, ripples may occur even if a piezoelectric material that has been subjected to reduction treatment is used. Furthermore, LT substrates doped with Fe to prevent pyroelectricity are also known. It has been found that even in this case, ripples may occur if the piezoelectric material becomes thin.

As a result of intensive research into this phenomenon, the inventor found that the ripple in the line L1 is caused by polarization reversal occurring in a portion of the piezoelectric body 7 due to heat and pyroelectric effects. Here, "heat" includes heat history during the fabrication of the acoustic wave element 1 that is added after the IDT electrode 7 is formed, and heat generated by application of a high frequency signal.

When the temperature of the acoustic wave element 1 rises rapidly due to the manufacturing process or the sudden application of a high frequency signal, charges are generated due to the pyroelectric effect, which causes a voltage to be generated between the IDT electrodes 9. Since the LT substrate has been subjected to reduction treatment, this voltage is not very high (estimated to be around 5 to 20 V), but at high temperatures, the coercive electric field of the LT substrate decreases, so polarization reversal easily occurs. It is thought that it will be put away.

Furthermore, if the piezoelectric body 7 is thin, the polarization inversion portion easily reaches the back surface, forming a stable 180° domain. For this reason, once the polarization inversion occurs, it becomes fixed and appears as ripples in the filter characteristics described above.

A similar situation occurs even when the piezoelectric body 7 is thick, but in this case, the polarization inversion portion does not reach the back surface of the piezoelectric body 7, so a stable 180° domain is not formed. Therefore, when the voltage due to the pyroelectric effect disappears or the temperature drops, the polarization inversion portion returns to its original state. Based on this principle, when the piezoelectric body 7 is thick, problems such as ripples in the filter characteristics described above are less likely to occur. However, even in this case, when power is applied during actual use, ripples occur in the passband and other frequencies, contributing to the deterioration of the performance of communication equipment.

Here, if the elastic wave element 1 shown in FIG. 1 does not include the intermediate portion 5, ripples may be less likely to occur. This means that even if the piezoelectric body 7 becomes thinner, the piezoelectric body and the Si substrate are directly bonded, so the surface potential is stabilized, and charges can be released to the side of the Si substrate, which has a certain level of conductivity. It is thought that this is because of this. Furthermore, since the thermal conductivity of the support substrate 3 is higher than that of the intermediate portion 5, the heat generated in the piezoelectric body 7 can be efficiently radiated to the support substrate 3 in the absence of the intermediate portion 5. Conceivable.

On the other hand, in the configuration shown in FIG. 1, since the piezoelectric body 7 is in contact with the insulating material (intermediate portion 5), the surface potential becomes even more unstable, and the pyroelectric effect is more likely to occur. Under such circumstances, it is presumed that a pyroelectric effect occurs due to heat, a voltage is generated at the IDT electrode 9, and polarization reversal occurs.

In this way, assuming that the ripple in the line L1 is due to the polarization reversal of the piezoelectric body 7, there is no contradiction in the above-mentioned phenomenon.

Therefore, in order to suppress polarization reversal, in the present disclosure, the Curie temperature X and impurity concentration Y of the piezoelectric body 7 are set to have a relationship that satisfies equation (1). The Curie temperature is one of the indicators indicating the characteristics of a piezoelectric material, and changes depending on the composition ratio of Li, Ta, and O, for example. By satisfying this relational expression between the Curie temperature and the impurity concentration, in the piezoelectric body 7, the composition ratio can be shifted from the stoichiometric value, and the composition ratio can be adjusted appropriately within a range that does not affect the electrical properties or crystallinity. By containing impurities in the piezoelectric body 7, distortions and defects can be generated in the crystal lattice of the piezoelectric crystal constituting the piezoelectric body 7. Thereby, it is possible to make it difficult to invert the crystal domains and to reduce the movement of the inverted domains.

It has been confirmed that by using such a piezoelectric body 7, a waveform like line L2 can be obtained without generating ripples like line L1. Note that the piezoelectric body 7 may be one that satisfies formula (1) and then undergoes a reduction treatment. Further, other impurities such as Fe may be doped.

An acoustic wave device 1 as described above was actually produced. Specifically, the film thickness of the piezoelectric body 7 was set to 0.8 μm (0.7p), and the film thickness of the intermediate portion 5 was set to 0.32 μm (0.3p). A power durability test was performed on such an acoustic wave element 1 by applying a high frequency signal having a frequency near the resonance frequency, and another case was performed on an acoustic wave element 1 with no thermal history. It was confirmed that no ripples occurred in both cases. On the other hand, as a comparative example, a similar test was conducted using a piezoelectric crystal that was only subjected to reduction treatment and did not satisfy formula (1) as the piezoelectric body 7, and it was found that the ripple of line L1 could be reproduced. It was confirmed.

Further, regarding the SAW device according to the comparative example, the IDT electrode 9 was removed by wet etching to expose the piezoelectric body 7, and the polarization state was measured using a piezoelectric force microscope (PFM). The results are shown in FIG. FIG. 4(a) is an AFM image of the vicinity of the location where the dummy electrode 93 was located on the surface of the piezoelectric body 7. Although the IDT electrode 9 itself has been removed, irregularities on the surface of the piezoelectric body 7 created during the manufacturing process have been detected, and traces of the IDT electrode 9 can be confirmed. In FIG. 4A, there is a bus bar 91 at the left end, from which a dummy electrode 93 extends to the right in the plane of the paper, and faces across a gap from an electrode finger 92 extending from the left side of the plane of the paper. FIG. 4(b) is a PFM image of the same location. From FIG. 4(b), the voltage V is negative along the dummy electrode and the counter electrode, centering around the gap between the dummy electrode 93 and the counter electrode, and the direction of polarization is reversed from other parts. was confirmed. In addition, in the acoustic wave element 1, no part where the polarity was reversed was observed.

In this way, in the dummy electrode 93, a voltage is generated due to pyroelectricity during heating and cooling, and at the same time, the direction of the electric field is along the polarization direction (the direction of the dummy electrode and its electrode fingers), so the polarization is reversed. It is assumed that this occurs. Therefore, in the case where the IDT electrode 9 includes the dummy electrode 93 and the IDT electrode 9 to which a voltage due to the pyroelectric effect may be applied, the acoustic wave element 1 is preferably provided with the piezoelectric body 7. In particular, polarization reversal is likely to occur in the IDT electrode 9 that is connected to a wiring with a large area.

<Piezoelectric body 7>
As described above, in order to suppress polarization reversal, it is necessary to control the composition value of the piezoelectric body 7 to a very small extent. Below, the characteristics of the piezoelectric body itself will be explained in detail.

As the following piezoelectric bodies, LT substrates of Samples 1 to 10 were prepared in the form of wafers. Regarding the composition values of samples 1 to 10, the Curie temperature, Ir content, Zr content, Fe content, and whether or not formula (1) is satisfied are displayed separated by "," in order.
Sample No.: (Curie temperature, Ir amount, Zr amount, Fe amount, formula (1))
Sample 1: (605.66,320,21000,60,○)
Sample 2: (605.61,280,6300,90,○)
Sample 3: (606.49,300,10000,70,○)
Sample 4: (606.64, 220, 2800, 41000, ○)
Sample 5: (605.62, 1300, 7800, 8700, ×)
Sample 6: (608.4, 670, 890, 1400, ×)
Sample 7: (607.16,790,170,1200,×)
Sample 8: (606.79, 1100, 290, 80, ×)
Sample 9: (609.1, 710, 100, 200, ×)
Sample 10: (608.77, 510, 800, 41000, x)

Whether formula (1) is satisfied is indicated by "〇" if it is satisfied, and by "x" otherwise. Note that the Ir content, Zr content, Fe content, etc. of each sample were calculated by quantitative analysis using inductively coupled plasma (ICP emission spectrometry) after dissolving the piezoelectric material.

Test IDT electrodes were provided on the piezoelectric bodies of Samples 1 to 10, and a voltage was applied to the IDT electrodes. The basic configuration of the test IDT electrode is the same as the IDT electrode 9 shown in FIG. 2, and the pitch p is 1 μm.

While varying the DC voltage from 0 V to 80 V for such a test IDT electrode, the frequency characteristics at each applied voltage were measured. When polarization inversion occurs in the piezoelectric material due to the applied voltage, the difference between the resonant frequency and the anti-resonant frequency (hereinafter referred to as Δf) becomes small, and therefore the voltage at which Δf changes is defined as a coercive electric field. Furthermore, it is thought that the larger the amount of change in Δf, the wider the polarization inversion region, so the amount of change in Δf when a certain voltage is applied can be used as an index of the ease of polarization inversion.

FIG. 5 shows the relationship between applied voltage and Δf. In FIG. 5, the horizontal axis shows the applied voltage (unit: V), and the vertical axis shows the initial Δf and the amount of change in Δf (unit: MHz) when the voltage is applied. As is clear from the figure, in samples 1 to 4 that satisfy formula (1), a constant Δf can be maintained regardless of the applied voltage (no polarization reversal occurs). On the other hand, it was confirmed that in Samples 5 to 10, Δf decreased when the applied voltage exceeded 30 V, and the amount of decrease increased in proportion to the magnitude of the applied voltage. Furthermore, it was confirmed that the coercive electric fields of Samples 5 to 10 were approximately the same. From this, it is inferred that polarization reversal can be suppressed if the coercive electric field is 40 V or more. In other words, if Δf is −1 (MHz) or less when the applied voltage is 40 V, it is presumed that polarization reversal can be suppressed. Note that this value is for room temperature, and as the temperature increases, this voltage decreases. For example, at 200° C., the voltage at which polarization inversion occurs is 10 V or less.

In more detail, comparing Samples 3 and 4 with Samples 7 and 8, even at the same Curie temperature, there are cases where the polarization is inverted and cases where the polarization is not reversed. This shows that whether polarization reversal occurs is not affected only by the Curie temperature.

Furthermore, when Samples 1 to 4 are compared with Sample 5, it can be seen that the correlation between the Curie temperature and the Ir amount is stronger than the relationship between the Curie temperature and the Zr amount.

Therefore, FIG. 6 shows the relationship between the magnitude of the change in Δf when the applied voltage is 50 V when the Curie temperature and the Ir amount are changed. In FIG. 6, the horizontal axis represents the Curie temperature, and the vertical axis represents the Ir amount, and the size of the bubble chart represents the magnitude of the change in Δf, that is, the ease of polarization reversal.

As is clear from this figure, there is a clear division between samples that undergo polarization reversal (samples 5 to 10) and samples that do not (samples 1 to 4). The relational expression between the Curie temperature X and the Ir amount Y that divides these two regions corresponds to equation (1).

Hereinafter, a mechanism for suppressing polarization reversal when formula (1) is satisfied will be considered.

In the LT crystal, the polarization reversal voltage is significantly different between the stoichiometric Li/Ta ratio (stoichiometric) and the Li/Ta ratio when the LT crystal is pulled (congruent). is known to be different. Specifically, it is known that the polarization inversion voltage is larger in the case of congruent than in the case of stoichiometric. Compared to the stoichiometric case, in the case of congruent, the Li/Ta ratio is smaller and Li is deficient, so there are more defects inside the crystal, which is presumed to make polarization reversal less likely to occur.

Here, in a piezoelectric material having a composition ratio near congruent, polarization reversal may or may not occur even if the Li/Ta ratio differs by only about 0.05%. Therefore, the inventor adopted the Curie temperature as a parameter that can verify this extremely small difference in composition ratio. Specifically, as the Li/Ta ratio goes from stoichiometric to congruent (as the Li/Ta ratio becomes smaller), the Curie temperature becomes lower.

FIG. 7 shows the relationship between the Curie temperature and the amount of change in Δf when a DC voltage of 50 V is applied. As is clear from Figure 7, a certain relationship has been confirmed between the Curie temperature and the ease of polarization reversal; for example, polarization reversal can be suppressed when the Curie temperature is 608°C or lower, more preferably 607°C or lower. It can be seen that there is a tendency. On the other hand, since polarization reversal cannot be suppressed in some cases even when the Curie temperature is low, it was inferred that there is an element other than Li vacancies that causes crystal defects. Therefore, we focused on the Ir content as an impurity. As shown in FIG. 8, there is a correlation between the Ir content and the ease of polarization reversal, and it can be seen that the larger the Ir content, the easier the polarization reversal tends to be. Although the details of the cause of this are not clear, it is thought that Ir neutralizes lattice defects caused by Li vacancies. In addition, Ir may be mixed in from the crucible during crystal growth of the piezoelectric material 7, so in order to produce a piezoelectric material 7 that is difficult to polarize, it is necessary to carefully control the conditions during the growth and mix Ir into the piezoelectric material 7. You need to make sure that you don't do that.

Here, since Ir is presumed to be a factor in crystal defects other than Li defects, the lower limit of the Ir content may be greater than 0 ng/g, but may be 100 ng/g or more. Further, from FIG. 8(a), it may be set to 400 ng/g or less.

Furthermore, as shown in FIG. 8(b), a certain relationship is confirmed between the Zr content and the ease of polarization reversal. Although Zr and Ir are the same tetravalent elements, it is presumed that doping with Zr can induce crystal defects caused by a factor different from that of Ir. Therefore, while formula (1) is satisfied, the Zr content may be set to 2000 ng/g or more. Note that the upper limit of the Zr amount may be less than 200000 ng/g to ensure crystallinity. This is because too much dopant (impurity) makes crystal growth difficult or makes the crystal brittle.

Furthermore, as is clear from the comparison with Samples 4 and 10, no significant correlation was confirmed between Fe content and polarization inversion. Similarly, even when the cut angle was changed between samples, no significant correlation was observed between the cut angle and polarization reversal.

Note that Samples 1 to 10 were wafer-shaped piezoelectric bodies. Therefore, regardless of the thickness of the piezoelectric material, polarization reversal itself can be suppressed by satisfying formula (1). Therefore, if a piezoelectric material 7 satisfying formula (1) is prepared as a substrate, pyroelectricity can be suppressed. , it is possible to provide a piezoelectric substrate that achieves stable piezoelectric characteristics.

On the other hand, in the case of a wafer-shaped piezoelectric material, it has been confirmed that even if polarization reversal occurs once, the value of Δf recovers after a certain period of time has passed after the voltage application is stopped. This is because, even if the polarization of the crystal on the surface where the IDT electrode is formed is reversed, it is assumed that the original polarization state is returned to the original polarization state due to the effects of the surrounding crystals whose polarization is not reversed, including the vicinity of the center of the thickness. When the thickness of the piezoelectric body is thin, the proportion of crystals having a normal polarization direction located around the polarization-inverted region decreases, and the polarization cannot return to its original state. As a result, when the thickness of the piezoelectric material becomes thinner, it is presumed that once polarization reversal occurs, it will not recover. Specifically, when the thickness of the piezoelectric material is 1λ or less (2p or less), by using the piezoelectric material 7 of the present disclosure, it is possible to provide the acoustic wave element 1 with less influence of polarization inversion. Using the piezoelectric bodies of samples 1 to 10, the acoustic wave element 1 shown in FIG. 1 was fabricated, and the frequency characteristics shown in FIG. In these cases, no ripples were measured, and it was confirmed that ripples occurred when piezoelectric bodies (samples 5 to 10) that did not satisfy formula (1) were used.

(other examples)
In the above-mentioned example, although it was set as the structure provided with the support board 3, it is not limited to this structure. For example, as shown in FIG. 9(a), the intermediate portion 5 is thick and the support substrate 3 is not provided, or as shown in FIG. 9(b), the support substrate 3 is not present directly below the intermediate portion 5. There may be. In FIG. 9, the structure may be such that the piezoelectric body 7 is supported by the support substrate 3 in the form of a membrane without the intermediate portion 5.

As an example shown in FIG. 9A, for example, a case where a sapphire substrate is used as the intermediate portion 5 can be illustrated.

Furthermore, as shown in FIG. 10, a plurality of layers may be interposed between the support substrate 3 and the intermediate portion 5. In FIG. 10, an acoustic multilayer film 15 may be included between the support substrate 3 and the piezoelectric body 7. If the piezoelectric body 7 is an LT substrate and its Euler angle is around (0, 25, 0), the acoustic velocity of the generated elastic wave will be high. The acoustic multilayer film 15 is required to confine this high-speed acoustic wave to the piezoelectric body 7 side.

The acoustic multilayer film 15 is formed by alternately laminating a plurality of high acoustic impedance layers 15a and low acoustic impedance layers 15b. Examples of the high acoustic impedance layer 15a include tantalum oxide and hafnium oxide. An example of the low acoustic impedance layer 15b is silicon oxide. This low acoustic impedance layer 15b functions as the intermediate portion 5.

In the case of such a configuration, a high-speed acoustic wave is reflected toward the piezoelectric body 7 and is not leaked to the supporting substrate 3, so that an acoustic wave element with low loss can be obtained. Since reversal can be suppressed, high electrical characteristics can be achieved.

In the above example, the piezoelectric body 7 and the intermediate part 5 are directly joined to each other. You can leave it there. For example, the conductive layer may have a thickness of 5 nm or less. In this case, the potential can be stabilized and the influence of pyroelectricity can be reduced. Similarly, a metal element such as Fe, Ni, or Cu may be dispersed between the piezoelectric body 7 and the intermediate portion 5. By scattering conductive elements, the potential on the second surface 7b of the piezoelectric body 7 can be stabilized. The dispersion concentration may be less than the number of atoms of the piezoelectric body 7 exposed at the bonding interface, for example, 1/10 or less, more preferably 1/1000 or less. A specific concentration may be, for example, 10 ¹⁴ atoms/cm ² or less.

Further, in the above example, an elastic wave resonator having an IDT electrode formed thereon is used as an acoustic wave element, but the present invention is not limited to this. The acoustic wave element (piezoelectric element) may be a resonator provided with a pair of electrodes facing each other in the thickness direction or in the plane direction.

1: Piezoelectric element (acoustic wave element)
3: Support substrate 5: Intermediate section 7: Piezoelectric body 9: Pair of electrodes (IDT electrodes)

Claims (11)

LiTaO ₃ A piezoelectric material made of single crystal,
an IDT electrode on the piezoelectric body,
The piezoelectric body is
When the Curie temperature is X and the Ir content is Yng/g,
0<Y<10^(-0.10034×X+63.67) and Y≦400ng/g,
The thickness is less than twice the pitch of the IDT electrodes.
Piezoelectric element. The piezoelectric substance satisfies Y≧220ng/g,
The piezoelectric element according to claim 1. The piezoelectric body has a Curie temperature of 607° C. or less.
A piezoelectric element according to claim 1 or 2. When Δf, which is the difference between the resonant frequency and the anti-resonant frequency, is measured while changing the value of the DC voltage applied to the IDT electrode at room temperature, the change in Δf is 1 MHz or less until the DC voltage value is 40V. is,
A piezoelectric element according to any one of claims 1 to 3. The IDT electrode has a plurality of electrode fingers,
having a gap located in the extending direction of the electrode finger,
In planar perspective from above,
The piezoelectric body that overlaps the gap does not have a polarization inversion part that reaches the lower surface of the piezoelectric body.
A piezoelectric element according to any one of claims 1 to 4. The IDT electrode has a pair of comb-teeth electrodes,
In planar perspective from above,
The piezoelectric body is
There is no polarization inversion part that reaches the lower surface of the piezoelectric body in the area where the pair of comb-teeth electrodes overlap with the opposing areas.
The piezoelectric element according to claim 5. In planar perspective from above,
The piezoelectric body is
There is no polarization inversion part in the area where the pair of comb-teeth electrodes overlap with the opposing areas.
The piezoelectric element according to claim 6. having an intermediate portion on the lower surface side of the piezoelectric body;
The intermediate portion includes a material having insulating properties.
A piezoelectric element according to any one of claims 1 to 7. a support substrate is provided on the lower surface side of the intermediate portion;
The intermediate portion includes a material having a low thermal conductivity compared to the support substrate.
The piezoelectric element according to claim 8. In planar perspective from above,
having a hollow portion between the piezoelectric body overlapping with the IDT electrode and a support substrate existing below the piezoelectric body;
The piezoelectric element according to claim 9. between the piezoelectric body and the support substrate,
having a high acoustic impedance layer and a low acoustic impedance layer,
The piezoelectric element according to claim 9.