Acoustic wave device with IDT electrode including al metal layer and high acoustic impedance metal layer

An acoustic wave device includes an energy confinement layer, a piezoelectric layer made of Y-cut X-propagation lithium tantalate having a cut angle in a range from about −10° to about 65°, and an IDT electrode. Electrode fingers of the IDT electrode include an Al metal layer and a high acoustic impedance metal layer having a Young's modulus equal to or more than about 200 GPa and an acoustic impedance higher than Al. The high acoustic impedance metal layer is closer to the piezoelectric layer than the Al metal layer. A wavelength specific film thickness tLT of the piezoelectric layer is expressed by tLT≤1λ. The total of normalized film thicknesses obtained by normalizing the film thickness of each layer of the electrode finger by a density and Young's modulus of the Al metal layer satisfies T≤0.1125tLT+0.0574.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave device.

2. Description of the Related Art

Existing acoustic wave devices have been widely used for a filter of a cellular phone or the like. International Publication No. 2012/086639, which will be described below, discloses an example of the acoustic wave device. In this acoustic wave device, a support substrate, a high-acoustic-velocity film, a low-acoustic-velocity film, and a piezoelectric film are laminated in this order, and an IDT (Inter Digital Transducer) electrode is provided on the piezoelectric film. A Q value is increased by providing the above-described laminated structure.

When a generally known Al electrode is applied as the IDT electrode of the acoustic wave device described above, an effect of increasing the Q value can be obtained. However, since the Young's modulus of Al is relatively small, there is a limit to the Q value that can be obtained, and such a Q value is sometimes insufficient for the desired Q value.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide acoustic wave devices each having an improved Q characteristic.

An acoustic wave device according to a preferred embodiment of the present invention includes an energy confinement layer, a piezoelectric layer on the energy confinement layer and made of Y-cut X-propagation lithium tantalate having a cut angle of equal to or more than about −10° and equal to or less than about 65°, and an IDT electrode on the piezoelectric layer, wherein the IDT electrode includes a plurality of electrode fingers, the plurality of electrode fingers including a multilayer body including an Al metal layer defined by an Al layer or an alloy layer including Al, and a high acoustic impedance metal layer that has a Young's modulus equal to or more than about 200 GPa and a higher acoustic impedance than Al, the high acoustic impedance metal layer is located closer to the piezoelectric layer than the Al metal layer, when a wavelength defined by an electrode finger pitch of the IDT electrode is denoted by λ and a wavelength specific film thickness of the piezoelectric layer is denoted by tLT, tLT≤1λ is satisfied, and a total of normalized film thicknesses obtained by normalizing a film thickness of each layer of the electrode finger by a density and a Young's modulus of the Al metal layer is denoted by T, the following Expression 1 is satisfied.
T≤0.1125tLT+0.0574  Expression 1

An acoustic wave device according to a preferred embodiment of the present invention includes an energy confinement layer, a piezoelectric layer on the energy confinement layer and made of Y-cut X-propagation lithium tantalate having a cut angle of equal to or more than about −10° and equal to or less than about 65°, and an IDT electrode on the piezoelectric layer, wherein the IDT electrode includes a plurality of electrode fingers, the plurality of electrode fingers including a multilayer body including an Al metal layer defined by an Al layer or an alloy layer including Al, and a high acoustic impedance metal layer that has a Young's modulus equal to or more than about 200 GPa and a higher acoustic impedance than Al, the high acoustic impedance metal layer is located closer to the piezoelectric layer than the Al metal layer, the high acoustic impedance metal layer is an Mo layer, a W layer, or an Ru layer, the following Expression 2 is satisfied, and a combination of each coefficient of Expression 2 and a metal of the high acoustic impedance metal layer is a combination shown in Table 1 below.
a0(2)(tLT−c0)2+a0(1)(tLT−c0)+b0≤aLT(2)(tLT−cLT)2+aLT(1)(tLT−cLT)+aM(2)(T−cM)2+aM(1)(T−cM)+dLT-M(tLT−cLT)(T−cM)+bExpression 2

With acoustic wave devices according to preferred embodiments of the present invention, Q characteristics are able to be improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be clarified by describing preferred embodiments of the present invention with reference to the drawings.

The preferred embodiments described in the present specification are exemplary, and partial replacement or combination of components between different preferred embodiments is possible.

FIG.1is a front sectional view of an acoustic wave device according to a first preferred embodiment of the present invention.

An acoustic wave device1includes a piezoelectric substrate2. The piezoelectric substrate2of the present preferred embodiment includes an energy confinement layer3and a piezoelectric layer7provided on the energy confinement layer3. The energy confinement layer3of the present preferred embodiment is a multilayer body including a high-acoustic-velocity support substrate5as a high-acoustic-velocity material layer and a low-acoustic-velocity film6provided on the high-acoustic-velocity support substrate5. The piezoelectric layer7is a piezoelectric layer using Y-cut X-propagation lithium tantalate having a cut angle of equal to or more than about −10° and equal to or less than about 65°, for example.

An IDT electrode8is provided on the piezoelectric layer7of the piezoelectric substrate2. An acoustic wave is excited by applying an AC voltage to the IDT electrode8. A pair of reflectors9A and9B are provided on both sides of the IDT electrode8in an acoustic wave propagation direction on the piezoelectric substrate2. The acoustic wave device1of the present preferred embodiment is an acoustic wave resonator, for example. However, the acoustic wave device1according to the present invention is not limited to an acoustic wave resonator, and may be, for example, a filter device or the like having a plurality of acoustic wave resonators.

The low-acoustic-velocity film6is a film having a relatively low acoustic velocity. More specifically, an acoustic velocity of a bulk wave propagating through the low-acoustic-velocity film6is lower than an acoustic velocity of a bulk wave propagating through the piezoelectric layer7. In the present preferred embodiment, the low-acoustic-velocity film6is, for example, a silicon oxide film. Silicon oxide is represented by SiOx. x is an arbitrary positive number. In the acoustic wave device1, for example, silicon oxide of the low-acoustic-velocity film6is SiO2. Note that the material of the low-acoustic-velocity film6is not limited to the above, and for example, a material including glass, silicon oxynitride, tantalum oxide, or a compound obtained by adding fluorine, carbon, boron, hydrogen, or a silanol group to silicon oxide as a main component can be used.

The high-acoustic-velocity material layer is a layer made of a material having a relatively high acoustic velocity. More specifically, an acoustic velocity of a bulk wave propagating through the high-acoustic-velocity material layer is higher than an acoustic velocity of an acoustic wave propagating through the piezoelectric layer7. As the material of the high-acoustic-velocity support substrate5that is the high-acoustic-velocity material layer, for example, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, silicon, sapphire, lithium tantalate, lithium niobate, crystal, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, a DLC (diamond-like carbon) film, diamond, or the like, and media containing these materials as a main component can be used.

Since the acoustic wave device1of the present preferred embodiment has a configuration in which the piezoelectric layer7is laminated on the energy confinement layer3that is a multilayer body including the high-acoustic-velocity support substrate5and the low-acoustic-velocity film6, the energy of the acoustic wave can be effectively confined to the piezoelectric layer7side. The low-acoustic-velocity film6may not be provided. In the present preferred embodiment, the piezoelectric layer7is indirectly provided on the high-acoustic-velocity material layer via the low-acoustic-velocity film6, but the piezoelectric layer7may be directly provided on the high-acoustic-velocity material layer.

FIG.2is a plan view of the acoustic wave device according to the first preferred embodiment of the present invention.

The IDT electrode8includes a first bus bar16and a second busbar17facing each other. The IDT electrode8includes a plurality of first electrode fingers18each including one end connected to the first busbar16. Further, the IDT electrode8includes a plurality of second electrode fingers19each including one end connected to the second busbar17. The plurality of first electrode fingers18and the plurality of second electrode fingers19are interdigitated with each other.

As illustrated inFIG.1, the IDT electrode8is provided of a multilayer body including an Al metal layer15and a high acoustic impedance metal layer14. The Al metal layer15is an Al layer or an alloy layer including Al, for example. In the present preferred embodiment, the Al metal layer15is an Al layer. The high acoustic impedance metal layer14in the IDT electrode8is a metal layer having a Young's modulus equal to or more than about 200 GPa and an acoustic impedance higher than that of Al. The high acoustic impedance metal layer14in the acoustic wave device1is, for example, an Mo layer, a W layer, or an Ru layer. However, the high acoustic impedance metal layer14is not limited to the above. The high acoustic impedance metal layer14is located closer to the piezoelectric layer7than the Al metal layer15.

When a wavelength defined by an electrode finger pitch of the IDT electrode8is λ and a wavelength specific film thickness of the piezoelectric layer7is tLT, tLT≤1λ is satisfied. The electrode finger pitch refers to a distance between the centers of the electrode fingers of the IDT electrode8.

The total of normalized film thicknesses obtained by normalizing the film thickness of each layer of the electrode finger of the IDT electrode8by the density and the Young's modulus of the Al metal layer15is denoted by T. T is the total of the above-described normalized film thicknesses of an arbitrary electrode finger among the plurality of electrode fingers included in the IDT electrode8. In the present preferred embodiment, T of any of the first electrode fingers18and T of any of the second electrode fingers19are the same or substantially the same. Here, it is assumed that the wavelength specific film thickness of an arbitrary metal type i is ti, the density is ρi, and the Young's modulus is Yi. i is a positive number of 0 or more, and i=0 means a metal type of the Al metal layer15. The wavelength specific film thickness of the Al metal layer15is to, the density is ρ0, and the Young's modulus is Y0. The inventors of preferred embodiments of the present invention discovered that, by being defined as described above, the total film thickness T of each layer of the plurality of first electrode fingers18and the plurality of second electrode fingers19of the IDT electrode8normalized by the density ρ0and the Young's modulus Y0of the Al metal layer15can be expressed as follows.

Hereinafter, the film thickness normalized by the density ρ0and the Young's modulus Y0of the Al metal layer15may be simply referred to as a normalized film thickness. Here, in the acoustic wave device1, the relationship between the total T of the normalized film thicknesses of each layer of the electrode finger of the IDT electrode8and the wavelength specific film thickness tLTof the piezoelectric layer7satisfies the following Expression 1.
T≤0.1125tLT+0.0574  Expression 1

Further, the acoustic wave device1satisfies the following Expression 2. In the case where the high acoustic impedance metal layer14is an Mo layer, in the case of a W layer, and in the case of an Ru layer, each coefficient of Expression 2 takes a value as shown in Table 2 below.
a0(2)(tLT−c0)2+a0(1)(tLT−c0)+b0≤aLT(2)(tLT−cLT)2+aLT(1)(tLT−cLT)+aM(2)(T−cM)2+aM(1)(T−cM)+dLT-M(tLT−cLT)(T−cM)+bExpression 2

However, as long as Expression 1 is satisfied, Expression 2 does not necessarily need to be satisfied.

The present preferred embodiment has the following configuration. 1) The first electrode finger18and the second electrode finger19of the IDT electrode8include a multilayer body including the Al metal layer15and the high acoustic impedance metal layer14, and the high acoustic impedance metal layer14is located closer to the piezoelectric layer7than the Al metal layer15. 2) The Young's modulus of the high acoustic impedance metal layer14is equal to or more than about 200 GPa. 3) tLT≤1λ and the above-described Expression 1 is satisfied. 4) The above-described Expression 2 is satisfied and a combination of each coefficient of Expression 2 and the metal of the high acoustic impedance metal layer14is a combination shown in Table 2. As a result, the Q characteristic can be improved and the energy loss can be effectively reduced. This will be described below.

Note that in the present specification, the Q characteristic is a characteristic represented by f×Q that is a product of a frequency f and a Q value. Hereinafter, the film thickness may be expressed as t/λ×100(%), where t is the film thickness.

With reference toFIGS.3A to3C, it will be described that 1) the high acoustic impedance metal layer14is located closer to the piezoelectric layer7than the Al metal layer15in the multilayer body forming the first electrode finger18and the second electrode finger19of the IDT electrode8, and 2) the Young's modulus of the high acoustic impedance metal layer14is equal to or more than about 200 GPa, thus being able to increase the Q value. With reference toFIG.4toFIG.10, it will be described that 3) tLT≤1λ and the Expression 1 is satisfied, thus being able to improve the Q characteristic and reduce the energy loss. Further, with reference toFIG.6and the like, 4) it is shown that the Expression 2 is satisfied and the combination of each coefficient of Expression 2 and the metal of the high acoustic impedance metal layer14is the combination shown in the above-described Table 2, thus being able to increase the Q value more reliably.

Here, a metal layer corresponding to the high acoustic impedance metal layer in the first preferred embodiment is referred to as an M layer. In order to obtain the relationship between the Young's modulus of the M layer and the Q value, a simulation by the finite element method was performed by changing the density and the Young's modulus of the M layer. Note that the above-described simulation was performed under the condition that the acoustic wave device did not include the low-acoustic-velocity film and the piezoelectric layer was directly provided on the high-acoustic-velocity support substrate. More specifically, the conditions are as follows.Layer configuration of IDT electrode: M layer/Al layer from the piezoelectric layer sideDensity of M layer: about 2.7 g/cm3, about 5.4 g/cm3, about 8.1 g/cm3, about 10.8 g/cm3, about 13.5 g/cm3, about 16.2 g/cm3, about 18.9 g/cm3, or about 21.6 g/cm3Film thickness of M layer: about 2%, about 4%, or about 6%Film thickness of Al layer: about 7%Material of piezoelectric layer: about 42° Y-cut lithium tantalate (42Y-LiTaO3)Film thickness of piezoelectric layer: about 0.3λLayer configuration of energy confinement layer: high-acoustic-velocity support substrateMaterial of high-acoustic-velocity support substrate: Z-cut X-propagation crystal

FIGS.3A to3Care diagrams showing the relationship between the Young's modulus of the M layer, which is a metal layer corresponding to the high acoustic impedance metal layer, and the Q value.FIG.3Ashows the dependence of the Q value on the Young's modulus of the M layer in each case where the film thickness of the M layer is about 2% and the density of the M layer is varied.FIG.3Bshows the dependence of the Q value on the Young's modulus of the M layer in each case where the film thickness of the M layer is about 4% and the density of the M layer is varied.FIG.3Cshows the dependence of the Q value on the Young's modulus of the M layer in each case where the film thickness of the M layer is about 6% and the density of the M layer is varied.

As shown inFIG.3A, it can be seen that the Q value increases stably when the Young's modulus of the M layer is equal to or more than about 200 GPa regardless of the density of the M layer. Similarly, as shown inFIGS.3B and3C, it can be seen that even in a case where the film thickness of the M layer is different, when the Young's modulus of the M layer is equal to or more than about 200 GPa, the Q value stably increases. Therefore, in the first preferred embodiment illustrated inFIG.1, since the Young's modulus of the high acoustic impedance metal layer14is equal to or more than about 200 GPa, the Q value can be stably increased.

FIG.4shows the Q characteristics of an acoustic wave device that differs from the first preferred embodiment in that the IDT electrode includes only an Al layer. The conditions of the acoustic wave device whose Q characteristics were measured are as follows. Note that a region where adjacent electrode fingers of the IDT electrode overlap each other when viewed in the acoustic wave propagation direction is referred to as an intersecting region. A dimension of the intersecting region along a direction orthogonal or substantially orthogonal to the acoustic wave propagation direction is defined as an intersecting width.Layer configuration of IDT electrode: Al layerWavelength λ of IDT electrode: about 2 μmNumber of pairs of electrode fingers of IDT electrode: 100 pairsIntersecting width of IDT electrode: about 15λNumber of electrode fingers of reflector: 10Material of piezoelectric layer: about 55° Y-cut lithium tantalate (55Y-LiTaO3)Film thickness of piezoelectric layer: about 0.15λ or more and about 0.3λ or lessLayer configuration of energy confinement layer: high-acoustic-velocity support substrate/low-acoustic-velocity filmMaterial of low-acoustic-velocity film: silicon oxide

FIG.4is a diagram showing the Q characteristics for each film thickness of the piezoelectric layer in the case where the IDT electrode includes only an Al layer. InFIG.4, the broken line indicates the results in the case where the film thickness of the piezoelectric layer is about 300 nm, the alternate long and short dash line indicates the results in the case where the film thickness of the piezoelectric layer is about 400 nm, the alternate long and two short dashes line indicates the results in the case where the film thickness of the piezoelectric layer is about 500 nm, and the solid line indicates the results in the case where the film thickness of the piezoelectric layer is about 600 nm.

As shown inFIG.4, it is understood that even when the film thickness of the piezoelectric layer is any of the film thicknesses, there is a film thickness of the electrode finger of the IDT electrode in which the Q characteristic has the maximum value. When the film thickness of the electrode finger is too thin with respect to the film thickness of the piezoelectric layer, a reflection coefficient becomes small, and thus the Q characteristic becomes low. On the other hand, when the film thickness of the electrode finger is too large, a large amount of energy is distributed in the metal layer of the IDT electrode having a relatively large loss of acoustic energy, and the Q characteristic is deteriorated. Therefore, there is a film thickness of the electrode finger in which the Q characteristic has the maximum value. In addition, when the film thickness of the piezoelectric layer is increased, a relatively large amount of acoustic energy is distributed in a piezoelectric single crystal having a small acoustic loss. Therefore, in the range in which energy confinement is established, the maximum value of the Q characteristic that can be achieved increases as the wavelength specific film thickness of the piezoelectric layer increases. Here, the film thickness of the electrode finger of the IDT electrode when the Q characteristic has the maximum value in the case where the IDT electrode includes only the Al layer is defined as T0. As the film thickness of the electrode finger increases to T0, the Q characteristic increases and an electric resistance value of the electrode finger decreases.

As shown inFIG.4, To varies depending on the film thickness of the piezoelectric layer. The inventors of preferred embodiments of the present invention derived the following approximate expression to obtain T0using the wavelength specific film thickness tLTof the piezoelectric layer. Note that in the following approximate expression, T0represents a wavelength specific film thickness.
T0≈0.1125tLT+0.0574

FIG.5andFIG.6show the Q characteristics of an acoustic wave device in which the IDT electrode includes a multilayer body including an Al layer as an Al metal layer and a Mo layer as a high acoustic impedance metal layer. The conditions of the acoustic wave device are as follows.Layer configuration of IDT electrode: Mo layer/Al layer from the piezoelectric layer sideWavelength λ of IDT electrode: about 2 μm Number of pairs of electrode fingers of IDT electrode: 100 pairsIntersecting width of IDT electrode: about 15λNumber of electrode fingers of reflector: 10Material of piezoelectric layer: about 55° Y-cut lithium tantalate (55Y-LiTaO3)Film thickness of piezoelectric layer: about 0.3λ or about 0.4λLayer configuration of energy confinement layer: high-acoustic-velocity support substrate/low-acoustic-velocity filmMaterial of low-acoustic-velocity film: silicon oxide

FIG.5andFIG.6, which will be described below, show changes in the Q characteristics when the total film thickness T of each layer of the electrode finger of the IDT electrode is changed by changing the film thickness of the Al layer in the case where the film thickness of the Mo layer is about 0%, about 1%, about 2%, or about 3%.

FIG.5is a diagram showing Q characteristics in the case where the IDT electrode includes only an Al layer or an Mo layer and an Al layer, and the piezoelectric layer has a film thickness of about 0.4λ.FIG.6is a diagram showing Q characteristics in the case where the IDT electrode includes only an Al layer or an Mo layer and an Al layer, and the piezoelectric layer has a film thickness of about 0.3λ. InFIG.5andFIG.6, the alternate long and short dash line indicates the result in the case where the film thickness of the Mo layer is about 1%, the alternate long and two short dashes line indicates the result in the case where the film thickness of the Mo layer is about 2%, and the solid line indicates the result in the case where the film thickness of the Mo layer is about 3%. The broken line indicates the result in the case where the IDT electrode includes only an Al layer. InFIG.5, the above-described film thickness T0of the electrode finger of the IDT electrode when the Q characteristic has the maximum value in the case where the IDT electrode includes only the Al layer is indicated by the straight broken line as a normalized film thickness. The same applies toFIG.6and the subsequent figures showing the Q characteristic.

As shown inFIG.5, it can be seen that in the case where the Mo layer as the high acoustic impedance metal layer is laminated closer to the piezoelectric layer than the Al layer, at least when the relationship of the normalized film thickness is T≤T0, the Q characteristic is superior to that in the case where the IDT electrode includes only the Al layer. The same applies to the case shown inFIG.6. Thus, in the case of T≤T0, the Q characteristic can be improved. Therefore, the energy loss can be reduced.

Here, in the relationship of T≤T0, Expression 1 can be derived by applying the above-described approximate expression of T0.
T≤0.1125tLT+0.0574  Expression 1

By satisfying Expression 1, the Q characteristic can be improved, and the energy loss can be reduced. Further, in the following description, it is shown that the Q characteristic can be improved by satisfying Expression 1 even in a case where the high acoustic impedance metal layer is other than the Mo layer.

The Q characteristics of the acoustic wave device under the same or substantially the same conditions as the conditions under which the Q characteristics shown inFIG.5andFIG.6were obtained except that the Mo layer of the IDT electrode was replaced with another high acoustic impedance metal layer, are shown inFIG.7toFIG.10below. In the case where the high acoustic impedance metal layer was a W layer, the film thickness of the W layer was about 0.5%, about 1%, or about 2%. In the case where the high acoustic impedance metal layer was an Ru layer, the film thickness of the Ru layer was about 1%, about 2%, or about 3%.

FIG.7is a diagram showing Q characteristics in the case where the IDT electrode includes only an Al layer or a W layer and an Al layer, and the piezoelectric layer has a film thickness of about 0.4λ.FIG.8is a diagram showing Q characteristics in the case where the IDT electrode includes only an Al layer or a W layer and an Al layer, and the piezoelectric layer has a film thickness of about 0.3λ. InFIG.7andFIG.8, the alternate long and short dash line indicates the result in the case the film thickness of the W layer is about 0.5%, the alternate long and two short dashes line indicates the result in the case where the film thickness of the W layer is about 1%, and the solid line indicates the result in the case where the film thickness of the W layer is about 2%. The broken line indicates the result in the case where the IDT electrode includes only the Al layer.

As shown inFIG.7, it can be seen that in the case where the W layer as the high acoustic impedance metal layer is laminated closer to the piezoelectric layer than the Al layer, at least when the relationship of the normalized film thickness is T≤T0, the Q characteristic is superior to that in the case where the IDT electrode includes only the Al layer. The same applies to the case shown inFIG.8. As described above, even in the case where the high acoustic impedance metal layer is the W layer, the Q characteristic can be improved and the energy loss can be reduced by satisfying the Expression 1.

FIG.9is a diagram showing Q characteristics in the case where the IDT electrode includes only an Al layer or an Ru layer and an Al layer, and the piezoelectric layer has a film thickness of about 0.4λ.FIG.10is a diagram showing Q characteristics in the case where the IDT electrode includes only an Al layer or an Ru layer and an Al layer, and the piezoelectric layer has a film thickness of about 0.3λ. InFIG.9andFIG.10, the alternate long and short dash line indicates the result in the case where the film thickness of the Ru layer is about 1%, the alternate long and two short dashes line indicates the result in the case where the film thickness of the Ru layer is about 2%, and the solid line indicates the result in the case where the film thickness of the Ru layer is about 3%. The broken line indicates the result in the case where the IDT electrode includes only the Al layer.

As shown inFIG.9, it can be seen that in the case where the Ru layer as the high acoustic impedance metal layer is laminated closer to the piezoelectric layer than the Al layer, at least when the relationship of the normalized film thickness is T≤T0, the Q characteristic is superior to that in the case where the IDT electrode includes only the Al layer. The same applies to the case shown inFIG.10. As described above, even in the case where the high acoustic impedance metal layer is the Ru layer, the Q characteristic can be improved and the energy loss can be reduced by satisfying the Expression 1.

Furthermore, the Q characteristic may be improved not only in the case where the total film thickness T of each layer of the electrode finger of the IDT electrode is equal to or less than the normalized film thickness T0, but also in the case where the total film thickness T of each layer of the electrode finger of the IDT electrode is larger than the normalized film thickness T0. The inventors of preferred embodiments of the present invention have discovered conditions under which the Q characteristic can be improved both in the case of T≤T0and in the case of T>T0. T0be more specific, the following Expression 2 is satisfied and the respective coefficients in Expression 2 and the metal of the high acoustic impedance metal layer are combinations as shown in Table 2, wherein it is possible to improve the Q characteristic both in the case of T≤T0and in the case of T>T0. That is, when T has any value, the Q characteristic can be improved. Therefore, the energy loss can be further reliably reduced.
a0(2)(tLT−c0)2+a0(1)(tLT−c0)+b0≤aLT(2)(tLT−cLT)2+aLT(1)(tLT−cLT)+aM(2)(T−cM)2+aM(1)(T−cM)+dLT-M(tLT−cLT)(T−cM)+bExpression 2

In the case where Expression 2 is satisfied and the respective coefficients in Expression 2 and the metal of the high acoustic impedance metal layer are the combinations shown in Table 2, Expression 1 may not be satisfied. However, it is preferable to satisfy Expression 1.

The Al metal layer15in the IDT electrode8illustrated inFIG.1is an Al layer, and the film thickness of the Al layer is preferably equal to or more than about 85 nm. As a result, the electric resistance value can be stabilized even when the film thickness of the Al layer varies during the formation of the Al layer. This will be described with reference toFIG.11below. Note thatFIG.11shows normalized conductivity obtained by normalized conductivity by the film thickness of the Al layer.

FIG.11is a diagram showing the relationship between the film thickness of the Al layer and the normalized conductivity. InFIG.11, each plot indicates the normalized conductivity at each film thickness of the Al layer. The broken line inFIG.11indicates the relationship of Expression 3 described later.

As shown inFIG.11, it can be seen that the normalized conductivity increases as the film thickness of the Al layer increases and the value approaches about 85 nm. It can be seen that the normalized conductivity is constant when the film thickness of the Al layer is equal to or more than about 85 nm. Therefore, by setting the film thickness of the Al layer to equal to or more than about 85 nm, the electric resistance value can be stabilized.

Note that the relationship between the film thickness of the Al layer and the normalized conductivity shown inFIG.11can be expressed by the following Expression 3. Here, it is assumed that i is the metal type, hiis the film thickness of the metal type i, κ is the conductivity, and κsatis a saturation conductivity. However, in the first preferred embodiment, the metal type i is Al.
κ(hi)=κsat{0.36 ln(hi)−0.6}  Expression 3

Expression 3 is an expression in the case where the film thickness hiis smaller than a critical film thickness hi-satat which the conductivity κ becomes the saturation conductivity κsat. When the film thickness hiexceeds the critical film thickness hi-sat, the relationship of κ(hi)=κsatis maintained for the conductivity κ. The critical film thickness hi-satis a film thickness hiwhen {0.36 ln (hi)−0.6} in Expression 3 is 1 and κ(hi)=κsatis established. More particularly, the critical film thickness hi-satis about 85.15 nm. As is clear from this, by setting the film thickness of the Al layer to be equal to or more than about 85 nm, the conductivity can be stabilized, and the electric resistance value can be stabilized.

As shown inFIG.12, when the film thickness of the LiTaO3film as the piezoelectric layer7is in the range of equal to or more than about 0.05λ and equal to or less than about 0.5λ, the fractional bandwidth significantly varies. Accordingly, the electromechanical coupling coefficient can be adjusted in a wider range. Therefore, in order to widen the adjustment range of the electromechanical coupling coefficient and the fractional bandwidth, the film thickness of the piezoelectric layer7is preferably in the range of equal to or more than about 0.05λ and equal to or less than about 0.5λ, for example.

As shown inFIG.13, when the film thickness of the LiTaO3film as the piezoelectric layer7is in the range of equal to or more than about 0.05λ and equal to or less than about 0.35 λ, the electromechanical coupling coefficient can be further increased. As a result, a state of spurious can be avoided. Therefore, the film thickness of the piezoelectric layer7is more preferably in the range of equal to or more than about 0.05λ and equal to or less than about 0.35λ, for example.

As illustrated inFIG.1, in the first preferred embodiment, the energy confinement layer3is a multilayer body including the high-acoustic-velocity support substrate5and the low-acoustic-velocity film6, but the configuration of the energy confinement layer3is not limited thereto. Hereinafter, first to third modifications of the first preferred embodiment in which the configuration of the energy confinement layer is different from that of the first preferred embodiment will be described. Also in the first to third modifications, similarly to the first preferred embodiment, the Q characteristic can be improved, and the energy loss can be reduced.

In the first modification illustrated inFIG.14, a piezoelectric substrate22A includes the high-acoustic-velocity support substrate5and the piezoelectric layer7provided directly on the high-acoustic-velocity support substrate5. The energy confinement layer of this modification is the high-acoustic-velocity support substrate5.

In the second modification illustrated inFIG.15, a piezoelectric substrate22B includes a support substrate24, a high-acoustic-velocity film25provided on the support substrate24, the low-acoustic-velocity film6provided on the high-acoustic-velocity film25, and the piezoelectric layer7provided on the low-acoustic-velocity film6. The high-acoustic-velocity material layer of this modification is the high-acoustic-velocity film25. An energy confinement layer23is a multilayer body of the high-acoustic-velocity film25and the low-acoustic-velocity film6.

As the material of the high-acoustic-velocity film25, for example, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, silicon, sapphire, lithium tantalate, lithium niobate, crystal, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, a diamond-like carbon (DLC) film, diamond, or the like, and media containing these materials as a main component can be used.

Examples of the material of the support substrate24include, for example, piezoelectric materials such as aluminum oxide, lithium tantalate, lithium niobate, and quartz; various ceramics such as alumina, magnesia, silicon nitride, aluminum nitride, silicon carbide, zirconia, cordierite, mullite, steatite, and forsterite; dielectrics such as sapphire, diamond, and glass; semiconductors such as silicon and gallium nitride; and resins.

In the third modification illustrated inFIG.16, a piezoelectric substrate22C includes the support substrate24, an acoustic reflection film26provided on the support substrate24, and the piezoelectric layer7provided on the acoustic reflection film26. The energy confinement layer of this modification is the acoustic reflection film26.

The acoustic reflection film26is a multilayer body including a plurality of acoustic impedance layers. T0be more specific, the acoustic reflection film26includes a low acoustic impedance layer27A and a low acoustic impedance layer27B that have a relatively low acoustic impedance, and a high acoustic impedance layer28A and a high acoustic impedance layer28B that have a relatively high acoustic impedance. In this modification, low acoustic impedance layers and high acoustic impedance layers are alternately laminated in the acoustic reflection film26. Note that the low acoustic impedance layer27A is located closest to the piezoelectric layer7in the acoustic reflection film26.

The acoustic reflection film26includes two low acoustic impedance layers and two high acoustic impedance layers. However, the acoustic reflection film26may include at least one low acoustic impedance layer and at least one high acoustic impedance layer.

As a material of the low acoustic impedance layer, for example, silicon oxide, aluminum, or the like can be used.

As a material of the high acoustic impedance layer, for example, a metal such as platinum or tungsten, or a dielectric such as aluminum nitride or silicon nitride can be used.