Acoustic wave device

An acoustic wave device includes: a first piezoelectric thin film resonator including a first lower electrode, a first upper electrode and a first piezoelectric film sandwiched between the first lower and upper electrodes; a decoupler film provided on the first upper electrode; and a second piezoelectric thin film resonator provided on the decoupler film and including a second lower electrode, a second upper electrode and a second piezoelectric film sandwiched between the second lower and upper electrodes, wherein the first piezoelectric film and the second piezoelectric film comprise aluminum nitride and include an element increasing a piezoelectric constant of the aluminum nitride.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-245452, filed on Nov. 1, 2010, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the present invention relates to acoustic wave devices.

BACKGROUND

A piezoelectric thin film resonator is used as a high frequency circuit of a cellular phone or the like. There are a film bulk acoustic resonator (FBAR) type and a solidly mounted resonator (SMR) type of piezoelectric thin film resonators. In late years, the balanced output is frequently requested at a reception terminal of an antenna duplexer. However, the filter using FBAR or SMR is not capable of converting unbalanced signals to balanced signals and the reverse. Thus, there has been developed a coupled resonator filter (CRF) structure capable of converting the unbalanced signals to the balanced signals and the reverse (for example, see non-Patent Document 1 (Improved Coupled Resonator Filter Performance using a Carbon-doped Oxide de-coupling Layer”, IEEE Ultrasonics Symp., 2009)). CRF has a plurality of piezoelectric thin film resonators and a decoupler film. The piezoelectric thin film resonators are stacked so as to interpose the decoupler film therebetween.

Non-Patent Document 2 (Advanced Materials 2009, 21, pp. 593-596) describes that the piezoelectric constant is increased by adding Sc (scandium) to aluminum nitride. Non-Patent Document 3 (Mater. Res. Soc Symp. Proc., Vol. 1129, 2009, pp. 21-25) describes that the piezoelectric constant is increased by adding Er (erbium) to aluminum nitride.

In CRF, two resonance characteristics of an anti-symmetry mode of the low frequency side and a symmetric mode of the high frequency side appear. When too large spacing between two resonance frequencies results in a loss in the vicinity of the center of the band, the loss in the vicinity of the center of the band can be suppressed by reducing the acoustic impedance of the decoupler film. However, the reduction in the acoustic impedance of the decoupler film will increase the loss resulting from the decoupler film itself.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided an acoustic wave device including: a first piezoelectric thin film resonator including a first lower electrode, a first upper electrode and a first piezoelectric film sandwiched between the first lower and upper electrodes, a decoupler film provided on the first upper electrode; and a second piezoelectric thin film resonator provided on the decoupler film and including a second lower electrode, a second upper electrode and a second piezoelectric film sandwiched between the second lower and upper electrodes, wherein the first piezoelectric film and the second piezoelectric film comprise aluminum nitride and include an element increasing a piezoelectric constant of the aluminum nitride.

DETAILED DESCRIPTION

First, a piezoelectric thin film resonator will be described. There are an FBAR type and an SMR type of resonators.FIG. 1Ais a plan view of an FBAR.FIG. 1Bis a sectional view taken along a A-A line ofFIG. 1A. The FBAR generally includes a substrate50, a lower electrode52, a piezoelectric film54, and an upper electrode56. The lower electrode52and the upper electrode56are provided on the substrate50to sandwich the piezoelectric film54. A cavity58penetrating through the substrate50is formed under a portion (a resonance area60) where the upper electrode56and the lower electrode52face each other across the piezoelectric film54.

FIG. 2is a sectional view of another exemplary FBAR. An upper surface of the substrate50is flat, and a the lower electrode52is formed so that a cavity58is formed between the substrate50and the lower electrode52. For example, the cavity58has a dome shape. The cavity58between the substrate50and the lower electrode52may be a recess formed in the substrate50.

FIG. 3is a sectional view of an SMR. Acoustic reflection films62are formed under the lower electrode52of the resonance area60. The acoustic reflection films62are films in which a film having a high acoustic impedance and a thickness of λ/4 and a film having a low acoustic impedance and a thickness of λ/4 are stacked alternately. Here, λ is the wavelength of the acoustic wave. In the piezoelectric thin film resonators illustrated inFIGS. 1A to 3A, Mo (molybdenum), W (tungsten), Ru (ruthenium), or the like are often used for the lower electrode52and the upper electrode56. AIN (aluminum nitride) is often used to form the piezoelectric film54. Si (silicon) is often sued to form the substrate50.

Next, a CRF will be described.FIG. 4is a sectional view of the CRF according to a comparative example 1. A first piezoelectric thin film resonator10and a second piezoelectric thin film resonator20are stacked on the substrate50. The first piezoelectric thin film resonator10includes a first lower electrode12, a first piezoelectric film14, and a first upper electrode16. The second piezoelectric thin film resonator20includes a second lower electrode22, a second piezoelectric film24, and a second upper electrode26. A single-layered decoupler film30is formed between the first piezoelectric thin film resonator10and the second piezoelectric thin film resonator20. The cavity58is provided under the first lower electrode12in the resonance area.

FIG. 5is a sectional view of a CRF according to a comparative example 2. As compared with the comparative example 1, the decoupler film30has multiple stacked films such as films32and34. The acoustic reflection films62are formed under the first lower electrode12. The other components are the same as those of the comparative example 1 illustrated inFIG. 4, and a description thereof is omitted here.

Since the decoupler film30is formed by the single film in the comparative example 1, a small number of manufacturing processes is used and the thickness of the film is controlled easily, as compared with the comparative example 2 that employs the decoupler film30composed of the multiple films. However, there is a problem that will be described below.

The decoupler film30formed by the single film is simulated.FIG. 6is a schematically sectional view of the CRF according to the comparative example 1 in simulation. A CRF100includes the first piezoelectric thin film resonator10, the second piezoelectric thin film resonator20, and the decoupler film30. The first piezoelectric thin film resonator10includes the first lower electrode12, the first upper electrode16and the first piezoelectric film14sandwiched between the electrodes12and16. The decoupler film30is provided on the first upper electrode16. The second piezoelectric thin film resonator20is provided on the decoupler film30, and includes the second lower electrode22, the second upper electrode26, and the second piezoelectric film24sandwiched between the electrodes22and26. The first piezoelectric thin film resonator10and the second piezoelectric thin film resonator20are stacked to sandwich the decoupler film30. The first lower electrode12and the second upper electrode26are connected to input/output terminals40and42, respectively. The first upper electrode16and the second lower electrode22are grounded.

The simulation is performed under the following conditions. Each of the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26is a Ru film having a thickness of 100 nm. Each of the first piezoelectric film14and the second piezoelectric film24is an AlN film having a thickness of 800 nm, and the decoupler film30is 450 nm thick. In the simulation, the acoustic impedance of the decoupler film30is changed.

FIG. 7Ais a graph of an impedance vs. frequency characteristic of the comparative example 1.FIG. 7Bis a graph of a passband (S21) characteristic of the comparative example 1. The impedance is a characteristic when the input/output terminal40is grounded. The passband characteristic corresponds to S21between the input/output terminals40and42. InFIGS. 7A and 7B, solid lines, broken lines, and dotted lines indicate simulation results under conditions that the acoustic impedance of the decoupler film30is 12.7 Mrayl, 7.6 Mrayl, and 2.5 Mrayl, respectively. It is to be noted that 12.7 Mrayl corresponds to the acoustic impedance of the silicon oxide film.

As illustrated inFIG. 7A, the CRF has a resonance characteristic of the anti-symmetric mode at the low frequency side (this frequency is referred to as far) and another resonance characteristic of the symmetry mode at the high frequency side (this frequency is referred to as fsr). A spacing between the resonance frequencies of the two modes fsr and far is referred to as a resonance frequency spacing Δf.FIG. 8illustrates a resonance frequency spacing ratio associated with the acoustic impedance of the decoupler film30. The resonance frequency spacing ratio is defined as (fsr−far)/(fsr+far)×2×100. As illustrated inFIG. 8, the resonance frequency spacing ratio simply increases as the acoustic impedance of the decoupler film30increases. As illustrated inFIG. 7B, a large spacing between the resonance frequencies leads to an increase in the loss at the band center of the communication band due to the impedance mismatch. On the other hand, a small spacing between the resonance frequencies reduces the loss at the band center of the communication band. Thus, according toFIGS. 7A to 8, a reduced acoustic impedance of the decoupler film30reduces the spacing Δf between the resonance frequencies, thereby reducing the loss at the band center of the communication band.

However, the simulation illustrated inFIGS. 7A to 8does not consider the loss of the decoupler film30itself.FIG. 9illustrates the amount of attenuation associated with the acoustic impedance of the decoupler film30.FIG. 9is described in the aforementioned Non-Patent Document 1. As illustrated inFIG. 9, the propagation loss of the decoupler film30increases as the acoustic impedance of the decoupler film30reduces. Thus, a simple change of the acoustic impedance of the decoupler film30as described above results in a trade-off relationship between the loss at the band center of the passband and the propagation loss.

For example, referring toFIG. 9, the decoupler film30having an acoustic impedance of 5 Mrayl or higher is capable of reducing the propagation loss. For example, a silicon oxide (for example, SiO2) film having an acoustic impedance of 13 Mrayl is capable of suppressing the propagation loss of the decoupler film30. However, this case increases the spacing Δf between the resonance frequencies, and thus increases the loss at the center of the passband.

As described above, the use of the single film for the decoupler film30results in the trade-off relationship between the loss at the band center of the passband and the propagation loss. According to aspects of embodiments described below, the above problems are taken into consideration.

An embodiment 1 improves the piezoelectric constant of a piezoelectric film. First, a description will be given of the significance of improving the piezoelectric constant of the piezoelectric film. A structure of each film used in simulation is the same as one of the comparative example 1. The decoupler film30is a silicon oxide film and has an acoustic impedance of 12.7 Mrayl.

FIG. 10Ais a graph of an impedance vs. frequency characteristic of the comparative example 1 in which the decoupler film30is a silicon oxide film. The spacing between the two modes is set to the spacing Δf between resonance frequencies.FIG. 10Bis a graph of the resonance frequency spacing ratio ((fsr−far)/(fsr+far)×2×100) associated with variations of the dielectric constant (white circle), the elastic constant (black circle), and the piezoelectric constant (white triangle) of the piezoelectric films14and24. The variations are values normalized by using, as reference values, the standard parameter values of aluminum nitride having an orientation with the main axis in the (002) direction. The standard values of the dielectric constant (∈33), the elastic constant (c33), and the piezoelectric constant (e33) are 8.91×1011F/m, 4.29×10−11Pa, and 1.55 C/m2, respectively. It is to be noted that the piezoelectric thin film resonator uses acoustic waves that are propagated upwardly and downwardly. Thus, the dielectric constant (∈33), the elastic constant (c33), and the piezoelectric constant (e33) mainly influence the property of aluminum nitride having the orientation with the main axis in the (002) direction. Therefore, these parameters are used in the simulation.

As illustrated inFIG. 10B, the spacing between resonance frequencies does not change greatly for variations of the dielectric constant and the piezoelectric constant. In contrast, the spacing between the resonance frequencies increases as the elastic constant decreases, and this is not preferable.

The electromechanical coupling constant k33is represented with the following expression, and a decrease in the elastic constant means an increase in the electromechanical coupling constant.
k332=e332/(∈33·c33)

FIG. 11Aillustrates a spacing ΔfL between the resonance frequency and the anti-resonance frequency in the symmetry mode at the low frequency side.FIG. 11Billustrates a resonance/anti-resonance frequency spacing ratio ((fL2−fL1)/(fL2+fL1)×2×100) associated with variations of the dielectric constant (white circle), the elastic constant (black circle), and the piezoelectric constant (white triangle) of the piezoelectric films14and24. The variations are values normalized by using, as reference values, the standard parameter values of aluminum nitride having an orientation with the main axis in the (002) direction. The spacing ΔfL between the resonance frequency and the anti-resonance frequency increases as the dielectric constant and the elastic constant reduce as illustrated inFIG. 11B. On the contrary, the spacing ΔfL between the resonance frequency and the anti-resonance frequency in the symmetry mode at the low frequency side increases as the piezoelectric constant increases. When the variations of the spacing ΔfL for the dielectric constant and the elastic constant are the same are those for the piezoelectric constant, the use of the piezoelectric constant increases the spacing ΔfL most efficiently. Decreases in the dielectric constant and the elastic constant, and an increase in the piezoelectric constant mean an increase in the electromechanical coupling constant k33.

FIG. 12Aillustrates a spacing ΔfH between the resonance frequency and the anti-resonance frequency in the symmetry mode at the high frequency side.FIG. 12Billustrates a resonance/anti-resonance frequency spacing ratio ((fH2−fH1)/(fH2+fH1)×2×100) associated with variations of the dielectric constant (white circle), the elastic constant (black circle), and the piezoelectric constant (white triangle). The variations are values normalized by using, as reference values, the standard parameter values of aluminum nitride having an orientation with the main axis in the (002) direction. The spacing ΔfH between the resonance frequency and the anti-resonance frequency increases as the dielectric constant and the elastic constant reduce as illustrated inFIG. 12B. In contrast, the spacing ΔfH between the resonance frequency and the anti-resonance frequency in the symmetry mode at the high frequency side increases as the piezoelectric constant increases. When the variations of the spacing ΔfH for the dielectric constant and the elastic constant are the same are those for the piezoelectric constant, the use of the piezoelectric constant increases ΔfH most efficiently. Decreases in the dielectric constant and the elastic constant, and an increase in the piezoelectric constant mean an increase in the electromechanical coupling constant k33.

Large spacings ΔfL and ΔfH between the resonance frequency and the anti-resonance frequency are large improve the loss at the center of the passband in the CRF. Therefore, improvement in the piezoelectric constant of the piezoelectric film reduces the loss of the center of the passband in the CRF most efficiently.

The simulation is performed under the following conditions. Each of the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26is a W film having a thickness of 85 nm. Each of the first piezoelectric film14and the second piezoelectric film24is an aluminum nitride film having a thickness of 680 nm. The decoupler film30is a silicon oxide film having a thickness of 385 nm. The piezoelectric constants of the first piezoelectric film14and the second piezoelectric film24are changed from the standard reference value of the piezoelectric constant (e33) of aluminum nitride.

FIG. 13Aillustrates an impedance vs. frequency characteristic.FIG. 13Billustrates a passband characteristic. InFIGS. 13A and 13B, solid lines, broken lines, and dotted lines indicate simulation results under conditions that a piezoelectric constant (e33) is 1.55 C/m2(the standard value of aluminum nitride), 1.55×1.5 C/m2(1.5 times the standard value of aluminum nitride), and 1.55×2 C/m2(twice the standard value of the aluminum nitride), respectively. When the piezoelectric constants of the first piezoelectric film14and the second piezoelectric film24increase as illustrated inFIG. 13A, the spacings Δf between the resonance frequencies in the two modes are substantially the same as each other, whereas the spacings ΔfL and ΔfH between the resonance frequency and the anti-resonance frequency in the two modes increase. When the piezoelectric constant of the piezoelectric film increases, the loss at the center of the passband reduces as illustrated inFIG. 13B.

FIG. 14illustrates an insertion loss at the center of the passband associated with a variation of the piezoelectric constant. The piezoelectric constant (e33) is normalized by 1.55 C/m2, which is the standard value of aluminum nitride. An increase in the piezoelectric constant decreases the insertion loss at the center of the passband, as illustrated inFIG. 14. When the variation of the piezoelectric constant is two, the insertion loss is the smallest. As the variation of the piezoelectric constant increases from two, the insertion loss increases.

For example, in order to realize an insertion loss of −6 dB or less at the band center of the passband, it is preferable that the piezoelectric constant (e33) should be 1.1 to 3.9 times the standard value of aluminum nitride. In order to realize an insertion loss of −4 dB or less, it is preferable that the piezoelectric constant (e33) should be 1.3 to 3.3 times the standard value of aluminum nitride.

As described above, the increase in the piezoelectric constants of the first piezoelectric film14and the second piezoelectric film24decreases the loss at the band center. For example, in order to increase the piezoelectric constant of the first piezoelectric film14and the second piezoelectric film24, Sc (scandium) or Er (erbium) is added to aluminum nitride as Non-Patent Documents 2 and 3.

In the embodiment 1, each of the first piezoelectric film14and the second piezoelectric film24is made of aluminum nitride including an element increasing the piezoelectric constant. In other words, as compared with the case without adding an element to the first piezoelectric film14or the second piezoelectric film24, the element is added so as to increase the piezoelectric constant. For example, scandium or erbium is added. This can suppress the loss at the band center of the passband. Besides scandium or erbium, it is possible to use another element that improves the piezoelectric constant of aluminum nitride.

The material constant such as the piezoelectric constant of aluminum nitride depends on the crystalline orientation. The crystalline orientation is greatly influenced by a roughness of an underlying film. For example, in cases where aluminum nitride is formed by sputtering, a bad roughness of the underlying film may degrade the orientation of aluminum nitride and decrease the piezoelectric constant. Thus, the first piezoelectric film14and the second piezoelectric film24may have different resonance characteristics. Thus, the amounts of the element added to the first piezoelectric film14and the second piezoelectric film24may be made different from each other. This allows the first piezoelectric film14and the second piezoelectric film24to have almost the same piezoelectric constants and almost the same resonance characteristics.

Also, the roughness of the upper film is subject to deterioration more easily than that of the lower film. Therefore, the first orientation of the second piezoelectric film24tends to be deteriorated more easily than that of the first piezoelectric film14. For this reason, the resonance characteristic of the second piezoelectric thin film resonator20tends to be deteriorated more easily than that of the piezoelectric thin film resonator10. Thus, the amount of the element added to the second piezoelectric film24is made different from that of the first piezoelectric film14. For example, an increased amount of the element is added to the second piezoelectric film24. It is thus possible to control the second piezoelectric thin film resonator20and the first piezoelectric film resonator10to have almost the same resonance characteristics.

An embodiment 2 has an exemplary structure in which the electrodes have different film thicknesses.FIG. 15illustrates a structure of a CRF according to the embodiment 2 in simulation. As illustrated inFIG. 15, T1indicates the film thickness of each of the first lower electrode12and the second upper electrode26, and T2indicates the film thickness of each of the first upper electrode16and the second lower electrode22. The other arrangements are the same as those illustrated inFIG. 1, and a description thereof is omitted here. The simulation is performed under the following conditions. Each of the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26is a Ru film. Each of the first piezoelectric film14and the second piezoelectric film24is an AIN film having a thickness of 800 nm and a piezoelectric constant (e33) of 1.55 C/m2. The decoupler film30is a silicon oxide film that is 450 nm thick. In the simulation, the film thicknesses T1of the first lower electrode12and the second upper electrode26and the film thicknesses T2of the first upper electrode16and the second lower electrode22are changed. The sum of T1and T2is fixedly set to 200 nm.

FIG. 16Ais a graph of an impedance vs. frequency characteristic.FIG. 16Bis a graph of a passband characteristic.FIG. 17is a graph of the spacing Δf between the resonance frequencies associated with T1/T2. InFIGS. 16A and 16B, solid lines indicate simulation results under conditions that T1is 100 nm and T2is 100 nm (T1/T2=1.0). Broken lines indicate simulation results under conditions that T1is 60 nm and T2is 140 nm (T1/T2=0.43). Dotted lines indicate simulation results under conditions that T1is 140 nm and T2is 60 nm (T1/T2=2.33). When T1/T2decreases, the spacing between the resonance frequencies in the two modes decreases as illustrated inFIGS. 16A and 17. Therefore, the loss at the band center decreases as illustrated inFIG. 16B.

In the embodiment 2, the film thicknesses of the first upper electrode16and the second lower electrode22are made larger than those of the first lower electrode12and the second upper electrode26. It is thus possible to reduce the loss at the band center of the passband. For example, when the first upper electrode16and the second lower electrode22are different in film thickness from each other and the first lower electrode12and the second upper electrode26are different in the film thickness from each other, it is preferable that the film thickness of a thinner one of the first upper electrode16and the second lower electrode22should be greater than that of a thicker one of the first lower electrode12and the second upper electrode26.

For example, when the decoupler film30such as the silicon oxide film having a high acoustic impedance is employed, the spacing between resonance frequencies is increased. Thus, T1/T2is decreased, and the loss at the center of the passband is thus suppressed. In contrast, the spacing between the resonance frequencies is decreased in the case of using the decoupler film30having a low acoustic impedance, which may be, for example, 2.5 Mrayl inFIG. 7B. Thus, T1/T2is increased, and broadband is achieved. As described above, the use of the decoupler film30having an acoustic impedance as small as not greater than 5 Mrayl makes it possible to set the thicknesses of the first upper electrode16and the second lower electrode22smaller than those of the first lower electrode12and the second upper electrode26.

As described above, the ratio of T1and T2is adjusted to control the spacing between the resonance frequencies, and the band characteristics of the CRF consequently. For the adjustment of T1and T2, the first upper electrode16and the second lower electrode22may have film thicknesses different from those of the first lower electrode12and the second upper electrode26.

An embodiment 3 has an exemplary structure in which the electrodes have different acoustic impedances. The simulation is performed by using a CRF having the same structure as one illustrated inFIG. 6under the following conditions. The first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26are made of Ru and are 100 nm thick. Each of the first piezoelectric film14and the second piezoelectric film24is an AIN film having a thickness of 800 nm and a piezoelectric constant (e33) equal to 1.5 times 1.55 C/m2. The decoupler film30is a silicon oxide film having a thickness of 450 nm. In the simulation, the acoustic impedances of the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26are changed. Since changing the acoustic impedances varies the frequencies, a thickness correction is made by multiplying the film thickness of each layer by an identical correction coefficient so that the frequencies are substantially the same as each other.

FIG. 18Ais a graph of an impedance vs. frequency characteristic.FIG. 18Bis a graph of a passband characteristic.FIG. 19is a view of a resonance frequency spacing ratio associated with the acoustic impedance of the electrode film. InFIGS. 18A and 18B, alternate long and short dash lines indicate simulation results under conditions that the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26have an acoustic impedance of 109 Mrayl and are made of Ir (iridium). Solid lines indicate simulation results under conditions that the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26have an acoustic impedance of 82 Mrayl and are is made of W (tungsten). Broken lines indicate simulation results under conditions that the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26have an acoustic impedance of 58 Mrayl and are made of Mo (molybdenum). Dotted lines indicate simulation results under conditions that the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26have an acoustic impedance of 23 Mrayl and are made of Ti (titanium). When the acoustic impedance of the electrodes increases, the spacing between the resonance frequencies decreases as illustrated inFIGS. 18A and 19. When the acoustic impedance of the electrodes increases, the loss at the band center of the passband reduces as illustrated inFIG. 18B.

Particularly, as illustrated inFIG. 19, when the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26have an acoustic impedance of not less than 50 Mrayl, the spacing between resonance frequencies is saturated. The acoustic impedance of the electrodes is preferably not less than 60 Mrayl, and is more preferably not less than 70 Mrayl. Table 1 illustrates the acoustic impedances of metallic elements.

It will be seen from Table 1 that it is preferable to include at least one of Ir (iridium), W (tungsten), Ru (ruthenium), Rh (rhodium), Mo (molybdenum), Pt (platinum), and Ta (tantalum) in the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26in order to set the acoustic impedances of these electrodes equal to or higher than 50 Mrayl. It is preferable that each of the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26is an Ir film, a W film, a Ru film, a Rh film, a Mo film, a Pt film, or a Ta film. More preferably, Ir is employed in the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26. Preferably, the first lower electrode12, the first upper electrode16, the second lower electrode22, and the second upper electrode26have an acoustic impedance of not greater than 120 Mrayl, since Ir has the highest acoustic impedance.

An embodiment 4 has an exemplary structure is an example where the CRFs according to the embodiments 1 to 3 are each formed on the substrate.FIGS. 20A to 21Bare sectional view of an acoustic wave device according to the embodiment 4. As illustrated inFIG. 20A, the first piezoelectric thin film resonator10and the second piezoelectric thin film resonator20are stacked on the substrate50so that the decoupler film30is interposed between the resonators10and20. The substrate50may be made of silicon, glass, or sapphire. In the resonance area, the cavity58is formed between the first lower electrode12and the substrate50. For example, the cavity58has a dome shape. Other components are the same as those illustrated inFIG. 6, and a description thereof is omitted here.

As illustrated inFIG. 20B, the substrate50has the cavity58penetrating therethrough in the resonance area. Other components are the same as those illustrated inFIG. 20A, and a explanation thereof is omitted here. As illustrated inFIG. 21A, the cavity58may be formed by a recess that is formed in an area according to the resonance area of the substrate50. As illustrated inFIG. 21B, instead of the cavity58, the acoustic reflection film62may be provided under the first lower electrode12. The structure of the acoustic reflection film62is the same as the structure illustrated inFIG. 3, and a description thereof is omitted here.

In the embodiments 1 to 4, the decoupler film30may be a multilayer film as illustrated inFIG. 5The decoupler film30of the multilayer film reduces the spacing between the resonance frequencies, and reduces the loss at the band center of the passband. In contrast, the decoupler film30of the single layered film may be formed easily. However, the spacing between resonance frequencies increases, and the loss at the band center of the passband increases. Therefore, when the decoupler film30is the single-layered film, it is preferable that the structure of one of the embodiments 1 to 4 is used to reduce the loss at the band center of the passband.

Also, as illustrated inFIGS. 7A to 8, the acoustic impedance of the decoupler film30that is not less than 5 Mrayl increases the spacing between the resonance frequencies and deteriorates the loss at the band center of the passband. In this case, it is preferable to employ one of the structures according to the embodiments 1 to 4. When the acoustic impedance of the decoupler film30is more than or equal to 7 Mrayl, it is preferable to employ one of the structures according to the embodiments 1 to 4. When the acoustic impedance of the decoupler film30is not less than 10 Mrayl, it is more preferable to employ one of the structures according to the embodiments 1 to 4.

When the decoupler film30includes silicon oxide, the spacing between resonance frequencies increases and the loss at the band center of the passband increases, as illustrated inFIGS. 7A and 8. Therefore, in this case, it is preferable to employ one of the structures of the embodiments 1 to 4. The decoupler film30may be the silicon oxide film or a film with silicon oxide added. For example, the silicon oxide film may include F (fluorine) for an improvement in temperature characteristics.

The embodiments of the present invention have been described. The present invention is not limited to these specific embodiments but may be varied or changed within the scope of the claimed invention.