Patent Grant
11456719
This application claims the benefit of priority to Japanese Patent Application No. 2019-106106 filed on Jun. 6, 2019. The entire contents of this application are hereby incorporated herein by reference.
The present invention relates to an acoustic wave device.
An acoustic wave device has been widely used, for example, as a filter for a mobile phone. International Publication No. 2012/086639 discloses an example of an acoustic wave device. This acoustic wave device has a multilayer body in which a high-acoustic-velocity support substrate, a low-acoustic-velocity film, and a piezoelectric film are laminated in this order and an interdigital transducer (IDT) electrode provided on the piezoelectric film. Due to the multilayer body, a Q factor increases.
However, use of the acoustic wave device described in International Publication No. 2012/086639 as a filter may undesirably deteriorate filter characteristics due to an occurrence of spurious modes outside a pass band of the filter.
Preferred embodiments of the present invention provide acoustic wave devices that are each able to significantly reduce or prevent an occurrence of a spurious mode outside a pass band.
An acoustic wave device according to a preferred embodiment of the present invention includes a high-acoustic-velocity support substrate; a low-acoustic-velocity film provided on the high-acoustic-velocity support substrate; a piezoelectric layer provided on the low-acoustic-velocity film; and an IDT electrode provided on the piezoelectric layer, wherein an acoustic velocity of a bulk wave propagating through the high-acoustic-velocity support substrate is higher than an acoustic velocity of an acoustic wave propagating through the piezoelectric layer, an acoustic velocity of a bulk wave propagating through the low-acoustic-velocity film is lower than an acoustic velocity of a bulk wave propagating through the piezoelectric layer, the low-acoustic-velocity film has a first portion and a second portion that is located closer to the high-acoustic-velocity support substrate than the first portion, the first portion and the second portion include the same or similar materials, and ρ1 and ρ2 are different where ρ1 is a density in the first portion of the low-acoustic-velocity film and ρ2 is a density in the second portion of the low-acoustic-velocity film.
According to the acoustic wave devices according to preferred embodiments of the present invention, an occurrence of a spurious mode outside a pass band is able to be significantly reduced or prevented.
The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.
Preferred embodiments of the present invention are described below with reference to the drawings.
The preferred embodiments below are illustrative, and an element in one preferred embodiment may be replaced or combined with an element in another preferred embodiment.
An acoustic wave device 1 includes a piezoelectric substrate 2. An IDT electrode 6 is provided on the piezoelectric substrate 2. An acoustic wave is excited by application of an alternating-current voltage to the IDT electrode 6. On the piezoelectric substrate 2, the IDT electrode 6 is sandwiched between a pair of reflectors 8 and 9 in a direction in which the acoustic wave propagates. That is, the acoustic wave device 1 according to the present preferred embodiment is an acoustic wave resonator. Note, however, that the acoustic wave device may be, for example, a filter device including an acoustic wave resonator.
The piezoelectric substrate 2 includes a high-acoustic-velocity support substrate 3, a low-acoustic-velocity film 4 provided on the high-acoustic-velocity support substrate 3, and a piezoelectric layer 5 provided on the low-acoustic-velocity film 4. The IDT electrode 6 is provided on the piezoelectric layer 5.
In the first preferred embodiment, the piezoelectric layer 5 is preferably a lithium tantalate layer, for example. A material of the piezoelectric layer 5 is not limited to lithium tantalate and may be, for example, a piezoelectric body such as, lithium niobate, zinc oxide, aluminum nitride, crystal, or PZT.
The low-acoustic-velocity film 4 is a film having a relatively low acoustic velocity. More specifically, an acoustic velocity of a bulk wave propagating through the low-acoustic-velocity film 4 is lower than an acoustic velocity of a bulk wave propagating through the piezoelectric layer 5. In the first preferred embodiment, the low-acoustic-velocity film 4 is preferably a silicon oxide film, for example. Silicon oxide is expressed by SiOx, where x is any positive number. Note, however, that a material of the low-acoustic-velocity film 4 is not limited to silicon oxide.
The low-acoustic-velocity film 4 includes a first layer 4A and a second layer 4B located closer to the high-acoustic-velocity support substrate 3 than the first layer 4A. More specifically, the second layer 4B is laminated on the high-acoustic-velocity support substrate 3, and the first layer 4A is laminated on the second layer 4B. The low-acoustic-velocity film 4 includes a first portion A located in the first layer 4A and a second portion B located in the second layer 4B. The first portion A is located at a center or approximate center in a thickness direction of the first layer 4A. The second portion B is located at a center or approximate center in a thickness direction of the second layer 4B. Note, however, that the first portion A may be any portion of the first layer 4A and the second portion B may be any portion of the second layer 4B.
The first layer 4A and the second layer 4B of the low-acoustic-velocity film 4 are made of the same or similar materials. Accordingly, the first portion A and the second portion B are also made of the same or similar materials. The “same or similar materials” as used herein refer to materials that include the same or similar element. For example, in a case where the low-acoustic-velocity film 4 is a silicon oxide film as in the first preferred embodiment, the first layer 4A and the second layer 4B are made of the same or similar materials even if x of SiOx of the first layer 4A and x of SiOx of the second layer 4B are different.
In the first preferred embodiment, ρ1 is preferably higher than ρ2, where ρ1 is a density in the first portion A of the low-acoustic-velocity film 4 and ρ2 is a density in the second portion B of the low-acoustic-velocity film 4. However, it is only necessary that the density ρ1 and the density ρ2 are different. Therefore, ρ1 may be lower than ρ2. The density ρ1 and the density ρ2 are able to be made different from one another by forming the low-acoustic-velocity film 4, for example, at different film formation speeds or pressures.
The high-acoustic-velocity support substrate 3 is a substrate having a relatively high acoustic velocity. More specifically, an acoustic velocity of a bulk wave propagating through the high-acoustic-velocity support substrate 3 is higher than an acoustic velocity of an acoustic wave propagating through the piezoelectric layer 5. The high-acoustic-velocity support substrate 3 may be a medium mainly including a material, for example, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, silicon, sapphire, lithium tantalate, lithium niobate, crystal, alumina, zirconia, cordierite, mullite, steatite, forsterite, magnesia, diamond-like carbon (DLC), or diamond.
Since the piezoelectric substrate 2 has a multilayer structure in which the high-acoustic-velocity support substrate 3, the low-acoustic-velocity film 4, and the piezoelectric layer 5 are laminated in this order, energy of an acoustic wave is able to be effectively confined on the piezoelectric layer 5 side. This multilayer structure of the piezoelectric substrate 2 is able to significantly increase a Q factor.
On the piezoelectric substrate 2, a dielectric film 7 covers the IDT electrode 6. In the first preferred embodiment, the dielectric film 7 is preferably a silicon oxide film, for example. A material of the dielectric film 7 is not limited to silicon oxide. Although the dielectric film 7 may be omitted, the dielectric film 7 is preferably provided to significantly reduce or prevent breaking of the IDT electrode 6.
As shown in
The IDT electrode 6, the reflector 8, and the reflector 9 may be a multilayer metal film including a plurality of metal layers laminated on each other or may be a single-layer metal film.
The density ρ1 in the first portion A and the density ρ2 in the second portion B of the low-acoustic-velocity film 4 are different. More specifically, the density ρ1 is preferably higher than the density ρ2. This difference in density significantly reduces or prevents an occurrence of a spurious mode outside a pass band. This is described below through comparison between the first preferred embodiment and a comparative example. The comparative example is different from the first preferred embodiment in that ρ1 is equal or substantially equal to ρ2.
Simulation was performed on a phase characteristic of a higher-order mode by setting the density ρ1 and the density ρ2 to be different in an acoustic wave device according to the first preferred embodiment. More specifically, simulation was performed on a phase characteristic of a higher-order mode that occurs in a band that is approximately 1.5 times higher than a pass band of the acoustic wave device. Simulation was also performed on the phase characteristic of the higher-order mode in an acoustic wave device according to the comparative example. Design parameters of the acoustic wave devices are as follows. In the following description, λ represents a wave length defined by an electrode finger pitch of an IDT electrode. Hereinafter, for example, 0.01 λ is sometimes referred to as 1% λ.
In the acoustic wave device according to the first preferred embodiment, the density ρ1 of the low-acoustic-velocity film was changed within a range of not less than about 2.12 g/cm3 and not more than about 2.30 g/cm3, and the density ρ2 of the low-acoustic-velocity film was changed within a range of not less than about 2.03 g/cm3 and not more than about 2.30 g/cm3. In the comparative example, the density ρ1 and the density ρ2 were changed within a range of not less than about 2.12 g/cm3 and not more than about 2.30 g/cm3.
Tables 1 to 3 show results of the first preferred embodiment and the comparative example. More specifically, Table 1 shows a result in a case where the density ρ1 is about 2.21 g/cm3. Table 2 shows a result in a case where the density ρ1 is about 2.12 g/cm3. Table 3 shows a result in a case where the density ρ1 is about 2.30 g/cm3. An absolute value of a difference between the density ρ1 and the density ρ2 is |Δρ|, an average of the density ρ1 and the density ρ2 is ρave, and a density difference ratio is |Δρ|/ρave×100(%).
As shown in Tables 1 to 3, a value of a phase of the higher-order mode in the first preferred embodiment is smaller than a value of a phase of the higher-order mode in the comparative example. That is, in the first preferred embodiment, an occurrence of a spurious mode outside a pass band is able to be significantly reduced or prevented.
As shown in Table 1, when the value of the density ρ1 is fixed, the higher-order mode is significantly reduced more as the density difference ratio becomes higher. The same is true for the results shown in Tables 2 and 3. The density difference ratio is preferably about 1% or more, for example, and more preferably about 4% or more, for example. This density difference ratio is able to further significantly reduce or prevent the higher-order mode. Note that an upper limit of the density difference ratio is not limited in particular but is preferably, for example, about 8% or less.
As shown in
A second preferred embodiment of the present invention is described in below with reference to
Also in the second preferred embodiment, the density ρ1 and the density ρ2 are different, and therefore an occurrence of a spurious mode outside a pass band is able to be significantly reduced or prevented. This is described below through comparison between the second preferred embodiment and a comparative example. The comparative example is different from the second preferred embodiment in that ρ1 is equal or substantially equal to ρ2.
Simulation was performed on a phase characteristic of a Rayleigh mode by setting the density ρ1 and the density ρ2 to be different in an acoustic wave device according to the second preferred embodiment. The Rayleigh mode occurs in a frequency band that is approximately 0.7 times higher than a pass band of the acoustic wave device. Simulation on a Rayleigh mode phase characteristic was also performed by an acoustic wave device according to the comparative example. Design parameters of the acoustic wave devices are as follows.
In the acoustic wave device according to the second preferred embodiment, the density ρ1 of the low-acoustic-velocity film was changed within a range of not less than about 2.12 g/cm3 and not more than about 2.30 g/cm3, and the density ρ2 was changed within a range of not less than about 2.12 g/cm3 and not more than about 2.48 g/cm3. In the comparative example, the density ρ1 and the density ρ2 of the low-acoustic-velocity film were changed within a range of not less than about 2.12 g/cm3 and not more than about 2.30 g/cm3.
Tables 4 to 6 show results of the second preferred embodiment and the comparative example. More specifically, Table 4 shows a result in a case where the density ρ1 is about 2.21 g/cm3. Table 5 shows a result in a case where the density ρ1 is about 2.12 g/cm3. Table 6 shows a result in a case where the density ρ1 is about 2.30 g/cm3.
As shown in Tables 4 to 6, a value of a phase of the Rayleigh mode in the second preferred embodiment is smaller than a value of a phase of the Rayleigh mode in the comparative example. That is, in the second preferred embodiment, an occurrence of a spurious mode outside a pass band is able to be significantly reduced or prevented.
As shown in Tables 4 to 6, in a case where the value of the density ρ1 is fixed, the Rayleigh mode is further significantly reduced as the density difference ratio becomes higher. The density difference ratio is preferably about 1% or more, for example, and more preferably about 4% or more, for example. This density difference ratio is able to further significantly reduce or prevent the Rayleigh mode. An upper limit of the density difference ratio is not limited in particular but is preferably, for example, about 8% or less.
Although the low-acoustic-velocity film 24 is a single-layer film in the modification of the first preferred embodiment shown in
While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2019-106106 | Jun 2019 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20130285768 | Watanabe et al. | Oct 2013 | A1 |
| 20140152146 | Kimura | Jun 2014 | A1 |
| Number | Date | Country |
|---|---|---|
| 2012086639 | Jun 2012 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20200389147 A1 | Dec 2020 | US |
