Bulk acoustic wave devices with sandwich electrodes for higher resonant frequencies and related methods of manufacture

Abstract

A bulk acoustic wave BAW device includes a piezoelectric layer (330) disposed between a first electrode layer (104A) and an interlayer electrode (106A). The interlayer electrode includes a first layer (114) of a first material having a first acoustic impedance and a second layer (116) of a second material having a second acoustic impedance that is less than the first acoustic impedance of the first layer. A second layer (116) of the interlayer electrode having a lower acoustic impedance is disposed between the first layer (114) and the piezoelectric layer (330). The combination of the interlayer electrode (106A) with the piezoelectric layer (330) and the first electrode (104A) may cause the BAW device to resonate at a frequency having a wavelength corresponding to an acoustic cavity length of the BAW device, depending on an acoustic mirror (124A, 126A) included on one side of the BAW device. The interlayer electrode has an acoustic thickness of 1 / 2 resonant frequency wavelength. If both electrodes are interlayer electrodes, the acoustic cavity length is approximately 1.5 times the resonant frequency wavelength.

Description

Technical Field

[0001] The technology of the present disclosure relates generally to microacoustic devices, and more particularly, to bulk acoustic wave (BAW) devices used in acoustic filters. Background Art

[0002] Wireless devices, such as cellular phones, communicate by sending and receiving electromagnetic waves over the air. Cellular phones are permitted to operate within a specific radio frequency range. The frequencies available for operation vary by geographic region (e.g., country) and are limited to certain frequency bands or ranges, which may be surrounded by (e.g., above and below) frequency bands allocated for other applications. Therefore, to avoid interference, cellular phones need to include frequency filters that allow certain frequencies while blocking others. The frequencies transmitted by wireless devices can be filtered by microacoustic devices that are small enough to fit in handheld devices.

[0003] Examples of microacoustic devices include surface acoustic wave (SAW) devices and bulk acoustic wave (BAW) devices. A BAW device receives an electrical signal that generates a varying (RF) electric field between two electrodes in the BAW device, causing the piezoelectric layer located between the electrodes to expand and contract to generate an acoustic wave with a resonant frequency that depends on the physical properties of the BAW device.

[0004] BAW devices include layers of piezoelectric material sandwiched between electrode layers. The thickness of these layers determines the frequency of resonance in the BAW device. As new generations of cellular technologies that adopt higher frequencies develop (e.g., 5G, 6G, 7G), the need to provide filters that can filter higher frequencies also needs to develop. Traditional BAW devices resonate at a frequency of twice the wavelength of the total thickness of the piezoelectric layer, upper electrode, and lower electrode. In order to make this type of traditional BAW device have a higher resonant frequency, the thickness of the piezoelectric layer and / or the electrode needs to be reduced. Devices with thinner electrode layers are more difficult to manufacture (e.g., to be manufactured to have a consistent thickness), and the resistance in the electrode increases. In order to reduce the overall thickness of the device without reducing the thickness of the electrodes, the piezoelectric layer must be thinner, which is also difficult to manufacture and increases the capacitance between the electrodes. To avoid increasing the capacitance, the device area can be reduced, but this increases the power density and limits the power handling capability of the device. Smaller devices are also poor thermal conductors, and thinner piezoelectric layers have lower breakdown voltages. Summary of the invention

[0005] Various aspects disclosed in the specific embodiments include a bulk acoustic wave (BAW) device with a sandwich electrode for a higher resonant frequency. A method for manufacturing a BAW device including a sandwich electrode is also disclosed. Conventional BAW devices used in wired and wireless communications resonate at a resonant frequency, and the wavelength at the resonant frequency is twice the length of the acoustic cavity of the BAW device. The acoustic cavity length is based on the total thickness of the first electrode, the piezoelectric layer, and the second electrode. In the exemplary BAW device disclosed herein, one of the electrodes is a sandwich electrode, and the thickness of the sandwich electrode corresponds to half the wavelength of the resonant frequency. The sandwich electrode includes an outer layer with a higher acoustic impedance and an inner layer with a lower acoustic impedance, wherein the inner layer is disposed between the outer layer and the piezoelectric layer (i.e., sandwiched between the outer layer and the piezoelectric layer). In this regard, the increase in the frequency of the BAW filter does not depend entirely on thinning the piezoelectric layer and the electrode. In some examples, the acoustic cavity length of the BAW device corresponds to a full wavelength of the resonant frequency, wherein half of the cavity length is the thickness of the sandwich electrode, and the other half is based on the thickness of the piezoelectric layer and the second electrode on the other side of the piezoelectric layer. In other examples, the second electrode is also an interlayer electrode, and the acoustic cavity length of the BAW device corresponds to one and a half times (1.5 times) the wavelength of the resonant frequency. In such other examples, each of the second interlayer electrode and the piezoelectric layer has a thickness corresponding to half the wavelength of the resonant frequency.

[0006] In one exemplary aspect, a BAW device is disclosed. The BAW device includes: a piezoelectric layer; a first electrode on a first side of the piezoelectric layer; and an interlayer electrode on a second side of the piezoelectric layer. The interlayer electrode includes: a first layer of a first material having a first acoustic impedance, and a second layer of a second material having a second acoustic impedance less than the first acoustic impedance, the second layer being disposed between the first layer and the piezoelectric layer.

[0007] In another exemplary aspect, a method of manufacturing a BAW device is disclosed. The method includes: forming a first electrode; forming a piezoelectric layer, the piezoelectric layer including a first side on the first electrode; and forming a sandwich electrode on a second side of the piezoelectric layer. The second electrode includes: a first layer including a first material, and a second layer of a second material, the second layer between the first layer and the piezoelectric layer. A first acoustic impedance of the first material is greater than a second acoustic impedance of the second material.

[0008] In another exemplary aspect, an acoustic filter is disclosed. The acoustic filter includes a first bulk acoustic wave (BAW) device and a second BAW device. Each of the first BAW device and the second BAW device includes: a piezoelectric layer; a first electrode on a first side of the piezoelectric layer; and an interlayer electrode on a second side of the piezoelectric layer. The interlayer electrode includes: a first layer of a first material having a first acoustic impedance, and a second layer of a second material having a second acoustic impedance less than the first acoustic impedance, the second layer being disposed between the first layer and the piezoelectric layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is an illustration of a cross-sectional view of exemplary bulk acoustic wave (BAW) devices of fixed mounted resonators (SMRs), each device including a sandwich electrode including an outer layer having a higher acoustic impedance and an inner layer having a lower acoustic impedance, wherein the inner layer is disposed between (i.e., sandwiched between) the outer layer and the piezoelectric layer to provide a higher resonant frequency;

[0010] Figure 2 Is manufacturing Figure 1 A flowchart of a method for an exemplary BAW device in;

[0011] Figure 3A yes Figure 1 A graphical representation of acoustic waves in a BAW device 100A including an acoustic mirror having a layer corresponding to one quarter (1 / 4) of the wavelength of a resonant frequency;

[0012] Figure 3B yes Figure 1 A graphical representation of acoustic waves in a BAW device 100B including an acoustic mirror having a layer corresponding to three quarters (3 / 4) of the wavelength of a resonant frequency;

[0013] Figure 4 is an illustration of a cross-sectional side view of an example of a film bulk acoustic resonator (FBAR) device including a sandwich electrode disclosed herein;

[0014] Figure 5A is an example of a ladder filter including a series BAW device and a shunt BAW device;

[0015] Figure 5B yes Figure 5A A cross-sectional side view of a series BAW device and a shunt BAW device;

[0016] Figure 6 is a block diagram of an exemplary wireless communication device including a sandwich electrode for higher resonant frequencies, including but not limited to Figure 1BAW devices in

[0017] Figure 7 is a block diagram of an exemplary processor-based system that may include a BAW device including sandwich electrodes for higher resonant frequencies, including but not limited to Figure 1 BAW devices in. DETAILED DESCRIPTION

[0018] Referring now to the accompanying drawings, several exemplary aspects of the present disclosure are described. The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any aspect described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects.

[0019] Various aspects disclosed in the specific embodiments include a bulk acoustic wave (BAW) device with a sandwich electrode for a higher resonant frequency. A method for manufacturing a BAW device including a sandwich electrode is also disclosed. Conventional BAW devices used in wired and wireless communications resonate at a resonant frequency at which the wavelength is twice the length of the acoustic cavity of the BAW device. The acoustic cavity length is based on the total thickness of the first electrode, the piezoelectric layer, and the second electrode. In an exemplary BAW device disclosed herein, one of the electrodes is a sandwich electrode, and the thickness of the sandwich electrode corresponds to half the wavelength of the resonant frequency. The sandwich electrode includes an outer layer having a higher acoustic impedance and an inner layer having a lower acoustic impedance, wherein the inner layer is disposed between the outer layer and the piezoelectric layer (i.e., sandwiched between the outer layer and the piezoelectric layer). In this regard, the increase in the frequency of the BAW filter does not entirely depend on thinning the piezoelectric layer and the electrode. In some examples, the acoustic cavity length of the BAW device corresponds to a full wavelength of the resonant frequency, wherein half of the cavity length is the thickness of the sandwich electrode, and the other half is based on the thickness of the piezoelectric layer and the second electrode on the other side of the piezoelectric layer. In other examples, the second electrode is also an interlayer electrode, and the acoustic cavity length of the BAW device corresponds to one and a half times (1.5 times) the wavelength of the resonant frequency. In such other examples, each of the second interlayer electrode and the piezoelectric layer has a thickness corresponding to half the wavelength of the resonant frequency.

[0020] Figure 1 1 is an illustration of a cross-sectional view of an example of bulk acoustic wave (BAW) devices 100A, 100B, and 100C, each device including a sandwich electrode with a thickness corresponding to half the wavelength of the resonant frequency. The sandwich electrode reduces the need to thin the electrodes and piezoelectric layers to achieve higher frequencies because the sandwich electrode changes the operation of the BAW device from resonating at a frequency having a wavelength corresponding to twice the length of the BAW device acoustic cavity (as in conventional devices) to resonating at a frequency having a wavelength corresponding to the length of the acoustic cavity. BAW device 100A is a first example in which a piezoelectric layer 102A is disposed between electrode 104A and sandwich electrode 106A. Figure 1 The electrode 104A is located on the bottom side PSA of the piezoelectric layer 102A. BOT The interlayer electrode 106A is located on the top side PSA of the piezoelectric layer 102A. TOP The bottom side PSA of the piezoelectric layer 102A BOT PSA with the top side of the piezoelectric layer 102 TOP As the name implies, the sandwich electrode 106A has more than one layer, including an outer layer 110A and an inner layer 112A. The inner layer 112A is disposed on the top side PSA of the outer layer 110A and the piezoelectric layer 102A. TOP In other examples, electrode 104A may be located on the top side PSA of piezoelectric layer 102A. TOP on the bottom side PSA, and the interlayer electrode 106A may be BOT superior.

[0021] The piezoelectric layer 102A, electrode 104A, and electrode 106A of the BAW device have a total thickness TA. The total thickness TA corresponds to the acoustic cavity length, which is equal to the full wavelength of the resonant frequency of the BAW device 100A. Specifically, the wavelength of the resonant frequency of the BAW device, i.e., the acoustic cavity length, is primarily determined by the thickness of all layers of the BAW device 100A and the materials they contain. The thickness of the passivation layer 113A and the isolation layer 132A form part of the acoustic cavity length, but these layers are much thinner than the piezoelectric layer 102A, the electrode 104A, and the electrode 106A, and only account for a small percentage of the total thickness TA.

[0022] At frequency FA, ​​each of piezoelectric layer 102A, electrode 104A, and electrode 106A constitutes a portion of the acoustic cavity length corresponding to the wavelength of the resonant frequency FA of BAW device 100A. This portion of the wavelength corresponds to the acoustic thickness, where the desired acoustic thickness of each layer determines the physical thickness based on the acoustic velocity of the layer material. The combined physical thickness of piezoelectric layer 102A, electrode 104A, and electrode 106A determines the total physical thickness TA.

[0023] Typically, the wavelength Lw of a layer of a given material in a BAW device is equal to the acoustic velocity Va of the material (e.g., in meters per second) divided by the frequency. The physical thickness of each layer is set to correspond to a certain fraction of the total wavelength. Thus, for example, the thickness T of the interlayer electrode 106A is 106A The fraction F1 of the wavelength corresponding to the resonant frequency FA, ​​where the fraction F1 is equal to the sum of the fraction F2 of the wavelength in the outer layer 110A plus the fraction F3 of the wavelength in the inner layer 112A. In other words, F1 = F2 + F3. The physical thickness T of the outer layer 110A E1 and the physical thickness T of the inner layer 112A E2 Determined by the following equation:

[0024] T E1 = (F2 x V 112A ) / FA; and T E2 = (F3 x V 110A ) / FA

[0025] where V 112A and V 110A are the acoustic velocities of the materials in the outer layer 110A and the inner layer 112A, respectively. The acoustic thickness of a layer is determined by multiplying the wavelength fraction (e.g., F2, F3) by the wavelength of the frequency FA.

[0026] The total thickness of the combined piezoelectric layer 102A and electrode 104A is TA - T 106A , corresponding to the remaining fraction F4 of the acoustic cavity length. In the BAW device 100A, one full wavelength at the frequency FA corresponds to the acoustic cavity length. Thus, the fraction F4 of the wavelength attributable to the piezoelectric layer 102A and the electrode 104A is equal to 1 - F1. The physical thickness of the combination of the piezoelectric layer 102A and the electrode 104A will depend on the materials of the piezoelectric layer 102 and the electrode 104A and the fraction of their corresponding wavelengths. The physical measurement T 106A of the interlayer electrode 106A can be greater than or less than the measurement TA - T 106A of the combined piezoelectric layer 102A and electrode 104A because the fractions F1 and F4 are not based solely on the physical layer thicknesses. It should be noted that the fraction F1 of the wavelength attributable to the interlayer electrode 106A plus the fraction F4 of the wavelength in the piezoelectric layer 102A and the second electrode 104A can be less than one (1) because a small portion of the acoustic cavity length is attributable to the passivation layers 113A and 132A of the BAW device 100A.

[0027] The acoustic velocities V 110A and V 112A depend on the materials of the outer layer 110A and the inner layer 112A. The acoustic thickness depends on their acoustic velocities V 110A and V 112A and their respective thicknesses. The outer layer 110A of the interlayer electrode 106A is a material layer 114 having a thickness T E1 , while the inner layer 112A is a material layer 116 having a thickness T E2 . The total thickness of the interlayer electrode T 106A is equal to T E1 + T E2. Material 114 may be a metal having an acoustic impedance AI1. The inner layer 112A of the sandwich electrode 106A has an acoustic impedance AI2. The acoustic impedance AI1 of material 114 is greater than the acoustic impedance AI2 of material 116. Material 114 also has a higher density than material 116. Material 114 of the outer layer 110A may also be a non-metallic material having a higher acoustic impedance. The acoustic impedances AI1, AI2 determine the acoustic velocities of the inner layer 112A and the outer layer 110A.

[0028] In some examples, the material 114 of the outer layer 110A may be molybdenum (Mo), tungsten (W), or gold (Au). Material 116 has a lighter density or molecular weight and may also have a lower resistivity than the material 114 of the outer layer 110A. In some examples, the material 114 of the inner layer 112A may be a metal or alloy including aluminum (Al), aluminum scandium (AlSc), aluminum copper (AlCu), beryllium (Be), or magnesium (Mg). The electrode 104A is formed of a material 118, which may be the same as or different from the material 114 of the outer layer 110A. In these examples, an example of the material of the piezoelectric layer 102A is aluminum scandium nitride (AlScN) (e.g., AlSc30N) or aluminum nitride (AlN), but the BAW device 100A is not limited thereto.

[0029] The BAW device 100A can be referred to as a fixed mounted resonator (SMR) because it is mounted on the substrate 122. The BAW device 100A also includes a first acoustic mirror 124A and a second acoustic mirror 126A for acoustic isolation from the substrate 122. The first acoustic mirror 124A and the second acoustic mirror 126A can also affect the acoustic cavity length of the BAW device 100A. The first acoustic mirror 124A includes an impedance layer 128A having a high acoustic impedance, which is separated from the electrode 104A by a layer 130A having a lower acoustic impedance. The impedance layer 128A and the layer 130A can be a metal material or a dielectric material (without considering the other layer). The impedance layer 128A has an acoustic thickness (WA / 4) of approximately one quarter (1 / 4) of the wavelength WA to reduce the propagation of acoustic waves from the BAW device 100A to the substrate 122. Layer 130A can be a dielectric layer, such as, for example, silicon dioxide (SiO2), and impedance layer 128A can be any material with high acoustic impedance, such as tungsten (W). Layer 130A can optionally be separated from electrode 104A by an isolation layer 132A, which can be provided to improve the structure of the bottom electrode to allow the growth of a higher quality piezoelectric layer. The second acoustic mirror 126A is similar to the first acoustic mirror 124A, having a layer 134A of lower acoustic impedance and an impedance layer 136A of higher acoustic impedance. The first acoustic mirror 124A and the second acoustic mirror 126A can be joined by an adhesive layer 138A, such as a titanium (Ti) layer. The second acoustic mirror 126A is coupled to substrate 122 by another adhesive (e.g., Ti) layer 140A and a layer 142A formed on substrate 122, such as a dielectric layer (e.g., SiO2). The materials identified herein are merely non-limiting examples of materials that can be used in an acoustic mirror of an SMR including a BAW device 100A.

[0030] Figure 1 The BAW device 100B includes a piezoelectric layer 102B disposed between an electrode 104B and an interlayer electrode 106B, wherein the interlayer electrode 106B has an outer layer 110B and a top surface PSB between the outer layer 110B and the piezoelectric layer 102B. TOP The thickness T of the sandwich electrode 106B is determined based on the acoustic velocities of the inner layer 112B and the outer layer 110B. 106B , to provide an acoustic thickness equal to approximately half of the total acoustic cavity length of the BAW device 100B. The combination of the piezoelectric layer 102B and the electrode 104B constitutes the remainder of the physical thickness TB and also constitutes the other half of the acoustic cavity length of the BAW device 100B, which corresponds to the wavelength WB of the resonant frequency FB.

[0031] The BAW device 100B is also an SMR disposed on the substrate 122, and differs from the BAW device 100 only in the acoustic mirrors 124B and 126B. In particular, the impedance layer 128B has an acoustic thickness (= (3 / 4xWB)) set to three quarters (WB / 4) of the wavelength WB to significantly reduce the propagation of acoustic waves from the BAW device 100B into the substrate 122. In some examples, the second acoustic mirror 126B may also include an impedance layer 136A having an acoustic thickness of (3 / 4xWB). The BAW device 100B also includes other layers corresponding to the layers of the BAW device 100A discussed above for the same purpose, which are not repeated here. The second acoustic mirror 126B is separated from the substrate 122 by a dielectric layer 142B, which includes SiO2 or another suitable material for coupling the acoustic mirror 126B to the substrate 122.

[0032] Figure 1 Also shown is a BAW device 100C, which includes a piezoelectric layer 102C disposed between an electrode 104C and an interlayer electrode 106C, wherein the interlayer electrode 106C has an outer layer 110C and an inner layer 112C. The inner layer 112C is located between the outer layer 110C and the top surface PSC of the piezoelectric layer 102C. TOP The BAW device 100C is different from the BAW devices 100A and 100B because the electrode 104C is a sandwich electrode 104C similar to the sandwich electrode 106C, rather than a single layer like the electrodes 104A and 104B. The sandwich electrode 104C includes an outer layer 150C and an inner layer 152C, wherein the inner layer 152C is disposed between the outer layer 150C and the bottom PSC layer 102C. BOT between.

[0033] The BAW device 100C has an acoustic cavity length based on the interlayer electrode 104C, the piezoelectric layer 102C, and the interlayer electrode 106C, wherein the acoustic cavity length corresponds to 1.5 times the wavelength WC of the resonant frequency FC. Each of the interlayer electrodes 104C and 106C and the piezoelectric layer 102C corresponds to an acoustic thickness WC / 2, which is half the wavelength WC of the resonant frequency FC of the BAW device 100C. Therefore, the total acoustic cavity length of the BAW device 100C is approximately equal to 1.5 times the wavelength WC of the resonant frequency FC (e.g., 3x(WC / 2)). In other words, the wavelength WC is two-thirds (2 / 3) of the total acoustic cavity length of the BAW device 100C.

[0034] The layers of the sandwich electrodes 104C and 106C correspond in composition to the layers of the sandwich electrode 106A. The outer layers 150C and 110C may include the same material 154 having a higher acoustic impedance CI1, while the inner layers 152C and 112C may include the same material 156 having a lower acoustic impedance CI2. Material 154 may be a metal or non-metal material having a higher acoustic impedance CI1 than material 156 having a lower impedance CI2. The difference in acoustic impedance may be due to the density of material 154 being higher than the density of material 156. For example, the outer layer 150C may include Mo or W, while the inner layer 152C may include Al, AlCu, AlSc, or Mg. Alternatively, the outer layer 110C and the inner layer 112C of the sandwich electrode 106C may be formed of or include a different material than the outer layer 150C and the inner layer 152C of the sandwich electrode 104C.

[0035] The BAW device 100C also includes acoustic mirrors 124C and 126C for isolation from the substrate 122. The acoustic mirrors 124C and 126C are the same as the acoustic mirrors 124 and 126A in the BAW device 100A. For example, the acoustic mirror 124C includes an impedance layer 128C having a high acoustic impedance. The impedance layer 128C is separated from the electrode 104A by a layer 130C having a lower acoustic impedance. The impedance layer 128C has a thickness (WC / 4) approximately equal to one-quarter (1 / 4) of the wavelength WC to significantly reduce the propagation of acoustic waves from the BAW device 100A to the substrate 122. The layers 128C and 130C can be formed of the same material as the above-mentioned layers 128A and 130A. The BAW device 100C also includes an isolation layer 132C to electrically insulate the acoustic mirror 124C from the interlayer electrode 104C and / or to dissipate heat from the interlayer electrode 104C.

[0036] As mentioned above, sound waves propagate in different materials at different speeds and wavelengths, depending on the frequency and type of material. Therefore, the acoustic thickness of a material layer represents the fraction of the wavelength of the frequency in the material, which depends on the acoustic velocity of the material. In this regard, in reference Figure 1 In the exemplary aspects of the BAW device 100A described in , for example, the total acoustic thickness of the piezoelectric layer 102A and the first electrode 104A is between 90% and 110% of the acoustic thickness of the first interlayer electrode 106A. In some examples, the total acoustic thickness of the piezoelectric layer 102A and the first electrode 104A is between 95% and 105% of the acoustic thickness of the first interlayer electrode 106A. In some examples, reference Figure 1In the BAW device 100C of FIG. 1 , the piezoelectric layer 102C includes a first acoustic thickness, the first electrode 104C includes a first acoustic thickness, and the first interlayer electrode 106C includes a first acoustic thickness. In some examples, the acoustic thickness of the first electrode 104C and the acoustic thickness of the first interlayer electrode 106C are each 90% to 110% of the acoustic thickness of the piezoelectric layer 102C. In some examples, reference Figure 1 In the BAW devices 100A and 100B, the acoustic thickness of the impedance layer 128A / 128B is one of 50% and 150% of the acoustic thickness of the first interlayer electrode 106A / 106B.

[0037] Figure 2 2 is a flow chart of a method for manufacturing a BAW device, comprising: forming a first electrode 104A (block 202), and forming a piezoelectric layer 102A, the piezoelectric layer 102A comprising a first side PSA on the first electrode 104A BOT (Block 204). The method includes: TOP The sandwich electrode 106A is formed on the first side PSA of the piezoelectric layer 102A (block 206). The sandwich electrode is formed by: TOP and forming an outer layer 110A including a first material 114 on the inner layer 112A, wherein a first acoustic impedance AI1 of the first material 114 is greater than a second acoustic impedance AI2 of the second material 116 (block 210).

[0038] Figure 3A and 3B A graphical representation of acoustic waves in BAW devices 100A and 100B are shown, which include impedance layers 128A, 128B having a thickness of one quarter of the resonant wavelength WA (eg, WA / 4) and three quarters of the resonant wavelength WB (eg, 3xWB / 4).

[0039] Figure 3A 106A and a portion 304A corresponding to the piezoelectric layer 102A and the electrode 104A. Portions 302 and 304 have the same acoustic length and therefore each constitutes approximately half of the total cavity length. As described above, the physical thickness of portions 302A and 304A will vary depending on the materials forming the layers and the fraction of the wavelength attributed to each layer. In this example, portions 302 and 304 correspond to fractions F1 and F4 of the wavelength WA at the resonant frequency FA. The fraction F1 of the wavelength WA at the resonant frequency FA corresponds to the physical thickness T of the interlayer electrode 106A represented by portion 302A. 106A , and portion 304A corresponds to a fraction F4 of the wavelength WA, where F1+F4=1, Figure 3AThe BAW device 100A is shown to have a resonant frequency at which a full wavelength equals the acoustic cavity length, which is twice the frequency of a conventional device of comparable thickness. Figure 3A The portions 306A and 308A of FIG. 1 show that the amplitude of the acoustic wave is reduced in the first acoustic mirror 124A and further dissipated in the second acoustic mirror 126A. As described above, the impedance layer 128A in the first acoustic mirror 124A has a thickness of about one quarter of the wavelength WA.

[0040] Figure 3B The portion 302B corresponding to the sandwich electrode 106B and the portion 304B corresponding to the combination of the piezoelectric layer 102B and the electrode 104B are included. The portions 302B and 304B each represent half of the total cavity length TB, which is approximately equal to the full wavelength WB of the resonant frequency FB. The impedance layer 128B in the first acoustic mirror 124B of the BAW device 100B has a thickness of approximately three quarters of the wavelength WA to reduce the amplitude of the acoustic wave further dissipated in the second acoustic mirror 126B.

[0041] Figure 4 4 is a cross-sectional side view of an FBAR type BAW device 400 corresponding to the SMR type BAW device 100A. The BAW device 400 is an example of providing isolation from the substrate 402 by an air cavity 404 instead of an acoustic mirror. The BAW device 400 is separated from the air cavity 404 by a diaphragm 406 supported by a frame 408. An electrode 410 is coupled to the diaphragm 406. The BAW device 400 includes: a piezoelectric layer 412 on the electrode 410, and a piezoelectric layer 412 on the top side P TOP In an alternative example, the positions of the electrode 410 and the sandwich electrode 414 can be reversed. In such an alternative, as long as the inner layer 416 with a lower acoustic impedance is disposed between the piezoelectric layer 412 and the outer layer 418 with a higher acoustic impedance than the inner layer 416, substantially the same resonant frequency F can be achieved. 400 , wherein the inner layer 416 and the outer layer 418 form an interlayer electrode 414. An insulating layer 420, such as silicon nitride (SiN), is disposed on the interlayer electrode 414.

[0042] Figure 5A is a schematic diagram of a ladder filter 500, which is an example of an acoustic filter, including a series BAW device 502 and a shunt BAW device 504 coupled between an input 506P / 506N and an output 508P / 508N. In other configurations, the ladder filter may include multiple series BAW devices and multiple shunt BAW devices. Figure 5B is a cross-sectional side view of a series BAW device 502 and a shunt BAW device 504, each of which may be Figure 1The BAW device 100B in FIG. 5 is a BAW device 100B (eg, having a mirror layer corresponding to ¾ of the resonant wavelength). The electrical interconnection of the ladder filter 500 is not Figure 5B It should be understood that the ladder filter corresponding to the ladder filter 500 can be made of any one of the BAW devices 100A, 100B or 100C.

[0043] Each of the BAW devices 502, 504 includes: a top side P of a piezoelectric layer 512 TOP The interlayer electrode 510 on the bottom side of the piezoelectric layer 512 BOT The BAW devices 502, 504 also include acoustic mirrors corresponding to the acoustic mirrors 124B and 126B in the BAW device 100B, which use an impedance layer having a thickness of ¾ of the wavelength WB to reduce acoustic wave propagation.

[0044] The desired operation of the ladder filter 500 is based on the resonant frequency difference between the series BAW device 502 and the shunt BAW device 504. The resonant frequency difference can be achieved by changing the ratio of the acoustic thickness of each layer of the sandwich electrode 510, as shown in FIG. Figure 5B When the acoustic thickness ratio of the sandwich electrodes in the tandem BAW device 502 and the shunt BAW device 504 is properly selected, many spurious modes outside the main passband can also be suppressed. The outer layer 520 of the sandwich electrode 410 in the shunt BAW device 502 has a thickness T 520 , and the inner layer 522 has a thickness T 522 The outer layer 540 of the interlayer electrode 510 in the tandem BAW device 504 has a thickness T 540 , and the inner layer 542 has a thickness T 542 In an oversimplified example, where the acoustic velocities of outer layer 520, inner layer 522, outer layer 540, and inner layer 542 are all the same, the ratio of their physical thicknesses will correspond to the ratio of their acoustic thicknesses. According to this example, Figure 5B Medium, thickness T 520 With thickness T 522 The ratio (i.e., T 520 / T 522 ) is different from the thickness T 540 With thickness T 542 The ratio (such as T 540 / T 542 In other words, even though the interlayer electrodes 510 in the shunt BAW device 502 and the tandem BAW device 504 each constitute approximately half of the acoustic cavity length of the shunt BAW device 502 and the tandem BAW device 504, respectively, the thickness T of the outer layer 520 is 520 The thickness T of the inner layer 522 522 The ratio may be different from the thickness T of the inner layer 542542 The thickness T of the outer layer 540 540 Thus, in this example, the acoustic ratios of the interlayer 510 in the shunt BAW device 502 and the tandem BAW device 504 are different. By adjusting the interlayer electrodes 510 of the shunt BAW device 502 and the tandem BAW device 504 to have different thickness ratios (e.g., to provide different acoustic thickness ratios), the resonant frequency difference can be adjusted to achieve the desired operation of the ladder filter 500. The ratio difference can be used to determine the bandwidth of the ladder filter 500. The resonant frequencies of the tandem BAW device 502 and the shunt BAW device 504 can be tuned by modifying the total thickness of the interlayer electrodes.

[0045] According to various aspects disclosed herein, an acoustic wave device having a tuned resonator piezoelectric thickness can be provided in or integrated into any processor-based device. Examples include, but are not limited to, a set-top box, an entertainment unit, a navigation device, a communication device, a fixed location data unit, a mobile location data unit, a global positioning system (GPS) device, a mobile phone, a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a tablet, a phablet, a server, a computer, a portable computer, a mobile computing device, a wearable computing device (e.g., a smart watch, a health or fitness tracker, glasses, etc.), a desktop computer, a personal digital assistant (PDA), a monitor, a computer monitor, a television, a tuner, a radio, a satellite radio, a music player, a digital music player, a portable music player, a digital video player, a video player, a digital video disc (DVD) player, a portable digital video player, an automobile, a vehicle component, an avionics system, a drone, and a multirotor.

[0046] Figure 6 An exemplary wireless communication device 600 is shown, which includes radio frequency (RF) components formed by one or more integrated circuits (ICs) 602, and may include exemplary BAW devices, such as Figure 1 As shown, and according to any aspect disclosed herein, in the BAW device, the wavelength of the resonant frequency is approximately equal to twice the thickness of the sandwich electrode on one side of the piezoelectric layer. As an example, the wireless communication device 600 may include or be provided in any of the above-mentioned devices. Figure 6 As shown, the wireless communication device 600 includes a transceiver 604 and a data processor 606. The data processor 606 may include a memory for storing data and program codes. The transceiver 604 includes a transmitter 608 and a receiver 610 that support two-way communication. In general, the wireless communication device 600 may include any number of transmitters 608 and / or receivers 610 for any number of communication systems and frequency bands. All or part of the transceiver 604 may be implemented on one or more analog ICs, RFICs, mixed signal ICs, etc.

[0047] The transmitter 608 or the receiver 610 may be implemented with a superheterodyne architecture or a direct conversion architecture. In a superheterodyne architecture, a signal is frequency converted between RF and baseband in multiple stages, for example, from RF to an intermediate frequency (IF) in one stage and then from IF to baseband in another stage. In a direct conversion architecture, a signal is frequency converted between RF and baseband in one stage. Superheterodyne and direct conversion architectures may use different circuit blocks and / or have different requirements. Figure 6 In the wireless communication device 600 in FIG. 1 , the transmitter 608 and the receiver 610 are implemented using a direct conversion architecture.

[0048] In the transmit path, the data processor 606 processes data to be transmitted and provides I and Q analog output signals to the transmitter 608. In the exemplary wireless communication device 600, the data processor 606 includes digital-to-analog converters (DACs) 612(1), 612(2) for converting digital signals generated by the data processor 606 into I and Q analog output signals, such as I and Q output currents, for further processing.

[0049] Within the transmitter 608, low pass filters 614(1), 614(2) filter the I and Q analog output signals, respectively, to remove undesired signals caused by the previous digital-to-analog conversion. The low pass filters 614(1), 614(2) may be implemented as a BAW filter package 603. Amplifiers (AMPs) 616(1), 616(2) amplify the signals from the low pass filters 614(1), 614(2), respectively, and provide I and Q baseband signals. An up converter 618 up-converts the I and Q baseband signals using I and Q transmit (TX) local oscillator (LO) signals from a TX LO signal generator 622 via mixers 620(1), 620(2) to provide an up-converted signal 624. A filter 626 filters the up-converted signal 624 to remove undesired signals caused by the up-conversion and noise in the receive band. A power amplifier (PA) 628 amplifies the up-converted signal 624 from filter 626 to obtain a desired output power level and provides a transmit RF signal. The transmit RF signal is routed through a duplexer or switch 630 and transmitted via an antenna 632. Either of the low pass filters 614(1) and 614(2) or filter 626 may be an acoustic wave filter (AW filter) package 603, which may include Figure 1 Any BAW device 100A, 100B, 100C.

[0050] In the receive path, antenna 632 receives the signal transmitted by the base station and provides a receive RF signal, which is routed through a duplexer or switch 630 and provided to a low noise amplifier (LNA) 634. The duplexer or switch 630 is designed to operate with a specific receive (RX) to TX duplexer frequency separation so that the RX signal is isolated from the TX signal. The receive RF signal is amplified by LNA 634 and filtered by filter 636 to obtain the desired RF input signal. Down-conversion mixers 638 (1), 638 (2) mix the output of filter 636 with the I and Q RX LO signals (i.e., LO_I and LO_Q) from RX LO signal generator 640 to generate I and Q baseband signals. The I and Q baseband signals are amplified by AMPs 642 (1), 642 (2) and further filtered by low pass filters 644 (1), 644 (2) to obtain I and Q analog input signals, which are provided to data processor 606. Any of the filter 636 and the low pass filters 644(1), 644(2) may be BAW filter packages 603. In this example, the data processor 606 includes analog-to-digital converters (ADCs) 646(1), 646(2) for converting analog input signals into digital signals for further processing by the data processor 606.

[0051] exist Figure 6 In the wireless communication device 600 of FIG. 6 , the TX LO signal generator 622 generates I and Q TX LO signals for up-conversion, and the RX LO signal generator 640 generates I or Q RX LO signals for down-conversion. Each LO signal is a periodic signal with a specific base frequency. The TX phase-locked loop (PLL) circuit 648 receives timing information from the data processor 606 and generates a control signal for adjusting the frequency and / or phase of the TX LO signal from the TX LO signal generator 622. Similarly, the RX PLL circuit 650 receives timing information from the data processor 606 and generates a control signal for adjusting the frequency and / or phase of the RX LO signal from the RX LO signal generator 640.

[0052] Each wireless communication device 600 may include an exemplary BAW device fabricated according to any aspect described herein, for example, wherein a top electrode is disposed on a surface (which is optionally planar) of a piezoelectric layer, such as Figure 1 and 4As shown in A-4H, and in accordance with any aspect disclosed herein, the piezoelectric layer is thinned in one area to tune the acoustic resonator to a different frequency, the wireless communication device 600 can be provided in or integrated into any processor-based device. Examples (but not limited to) include: a set-top box, an entertainment unit, a navigation device, a communication device, a fixed location data unit, a mobile location data unit, a global positioning system (GPS) device, a mobile phone, a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a tablet, a tablet phone, a server, a computer, a portable computer, a mobile computing device, a wearable computing device (e.g., a smart watch, a health or fitness tracker, glasses, etc.), a desktop computer, a personal digital assistant (PDA), a monitor, a computer monitor, a television, a tuner, a radio device, a satellite radio device, a music player, a digital music player, a portable music player, a digital video player, a video player, a digital video disc (DVD) player, a portable digital video player, an automobile, a vehicle component, an avionics system, a drone, and a multirotor.

[0053] Figure 7 An example of a processor-based system 700 is shown, which includes circuits including exemplary BAW devices, such as Figure 1 As shown and in accordance with any aspects disclosed herein, in the BAW device, the wavelength of the resonant frequency is approximately equal to twice the thickness of the interlayer electrode on one side of the piezoelectric layer. In this example, the processor-based system 700 includes one or more central processor units (CPUs) 702, which may also be referred to as CPUs or processor cores, each of which includes one or more processors 704. The CPU 702 may have a cache memory 706 coupled to the processor 704 for quickly accessing temporarily stored data. The CPU 702 is coupled to a system bus 708 and may be mutually coupled to master devices and slave devices included in the processor-based system 700. As is well known, the CPU 702 communicates with these other devices by exchanging address, control, and data information on the system bus 708. For example, the CPU 702 may transmit a bus transaction request to a memory controller 710, which is an example of a slave device. Although in Figure 7 It is not shown, but multiple system buses 708 may be provided; each system bus 708 forming a different structure.

[0054] Other master devices and slave devices may be connected to the system bus 708. Figure 7As shown, as an example, these devices may include: a memory system 712, which includes a memory controller 710 and one or more memory arrays 714, one or more input devices 716, one or more output devices 718, one or more network interface devices 720, and one or more display controllers 722. The input device 716 may include any type of input device, including but not limited to: input keys, switches, voice processors, etc. The output device 718 may include any type of output device, including but not limited to: audio, video, other visual indicators, etc. The network interface device 720 can be any device configured to allow data exchange with the network 724. The network 724 can be any type of network, including but not limited to: wired or wireless network, private or public network, local area network (LAN), wireless local area network (WLAN), wide area network (WAN), Bluetooth TM Networks and the Internet. Network interface device 720 may be configured to support any type of communication protocol desired.

[0055] The CPU 702 may also be configured to access a display controller 722 via the system bus 708 to control information sent to one or more displays 726. The display controller 722 sends information to the display 726 for display via one or more video processors 728, which processes the information to be displayed into a format suitable for the display 726. The display 726 may include any type of display, including but not limited to a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, or a light emitting diode (LED) display.

[0056] Those skilled in the art will further understand that the various illustrative logic boxes, modules, circuits and algorithms described in conjunction with the various aspects disclosed herein can be implemented as electronic hardware, stored in a memory or another computer-readable medium and executed by a processor or other processing device, or a combination of the two. As an example, the master device and slave device described herein can be used in any circuit, hardware component, IC or IC chip. The memory disclosed herein can be a memory of any type and size, and can be configured to store any type of information required. In order to clearly illustrate this interchangeability, the functions of various illustrative components, frames, modules, circuits and steps have been generally described above. How to implement this function depends on specific applications, design choices and / or design constraints imposed on the entire system. Those skilled in the art can implement the described functions in different ways for each specific application, but such implementation decisions should not be interpreted as causing deviations from the scope of the present disclosure.

[0057] The various illustrative logic blocks, modules, and circuits described in conjunction with the various aspects disclosed herein may be implemented or performed using a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. The processor may be a microprocessor, but in an alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. The processor may also be implemented as a combination of computing devices (e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration).

[0058] The various aspects disclosed herein may be embodied in hardware and instructions, which are stored in hardware and may reside in, for example, random access memory (RAM), flash memory, read-only memory (ROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), registers, hard disks, removable disks, CD-ROMs, or any other form of computer-readable medium known in the art. An exemplary storage medium is coupled to a processor so that the processor can read information from the storage medium and write information to the storage medium. In an alternative, the storage medium may be integrated into the processor. The processor and storage medium may reside in an ASIC. The ASIC may reside in a remote station. Alternatively, the processor and storage medium may reside in a remote station, a base station, or a server as discrete components.

[0059] It should also be noted that the operational steps described in any exemplary aspects of this article are all for providing examples and discussions. The described operations can be performed in many different sequences in addition to the sequence shown. In addition, the operations described in a single operational step can actually be performed in several different steps. In addition, one or more operational steps discussed in the exemplary aspects can be combined. It should be understood that the operational steps shown in the flow chart may be subject to many different modifications, which is obvious to those skilled in the art. Those skilled in the art will also understand that various technologies and means can be used to represent information and signals. For example, the data, instructions, commands, information, signals, bits, symbols and chips mentioned throughout the description above can be represented by voltage, current, electromagnetic waves, magnetic fields or magnetic particles, light fields or optical particles or any combination thereof.

[0060] The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be apparent to those skilled in the art, and the general principles defined herein may be applied to other variations. Therefore, the disclosure is not intended to be limited to the examples and designs described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

[0061] Implementation examples are described in the following numbered clauses: 1. A bulk acoustic wave (BAW) device, comprising: Piezoelectric layer; a first electrode on a first side of the piezoelectric layer; and a first interlayer electrode on a second side of the piezoelectric layer, the first interlayer electrode comprising: a first layer of a first material having a first acoustic impedance; and A second layer of a second material having a second acoustic impedance less than the first acoustic impedance, the second layer being disposed between the first layer and the piezoelectric layer. 2. A BAW device according to clause 1, wherein: The first material and the first electrode include molybdenum (Mo); and The second material includes aluminum and copper (AlCu). 3. A BAW device according to clause 1 or clause 2, wherein the first electrode comprises a second interlayer electrode, further comprising: a third layer of the first material; and A fourth layer of the second material is disposed between the third layer and the piezoelectric layer. 4. A BAW device according to any one of clauses 1 to 3, further comprising: a substrate adjacent to one of the first electrode and the first interlayer electrode; and An air cavity is between the substrate and the one of the first electrode and the first interlayer electrode. 5. The BAW device according to any one of clauses 1 to 3, further comprising: an acoustic mirror disposed on one of the first electrode and the first interlayer electrode. 6. A BAW device according to clause 5, wherein the acoustic mirror comprises: a first mirror layer comprising one of a first dielectric layer and a first metal layer; and A second mirror layer includes one of a second dielectric layer and a second metal layer. 7. The BAW device according to clause 6, wherein the first dielectric layer comprises silicon dioxide (SiO2 ), and the first metal layer includes tungsten (W). 8. A BAW device according to any one of clauses 1 to 7, integrated into a semiconductor die. 9. A BAW device according to any one of clauses 1 to 8, which is integrated into a device selected from the group consisting of: a set-top box; an entertainment unit; a navigation device; a communication device; a fixed location data unit; a mobile location data unit; a global positioning system (GPS) device; a mobile phone; a cellular phone; a smartphone; a Session Initiation Protocol (SIP) phone; a tablet computer; a tablet phone; a server; a computer; a portable computer; a mobile computing device; a wearable computing device; a desktop computer; a personal digital assistant (PDA); a monitor; a computer monitor; a television; a tuner; a radio device; a satellite radio device; a music player; a digital music player; a portable music player; a digital video player; a video player; a digital video disc (DVD) player; a portable digital video player; an automobile; a vehicle component; an avionics system; an unmanned aerial vehicle; and a multirotor aircraft. 10. A method for manufacturing a BAW device, the method comprising: forming a first electrode; forming a piezoelectric layer including a first side on the first electrode; and forming an interlayer electrode on a second side of the piezoelectric layer, comprising: forming a second layer of a second material on the second side of the piezoelectric layer; and forming a first layer including a first material on the second layer, The first acoustic impedance of the first material is greater than the second acoustic impedance of the second material. 11. An acoustic filter, comprising: The first bulk acoustic wave (BAW) devices; and A second BAW device; Wherein, each of the first BAW device and the second BAW device comprises: Piezoelectric layer; a first electrode on a first side of the piezoelectric layer; and a sandwich electrode on a second side of the piezoelectric layer, the sandwich electrode comprising: a first layer of a first material having a first acoustic impedance; and A second layer of a second material having a second acoustic impedance less than the first acoustic impedance, the second layer being disposed between the first layer and the piezoelectric layer. 12. An acoustic filter according to clause 11, wherein a first ratio of the thickness of the first layer to the thickness of the second layer of the interlayer electrode in the first BAW device is different from a second ratio of the thickness of the first layer to the thickness of the second layer of the interlayer electrode in the second BAW device. 13. The acoustic filter according to clause 11 or clause 12, wherein in one of the first BAW device and the second BAW device, a thickness of the second layer of the interlayer electrode is greater than a thickness of the first layer of the interlayer electrode. 14. The acoustic filter according to any one of clauses 11 to 13, wherein, in each of the first BAW device and the second BAW device, the first electrode comprises: a third layer of the first material; and A fourth layer of the second material is disposed between the third layer and the piezoelectric layer. 15. The acoustic filter according to any one of clauses 11 to 14, further comprising a ladder filter, wherein: The first BAW device is a series coupled device; and The second BAW device is a shunt-coupled device.

Claims

1. A bulk acoustic wave (BAW) device, include: Piezoelectric layer; a first electrode on a first side of the piezoelectric layer; as well as a first interlayer electrode on a second side of the piezoelectric layer, the first interlayer electrode comprising: a first layer of a first material having a first acoustic impedance; as well as A second layer of a second material having a second acoustic impedance less than the first acoustic impedance, the second layer being disposed between the first layer and the piezoelectric layer.

2. The BAW device according to claim 1, in: The first material and the first electrode include molybdenum (Mo); and The second material includes aluminum and copper (AlCu).

3. The BAW device according to claim 1, in, The first electrode includes a second interlayer electrode, further comprising: a third layer of the first material; and A fourth layer of the second material is disposed between the third layer and the piezoelectric layer.

4. The BAW device according to claim 1, further comprising: include: a substrate adjacent to one of the first electrode and the first interlayer electrode; as well as An air cavity is between the substrate and the one of the first electrode and the first interlayer electrode.

5. The BAW device according to claim 1, further comprising: include: An acoustic mirror is disposed on one of the first electrode and the first interlayer electrode.

6. The BAW device according to claim 5, in, The acoustic mirror comprises: a first mirror layer comprising one of a first dielectric layer and a first metal layer; and A second mirror layer includes one of a second dielectric layer and a second metal layer.

7. The BAW device according to claim 6, in, The first dielectric layer includes silicon dioxide (SiO 2 ), and the first metal layer includes tungsten (W).

8. The BAW device of claim 1 integrated into a semiconductor die.

9. The BAW device of claim 1 , integrated into a device selected from the group consisting of: a set-top box; an entertainment unit; a navigation device; a communication device; a fixed location data unit; a mobile location data unit; a global positioning system (GPS) device; a mobile phone; a cellular phone; a smartphone; a session initiation protocol (SIP) phone; a tablet computer; a phablet; a server; a computer; a portable computer; a mobile computing device; a wearable computing device; a desktop computer; a personal digital assistant (PDA); a monitor; a computer monitor; a television; a tuner; a radio device; a satellite radio device; a music player; a digital music player; a portable music player; a digital video player; a video player; a digital video disc (DVD) player; a portable digital video player; an automobile; a vehicle component; an avionics system; an unmanned aerial vehicle; and a multirotor aircraft.

10. A method for manufacturing a BAW device, the method include: forming a first electrode; forming a piezoelectric layer, the piezoelectric layer comprising a first side on the first electrode; as well as forming an interlayer electrode on a second side of the piezoelectric layer, comprising: forming a second layer of a second material on the second side of the piezoelectric layer; and forming a first layer including a first material on the second layer, The first acoustic impedance of the first material is greater than the second acoustic impedance of the second material.

11. An acoustic filter, include: The first bulk acoustic wave (BAW) device; as well as A second BAW device; Wherein, each of the first BAW device and the second BAW device comprises: Piezoelectric layer; a first electrode on a first side of the piezoelectric layer; and a sandwich electrode on a second side of the piezoelectric layer, the sandwich electrode comprising: a first layer of a first material having a first acoustic impedance; and A second layer of a second material having a second acoustic impedance less than the first acoustic impedance, the second layer being disposed between the first layer and the piezoelectric layer.

12. The acoustic filter according to claim 11, in, A first ratio of a thickness of the first layer to a thickness of the second layer of the interlayer electrode in the first BAW device is different from a second ratio of a thickness of the first layer to a thickness of the second layer of the interlayer electrode in the second BAW device.

13. The acoustic filter according to claim 11, in, In one of the first BAW device and the second BAW device, a thickness of the second layer of the interlayer electrode is greater than a thickness of the first layer of the interlayer electrode.

14. The acoustic filter according to claim 11, in, In each of the first BAW device and the second BAW device, the first electrode includes: a third layer of the first material; and A fourth layer of the second material is disposed between the third layer and the piezoelectric layer.

15. The acoustic filter according to claim 11, further comprising a ladder filter, in: The first BAW device is a series coupled device; and The second BAW device is a shunt-coupled device.

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