Acoustic resonator

Abstract

The invention relates to an acoustic resonator. An acoustic resonator includes a first piezoelectric layer, a second piezoelectric layer, a coupler layer, a first electrode, and a second electrode. The first piezoelectric layer has a first polarity. The second piezoelectric layer has a second polarity opposite to the first polarity. The coupler layer is between the first piezoelectric layer and the second piezoelectric layer. The first electrode is opposite the coupler layer on the first piezoelectric layer. The second electrode is opposite the coupler layer on the second piezoelectric layer.

Description

[0001] This application is a divisional application of a Chinese patent application with an application number of 201910229019.6 and an invention title of "Acoustic Resonator", which was filed on March 25, 2019.

[0002] Related Applications

[0003] This application claims the benefit of Provisional Patent Application Serial No. 62 / 649,343, filed on March 28, 2018, the disclosure of which is hereby incorporated by reference in its entirety. Technical Field

[0004] The present disclosure relates to bulk acoustic wave (BAW) resonators that can operate in higher-order modes with enhanced piezoelectric coupling. Background Art

[0005] Figure 1 A fundamental-mode bulk acoustic wave (BAW) resonator 10 is shown. The fundamental-mode BAW resonator 10 includes a piezoelectric layer 12 between a first electrode 14 and a second electrode 16. The thicknesses of the piezoelectric layer 12, the first electrode 14, and the second electrode 16 are shown as a distance from the center of the thickness of the fundamental-mode BAW resonator 10, where d is half the thickness of the piezoelectric layer 12 and t is the thickness of both the first electrode 14 and the second electrode 16.

[0006] Figure 2 The stress distribution (solid line 18) and displacement distribution (dashed line 20) of the fundamental-mode BAW resonator 10 are shown. As shown, the fundamental-mode BAW resonator 10 operates in a fundamental mode, where the stress distribution corresponding to half the wavelength of the sine wave fits the thickness of the piezoelectric layer 12. The effective electromechanical coupling of the fundamental-mode BAW resonator 10 depends on the integral of the stress distribution over the thickness of the piezoelectric layer 12. Generally, a higher electromechanical coupling coefficient is desired. Although not shown, those skilled in the art will understand that the stress distribution may become steeper at the contact surfaces between the piezoelectric layer 12, the first electrode 14, and the second electrode 16.

[0007] The possibility of exciting higher-order modes in a BAW resonator allows a filter to operate at a higher frequency than in the case of using a conventional fundamental-mode BAW resonator, while maintaining a reasonable quality factor, size, and electrode thickness. In higher-order modes, a stress distribution corresponding to an integer multiple of the fundamental-mode frequency is excited in the piezoelectric layer. For example, in the second-order mode (also referred to herein as the second overtone mode), the entire wavelength of the sine wave (twice the fundamental-mode frequency) fits the thickness of the piezoelectric layer.

[0008] While BAW resonators capable of operating in higher order modes allow for improved performance at high frequencies, attempting to fabricate such devices provides very poor electromechanical coupling compared to fundamental mode devices such as the fundamental mode BAW resonator 10 discussed above. Accordingly, there is a need for BAW resonators capable of operating in higher order modes, such as second order modes with improved electromechanical coupling. SUMMARY OF THE INVENTION

[0009] In one embodiment, an acoustic resonator includes a first piezoelectric layer, a second piezoelectric layer, a coupler layer, a first electrode, and a second electrode. The first piezoelectric layer has a first polarity. The second piezoelectric layer has a second polarity opposite the first polarity. The coupler layer is between the first piezoelectric layer and the second piezoelectric layer. The first electrode is on the first piezoelectric layer opposite the coupler layer. The second electrode is on the second piezoelectric layer opposite the coupler layer. Providing the coupler layer between the first piezoelectric layer and the second piezoelectric layer increases the electromechanical coupling coefficient of the acoustic resonator, thereby improving the performance of the acoustic resonator.

[0010] Those skilled in the art will appreciate the scope of the present disclosure and recognize additional aspects thereof after reading the detailed description of the preferred embodiments in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several aspects of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0012] Figure 1 A fundamental mode bulk acoustic wave (BAW) resonator is shown.

[0013] Figure 2 The stress response and displacement distribution of the fundamental mode BAW resonator are shown.

[0014] Figure 3 A second overtone BAW resonator according to one embodiment of the present disclosure is shown.

[0015] Figure 4 The stress distribution and displacement distribution of the second overtone BAW resonator according to one embodiment of the present disclosure are shown.

[0016] Figure 5 A second overtone BAW resonator according to one embodiment of the present disclosure is shown.

[0017] Figure 6 The stress distributions of two second overtone BAW resonators according to various embodiments of the present disclosure are shown.

[0018] Figure 7 A graph showing the performance characteristics of several second overtone BAW resonators according to various embodiments of the present disclosure.

[0019] Figure 8 Shows a second overmode BAW resonator according to an embodiment of the present disclosure.

[0020] Figure 9 Shows a second overmode BAW resonator according to an embodiment of the present disclosure. Detailed Description

[0021] The embodiments set forth below represent the necessary information that enables those skilled in the art to practice the embodiments and show the best mode of practicing the embodiments. After reading the following description with reference to the drawings, those skilled in the art will understand the concepts of the present disclosure and will recognize applications of these concepts that are not specifically set forth herein. It should be understood that these concepts and applications are within the scope of the present disclosure and the appended claims.

[0022] It should be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, without departing from the scope of the present disclosure, a first element may be named a second element, and similarly, a second element may be named a first element. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0023] It should be understood that when an element such as a layer, region, or substrate is referred to as being "on" or extending "onto" another element, it may be directly on the other element or directly extend onto the other element, or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or "directly extending onto" another element, no intervening elements are present. Similarly, it should be understood that when an element such as a layer, region, or substrate is referred to as being "above" or extending "above" another element, it may be directly above the other element or directly extend above the other element, or intervening elements may also be present. In contrast, when an element is referred to as being "directly above" or "directly extending above" another element, no intervening elements are present. It should also be understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, no intervening elements are present.

[0024] Relative terms such as "below", "above", "upper", "lower", "horizontal", or "vertical" are used herein to describe the relationship of one element, layer, or region illustrated in the figures to another element, layer, or region. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

[0025] The terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the disclosure. As used herein, unless the context clearly dictates otherwise, the singular forms "a" and "the" are intended to include the plural forms as well. It should also be understood that when used herein, the terms "comprises", "comprising", "includes", and "including" specify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0026] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It should also be understood that the terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art, and should not be interpreted in an idealized or overly formal sense, unless expressly so defined herein.

[0027] Figure 3 A second overmode bulk acoustic wave (BAW) resonator 22 according to an embodiment of the present disclosure is shown. The second overmode BAW resonator 22 includes a first piezoelectric layer 24, a second piezoelectric layer 26 on the first piezoelectric layer 24, a first electrode 28 on the first piezoelectric layer 24 and opposite the second piezoelectric layer 26, and a second electrode 30 on the second piezoelectric layer 26 and opposite the first piezoelectric layer 24. The thicknesses of the first piezoelectric layer 24, the second piezoelectric layer 26, the first electrode 28, and the second electrode 30 are shown as a distance from the center of the thickness of the BAW resonator 10, where d' is the thickness of the first piezoelectric layer 24 and the second piezoelectric layer 26 and t is the thickness of both the first electrode 28 and the second electrode 30.

[0028] Note that the first piezoelectric layer 24 has a first polarity, while the second piezoelectric layer 26 has a second polarity opposite to the first polarity. This allows the second overmode BAW resonator 22 to excite a second-order mode, as described below. In one embodiment, the first piezoelectric layer 24 and the second piezoelectric layer 26 are aluminum nitride (AlN) layers with opposite polarities. For example, the first piezoelectric layer 24 can be a nitrogen-polarity layer of aluminum nitride (c-AlN), while the second piezoelectric layer 26 can be an aluminum-polarity layer of aluminum nitride (f-AlN). The aluminum nitride can be undoped or can be doped with one or more of scandium (Sc), erbium (Er), magnesium (Mg), hafnium (Hf), etc. The first electrode 28 and the second electrode 30 can be metal layers. For example, the first electrode 28 and the second electrode 30 can be aluminum (Al), molybdenum (Mo), tungsten (W), etc. The thicknesses (d') of the first piezoelectric layer 24 and the second piezoelectric layer 26 and the thicknesses (t) of the first electrode 28 and the second electrode 30 can be selected to provide specific electrical and / or acoustic properties in order to change one or more operating parameters of the second overmode BAW resonator 22.

[0029] Figure 4 The stress distribution (solid line 32) and displacement distribution (dashed line 34) of the second overmode BAW resonator 22 are shown. As shown, the BAW resonator operates in a second-order mode (also referred to herein as the second overmode), where the stress distribution corresponding to the full wavelength of the sine wave is adapted to the combined thickness of the first piezoelectric layer 24 and the second piezoelectric layer 26. Although not shown, those skilled in the art will understand that the stress distribution may become steeper at the contact surface between the first piezoelectric layer 24 and the first electrode 28 and at the contact surface between the second piezoelectric layer 26 and the second electrode 30.

[0030] As described above, one problem faced by BAW resonators attempting to excite higher-order modes is that they typically have a lower electromechanical coupling coefficient than their fundamental-mode counterparts This is illustrated by the comparison of the fundamental-mode BAW resonator 10 and the second overmode BAW resonator 22 discussed above in the background art. Assuming that the electrodes in both the fundamental-mode BAW resonator 10 and the second overmode BAW resonator 22 have the same material properties as the piezoelectric layers therein, the displacement distribution u(z) of the device can be expressed according to Equation (1):

[0031]

[0032] where a is the mode amplitude and the wave vector k p is obtained by using the stress-free boundary condition T(±d + t) = 0, such that for the fundamental-mode BAW resonator 10, k p = π / 2(d + t), and for the second overmode BAW resonator 22, k p= π / (d'+t). The basic method for calculating the effective electromechanical coupling of these structures is derived from the Belincourt formula, which compares the stored energies in the electrical and mechanical domains. Although more complex methods can be used to calculate the treatment shown below provides an intuitive understanding of the principles discussed herein. Using the Belincourt relation, the electromechanical coupling coefficient is expressed according to Equation (2):

[0033]

[0034] where U m is the energy stored in the piezoelectric layer through the interchange between the electrical and mechanical domains, and is expressed according to Equation (3):

[0035]

[0036] U e is the mechanical energy stored together with the piezoelectric layer, and is expressed according to Equation (4):

[0037]

[0038] And U d is the electrical energy stored in the dielectric, and is expressed according to Equation (5):

[0039] U d = ∈E 2 d (5)

[0040] where e(z) is the piezoelectric constant, E is the electric field, and c p and ∈ are the electrical constant and dielectric constant of the piezoelectric layer. For a piezoelectric film such as aluminum nitride, e(z) is e 33 , which is the piezoelectric constant along the c-axis. Substituting the equation of u(z) in Equation (1) into Equation (2), the electromechanical coupling coefficient of the fundamental mode BAW resonator 10 can be expressed according to Equation (6):

[0041]

[0042] And the electromechanical coupling coefficient of the second overtone BAW resonator 10 can be expressed according to Equation (7):

[0043]

[0044] The relationship between the electromechanical coupling coefficients of the fundamental mode BAW resonator 10 and the second overtone BAW resonator 22 can be expressed according to Equation (8):

[0045]

[0046] Equation (8) shows that the second overmode BAW resonator 22 always has a lower electromechanical coupling coefficient than the fundamental mode BAW resonator 10. For most applications, typical values of m are between 0.2 and 0.3, in which case the electromechanical coupling coefficient of the second overmode BAW resonator is only 80% to 90% of the electromechanical coupling coefficient of the fundamental mode BAW resonator 10.

[0047] To improve the electromechanical coupling coefficient of the second overmode BAW resonator 22, a coupler layer 36 is added between the first piezoelectric layer 24 and the second piezoelectric layer 26, as Figure 5 shown. The thickness of the coupler layer is As described below, the coupler layer 36 provides a desired acoustic impedance between the first piezoelectric layer 24 and the second piezoelectric layer 26 to increase the integral of the stress distribution of the second overmode BAW resonator 22, thereby improving the electromechanical coupling coefficient of the second overmode BAW resonator. In an alternative embodiment, the coupler layer can be used for two purposes: (i) enhancing the effective electromechanical coupling through its acoustic impedance, and (ii) acting as a layer that causes a polarity inversion of the second piezoelectric layer deposited on top of it.

[0048] Equation (1) can be rewritten to represent the displacement distribution u(z) of the second overmode BAW resonator 22 including the coupler layer 36, as shown in Equation (9):

[0049]

[0050] where k p is the wave vector in the first piezoelectric layer 24 and the second piezoelectric layer 26, and k c is the wave vector in the coupler layer 36. At , using the displacement and stress continuous boundary conditions, Equation (10) is obtained:

[0051]

[0052] where Z p is the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26, and Z c is the acoustic impedance of the coupler layer 36. The electromechanical energy U m of the second overmode BAW resonator 22 in the absence of the coupler layer 36 is expressed according to Equation (11):

[0053]

[0054] and the electromechanical energy U m of the second overmode BAW resonator 22 in the presence of the coupler layer 36 is expressed according to Equation (12):

[0055]

[0056] Although due to the complex relationship between k c and k p it is difficult to obtain a closed-form solution for the elastic energy U e numerical methods can be used to solve equations (9) through (12) to obtain the modal vibration mode u(z) and the stress distribution σ(z). Using u(z), equations (2) through (5) can be used to calculate the electromechanical coupling to show that the coupler layer 36 increases the electromechanical coupling coefficient of the second overmode BAW resonator 22.

[0057] Figure 6 The stress distribution (solid line 38) of the second overmode BAW resonator 22 including the coupler layer 36 and the displacement distribution (dashed line 40) of the second overmode BAW resonator 22 in the absence of the coupler layer 36 are shown. As shown, the coupler layer 36 forces additional acoustic energy into the first piezoelectric layer 24 and the second piezoelectric layer 26, thereby increasing the integral of the stress distribution and thus increasing the electromechanical coupling coefficient. Although not shown, those skilled in the art will understand that the stress distribution may become steeper at the contact surface between the first piezoelectric layer 24 and the first electrode 28 and at the contact surface between the second piezoelectric layer 26 and the second electrode 30.

[0058] The relationship between the acoustic impedance (Z c ) of the coupler layer 36 and the acoustic impedance (Z p ) of the first piezoelectric layer 24 and the second piezoelectric layer 26 can change the electromechanical coupling coefficient of the second overmode BAW resonator 22. In various embodiments, when the acoustic impedance of the coupler layer 36 divided by the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26 (Z c / Z p ) is greater than 1.0, greater than 1.5, greater than 2.0, and greater than 3.0, a desired electromechanical coupling coefficient can be achieved. The acoustic impedance of the coupler layer 36 divided by the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26 (Z c / Z p ) can be restricted by the available acoustic impedance of the material of the coupler layer 36, and thus in various embodiments, it can be less than 10.0, less than 8.0, and less than 6.0. In addition, the relationship between the thickness of the coupler layer 36 and the thickness (t) of the first electrode 28 and the second electrode 30 can also change the electromechanical coupling coefficient. When the thickness of the coupler layer 36 divided by the thickness of the first electrode 28 and the second electrode 30 is between 0.1 and 0.4, and more specifically between 0.1 and 0.2, between 0.1 and 0.3, between 0.2 and 0.3, between 0.2 and 0.4, and between 0.3 and 0.4, a desired electromechanical coupling coefficient can be achieved. Figure 7is a graph showing these relationships. Specifically, Figure 7 shows on the x-axis the thickness of the coupler layer 36 divided by the thicknesses of the first electrode 28 and the second electrode 30 and on the y-axis shows the improvement ratio of the electromechanical coupling coefficient of the second overmode BAW resonator 22 in the presence of the coupler layer 36 relative to in the absence of the coupler layer 36 (i.e., the electromechanical coupling coefficient of the second overmode BAW resonator 22 divided by the electromechanical coupling coefficient of the second overmode BAW resonator 22 in the absence of the coupler layer 36 ). Each line in the graph shows a specific relationship between the acoustic impedance of the coupler layer 36 and the acoustic impedances of the first piezoelectric layer 24 and the second piezoelectric layer 26 (Z c / Z p ).

[0059] In one embodiment, the coupler layer 36 is a metal layer. The coupler layer 36 can be aluminum (Al), molybdenum (Mo), tungsten (W), or osmium (Os). Depending on the material selected for the coupler layer 36, the thickness of the coupler layer 36 can be modified to provide a desired acoustic response to increase the electromechanical coupling coefficient of the second overmode BAW resonator 22. For a coupler layer 36 with a thickness of 60 nm and first and second piezoelectric layers 24 and 26 of aluminum nitride (AlN) with a thickness of 700 nm, the acoustic impedance of the coupler layer 36 divided by the acoustic impedances of the first piezoelectric layer 24 and the second piezoelectric layer 26 (Z c / Z p)For aluminum (Al) coupler layer 36 it is 0.50, for molybdenum (Mo) coupler layer 36 it is 1.90, for tungsten (W) coupler layer 36 it is 2.75, and for osmium (Os) coupler layer 36 it is 3.25. In various embodiments, the thickness of the first piezoelectric layer 24 and the second piezoelectric layer 26 can be from 350 nm to 1050 nm, the thickness of the first electrode 28 and the second electrode 30 can be between 100 nm and 300 nm, and the thickness of the coupler layer 36 can be between 30 nm and 90 nm. The thickness of the coupler layer 36 can include any sub-range within this range such that, in various embodiments, the thickness of the coupler layer 36 can be between 30 nm and 40 nm, between 30 nm and 50 nm, between 30 nm and 60 nm, between 30 nm and 70 nm, between 30 nm and 80 nm, between 40 nm and 50 nm, between 40 nm and 60 nm, between 40 nm and 70 nm, between 40 nm and 80 nm, between 40 nm and 90 nm, between 50 nm and 60 nm, between 50 nm and 70 nm, between 50 nm and 80 nm, between 50 nm and 90 nm, between 60 nm and 70 nm, between 60 nm and 80 nm, between 60 nm and 90 nm, between 70 nm and 80 nm, between 70 nm and 90 nm, and between 80 nm and 90 nm. Due to the excitation of the second-order mode in the second overmode BAW resonator 22, the device can provide a resonance frequency greater than about 3.0 GHz. Thus, the second overmode BAW resonator 22 can be very useful in high-frequency applications.

[0060] The second overmode BAW resonator 22 can be a solidly mounted resonator (SMR), such as Figure 8 shown. In this embodiment, the second overmode BAW resonator 22 is disposed on a substrate 42, and a plurality of insertion layers 44 are disposed above the substrate 42 such that the insertion layers 44 are located between the second overmode BAW resonator 22 and the substrate 42. The insertion layers 44 can include a plurality of alternating layers of high acoustic impedance materials and low acoustic impedance materials. Those skilled in the art will readily understand the details of the substrate 42 and the insertion layers 44, and thus will not be discussed here.

[0061] The second overmode BAW resonator 22 can also be a thin film bulk acoustic resonator (FBAR), such as Figure 9 shown. In this embodiment, the second overmode BAW resonator 22 is disposed on a support layer 46 and is suspended above a cavity 48 by a substrate 50. Those skilled in the art will readily understand the details of the support layer 46, the cavity 48, and the substrate 50, and thus will not be discussed here.

[0062] Although not shown, the second overmode BAW resonator 22 can be used in any number of different support structures to form any number of different circuit topologies. In various embodiments, the second overmode BAW resonator 22 can be electrically or acoustically coupled to one or more other resonators or components to form a filtering circuit, such as a duplexer, multiplexer, etc. Those skilled in the art will readily understand the details of these structures and thus they will not be discussed here.

[0063] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered to be within the scope of the concepts disclosed herein and the appended claims.

Claims

1. A second-order mode bulk acoustic wave (BAW) resonator, comprising: A first piezoelectric layer having a first polarity; A second piezoelectric layer having a second polarity, wherein the first polarity and the second polarity have opposite signs of piezoelectric constants; A coupler layer between the first piezoelectric layer and the second piezoelectric layer; A first electrode on the first piezoelectric layer opposite to the coupler layer; And A second electrode on the second piezoelectric layer opposite to the coupler layer; Wherein the first polarity and the second polarity having opposite signs of piezoelectric constants configure the second-order mode BAW resonator to excite a second-order mode through the first piezoelectric layer, the coupler layer, and the second piezoelectric layer.

2. The second-order mode BAW resonator according to claim 1, wherein: The coupler layer has a first acoustic impedance; and The first piezoelectric layer and the second piezoelectric layer have a second acoustic impedance such that the first acoustic impedance divided by the second acoustic impedance is greater than 1.

0.

3. The second-order mode BAW resonator according to claim 2, wherein the first acoustic impedance divided by the second acoustic impedance is greater than 1.

5.

4. The second-order mode BAW resonator according to claim 2, wherein the first acoustic impedance divided by the second acoustic impedance is greater than 2.

0.

5. The second-order mode BAW resonator according to claim 2, wherein the first acoustic impedance divided by the second acoustic impedance is greater than 3.

0.

6. The second-order mode BAW resonator according to claim 2, wherein the coupler layer is a metal layer.

7. The second-order mode BAW resonator according to claim 6, wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

8. The second-order mode BAW resonator according to claim 7, wherein the first electrode and the second electrode are tungsten.

9. The second-order mode BAW resonator according to claim 6, wherein the coupler layer is one of molybdenum, tungsten, and osmium.

10. The second-order mode BAW resonator according to claim 9, wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

11. The second-order mode BAW resonator according to claim 10, wherein the first electrode and the second electrode are tungsten.

12. The second-order mode BAW resonator according to claim 2, wherein: The thicknesses of the first piezoelectric layer and the second piezoelectric layer are between 350 nm and 1050 nm; and The thickness of the coupler layer is between 30 nm and 120 nm.

13. The second-order mode BAW resonator according to claim 2, wherein: The coupler layer has a first thickness; and The first electrode and the second electrode have a second thickness such that the first thickness divided by the second thickness is between 0.1 and 0.

4.

14. The second-order mode BAW resonator according to claim 13, wherein: The thicknesses of the first piezoelectric layer and the second piezoelectric layer are between 350 nm and 1050 nm; The first thickness of the coupler layer is between 30 nm and 120 nm; and The second thickness of the first electrode and the second electrode is between 100 nm and 300 nm.

15. The second overmode BAW resonator according to claim 14, wherein: The first piezoelectric layer and the second piezoelectric layer are aluminum nitride; The coupler layer is one of molybdenum, tungsten, and osmium; and The first electrode and the second electrode are tungsten.

16. The second overmode BAW resonator according to claim 13, wherein the first thickness divided by the second thickness is between 0.2 and 0.

3.

17. The second overmode BAW resonator according to claim 16, wherein: The thickness of the first piezoelectric layer and the second piezoelectric layer is between 350 nm and 1050 nm; The first thickness of the coupler layer is between 30 nm and 120 nm; and The second thickness of the first electrode and the second electrode is between 100 nm and 300 nm.

18. The second overmode BAW resonator according to claim 17, wherein: The first piezoelectric layer and the second piezoelectric layer are aluminum nitride; The coupler layer is one of molybdenum, tungsten, and osmium; and The first electrode and the second electrode are tungsten.

19. The second overmode BAW resonator according to claim 1, wherein the coupler layer provides an acoustic impedance between the first piezoelectric layer and the second piezoelectric layer to provide an increased integral of the stress distribution of the second overmode BAW resonator.

20. The second overmode BAW resonator according to claim 19, wherein the acoustic impedance provided by the coupler layer provides enhanced effective electromechanical coupling of the second overmode BAW resonator.

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