Acoustic resonator

Abstract

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 to the coupler layer on the first piezoelectric layer. The second electrode is opposite to the coupler layer on the second piezoelectric layer.

Description

[0001] Related Applications

[0002] This application claims the benefit of provisional patent application serial number 62 / 649,343 filed on March 28, 2018, the disclosure of which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present disclosure relates to bulk acoustic wave (BAW) resonators capable of operating in higher order modes with enhanced piezoelectric coupling. Background Art

[0004] 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 thickness 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.

[0005] 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 the fundamental mode, where the stress distribution corresponding to half the wavelength of the sine wave is adapted to 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 appreciate that the stress distribution may become steeper at the interface between the piezoelectric layer 12, the first electrode 14, and the second electrode 16.

[0006] The possibility of exciting higher-order modes in a BAW resonator allows the filter to operate at higher frequencies than would be the case with a conventional fundamental-mode BAW resonator, while maintaining reasonable quality factors, dimensions, and electrode thicknesses. In higher-order modes, stress distributions corresponding to integer multiples of the fundamental-mode frequency are excited in the piezoelectric layer. For example, in the second-order mode (also referred to herein as the second overmode), the entire wavelength of the sine wave (twice the fundamental-mode frequency) fits into the thickness of the piezoelectric layer.

[0007] While BAW resonators capable of operating in higher order modes allow for improved performance at high frequencies, attempts to fabricate such devices provide very poor electromechanical coupling compared to fundamental mode devices such as the fundamental mode BAW resonator 10 discussed above. Therefore, there is a need for BAW resonators capable of operating in higher order modes, such as the second order mode, with improved electromechanical coupling. Summary of the invention

[0008] In one embodiment, the 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 to the coupler layer on the first piezoelectric layer. The second electrode is opposite to the coupler layer on the second piezoelectric 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.

[0009] Those skilled in the art will understand the scope of the present disclosure and recognize additional aspects of the present disclosure after reading the following detailed description of the preferred embodiments in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate several aspects of the disclosure, and together with the description, serve to explain the principles of the disclosure.

[0011] Figure 1 A fundamental mode bulk acoustic wave (BAW) resonator is shown.

[0012] Figure 2 The stress response and displacement distribution of the fundamental mode BAW resonator are shown.

[0013] Figure 3 A second overmode BAW resonator according to one embodiment of the present disclosure is shown.

[0014] Figure 4 Stress and displacement distributions of a second over-mode BAW resonator according to one embodiment of the present disclosure are shown.

[0015] Figure 5 A second overmode BAW resonator according to one embodiment of the present disclosure is shown.

[0016] Figure 6 Stress distributions of two second overmode BAW resonators according to various embodiments of the present disclosure are shown.

[0017] Figure 7 is a graph illustrating performance characteristics of several second overmode BAW resonators according to various embodiments of the present disclosure.

[0018] Figure 8 A second overmode BAW resonator according to one embodiment of the present disclosure is shown.

[0019] Fig. 9 A second overmode BAW resonator according to one embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0020] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments, and illustrate the best mode for practicing the embodiments. After reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the present disclosure, and will recognize the application of these concepts not specifically proposed herein. It should be understood that these concepts and applications are within the scope of the present disclosure and the appended claims.

[0021] It should be understood that although the terms first, second, etc. may be used in this article to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. For example, without departing from the scope of the present disclosure, the first element may be named as the second element, and similarly, the second element may be named as the first element. As used herein, the term "and / or" includes any and all combinations of one or more of the related listed items.

[0022] It should be understood that when an element such as a layer, region, or substrate is referred to as being "on" or extending "on" another element, it may be directly on or directly extending to another element, or there may also be an intervening element. In contrast, when an element is referred to as "directly on" another element or extending "directly" to another element, there is no intervening element. Similarly, it should be understood that when an element such as a layer, region, or substrate is referred to as being "above" another element or extending "above another element", it may be directly above or directly above another element, or there may also be an intervening element. In contrast, when an element is referred to as extending "directly above" or "directly above another element", there is no intervening element. 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 another element, or there may be an intervening element. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there is no intervening element.

[0023] Relative terms such as "below" or "above" or "up" or "lower" or "horizontal" or "vertical" may be used herein to describe the relationship of one element, layer or region to another element, layer or region illustrated in the figures. 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.

[0024] The terms used herein are only used for the purpose of describing specific embodiments, and are not intended to limit the present disclosure. As used herein, unless the context clearly indicates otherwise, the singular forms "one" and "said" are intended to include plural forms as well. It should also be understood that when used herein, the terms "include", "comprise", "contain" indicate the presence of the features, wholes, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, parts and / or their groups.

[0025] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those commonly understood by those skilled in the art to which the present disclosure belongs. It should also be understood that the terms used herein should be interpreted as having the same meaning as they have in the context of this specification and the relevant art, and should not be interpreted in an idealized or overly formal sense, unless explicitly defined in this article.

[0026] Figure 3 A second over-molded bulk acoustic wave (BAW) resonator 22 according to one embodiment of the present disclosure is shown. The second over-molded 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 opposite the second piezoelectric layer 26, and a second electrode 30 on the second piezoelectric layer 26 opposite the first piezoelectric layer 24. The thickness 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.

[0027] It is noteworthy 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 over-mode 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 (c-AlN) of aluminum nitride, and the second piezoelectric layer 26 can be an aluminum polarity layer (f-AlN) of aluminum nitride. 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 thickness (d′) of the first and second piezoelectric layers 24 and 26 and the thickness (t) of the first and second electrodes 28 and 30 may be selected to provide specific electrical and / or acoustic properties in order to alter one or more operating parameters of the second overmode BAW resonator 22 .

[0028] 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 thickness of the combination 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 interface of the first piezoelectric layer 24 and the first electrode 28 and at the interface of the second piezoelectric layer 26 and the second electrode 30.

[0029] As mentioned above, one problem faced by BAW resonators attempting to excite higher-order modes is that they typically have lower electromechanical coupling coefficients than their fundamental-mode counterparts. This is illustrated by comparing the fundamental mode BAW resonator 10 discussed above in the background with the second overmode BAW resonator 22. 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):

[0030]

[0031] 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, so that for the fundamental mode BAW resonator 10, k p =π / 2(d+t), and for the second over-mode 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 energy in the electrical and mechanical domains. Although more sophisticated methods can be used to calculate However, the treatment shown below provides an intuitive understanding of the principles discussed in this article. Using the Belincourt relation, the electromechanical coupling coefficient According to equation (2):

[0032]

[0033] Among them U m is the energy stored in the piezoelectric layer through the exchange between the electrical and mechanical domains and is expressed according to equation (3):

[0034]

[0035] U e is the mechanical energy stored with the piezoelectric layer and is expressed according to equation (4):

[0036]

[0037] And U d is the electrical energy stored in the dielectric and is expressed according to equation (5):

[0038] U d =∈E 2 d (5)

[0039] 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):

[0040]

[0041] And the electromechanical coupling coefficient of the second over-mode BAW resonator 10 can be expressed according to equation (7):

[0042]

[0043] The relationship between the electromechanical coupling coefficients of the fundamental mode BAW resonator 10 and the second overmode BAW resonator 22 can be expressed according to equation (8):

[0044]

[0045] 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 that of the fundamental mode BAW resonator 10.

[0046] In order to improve the electromechanical coupling coefficient of the second over-mode BAW resonator 22, a coupler layer 36 is added between the first piezoelectric layer 24 and the second piezoelectric layer 26, such as Figure 5 As shown. The thickness of the coupler layer is 21. 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 over-mode BAW resonator 22, thereby improving the electromechanical coupling coefficient of the second over-mode BAW resonator. In alternative embodiments, the coupler layer can serve two purposes: (i) to enhance effective electromechanical coupling through its acoustic impedance, and (ii) to act as a layer that causes polarity reversal of the second piezoelectric layer deposited on top of it.

[0047] 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):

[0048]

[0049] 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. When z = ±l, using displacement and stress continuity boundary conditions, equation (10) is obtained:

[0050] Z p tan k p l=Z c tan k c l (10)

[0051] 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 of the second overmode BAW resonator 22 in the absence of the coupler layer 36 m According to equation (11):

[0052]

[0053] And the electromechanical energy U of the second overmode BAW resonator 22 in the presence of the coupler layer 36 m According to equation (12):

[0054]

[0055] Although due to k c and k p The complex relationship between elastic energy U e A closed-form solution is difficult, but numerical methods can be used to solve equations (9) to (12) to obtain the modal vibration shape u(z) and the stress distribution σ(z). Using u(z), the electromechanical coupling can be calculated using equations (2) to (5) to show that the coupler layer 36 increases the electromechanical coupling coefficient of the second over-mode BAW resonator 22.

[0056] Figure 6The stress distribution (solid line 38) of the second over-mode BAW resonator 22 including the coupler layer 36 and the displacement distribution (dashed line 40) of the second over-mode 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 interface of the first piezoelectric layer 24 and the first electrode 28 and at the interface of the second piezoelectric layer 26 and the second electrode 30.

[0057] The acoustic impedance (Z c ) and the acoustic impedance (Z p ) can change the electromechanical coupling coefficient of the second over-mode BAW resonator 22. In various embodiments, when the acoustic impedance of the coupler layer 36 is 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, the desired electromechanical coupling coefficient can be achieved. The acoustic impedance of the coupler layer 36 is divided by the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26 (Z c / Z p ) may be constrained by the acoustic impedance available from the material of the coupler layer 36, and thus, in various embodiments, may be less than 10.0, less than 8.0, and less than 6.0. In addition, the relationship between the thickness (2l) of the coupler layer 36 and the thickness (t) of the first electrode 28 and the second electrode 30 may also change the electromechanical coupling coefficient. The desired electromechanical coupling coefficient may be achieved when the thickness of the coupler layer 36 divided by the thickness of the first electrode 28 and the second electrode 30 (2l / t) 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. Figure 7 is a graph showing these relationships. Specifically, Figure 7 is a graph showing the thickness of the coupler layer 36 divided by the thickness of the first electrode 28 and the second electrode 30 (2l / t) on the x-axis and showing the improvement ratio of the electromechanical coupling coefficient of the second over-mode BAW resonator 22 in the presence of the coupler layer 36 relative to the absence of the coupler layer 36 on the y-axis (i.e., the electromechanical coupling coefficient of the second over-mode BAW resonator 22 in the presence of the coupler layer 36 divided by the electromechanical coupling coefficient of the second over-mode BAW resonator 22 in the absence of the coupler layer 36 Each line in the graph shows a specific relationship (Z) between the acoustic impedance of the coupler layer 36 and the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26. c / Z p ).

[0058] 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 so that the electromechanical coupling coefficient of the second over-mode BAW resonator 22 is increased. For a coupler layer 36 having a thickness of 60 nm and a first piezoelectric layer 24 and a second piezoelectric layer 26 of aluminum nitride (AlN) having a thickness of 700 nm, 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 0.50 for aluminum (Al) coupler layer 36, 1.90 for molybdenum (Mo) coupler layer 36, 2.75 for tungsten (W) coupler layer 36, and 3.25 for osmium (Os) coupler layer 36. In various embodiments, the thickness of first piezoelectric layer 24 and second piezoelectric layer 26 may be 350 nm to 1050 nm, the thickness of first electrode 28 and second electrode 30 may be between 100 nm and 300 nm, and the thickness of coupler layer 36 may be between 30 nm and 90 nm. The thickness of the coupler layer 36 can include any sub-range within this range, so that in various embodiments, the thickness of the coupler layer 36 can be between 30nm and 40nm, between 30nm and 50nm, between 30nm and 60nm, between 30nm and 70nm, between 30nm and 80nm, between 40nm and 50nm, between 40nm and 60nm, between 40nm and 70nm, between 40nm and 80nm, between 40nm and 90nm, between 50nm and 60nm, between 50nm and 70nm, between 50nm and 80nm, between 50nm and 90nm, between 60nm and 70nm, between 60nm and 80nm, between 60nm and 90nm, between 70nm and 80nm, between 70nm and 90nm, and between 80nm and 90nm. The device may provide a resonant frequency greater than about 3.0 GHz due to the excitation of the second-order mode in the second overmode BAW resonator 22. Therefore, the second overmode BAW resonator 22 may be very useful in high frequency applications.

[0059] The second over-mode BAW resonator 22 may be a solid-state mounted resonator (SMR), such as Figure 8As shown. In this embodiment, the second over-mode BAW resonator 22 is disposed on a substrate 42, and a plurality of insertion layers 44 are disposed above the substrate 42, so that the insertion layers 44 are located between the second over-mode BAW resonator 22 and the substrate 42. The insertion layers 44 may include a plurality of alternating layers of high acoustic impedance material and low acoustic impedance material. Those skilled in the art will readily understand the details of the substrate 42 and the insertion layers 44, and therefore will not be discussed here.

[0060] The second over-mode BAW resonator 22 may also be a thin film bulk acoustic resonator (FBAR), such as Fig. 9 In this embodiment, the second over-mode BAW resonator 22 is disposed on a support layer 46 and suspended above an air cavity 48 by a substrate 50. The details of the support layer 46, the air cavity 48 and the substrate 50 will be readily understood by those skilled in the art and will not be discussed here.

[0061] Although not shown, the second over-mode 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 over-mode BAW resonator 22 can be electrically or acoustically coupled with one or more other resonators or components to form a filtering circuit, such as a duplexer, a multiplexer, etc. The details of these structures will be readily understood by those skilled in the art and are therefore not discussed here.

[0062] 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 over-mode bulk acoustic wave resonator, comprising: a first piezoelectric layer, the 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, the first electrode being opposite the coupler layer on the first piezoelectric layer; as well as a second electrode on the second piezoelectric layer opposite the coupler layer; wherein the second over-mode BAW resonator is configured to excite a second-order mode through the first piezoelectric layer, the coupler layer, and the second piezoelectric layer, wherein the coupler layer has a first acoustic impedance; and The first piezoelectric layer and the second piezoelectric layer each have a second acoustic impedance, such that the first acoustic impedance divided by the second acoustic impedance is greater than 1.

0.

2. The second overmode BAW resonator of claim 1, wherein the first acoustic impedance divided by the second acoustic impedance is greater than 1.

5.

3. The second overmode BAW resonator of claim 1, wherein the first acoustic impedance divided by the second acoustic impedance is greater than 2.

0.

4. The second overmode BAW resonator of claim 1, wherein the first acoustic impedance divided by the second acoustic impedance is greater than 3.

0. The second overmode BAW resonator of claim 1 , wherein the coupler layer is a metal layer. 6 . The second over-mode BAW resonator of claim 5 , wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride. 7 . The second overmode BAW resonator of claim 6 , wherein the first electrode and the second electrode are tungsten.

8. The second overmode BAW resonator of claim 5, wherein the coupler layer is one of molybdenum, tungsten and osmium. 9 . The second over-mode BAW resonator of claim 8 , wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

10. The second overmode BAW resonator of claim 9, wherein the first electrode and the second electrode are tungsten.

11. The second overmode BAW resonator of claim 1, wherein: The thickness of the first piezoelectric layer and the second piezoelectric layer is between 350 nm and 1050 nm; and The coupler layer has a thickness between 30 nm and 120 nm.

12. The second overmode BAW resonator of claim 1, 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.

13. The second overmode BAW resonator of claim 12, wherein: The thickness of the first piezoelectric layer and the second piezoelectric layer is between 350nm and 1050nm; 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.

14. The second overmode BAW resonator of claim 13, 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.

15. The second overmode BAW resonator of claim 12, wherein the first thickness divided by the second thickness is between 0.2 and 0.

3.

16. The second overmode BAW resonator of claim 15, wherein: The thickness of the first piezoelectric layer and the second piezoelectric layer is between 350nm and 1050nm; 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.

17. The second overmode BAW resonator of claim 16, 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.

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

19. The second overmode BAW resonator of claim 18, wherein the acoustic impedance provided by the coupler layer provides enhanced effective electromechanical coupling of the second overmode BAW resonator.

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