Bulk acoustic wave (BAW) resonator

Abstract

To provide a bulk acoustic wave (BAW) resonator capable of operating in higher order modes with enhanced piezoelectric coupling.SOLUTION: A second overmode bulk acoustic wave (BAW) resonator 22 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.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] Cross - reference to Related Applications This application claims the benefit of Provisional Patent Application No. 62 / 649,343, filed Mar. 28, 2018, the disclosure of which is hereby incorporated by reference in its entirety.

[0002] The present disclosure relates to a bulk acoustic wave (BAW) resonator capable of operating in a higher - order mode with improved piezoelectric coupling.

Background Art

[0003] FIG. 1 shows a fundamental - mode bulk acoustic wave (BAW) resonator 10. 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 with respect to the 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.

[0004] FIG. 2 shows the stress profile (solid line 18) and displacement profile (dashed line 20) of the fundamental - mode BAW resonator 10. As shown, the fundamental - mode BAW resonator 10 operates in a fundamental mode where a stress profile that matches half of the wavelength of a sine curve fits within the thickness of the piezoelectric layer 12. The effective electromechanical coupling (k 2 eff ) of the fundamental - mode BAW resonator 10 depends on the entire stress profile across the thickness of the piezoelectric layer 12. Generally, a higher electromechanical coupling coefficient is desirable. Although not shown, those skilled in the art will recognize that the stress profile can have a steep gradient at the contact interfaces between the piezoelectric layer 12, the first electrode 14, and the second electrode 16.

[0005] As a possibility of exciting higher-order modes of a BAW resonator, when a conventional fundamental-mode BAW resonator is used while maintaining a reasonable quality factor, size, and electrode thickness, a filter that can operate at a higher frequency is expected. In the higher-order mode, a stress profile that coincides with an integer multiple of the fundamental-mode frequency is excited within the piezoelectric layer. For example, in the second mode, which is also referred to herein as the second overtone mode, the full wavelength of the sine curve (twice the fundamental-mode frequency) is adapted within the thickness range of the piezoelectric layer.

[0006] Although a BAW resonator capable of operating in a higher-order mode is desirable for improving performance at high frequencies, attempts to create such a device have provided significantly insufficient electromechanical coupling compared to fundamental-mode devices such as the fundamental-mode BAW resonator 10 described above. Therefore, there is a need for a BAW resonator capable of operating in a higher-order mode such as the second mode with improved electromechanical coupling.

SUMMARY OF THE INVENTION

[0007] 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 on the opposite side of 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 on the opposite side of the coupler layer. The second electrode is on the second piezoelectric layer on the opposite side of the coupler layer. Providing a coupler layer between the first piezoelectric layer and the second piezoelectric layer increases the electromechanical coupling coefficient of the acoustic resonator, thereby improving its performance.

[0008] Those skilled in the art will recognize the scope of the present disclosure and understand its additional aspects after reading the following detailed description of the preferred embodiments related to the accompanying drawings.

[0009] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate some aspects of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]

Figure 1

Figure 2

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Figure 5

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Figure 8

Figure 9

DETAILED DESCRIPTION OF THE INVENTION

[0011] The embodiments described below represent the necessary information that enables those skilled in the art to practice the embodiments and to show the best mode of practicing the embodiments. Reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the present disclosure and, in particular, will recognize the application of these concepts not addressed herein. It should be understood that these concepts and their applications are within the scope of the present disclosure and within the scope of the appended claims.

[0012] The terms first, second, etc. may be used herein to describe various elements, but it will be understood that 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 could be named a second element, and similarly, a second element could 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.

[0013] When an element such as a layer, region, or substrate is said to be "on" or to extend "onto" another element, it will be understood that it is directly on or could directly extend onto the other element, or intervening elements may also be present. In contrast, when an element is said to be "directly on" or to directly extend "onto" another element, no intervening elements are present. Similarly, when an element such as a layer, region, or substrate is said to be "across" or to extend "across" another element, it will be understood that it is directly across or could directly extend across the other element, or intervening elements may also be present. In contrast, when an element is said to be "directly across" or to directly extend "across" another element, no intervening elements are present. Also, when an element is "connected" or "coupled" to another element, it will be understood that it could be directly connected or directly coupled to the other element, or intervening elements may be present. In contrast, when an element is said to be "directly connected" or "directly coupled" to another element, no intervening elements are present.

[0014] Relative terms such as "below" or "above", or "upper" or "lower", or "horizontal" or "vertical", etc. are with respect to another element, layer, or region as shown in the figures It may be used in this specification to describe the relationship of one element, layer, or region to another. It will be understood that these terms, as well as those described above, are intended to encompass different orientations of the device in addition to the orientations depicted in the figures.

[0015] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly dictates otherwise. Furthermore, the terms "comprises", "comprising", "includes", and / or "including", when used herein, 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.

[0016] Unless defined otherwise, 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. Further, the terms used herein should be interpreted as having a meaning that is consistent with the meaning in the context of this specification and the relevant prior art, and thus should not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0017] Figure 3 shows a second overtone bulk acoustic wave (BAW) resonator 22 according to an embodiment of the present disclosure. The second overtone 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 on the opposite side of the second piezoelectric layer 26, and a second electrode 30 on the second piezoelectric layer 26 on the opposite side of 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 with respect to the 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.

[0018] In particular, the first piezoelectric layer 24 has a first polarity, while the second piezoelectric layer 26 has a second polarity on the opposite side of the first polarity. This enables the second overtone BAW resonator 22 to excite the second mode, as will be described below. In one embodiment, the first piezoelectric layer 24 and the second piezoelectric layer 26 are aluminum nitride (AlN) layers having opposite polarities. For example, the first piezoelectric layer 24 can be a nitrogen-polar layer of aluminum nitride (c-AlN), while the second piezoelectric layer 26 can be an aluminum-polar layer of aluminum nitride (f-AlN). The aluminum nitride can be undoped or 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 change one or more operating parameters of the second overtone BAW resonator 22 and to provide specific electrical and / or acoustic characteristics.

[0019] Figure 4 shows the stress profile (solid line 32) and displacement profile (dashed line 34) of the second overmode BAW resonator 22. As shown, the BAW resonator operates in the second mode (also referred to herein as the second overmode), and a stress profile that matches the full wavelength of the sine curve fits within the thickness range of the combination of the first piezoelectric layer 24 and the second piezoelectric layer 26. Although not shown, one skilled in the art will recognize that the stress profile can have a steep gradient at the contact interface between the first piezoelectric layer 24 and the first electrode 28, and at the contact interface between the second piezoelectric layer 26 and the second electrode 30.

[0020] As described above, one problem faced by a BAW resonator when attempting to excite a higher-order mode is that the BAW resonator generally has an electromechanical coupling coefficient (k 2 eff ) that is smaller than some of those in its fundamental mode. This is shown by the comparison between the fundamental mode BAW resonator 10 and the second overmode BAW resonator 22 described in the above "Background Art". Assuming that both the electrodes of the fundamental mode BAW resonator 10 and the second overmode BAW resonator 22 have the same material properties as the piezoelectric layer(s) therein, the displacement profile u(z) of the device can be represented by Equation (1).

Equation

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[0021] To improve the electromechanical coupling coefficient of the second overtone mode BAW resonator 22, the coupler layer 36 is added between the first piezoelectric layer 24 and the second piezoelectric layer 26, as shown in FIG. 5. The coupler layer has a thickness of 2l. As will be explained below, the coupler layer 36 provides a desired acoustic impedance between the first piezoelectric layer 24 and the second piezoelectric layer 26 in order to increase the overall stress profile of the second overtone mode BAW resonator 22, thereby improving its electromechanical coupling coefficient. In an alternative embodiment, the coupler layer can serve the role of (i) improving the effective electromechanical coupling by its acoustic impedance and (ii) causing a polarity inversion for the second piezoelectric layer deposited thereon.

[0022] Equation (1) can be rewritten to represent the displacement profile u(z) of the second overtone mode BAW resonator 22 including the coupler layer 36, as expressed by Equation (9).

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[0023] FIG. 6 shows the stress profile (solid line 38) of the second overtone BAW resonator 22 including the coupler layer 36 and the displacement profile (dashed line 40) of the second overtone BAW resonator 22 without the coupler layer 36. As shown, the coupler layer 36 pushes additional acoustic energy into the first piezoelectric layer 24 and the second piezoelectric layer 26, increasing the overall stress profile and, thus, the electromechanical coupling coefficient. Although not shown, those skilled in the art will recognize that the stress profile can have a steep gradient at the contact interfaces of the first piezoelectric layer 24 and the first electrode 28 and the contact interfaces of the second piezoelectric layer 26 and the second electrode 30.

[0024] 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 overtone BAW resonator 22. The value (Z c / Z p ) obtained by dividing the acoustic impedance of the coupler layer 36 by the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26 can reach a desirable electromechanical coupling coefficient when it is greater than 1.0, greater than 1.5, greater than 2.0, and greater than 3.0 in various embodiments. The value (Z c / Z p) may be constrained by the acoustic impedance available for the material of the coupler layer 36, and thus may be less than 10.0, less than 8.0, or less than 6.0, in various embodiments. 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 modify the electromechanical coupling coefficient. Desired electromechanical coupling coefficients may be achieved when the value (2l / t) of 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, more specifically, between 0.1 and 0.2, 0.1 and 0.3, 0.2 and 0.3, 0.2 and 0.4, and 0.3 and 0.4. FIG. 7 is a graph illustrating these relationships. Specifically, FIG. 7 plots the x-axis (the thickness (2l / t) of the coupler layer 36 divided by the thickness of the first electrode 28 and the second electrode 30) and the y-axis (the thickness without the coupler layer 36). Improved ratio of the electromechanical coupling coefficient for the second overmoded BAW resonator 22 with the coupler layer 36 to that of the first overmoded BAW resonator 22 (i.e., the electromechanical coupling coefficient (k 2 eff,c ) is the electromechanical coupling coefficient (k 2 eff,nc ) divided by ). Each line on the graph represents a particular relationship (Z c / Z p ) is shown.

[0025] 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 in order to increase the electromechanical coupling coefficient of the second overtone BAW resonator 22. For a coupler layer 36 with a thickness of 60 nm, and a first piezoelectric layer 24 and a second piezoelectric layer 26 of aluminum nitride (AlN) with a thickness of 700 nm, the value (Z c / Z p ) obtained by dividing the acoustic impedance of the coupler layer 36 by the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26 is 0.50 for the aluminum (Al) coupler layer 36, 1.90 for the molybdenum (Mo) coupler layer 36, 2.75 for the tungsten (W) coupler layer 36, and 3.25 for the osmium (Os) coupler layer 36. 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 from 100 nm to 300 nm, and the thickness of the coupler layer 36 can be from 30 nm to 90 nm. The thickness of the coupler layer 36 can include any partial range within this range. At this time, the thickness of the coupler layer 36 can be, in various embodiments, 30 nm to 40 nm, 30 nm to 50 nm, 30 nm to 60 nm, 30 nm to 70 nm, 30 nm to 80 nm, 40 nm to 50 nm, 40 nm to 60 nm, 40 nm to 70 nm, 40 nm to 80 nm, 40 nm to 90 nm, 50 nm to 60 nm, 50 nm to 70 nm, 50 nm to 80 nm, 50 nm to 90 nm, 60 nm to 70 nm, 60 nm to 80 nm, 60 nm to 90 nm, 70 nm to 80 nm, 70 nm to 90 nm, and 80 nm to 90 nm. Due to the excitation of the second mode of the second overtone BAW resonator 22, the device can provide a resonance frequency greater than about 3.0 GHz. Therefore, the second overtone BAW resonator 22 can be very useful in high-frequency applications.

[0026] The second overtone BAW resonator 22 can be a solidly mounted resonator (SMR), as shown in FIG. 8. In such an embodiment, the second overtone BAW resonator 22 is provided on a substrate 42 such that several intervening layers 44 are positioned between the second overtone BAW resonator 22 and the substrate 42. The intervening layer 44 can include several alternating layers of high and low acoustic impedance materials. Details of the substrate 42 and the intervening layer 44 will be readily appreciated by those skilled in the art and thus will not be discussed in detail herein.

[0027] The second overtone BAW resonator 22 can also be a thin film bulk acoustic resonator (FBAR), as shown in FIG. 9. In such an embodiment, the second overtone BAW resonator 22 is provided on a support layer 46 and is floated over an air cavity 48 by a substrate 50. Details of the support layer 46, the air cavity 48, and the substrate 50 will be readily appreciated by those skilled in the art and thus will not be discussed in detail herein.

[0028] Although not shown, the second overtone BAW resonator 22 can be used with any number of different support structures to create any number of different circuit topologies. In various embodiments, the second overtone BAW resonator 22 can be coupled either electrically or acoustically with one or more other resonators or components that form filtering circuits such as duplexers, multiplexers, etc. Details of these structures will be readily appreciated by those skilled in the art and thus will not be discussed in detail herein. The details of these structures will be readily appreciated by those skilled in the art and thus will not be discussed in detail herein.

[0029] 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 scope of the following claims.

Claims

1. An acoustic resonator, comprising: a first piezoelectric layer having a first polarity; a second piezoelectric layer having a second polarity opposite to the first polarity; a coupler layer between the first piezoelectric layer and the second piezoelectric layer; a first electrode on the first piezoelectric layer on the opposite side of the coupler layer; a second electrode on the second piezoelectric layer on the opposite side of the coupler layer; The acoustic resonator.

2. The coupler layer has a first acoustic impedance, 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. The acoustic resonator according to claim 1.

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

5.

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

0.

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

0.

6. The acoustic resonator according to claim 2, wherein the coupler layer is a metal layer.

7. The acoustic resonator according to claim 6, wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

8. The acoustic resonator according to claim 7, wherein the first electrode and the second electrode are tungsten.

9. The acoustic resonator according to claim 6, wherein the coupler layer is one of molybdenum, tungsten, and osmium.

10. The acoustic resonator according to claim 9, wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

11. The acoustic resonator according to claim 10, wherein the first electrode and the second electrode are tungsten.

12. The thickness of the first piezoelectric layer and the second piezoelectric layer is 350 nm to 1050 nm, The thickness of the coupler layer is 30 nm to 120 nm. The acoustic resonator according to claim 2.

13. The coupler layer has a first thickness, The first electrode and the second electrode have a second thickness such that the first thickness divided by the second thickness is 0.1 to 0.

4. The acoustic resonator according to claim 2.

14. The thicknesses of the first piezoelectric layer and the second piezoelectric layer are from 350 nm to 1050 nm, the thickness of the coupler layer is from 30 nm to 120 nm, and the thicknesses of the first electrode and the second electrode are from 100 nm to 300 nm. The acoustic resonator according to claim 13.

15. 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. The acoustic resonator according to claim 14.

16. The first thickness divided by the second thickness is from 0.2 to 0.

3. The acoustic resonator according to claim 13.

17. The thicknesses of the first piezoelectric layer and the second piezoelectric layer are from 350 nm to 1050 nm, the thickness of the coupler layer is from 30 nm to 120 nm, and the thicknesses of the first electrode and the second electrode are from 100 nm to 300 nm. The acoustic resonator according to claim 16.

18. 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. The acoustic resonator according to claim 17.

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