Bulk Acoustic Wave (BAW) Resonators

Abstract

To increase an electromechanical coupling coefficient of an acoustic resonator, thereby improving its performance.SOLUTION: An acoustic resonator 22 includes a first piezoelectric layer 24, a second piezoelectric layer 26, a coupler layer 36, a first electrode 28, and a second electrode 30. The first piezoelectric layer 24 has a first polarity. The second piezoelectric layer 26 has a second polarity opposite the first polarity. The coupler layer 36 is between the first piezoelectric layer 24 and the second piezoelectric layer 26. The first electrode 28 is on the first piezoelectric layer 24 opposite the coupler layer 36. The second electrode 30 is on the second piezoelectric layer 26 opposite the coupler layer 36.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 March 28, 2018, the disclosure of which is incorporated by reference herein in its entirety.

[0002] The present disclosure relates to bulk acoustic wave (BAW) resonators capable of operating in higher order modes with improved piezoelectric coupling. [Background technology]

[0003] 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 the half-thickness of the piezoelectric layer 12 and t is the thickness of both the first electrode 14 and the second electrode 16.

[0004] 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 matching half the wavelength of a sinusoid is fitted within the thickness of the piezoelectric layer 12. The effective electromechanical coupling (k 2 eff ) depends on the overall stress profile across the thickness of the piezoelectric layer 12. Generally, a higher electromechanical coupling coefficient is desirable. Although not shown, one skilled in the art will recognize that the stress profile can be steep at the contact interfaces between the piezoelectric layer 12 and the first and second electrodes 14 and 16.

[0005] With the possibility of exciting higher order modes of the BAW resonator, we expect the filter to be able to operate at higher frequencies if a conventional fundamental mode BAW resonator is used while maintaining a reasonable quality factor, size, and electrode thickness. In the higher order modes, stress profiles that correspond 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, a full wavelength of a sinusoid (twice the fundamental mode frequency) is fitted within the thickness of the piezoelectric layer.

[0006] BAW resonators capable of operating in higher order modes offer the promise of improved performance at high frequencies, but attempts to create such devices have provided significantly poorer electromechanical coupling compared to fundamental mode devices such as the above-described fundamental mode BAW resonator 10. Thus, there is a need for a BAW resonator capable of operating in higher order modes, such as the second mode, with improved electromechanical coupling. Summary of the Invention

[0007] 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 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 its performance.

[0008] Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in conjunction with the accompanying drawing figures.

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

[0010] [Figure 1] A fundamental mode bulk acoustic wave (BAW) resonator is shown. [Diagram 2] 1 shows the stress response and displacement profile of a fundamental mode BAW resonator. [Diagram 3] 3 illustrates a second overmoded BAW resonator according to one embodiment of the present disclosure. [Figure 4] 13 illustrates stress and displacement profiles of a second overmoded BAW resonator according to one embodiment of the present disclosure. [Diagram 5] 3 illustrates a second overmoded BAW resonator according to one embodiment of the present disclosure. [Figure 6] 13 illustrates stress profiles for two second overmoded BAW resonators according to various embodiments of the present disclosure. [Figure 7] 11 is a graph illustrating performance characteristics of several second overmoded BAW resonators according to various embodiments of the present disclosure. [Figure 8] 3 illustrates a second overmoded BAW resonator according to one embodiment of the present disclosure. [Figure 9] 3 illustrates a second overmoded BAW resonator according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The embodiments described below represent the necessary information to enable those skilled in the art to practice the embodiments and to show the best mode of practicing the embodiments. Upon 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 particularly recognize applications of these concepts not addressed herein. It is to be understood that these concepts and applications are within the scope of the present disclosure and the scope of the appended claims.

[0012] It will be understood that although terms such as first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are used only to distinguish one element from another. For example, a first element may be named a second element, and similarly, a second element may be named a first element, without departing from the scope of the present disclosure. 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 extending "onto" another element, it will be understood that it may be directly on or extending directly onto the other element, or intervening elements may also be present. In contrast, when an element is said to be "directly on" or extending "directly onto" another element, there are no intervening elements present. Similarly, when an element such as a layer, region, or substrate is said to be "on" or extending "over" another element, it will be understood that it may be directly on or extending directly onto the other element, or intervening elements may also be present. In contrast, when an element is said to be "directly on" or extending "directly onto" another element, there are no intervening elements present. Also, when an element is "connected" or "coupled" to another element, it will be understood that it may be directly connected or coupled to the other element, or there may be intervening elements. In contrast, when an element is said to be "directly connected" or "directly coupled" to another element, there are no intervening elements present.

[0014] Relative terms such as "below" or "upper," or "upper" or "lower," or "horizontally" or "vertically" refer to another element, layer, or region as shown in the figures. may be used herein to describe the relationship of an element, layer, or region to another. It will be understood that these terms, and those described above, are intended to encompass different orientations of the device in addition to the orientation 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 disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, it will be understood that the terms "comprises", "comprising", "includes" and / or "including" as used herein specify the presence of stated features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0016] Unless otherwise defined, all terms used herein (including technical terms and specific terms) have the same meaning as commonly understood by those skilled in the art to which this disclosure belongs. Furthermore, it will be understood that the terms used herein should be interpreted as having a meaning consistent with the meaning associated with the present specification and the prior art, and therefore will not be interpreted in an idealized or overly formal sense unless expressly defined herein.

[0017] 3 illustrates a second overmoded bulk acoustic wave (BAW) resonator 22 according to an embodiment of the present disclosure. The second overmoded 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 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 opposite the first polarity. This allows the second overmoded 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 pole layer of aluminum nitride (c-AlN), while the second piezoelectric layer 26 can be an aluminum pole 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), and the like. 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), and the like. 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 to modify one or more operating parameters of the second overmoded BAW resonator 22.

[0019] 4 illustrates the stress profile (solid line 32) and displacement profile (dashed line 34) of the second overmoded BAW resonator 22. As shown, the BAW resonator operates in a second order mode (also referred to herein as the second overmoded) and a stress profile that matches a full wavelength of a sinusoid is met within the combined thickness of the first piezoelectric layer 24 and the second piezoelectric layer 26. Although not shown, one skilled in the art will appreciate that the stress profile is consistent with the full wavelength of the first piezoelectric layer 24. It will be appreciated that the contact interface of the first electrode 28 and the contact interface of the second piezoelectric layer 26 and the second electrode 30 may be steep.

[0020] As explained above, one problem facing BAW resonators when attempting to excite higher order modes is that the BAW resonators generally have smaller electromechanical coupling coefficients (k 2 eff ). This is illustrated by a comparison of the fundamental mode BAW resonator 10 and the second overmoded BAW resonator 22 described in the "Background" section above. Assuming that the electrodes of both the fundamental mode BAW resonator 10 and the second overmoded 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):

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[0021] To improve the electromechanical coupling coefficient of the second overmoded BAW resonator 22, a 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 2l. As explained 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 overall stress profile of the second overmoded BAW resonator 22, thereby improving its electromechanical coupling coefficient. In an alternative embodiment, the coupler layer may serve two purposes: (i) to improve the effective electromechanical coupling by its acoustic impedance, and (ii) to act as a layer that creates a polarity reversal with respect to the second piezoelectric layer deposited on top of it.

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

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[0023] 6 shows the stress profile (solid line 38) of the second overmoded BAW resonator 22 including the coupler layer 36 and the displacement profile (dashed line 40) of the second overmoded 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, one skilled in the art will recognize that the stress profile can be steeper at the contact interface of the first piezoelectric layer 24 and the first electrode 28 and the contact interface of the second piezoelectric layer 26 and the second electrode 30.

[0024] The acoustic impedance of the coupler layer 36 (Z c ) and the acoustic impedance (Z p ) can change the electromechanical coupling coefficient of the second overmoded BAW resonator 22. The value of 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 ) may be greater than 1.0, greater than 1.5, greater than 2.0, and greater than 3.0 in various embodiments. 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) 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 reached 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, 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 shows a graph of 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 the thickness of the coupler layer 36 divided by the thickness of the first electrode 28 and the second electrode 30 on the y-axis (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 with the coupler layer 36 (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 may 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 may be modified to provide a desired acoustic response to increase the electromechanical coupling coefficient of the second overmoded BAW resonator 22. 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 an aluminum (Al) coupler layer 36, 1.90 for a molybdenum (Mo) coupler layer 36, 2.75 for a tungsten (W) coupler layer 36, and 3.25 for an osmium (Os) coupler layer 36. In various embodiments, the first and second piezoelectric layers 24 and 26 may have thicknesses between 350 nm and 1050 nm, the first and second electrodes 28 and 30 may have thicknesses between 100 nm and 300 nm, and the coupler layer 36 may have a thickness between 30 nm and 90 nm. The thickness of the coupler layer 36 may include any subrange within this range, whereby the thickness of the coupler layer 36 may be 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, in various embodiments. Due to excitation of the second mode of the second overmoded BAW resonator 22, the device may provide a resonant frequency greater than about 3.0 GHz. Therefore, the second overmoded BAW resonator 22 can be very useful in high frequency applications.

[0026] The second overmoded BAW resonator 22 may be a solidly mounted resonator (SMR), as shown in Fig. 8. In such an embodiment, the second overmoded BAW resonator 22 is provided on a substrate 42, with several intervening layers 44 provided to be located between the second overmoded BAW resonator 22 and the substrate 42. The intervening layers 44 may include several alternating layers of high and low acoustic impedance materials. Details of the substrate 42 and the intervening layers 44 will be readily appreciated by those skilled in the art and therefore will not be discussed in detail herein.

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

[0028] Although not shown, the second overmoded BAW resonator 22 may be used with any number of different support structures to create any number of different circuit topologies. In various embodiments, the second overmoded BAW resonator 22 may be electrically or acoustically coupled to one or more other resonators or components to form a filtering circuit, such as a duplexer, multiplexer, etc. The details of these structures will be readily appreciated by those skilled in the art and therefore 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, and all such improvements and modifications are deemed to be within the scope of the concepts disclosed herein and the claims that follow.

Claims

1. a second overmoded bulk acoustic wave (BAW) resonator, a first piezoelectric layer having a first polarity; a second piezoelectric layer having a second polarity, the first polarity and the second polarity having 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 the coupler layer; and a second electrode on the second piezoelectric layer opposite the coupler layer; and the second overmoded BAW resonator is configured to excite a second order mode through the first piezoelectric layer, the coupler layer, and the second piezoelectric layer; the coupler layer has a first acoustic impedance, the coupler layer being a metal layer having a first thickness modified to increase an electromechanical coupling coefficient of the resonator according to a selected metal; the first piezoelectric layer and the second piezoelectric layer are aluminum nitride; the first electrode and the second electrode have a second thickness, the first thickness divided by the second thickness being between 0.1 and 0.4; A second overmoded BAW resonator, wherein the first piezoelectric layer and the second piezoelectric layer have a second acoustic impedance, and the first acoustic impedance divided by the second acoustic impedance is greater than 1.

0.

2. A second overmoded BAW resonator as described in claim 1, wherein a value obtained by dividing the first acoustic impedance by the second acoustic impedance is greater than 1.

5.

3. A second overmoded BAW resonator as described in claim 1, wherein a value obtained by dividing the first acoustic impedance by the second acoustic impedance is greater than 2.

0.

4. A second overmoded BAW resonator as described in claim 1, wherein a value obtained by dividing the first acoustic impedance by the second acoustic impedance is greater than 3.

0.

5. The second overmoded BAW resonator of claim 1 , wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

6. The second overmoded BAW resonator of claim 5 , wherein the first electrode and the second electrode are tungsten.

7. The second overmoded BAW resonator of claim 1 , wherein the coupler layer is one of molybdenum, tungsten, and osmium.

8. The second overmoded BAW resonator of claim 7 , wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

9. The second overmoded BAW resonator of claim 8 , wherein the first electrode and the second electrode are tungsten.

10. the first piezoelectric layer and the second piezoelectric layer have a thickness of 350 nm to 1050 nm; The thickness of the coupler layer is 30 nm to 120 nm.

2. A second overmoded BAW resonator according to claim 1.

11. the first piezoelectric layer and the second piezoelectric layer have a thickness of 350 nm to 1050 nm; the first thickness of the coupler layer is between 30 nm and 90 nm; The second thickness of the first electrode and the second electrode is 100 nm to 300 nm.

2. A second overmoded BAW resonator according to claim 1.

12. the coupler layer is one of molybdenum, tungsten, and osmium; the first electrode and the second electrode are tungsten; The second overmoded BAW resonator of claim 11.

13. The second overmoded BAW resonator of claim 1, wherein the value obtained by dividing the first thickness by the second thickness is between 0.2 and 0.

3.

14. the first piezoelectric layer and the second piezoelectric layer have a thickness of 350 nm to 1050 nm; the first thickness of the coupler layer is between 30 nm and 120 nm; The second overmoded BAW resonator of claim 13, wherein the second thickness of the first electrode and the second electrode is between 100 nm and 300 nm.

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

16. The second overmoded BAW resonator of claim 1 , wherein the coupler layer provides an acoustic impedance between the first piezoelectric layer and the second piezoelectric layer.

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