Bulk ultrasonic (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 of related applications This application claims the benefit of Provisional Patent Application No. 62 / 649,343, filed on 28 March 2018, whose disclosure is incorporated herein by reference in its entirety.

[0002] This disclosure relates to a bulk ultrasonic (BAW) resonator capable of operating in higher-order modes with improved piezoelectric coupling. [Background technology]

[0003] Figure 1 shows a fundamental mode bulk ultrasonic (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] Figure 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 in which the stress profile, which matches half the wavelength of the sinusoidal curve, fits within the thickness range of the piezoelectric layer 12. Effective electromechanical coupling (k 2 eff The stress profile 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 be steep at the contact interface between the piezoelectric layer 12 and the first electrode 14 and the second electrode 16.

[0005] As a possibility for exciting higher-order modes in a BAW resonator, when a conventional fundamental-mode BAW resonator is used while maintaining reasonable quality factors, size, and electrode thickness, it is expected that a filter capable of operating at higher frequencies can be anticipated. In higher-order modes, stress profiles that coincide with integer multiples of the fundamental mode frequency are excited within the piezoelectric layer. For example, in the second-order mode, also referred to herein as the second overmode, the entire wavelength of the sinusoidal curve (twice the fundamental mode frequency) is fitted within the thickness of the piezoelectric layer.

[0006] While BAW resonators capable of operating in higher-order modes offer the potential for improved performance at high frequencies, attempts to create such devices provide considerably insufficient electromechanical coupling compared to fundamental-mode devices such as the fundamental-mode BAW resonator 10 described 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. [Overview of the Initiative]

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

[0008] Those skilled in the art will understand the scope of this disclosure and its additional embodiments after reading the following detailed description of preferred embodiments relating to the attached drawings.

[0009] The accompanying drawings incorporated into and forming part of this specification illustrate several aspects of this disclosure and, together with the description thereof, serve to illustrate the principles of this disclosure. [Brief explanation of the drawing]

[0010] [Figure 1] Shows a fundamental mode bulk acoustic wave (BAW) resonator. [Figure 2] Shows the stress response and displacement profile of a fundamental mode BAW resonator. [Figure 3] Shows a second overtone BAW resonator according to an embodiment of the present disclosure. [Figure 4] Shows the stress profile and displacement profile of a second overtone BAW resonator according to an embodiment of the present disclosure. [Figure 5] Shows a second overtone BAW resonator according to an embodiment of the present disclosure. [Figure 6] Shows the stress profiles of two second overtone BAW resonators according to various embodiments of the present disclosure. [Figure 7] A graph showing the performance characteristics of several second overtone BAW resonators according to various embodiments of the present disclosure. [Figure 8] Shows a second overtone BAW resonator according to an embodiment of the present disclosure. [Figure 9] Shows a second overtone BAW resonator according to an embodiment of the present disclosure.

Mode for Carrying Out the Invention

[0011] The embodiments described below represent the necessary information that enables those skilled in the art to practice these embodiments and to show the best mode of practicing these 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] While terms such as "first," "second," etc., may be used herein to describe various elements, it will be understood that these elements should not be limited by these terms. These terms are used solely to distinguish one element from another. For example, without departing from the scope of this disclosure, a first element may be named a second element, and similarly, a second element may be named a first element. The terms "and / or" as used herein include any and all combinations of one or more related enumerated items.

[0013] When an element such as a layer, region, or substrate is said to be "on" another element, or to extend "onto" another element, it will be understood that it is directly on top of the other element, or may extend directly onto it, or that an intervening element may also exist. In contrast, when an element is said to be "directly on" another element, or to extend "directly onto" another element, there is no intervening element. Similarly, when an element such as a layer, region, or substrate is said to exist "over" another element, or to extend "over" another element, it will be understood that it is directly on another element, or may extend directly onto it, or that an intervening element may also exist. In contrast, when an element is said to exist "over" another element, or to extend "over" another element, there is no intervening element. Furthermore, when an element is “connected” or “joined” to another element, it will be understood that it is directly connected to the other element, or may be directly joined to it, or that an intermediary element may exist. In contrast, when an element is said to be “directly connected” to another element or “directly joined” to it, there is no intermediary element.

[0014] Relative terms such as "downward" or "upward," or "upper side" or "lower side," or "horizontally" or "vertically" refer to other elements, layers, or regions as shown in the diagram. These terms may be used herein to describe the relationship between a single element, layer, or region. 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 figure.

[0015] The technical terms used herein are for the purpose of describing only specific embodiments and are not intended to limit the disclosure. The singular forms “a,” “an,” and “the” as used herein are also intended to include the plural form unless otherwise explicitly indicated in the context. Furthermore, it will be understood that the terms “comprises,” “comprising,” “includes,” and / or “including,” when used herein, specify the presence of the described features, integers, steps, actions, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, actions, elements, components, and / or groups thereof.

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

[0017] Figure 3 shows a second overmode bulk ultrasonic (BAW) resonator 22 according to one embodiment of the present disclosure. 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 opposite to the second piezoelectric layer 26, and a second electrode 30 on the second piezoelectric layer 26 opposite to 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 to the first polarity. This allows the second overmode BAW resonator 22 to excite the second mode, as described below. In one embodiment, the first piezoelectric layer 24 and the second piezoelectric layer 26 are aluminum nitride (AIN) layers with opposite polarities. For example, the first piezoelectric layer 24 may be a nitrogen electrode layer of aluminum nitride (c-AlN), while the second piezoelectric layer 26 may be an aluminum electrode layer of aluminum nitride (f-AlN). The aluminum nitride may be undoped or doped with one or more of the following: scandium (Sc), erbium (Er), magnesium (Mg), hafnium (Hf), etc. The first electrode 28 and the second electrode 30 may be metallic layers. For example, the first electrode 28 and the second electrode 30 may 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 may be selected to provide specific electrical and / or acoustic characteristics in order to modify one or more operating parameters of the second overmode BAW resonator 22.

[0019] FIG. 4 shows the stress profile (solid line 32) and displacement profile (dashed line 34) of the second overtone BAW resonator 22. As shown, the BAW resonator operates in the second mode (also referred to herein as the second overtone), and the stress profile that matches the full wavelength of the sine curve fits within the range of the combined thickness of the first piezoelectric layer 24 and the second piezoelectric layer 26. Although not shown, those 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 its fundamental modes. This is shown by comparing the fundamental mode BAW resonator 10 and the second overtone BAW resonator 22 described in the above "Background Art". Assuming that both the electrodes of the fundamental mode BAW resonator 10 and the second overtone 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 overmode BAW resonator 22, a coupler layer 36 is added between the first piezoelectric layer 24 and the second piezoelectric layer 26, as shown in Figure 5. The coupler layer has a thickness of 2l. 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 overall stress profile of the second overmode BAW resonator 22, thereby improving its electromechanical coupling coefficient. In alternative embodiments, the coupler layer may serve two purposes: (i) to improve effective electromechanical coupling by its acoustic impedance, and (ii) to serve as a layer that causes a reversal of polarity 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 overmode BAW resonator 22, which includes the coupler layer 36, as expressed by equation (9).

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[0023] Figure 6 shows the stress profile of the second overmode BAW resonator 22 including the coupler layer 36 (solid line 38) and the displacement profile of the second overmode BAW resonator 22 without the coupler layer 36 (dashed line 40). As shown, the coupler layer 36 pushes further acoustic energy into the first piezoelectric layer 24 and the second piezoelectric layer 26, increasing the overall stress profile and, consequently, the electromechanical coupling coefficient. Although not shown, those skilled in the art will recognize that the stress profile can be steeper 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.

[0024] Acoustic impedance (Z) of coupler layer 36 c ) and the acoustic impedance (Z) of the first piezoelectric layer 24 and the second piezoelectric layer 26 p The relationship between (Z) can change the electromechanical coupling coefficient of the second overmode BAW resonator 22. The value obtained by dividing the acoustic impedance of the coupler layer 36 by the acoustic impedances of the first piezoelectric layer 24 and the second piezoelectric layer 26 (Z) c / Z p In various embodiments, a desirable electromechanical coupling coefficient can be achieved when the value (Z) is greater than 1.0, greater than 1.5, greater than 2.0, and greater than 3.0. The value obtained by dividing the acoustic impedance of the coupler layer 36 by the acoustic impedances of the first piezoelectric layer 24 and the second piezoelectric layer 26 is Z. c / Z pThe x-axis can be constrained by the acoustic impedance available to the material of the coupler layer 36 in various embodiments, and thus may be less than 10.0, less than 8.0, or less than 6.0. The relationship between the thickness of the coupler layer 36 (2l) and the thicknesses (t) of the first electrode 28 and the second electrode 30 can also change the electromechanical coupling coefficient. A desirable electromechanical coupling coefficient can be achieved when the value obtained by dividing the thickness of the coupler layer 36 by the thicknesses 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 and 0.4. Figure 7 is a graph showing these relationships. Specifically, Figure 7 has the x-axis (thickness of the coupler layer 36 divided by the thicknesses of the first electrode 28 and the second electrode 30 (2l / t)) and the y-axis (where the coupler layer 36 is absent). The improved ratio of the electromechanical coupling coefficient for the second overmode BAW resonator 22 having a coupler layer 36 (i.e., the electromechanical coupling coefficient of the second overmode BAW resonator 22 having a coupler layer 36 (k 2 eff,c ) is the electromechanical coupling coefficient (k) of the second overmode BAW resonator 22 which lacks the coupler layer 36. 2 eff,nc This graph shows the value divided by (Z). Each line in the graph represents a specific relationship (Z) 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. c / Z p ) indicates.

[0025] In one embodiment, the coupler layer 36 is a metallic layer. The coupler layer 36 may be aluminum (Al), molybdenum (Mo), tungsten (W), or osmium (Os). Depending on the material chosen for the coupler layer 36, the thickness of the coupler layer 36 may be modified to increase the electromechanical coupling coefficient of the second overmode BAW resonator 22 and to provide a desired acoustic response. With respect to 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 is a value obtained by dividing the acoustic impedance of the first piezoelectric layer 24 and the second piezoelectric layer 26 by (Z c / Z p The coefficient of the coupler layer 36 is 0.50 for aluminum (Al), 1.90 for molybdenum (Mo), 2.75 for tungsten (W), and 3.25 for osmium (Os). In various embodiments, the thickness of the first piezoelectric layer 24 and the second piezoelectric layer 26 may be 350 nm to 1050 nm, the thickness of the first electrode 28 and the second electrode 30 may be 100 nm to 300 nm, and the thickness of the coupler layer 36 may be 30 nm to 90 nm. The thickness of the coupler layer 36 may include any partial range within this range, in which case the thickness of the coupler layer 36 may be 30nm-40nm, 30nm-50nm, 30nm-60nm, 30nm-70nm, 30nm-80nm, 40nm-50nm, 40nm-60nm, 40nm-70nm, 40nm-80nm, 40nm-90nm, 50nm-60nm, 50nm-70nm, 50nm-80nm, 50nm-90nm, 60nm-70nm, 60nm-80nm, 60nm-90nm, 70nm-80nm, 70nm-90nm, and 80nm-90nm. Due to the excitation of the second mode of the second overmode BAW resonator 22, the device may provide a resonant frequency greater than approximately 3.0 GHz. Therefore, the second overmode BAW resonator 22 can be very useful in high-frequency applications.

[0026] The second overmode BAW resonator 22 may be a rigidly mounted resonator (SMR), as shown in Figure 8. In this embodiment, the second overmode BAW resonator 22 is provided on a substrate 42 such that several intervening layers 44 are located between the second overmode 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 apparent to those skilled in the art and are therefore not discussed in detail herein.

[0027] The second overmode BAW resonator 22 may also be a thin-film bulk ultrasonic resonator (FBAR), as shown in Figure 9. In this embodiment, the second overmode BAW resonator 22 is provided on a support layer 46 and floats over an air cavity 48 by a substrate 50. Details of the support layer 46, air cavity 48, and substrate 50 will be readily apparent to those skilled in the art and are therefore not discussed in detail herein.

[0028] Although not shown, the second overmode BAW resonator 22 can be used in any number of different support structures to create any number of different circuit topologies. In various embodiments, the second overmode BAW resonator 22 is electrically or acoustically connected to one or more other resonators or components that form a filtering circuit such as a duplexer or multiplexer. They can be joined in any way. Details of these structures will be readily apparent to those skilled in the art and are therefore not discussed in detail herein.

[0029] Those skilled in the art will recognize improvements and modifications to preferred embodiments of this disclosure. All such improvements and modifications are deemed to fall within the scope of the concepts disclosed herein and the claims that follow.

Claims

1. A second overmode bulk ultrasonic (BAW) resonator, 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 piezoelectric constants with opposite signs, 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, The second overmode bulk ultrasonic resonator is configured to excite the secondary mode by the first piezoelectric layer, the coupler layer, and the second piezoelectric layer, wherein the first and second polarities have piezoelectric constants of opposite signs. The coupler layer has a first acoustic impedance, The first piezoelectric layer and the second piezoelectric layer have a second acoustic impedance, and the value obtained by dividing the first acoustic impedance by the second acoustic impedance is greater than 1.

0. The coupler layer is one of molybdenum, tungsten, and osmium. A second overmode BAW resonator in which the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

2. The second overmode BAW resonator according to claim 1, wherein the value obtained by dividing the first acoustic impedance by the second acoustic impedance is greater than 1.

5.

3. The second overmode BAW resonator according to claim 1, wherein the value obtained by dividing the first acoustic impedance by the second acoustic impedance is greater than 2.

0.

4. The second overmode BAW resonator according to claim 1, wherein the value obtained by dividing the first acoustic impedance by the second acoustic impedance is greater than 3.

0.

5. The second overmode BAW resonator according to claim 1, wherein the coupler layer is a metal layer.

6. The second overmode BAW resonator according to claim 5, wherein the first piezoelectric layer and the second piezoelectric layer are aluminum nitride.

7. The second overmode BAW resonator according to claim 6, wherein the first electrode and the second electrode are made of tungsten.

8. The second overmode BAW resonator according to claim 1, wherein the first electrode and the second electrode are made of tungsten.

9. A second overmode BAW resonator according to claim 1, The thickness of the first piezoelectric layer and the second piezoelectric layer is 350 nm to 1050 nm. A second overmode BAW resonator, wherein the thickness of the coupler layer is 30 nm to 120 nm.

10. A second overmode BAW resonator according to claim 1, The coupler layer has a first thickness, A second overmode BAW resonator, wherein the first electrode and the second electrode have a second thickness, and the value obtained by dividing the first thickness by the second thickness is between 0.1 and 0.

4.

11. A second overmode BAW resonator according to claim 10, The thickness of the first piezoelectric layer and the second piezoelectric layer is 350 nm to 1050 nm. The first thickness of the coupler layer is 30 nm to 120 nm. A second overmode BAW resonator in which the second thickness of the first electrode and the second electrode is 100 nm to 300 nm.

12. A second overmode BAW resonator according to claim 11, The first piezoelectric layer and the second piezoelectric layer are aluminum nitride. The coupler layer is one of molybdenum, tungsten, and osmium. A second overmode BAW resonator in which the first and second electrodes are tungsten.

13. The second overmode BAW resonator according to claim 10, wherein the value obtained by dividing the first thickness by the second thickness is 0.2 to 0.

3.

14. The thickness of the first piezoelectric layer and the second piezoelectric layer is 350 nm to 1050 nm. The first thickness of the coupler layer is 30 nm to 120 nm, and The second overmode BAW resonator according to claim 13, wherein the second thickness of the first electrode and the second electrode is 100 nm to 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 second overmode BAW resonator according to claim 14, wherein the first electrode and the second electrode are made of tungsten.

16. 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.

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