Bipolar boundary regions in piezoelectric devices

Abstract

The present application relates to a bipolar boundary region in a piezoelectric device. An acoustic device includes a base structure and a transducer disposed above the base structure. The base structure includes a piezoelectric layer located between an upper electrode and a lower electrode. The piezoelectric layer has an active portion located within an active region of the transducer and a bipolar boundary portion located within a boundary region of the transducer. The piezoelectric material in the active portion has a first polarization. The bipolar boundary portion has a first subportion and a second subportion located above or below the first subportion. The piezoelectric material in the first subportion has the first polarization, and the piezoelectric material in the second subportion has a second polarization opposite to the first polarization.

Description

[0001] Related Applications

[0002] This application claims the benefit of provisional patent application serial number 62 / 779,605, filed on December 14, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to piezoelectric devices employing piezoelectric films, and in particular to such devices having bipolar boundary regions. Background Art

[0004] Acoustic wave resonators, particularly bulk acoustic wave (BAW) resonators, are used in a variety of high-frequency communication applications. In particular, BAW resonators are often used in filter networks that operate at frequencies above 1.5 GHz and require a flat passband, have unusually steep filter skirts and square shoulders at the upper and lower ends of the passband, and provide excellent rejection outside the passband. BAW-based filters also have relatively low insertion loss, their size tends to decrease with increasing operating frequency, and are relatively stable over a wide temperature range. As a result, BAW-based filters are the filter of choice for many third-generation (3G) wireless devices and fourth-generation (4G) wireless devices, and are destined to dominate filter applications for fifth-generation (5G) wireless devices. Most of these wireless devices support cellular, wireless fidelity (Wi-Fi), Bluetooth, and / or near-field communications on the same wireless device, and therefore, present extremely challenging filtering requirements. While these demands continue to increase the complexity of wireless devices, there is a constant need to improve the performance of BAW resonators and BAW-based filters, and reduce the cost and size associated with them. Summary of the invention

[0005] An acoustic device includes a base structure and a transducer disposed above the base structure. The base structure includes a piezoelectric layer located between an upper electrode and a lower electrode. The piezoelectric layer has an active portion located within an active region of the transducer and a bipolar boundary portion located within a boundary region of the transducer. The piezoelectric material in the active portion has a first polarization. The bipolar boundary portion has a first sub-portion and a second sub-portion located above or below the first sub-portion. The piezoelectric material in the first sub-portion has a first polarization, and the piezoelectric material in the second sub-portion has a second polarization opposite to the first polarization.

[0006] After reading the following detailed description of the preferred embodiments in conjunction with the accompanying drawings, those skilled in the art will understand the scope of the present disclosure and realize other aspects of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

[0008] Figure 1 A conventional bulk acoustic wave (BAW) resonator is shown.

[0009] Figure 2 is a graph of the magnitude and phase of the impedance versus frequency response of an ideal BAW resonator as a function of frequency.

[0010] FIG. 3A to FIG. 3C are phase response plots for various BAW resonator configurations.

[0011] Figure 4 A conventional BAW resonator with boundary rings is shown.

[0012] Figure 5A It is a schematic diagram of a traditional ladder network.

[0013] Figure 5B and Figure 5C yes Figure 5A The frequency response of the BAW resonator in the traditional ladder network and Figure 5A Frequency response plot of a traditional ladder network.

[0014] FIG. 6A to FIG. 6E yes Figure 5A The equivalent circuit of the ladder network at mid-frequency points 1, 2, 3, 4, and 5 is as follows: Figure 5C as shown in .

[0015] Figure 7 A piezoelectric device according to a first embodiment of the present disclosure is shown, which provides a bipolar portion in a boundary region of a piezoelectric layer.

[0016] Fig. 8A and Figure 8B Diagram showing the different polar orientations of aluminum nitride.

[0017] Fig. 9A The quality factor of a BAW resonator with a boundary ring and a BAW resonator with a bipolar boundary portion according to one embodiment is depicted as a function of frequency shift under mass loading.

[0018] Fig. 9B Depicted are boundary mode amplitudes versus frequency shift under mass loading for a BAW resonator with boundary rings and a BAW resonator with a bipolar boundary section according to one embodiment.

[0019] Fig. 9C The boundary mode content of a BAW resonator equipped with boundary rings and bipolar boundary sections according to one embodiment is shown.

[0020] Fig. 10A , Fig. 10B and Fig. 10C shows the series resonant frequency (f s ) near the series resonant frequency (f s ) and the parallel resonant frequency (f p ) and at the parallel resonant frequency (f p ) in the vicinity of lateral (spurious) modes.

[0021] Fig.11 A piezoelectric device according to a second embodiment of the present disclosure is shown, which provides a boundary ring and a bipolar portion in a boundary region of a piezoelectric layer.

[0022] Fig.12 A piezoelectric device according to a third embodiment of the present disclosure is shown, which provides a bipolar portion in a boundary region of a piezoelectric layer.

[0023] Fig.13 A piezoelectric device according to a fourth embodiment of the present disclosure is shown, which provides a bipolar portion in a boundary region of a piezoelectric layer.

[0024] Fig.14 A piezoelectric device according to a fifth embodiment of the present disclosure is shown, which provides a bipolar portion in a boundary region of a piezoelectric layer.

[0025] Fig.15 A piezoelectric device according to a sixth embodiment of the present disclosure is shown, which provides a bipolar portion in a boundary region of a piezoelectric layer. DETAILED DESCRIPTION

[0026] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments, and show the best mode of practicing the embodiments. When reading the following description according to the accompanying drawings, those skilled in the art will understand the conception of the present disclosure, and will recognize the application of these concepts not particularly proposed herein. It should be understood that these concepts and applications are encompassed within the scope of the present disclosure and the appended claims.

[0027] It will be understood that, although the terms first, second, etc. can be used here 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 can be referred to as the second element, and similarly, the second element can be referred to as the first element. As used herein, the term "and / or" includes any and all combinations of one or more of the items listed in the relevant.

[0028] It will 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 can be directly on or directly on another element, or there can also be an intermediate element. On the contrary, when an element is referred to as "directly on" or "extending directly on another element", there is no intermediate element. Similarly, it will be understood that when an element such as a layer, region or substrate is referred to as being "above" or "extending" on another element, it can be directly above or directly above another element, or there can also be an intermediate element. On the contrary, when an element is referred to as being "directly above" or "extending directly above another element", there is no intermediate element. It should also be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to another element, or there can be an intermediate element. On the contrary, when an element is referred to as being "directly connected" or "directly coupled" to another element, there is no intermediate element.

[0029] Relative terms such as "below" or "above" or "above" or "horizontally" or "vertically" may be used herein to describe the relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms, as well as those discussed above, are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

[0030] The terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the present disclosure. As used herein, the singular forms "a", "an", and "the" are also intended to include the plural forms, unless the context clearly indicates otherwise. It will be further understood that when used herein, the terms "comprises, comprising" and / or "includes, including" specify the presence of the features, integers, steps, operations, elements, and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts, and / or groups thereof.

[0031] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure belongs. It will be further understood that, unless expressly defined herein, the terms used herein should be interpreted as having a meaning consistent with that in the context of this specification and the relevant art, and will not be interpreted in an idealized or overly formal sense.

[0032] The present disclosure relates to a piezoelectric device having a unique piezoelectric layer and a method for manufacturing the piezoelectric device. The piezoelectric device includes a base structure and a transducer disposed above the base structure. The base structure includes a piezoelectric layer located between an upper electrode and a lower electrode. The piezoelectric layer has an active portion located within an active region of the transducer and a bipolar boundary portion located within a boundary region of the transducer. The piezoelectric material in the active portion has a first polarization. The bipolar boundary portion has a first subportion and a second subportion located above or below the first subportion. The piezoelectric material in the first subportion has a first polarization, and the piezoelectric material in the second subportion has a second polarization opposite to the first polarization.

[0033] Piezoelectric devices can be implemented in various devices such as bulk acoustic wave (BAW) resonators. Before delving into the details of unique piezoelectric films, an overview of BAW resonators and their operation is provided. BAW resonators are used in a variety of high frequency filter applications. Figure 1 An exemplary BAW resonator 10 is shown in FIG. The BAW resonator 10 is a firmly mounted resonator type BAW resonator 10 and generally includes a substrate 12, a reflector 14 mounted above the substrate 12, and a transducer 16 mounted above the reflector 14. The transducer 16 is placed on the reflector 14 and includes a piezoelectric layer 18 sandwiched between an upper electrode 20 and a lower electrode 22. The upper electrode 20 and the lower electrode 22 may be formed of materials such as tungsten (W), molybdenum (Mo), platinum (Pt), and the piezoelectric layer 18 may be formed of aluminum nitride (AlN), zinc oxide (ZnO), or other suitable piezoelectric materials. Although in Figure 1 1 and 2. The piezoelectric layer 18, the upper electrode 20, and / or the lower electrode 22 are shown as comprising a single layer, but the piezoelectric layer 18, the upper electrode 20, and / or the lower electrode 22 may comprise multiple layers of the same material, multiple layers in which at least two layers are different materials, or multiple layers in which each layer is a different material. The combination of the substrate 12 and the reflector 14 is generally referred to as the base structure of the BAW resonator 10, and thus the transducer 16 is located on or above the base structure.

[0034] Continue to refer Figure 1 , the BAW resonator 10 is divided into an active region 24 and an outer region 26. The active region 24 generally corresponds to the portion of the BAW resonator 10 where the upper electrode 20 and the lower electrode 22 overlap and also includes the layers below the overlapping upper electrode 20 and the lower electrode 22. The outer region 26 corresponds to the portion of the BAW resonator 10 surrounding the active region 24. The portion of the piezoelectric layer 18 located in the active region 24 is referred to as the active portion SA, and the portion of the piezoelectric layer 18 located in the outer region 26 is referred to as the outer portion SO.

[0035] For the BAW resonator 10, applying an electrical signal between the upper electrode 20 and the lower electrode 22 excites acoustic waves in the piezoelectric layer 18. These acoustic waves propagate primarily vertically. A primary goal of BAW resonator design is to confine these vertically propagating acoustic waves within the transducer 16. The upwardly propagating acoustic waves are reflected back into the transducer 16 through the air-metal boundary at the top surface of the upper electrode 20. The downwardly propagating acoustic waves are reflected back into the transducer 16 through the reflector 14 or through an air cavity (not shown) just below the transducer in a thin film BAW resonator (FBAR).

[0036] The reflector 14 is typically formed of a stack of reflector layers (RL) 28 that alternate in material composition to produce a significant reflection coefficient at the junction of adjacent reflector layers 28. Typically, the reflector layers 28 alternate between materials with high and low acoustic impedances, such as tungsten (W) and silicon dioxide (SiO2). Figure 1 Only five reflector layers 28 are shown in FIG. 1 , but the number of reflector layers 28 and the structure of the reflector 14 vary between different designs.

[0037] exist Figure 2 The magnitude (Z) and phase (φ) of the electrical impedance of a relatively ideal BAW resonator are provided as a function of frequency. The solid line represents the magnitude of the electrical impedance (Z), while the dashed line represents the phase (φ) of the electrical impedance. The BAW resonator 10 is unique in that it has both a resonant frequency and an anti-resonant frequency. The resonant frequency is often referred to as the series resonant frequency (f s ), while the anti-resonance frequency is usually called the parallel resonant frequency (f p When the impedance or reactance of the BAW resonator 10 approaches zero, a series resonance frequency (f s The parallel resonant frequency (f ) occurs when the magnitude of the impedance or reactance of the BAW resonator 10 peaks at a relatively high level. p ). Usually, the series resonant frequency (f s ) is a function of the thickness of the piezoelectric layer 18 and the mass of the upper electrode 20 and the lower electrode 22.

[0038] With respect to phase, the BAW resonator 10 acts like an inductor at the series resonant frequency (f s ) and the parallel resonant frequency (f p ) provides a 90° phase shift between the two terminals. In contrast, the BAW resonator 10 acts like a capacitor, which is s ) and the parallel resonant frequency (f p ) provides a -90° phase shift above the series resonant frequency (f s ) presents very low, near-zero resistance at the parallel resonant frequency (f p) presents a very high resistance at ). The electrical properties of the BAW resonator 10 lend themselves to very high Q (quality factor) inductance over a relatively short frequency range, which has proven to be very beneficial in high frequency filter networks, particularly those operating at frequencies of 1.8 GHz and above.

[0039] Unfortunately, Figure 2 The phase (φ) curve of represents the ideal phase curve. In practice, approaching this ideal is challenging. Figure 3A Shows Figure 1 Typical phase curve of the BAW resonator 10. Figure 3A The phase curve is not a smooth curve, but includes the series resonant frequency (f s ) or less, the series resonant frequency (f s ) and the parallel resonant frequency (f p ) and the parallel resonant frequency (f p ) above. The ripple is the result of spurious modes caused by imaginary resonances occurring at the corresponding frequencies. Although the vast majority of acoustic waves in the BAW resonator 10 propagate vertically, various boundary conditions about the transducer 16 result in the propagation of lateral (horizontal) acoustic waves, which are referred to as lateral standing waves. The presence of these lateral standing waves reduces the potential Q associated with the BAW resonator 10.

[0040] like Figure 4 As shown, a boundary (BO) ring 30 is formed on or inside the upper electrode 20 to suppress certain spurious modes. Figure 3B The spurious modes suppressed by the BO ring 30, as highlighted by circles A and B in the phase curve of FIG. 3, are those above the series resonant frequency (f s Circle A shows the suppression of ripple in the passband of the phase curve and, therefore, the suppression of spurious modes located at the series resonant frequency (f s ) and the parallel resonant frequency (f p ). Circle B shows the suppression of ripple and hence the suppression of the frequency above the parallel resonant frequency (f p ) to suppress the spurious modes. It is worth noting that the upper shoulder of the passband is just below the parallel resonant frequency (f p ) and spurious modes above the passband are suppressed, such as by the series resonant frequency (f s ) and the parallel resonant frequency (f p ) and above the parallel resonant frequency (f p ) as evidenced by a smooth or essentially ripple-free phase curve.

[0041] The BO ring 30 corresponds to the mass loading of the portion of the upper electrode 20 extending around the periphery of the active region 24. The BO ring 30 may correspond to a thickened portion of the upper electrode 20 or the application of an additional layer of a suitable material above the upper electrode 20. The portion of the BAW resonator 10 that includes and is located below the BO ring 30 is referred to as a boundary (BO) region 32. The portion of the piezoelectric layer 18 that is located within the BO region 32 is referred to as a boundary portion SB. Thus, the BO region 32 is generally located between the active region 24 and the outer region 26, and the boundary portion SB is located between the active portion SA and the outer portion SO.

[0042] Although the BO ring 30 can effectively suppress the frequency above the series resonance frequency (f s ) but the BO ring 30 is usually s ) has much less influence than the spurious modes, such as Figure 3B A technique called diffraction imaging is often used to suppress the frequency below the series resonance frequency (f s )'s false modality.

[0043] The diffraction imaging method acts to reduce any lateral symmetry in the BAW resonator 10 or at least its transducer 16. The lateral symmetry corresponds to the footprint of the transducer 16, and avoiding lateral symmetry corresponds to avoiding symmetry associated with the sides of the footprint. For example, a footprint corresponding to a pentagon rather than a square or rectangle can be selected. Avoiding symmetry helps reduce the presence of lateral standing waves in the transducer 16. Figure 3C Circle C shows the effect of diffraction imaging, where the frequency below the series resonance frequency (f s ) is suppressed. Assuming that no BO ring 30 is provided, then Figure 3C It can be easily seen that the diffraction imaging method cannot suppress those frequencies above the series resonance frequency (f s Therefore, a typical BAW resonator 10 uses both the diffraction imaging method and the BO ring 30.

[0044] As previously mentioned, the BAW resonator 10 is typically used in filter networks that operate at high frequencies and require high Q values. Figure 5A The basic ladder network LN is shown in FIG. The ladder network LN includes two series resonators B SER and two shunt resonators B SH , which are arranged in a conventional ladder configuration. Typically, the series resonator B SER have the same or similar first frequency response, while the shunted resonator B SH Having a second frequency response that is the same as or similar to the first frequency response, such as Figure 5B In many applications, the shunt resonator B SH is the series resonator BSER Therefore, the series resonator B SER and shunt resonator B SH The frequency responses of the shunt resonators are usually very similar, but offset relative to each other so that the parallel resonant frequency (f p,SH ) is similar to the series resonator B SER The series resonant frequency (f s,SER ). Note that the shunt resonator B SH The series resonant frequency (f s,SH ) is smaller than the series resonator B SER The series resonant frequency (f s,SER ). Shunt resonator B SH The parallel resonant frequency (f p,SH ) is smaller than the series resonator B SER The parallel resonant frequency (f p,SER ).

[0045] Figure 5C and Figure 5B The response of the ladder network LN is shown. SH The series resonant frequency (f s,SH ) corresponds to the lower end of the passband skirt (Phase 2), and the series resonator B SER The parallel resonant frequency (f p,SER ) corresponds to the high end of the passband skirt (phase 4). The substantially aligned series resonator B SER The series resonant frequency (f s,SER ) and shunt resonator B SH The parallel resonant frequency (f p,SH ) falls within the passband. FIG. 6A to FIG. 6E The equivalent circuits of the five phases of the response of the ladder network LN are provided. In the first phase ( Figure 5C , Fig. 6A During phase 1), the function of the ladder network LN is to attenuate the input signal. SH The series resonant frequency (f s,SH ), shunt resonator B SH The impedance of the shunt resonator B drops sharply, making SH Essentially at the series resonant frequency (f s,SH ) Figure 5C , Figure 6B Phase 2) provides a short circuit to ground. In the shunt resonator B SH The series resonant frequency (f s,SH )(Phase 2), the input signal is essentially blocked from the output of the ladder network LN.

[0046] In the shunt resonator B SHThe series resonant frequency (f s,SH ) and the series resonator B corresponding to the passband SER The parallel resonant frequency (f p,SER ), the input signal is passed to the output ( Figure 5C , Figure 6C Phase 3). In the passband, the series resonator B SER presents a relatively low impedance, while the shunt resonator B SH presents a relatively high impedance, the combination of which results in a flat passband with steep low-side and high-side skirts. As the series resonator B is approached SER The parallel resonant frequency (f p,SER ), series resonator B SER The impedance of the series resonator B becomes very high. SER The parallel resonant frequency (f p,SER ) Figure 5C , Fig.6D The phase 4) is basically an open circuit. In the series resonator B SER The parallel resonant frequency (f p,SER ) (Phase 4), the input signal is again substantially blocked from the output of the ladder network LN. Figure 5C , Fig. 6E Phase 5), the role of the ladder network LN is to attenuate the input signal in a similar manner as provided in Phase 1. With the series resonator B SER The parallel resonant frequency (f p,SER ) after the series resonator B SER The impedance of the shunt resonator B is reduced SH Therefore, the ladder network LN is used to shunt the resonator B SH The series resonant frequency (f s,SH ) and the series resonator B SER The parallel resonant frequency (f p,SER ) provides a high Q passband between the shunt resonator B. SH The series resonant frequency (f s,SH ) and the parallel resonant frequency (f p,SER ) provide extremely high attenuation. The ladder network LN is located below the shunt resonator B SH The series resonant frequency (f s,SH ) and above the series resonator B SER The parallel resonant frequency (f p,SER ) provides good attenuation.

[0047] In a single modern communication system such as a mobile phone, various filters require passbands with different bandwidths and centered at different frequencies. The center frequency of a filter using a BAW resonator 10 is primarily determined by the thickness of the individual layers of the transducer 16, and in particular the thickness of the piezoelectric layer 18. The passband bandwidth and shape of the band edges of the filter are primarily determined by the electromechanical coupling coefficient k of the piezoelectric layer 18. The electromechanical coupling coefficient k is a measure of the effectiveness of the piezoelectric layer in converting electrical energy into mechanical energy (or vice versa). Different piezoelectric materials or material compositions typically have different electromechanical coupling coefficients k.

[0048] For passbands with bandwidths less than 100 MHz, aluminum nitride (AlN) is a common choice for the piezoelectric layer 18. For passbands with bandwidths greater than 100 MHz, new piezoelectric materials that provide an increased electromechanical coupling coefficient k are currently being employed. These new piezoelectric materials include, but are not limited to, aluminum nitride that has been doped with one or more transition metals (such as scandium (Sc), yttrium (Y), magnesium (Mg), zirconium (Zr), etc.) alone or in combination with other materials (such as erbium (Er), magnesium (Mg), etc.). Exemplary piezoelectric materials include, but are not limited to, ScAlN, YAlN, [Mg][Zr]AlN, [Sc][Er]AlN, etc.

[0049] However, each of these piezoelectric materials has a fairly specific electromechanical coupling coefficient k. Therefore, designers currently must select a specific piezoelectric material and then design the rest of the BAW resonator 10 and the filter incorporating the BAW resonator 10 around the electromechanical coupling coefficient k of the selected piezoelectric material. In other words, the choice of piezoelectric material for the piezoelectric layer 18 limits the electromechanical coupling coefficient k, and therefore ultimately limits the designer's ability to optimize the performance of the entire filter design. Further, the designer would benefit from a technique for providing electromechanical coupling in certain regions of the piezoelectric layer 18 and providing essentially zero electromechanical coupling in other regions of the piezoelectric layer 18. For example, one would want to provide electromechanical coupling at a desired level in the active region 24 of the BAW resonator 10 and provide little or no electromechanical coupling in the external region 26 and / or the BO region 32.

[0050] The electromechanical coupling coefficient of a material is a function of the piezoelectric properties of the material. Thus, non-piezoelectric materials exhibit little or no electromechanical coupling and therefore have an electromechanical coupling coefficient k that is zero or close to zero. Piezoelectric materials exhibit an electromechanical coupling coefficient k that is based at least in part on the piezoelectric properties of the material.

[0051] A technique for providing both a piezoelectric portion or region and a non-piezoelectric portion or region in the piezoelectric layer 18 is described below. As described above, multiple BAW resonators 10 are often used in combination to form a ladder network LN, etc. In many cases, multiple BAW resonators 10 for forming a ladder network LN are integrated on a single die, wherein the transducers 16 of different BAW resonators 10 share a common substrate 12, a reflector 14, etc. Further, the piezoelectric layer 18, the upper electrode 20, and the lower electrode 22 are respectively formed from a common material layer by appropriate coating processes and etching processes.

[0052] refer to Figure 7 , the present disclosure relates to a piezoelectric layer 18, wherein the portion of the piezoelectric layer 18 located in the BO region 32 is bipolar. This bipolar portion is referred to as a bipolar boundary portion BSB. In most embodiments, the active portion SA in the active region 24 remains unipolar. In different embodiments, the outer portion SO in the outer region 26 can be unipolar or bipolar. Piezoelectric materials have an inherent polar orientation, and in Figure 7 The direction of the polar orientation of one embodiment is shown by arrows in FIG. For BAW resonators, vertical polarization ("up" or "down") is important for electromechanical conversion. Within the active portion SA and the outer portion SO of the piezoelectric layer 18, the polar orientation is constant in each portion and is therefore unipolar.

[0053] In contrast, the bipolar boundary portion BSB of the piezoelectric layer 18 is formed to have sub-portions 34, 36 having opposite polarizations, as indicated by opposite arrows within those portions. The sub-portions 34, 36 of the bipolar boundary portion BSB may be formed of the same material as the rest of the piezoelectric layer 18, but are formed to have opposite polarizations. The inversion layer IL represents the boundary between the sub-portions 34, 36 of the bipolar boundary portion BSB. The inversion layer IL may simply represent the actual structure of the polarization transition level or one or more layers that actually triggers polarization reversal during manufacturing, which will be described in further detail below.

[0054] Because the polarity change does not affect the acoustic properties of the piezoelectric layer, the active portion SA and the bipolar boundary portion SB are acoustically similar, and there is substantially no additional mass loading within the bipolar boundary portion BSB of the piezoelectric layer 18 caused by making the boundary portion SB bipolar. Thus, the bipolar boundary portion BSB limits the lateral acoustic energy with or without the need for mass loading, thereby potentially eliminating the need for the BO ring 30, which is not present in the piezoelectric layer 18. Figure 7 In an embodiment of the present invention, the quality factor Q obtained is better than or comparable to the value achieved by the design incorporating the BO ring 30.

[0055] The incorporation of the bipolar boundary section BSB does not cause the series resonant frequency (fs ) produces a boundary (BO) mode near the series resonant frequency (f s ). Instead, the BO mode is shifted above the active region series resonant frequency (f s ) to several gigahertz, thereby having essentially no effect on the performance of filters employing these BAW resonators 10. The use of the bipolar boundary portion BSB also provides better or comparable reduction of spurious transverse wave modes than conventional designs incorporating BO rings 30. Further details on these and other benefits will be provided subsequently.

[0056] Common piezoelectric materials for the piezoelectric layer 18 include III-V nitrides, primarily aluminum nitride (AlN) and AlN doped with transition metals such as Sc, Er, Mg, Hf, etc. If the piezoelectric layer 18 is formed of AlN, the direction of the Al-N bonds along the c-axis of the AlN crystal structure determines the polarization or polarity direction of the AlN crystal structure. The polarization of the material can be reversed by flipping the direction of the Al-N bonds during the growth of the piezoelectric layer 18. There are several processing methods to achieve polarization reversal of III-V nitrides. For example, AlN grows with N polarity (N pole) on and above certain metal surfaces such as W, Cu, and Mo, and grows with Al polarity (Al pole) on surfaces such as Al, Ru, and RuO2. During the growth of the piezoelectric layer 18, an appropriate inversion layer IL can be coated in the BO region 32 and photolithographically patterned to achieve Figure 7 The bipolar boundary portion BSB is shown.

[0057] For the embodiment shown, a firmly mounted resonator or FBAR base structure is provided. The lower electrode 22 is formed over the base structure by coating an appropriate metal layer or the like over the piezoelectric layer base structure and then etching the metal layer away from the lower electrode 22. Next, the lower portion of the piezoelectric layer 18 is grown to a first thickness over the lower electrode 22 and through the outer region 26, the BO region 32, and the active region 24 at a first polarization. At this level, an appropriate metal layer or the like is coated over the piezoelectric layer 18 and then etched away from the inversion layer IL. As previously described, the inversion layer IL is located in the BO region 32.

[0058] Next, the growth process of the piezoelectric layer 18 is resumed so that the upper portion of the piezoelectric layer 18 grows over the lower portion of the piezoelectric layer 18 through the outer region 26, the BO region 32, and the active region 24 until the piezoelectric layer 18 reaches a second thickness. It is noteworthy that the polarization of the piezoelectric layer 18 grown over the inversion layer IL is reversed, and therefore, the piezoelectric layer 18 over the inversion layer IL and within the BO region 32 has a second polarization opposite to the first polarization. The polarization of the piezoelectric layer 18 grown in the active portion SA and the outer portion SO is not reversed, and therefore has a first polarization. An upper electrode 20 is formed over the piezoelectric layer 18 by coating an appropriate metal layer, etc. over the piezoelectric layer and then etching the metal layer in a manner away from the upper electrode 20.

[0059] Fig. 8A The aluminum polar (Al polar) orientation is shown, where the growth direction is along the c-axis of the atomic structure. For the Al polar atomic structure, the Al-N atomic bonds along the c-axis cause the Al atoms to be located below the N atoms. The net polarity P NET is reversed, as indicated by the downward arrow. Figure 8B As shown, for the nitrogen polar (N polar) orientation with the growth direction along the c-axis, the situation is the opposite. For the N polar atomic structure, the Al-N atomic bonds along the c-axis cause the N atoms to be located below the Al atoms. The net polarity P NET Aligned with the growth direction, as indicated by the upward arrow. Therefore, the polarization of the Al pole orientation is opposite to the polarization of the N pole orientation. Although aluminum nitride is used in this example, other piezoelectric compounds can be used for the piezoelectric material, and an appropriate metal such as the metal of the compound can be used to form the inversion layer IL.

[0060] The thickness ratio of the subsections 34, 36 with opposite polarizations can be selected to cancel the fundamental BAW mode in the BO region 32. The thickness of the individual layers can be, but need not be, 1:1. The specific ratio may depend on the stacking of materials and elements below and above the piezoelectric layer 18. Typically, for most BAW resonators, the ratio is in the range of 0.7:1 to 1.3:1, while other embodiments may require a narrower range of 0.8:1 to 1.2:1, 0.9:1 to 1.1:1, 0.95:1 to 1.05:1, etc. Other wider or narrower ranges are possible and are considered to be covered by the scope of the present disclosure. Although the bipolar boundary section BSB is shown to switch polarization only once, multiple polarity switching throughout the thickness of the piezoelectric layer 18 (i.e., two or more alternations in each subsection 34, 36) is also conceivable, as further described subsequently. The aforementioned thickness ratios also apply to the cumulative thickness of the individual subsections 34, 36.

[0061] When using a conventional BO ring 30, such as Figure 4As shown, the additional mass loading provided by the BO ring 30 in the BO region 32 introduces a downward frequency shift in the BO region 32 of the flexural BAW mode. The resulting change in acoustic dispersion between the active region 24 and the BO region 32 is exploited to confine the lateral acoustic energy within the active portion SA of the piezoelectric layer 18. Fig. 9A The downshift in frequency is plotted against the quality factor Qp at the optimum BO ring 30 width. The amount of mass loading provided by the BO ring 30 is critical to achieving better Qp.

[0062] For the present disclosure, the dipole boundary portion BSB limits the energy of the side waves without significantly changing the acoustic dispersion from the active portion SA. Fig. 9A Significantly higher Qp values ​​are obtained with negligible mass loading. Although polarization reversal does not induce changes in mechanical dispersion, several modes are not supported by the bipolar boundary part BSB due to the electrical boundary conditions. The antisymmetric modes (along the z direction) in the active part SA do not propagate into the bipolar boundary part BSB and therefore remain confined within the active part SA.

[0063] The active region 24 of the BAW resonator is at the fundamental series resonant frequency (f s ) resonates at the bottom. The novelty of the dipole boundary section BSB is that it does not support the fundamental acoustic tone. In a structure where the subsections 34, 36 have opposite polarizations, the fundamental mode resonance has zero coupling (k2e). Regardless of the mass loading added to or by the dipole boundary section BSB, the series resonance frequency (f s ). Therefore, no modes are generated from the BO region 32 at frequencies that are critical to filters made from such BAW resonators 10. Fig. 9B It is shown that when the BO region 32 is designed as a bipolar boundary portion BSB, the BO mode is effectively suppressed in the BO region 32. s ), since the bipolar BO region 32 does not participate in the electromechanical conversion, the effective piezoelectric coupling can be reduced by 4% or more.

[0064] The bipolar boundary section BSB does support second overtone resonances. These resonances usually occur at frequencies above the fundamental series resonant frequency (f s Since most BAW resonators 10 operate at frequencies above 1500 MHz, the second overtone from the BO region 32 occurs at a frequency that is twice the fundamental series resonant frequency (f s ) at least several thousand GHz. Fig. 9CThe BO mode content of a BAW resonator 10 equipped with a BO ring 30 and a bipolar boundary section BSB, respectively, is shown. As shown, the use of a bipolar boundary section BSB solves the problem of the BO mode falling within the filter passband and enables better design of BAW filters of any bandwidth. In addition, the use of a bipolar boundary section BSB facilitates multiplexing with closely spaced frequency bands, as is often required for duplexers and multiplexers.

[0065] Furthermore, using the bipolar boundary portion BSB to define the BO region 32 provides better lateral (spurious) mode suppression than using only the BO ring 30 . Fig. 10A , Fig. 10B and Fig. 10C The best case scenarios for the two configurations are compared. Fig. 10A , Fig. 10B and Fig. 10C In the series resonant frequency (f s ) below the series resonant frequency (f s ) to the parallel resonant frequency (f p ) and just at the parallel resonant frequency (f p ) shows the spurious content above. These three frequency regions are critical to shaping the filter passband. Note that in all three frequency regions, the design with the bipolar boundary section BSB is better at suppressing the spurious content than the design with the BO ring 30. The BAW resonator 10 incorporating the bipolar boundary section BSB has the potential to allow the filter fabricated therefrom to achieve a passband free of spurious modes.

[0066] Fig.11 The BAW resonator 10 is shown incorporating a BO ring 30 over a bipolar boundary portion BSB of a BO region 32 . Fig.12 A BAW resonator 10 is shown that incorporates an inversion layer IL that is thick enough to provide mass loading within the bipolar boundary portion BSB. Thus, the inversion layer IL in this embodiment is a thickened structure of one or more layers having a desired mass and disposed between the sub-portions 34, 36 within the bipolar boundary portion BSB. The inversion layer IL in this embodiment can replace the BO ring 30 and provide its mass loading. Further, the inversion layer IL can be provided together with the BO ring 30 located on top of the bipolar portion BSB.

[0067] Any of the foregoing embodiments may be implemented in a FBAR configuration, wherein the reflector 14 is substantially replaced by an air gap 38, such as Fig.13 As shown. The basic structure of the FBAR embodiment includes a substrate 12 and an air gap 38. Thus, the transducer 16 is substantially located on or above the basic structure. In this embodiment, a BO ring 30 and an inversion layer IL are provided, which can be configured to add additional mass.

[0068] There are many potential alternatives to the above described embodiments. Fig.14 As shown, the sub-portions 34, 36 of the BO region 32 may exchange polarization relative to the above-described embodiments. As previously noted, the bipolar boundary portion BSB may have more than two sub-portions. Fig.15 As shown, the bipolar boundary portion BSB is divided into four sub-portions 40, 42, 44, 46, which are separated by three inversion layers IL. The inversion layer IL promotes the polarization exchange of the piezoelectric material within the bipolar boundary portion BSB during the growth or coating process of the piezoelectric layer 18. The sub-portions 40, 44 have a first polarization, while the sub-portions 42, 46 have a second polarization opposite to the first polarization. In this embodiment, the outer portion SO and the active portion SA also have a second polarization; however, all of these polarizations can be exchanged. Further, the polarization of the sub-portions 40, 42, 44, 46 can be exchanged. The number and thickness of the sub-portions 40, 42, 44, 46 can vary between different embodiments depending on the performance indicators required by the application.

[0069] 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 piezoelectric device, comprising: Infrastructure; as well as A transducer located above the base structure comprises: Lower electrode; a piezoelectric layer located above the lower electrode and comprising a piezoelectric material; and an upper electrode located above the piezoelectric layer, wherein: The piezoelectric layer has an active portion located within an active region of the transducer and a bipolar boundary portion located within a boundary region of the transducer; The piezoelectric material in the active portion has a first polarization such that the active region is unipolar; The bipolar boundary portion includes a first sub-portion and a second sub-portion located above or below the first sub-portion; and The piezoelectric material in the first subsection has the first polarization, and the piezoelectric material in the second subsection has a second polarization opposite to the first polarization. 2 . The piezoelectric device of claim 1 , further comprising a first inversion layer located between the first subsection and the second subsection.

3. The piezoelectric device according to claim 2, wherein: The first inversion layer provides mass loading in the boundary region. 4 . The piezoelectric device of claim 3 , further comprising a boundary ring located within the boundary region and above the piezoelectric layer, wherein the boundary ring provides mass loading. 5 . The piezoelectric device of claim 1 , further comprising a boundary ring located within the boundary region and above the piezoelectric layer, wherein the boundary ring provides mass loading.

6. The piezoelectric device according to claim 1, wherein The piezoelectric material is formed of a compound including a metal element and a non-metal element.

7. The piezoelectric device according to claim 6, wherein: The metal element is a Group III element and the non-metal element is a Group V element.

8. The piezoelectric device according to claim 1, wherein The piezoelectric material includes AlN doped with a transition metal including at least one of scandium, erbium, magnesium, and hafnium.

9. The piezoelectric device according to claim 1, wherein: The piezoelectric material has an outer portion within the outer region such that the boundary region is located between the outer region and the active region.

10. The piezoelectric device according to claim 9, wherein: The piezoelectric material in the outer portion has the first polarization.

11. The piezoelectric device according to claim 1, wherein: The base structure includes a substrate and a reflector including a plurality of reflector layers located above the substrate such that the device is a BAW resonator based on a firmly mounted resonator.

12. The piezoelectric device according to claim 1, wherein The base structure includes a substrate providing an air gap below the transducer such that the device is a film bulk acoustic resonator (FBAR).

13. The piezoelectric device according to claim 1, wherein: The dipole boundary portion includes at least two further subportions located above or below the first subportion and the second subportion, the at least two further subportions alternating between the first polarization and the second polarization.

14. The piezoelectric device according to claim 1, wherein: An outer portion of the piezoelectric layer has only one of the first polarization and the second polarization.

15. The piezoelectric device according to claim 1, wherein: A ratio of a thickness of the first sub-portion to a thickness of the second sub-portion is between 0.7:1.0 and 1.3:1.

0.

16. A method for forming a piezoelectric device, comprising: Provide infrastructure; forming a lower electrode above the base structure; forming a lower portion of the piezoelectric layer over the lower electrode and through an outer region, a boundary region, and an active region, wherein the active region is located inside the boundary region and the boundary region is located inside the outer region; forming an inversion layer in the boundary region over the lower portion of the piezoelectric layer; An upper portion of the piezoelectric layer is formed above the inversion layer and through the outer region, the boundary region, and the active region, wherein: The lower portion of the piezoelectric layer located below the inversion layer and within the boundary region has a first polarization, and The upper portion of the piezoelectric layer located above the inversion layer and within the boundary region has a second polarization opposite to the first polarization; and forming an upper electrode over the upper portion of the piezoelectric layer; Wherein, the active portion of the piezoelectric layer has only one of the first polarization and the second polarization.

17. The method according to claim 16, wherein: The active portion and the outer portion of the piezoelectric layer each have only one of the first polarization and the second polarization.

18. The method according to claim 16, wherein: An outer portion of the piezoelectric layer has only one of the first polarization and the second polarization.

19. The method according to claim 16, wherein: The base structure includes a substrate and a reflector including a plurality of reflector layers located above the substrate such that the device is a BAW resonator based on a firmly mounted resonator.

20. The method according to claim 16, wherein: The base structure includes a substrate providing an air gap below the transducer such that the device is a film bulk acoustic resonator (FBAR).

21. The method according to claim 16, wherein: The inversion layer provides mass loading in the boundary region.

22. The method according to claim 16, wherein: A ratio of the thickness of the upper portion to the thickness of the lower portion is between 0.7:1 and 1.3:1.0.

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