Bipolar boundary regions in piezoelectric devices

Abstract

The invention relates to a bipolar boundary region in a piezoelectric device. An acoustic device includes a base structure and a transducer disposed over the base structure. The base structure includes a piezoelectric layer 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 the first polarization and the piezoelectric material in the second sub-portion has a second polarization opposite the first polarization.

Description

[0001] This application is a divisional application of the patent application for invention with the application date of December 16, 2019, application number 201911291705.2, and invention title "Bipolar Boundary Region in a Piezoelectric Device".

[0002] Related Applications

[0003] This application claims the benefit of Provisional Patent Application Serial No. 62 / 779,605, filed on December 14, 2018, the entire content of which is incorporated herein by reference. Technical Field

[0004] The present disclosure relates to piezoelectric devices employing piezoelectric films, and more particularly to such devices having a bipolar boundary region. Background Art

[0005] Acoustic wave resonators, particularly bulk acoustic wave (BAW) resonators, are used in a variety of high-frequency communication applications. In particular, BAW resonators are commonly used in filter networks operating at frequencies above 1.5 GHz and requiring a flat passband, with extremely steep filter skirts and square shoulders at the upper and lower ends of the passband, and providing excellent rejection outside the passband. BAW-based filters also have relatively low insertion losses, their size tends to decrease as the operating frequency increases, and they are relatively stable over a wide temperature range. Thus, BAW-based filters are the filter of choice for many third-generation (3G) and fourth-generation (4G) wireless devices and are destined to dominate filter applications in fifth-generation (5G) wireless devices. Most of these wireless devices support cellular, Wi-Fi, Bluetooth, and / or near-field communication on the same wireless device, and thus, extremely challenging filtering requirements are presented. Despite these increasing demands adding complexity to wireless devices, there is always a need to improve the performance of BAW resonators and BAW-based filters and reduce the cost and size associated therewith. Summary of the Invention

[0006] An acoustic device includes a substrate and a transducer disposed above the substrate. The substrate includes a piezoelectric layer between an upper electrode and a lower electrode. The piezoelectric layer has an active portion within an active region of the transducer and a bipolar boundary portion 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 disposed 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.

[0007] 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

[0008] The drawings included in this specification and forming a part of this specification illustrate several aspects of the present disclosure and, together with the specification, are used to explain the principles of the present disclosure.

[0009] Figure 1 Shows a conventional bulk acoustic wave (BAW) resonator.

[0010] Figure 2 Is a graph of the magnitude and phase of the impedance of an ideal BAW resonator as a function of frequency versus frequency.

[0011] Figures 3A to 3C Is a phase response diagram of various BAW resonator configurations.

[0012] Figure 4 Shows a conventional BAW resonator with a border ring.

[0013] Figure 5A Is a schematic diagram of a conventional ladder network.

[0014] Figure 5B And Figure 5C Is Figure 5A The frequency response diagram of the BAW resonator in the conventional ladder network of Figure 5A And the frequency response diagram of the conventional ladder network of

[0015] Figures 6A to 6E Is Figure 5A The equivalent circuit of the ladder network at frequency points 1, 2, 3, 4, and 5 in Figure 5C As shown in

[0016] Figure 7 Shows a piezoelectric device according to a first embodiment of the present disclosure, which provides a bipolar portion in the boundary region of the piezoelectric layer.

[0017] Figure 8A And Figure 8B Illustrate different polar orientations of aluminum nitride.

[0018] Figure 9A Depicts the relationship between the quality factor and the frequency shift under mass loading of a BAW resonator with a border ring and a BAW resonator with a bipolar boundary portion according to one embodiment.

[0019] Figure 9B Depicts the relationship between the boundary mode amplitude and the frequency shift under mass loading of a BAW resonator with a border ring and a BAW resonator with a bipolar boundary portion according to one embodiment.

[0020] Figure 9C Shows the boundary mode content of a BAW resonator equipped with a boundary ring and a bipolar boundary portion according to one embodiment.

[0021] Figure 10A 、 Figure 10B and Figure 10C Shows transverse (spurious) mode suppression according to one embodiment near the series resonance frequency (f s ), between the series resonance frequency (f s ) and the parallel resonance frequency (f p ), and near the parallel resonance frequency (f p ).

[0022] Figure 11 Shows a piezoelectric device according to a second embodiment of the present disclosure, which provides a boundary ring and a bipolar portion in the boundary region of a piezoelectric layer.

[0023] Figure 12 Shows a piezoelectric device according to a third embodiment of the present disclosure, which provides a bipolar portion in the boundary region of a piezoelectric layer.

[0024] Figure 13 Shows a piezoelectric device according to a fourth embodiment of the present disclosure, which provides a bipolar portion in the boundary region of a piezoelectric layer.

[0025] Figure 14 Shows a piezoelectric device according to a fifth embodiment of the present disclosure, which provides a bipolar portion in the boundary region of a piezoelectric layer.

[0026] Figure 15 Shows a piezoelectric device according to a sixth embodiment of the present disclosure, which provides a bipolar portion in the boundary region of a piezoelectric layer. Detailed Description

[0027] The embodiments set forth below represent the necessary information enabling those skilled in the art to practice the embodiments and show the best mode of practicing the embodiments. When reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the present disclosure and will recognize applications of these concepts not particularly set forth herein. It should be understood that these concepts and applications are covered by the present disclosure and the appended claims.

[0028] It will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, without departing from the scope of the present disclosure, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0029] 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 extend on the other element, or there may also be intervening elements. In contrast, when an element is referred to as being "directly on" or "directly extending on" another element, there are no intervening elements. Similarly, it will be understood that when an element such as a layer, region, or substrate is referred to as being "above" or "extending above" another element, it can be directly above or directly extend above the other element, or there may also be intervening elements. In contrast, when an element is referred to as being "directly above" or "directly extending above" another element, there are no intervening elements. 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 the other element, or there may be intervening elements. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements.

[0030] Relative terms such as "lower than", "higher than", "above", "below", "horizontal", or "vertical" may be used herein to describe the relationship of one element, layer, or region shown in the figures to another element, layer, or region. It will be understood that these terms, as well as those discussed above, are intended to cover different orientations of the device other than the orientations depicted in the figures.

[0031] The terms used herein are for the purpose of describing particular 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", "includes", "including" specify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0032] 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 this disclosure belongs. It will be further understood that terms used herein, unless explicitly defined herein, are to be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense.

[0033] This disclosure relates to a piezoelectric device having a unique piezoelectric layer and a method of manufacturing the piezoelectric device. The piezoelectric device includes an infrastructure and a transducer disposed above the infrastructure. The infrastructure includes a piezoelectric layer located between an upper electrode and a lower electrode. The piezoelectric layer has an active portion within an active region of the transducer and a bipolar boundary portion 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 the first polarization, and the piezoelectric material in the second sub-portion has a second polarization opposite to the first polarization.

[0034] The piezoelectric device can be implemented in various devices such as a bulk acoustic wave (BAW) resonator. Before delving into the details of the unique piezoelectric thin film, an overview of the BAW resonator and its operation is provided. The BAW resonator is used in a variety of high-frequency filter applications. Figure 1 An exemplary BAW resonator 10 is shown in. 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 can be formed of materials such as tungsten (W), molybdenum (Mo), platinum (Pt), etc., and the piezoelectric layer 18 can be formed of aluminum nitride (AlN), zinc oxide (ZnO), or other suitable piezoelectric materials. Although shown as including a single layer in Figure 1 it can include multiple layers of the same material, multiple layers in which at least two layers are of different materials, or multiple layers in which each layer is of a different material. The combination of the substrate 12 and the reflector 14 is generally referred to as the infrastructure of the BAW resonator 10, and thus the transducer 16 is located on or above the infrastructure.

[0035] Continuing to refer to 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 further includes the portion of the layer below the overlapping upper electrode 20 and lower electrode 22. The outer region 26 corresponds to the portion of the BAW resonator 10 that surrounds 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.

[0036] For the BAW resonator 10, an electrical signal is applied between the upper electrode 20 and the lower electrode 22 to excite acoustic waves in the piezoelectric layer 18. These acoustic waves mainly propagate vertically. The main objective of the BAW resonator design is to confine these vertically propagating acoustic waves within the transducer 16. The upward 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 downward propagating acoustic waves are reflected back into the transducer 16 through the reflector 14 or through an air cavity (not shown) located just below the transducer in a thin film bulk acoustic resonator (FBAR).

[0037] The reflector 14 is generally formed by stacking reflector layers (RL) 28 that alternate in material composition to produce a significant reflection coefficient at the junctions of adjacent reflector layers 28. Typically, the reflector layers 28 alternate between materials having a high acoustic impedance and a low acoustic impedance, such as tungsten (W) and silicon dioxide (SiO 2 ). Although only five reflector layers 28 are shown in Figure 1 , the number of reflector layers 28 and the structure of the reflector 14 vary between different designs.

[0038] In Figure 2 , the magnitude (Z) and phase (φ) of the impedance of a relatively ideal BAW resonator are provided as a function of frequency. The solid line represents the magnitude (Z) of the impedance, while the dashed line represents the phase (φ) of the impedance. The BAW resonator 10 is unique in that it has both a resonant frequency and an anti-resonant frequency. The resonant frequency is commonly referred to as the series resonant frequency (f s ), while the anti-resonant frequency is commonly referred to as the parallel resonant frequency (f p ). When the magnitude of the impedance or reactance of the BAW resonator 10 approaches zero, the series resonant frequency (f s ) occurs. When the magnitude of the impedance or reactance of the BAW resonator 10 reaches a peak at a relatively high level, the parallel resonant frequency (f p ) appears. Generally, 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.

[0039] 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.

[0040] 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.

[0041] 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) suppression of spurious modes. It is noted that spurious modes just below the parallel resonance frequency (f p ) in the upper passband and spurious modes above the passband are suppressed, as evidenced by the smooth or substantially ripple-free phase curve between the series resonance frequency (f s ) and the parallel resonance frequency (f p ), and above the parallel resonance frequency (f p ).

[0042] The BO loop 30 corresponds to the mass loading of the portion of the upper electrode 20 that extends peripherally around the active region 24. The BO loop 30 can 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 loop 30 is referred to as the boundary (BO) region 32. The portion of the piezoelectric layer 18 that is located within the BO region 32 is referred to as the boundary portion SB. Thus, the BO region 32 is generally located between the active region 24 and the external region 26, and the boundary portion SB is located between the active portion SA and the outer portion SO.

[0043] Although the BO loop 30 can effectively suppress spurious modes above the series resonance frequency (f s ), the BO loop 30 generally has much less effect on those spurious modes below the series resonance frequency (f s ), as Figure 3B shown. A technique called diffraction imaging is typically used to suppress spurious modes below the series resonance frequency (f s ).

[0044] Diffraction imaging acts to reduce any lateral symmetry in the BAW resonator 10 or at least its transducer 16. Lateral symmetry corresponds to the coverage area of the transducer 16, and avoiding symmetry corresponds to avoiding lateral symmetry associated with the sides of the coverage area. For example, a coverage area corresponding to a pentagon rather than a square or rectangle can be selected. Avoiding symmetry helps to reduce the presence of lateral standing waves in the transducer 16. Figure 3C The circle C in shows the effect of diffraction imaging, where spurious modes below the series resonance frequency (f s ) are suppressed. Assuming that the BO loop 30 is not provided, it can be easily seen in Figure 3C that diffraction imaging cannot suppress those spurious modes above the series resonance frequency (f s ). Thus, a typical BAW resonator 10 employs both diffraction imaging and the BO loop 30.

[0045] As previously described, the BAW resonator 10 is typically used in filter networks that operate at high frequencies and require high Q values. Figure 5AThe basic ladder network LN is shown. 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 resonators B SER have the same or similar first frequency response, while the shunt resonators B SH have the same or similar second frequency response different from the first frequency response, as Figure 5B shown. In various applications, the shunt resonators B SH are detuned versions of the series resonators B SER . Thus, the frequency responses of the series resonators B SER and the shunt resonators B SH are generally very similar, but are offset relative to each other such that the parallel resonance frequency (f p,SH ) of the shunt resonator is approximately equal to the series resonance frequency (f SER ) of the series resonator B s,SER . Note that the series resonance frequency (f SH ) of the shunt resonator B s,SH is less than the series resonance frequency (f SER ) of the series resonator B s,SER . The parallel resonance frequency (f SH ) of the shunt resonator B p,SH is less than the parallel resonance frequency (f SER ) of the series resonator B p,SER .

[0046] Figure 5C is associated with Figure 5B and shows the response of the ladder network LN. The series resonance frequency (f SH ) of the shunt resonator B s,SH corresponds to the lower end of the passband skirt (phase 2), and the parallel resonance frequency (f SER ) of the series resonator B p,SER corresponds to the upper end of the passband skirt (phase 4). The series resonance frequency (f SER ) of the substantially aligned series resonator B s,SER and the parallel resonance frequency (f SH ) of the shunt resonator B p,SH fall within the passband. Figures 6A to 6E provides the equivalent circuits for five phases of the response of the ladder network LN. During the first phase ( Figure 5C 、 Figure 6A phase 1 of ), the function of the ladder network LN is to attenuate the input signal. As the series resonance frequency (f SH ) of the shunt resonator B s,SH is approached, the shunt resonator B SHThe impedance drops sharply, causing the shunt resonator B SH to provide a short circuit to ground substantially at the series resonance frequency (f s,SH ) of the shunt resonator ( Figure 5C , Figure 6B , phase 2). At the series resonance frequency (f SH ) of the shunt resonator B ( s,SH , phase 2), the input signal is substantially blocked from the output of the ladder network LN.

[0047] Between the series resonance frequency (f SH ) of the shunt resonator B and the parallel resonance frequency (f s,SH ) of the shunt resonator B corresponding to the passband ( SER ), the input signal is transmitted to the output with relatively little or zero attenuation ( p,SER , Figure 5C , Figure 6C , phase 3). Within the passband, the series resonator B ( SER ) presents a relatively low impedance, while the shunt resonator B ( SH ) presents a relatively high impedance, and the combination of the two results in a flat passband with steep low-side and high-side skirts. As the parallel resonance frequency (f SER ) of the series resonator B ( p,SER ) is approached, the impedance of the series resonator B ( SER ) becomes very high, causing the series resonator B ( SER ) to present substantially an open circuit at the parallel resonance frequency (f p,SER ) of the series resonator ( Figure 5C , Figure 6D , phase 4). At the parallel resonance frequency (f SER ) of the series resonator B ( p,SER ) (phase 4), the input signal is again substantially blocked from the output of the ladder network LN. In the last phase ( Figure 5C , Figure 6E , phase 5), the ladder network LN serves to attenuate the input signal in a manner similar to that provided in phase 1. As the parallel resonance frequency (f SER ) of the series resonator B passes, the impedance of the series resonator B decreases, and the impedance of the shunt resonator B ( p,SER ) normalizes. Thus, the ladder network LN is used to provide a high-Q passband between the series resonance frequency (f SER ) of the shunt resonator B and the parallel resonance frequency (f SH ) of the series resonator B. The ladder network LN is at the series resonance frequency (f SH ) of the shunt resonator B s,SH ) and the parallel resonance frequency (f SER ) of the series resonator B p,SER ) to provide a high-Q passband. The ladder network LN is at the series resonance frequency (f SH ) of the shunt resonator Bs,SH ) and the parallel resonance frequency (f p,SER ) both provide extremely high attenuation. The ladder network LN provides good attenuation below the series resonance frequency (f SH ) of the shunt resonator B s,SH ) and above the parallel resonance frequency (f SER ) of the series resonator B p,SER ).

[0048] In a single modern communication system such as a mobile phone, various filters need passbands having different bandwidths and centered at different frequencies. The center frequency of a filter employing the BAW resonator 10 is mainly determined by the thicknesses of the respective layers of the transducer 16, and in particular the thickness of the piezoelectric layer 18. The passband bandwidth and shape of the filter's band edges are mainly determined by the electromechanical coupling coefficient k of the piezoelectric layer 18. The electromechanical coupling coefficient k is a measure of the effectiveness with which the piezoelectric layer converts electrical energy into mechanical energy (and vice versa). Different piezoelectric materials or material compositions typically have different electromechanical coupling coefficients k.

[0049] 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 employed. These new piezoelectric materials include, but are not limited to, aluminum nitride 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.

[0050] However, each of these piezoelectric materials has a rather specific electromechanical coupling coefficient k. Therefore, designers currently must select a specific piezoelectric material and then design the remainder of the BAW resonator 10 and the filter employing the BAW resonator 10 around the electromechanical coupling coefficient k of the selected piezoelectric material. In other words, the choice of the piezoelectric material of the piezoelectric layer 18 limits the electromechanical coupling coefficient k and thus ultimately limits the ability of the designer to optimize the performance of the overall filter design. Further, designers would benefit from a technique for providing electromechanical coupling in certain regions of the piezoelectric layer 18 and providing substantially zero electromechanical coupling in other regions of the piezoelectric layer 18. For example, one would like 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 outer region 26 and / or the BO region 32.

[0051] The electromechanical coupling factor of a material is a function of the piezoelectric properties of the material. Thus, non-piezoelectric materials exhibit little or no electromechanical coupling and thus have an electromechanical coupling factor k that is zero or near zero. Piezoelectric materials exhibit an electromechanical coupling factor k that is at least partially based on the piezoelectric properties of the material.

[0052] A technique for simultaneously providing piezoelectric and non-piezoelectric portions or regions in a 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, the multiple BAW resonators 10 used to form the ladder network LN are integrated on a single die, where the transducers 16 of different BAW resonators 10 share a common substrate 12, reflector 14, etc. Further, the piezoelectric layer 18, upper electrode 20, and lower electrode 22 are formed from a common material layer by appropriate coating and etching processes, respectively.

[0053] Reference Figure 7 , the present disclosure relates to a piezoelectric layer 18 in which a portion of the piezoelectric layer 18 located in the BO region 32 is bipolar. This bipolar portion is referred to as the 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 polarization orientation, and the direction of the polarization orientation of one embodiment is shown by arrows in Figure 7 . 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 polarization orientation is constant within each portion and is thus unipolar.

[0054] In contrast, the bipolar boundary portion BSB of the piezoelectric layer 18 is formed to have sub-portions 34, 36 that have opposite polarizations, as indicated by the opposite arrows within those portions. The sub-portions 34, 36 of the bipolar boundary portion BSB can 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 can simply represent the polarization transition level or the actual structure of one or more layers that actually trigger polarization inversion during manufacturing, which will be described in further detail below.

[0055] 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 a mass load, potentially eliminating the need for the BO ring 30, which is not present inFigure 7 In the embodiments, the resulting quality factor Q is better than or comparable to the value achieved by the design incorporating the BO loop 30.

[0056] The incorporation of the bipolar boundary section BSB does not generate a boundary (BO) mode near the series resonance frequency (f s ). "Near" is defined as within 300 MHz of the series resonance frequency (f s ). Instead, the BO mode is shifted to several gigahertz above the series resonance frequency (f s ) of the active region, thus having substantially no effect on the performance of filters employing these BAW resonators 10. The use of the bipolar boundary section BSB also better reduces spurious transverse wave modes or can be comparable to conventional designs incorporating the BO loop 30. Further details regarding these and other benefits will be provided subsequently.

[0057] Common piezoelectric materials for the piezoelectric layer 18 include III-V nitrides, mainly 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 bond 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 bond during the growth of the piezoelectric layer 18. There are several processing methods to achieve the polarization reversal of III-V nitrides. For example, AlN grows in the N-polarity (N-pole) on and above certain metal surfaces such as W, Cu, and Mo, while grows in the Al-polarity (Al-pole) on surfaces such as Al, Ru, and RuO 2 . During the growth process of the piezoelectric layer 18, an appropriate inversion layer IL can be coated in the BO region 32 and lithographically patterned to achieve Figure 7 the bipolar boundary section BSB shown.

[0058] For the illustrated embodiments, a firmly mounted resonator or FBAR infrastructure is provided. By coating an appropriate metal layer etc. above the piezoelectric layer infrastructure and then etching the metal layer in a way that leaves the lower electrode 22, the lower electrode 22 is formed above the infrastructure. Next, the lower part of the piezoelectric layer 18 is grown to a first thickness above the lower electrode 22 with a first polarization and through the outer region 26, the BO region 32, and the active region 24. At this level, an appropriate metal layer etc. is coated above the piezoelectric layer 18 and then etched in a way that leaves the inversion layer IL. As previously mentioned, the inversion layer IL is located in the BO region 32.

[0059] Next, resume the growth process of the piezoelectric layer 18 such that the upper portion of the piezoelectric layer 18 grows through the outer region 26, the BO region 32, and the active region 24 above the lower portion of the piezoelectric layer 18 until the piezoelectric layer 18 reaches a second thickness. It should be noted that the polarization of the piezoelectric layer 18 grown above the inversion layer IL is inverted. Thus, the piezoelectric layer 18 above 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 inverted and thus has the first polarization. An upper electrode 20 is formed above the piezoelectric layer 18 by coating an appropriate metal layer or the like over the piezoelectric layer and then etching the metal layer away from the upper electrode 20.

[0060] Figure 8A The Al-pole (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 bond along the c-axis places the Al atoms below the N atoms. The net polarity P NET is inverted, as shown by the downward arrow. As Figure 8B shown, for the N-pole (N-polar) orientation with the growth direction along the c-axis, the situation is reversed. For the N-polar atomic structure, the Al-N atomic bond along the c-axis places the N atoms below the Al atoms. The net polarity P NET is aligned with the growth direction, as shown by the upward arrow. Thus, the polarization of the Al-polar orientation is opposite to the polarization of the N-polar 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.

[0061] The thickness ratio of the sub-parts 34, 36 with opposite polarizations can be selected to cancel the fundamental BAW mode in the BO region 32. The thickness of each layer can be, but does not have to be, 1:1. The specific ratio can depend on the stack of materials and components below and above the piezoelectric layer 18. Generally, 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 such as 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 within the scope of the present disclosure. Although the bipolar boundary portion BSB is shown to switch polarization only once, multiple polarity switches (i.e., two or more alternations in each sub-part 34, 36) throughout the thickness of the piezoelectric layer 18 are also conceivable, as described further subsequently. The aforementioned thickness ratio also applies to the cumulative thickness of each sub-part 34, 36.

[0062] When using a conventional BO loop 30, 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 variation generated in the acoustic dispersion between the active region 24 and the BO region 32 is utilized to confine the lateral acoustic energy within the active portion SA of the piezoelectric layer 18. Figure 9A The downward shift of the frequency is plotted against the quality factor Qp at the optimal BO ring 30 width. The magnitude of the mass loading provided by the BO ring 30 is crucial for obtaining a better Qp.

[0063] For the present disclosure, the bipolar boundary portion BSB confines the energy of the lateral wave without significantly altering the acoustic dispersion from the active portion SA. In Figure 9A significantly higher Qp values are obtained with negligible mass loading. Although the polarization reversal does not cause a change in the mechanical dispersion, due to the electrical boundary conditions, the bipolar boundary portion BSB does not support several modes. The antisymmetric mode (along the z direction) in the active portion SA does not propagate into the bipolar boundary portion BSB and thus remains confined within the active portion SA.

[0064] The active region 24 of the BAW resonator resonates at the fundamental series resonance frequency (f s ). The novelty of the bipolar boundary portion BSB is that it does not support the fundamental acoustic tone. In the structure where the sub - portions 34, 36 have opposite polarizations, the fundamental mode resonance has zero coupling (k2e). Regardless of the mass loading added to or by the bipolar boundary portion BSB, modes near the series resonance frequency (f s ) are not excited in the BO region 32. Thus, modes are not generated from the BO region 32 at frequencies crucial for filters made from such BAW resonators 10. Figure 9B It is shown that when the BO region 32 is designed as the bipolar boundary portion BSB, the BO mode is effectively suppressed in the BO region 32. At the fundamental series resonance frequency (f s ), since the bipolar BO region 32 does not participate in the electromechanical conversion, the effective piezoelectric coupling reduction can reach 4% or more.

[0065] The bipolar boundary portion BSB does support the second - overtone resonance. These resonances typically occur at frequencies greater than twice the fundamental series resonance frequency (f s ). Since most BAW resonators 10 operate at frequencies above 1500 MHz, the second overtone from the BO region 32 occurs at least several gigahertz away from the fundamental series resonance frequency (f s ). Figure 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.

[0066] 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 . Figure 10A , Figure 10B and Figure 10C The best case scenarios for the two configurations are compared. Figure 10A , Figure 10B and Figure 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.

[0067] Figure 11 The BAW resonator 10 is shown incorporating a BO ring 30 over a bipolar boundary portion BSB of a BO region 32 . Figure 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.

[0068] 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 Figure 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.

[0069] There are various potential alternatives to the embodiments described above. For example, as Figure 14 shown, sub-parts 34, 36 of the BO region 32 can have their polarizations exchanged relative to the above embodiments. As previously pointed out, the bipolar boundary part BSB can have more than two sub-parts. As Figure 15 shown, the bipolar boundary part BSB is divided into four sub-parts 40, 42, 44, 46, which are separated by three inversion layers IL. The inversion layers IL promote polarization exchange of the piezoelectric material within the bipolar boundary part BSB during the growth or coating process of the piezoelectric layer 18. Sub-parts 40, 44 have a first polarization, while sub-parts 42, 46 have a second polarization opposite to the first polarization. In this embodiment, the outer part SO and the active part SA also have the second polarization; however, all of these polarizations can be exchanged. Further, the polarizations of sub-parts 40, 42, 44, 46 can be exchanged. The number and thickness of sub-parts 40, 42, 44, 46 can vary between different embodiments according to the performance metrics required for the application.

[0070] 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 included within the spirit of the disclosure herein and the scope of the appended claims.

Claims

1. A piezoelectric device, comprising: a base structure; and a transducer located above the base structure, comprising: a 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 within the active region of the transducer and a bipolar boundary portion within the boundary region of the transducer; the piezoelectric material in the active portion has a first polarization; the bipolar boundary portion includes 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 the first polarization, and the piezoelectric material in the second sub-portion has a second polarization opposite to the first polarization; and the outer portion of the piezoelectric layer has only one of the first polarization and the second polarization.

2. The piezoelectric device according to claim 1, further comprising a first inversion layer located between the first sub-portion and the second sub-portion.

3. The piezoelectric device according to claim 2, wherein, the first inversion layer provides a mass load within the boundary region.

4. The piezoelectric device according to claim 3, further comprising a boundary ring located within the boundary region and above the piezoelectric layer, wherein the boundary ring provides a mass load.

5. The piezoelectric device according to claim 1, further comprising a boundary ring located within the boundary region and above the piezoelectric layer, wherein the boundary ring provides a mass load.

6. The piezoelectric device according to claim 1, wherein, the piezoelectric material is formed of a compound comprising 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 comprises aluminum nitride AlN doped with a transition metal comprising at least one of scandium, erbium, magnesium, and hafnium.

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

10. The piezoelectric device according to claim 1, 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, the reflector including a plurality of reflector layers located above the substrate such that the device is a bulk acoustic wave 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 thin film bulk acoustic wave resonator FBAR.

13. The piezoelectric device according to claim 1, wherein, the bipolar boundary portion includes at least two additional sub-portions located above or below the first sub-portion and the second sub-portion, the at least two additional sub-portions alternating between the first polarization and the second polarization.

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

0.

15. A method for forming a piezoelectric device, comprising: providing a substrate; forming a lower electrode above the substrate; forming a lower portion of a piezoelectric layer above 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 within the boundary region above the lower portion of the piezoelectric layer; forming an upper portion of the piezoelectric layer 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, the upper portion of the piezoelectric layer located above the inversion layer and within the boundary region has a second polarization opposite to the polarization of the first polarization; and the outer portion of the piezoelectric layer located in the outer region has only one of the first polarization and the second polarization; and forming an upper electrode above the upper portion of the piezoelectric layer.

16. The method according to claim 15, wherein, the active region and the outer portion of the piezoelectric layer have the first polarization.

17. The method according to claim 15, wherein, the piezoelectric device is a bulk acoustic wave resonator or a thin - film bulk acoustic wave resonator FBAR based on a solidly mounted resonator.

18. A wireless device, comprising: one or more piezoelectric devices, wherein at least one of the one or more piezoelectric devices comprises: a substrate; and a transducer located above the substrate, comprising: a 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 within the active region of the transducer and a bipolar boundary portion within the 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 - part and a second sub - part located above or below the first sub - part; and the piezoelectric material in the first sub - part has the first polarization, and the piezoelectric material in the second sub - part has a second polarization opposite to the first polarization.

19. The wireless device according to claim 18, wherein, the outer portion of the piezoelectric layer has only one of the first polarization and the second polarization, and the outer portion is within an outer region such that the boundary region is located between the outer region and the active region.

20. The wireless device according to claim 18, wherein, the at least one piezoelectric device is a bulk acoustic wave resonator or a thin - film bulk acoustic wave resonator FBAR based on a solidly mounted resonator.

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