ESD protection layer in electrically conductive bragg reflector for BAW devices

Abstract

The present disclosure relates to a bulk acoustic wave (BAW) device with electrostatic discharge (ESD) protection. The disclosed BAW device includes a substrate (106), an ESD protection layer (106) over the substrate (106), a first bottom reflector (109-1) over the ESD protection layer (108), a first bottom electrode (112-1) over the first bottom reflector (109-1), a first top electrode (118-1) vertically aligned with the first bottom electrode (112-1), and a piezoelectric layer (116C) vertically between the first bottom electrode (112-1) and the first top electrode (118-1). Herein, the ESD protection layer (108) is electrically insulating and eligible to achieve at least a 400 V breakdown voltage with a 200 nm thickness. The first bottom reflector (109-1) includes a stack of alternating high acoustic impedance conductive layers (120A-E) and low acoustic impedance conductive layers (120A-E). The first bottom electrode (112-1) is electrically connected to the first bottom reflector (109-1). A breakdown voltage of the piezoelectric layer (116C) is no more than the breakdown voltage of the ESD protection layer (108).

Description

ESD PROTECTION LAYER IN ELECTRICALLY CONDUCTIVE BRAGG REFLECTOR FOR BAW DEVICESRelated Applications

[0001] This application claims the benefit of provisional patent application serial number 63 / 674,480, filed July 23, 2024, the disclosure of which is hereby incorporated herein by reference in its entirety.Field of the Disclosure

[0001] The present disclosure relates to a bulk acoustic wave (BAW) device with one or more BAW resonators, each of which includes at least one electrically conductive reflector with an electrostatic discharge (ESD) protection layer.Background

[0002] Due to their small size, high Q values, and very low insertion losses at microwave frequencies, particularly those above 1.5 Gigahertz (GHz), bulk acoustic wave (BAW) filters incorporating BAW resonators have been widely used in many modern wireless applications. In particular, the BAW filters are the filters of choice for many 3rdGeneration (3G) and 4thGeneration (4G) wireless devices, and are destined to dominate filter applications for 5th 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 as such, pose extremely challenging filtering demands. While these demands keep raising the complexity of the wireless devices, there is a constant need to improve the performance of BAW resonators and BAW-based filters as well as decrease the cost and size associated therewith.

[0003] Electrical loss in the BAW filters, or other BAW devices, can negatively affect performance, such as Q value. To meet filtering requirements in certain applications (e.g., 5G networks), the BAW filters operate at higher frequencies (e.g., greater than 5 GHz), which may require thin layers of electrodes. However,reducing the thickness of the electrodes may result in increased resistance and / or electrical loss.

[0004] Accordingly, there remains a need for improved BAW device designs that can accommodate reductions in electrode thickness to meet performance requirements (e.g., filtering requirements), while preventing increases in resistance and / or electrical loss in order to retain high Q values. Further, there is also a need to avoid ESD failures in the final product.Summary

[0005] The present disclosure relates to a bulk acoustic wave (BAW) device with one or more BAW resonators, each of which includes at least one electrically conductive reflector with an electrostatic discharge (ESD) protection layer. The disclosed BAW device includes a substrate, an ESD protection layer formed over the substrate, and a first BAW resonator that is composed of a first bottom reflector, a first bottom electrode, a first top electrode, and a first piezoelectric portion. Herein, the ESD protection layer is electrically insulating and eligible to achieve at least a 400 V breakdown voltage with a 200 nm thickness. Within the first BAW resonator, the first bottom reflector is formed over the ESD protection layer and includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers.The first bottom electrode is formed over the first bottom reflector and electrically connected to the first bottom reflector. The first top electrode is vertically aligned with the first bottom electrode, the first piezoelectric portion is vertically between the first bottom electrode and the first top electrode, and a breakdown voltage of the first piezoelectric portion is no more than the breakdown voltage of the ESD protection layer.

[0006] In one embodiment of the BAW device, the first BAW resonator further includes a first bottom dielectric layer and a first bottom connection structure. Herein, the first bottom dielectric layer is formed directly over the first bottom reflector and partially covers a top surface of the first bottom reflector. The first bottom electrode is formed directly over the first bottom dielectric layer. The firstbottom connection structure is configured to electrically connect the first bottom electrode with the first bottom reflector.

[0007] In one embodiment of the BAW device, the first bottom connection structure extends from the first bottom electrode, along sides of the first bottom dielectric layer, and towards the top surface of the first bottom reflector.

[0008] In one embodiment of the BAW device, the first bottom connection structure is composed of conductive vias.

[0009] In one embodiment of the BAW device, the first bottom connection structure and the first bottom electrode are formed from the same bottom electrode layers. The first bottom connection structure is in contact with a portion of the top surface of the first bottom reflector, which is not covered by the first bottom dielectric layer.

[0010] In one embodiment of the BAW device, the bottom electrode layers include at least a first bottom electrode layer and a second bottom electrode layer. The second bottom electrode layer fully covers the sides and the top surface of the first bottom dielectric layer and is in contact with the portion of the top surface of the first bottom reflector that is not covered by the first bottom dielectric layer. The first bottom electrode layer is formed over and fully covers the second bottom electrode layer.

[0011] In one embodiment of the BAW device, the first bottom electrode layer is formed of tungsten (W), molybdenum (Mo), or platinum (Pt), and the second bottom electrode layer is formed of aluminum copper (AICu) or titanium (Ti).

[0012] In one embodiment of the BAW device, the ESD protection layer is capable of being tuned to have an opposite deposition stress direction to the first bottom reflector.

[0013] In one embodiment of the BAW device, the ESD protection layer has a high thermal conductivity larger than 300W7mK.

[0014] In one embodiment of the BAW device, the ESD protection layer is formed of aluminum nitride (AIN).

[0015] In one embodiment of the BAW device, the high acoustic impedance conductive layers are formed of W, Mo, or Pt. The low acoustic impedance conductive layers are formed of aluminum (Al) or titanium (Ti).

[0016] In one embodiment of the BAW device, the first top electrode is composed of at least a first top electrode layer and a second top electrode layer. The first top electrode layer is formed directly over the first piezoelectric portion, and the second top electrode layer is formed over the first top electrode layer. The first top electrode layer is formed of W, Mo, or Pt, and the second top electrode layer is formed of AICu or Ti.

[0017] In one embodiment of the BAW device, the first BAW resonator further includes a border ring (BO) formed on or within the first top electrode to suppress spurious modes.

[0018] In one embodiment of the BAW device, the first BAW resonator further includes a first top reflector. Herein, the first top reflector includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers. The first top reflector is formed over and electrically connected to the first top electrode.

[0019] In one embodiment of the BAW device, the first BAW resonator further includes a first top dielectric layer and a first top connection structure. Herein, the first top dielectric layer is formed directly over the first top electrode, and a peripheral portion of a top surface of the first top electrode is not covered by the first top dielectric layer. The first top reflector is formed directly over the first top dielectric layer. The first top connection structure extends from a peripheral portion of a bottom surface of the first top reflector, along sides of the first top dielectric layer, and toward the peripheral portion of the top surface of the first top electrode, which is not covered by the first top dielectric layer. The first top connection structure is configured to electrically connect the first top electrode with the first top reflector.

[0020] In one embodiment of the BAW device, the first top connection structure and a bottommost one of the alternating high acoustic impedanceconductive layers and low acoustic impedance conductive layers are formed of a same conductive material.

[0021] In one embodiment of the BAW device, the first top connection structure and the bottommost one of the alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers are formed of W, Mo, or Pt.

[0022] According to one embodiment, the BAW device further includes a second electronic component formed over the ESD protection layer.

[0023] In one embodiment of the BAW device, the second electronic component is a second BAW resonator, which includes a second bottom reflector, a second bottom electrode, a second top electrode, and a second piezoelectric portion. The second bottom reflector is formed over the ESD protection layer and includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers. The second bottom electrode is formed over the second bottom reflector and electrically connected to the second bottom reflector. The second top electrode is vertically aligned with the second bottom electrode, while the second piezoelectric portion is vertically between the second bottom electrode and the second top electrode. Herein, the second piezoelectric portion and the first piezoelectric portion are formed from a same common piezoelectric layer that has a breakdown voltage no more than the breakdown voltage of the ESD protection layer.

[0024] In one embodiment of the BAW device, the second electronic component has no piezoelectric portion and is a lead or a probe pad, which is electrically connected to the first bottom electrode of the first BAW resonator and electrically isolated to at least one surrounding BAW resonator other than the first BAW resonator.

[0025] According to one embodiment, an exemplary method of fabricating a BAW device starts with providing a substrate. An ESD protection layer is then deposited over the substrate. Herein, the ESD protection layer is electrically insulating and eligible to achieve at least a 400 V breakdown voltage with a 200 nm thickness. Next, one or more conductive reflectors, each of which includes astack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers, are deposited over the ESD protection layer. One or more bottom electrodes are then formed over the one or more conductive reflectors, respectively. Each of the one or more bottom electrodes is electrically connected to a corresponding one of the one or more conductive reflectors. A piezoelectric layer is formed over each of the one or more bottom electrodes. After the piezoelectric layer is formed, one or more top electrodes are formed over the piezoelectric layer and aligned with the one or more bottom electrodes, respectively. A breakdown voltage of the piezoelectric layer is no more than the breakdown voltage of the ESD protection layer.

[0026] According to one embodiment, a system includes radio-frequency (RF) input circuitry, RF output circuitry, and filter circuitry connected between the RF input circuitry and the RF output circuitry. Herein, the filter circuitry has at least one BAW device, which includes a substrate, an ESD protection layer formed over the substrate, and a first BAW resonator with a first bottom reflector, a first bottom electrode, a first top electrode, and a first piezoelectric portion. The ESD protection layer is electrically insulating and eligible to achieve at least a 400 V breakdown voltage with a 200 nm thickness. The first bottom reflector of the first BAW resonator is formed over the ESD protection layer and includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers. The first bottom electrode is formed over the first bottom reflector and electrically connected to the first bottom reflector. The first top electrode is vertically aligned with the first bottom electrode, and the first piezoelectric portion is vertically between the first bottom electrode and the first top electrode. A breakdown voltage of the first piezoelectric portion is no more than the breakdown voltage of the ESD protection layer.

[0027] In another aspect, any of the foregoing aspects individually or together, and / or various separate aspects and features as described herein, may be combined for additional advantage. Any of the various features and elements as disclosed herein may be combined with one or more other disclosed features and elements unless indicated to the contrary herein.

[0028] Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.Brief Description of the Drawing Figures

[0029] The accompanying drawing figures 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.

[0030] Figure 1 is a diagram illustrating a conventional bulk acoustic wave (BAW) resonator.

[0031] Figure 2 is a diagram graphically illustrating the magnitude and phase of the electrical impedance as a function of the frequency for a relatively ideal BAW resonator.

[0032] Figures 3A-3C are diagrams graphically illustrating phase curves for various conventional BAW resonators.

[0033] Figure 4 is a diagram illustrating a conventional BAW resonator with a top electrode including a border ring (BO).

[0034] Figure 5A is a schematic diagram of a conventional ladder network.

[0035] Figures 5B and 5C are graphs of a frequency response for BAW resonators in the conventional ladder network of Figure 5A and a frequency response for the conventional ladder network of Figure 5A, respectively.

[0036] Figures 6A-6E are circuit equivalents for the ladder network of Figure 5A at the frequency points 1 , 2, 3, 4, and 5, which are identified in Figure 5C.

[0037] Figure 7 is a cross-sectional diagram illustrating an exemplary BAW device with one or more BAW resonators, each of which includes an electrically conductive reflector with an electrostatic discharge (ESD) protection layer according to embodiments of the present disclosure.

[0038] Figures 8A-8B illustrate breakdown voltages of aluminum nitride and different types of oxides.

[0039] Figures 9A-9B illustrate wafer shapes based on compressive stress and tensile stress.

[0040] Figures 10A-1 OC illustrate a shape change of a device wafer.

[0041] Figure 11 illustrates each BAW resonator included in the exemplary BAW device having two electrically conductive reflectors according to embodiments of the present disclosure.

[0042] Figure 12 illustrates a flowchart of an exemplary method of fabricating the BAW device shown in Figure 7 according to some embodiments of the present disclosure.

[0043] Figure 13 illustrates a block diagram of an exemplary system that includes at least one BAW device shown in Figure 7 or Figure 11 .

[0044] Figure 14 illustrates a block diagram of an exemplary communication device that includes at least one BAW device shown in Figure 7 or Figure 11 .

[0045] It will be understood that for clear illustrations, Figures 1-14 may not be drawn to scale.Detailed Description

[0046] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0047] 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, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departingfrom the scope of the present disclosure. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0048] It will be understood that when an element such as a layer, region, or substrate is referred to as being "on" or extending "onto" another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or extending "directly onto" another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being "over" or extending "over" another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly over" or extending "directly over" another element, there are no intervening elements present. It will 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 intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

[0049] Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a 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 and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

[0050] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes," and / or "including" when used herein specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude thepresence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0051] 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 should 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 unless expressly so defined herein.

[0052] Embodiments are described herein with reference to schematic illustrations of embodiments of the disclosure. As such, the actual dimensions of the layers and elements can be different, and variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and / or tolerances, are expected. For example, a region illustrated or described as square or rectangular can have rounded or curved features, and regions shown as straight lines may have some irregularity. Thus, the regions illustrated in the figures are schematic and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the disclosure. Additionally, sizes of structures or regions may be exaggerated relative to other structures or regions for illustrative purposes and, thus, are provided to illustrate the general structures of the present subject matter and may or may not be drawn to scale. Common elements between figures may be shown herein with common element numbers and may not be subsequently redescribed.

[0053] The present disclosure relates to a bulk acoustic wave (BAW) device having one or more BAW resonators, each of which includes at least one all- metal Bragg reflector with an electrostatic discharge (ESD) protection layer. The disclosed BAW resonator utilizes the all-metal Bragg reflector to compensate for reductions in electrode thickness, which not only meets performance requirements (e.g., filtering requirements), but also prevents increases in resistance and / or electrical losses in order to retain high Q values. In addition,the disclosed BAW resonator further introduces an ESD protection layer to electrically isolate the all-metal Bragg reflector from a substrate of the BAW resonator, so as to avoid ESD failures. The ESD protection layer is thermally conductive and electrically insulating, and helps tune a wafer bow to improve processability.

[0054] Prior to delving into the details of these concepts, an overview of BAW resonators and filters that employ BAW resonators is provided. The BAW resonators are used in many high-frequency filter applications. An exemplary BAW resonator 10 is illustrated in Figure 1 . The BAW resonator 10 is a solidly mounted resonator (SMR) type BAW resonator 10 and generally includes a substrate 12, a reflector 14 mounted over the substrate 12, and a transducer 16 mounted over the reflector 14. The transducer 16 rests on the reflector 14 and includes a piezoelectric layer 18, which is sandwiched between a top electrode 20 and a bottom electrode 22. The top and bottom electrodes 20, 22 may be formed of tungsten (W), molybdenum (Mo), platinum (Pt), or like materials, and the piezoelectric layer 18 may be formed of aluminum nitride (AIN), zinc oxide (ZnO) or other appropriate piezoelectric materials. Although shown as a single layer in Figure 1 , the piezoelectric layer 18, the top electrode 20, and / or the bottom electrode 22 may each include 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.

[0055] The BAW resonator 10 is divided into an active region 24 and an outside region 26. The active region 24 generally corresponds to the section of the BAW resonator 10 where the top and bottom electrodes 20, 22 overlap and also includes the layers below the overlapping top and bottom electrodes 20, 22. The outside region 26 corresponds to the section of the BAW resonator 10 that surrounds the active region 24.

[0056] For the BAW resonator 10, applying electrical signals across the top electrode 20 and the bottom electrode 22 excites acoustic waves in the piezoelectric layer 18. These acoustic waves primarily propagate vertically. A primary goal in BAW resonator design is to confine these vertically propagatingacoustic waves in the transducer 16. Acoustic waves traveling upwardly are reflected back into the transducer 16 by an air-metal boundary at a top surface of the top electrode 20. Acoustic waves traveling downwardly are reflected back into the transducer 16 by the reflector 14, or by an air cavity, which is provided just below the transducer 16 in a film bulk acoustic resonator (FBAR).

[0057] The reflector 14 is typically formed by a stack of reflector layers (RLs) 28A through 28E (referred to generally as reflector layers 28), which 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 having high and low acoustic impedances, such as W and silicon dioxide (SiC>2) . While only five reflector layers 28 are illustrated in Figure 1 , the number of reflector layers 28 and the structure of the reflector 14 will vary from one design to another.

[0058] The magnitude (Z) and phase (<J>) of the electrical impedance as a function of the frequency (GHz) for a relatively ideal BAW resonator 10 is provided in Figure 2. The magnitude (Z) of the electrical impedance is illustrated by the solid line, while the phase ($) of the electrical impedance is illustrated by the dashed line. A unique feature of the BAW resonator 10 is that it has both a resonance frequency and an antiresonance frequency. The resonance frequency is typically referred to as the series resonance frequency (fs), and the antiresonance frequency is typically referred to as the parallel resonance frequency (fp). The series resonance frequency ( / s) occurs when the magnitude of the impedance, or reactance, of the BAW resonator 10 approaches zero. The parallel resonance frequency (fp) occurs when the magnitude of the impedance, or reactance, of the BAW resonator 10 peaks at a significantly high level. In general, the series resonance frequency (fs) is a function of the thickness or height of the piezoelectric layer 18 and the mass of the top and bottom electrodes 20, 22.

[0059] For the phase (<|>), the BAW resonator 10 acts like an inductance that provides a 90° phase shift between the series resonance frequency ( / s) and the parallel resonance frequency (fp). In contrast, the BAW resonator 10 acts like acapacitance that provides a -90° phase shift below the series resonance frequency (fs) and above the parallel resonance frequency (fp). The BAW resonator 10 presents a very low, near zero, resistance at the series resonance frequency (fs), and a very high resistance at the parallel resonance frequency (fp). The electrical nature of the BAW resonator 10 lends itself to the realization of a very high quality factor (Q) inductance over a relatively short range of frequencies, which has proven to be very beneficial in high frequency filter networks, especially those operating at frequencies around 1 .8 GHz and above.

[0060] Unfortunately, the phase ($) curve of Figure 2 is representative of an ideal phase curve. In reality, approaching this ideal is challenging. A typical phase curve for the BAW resonator 10 of Figure 1 is illustrated in Figure 3A. Instead of being a smooth curve, the phase curve of Figure 3A includes a ripple below the series resonance frequency (fs), between the series resonance frequency (fs) and the parallel resonance frequency (fp), and above the parallel resonance frequency (fp). The ripple is the result of spurious modes, which are caused by spurious resonances that occur in corresponding frequencies. While the vast majority of the 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 quality factor (Q) associated with the BAW resonator 10.

[0061] As illustrated in Figure 4, a border ring (BO) ring 30 is formed on or within (not shown) the top electrode 20 to suppress certain spurious modes. The spurious modes that are suppressed by the BO ring 30 are those above the series resonance frequency (fs), as highlighted by circles A and B in the phase curve of Figure 3B. Circle A shows a suppression of the ripple, and thus the spurious mode, in the passband of the phase curve, which resides between the series resonance frequency (fs) and the parallel resonance frequency (fP). Circle B shows suppression of the ripple, and thus the spurious modes, above the parallel resonance frequency (fp). Notably, the spurious mode in the upper shoulder of the passband, which is just below the parallel resonance frequency(fp), and the spurious modes above the passband are suppressed, as evidenced by the smooth or substantially ripple free phase curve between the series resonance frequency (fs) and the parallel resonance frequency (fp) and above the parallel resonance frequency (fp).

[0062] The BO ring 30 corresponds to a mass loading of a portion of the top electrode 20 that extends about a periphery of the active region 24. In this regard, the BO ring 30 with mass loading forms a raised frame that is arranged about a periphery of the top electrode 20. The BO ring 30 may correspond to a thickened portion of the top electrode 20 or the application of additional layers of an appropriate material over the top electrode 20. The portion of the BAW resonator 10 that includes and resides below the BO ring 30 is referred to as a BO region 32. Accordingly, the BO region 32 corresponds to an outer perimeter portion of the active region 24 and resides inside the active region 24. In addition, a central region 34 of the BAW resonator 10 is defined laterally inside of the BO region 32 and is not covered by the BO ring 30.

[0063] While the BO ring 30 is effective at suppressing spurious modes above the series resonance frequency ( / s), the BO ring 30 has little or no impact on those spurious modes below the series resonance frequency (fs), as shown in Figure 3B. A technique referred to as apodization is often used to suppress the spurious modes that fall below the series resonance frequency (fs).

[0064] Apodization works to avoid, or at least significantly reduce, any lateral symmetry in the BAW resonator 10, or at least in the transducer 16 thereof. Lateral symmetry corresponds to the footprint of the transducer 16, and avoiding the lateral symmetry corresponds to avoiding symmetry associated with the sides of the footprint. For example, one may choose a footprint that corresponds to a pentagon instead of a square or rectangle. Avoiding symmetry helps reduce the presence of lateral standing waves in the transducer 16. Circle C of Figure 3C illustrates the effect of apodization in which the spurious modes below the series resonance frequency (fs) are suppressed. Assuming that no BO ring 30 is provided, one can readily see in Figure 3C that apodization fails to suppressthose spurious modes above the series resonance frequency (fs). As such, the typical BAW resonator 10 employs both apodization and the BO ring 30.

[0065] As noted above, BAW resonators like the BAW resonators 10 in Figures 1 and 4 are often used in filter networks that operate at high frequencies and require high Q values. A basic ladder network 40 is illustrated in Figure 5A. The ladder network 40 includes two series resonators BSER and two shunt resonators BSH, which are arranged in a traditional ladder configuration. Typically, the series resonators BSER have the same or similar first frequency response, and the shunt resonators BSH have the same or similar second frequency response, which is different than the first frequency response, as shown in Figure 5B. In many applications, the shunt resonators BSH are a detuned version of the series resonators BSER. AS a result, the frequency responses for the series resonators BSE and the shunt resonators BSH are generally very similar, yet shifted relative to one another such that the parallel resonance frequency (fp,sn), of the shunt resonators approximates the series resonance frequency (fs.sER), of the series resonators BSER. Note that the series resonance frequency (fs,sn) of the shunt resonators BSH is less than the series resonance frequency (fs.sER) of the series resonators BSER. The parallel resonance frequency (fp,sH) of the shunt resonators BSH is less than the parallel resonance frequency (fp.sER) of the series resonators BSER.

[0066] Figure 5C is associated with Figure 5B and illustrates the response of the ladder network 40. The series resonance frequency (fs,sn) of the shunt resonators BSH corresponds to the low side of the passband’s skirt (phase 2), and the parallel resonance frequency (fp.sER) of the series resonators BSER corresponds to the high side of the passband’s skirt (phase 4). The substantially aligned series resonance frequency (fs.sER) of the series resonators BSER and the parallel resonance frequency (fp,sn) of the shunt resonators BSH fall within the passband.

[0067] Figures 6A through 6E provide circuit equivalents for the five phases of the response of the ladder network 40. During the first phase (phase 1 , Figures 5C, 6A), the ladder network 40 functions in order to attenuate the input signal. Asthe series resonance frequency (fs,sn) of the shunt resonators BSH is approached, the impedance of the shunt resonators BSH drops precipitously, such that the shunt resonators BSH essentially provide a short to ground at the series resonance frequency (fs,sn) of the shunt resonators (phase 2, Figures 5C, 6B). At the series resonance frequency (fs,sn) of the shunt resonators BSH (phase 2), the input signal is essentially blocked from the output of the ladder network 40.

[0068] Between the series resonance frequency (fs,sn) of the shunt resonators BSH and the parallel resonance frequency (fp.sER) of the series resonators BSER, which corresponds to the passband, the input signal is passed to the output with relatively little or no attenuation (phase 3, Figures 5C, 6C). Within the passband, the series resonators BSER present a relatively low impedance, while the shunt resonators BSH present a relatively high impedance, wherein the combination of the two leads to a flat passband with steep low and high-side skirts. As the parallel resonance frequency (fp.sER) of the series resonators BSER is approached, the impedance of the series resonators BSER becomes very high, such that the series resonators BSER essentially present themselves as open at the parallel resonance frequency (fp.sER) of the series resonators (phase 4, Figures 5C, 6D). At the parallel resonance frequency (fp.sER) of the series resonators BSER (phase 4), the input signal is again essentially blocked from the output of the ladder network 40. During the final phase (phase 5, Figures 5C, 6E), the ladder network 40 functions in order to attenuate the input signal, in a similar fashion to that provided in phase 1 . As the parallel resonance frequency (fp.sER) of the series resonators BSER is passed, the impedance of the series resonators BSER decreases, and the impedance of the shunt resonators BSH normalizes. Thus, the ladder network 40 functions in order to provide a high Q passband between the series resonance frequency (fs,sn) of the shunt resonators BSH and the parallel resonance frequency (fp.sER) of the series resonators BSER. The ladder network 40 provides extremely high attenuation at both the series resonance frequency (fs,sH) of the shunt resonators BSH and the parallel resonance frequency (fp.sER) of the series resonators. The ladder network 40 provides good attenuation belowthe series resonance frequency (fs,sn) of the shunt resonators BSH and above the parallel resonance frequency (fp.sER) of the series resonators BSER.

[0069] In order to meet filtering requirements in certain applications (e.g., 5G networks), the BAW filters need to operate at higher frequencies (e.g., greater than 5 GHz), which may require thin layers of the piezoelectric and the top and bottom electrodes (e.g., frequency scale as 1 / thickness). However, reducing the thickness of the electrodes may result in increased resistance and / or electrical loss. Figure 7 illustrates a cross-sectional diagram of a BAW device 100 (e.g., a BAW filter) including one or more BAW resonators, each of which can accommodate reductions in electrode thickness to meet filtering requirements, prevent increases in electrical loss to retain a high Q value, and avoid ESD failures according to some embodiments of the present disclosure. For the purpose of this illustration, the BAW device 100 includes two BAW resonators 102 (e.g., a first BAW resonator 102-1 and a second BAW resonator 102-2) formed over a substrate 106 via an ESD protection layer 108. In different applications, the BAW device 100 may include fewer or more BAW resonators.

[0070] In detail, the substrate 106 may be formed of silicon, or other semiconductor materials. The ESD protection layer 108 formed over the substrate 106 is an isolation layer with high breakdown strength. Herein and hereafter, the isolation layer with high breakdown strength refers to a layer that is electrically insulating and can achieve at least a 400 V or at least a 500 V breakdown voltage with a thickness as thin as 200 nm. In addition, it is desired that the ESD protection layer 108 has a relatively high thermal conductivity, such as larger than 300W / mK. For a non-limiting example, the ESD protection layer 108 is formed of aluminum nitride (AIN) with a thickness up to 1.5 pm (e.g., 200nm and 1000nm). AIN is a good electrical insulator with a high thermal conductivity of up to 321 W / (m K) and has a high breakdown voltage strength. Figure 8A illustrates a relationship between a breakdown voltage (V) and a thickness of AIN, while Figure 8B illustrates a relationship between a breakdown voltage (V) and a thickness of oxides. It is clear that AIN can sustain a larger voltage for a thinner thickness as compared to various oxides. For a samebreakdown voltage 300V, AIN only needs about 250nm, while an oxide (e.g., vapor-deposited oxide) at least needs about 0.3 pm. Moreover, the poor thermal conductivity of oxides also makes AIN an optimal material choice for high performance resonators (more details are described in the following paragraphs).

[0071] The first BAW resonator 102-1 includes a first bottom reflector 109-1 formed over the ESD protection layer 108, a first bottom dielectric layer 110-1 over the first bottom reflector 109-1 , a first bottom electrode 112-1 over the first bottom dielectric layer 110-1 , a first bottom connection structure 114-1 , a first piezoelectric portion 116-1 over the first bottom electrode 112-1 , and a first top electrode 118-1. In some embodiments, the first bottom reflector 109-1 is a Bragg reflector and is composed of a stack of reflector layers 120A through 120E (referred to generally as reflector layers 120), each of which is electrically conductive. The reflector layers 120 alternate between different electrically conductive materials (e.g. different metal materials) having high and low acoustic impedances, so as to produce a significant reflection coefficient at a junction of adjacent reflector layers 120. The electrically conductive materials with high acoustic impedance may be W, Mo, or Pt, and the electrically conductive materials with low acoustic impedance may be aluminum (Al) or titanium (Ti). In a non-limiting example, a first reflector layer 120A at a top portion of the first bottom reflector 109-1 is formed of W, a second reflector layer 120B directly underneath the first reflector layer 120A is formed of Al, a third reflector layer 120C directly underneath the second reflector layer 120B is formed of W, a fourth reflector layer 120D directly underneath the third reflector layer 120C is formed of Al, and a fifth reflector layer 120C, which is directly underneath the fourth reflector layer 120D and directly over the ESD protection layer 108, is formed of W. While only five reflector layers 120 are illustrated in Figure 7, the number of reflector layers 120 and the sequence of high / low impedance materials within the first bottom reflector 109-1 will vary from one design to another.

[0072] The first bottom dielectric layer 110-1 is formed directly over the first reflector layer 120A without fully covering a top surface of the first reflector layer 120A (i.e., a top surface of the first bottom reflector 109-1 ). The first bottomdielectric layer 110-1 may be formed of SiCh. The first bottom electrode 112-1 is formed over the first bottom dielectric layer 110-1 , while the first bottom connection structure 114-1 extends from the first bottom electrode 112-1 , along sides of the first bottom dielectric layer 110-1 , and towards the top surface of the first reflector layer 120A of the first bottom reflector 109-1 . The first bottom connection structure 114-1 is configured to provide an electrical connection between the first bottom electrode 112-1 and the first bottom reflector 109-1 . In some embodiments, the first bottom electrode 112-1 and the first bottom connection structure 114-1 are formed from the same two bottom electrode layers 122 (e.g., a first bottom electrode layer 122A and a second bottom electrode layer 122B). The second bottom electrode layer 122B fully covers the sides and the top surface of the first bottom dielectric layer 110-1 , and is in contact with positions of the top surface of the first reflector layer 120A that are not covered by the first bottom dielectric layer 110-1. The first bottom electrode layer 122A is formed over and fully covers the second bottom electrode layer 122B. The first bottom electrode layer 122A may be formed of W, Mo, or Pt, while the second bottom electrode layer 122B may be formed of aluminum copper (AICu) or titanium (Ti). In different applications, the first bottom electrode 112-1 may still be composed of the first and second bottom electrode layer 122A and 122B, while the first bottom connection structure 114-1 may be composed of conductive vias or other electrical connecting approaches (not shown).

[0073] It is known that within a BAW resonator, a thin thickness of an electrode will result in a relatively high resistance and / or electrical loss (leading to low Q factors), since current has only a narrow conductive path to pass through. However, simply increasing the thickness of the electrode may not meet certain acoustic performance requirements (e.g., high frequency filtering). It is because the resonance frequency of the BAW resonator is very sensitive to the thickness of the electrode. A small thickness increment of the electrode may result in a significant reduction in frequency. A thickness of a reflector of the BAW resonator, on the other hand, has a relatively small impact on the resonate frequency. Therefore, electrically connecting the electrode to the resonator canachieve a thicker effective electrode (electrode + reflector) for reduced resistance / electrical loss without significantly affecting the resonate frequency range. Herein, with the first bottom connection structure 114-1 , the first bottom electrode 112-1 is electrically connected to the first bottom reflector 109-1 to achieve a thicker effective bottom electrode. The current received from the first bottom electrode 112-1 can pass through a combination of the first bottom electrode 112-1 and the first bottom reflector 109-1 . In addition, the first bottom dielectric layer 110-1 is configured to compensate for a frequency shift caused by the first bottom reflector 109-1 .

[0074] The first piezoelectric portion 116-1 is formed over the first bottom electrode 112-1 , and may be formed of AIN, scandium-doped aluminum nitride (ScAIN), magnesium hydrofluoric acid aluminum nitride (MgHfAIN), magnesium zirconium aluminum nitride (MgZrAIN), or magnesium titanium aluminum nitride (MgTiAIN). The first piezoelectric portion 116-1 may have a thickness up to 1 .4 pm. In some cases, the first piezoelectric portion 116-1 may be thinner than 100 nm. The first top electrode 118-1 is formed over the first piezoelectric portion 116-1 and is vertically aligned with the first bottom electrode 112-1. The first top electrode 118-1 may be composed of a first top electrode layer 124A formed over the first piezoelectric portion 116-1 and a second top electrode layer 124B formed over and fully covering the first top electrode layer 124A. The first top electrode layer 124A may be formed of W, Mo, or Pt, while the second top electrode layer 124B may be formed of AICu or Ti. In some applications, the first top electrode 118-1 may also include an electrode seed layer (not shown) vertically between the first top electrode layer 124A and the second top electrode layer 124B and formed of titanium tungsten (TiW) or Ti.

[0075] An active region 126-1 of the first BAW resonator 102-1 corresponds to a section of the BAW resonator 10 where the first top and first bottom electrodes 118-1 and 112-1 overlap and also includes the layers below the overlapping the first top and first bottom electrodes 118-1 and 112-1. In some applications, a first BO ring 128-1 is formed on or within (not shown) the first top electrode 118-1 to suppress certain spurious modes. The first BO ring 128-1 corresponds to a massloading of a portion of the first top electrodes 118-1 that extends about a periphery of the active region 128. In this regard, the first BO ring 128-1 may correspond to a thickened portion of the first top electrodes 118-1 or the application of additional layers of an appropriate material (e.g. silicon dioxide, silicon nitride, aluminum nitride, or combinations thereof) over the first top electrodes 118-1. In some embodiments, the first BO ring 128-1 may have a dual-step configuration.

[0076] The second BAW resonator 102-2 may have a similar configuration as the first BAW resonator 102-1 and is formed over the ESD protection layer 108. The second BAW resonator 102-2 includes a second bottom reflector 109-2 formed over the ESD protection layer 108, a second bottom dielectric layer 110-2 over the second bottom reflector 109-2, a second bottom electrode 112-2 over the second bottom dielectric layer 110-2, a second bottom connection structure 114-2, a second piezoelectric portion 116-2 over the second bottom electrode 112-2, and a second top electrode 118-2. Herein, the second bottom reflector 109-2, the second bottom dielectric layer 110-2, the second bottom electrode 112-2, the second bottom connection structure 114-2, the second piezoelectric portion 116-2, and the second top electrode 118-2 have similar or identical features / structures as the first bottom reflector 109-1 , the first bottom dielectric layer 110-1 , the first bottom electrode 112-1 , the first bottom connection structure 114-1 , the first piezoelectric portion 116-1 , and the first top electrode 118-1 within the first BAW resonator 102-1 , respectively. In some applications, the first BAW resonator 102-1 and the second BAW resonator 102-2 may be identical, while in some applications, the first BAW resonator 102-1 and the second BAW resonator 102-2 may have different sizes and may utilize different materials. For a nonlimiting example, the active region 126-1 of the first BAW resonator 102-1 may be smaller than an active region 126-2 of the second BAW resonator 102-2. For another non-limiting example, the first bottom reflector 109-1 of the first BAW resonator 102-1 and the second bottom reflector 109-2 of the second BAW resonator 102-2 may include different low / high impedance materials.

[0077] The first piezoelectric portion 116-1 in the first BAW resonator 102-1 and the second piezoelectric portion 116-2 in the second BAW resonator 102-2 may be portions of a common piezoelectric layer 116C (i.e. , the first BAW resonator 102-1 and the second BAW resonator 102-2 share the same common piezoelectric layer 116C). In some applications, the ESD protection layer 108 may be a continuous layer as shown in Figure 7. In some applications, the ESD protection layer 108 may include discrete sections, where each bottom reflector 109 is confined within a corresponding ESD protection section (not shown). The ESD protection layer 108 is configured to isolate the corresponding conductive bottom reflector 109 from the substrate 106, so as to avoid undesired shorting to other electronic components (e.g., another BAW resonator) through the substrate 106. In addition, the ESD protection layer 108 is also desired to be thermally conductive (e.g., > 300W / mK), so as to be able to dissipate the heat generated in the one or more BAW resonators 102.

[0078] Herein, within each BAW resonator 102, a breakdown voltage of one piezoelectric portion 116 is always no more than the breakdown voltage of the ESD protection layer 108. If the piezoelectric portions 116 and the ESD protection layer 108 are formed of a same material, the thickness of each piezoelectric portion 116 is no more than the thickness of the ESD protection layer 108. As such, defections of the piezoelectric portion 116 will be the dominant failure in the BAW device 100 (e.g., the defections of the piezoelectric portion 116 happen before or at least at the same time as the ESD protection layer 108 breakdown). In other words, when a voltage smaller than the breakdown voltage of the piezoelectric portion 116 is applied to the BAW resonator 102, the ESD protection layer 108 will never break down and will not be electrically connected to the substrate 106, and thus the BAW resonator 102 will function properly without undesired electrical shorting.

[0079] It is known that deposition of most metals is often accompanied by film residual stress, which manifests itself by bowing a device wafer providing BAW devices (e.g., the BAW device 100). A compressive stress bows the device wafer in the shape of a dome and a tensile stress bows the device wafer in the shapeof a bowl as shown in Figures 9A and 9B. Ideally, the device wafer is desired to be flat to make the device wafer processable on semiconductor tools. However, the metal reflector layers (e.g., the reflector layers 120) in a reflector (e.g., the first / second bottom reflector 109-1 / 109-2) of the BAW device need to be grown with tensile stress to obtain superior performance. The tensile stress makes the device wafer bowl shaped (Figure 9B) which makes it difficult to process. With the choice of AIN material as the ESD protection layer 108, it is possible to tune the stress of AIN to make it compressive (e.g., making the device wafer dome shaped). A relatively thick AIN layer (e.g. ,>200 nm) helps with such tuning. Once the device wafer obtains the shape of a dome from the AIN layer, subsequent deposition of the metal reflector layers reduces the dome and makes the device wafer relatively flat. Figures 10A-10C illustrate a shape change of the device wafer compared to an ideal wafer bow. Figure 10A shows a wafer shape when a metal reflector is deposited without any AIN stress tuning, Figure 10B shows a wafer shape when only a tuned AIN layer is deposited (before the metal reflector is deposited), and Figure 10C shows a wafer shape after both the tuned AIN layer and the metal reflector are deposited. It is clear that a final wafer with both the tuned AIN layer and the metal reflector has a very low bow, which makes the final wafer processable on semiconductor tools.

[0080] Furthermore, within the BAW device 100 (back to Figure 7), besides the first and second BAW resonators 102-1 and 102-2, there might also be bottom isolation sections 130 and a passivation layer 132. The bottom isolation sections 130 may be formed of silicon oxide, and are filled vertically between the common piezoelectric layer 116C and the substrate 106 to separate the first and second BAW resonators 102-1 and 102-2. If the ESD protection layer 108 is a continuous layer, the bottom isolation sections 130 are formed vertically between the ESD protection layer 108 and the common piezoelectric layer 116C, and surround the bottom reflector 109, the bottom electrode 112, and the bottom connection structure 114 of each BAW resonator 102. The passivation layer 132 may be formed of Silicon Nitride (SiN), SiO , or Silicon Oxynitride (SiON), with a thickness between 250 A and 5000 A, and fully covers each top electrode 118and portions of the common piezoelectric layer 116C that are not covered by any top electrode 118. The passivation layer 132 is configured to protect the BAW device 100 from an external environment.

[0081] In some applications, each BAW resonator 102-1 / 102-2 may include two reflectors instead of one reflector, as illustrated in Figure 11 . For the purpose of this illustration, besides the first bottom reflector 109-1 , the first bottom dielectric layer 110-1 , the first bottom electrode 112-1 , the first bottom connection structure 11 -1 , the first piezoelectric portion 116-1 , and the first top electrode 118-1 , the first BAW resonator 102-1 further includes a first top reflector 134-1 , a first top dielectric layer 136-1 , and a first top connection structure 138-1 .

[0082] The first top dielectric layer 136-1 is formed directly over the first top electrode 118-1 (i.e., directly over the second top electrode layer 124B) without fully covering a top surface of the first top electrode 118-1. In particular, a peripheral portion of the top surface of the first top electrode 118-1 is not covered by the first top dielectric layer 136-1. The first top dielectric layer 136-1 may be formed of SiO2.

[0083] The first top reflector 134-1 , which might be a Bragg reflector and composed of a stack of conductive reflector layers 140A through OE (referred to generally as reflector layers 140), is formed over the first top dielectric layer 136-1 . The reflector layers 140 alternate between different electrically conductive materials (e.g. different metal materials) having high and low acoustic impedances, so as to produce a significant reflection coefficient at a junction of adjacent reflector layers 140. The electrically conductive materials with high acoustic impedance may be W, Mo, or Pt, and the electrically conductive materials with low acoustic impedance may be aluminum (Al) or Ti. In a nonlimiting example, a first reflector layer 140A that is located at a bottom portion of the first top reflector 134-1 and directly formed over the first top dielectric layer 136-1 is formed of W, a second reflector layer 140B directly over the first reflector layer 140A is formed of Al, a third reflector layer 140C directly over the second reflector layer OB is formed of W, a fourth reflector layer MOD directly over the third reflector layer 140C is formed of Al, and a fifth reflector layer MOE, which isdirectly over the fourth reflector layer 1400 and at a top portion of the first top reflector 134-1 , is formed of W. While only five reflector layers 140 are illustrated in Figure 11 , the number of reflector layers 140 and the sequence of high / low impedance materials within the first top reflector 134-1 will vary from one design to another.

[0084] In addition, the first top connection structure 138-1 extends from a peripheral portion of a bottom surface of the first reflector layer 140A, along sides of the first top dielectric layer 136-1 , and toward the peripheral portion of the top surface of the first top electrode 118-1 , which is not covered by the first top dielectric layer 136-1 . The first top connection structure 138-1 is configured to provide an electrical connection between the first top electrode 118-1 and the first top reflector 134-1 . In some embodiments, the first top connection structure 138-1 and the first reflector layer 140A are formed by a same deposition process and include the same conductive material, such as W, Mo, or Pt.

[0085] As described above, with the first top connection structure 138-1 , the first top electrode 118-1 is electrically connected to the first top reflector 134-1 to achieve a thicker effective top electrode. The current received from the first top electrode 118-1 can pass through a combination of the first top electrode 118-1 and the first top reflector 134-1 . As such, acoustic performance requirements (e.g., high frequency operation) can be met without sacrificing electrical loss. In addition, the first top dielectric layer 136-1 is configured to compensate for a frequency shift caused by the first top reflector 134-1 . Herein, the active region 126-1 of the first BAW resonator 102-1 corresponds to a section of the first BAW resonator 102-1 where the first top and first bottom electrodes 118-1 and 112-1 overlap and also includes the layers below, in-between, and above the overlapping first top and first bottom electrodes 118-1 and 112-1. In this illustration, the first BO ring 128-1 is omitted in the BAW device 100.

[0086] The second BAW resonator 102-2 may have a similar configuration as the first BAW resonator 102-1 . Besides the second bottom reflector 109-2, the second bottom dielectric layer 110-2, the second bottom electrode 112-2, the second bottom connection structure 114-2, the second piezoelectric portion 116-2, and the second top electrode 118-2, the second BAW resonator 102-2 further includes a second top reflector 134-2, a second top dielectric layer 136-2, and a second top connection structure 138-2. Herein, the second top reflector 134-2, the second top dielectric layer 136-2, and the second top connection structure 138-2 have similar or identical features / structures as the first top reflector 134-1 , the first top dielectric layer 136-1 , and the first top connection structure 138-1 within the first BAW resonator 102-1 , respectively. In some applications, the first BAW resonator 102-1 and the second BAW resonator 102-2 may be identical, while in some applications, the first BAW resonator 102-1 and the second BAW resonator 102-2 may have different sizes and may utilize different materials. For a non-limiting example, the active region 126-1 of the first BAW resonator 102-1 may be smaller than an active region 126-2 of the second BAW resonator 102-2. For another non-limiting example, the first top reflector 134-1 of the first BAW resonator 102-1 and the second top reflector 134-2 of the second BAW resonator 102-2 may include different low / high impedance materials.

[0087] In this illustration, the BAW device 100 further includes top isolation sections 142 and the passivation layer 132. The top isolation sections 142 surround the top reflector 134, the top electrode 118, and the top connection structure 138 of each BAW resonator 102, so as to separate the first and second BAW resonators 102-1 and 102-2. The passivation layer 132 is formed over the top isolation sections 142 to encapsulate each top reflector 134, so as to protect the BAW device 100 from an external environment. The top isolation sections 142 may be formed of silicon oxide, while the passivation layer 132 may be formed of SiN, SiO2, or SiON with a thickness between 250 A and 5000 A. In different applications, the top isolation sections 142 and the passivation layer 132 might be omitted.

[0088] Furthermore, in some applications, the second BAW resonator 102-2 may be replaced by a non-resonating electronic component (i.e. , an electronic component having no piezoelectric portion), such as a lead, a probe pad or any other appropriate electronic component (not shown). This non-resonating electronic component is still formed over the ESD protection layer 108, and theESD protection layer 108 is configured to isolate the non-resonating electronic component from the substrate 106, so as to avoid undesired shorting to other electronic components through the substrate 106. For a non-limiting example, if the non-resonating electronic component is a lead / probe pad that resides on the ESD protection layer 108 and is electrically connected to the first bottom electrode 112-1 of the first BAW resonator 102-1 (e.g., through the first bottom reflector 109-1 and the first bottom connection structure 114-1 ), then such lead / probe pad is electrically isolated from at least one surrounding BAW resonator ( excluding the first BAW resonator 102-1 , not shown). If the nonresonating electronic component is not connected to the first BAW resonator 102-1 , the non-resonating electronic component is electrically isolated from the first BAW resonator 102-1 by the ESD protection layer 108.

[0089] Figure 12 illustrates a flowchart of an exemplary method of fabricating the BAW device 100 shown in Figure 7 according to some embodiments of the present disclosure. Although the process steps are illustrated in a series, the process steps are not necessarily order dependent. Some steps may be taken in a different order than that presented. Further, processes within the scope of this disclosure may include fewer or more steps than those illustrated in Figure 12.

[0090] Initially, a substrate (e.g., the common substrate 106C) is provided (step 200). An ESD protection layer (e.g., the common ESD protection layer 108C) is then deposited over the substrate (step 202). Herein, the ESD protection layer is electrically insulating and can achieve at least a 400 V or at least a 500 V breakdown voltage with a thickness as thin as 200 nm. Next, one or more conductive reflectors (e.g., the first and / or second bottom reflectors 109- 1 and / or 109-2) are deposited over the ESD protection layer (step 204).Typically, the conductive reflectors are composed of a stack of metal layers, and the deposition of the metal layers is often accompanied by film residual stresses, which manifest by bowing a device wafer. Ideally, the device wafer is desired to be flat to make the device wafer processable on semiconductor tools. Therefore, it is also desired that the ESD protection layer is capable of being tuned to have an opposite deposition stress direction to the one or more conductive reflectors.As such, after depositing the ESD protection layer and the one or more conductive reflectors, the device wafer is relatively flat.

[0091] After the one or more conductive reflectors are deposited, one dielectric layer (e.g., the first / second bottom dielectric layer 110-1 / 110-2) is formed over each conductive reflector without fully covering a top surface of such conductive reflector (step 206). One bottom electrode (e.g., the first / second bottom electrode 112-1 / 112-2) is formed over each dielectric layer (step 208), and one bottom connection structure (e.g., the first / second bottom connection structure 114-1 / 114-2) is formed to electrically connect the bottom electrode to a corresponding conducive reflector (step 210). In case the bottom connection structure and the bottom electrode are formed from the same electrode layers (e.g., the first bottom electrode layer 122A and the second bottom electrode layer 122B), the bottom connection structure is formed simultaneously with the bottom electrode.

[0092] Next, isolation sections (e.g., the isolation sections 130) are formed over the ESD protection layer to surround each combination of one conductive reflector and the corresponding bottom electrode and bottom connection structure (step 212). Top surfaces of the isolation sections are coplanar with a top surface of each bottom electrode. A piezoelectric layer (e.g., the common piezoelectric layer 116C) is then formed directly over each bottom electrode (step 214). The piezoelectric layer is also directly over the isolation sections. Corresponding to each bottom electrode, one top electrode (e.g., the first / second top electrode 118-1 / 118-2) is formed on a top surface of the piezoelectric layer and aligned with a corresponding bottom electrode (e.g., the first / second bottom electrode 112-1 / 112-2, step 216). As such, one or more BAW resonators (e.g., the first / second resonator 102-1 / 102-2) are formed over the ESD protection layer. Lastly, a passivation layer (e.g., the passivation layer 132) is formed to cover each top electrode and exposed portions of the top surface of the piezoelectric layer (step 218).

[0093] Figure 13 illustrates a block diagram of an example system 600 that includes at least one BAW device 100 shown in Figure 7 or Figure 11 . Thesystem 600 includes radio frequency (RF) input circuitry 602 connected to filter circuitry 604. In certain embodiments, the RF input circuitry 602 includes a transceiver. For the purpose of this illustration, the filter circuitry 604 includes three filters 606A, 606B, and 606C. Herein, one or more of the filters 606A, 606B, and 606C may be acoustic filters, which are implemented by the BAW device 100. In different applications, the filter circuitry 604 may include fewer or more filters. In one embodiment, each of the filters 606A, 606B, and 606C may be a lowpass filter, a high-pass filter, a notch filter, or a bandpass filter, and the RF switch structures 606A, 606B, and 606C may be connected in a cascaded arrangement. The filter types that are included in the filter circuitry 604 may be based at least on the rejection requirements of the system 600.

[0094] The filter circuitry 604 is connected to an RF output circuitry 608. In certain embodiments, the RF output circuitry 608 includes an antenna. The RF input circuitry 602 and / or the RF output circuitry 608 may include additional or different components in other embodiments.

[0095] Figure 14 illustrates a block diagram of an exemplary communication device 700, in which at least one acoustic filter implemented by the BAW device 100 as shown in Figure 7 and Figure 11 can be provided. Herein, the communication device 700 can be any type of communication device, such as mobile terminals, smart watches, tablets, computers, navigation devices, access points, base stations (e.g., eNB or gNB), and any other type of wireless communication devices that support wireless communications, such as cellular, wireless local area network (WLAN), Bluetooth, Ultra-wideband (UWB), and near field communications. The communication device 700 will generally include a control system 702, a baseband processor 704, transmit circuitry 706, receive circuitry 708, antenna switching circuitry 710, multiple antennas 712, and user interface circuitry 714. In a non-limiting example, the control system 702 can be a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), as an example. In this regard, the control system 702 can include at least a microprocessor(s), an embedded memory circuit(s), and a communication bus interface(s). The receive circuitry 708 receives radiofrequency signals via the antennas 712 and through the antenna switching circuitry 710 from one or more base stations. A low noise amplifier and a filter of the receive circuitry 708 cooperate to amplify and remove broadband interference from the received signal for processing. Down conversion and digitization circuitry (not shown) will then down convert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams using an analog-to-digital converter(s) (ADC).

[0096] The baseband processor 704 processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations, as will be discussed in greater detail below. The baseband processor 704 is generally implemented in one or more digital signal processors (DSPs) and ASICs.

[0097] For transmission, the baseband processor 704 receives digitized data, which may represent voice, data, or control information, from the control system 702, which it encodes for transmission. The encoded data is output to the transmit circuitry 706, where a digital-to-analog converter(s) (DAC) converts the digitally encoded data into an analog signal and a modulator modulates the analog signal onto a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier will amplify the modulated carrier signal to a level appropriate for transmission and deliver the modulated carrier signal to the antennas 712 through the antenna switching circuitry 710 to the antennas 712. The multiple antennas 712 and the replicated transmit and receive circuitries 706, 708 may provide spatial diversity. Modulation and processing details will be understood by those skilled in the art. In some embodiments, the at least one acoustic filter implemented by the BAW device 100 may be provided in any one or more of the circuitries in the communication device 700, such as the transmit circuitry 706, the receive circuitry 708, and / or the antenna switching circuitry 710.

[0098] It is contemplated that any of the foregoing aspects, and / or various separate aspects and features as described herein, may be combined for additional advantage. Any of the various embodiments as disclosed herein maybe combined with one or more other disclosed embodiments unless indicated to the contrary herein.

[0099] 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 within the scope of the concepts disclosed herein and the claims that follow.

Claims

AMENDED CLAIMS received by the International Bureau on 25 November 2025 (25.11 .2025)ClaimsWhat is claimed is:1 . A bulk acoustic wave (BAW) device, comprising:• a substrate;• an electrostatic discharge (ESD) protection layer formed over and in contact with the substrate, wherein the ESD protection layer is electrically insulating and has a thermal conductivity larger than 300W / mK; and• a first BAW resonator that is composed of a first bottom reflector, a first bottom electrode, a first top electrode, and a first piezoelectric portion, wherein:• the first bottom reflector is formed over and in contact with the ESD protection layer and includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers;• the first bottom electrode is formed over the first bottom reflector and electrically connected to the first bottom reflector;• the first top electrode is vertically aligned with the first bottom electrode; and• the first piezoelectric portion is vertically between the first bottom electrode and the first top electrode, wherein a breakdown voltage of the first piezoelectric portion is no more than a breakdown voltage of the ESD protection layer.

2. The BAW device of claim 1 wherein the first BAW resonator further includes a first bottom dielectric layer and a first bottom connection structure, wherein:• the first bottom dielectric layer is formed directly over the first bottom reflector and partially covers a top surface of the first bottom reflector;• the first bottom electrode is formed directly over the first bottom dielectric layer; and• the first bottom connection structure is configured to electrically connect the first bottom electrode with the first bottom reflector.

3. The BAW device of claim 2 wherein the first bottom connection structure extends from the first bottom electrode, along sides of the first bottom dielectric layer, and towards the top surface of the first bottom reflector.

4. The BAW device of claim 3 wherein the first bottom connection structure is composed of conductive vias.

5. The BAW device of claim 3 wherein:• the first bottom connection structure and the first bottom electrode are formed from same bottom electrode layers, and• the first bottom connection structure is in contact with a portion of the top surface of the first bottom reflector, which is not covered by the first bottom dielectric layer.

6. The BAW device of claim 5 wherein:• the bottom electrode layers include at least a first bottom electrode layer and a second bottom electrode layer;• the second bottom electrode layer fully covers the sides and the top surface of the first bottom dielectric layer and is in contact with the portion of the top surface of the first bottom reflector that is not covered by the first bottom dielectric layer; and• the first bottom electrode layer is formed over and fully covers the second bottom electrode layer.

7. The BAW device of claim 6 wherein the first bottom electrode layer is formed of W, Mo, or Pt, and the second bottom electrode layer is formed of aluminum copper (AICu) or titanium (Ti).

8. The BAW device of claim 1 wherein the ESD protection layer is capable of being tuned to have an opposite deposition stress direction to the first bottom reflector.

9. The BAW device of claim 1 wherein the ESD protection layer is formed of aluminum nitride (AIN).

10. The BAW device of claim 1 wherein:• the high acoustic impedance conductive layers are formed of tungsten (W), molybdenum (Mo), or platinum (Pt); and• the low acoustic impedance conductive layers are formed of aluminum (Al) or titanium (Ti).11 . The BAW device of claim 1 wherein:• the first top electrode is composed of at least a first top electrode layer and a second top electrode layer;• the first top electrode layer is formed directly over the first piezoelectric portion, and the second top electrode layer is formed over the first top electrode layer; and• the first top electrode layer is formed of W, Mo, or Pt, and the second top electrode layer is formed of AICu or Ti.

12. The BAW device of claim 11 wherein the first BAW resonator further includes a border ring (BO) formed on or within the first top electrode to suppress spurious modes.

13. The BAW device of claim 1 wherein the first BAW resonator further includes a first top reflector, wherein:• the first top reflector includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers; and• the first top reflector is formed over and electrically connected to the first top electrode.

14. The BAW device of claim 13 wherein the first BAW resonator further includes a first top dielectric layer and a first top connection structure, wherein:• the first top dielectric layer is formed directly over the first top electrode;• a peripheral portion of a top surface of the first top electrode is not covered by the first top dielectric layer;• the first top reflector is formed directly over the first top dielectric layer;• the first top connection structure extends from a peripheral portion of a bottom surface of the first top reflector, along sides of the first top dielectric layer, and toward the peripheral portion of the top surface of the first top electrode, which is not covered by the first top dielectric layer; and• the first top connection structure is configured to electrically connect the first top electrode with the first top reflector.

15. The BAW device of claim 14 wherein the first top connection structure and a bottommost one of the alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers are formed of a same conductive material.

16. The BAW device of claim 15 wherein the first top connection structure and a bottommost one of the alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers are formed of W, Mo, or Pt.

17. The BAW device of claim 1 further includes a second electronic component formed over the ESD protection layer.

18. The BAW device of claim 17 wherein the second electronic component is a second BAW resonator, which comprises:• a second bottom reflector formed over the ESD protection layer and including a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers;• a second bottom electrode formed over the second bottom reflector and electrically connected to the second bottom reflector;• a second top electrode vertically aligned with the second bottom electrode; and• a second piezoelectric portion vertically between the second bottom electrode and the second top electrode, wherein the second piezoelectric portion and the first piezoelectric portion are formed from a same common piezoelectric layer that has a breakdown voltage no more than the breakdown voltage of the ESD protection layer.

19. The BAW device of claim 17 wherein the second electronic component has no piezoelectric portion and is a lead or a probe pad, which is electrically connected to the first bottom electrode of the first BAW resonator and electrically isolated to at least one surrounding BAW resonator other than the first BAW resonator.

20. A method of fabricating a bulk acoustic wave (BAW) device comprising:• providing a substrate;• depositing an electrostatic discharge (ESD) protection layer directly over the substrate, wherein the ESD protection layer is electrically insulating and has a thermal conductivity larger than 300W / mK;• depositing one or more conductive reflectors directly over the ESD protection layer, wherein each of the one or more conductive reflectors includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers;• forming one or more bottom electrodes over the one or more conductive reflectors, respectively, wherein each of the one or more bottom electrodes is electrically connected to a corresponding one of the one or more conductive reflectors;• forming a piezoelectric layer over each of the one or more bottom electrodes; and• forming one or more top electrodes over the piezoelectric layer, each of the one or more top electrodes being aligned with a corresponding one of the one or more bottom electrodes, wherein a breakdown voltage of the piezoelectric layer is no more than a breakdown voltage of the ESD protection layer.21 . A system, comprising:• radio-frequency (RF) input circuitry;• RF output circuitry; and• filter circuitry, which includes at least one bulk acoustic wave (BAW) device, connected between the RF input circuitry and the RF output circuitry, wherein the at least one BAW device comprises:• a substrate;• an electrostatic discharge (ESD) protection layer formed over and in contact with the substrate, wherein the ESD protection layer is electrically insulating and has a thermal conductivity larger than 300W / mK; and• a first BAW resonator that includes a first bottom reflector, a first bottom electrode, a first top electrode, and a first piezoelectric portion, wherein:• the first bottom reflector is formed over and in contact with the ESD protection layer and includes a stack of alternating high acoustic impedance conductive layers and low acoustic impedance conductive layers;• the first bottom electrode is formed over the first bottom reflector and electrically connected to the first bottom reflector;• the first top electrode is vertically aligned with the first bottom electrode; and• the first piezoelectric portion is vertically between the first bottom electrode and the first top electrode, wherein a breakdown voltage of the first piezoelectric portion is no more than a breakdown voltage of the ESD protection layer.STATEMENT UNDER ARTICLE 19(1)Applicant has submitted amendments under Article 19 PCT for the abovereferenced application.Applicant has amended original claim 1, and cancelled original claim 9. In addition, Applicant has amended renumbered claims 12, 14-16, and 18-21 (original claims 13, 15-17, and 19-22, respectively).Claim 1 has been amended to clarify both the characteristics and the placement of the claimed electrostatic discharge (ESD) protection layer, thereby distinguishing it from a conventional reflector layer. In particular, the claimed ESD protection layer is required (1) to be positioned directly between the first bottom reflector and the substrate, (2) to be electrically insulating with a breakdown voltage no smaller than that of the first piezoelectric portion, and (3) to have a thermal conductivity larger than 300W / mK. Accordingly, the claimed ESD protection layer is different from an arbitrary dielectric layer that has a thermal conductivity below 300W / mK or that is not disposed directly between a reflector and a substrate.Renumbered claims 12, 14-16, and 18-19 (original claims 13, 15-17, and 19-20, respectively) have been amended to change their dependencies.Renumbered claims 20-21 have been amended to include similar features to claim

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