# Patent

## Title
Acoustic resonator structure comprising a bridge

## Abstract

An acoustic resonator comprises a first electrode a second electrode and a piezoelectric layer disposed between the first and second electrodes. The acoustic resonator further comprises a reflective element disposed beneath the first electrode, the second electrode and the piezoelectric layer. An overlap of the reflective element, the first electrode, the second electrode and the piezoelectric layer comprises an active area of the acoustic resonator. The acoustic resonator also comprises a bridge adjacent to a termination of the active area of the acoustic resonator.

## Background
BACKGROUND
In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently, resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Film Bulk Acoustic Resonator (FBAR). The FBAR has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The FBAR includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns and length and width dimensions of hundreds of microns. FBARs beneficially provide a comparatively compact alternative to known resonators.
Desirably, the bulk acoustic resonator excites only thickness-extensional (TE) modes, which are longitudinal mechanical waves having propagation (k) vectors in the direction of propagation. The TE modes desirably travel in the direction of the thickness (e.g., z-direction) of the piezoelectric layer.
Unfortunately, besides the desired TE modes there are lateral modes, known as Rayleigh-Lamb modes, generated in the acoustic stack as well. The Rayleigh-Lamb modes are mechanical waves having k-vectors that are perpendicular to the direction of TE modes, the desired modes of operation. These lateral modes travel in the areal dimensions (x, y directions of the present example) of the piezoelectric material. Among other adverse effects, lateral modes deleteriously impact the quality (Q) factor of an FBAR device. In particular, the energy of Rayleigh-Lamb modes is lost at the interfaces of the FBAR device. As will be appreciated, this loss of energy to spurious modes is a loss in energy of desired longitudinal modes, and ultimately a degradation of the Q-factor.
FBARs comprise an active area, and connections to and from the active area can increase losses, and thereby degrade the Q factor. For example, in transition regions between the active area and the connections, defects may form in the piezoelectric layer during fabrication. These defects can result in acoustic loss, and as a result, a reduction in the Q factor.
What is needed, therefore, are an acoustic resonator structure and electrical filter that overcomes at least the known shortcomings described above.
SUMMARY
In accordance with a representative embodiment, an acoustic resonator comprises: a first electrode; a second electrode; a piezoelectric layer disposed between the first and second electrodes; and a reflective element disposed beneath the first electrode, the second electrode and the piezoelectric layer. An overlap of the reflective element, the first electrode, the second electrode and the piezoelectric layer defines an active area of the acoustic resonator. The first electrode substantially covers the reflective element, and the piezoelectric layer extends over an edge of the first electrode. The acoustic resonator also comprises a bridge adjacent to a termination of the active area of the acoustic resonator, and the bridge overlaps a portion of the first electrode.
In accordance with another representative embodiment, a film bulk acoustic resonator (FBAR) comprises: a first electrode; a second electrode; a piezoelectric layer disposed between the first and second electrodes; and a cavity disposed beneath the first electrode, the second electrode and the piezoelectric layer. An overlap of the cavity, the first electrode, the second electrode and the piezoelectric layer defines an active area of the acoustic resonator. The first electrode substantially covers the cavity, and the piezoelectric layer extends over an edge of the first electrode. The FBAR also comprises a bridge adjacent to a termination of the active area of the acoustic resonator. The bridge overlaps a portion of the first electrode.
In accordance with yet another representative embodiment, a filter element comprises an acoustic resonator. The acoustic resonator comprises: a first electrode; a second electrode; a piezoelectric layer disposed between the first and second electrodes; and a reflective element disposed beneath the first electrode, the second electrode and the piezoelectric layer. An overlap of the reflective element, the first electrode, the second electrode and the piezoelectric layer defines an active area of the acoustic resonator. The first electrode substantially covers the reflective element, and the piezoelectric layer extends over an edge of the first electrode. The acoustic resonator also comprises a bridge adjacent to a termination of the active area of the acoustic resonator, and the bridge overlaps a portion of the first electrode.



BRIEF DESCRIPTION OF THE DRAWINGS
The illustrative embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
FIG. 1 shows a cross-sectional view of an acoustic resonator in accordance with a representative embodiment.
FIG. 2 shows a top view of an acoustic resonator in accordance with a representative embodiment.
FIG. 3 shows a graph of the Q-factor versus a spacing between the air-bridge and the lower electrode of an FBAR in accordance with a representative embodiment.
FIG. 4 shows a graph of the effective coupling coefficient (kt2) versus a spacing between the air-bridge and the lower electrode of an acoustic resonator in accordance with a representative embodiment.
FIG. 5 shows a cross-sectional view of an FBAR in accordance with a representative embodiment.
FIG. 6 shows a graph of the parallel impedance (Rp) for FBARs, including certain FBARs of representative embodiments.
FIG. 7 shows a graph of the effective coupling coefficient (kt2) for FBARs, including certain FBARs of representative embodiments.



DEFINED TERMINOLOGY
It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. The defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings.
As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices.
As used in the specification and appended claims, and in addition to their ordinary meanings, the terms ‘substantial’ or ‘substantially’ mean to within acceptable limits or degree. For example, ‘substantially cancelled’ means that one skilled in the art would consider the cancellation to be acceptable.
As used in the specification and the appended claims and in addition to its ordinary meaning, the term ‘approximately’ means to within an acceptable limit or amount to one having ordinary skill in the art. For example, ‘approximately the same’ means that one of ordinary skill in the art would consider the items being compared to be the same.
DETAILED DESCRIPTION
In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of illustrative embodiments according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparati and methods may be omitted so as to not obscure the description of the illustrative embodiments. Such methods and apparati are clearly within the scope of the present teachings.
FIG. 1 is a cross-sectional view of an acoustic resonator 100 in accordance with an illustrative embodiment. Illustratively, the acoustic resonator 100 is an FBAR structure. The acoustic resonator 100 comprises a substrate 101. A first electrode 102 is disposed over the substrate 101. A piezoelectric layer 103 is disposed over the first electrode 102. A second electrode 104 is disposed over the first electrode 102. The piezoelectric layer 103 has a first surface in contact with the first electrode 102 and a second surface in contact with the second electrode 104. The first and second electrodes 102, 104 include an electrically conductive material and provide an oscillating electric field in the y-direction, which is the direction of the thickness of the piezoelectric layer 103. In the present illustrative embodiment, the y-axis is the axis for the TE (longitudinal) mode(s) for the resonator.
The piezoelectric layer 103 and first and second electrodes 102,104 are suspended over a cavity 105 formed by selective etching of the substrate 101. The cavity 105 may be formed by a number of known methods, for example as described in commonly assigned U.S. Pat. No. 6,384,697 to Ruby, et al. The region of overlap of the first and second electrodes 102, 104, the piezoelectric layer 103 and the cavity 105 is referred to as the active area of the acoustic resonator 100. Accordingly, the acoustic resonator 100 is a mechanical resonator, which can be electrically coupled via the piezoelectric layer 103. Other suspension schemes that foster mechanical resonance by FBARs are contemplated. For example, the acoustic resonator 100 can be located over a mismatched acoustic Bragg reflector (not shown) formed in or on the substrate 101. This type of FBAR is sometimes referred to as a solid mount resonator (SMR) and, for example, may be as described in U.S. Pat. No. 6,107,721 to Lakin, the disclosure of which is specifically incorporated into this disclosure by reference in its entirety.
By contrast, an inactive area of the acoustic resonator 100 comprises a region of overlap between first electrode 102, or second electrode 104, or both, and the piezoelectric layer 103 not disposed over the cavity 105 or other suspension structure. As described more fully below, it is beneficial to the performance of the resonator to reduce the area of the inactive region of the acoustic resonator 100 to the extent practical.
When connected in a selected topology, a plurality of acoustic resonators 100 can act as an electrical filter. For example, the acoustic resonators 100 may be arranged in a ladder-filter arrangement, such as described in U.S. Pat. No. 5,910,756 to Ella, and U.S. Pat. No. 6,262,637 to Bradley, et al. The electrical filters may be used in a number of applications, such as in duplexers.
The acoustic resonator 100 may be fabricated according to known semiconductor processing methods and using known materials. Illustratively, the acoustic resonator 100 may be fabricated according to the teachings of U.S. Pat. Nos. 5,587,620; 5,873,153; 6,384,697; 6,507,983; and 7,275,292 to Ruby, et al.; and 6,828,713 to Bradley, et. al. The disclosures of these patents are specifically incorporated herein by reference. It is emphasized that the methods and materials described in these patents are representative and other methods of fabrication and materials within the purview of one of ordinary skill in the art are contemplated.
The acoustic resonator 100 also comprises a bridge 106 provided on a side of the acoustic resonator 100 that connects to a contact 107. The contact 107 is connected to a signal line (not shown) and electronic components (not shown) are selected for the particular application of the acoustic resonator 100. This portion of the acoustic resonator 100 is often referred to as the interconnection side of the acoustic resonator 100. By contrast, the second electrode 104 terminates at a position 108 over the cavity 105 in order to minimize the inactive area of the acoustic resonator 100 as described below. The position 108 opposes the interconnection side of the acoustic resonator 100.
The bridge 106 comprises a gap 109 formed beneath a portion of the second electrode 104. Illustratively, and as described below, after removal of a sacrificial layer (not shown) provided in the formation of the gap 109, the gap 109 comprises air. However, the gap 109 may comprise other materials including low acoustic impedance materials, such as carbon (C) doped SiO2, which is also referred as Black-diamond; or dielectric resin commercially known as SiLK; or benzocyclobutene (BCB). Such low acoustic impedance materials may be provided in the gap 109 by known methods. The low acoustic impedance material may be provided after removal of sacrificial material used to form the gap 109 (as described below), or may be used instead of the sacrificial material in the gap 109, and not removed.
In a representative embodiment, the bridge 106 is formed by providing a sacrificial layer (not shown) over the first electrode 102 and a portion of the piezoelectric layer 103 on the interconnection side and forming the second electrode 104 over the sacrificial layer. Illustratively, the sacrificial material comprises phosphosilicate glass (PSG), which illustratively comprises 8% phosphorous and 92% silicon dioxide. Subsequent layers such as the piezoelectric layer 103 and the second electrode 104 are deposited, sequentially, upon the PSG until the final structure is developed. Notably a seed layer (not shown) may be provided over the first electrode 102 before depositing the piezoelectric layer 103, and a passivation layer (not shown) may be deposited over the second electrode 104. After the formation of the structure comprising the bridge 106, the PSG sacrificial layer is etched away illustratively with hydrofluoric acid leaving the free-standing bridge 106. In a representative embodiment, the sacrificial layer disposed in the cavity 105 and the sacrificial layer beneath the bridge 106 are removed in the same process step, with the latter leaving the gap 109 comprising air.
The piezoelectric layer 103 comprises a transition 110 formed during the formation of the piezoelectric layer 103 over the first electrode 102 and the substrate 101. The piezoelectric layer 103 at the transition 110 often comprises material defects and voids, particularly structural defects such as lattice defects and voids. These defects and voids can result in losses of acoustic energy of the mechanical waves propagating in the piezoelectric material. As should be appreciated, acoustic energy loss results in a reduction in the Q-factor of the acoustic resonator 100. However, and as described below, by separating the second electrode 104 from the piezoelectric layer 103 in a region 111 of the gap 109 where the transition 110 occurs, the portion of the active region of the acoustic resonator 100 necessarily does not include the transition 110 of the piezoelectric layer 103 that includes the defects and voids therein. As a result, acoustic losses due to the defects and voids in the piezoelectric layer 103 at the transition 110 are reduced and the Q-factor is improved compared to known resonators, such as known FBARs.
Additionally, and beneficially, the bridge 106 provides an acoustic impedance mismatch at the boundary of the active region on the interconnection side of the acoustic resonator 100. This acoustic impedance mismatch results in the reflection of acoustic waves at the boundary that may otherwise propagate out of the active region and be lost, resulting in energy loss. By preventing such losses, the bridge 106 results in an increased Q-factor in the acoustic resonator 100. Moreover, the termination of the second electrode 104 at position 108 terminates the active region of the acoustic resonator 100 and reduces losses by creating an acoustic impedance mismatch. This also provides an improvement in the Q-factor.
In addition to terminating the active region of the acoustic resonator 100 before the transition 110, the bridge 106 also reduces the area of an inactive region of the acoustic resonator 100. The inactive region of the FBAR 100 creates a parasitic capacitance, which in an equivalent circuit is electrically in parallel with the intrinsic capacitance of the active region of the FBAR. This parasitic capacitance degrades the effective coupling coefficient (kt2), and therefore it is beneficial to reduce the parasitic capacitance. Reducing the area of the inactive region reduces the parasitic capacitance, and thereby improves the effective coupling coefficient (kt2).
FIG. 1 also shows a first line 112 and a second line 113 separated by a distance ‘d.’ The distance ‘d’ represents the distance between the edge of the first electrode 102 at the first line 112 (arbitrarily selected as x=0 for this discussion) to the beginning point of the bridge 106 at the second line 113. As the position of the second line 113 becomes larger (more negative), the Q-factor increases. The effective coupling coefficient (kt2) also increases with separation of first and second lines 112, 113 to an extent. Thus, the selection of a particular distance ‘d’ results in an improvement in Q due to reduced acoustic losses due to the reduction in the inactive FBAR area and the improvement in kt2 from the reduction of the parasitic capacitance.
FIG. 2 shows a top view of the acoustic resonator 100 of FIG. 1. Notably, the cross-sectional view of the acoustic resonator 100 shown in FIG. 1 is taken along the line 1-1. The second electrode 104 of the present embodiment is apodized to reduce acoustic losses. Further details of the use of apodization in acoustic resonators may be found in commonly owned U.S. Pat. No. 6,215,375 to Larson III, et al; or in commonly owned U.S. Pat. No. 7,629,865 entitled “Piezoelectric Resonator Structures and Electrical Filters” filed May 31, 2006, to Richard C. Ruby. The disclosures of U.S. Pat. Nos. 6,215,375 and 7,629,865 are specifically incorporated herein by reference in their entirety.
The fundamental mode of the acoustic resonator 100 is the longitudinal extension mode or “piston” mode. This mode is excited by the application of a time-varying voltage to the two electrodes at the resonant frequency of the acoustic resonator 100. The piezoelectric material converts energy in the form of electrical energy into mechanical energy. In an ideal FBAR having infinitesimally thin electrodes, resonance occurs when the applied frequency is equal to the velocity of sound of the piezoelectric medium divided by twice the thickness of the piezoelectric medium: f=vac(2*T), where T is the thickness of the piezoelectric medium and vac is the acoustic phase velocity. For resonators with finite thickness electrodes, this equation is modified by the weighted acoustic velocities and thicknesses of the electrodes.
A quantitative and qualitative understanding of the Q of a resonator may be obtained by plotting on a Smith Chart the ratio of the reflected energy to applied energy as the frequency is varied for the case in which one electrode is connected to ground and another to signal, for an FBAR resonator with an impedance equal to the system impedance at the resonant frequency. As the frequency of the applied energy is increased, the magnitude/phase of the FBAR resonator sweeps out a circle on the Smith Chart. This is referred to as the Q-circle. Where the Q-circle first crosses the real axes (horizontal axes), this corresponds to the series resonance frequency fs. The real impedance (as measured in Ohms) is Rs. As the Q-circle continues around the perimeter of the Smith chart, it again crosses the real axes. The second point at which the Q circle crosses the real axis is labeled fp, the parallel or anti-resonant frequency of the FBAR. The real impedance at fp is Rp.
Often it is desirable to minimize Rs while maximizing Rp. Qualitatively, the closer the Q-circle “hugs” the outer rim of the Smith chart, the higher the Q-factor of the device. The Q-circle of an ideal lossless resonator would have a radius of one and would be at the edge of the Smith chart. However, as noted above, there are energy losses that impact the Q of the device. For instance, and in addition to the sources of acoustic losses mentioned above, Rayleigh-Lamb (lateral or spurious) modes are in the x,y dimensions of the piezoelectric layer 103. These lateral modes are due to interfacial mode conversion of the longitudinal mode traveling in the z-direction; and due to the creation of non-zero propagation vectors, kx and ky, for both the TE mode and the various lateral modes (e.g., the S0 mode and the zeroth and first flexure modes, A0 and A1), which are due to the difference in effective velocities between the regions where electrodes are disposed and the surrounding regions of the resonator where there are no electrodes.
Regardless of their source, the lateral modes are parasitic in many resonator applications. For example, the parasitic lateral modes couple at the interfaces of the resonator and remove energy available for the longitudinal modes and thereby reduce the Q-factor of the resonator device. Notably, as a result of parasitic lateral modes and other acoustic losses sharp reductions in Q can be observed on a Q-circle of the Smith Chart of the S11 parameter. These sharp reductions in Q-factor are known as “rattles” or “loop-de-loops,” which are shown and described in commonly owned U.S. Pat. No. 7,280,007, referenced below.
As described more fully in U.S. Pat. Nos. 6,215,375 and 7,629,865, the apodized first and second electrodes 102, 104 cause reflections of the lateral modes at the interfaces of the resonator to interfere non-constructively, and therefore reduce the magnitude of lateral modes which otherwise result in more viscous energy dissipation. Beneficially, because these lateral modes are not coupled out of the resonator and developed to higher magnitude, energy loss can be mitigated with at least a portion of the reflected lateral modes being converted to longitudinal modes through mode conversion. Ultimately, this results in an overall improvement in the Q-factor.
FIG. 3 shows a graph of the Q-factor versus a spacing (‘d’ in FIG. 1) between the bridge 106 and the first electrode 102 of acoustic resonator 100 in accordance with a representative embodiment. When the spacing ‘d’ is selected to be zero, second line 113 is aligned with first line 112, and the region 111 is eliminated. When the spacing ‘d’ is selected to be positive, the second line 113 now has a positive x-coordinate (i.e., second line 113 is located to the right of first line 112 in FIG. 1). In either case, the acoustic losses due to defects in the transition 110 and a comparative increase in the area of the inactive region of the acoustic resonator 100 combine to result in a Q-factor that is comparatively low. By contrast, when the spacing d is selected to be ‘negative’ the inactive area is decreased (i.e., second line 113 is located to the left of first line 112 in FIG. 1), with the bridge 106 and region 111 comprising a comparatively increased dimension. As can be seen in region 302 of FIG. 3, the Q-factor increases to a maximum value at 303. As should be appreciated, the reduction in the inactive area of the acoustic resonator 100 on the interconnection side of the acoustic resonator 100, and the forming of the region 111 results in a decrease in losses due to defects and an acoustic impedance mismatch at the boundary of the active region of the acoustic resonator 100 at the interconnection side of the acoustic resonator 100.
FIG. 4 shows a graph of the effective coupling coefficient (kt2) versus a spacing (‘d’ in FIG. 1) between the air-bridge (e.g., 106) and the lower electrode (e.g., 102) of an acoustic resonator 100 in accordance with a representative embodiment. When the spacing is selected to be zero, second line 113 is aligned with first line 112 and the region 111 is eliminated. When the spacing ‘d’ is selected to be positive, the second line 113 now has a positive x-coordinate (i.e., second line 113 is to the right of first line 112 in FIG. 1). In either case, the area of the inactive region results in an increase in the parasitic capacitance. This increased parasitic capacitance provides a comparatively reduced effective coupling coefficient (kt2) as shown in region 401 of FIG. 4. By contrast, when the spacing d is selected to be ‘negative’ (i.e., second line 113 is to the left of first line 112 in FIG. 1) the inactive area is decreased, with the bridge 106 and region 111 increasing in dimension. This results in a reduction in the parasitic capacitance, and a comparative increase in the effective coupling coefficient (kt2) as shown in region 402 of FIG. 4. A maximum value of the effective coupling coefficient (kt2) is reached at 403 in FIG. 4. In region 404 of FIG. 4, the effective coupling coefficient (kt2) begins to drop. Likely, this is due to the fact that region 111 further increases over cavity 105, resulting in an increase in the area of the parasitic capacitor formed by the bridge 106 and the gap 109, thus increasing the parasitic capacitance in parallel with the intrinsic capacitance of the acoustic resonator 100. Moreover, as region 111 increases within cavity 105, the active resonator area also decreases, resulting in a decrease in kt2. Accordingly, in region 404, the effective coupling coefficient (kt2) decreases as region 111 increases. As should be appreciated from the discussion of FIGS. 3 and 4, the selection of the distance ‘d’ in FIG. 1 allows for the selection of an acceptable increase in both the Q-factor and the effective coupling coefficient (kt2).
FIG. 5 is a cross-sectional view of an acoustic resonator 500 in accordance with an illustrative embodiment. The acoustic resonator 500, which is illustratively an FBAR, shares many common features with the acoustic resonator 100 described previously. Many of these common details are not repeated in order to avoid obscuring the presently described embodiments.
The acoustic resonator 500 comprises a selective recess 501 (often referred to as an ‘innie’) and a frame element 502 (also referred to as an ‘outie’). The recess 501 and frame element 502 provide an acoustic mismatch at the perimeter of the second electrode 104, improve signal reflections and reduce acoustic losses. Ultimately, reduced losses translate into an improved Q-factor of the device. While the recess 501 and the frame element 502 are shown on the second electrode 104, these features may instead be provided on the first electrode 102, or selectively on both the first and second electrodes 102,104. Further details of the use, formation and benefits of the recess 501 and the frame element 502 are found for example, in commonly owned U.S. Pat. No. 7,280,007 entitled “Thin Film Bulk Acoustic Resonator with a Mass Loaded Perimeter” to Feng, et al.; and commonly owned U.S. Patent Application Publication 20070205850 entitled “Piezoelectric Resonator Structure and Electronic Filters having Frame Elements” to Jamneala, et al. The disclosures of this patent and patent application publication are specifically incorporated herein by reference.
FIG. 6 shows a graph of the impedance Rp at parallel resonance for FBARs, including certain acoustic resonators of representative embodiments. Notably, Rp of a known resonator is shown at 601; Rp of an acoustic resonator comprising the bridge 106 (e.g., acoustic resonator 100 of FIG. 1) is shown at 602; Rp of an acoustic resonator comprising recesses and frame elements (with no bridge) is shown at 603; and Rp of an acoustic resonator comprising both the bridge 106, and the recess 501 and the frame element 502 (e.g., acoustic resonator 500 of FIG. 5) is shown at 604.
The known resonator typically has Rp (shown at 601) of approximately 2000 Ohm. The addition of a recess and a frame element in a known FBAR increases Rp by approximately 1 kΩ as shown at 602. Similarly, the addition of the bridge 106 but not the recess 501 and or the frame element 502 increases Rp by approximately 1 kΩ as shown at 603. However, when adding combined features of the bridge 106 and the recess 501 and frame element 502, the overall parallel resonance Rp improves by nearly 3 kΩ (over that of the known resonator) as shown at 604. Accordingly, the bridge 106, and the recess 501 and frame element 502 provide a synergistic increase in the parallel resonance Rp, as is evident by a comparison of 601 and 604 in FIG. 6. It is beneficial to have comparatively high Rp in FBARs to provide comparatively low insertion loss and comparatively ‘fast’ roll off filters comprising such FBARs.
As is known, although the use of recesses such as recess 501 results in a comparatively small increase in the effective coupling coefficient (kt2), there can be a degradation in the effective coupling coefficient (kt2) as a result of the frame elements and recesses. In some applications, it may be useful to mitigate this degradation, even though the improvement in the Q-factor may not be as great. For instance, it is known that the bandwidth of an FBAR filter is related to kt2. As such, a degradation of kt2 can reduce the bandwidth of the FBAR filter. Certain representative embodiments described presently provide somewhat of a trade-off of an acceptable Q-factor and an acceptable degradation of kt2.
FIG. 7 shows a graph of the effective coupling coefficient (kt2) for FBARs, including certain acoustic resonators of representative embodiments. The effective coupling coefficient (kt2) of a known resonator is shown at 701; the effective coupling coefficient (kt2) of an acoustic resonator comprising bridge 106 (e.g., acoustic resonator 100 of FIG. 1) is shown at 702; the effective coupling coefficient (kt2) a known acoustic resonator comprising a recess and a frame element (but with no bridge) is shown at 703; and the effective coupling coefficient (kt2) of an acoustic resonator comprising both a bridge 106, and the recess 501 and the frame element 502 (e.g., acoustic resonator 500 of FIG. 5) is shown at 704.
The effective coupling coefficient (kt2) of the known resonator is approximately 5.3 as shown at 701. The addition of the bridge 106 improves the effective coupling coefficient (kt2) to 5.4 as shown at 702. However, adding recesses and frame elements and no airbridge will result in an effective coupling coefficient (kt2) of approximately 5.15 as shown at 703. Finally, incorporating the bridge 106, the recess 501 and the frame element 502 result in an effective coupling coefficient (kt2) (shown at 704) that is substantially the same as the known FBAR. Thus, the positive impact on the effective coupling coefficient (kt2) from the bridge 106 must be contrasted with the negative impact of recesses and frame elements on the effective coupling coefficient (kt2).
In accordance with illustrative embodiments, acoustic resonators for various applications such as in electrical filters are described having a bridge. One of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. These and other variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.

## Claims

1. An acoustic resonator, comprising:
a first electrode;
a second electrode;
a piezoelectric layer disposed between the first and second electrodes;
a reflective element disposed beneath the first electrode, the second electrode and the piezoelectric layer, an overlap of the reflective element, the first electrode, the second electrode and the piezoelectric layer defining an active area of the acoustic resonator, wherein the first electrode substantially covers the reflective element, and the piezoelectric layer extends over an edge of the first electrode; and
a bridge adjacent to a termination of the active area of the acoustic resonator, wherein the bridge overlaps a portion of the first electrode.

2. An acoustic resonator as claimed in claim 1, further comprising an electrical connection to one of a plurality of sides of the second electrode, wherein the bridge is disposed between the connection and the one side of the second electrode.

3. An acoustic resonator as claimed in claim 1, wherein adjacent to termination of the active area of the acoustic resonator, the piezoelectric layer comprises a transition comprising defects.

4. An acoustic resonator as claimed in claim 3, wherein the second electrode does not contact the transition.

5. An acoustic resonator as claimed in claim 1, wherein the bridge comprises a gap.

6. An acoustic resonator as claimed in claim 5, wherein the gap comprises a region between the second electrode and the piezoelectric layer.

7. An acoustic resonator as claimed in claim 6, wherein adjacent to termination of the active area of the acoustic resonator, the piezoelectric layer comprises a transition comprising defects, and the transition is disposed beneath the region of the gap.

8. An acoustic resonator as claimed in claim 1, wherein the second electrode comprises an upper surface with a side and a recess is disposed along the side.

9. An acoustic resonator as claimed in claim 1, wherein the second electrode comprises an upper surface with a side, and a frame element is disposed along the side.

10. A film bulk acoustic resonator (FBAR), comprising:
a first electrode;
a second electrode;
a piezoelectric layer disposed between the first and second electrodes;
a cavity disposed beneath the first electrode, the second electrode and the piezoelectric layer, an overlap of the cavity, the first electrode, the second electrode and the piezoelectric layer defining an active area of the acoustic resonator, wherein the first electrode substantially covers the cavity, and the piezoelectric layer extends over an edge of the first electrode; and
a bridge adjacent to a termination of the active area of the acoustic resonator, wherein the bridge overlaps a portion of the first electrode.

11. An FBAR as claimed in claim 10, further comprising an electrical connection to one of a plurality of sides of the second electrode, wherein the bridge is disposed between the connection and the one side of the second electrode.

12. An FBAR as claimed in claim 10, wherein adjacent to termination of the active area of the acoustic resonator, the piezoelectric layer comprises a transition comprising defects.

13. An FBAR as claimed in claim 12, wherein the second electrode does not contact the transition.

14. An FBAR as claimed in claim 10, wherein the bridge comprises a gap.

15. An FBAR as claimed in claim 14, wherein the gap comprises a region between the second electrode and the piezoelectric layer.

16. An FBAR as claimed in claim 15, wherein adjacent to termination of the active area of the acoustic resonator, the piezoelectric layer comprises a transition comprising defects, and the transition is disposed beneath the region of the gap.

17. An FBAR as claimed in claim 10, wherein the second electrode comprises an upper surface with a side and a recess is disposed along the side.

18. An FBAR as claimed in claim 10, wherein the second electrode comprises an upper surface with a side, and a frame element is disposed along the side.

19. An FBAR as claimed in claim 10, further comprising a low acoustic impedance material beneath the bridge.

20. A filter element comprising an acoustic resonator, the acoustic resonator comprising:
a first electrode;
a second electrode;
a piezoelectric layer disposed between the first and second electrodes;
a reflective element disposed beneath the first electrode, the second electrode and the piezoelectric layer, an overlap of the reflective element, the first electrode, the second electrode and the piezoelectric layer defining an active area of the acoustic resonator, wherein the first electrode substantially covers the reflective element, and the piezoelectric layer extends over an edge of the first electrode; and
a bridge adjacent to a termination of the active area of the acoustic resonator, wherein the bridge overlaps a portion of the first electrode.