Patent Publication Number: US-2023136934-A1

Title: Steep skirt combination microelectromechanical system cavity filter and bulk acoustic wave filter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Pat. Application Serial No. 63/274,588, titled “STEEP SKIRT COMBINATION MICROELECTROMECHANICAL SYSTEM CAVITY FILTER AND BULK ACOUSTIC WAVE FILTER,” filed Nov. 2, 2021, the entire contents of which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     Field 
     Aspects and embodiments disclosed herein relate to passband filters, such as for use in radio-frequency front end (RFFE) modules. Aspects and embodiments disclosed herein also relate to radio-frequency (RF) modules and wireless devices comprising passband filters. 
     Description of the Related Technology 
     Acoustic wave devices may be utilized as components of filters in radio frequency electronic systems. For instance, filters in a radio frequency front end of a mobile phone can include acoustic wave filters. Filters include passband filters. The acoustic wave devices that comprise a filter affect its performance and the range of frequencies in which it can operate. 
     A type of acoustic wave device commonly used in passband filters is a bulk acoustic wave resonator (BAW). This type of resonator has a robust structure and may be formed using well-known processes. 
     Another form of filter that is operable at high frequencies is a cavity filter. A cavity filter has a resonant frequency that is determine by the geometry of the cavity or cavities included in the filter. 
     With the advent of 5G New Radio (5G NR), new frequency ranges have been opened up for use, including Frequency Range 1 (FR1) between approximately 0.4 and 7.1 GHz and Frequency Range 2 (FR2) which spans approximately the range 24 GHz to 53 GHz. New filters are desired that are adapted to work at these frequency ranges, as well as filters that produce a cleaner, better quality signal. 
     SUMMARY 
     In accordance with one aspect, there is provided a combination filter. The combination filter comprises a notch filter formed of acoustic wave resonators and a cavity filter electrically in series with the notch filter to provide for the combination filter to operate at higher powers and frequencies. 
     In some embodiments, the notch filter is formed of bulk acoustic wave resonators. 
     In some embodiments, the bulk acoustic resonators include piezoelectric films having multiple sub-layers having different crystallographic orientations that provide for the bulk acoustic resonators to operate in one of a second overtone mode or a third overtone mode. 
     In some embodiments, the bulk acoustic resonators include piezoelectric films having two sub-layers having different crystallographic orientations. 
     In some embodiments, the bulk acoustic resonators include piezoelectric films having four sub-layers, adjacent ones of the four sub-layers having different crystallographic orientations. 
     In some embodiments, the notch filter is formed of lamb wave resonators. 
     In some embodiments, the cavity filter is a microelectromechanical system cavity filter. 
     In some embodiments, an upper end of a stopband of the notch filter is aligned with a lower end of a passband of the cavity filter. 
     In some embodiments, a lower end of a stopband of the notch filter is aligned with an upper end of a passband of the cavity filter. 
     In some embodiments, the notch filter is a first notch filter and the combination filter further comprises a second notch filter in series with the first notch filter and cavity filter. 
     In some embodiments, an upper end of a stopband of the first notch filter is aligned with a lower end of a passband of the cavity filter and a lower end of a stopband of the second notch filter is aligned with an upper end of the passband of the cavity filter. 
     In some embodiments, the combination filter has an operating frequency between about 0.4 and about 7.1 GHz. 
     In some embodiments, the combination filter has an operating frequency between about 24 GHz and about 53 GHz. 
     In some embodiments, the combination filter exhibits a steeper skirt than the cavity filter alone. 
     In some embodiments, the combination filter exhibits a greater power handling ability at a given frequency than a ladder filter formed of acoustic wave resonators operating at the given frequency. 
     In some embodiments, the combination filter is included in a radio frequency front end module. 
     In some embodiments, the radio frequency front end module of claim has an operating frequency within one of the 5G FR1 band or the 5G FR2 band. 
     In some embodiments, the radio frequency front end module is included in an electronic device. 
     In some embodiments, the radio frequency front end module is included in a cellular telephone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
         FIG.  1 A  illustrates an example of a microelectromechanical system (MEMS) cavity filter; 
         FIG.  1 B  illustrates the transmitted and returned power versus frequency curves for an example of a MEMS cavity filter; 
         FIG.  1 C  is a simplified illustration of the shape of an admittance versus frequency curve for an example of MEMS cavity filter; 
         FIG.  2 A  is a schematic diagram of an example of a ladder filter; 
         FIG.  2 B  is a simplified illustration of the shape of an admittance versus frequency curve for the ladder filter of  FIG.  2 A ; 
         FIG.  3 A  is a schematic diagram of a notch filter; 
         FIG.  3 B  is a simplified illustration of the shape of an admittance versus frequency curve for an example of a notch filter; 
         FIG.  4    is a cross-sectional diagram of a bulk acoustic wave resonator (BAW); 
         FIG.  5    is a cross-sectional diagram of a BAW identifying a portion of the active region of the BAW; 
         FIG.  6 A  illustrates an example of an electrode and piezoelectric film stack that may be present within the region of the BAW identified in  FIG.  5    that causes the BAW to operate in a thickness extension mode; 
         FIG.  6 B  illustrates an example of the frequency response of a BAW operating in a thickness extension mode; 
         FIG.  7 A  illustrates an example of an electrode and piezoelectric film stack including a multilayer piezoelectric film that may be present within the region of the BAW identified in  FIG.  5    that causes the BAW to operate in a second overtone mode; 
         FIG.  7 B  illustrates an example of the frequency response of a BAW operating in a second overtone mode; 
         FIG.  8 A  illustrates another example of an electrode and piezoelectric film stack including a multilayer piezoelectric film that may be present within the region of the BAW identified in  FIG.  5    that causes the BAW to operate in a second overtone mode; 
         FIG.  8 B  illustrates another example of an electrode and piezoelectric film stack including a multilayer piezoelectric film that may be present within the region of the BAW identified in  FIG.  5    that causes the BAW to operate in a second overtone mode; 
         FIG.  9 A  illustrates an example of an electrode and piezoelectric film stack including a multilayer piezoelectric film that may be present within the region of the BAW identified in  FIG.  5    that causes the BAW to operate in a third overtone mode; 
         FIG.  9 B  illustrates another example of an electrode and piezoelectric film stack including a multilayer piezoelectric film that may be present within the region of the BAW identified in  FIG.  5    that causes the BAW to operate in a third overtone mode; 
         FIG.  9 C  illustrates an example of the frequency response of a BAW operating in a third overtone mode; 
         FIG.  10 A  is a cross-sectional diagram of an example of a Lamb wave resonator; 
         FIG.  10 B  illustrates an example of an interdigital electrode structure that may be included in an example of a Lamb wave resonator; 
         FIGS.  10 C -  10 G  are cross-sectional diagrams of other examples of Lamb wave resonators; 
         FIG.  11    is a schematic diagram of a combination filter including a BAW notch filter and a MEMS cavity filter in series; 
         FIG.  12 A  illustrates one way the frequency responses of the BAW notch filter and the MEMS cavity filter of the combination filter of  FIG.  11    may be combined to define a frequency response for the combination filter; 
         FIG.  12 B  illustrates another way the frequency responses of the BAW notch filter and the MEMS cavity filter of the combination filter of  FIG.  11    may be combined to define a frequency response for the combination filter; 
         FIG.  12 C  is a schematic diagram of a combination filter including two BAW notch filters and a MEMS cavity filter in series; 
         FIG.  12 D  illustrates a frequency response for the combination filter of  FIG.  12 C ; 
         FIG.  13    is a schematic of a radio frequency front end module incorporating aspects disclosed herein; and 
         FIG.  14    is a schematic of a wireless device incorporating aspects disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary aspects and embodiments, and advantages of these exemplary aspects and embodiments are discussed in detail below. Embodiments disclosed herein may be combined with other embodiments in any manner consistent with at least one of the principles disclosed herein, and references to “an embodiment,” “some embodiments,” “an alternate embodiment,” “various embodiments,” “one embodiment,” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one embodiment. The appearances of such terms herein are not necessarily all referring to the same embodiment. 
     Aspects and embodiments described herein are directed to a passband filter, such as for use in radio-frequency (RF) implementations including, for example, radio-frequency front end (RFFE) modules, having a steep skirt and able to operate at higher frequencies with better power handling than many conventional radio frequency filters. 
     It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. 
     A number of different filter architectures capable of filtering radio-frequency signals are known in the art. Different filter architectures have different advantages and disadvantages. One type of filter architecture capable of filtering radio-frequency signals is a cavity filter. Cavity filters, or more specifically for the purposes of this disclosure, microelectromechanical system (MEMS) cavity filters can operate at very high frequencies at high power with little insertion loss. One example of a MEMS cavity filter, as described in  MEMS Bandpass Filter Based On Substrate Integrated Waveguide Structure , Ze-hua Cao et al., Fundamental Science on EHF Laboratory, University of Electronic Science and Technology of China, is illustrated in  FIG.  1 A . Examples of MEMS cavity filters are capable of handling higher power signals without damage than many other filter types. A disadvantage of MEMS cavity filters is that their rejection performance out of band, or away from their resonance frequency is not as good as many other types of RF filters because MEMS cavity filters exhibit resonance but not anti-resonance. The frequency response of an example of a MEMS cavity filter is illustrated in  FIG.  1 B , showing the transmitted power S 21  curve and the returned power S 11  curve. The transmitted power (admittance) curve is shown more schematically in  FIG.  1 C . The admittance of the cavity filter falls off gradually on either side of the passband, making it difficult to operate the cavity filter to pass only a narrow range of frequencies. The cavity filter can be described as having a wide skirt, where the term “skirt” refers to the drop in admittance on either side of the passband or resonant frequency of the filter. 
     Another type of filter for RF signals may be formed from acoustic wave resonators. One type of acoustic wave resonator-based RF filter is known as a ladder filter. One example of a ladder filter is illustrated in  FIG.  2 A . The filter of  FIG.  2 A  includes a plurality of resonators R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R 10 , and R 11 . Resonators R 1 , R 3 , R 5 , R 7 , R 9 , and R 11  are connected in series between the input port (IN) and the output port (OUT). Resonators R 2 , R 4 , R 6 , R 8 , and R 10  are connected between adjacent pairs of resonators R 1 , R 3 , R 5 , R 7 , R 9 , and R 11  and ground. Resonators R 2 , R 4 , R 6 , R 8 , and R 10  may also be referred to as shunt resonators. In some embodiments, each of the series resonators R 1 , R 3 , R 5 , R 7 , R 9 , and R 11  may have the same resonant frequency. Each of the plurality of shunt resonators R 2 , R 4 , R 6 , R 8 , and R 10  have a resonant frequency below the resonant frequency of each of the series arm resonators R 1 , R 3 , R 5 , R 7 , R 9 , and R 11 . At least one of the plurality of shunt resonators R 2 , R 4 , R 6 , R 8 , and R 10  may have a first resonant frequency different from resonant frequencies of others of the plurality of shunt resonators. At least one of the plurality of shunt resonators R 2 , R 4 , R 6 , R 8 , and R 10  may have a second resonant frequency different from the first resonant frequency and different from the resonant frequencies of others of the plurality of shunt resonators. 
     Filters having an architecture similar to that illustrated in  FIG.  2 A  may act as passband filters, and are often provided in radio-frequency front end (RFFE) modules. Such a passband filter has a frequency response as illustrated in  FIG.  2 B  in which the passband is indicated by the frequency range in the circled area “E”. The filter allows a range of frequencies through, with signals having frequencies outside of this range being suppressed. The series resonators R 1 , R 3 , R 5 , R 7 , R 9 , and R 11  define the upper frequency limit of the passband filter, while the shunt resonators R 2 , R 4 , R 6 , R 8 , and R 10  define the lower frequency limit. 
     Ladder filters as illustrated in  FIG.  2 A  may be formed from acoustic wave resonators including surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators. Such ladder filters may exhibit certain disadvantages making them less than ideal for operating at frequencies and power ratings desired for 5G devices. The frequency at which SAW resonators may operate is determined in part by how closely the electrodes of the SAW resonators may be spaced from one another. A closer spacing of electrodes provides for a SAW filter to operate at a higher resonant and anti-resonant frequency. However, as the spacing between electrodes becomes smaller, a SAW resonator generally becomes less robust for operating with high power signals because the higher the power the signal is, the more likely that arcing may occur across closely spaced electrodes. A ladder filter formed of SAW resonators thus may have difficulty operating at both high frequencies and high power. A ladder filter formed of BAW resonators may exhibit similar disadvantages. In BAW resonators the frequency at which BAW resonators may operate is determined in part by how thin the piezoelectric film of the BAW resonator may be formed. A thinner piezoelectric film layer generally provides for a BAW to operate at a higher resonant and anti-resonant frequency. However, as the thickness of the piezoelectric film layer becomes smaller, a BAW generally becomes less robust for operating with high power signals because the higher the power the signal is, the more likely that arcing may occur through the piezoelectric film layer. A ladder filter formed of BAW resonators thus may have difficulty operating at both high frequencies and high power. 
     Acoustic wave resonators may also be arranged as a notch filter which may be considered the opposite of a passband filter. Rather than defining a passband, a notch filter defines a frequency region in which a radio frequency signal is attenuated. One example arrangement of acoustic wave resonators forming a notch filter is illustrated in  FIG.  3 A . The notch filter includes a series resonator S 1  and shunt resonators S 2 , S 3  coupled between the input and output sides of the series resonator S 1  and ground. The frequency response of the notch filter is exhibited schematically in  FIG.  3 B . 
     As discussed above one type of acoustic wave resonator that may be used to form an acoustic wave filter is a BAW.  FIG.  4    is cross-sectional view of an example of a BAW, indicated generally at  200 , specifically a film bulk acoustic wave resonator (FBAR). The BAW  200  is disposed on a substrate  210 , for example, a silicon substrate that may include a dielectric surface layer  210 A of, for example, silicon dioxide. The BAW  200  includes a layer or film of piezoelectric material  215 , for example, aluminum nitride (AlN). A top electrode  220  is disposed on top of a portion of the layer or film of piezoelectric material  215  and a bottom electrode  225  is disposed on the bottom of a portion of the layer or film of piezoelectric material  215 . The top electrode  220  may be formed of, for example, ruthenium (Ru). The bottom electrode  225  may include a layer  225 A of Ru disposed in contact with the bottom of the portion of the layer or film of piezoelectric material  215  and a layer  225 B of titanium (Ti) disposed on a lower side of the layer  225 A of Ru opposite a side of the layer  225 A of Ru in contact with the bottom of the portion of the layer or film of piezoelectric material  215 . Each of the top electrode  220  and the bottom electrode  225  may be covered with a layer of dielectric material  230 , for example, silicon dioxide. An air cavity  235  is defined beneath the layer of dielectric material  230  covering the bottom electrode  225  and the surface layer  210 A of the substrate  210 . A bottom electrical contact  240  formed of, for example, copper may make electrical connection with the bottom electrode  225  and a top electrical contact  245  formed of, for example, copper may make electrical connection with the top electrode  220 . 
     The BAW  200  may include a central region  250  including a main active domain in the layer or film of piezoelectric material  215  in which a main acoustic wave is excited during operation. A recessed frame region or regions  255  may bound and define the lateral extent of the central region  250 . The recessed frame region(s)  255  may be defined by areas that have a thinner layer of dielectric material  230  on top of the top electrode  220  than in the central region  250 . The dielectric material layer  230  in the recessed frame region(s)  255  may be from about 10 nm to about 200 nm thinner than the dielectric material layer  230  in the central region  250  and/or the difference in thickness of the dielectric material in the recessed frame region(s)  255  vs. in the central region  250  may cause the resonant frequency of the device in the recessed frame region(s)  255  to be between about 5 MHz to about 50 MHz higher than the resonant frequency of the device in the central region  250 . A raised frame region or regions  260  may be defined on an opposite side of the recessed frame region(s)  255  from the central region  250  and may directly abut the outside edge(s) of the recessed frame region(s)  255 . The raised frame region(s)  260  may be defined by areas where the top electrode  220  is thicker than in the central region  250  and in the recessed frame region(s)  255 . The top electrode  220  may have the same thickness in the central region  250  and in the recessed frame region(s)  255  but a greater thickness in the raised frame region(s)  260 . The top electrode  220  may be between about 50 nm and about 500 nm thicker in the raised frame region(s)  260  than in the central region  250  and/or in the recessed frame region(s)  255 . The raised frame region(s) may be, for example, 1 µm or more in width. 
     The recessed frame region(s)  255  and the raised frame region(s)  260  may contribute to dissipation or scattering of transverse acoustic waves generated in the BAW  200  during operation and/or may reflect transverse waves propagating outside of the recessed frame region(s)  255  and the raised frame region(s)  260  and prevent these transverse acoustic waves from entering the central region and inducing spurious signals in the main active domain region of the BAW. Without being bound to a particular theory, it is believed that due to the thinner layer of dielectric material  230  on top of the top electrode  220  in the recessed frame region(s)  255 , the recessed frame region(s)  255  may exhibit a higher velocity of propagation of acoustic waves than the central region  250 . Conversely, due to the increased thickness and mass of the top electrode  220  in the raised frame region(s)  260 , the raised frame regions(s)  260  may exhibit a lower velocity of propagation of acoustic waves than the central region  250  and a lower velocity of propagation of acoustic waves than the recessed frame region(s)  255 . The discontinuity in acoustic wave velocity between the recessed frame region(s)  255  and the raised frame region(s)  260  creates a barrier that scatters, suppresses, and/or reflects transverse acoustic waves. 
       FIG.  5    illustrates a BAW  200  that is generally the same as that discussed in relation to  FIG.  4   , and is mounted on a substrate  301 , such as a silicon wafer, and comprises a top electrode  220  and a bottom electrode  225  disposed either side of piezoelectric layer  215 . The BAW further comprises a cavity  235  and has a layer of dielectric material  230  disposed over the top electrode  220 . The difference of the BAW  200  of  FIG.  5    compared to that of  FIG.  4    will be discussed in relation to  FIGS.  6 A- 10   . 
       FIG.  6 A  illustrates the region of box E in  FIG.  5    that may be provided in a conventional BAW such as illustrated in  FIG.  4   . It comprises a single piezoelectric layer  215 , bounded by a top electrode  220  and a bottom electrode  225 .  FIG.  6 B  illustrates the frequency response of such a device. The y-axis of  FIG.  6 B  represents the admittance of the 1-port resonator, while the x-axis represents frequency. As can be seen, the resonant frequency lies between 2 GHz and 3 GHz (for the selected thickness of the piezoelectric layer  215 ). Such a mode of operation is known as a thickness extension (TE) mode. 
       FIG.  7 A  illustrates a variation whereby the piezoelectric layer  215  is divided into two sub-layers  215   a ,  215   b . The layers are distinguished by having different polarizations, as illustrated in the example of  FIG.  7 A  wherein the piezoelectric layers are AlN layers having different orientations of the crystallographic C-axis, the glide plane in the c-direction to the normal of the crystal structure, in each sub-layer  215   a ,  215   b . Looking at the graph of  FIG.  7 B , the effect of dividing the piezoelectric layer  215  into two sub-layers  215   a ,  215   b  is apparent. The resonant frequency, for the same total thickness of the piezoelectric layer  215  as in a BAW having the piezoelectric film structure  215  illustrated in  FIG.  6 A , is greatly increased, now lying between 6 GHz and 7 GHz. Such a mode of operation is known as an overtone mode, specifically the second overtone mode. In this manner, by utilizing the overtone mode by having two (or more) layers of piezoelectric material with different polarizations, a BAW can be provided to operate at a higher frequency region than a BAW having a homogeneous single piezoelectric film layer. 
       FIGS.  8 A and  8 B  illustrate additional configurations of the piezoelectric layers in a BAW as illustrated in  FIG.  5    that may cause the BAW to operate in a second overtone mode. In  FIG.  8 A , the C axes of the two sub-layers  215   a ,  215   b  are oriented opposite to the orientation illustrated in  FIG.  7 A . In  FIG.  8 B , the piezoelectric layer is split into four sublayers  215   a - 215   d , with adjacent sublayers having opposite C-axis orientations. 
     The piezoelectric film of a BAW can also be split into sub-layers with C-axis orientations that cause the BAW to operate in a third overtone mode.  FIGS.  9 A and  9 B  illustrate BAW piezoelectric layer structures  215  each broken into four sublayers  215   a - 215   d , with the C-axis orientation of each sublayer being opposite to the C-axis orientation of an adjacent sublayer.  FIG.  9 C  illustrates the admittance curve of an example BAW operating in the third overtone mode. The resonant frequency, for the same total thickness of the piezoelectric layer  215  as in a BAW having the piezoelectric film structure  215  illustrated in  FIG.  6 A , is greatly increased over that of BAWS operating in either the thickness extension mode or the second overtone mode, now lying between 10 GHz and 11 GHz. 
     BAW resonators can thus be operated at very high frequencies when utilizing overtone modes of operation. One advantage of BAW resonators over cavity resonators is that they can be used to form a filter having a passband with a sharp skirt because BAW resonators exhibit both resonant and antiresonant frequencies which can be utilized to define the upper and lower ends of a passband of a filter including BAW resonators. BAW resonators can also generally be made smaller than cavity resonators for the same resonant frequency. A disadvantage of BAW resonators as compared to cavity resonators is that they may exhibit higher insertion loss when operating in overtone mode than a cavity filter operating in the same frequency range. Also, as discussed above, BAW resonators are generally less capable of handling high power signals at high frequencies due to the reduction in thickness of the piezoelectric film between the electrodes of the resonator that may be used to increase the operating frequency. A thinner piezoelectric layer may lead to a higher operating frequency of a BAW, but a greater possibility of electrical breakdown across the electrodes and through the piezoelectric film. There is also a limit to how high the operating frequency of a BAW may be made due to limits in reliability when utilizing very thin piezoelectric films. Further, the electromagnetic coupling coefficient k 2  decreases with decreasing film thickness which causes the bandwidth of a filter formed of BAWs operating at high frequencies to be narrow. 
     Another form of acoustic wave resonator is a Lamb wave resonator. A Lamb wave resonator is also a type of bulk acoustic wave resonator. Lamb wave resonators can operate at higher frequency than film bulk acoustic wave resonators (FBARs), because they typically do not rely only a thin film of piezoelectric material to define their operating frequencies, and they can handle higher powers. Furthermore, they typically exhibit a higher k 2  than FBARs. However, Lamb wave resonators are a more complex design to manufacture than FBARs using current manufacturing processes. 
       FIG.  10 A  is a cross-sectional diagram of a Lamb wave resonator  300 . The Lamb wave resonator  300  is implemented on a substrate  309 . The Lamb wave resonator  300  includes a piezoelectric layer  305 , an interdigital transducer electrode (IDT) electrode  306  on the piezoelectric layer  305 , and a lower electrode  307 . The piezoelectric layer  305  can be a thin film. The piezoelectric layer  305  can be an aluminum nitride (AlN) layer. In other instances, the piezoelectric layer  305  can be any suitable piezoelectric layer. For example, the piezoelectric layer  305  can be a lithium niobate layer (LiNbO 3 ) or a lithium tantalate (LiTaO 3 ) layer. The operating frequency of the Lamb wave resonator  300  can be based on the geometry of the IDT electrode  306 . In some instances, the illustrated IDTs of the Lamb wave resonator  300  represent two sets of IDTs. The lower electrode  307  can be grounded in certain instances. In some other instances, the lower electrode  307  can be floating. An air cavity  308  is disposed between the lower electrode  307  and the substrate  309 . Any suitable cavity can be implemented in place of the air cavity  308 . The substrate  309  can be a semiconductor substrate. For example, the substrate  309  can be a silicon substrate. The substrate  309  can be any other suitable substrate, such as a quartz substrate, a sapphire substrate, or a spinel substrate. 
     In some embodiments, the IDT electrode  306  may be include a plurality of interdigitated electrode fingers  310 A,  310 B extending from respective bus bar electrodes  312 A,  312 B and disposed between reflector gratings  314  including reflector fingers  316  and reflector bus bars  318 , as illustrated in plan view in  FIG.  10 B  and cross-sectional view in  FIG.  10 C . 
     Another example of a Lamb waver resonator is illustrated in cross-section in  FIG.  10 D . The Lamb wave resonator of  FIG.  10 D  differs from that illustrated in  FIG.  10 C  in that the lower electrode  307  is omitted. 
     In other embodiments, the lower electrode  307  may be replaced with a lower IDT electrode  306 ′ and associated reflector gratings  314 ′. The upper and lower IDT electrodes  306 ,  306 ′ may include fingers  310  to which the same voltage polarity is applied during operation that are either horizontally offset from one another ( FIG.  10 E ) or vertically aligned with one another ( FIG.  10 F ). In other embodiments an opposite voltage polarity may be applied to each of the fingers of the upper IDT electrode  306  than the voltage polarity applied to each of the fingers of the lower IDT electrode  306 ′ during operation ( FIG.  10 G ). The substrates  309  are omitted from  FIGS.  10 E- 10 G  for clarity. 
     A notch filter formed of Lamb wave resonators as described herein may exhibit similar admittance vs. frequency characteristics as a notch filter formed of BAWs, for example, as illustrated in  FIG.  3 B  above. 
     In some embodiments, different forms of filters are combined to form a combination filter having the advantages of each of the different forms of filters. In one example, a notch filter formed of bulk acoustic wave resonators, for example, BAWs or Lamb wave resonators as disclosed herein may be arranged electrically in series with a MEMS cavity filter to form a combination filter as illustrated in  FIG.  11   . Such a combination filter may provide benefits associated with both a filter formed from bulk acoustic wave resonators (BAWs) and a MEMS cavity filter. 
     As discussed above, the stop band for a BAW notch filter has steep skirts while a MEMS cavity filter may have a wide skirt. If one were to combine a BAW notch filter and a MEMS cavity filter in series as illustrated in  FIG.  11    and select BAW resonators for the notch filter and a cavity filter such that the upper edge of the stop band of the BAW notch filter aligned with the lower end passband of the cavity filter, one could obtain a combination filter with a steeper lower skirt than exhibited by the cavity filter alone. This is illustrated schematically in  FIG.  12 A . Similarly, if one were to combine a BAW notch filter and a MEMS cavity filter in series as illustrated in  FIG.  11    and select BAW resonators for the notch filter and a cavity filter such that the lower edge of the stop band of the BAW notch filter aligned with the upper end of the passband of the cavity filter, one could obtain a combination filter with a steeper upper skirt than exhibited the cavity filter alone. This is illustrated schematically in  FIG.  12 B . If one used two notch filters in series with a cavity filter as illustrated in  FIG.  12 C , selected BAW resonators for one notch filter such that the upper edge of the stop band of the BAW notch filter aligned with the lower end passband of the cavity filter, and selected BAW resonators for the other notch filter such that the lower edge of the stop band of the other BAW notch filter aligned with the upper end passband of the cavity filter, one could obtain a combination filter combining the effects illustrated in  FIGS.  12 A and  12 B  with a passband with sharp upper and lower skirts as illustrated schematically in  FIG.  12 D . The cavity filter may be disposed between the two notch filters as illustrated. It should be understood that either terminal of embodiments of the combined filter may be the input and the other terminal the output. 
     A combination filter as disclosed herein exhibits advantages of both BAW filters and MEMS cavity filters while avoiding at least some of the disadvantages of each type of filter. The cavity filter is responsible for defining the passband of the combination filter and can operate with high power within this passband. The BAW filters, which are often less capable of handling the same amount of power as a cavity filter, are arranged in notch filter configurations and help define the skirts of the combined filter and thus do not need to handle high amounts of power. The cavity filter can operate at high frequencies, as can the BAW filters when operated in overtone mode. 
     Combination filters as disclosed herein including both BAW or Lamb wave notch filters and MEMS cavity filters may be incorporated into a radio-frequency front end (RFFE) module. An exemplary RFFE module is shown in  FIG.  13   . This figure illustrates a front end module  2200 , connected between an antenna  2310  and a transceiver  2230 . The front end module  2200  includes a duplexer  2210  in communication with an antenna switch  2250 , which itself is in communication with the antenna  2310 . 
     As illustrated, the transceiver  2230  comprises a transmitter circuit  2232 . Signals generated for transmission by the transmitter circuit  2232  are received by a power amplifier (PA) module  2260  within the front end module  220  which amplifies the generated signals from the transceiver  2230 . The PA module  2260  can include one or more PAs. The PA module  2260  can be used to amplify a wide variety of RF or other frequency-band transmission signals. For example, the PA module  2260  can receive an enable signal that can be used to pulse the output of the PE to aid in transmitting a wireless local area network (WLAN) signal or any other suitable pulsed signal. The PA module  2260  can be configured to amplify any of a variety of types of signal, including, for example, a Global System for Mobile (GSM) signal, a code division multiple access (CDMA) signal, a W-CDMA signal, a Long Term Evolution (LTE) signal, or an EDGE signal. In certain embodiments, the PA module  2260  and associated components including switches and the like can be fabricated on gallium arsenide (GaAs) substrates using, for example, high electron mobility transistors (pHEMT) or insulated-gate bipolar transistors (BiFET), or on a silicon substrate using complementary metal-oxide semiconductor (CMOS) field effect transistors (FETs). 
     Still referring to  FIG.  13   , the front end module  2200  may further include a low noise amplifier (LNA) module  2270 , which amplifies received signals from the antenna  2310  and provides the amplified signals to the receiver circuit  2234  of the transceiver  2230 . 
       FIG.  14    is a schematic diagram of a wireless device  1100  that can incorporate aspects of combination filters as disclosed herein. The wireless device  1100  can be, for example but not limited to, a portable telecommunication device such as, a mobile cellular-type telephone. The wireless device  1100  can include a microphone arrangement  1100 , and may include one or more of a baseband system  1101 , a transceiver  1102 , a front end system  1103 , one or more antennas  1104 , a power management system  1105 , a memory  1106 , a user interface  1107 , a battery  1108 , and audio codec  1109 . The microphone arrangement may supply signals to the audio codec  109  which may encode analog audio as digital signals or decode digital signals to analog. The audio codec  1109  may transmit the signals to a user interface  1107 . The user interface  1107  transmits signals to the baseband system  1101 . The transceiver  1102  generates RF signals for transmission and processes incoming RF signals received from the antennas. The front end system  1103  aids in conditioning signals transmitted to and/or received from the antennas  1104 . The antennas  1104  can include antennas used for a wide variety of types of communications. For example, the antennas  1104  can include antennas  1104  for transmitting and/or receiving signals associated with a wide variety of frequencies and communications standards. The baseband system  1101  is coupled to the user interface to facilitate processing of various user input and output, such as voice and data. The baseband system  1101  provides the transceiver  1102  with digital representations of transmit signals, which the transceiver  1102  processes to generate RF signals for transmission. The baseband system  1101  also processes digital representations of received signals provided by the transceiver  1102 . 
     As shown in  FIG.  14   , the baseband system  1101  is coupled to the memory  1106  to facilitate operation of the wireless device  1100 . The memory  1106  can be used for a wide variety of purposes, such as storing data and/or instructions to facilitate the operation of the wireless device  1100  and/or to provide storage of user information. The power management system  1105  provides a number of power management functions of the wireless device  1100 . The power management system  1105  receives a battery voltage from the battery  1108 . The battery  1108  can be any suitable battery for use in the wireless device, including, for example, a lithium-ion battery. 
     Embodiments of the combination filters described herein may be incorporated into the wireless device  1100  of  FIG.  14   , in particular may be incorporated into the front end system  1103   
     It should be appreciated that the resonators illustrated herein along with other structures illustrated in the other figures accompanying this disclosure, are illustrated in a highly simplified form. The relative dimensions of the different features are not shown to scale. Further, embodiments comprising elements of the resonators, filters, combined filters, or devices disclosed herein may include additional features or layers not illustrated or may lack one or more features or layers illustrated herein. 
     Having described above several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.