Patent Publication Number: US-2021184647-A1

Title: Interdigital transducer arrangements for surface acoustic wave devices

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 16/156,014, which claims the benefit of provisional patent application Ser. No. 62/698,509, filed Jul. 16, 2018, the disclosures of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to acoustic wave devices, and particularly to interdigital transducer (IDT) arrangements for surface acoustic wave (SAW) devices. 
     BACKGROUND 
     Acoustic wave devices are widely used in modern electronics. At a high level, acoustic wave devices include a piezoelectric material in contact with one or more electrodes. Piezoelectric materials acquire a charge when compressed, twisted, or distorted, and similarly compress, twist, or distort when a charge is applied to them. Accordingly, when an alternating electrical signal is applied to the one or more electrodes in contact with the piezoelectric material, a corresponding mechanical signal (i.e., an oscillation or vibration) is transduced therein. Based on the characteristics of the one or more electrodes on the piezoelectric material, the properties of the piezoelectric material, and other factors such as the shape of the acoustic wave device and other structures provided on the device, the mechanical signal transduced in the piezoelectric material exhibits a frequency dependence on the alternating electrical signal. Acoustic wave devices leverage this frequency dependence to provide one or more functions. 
     Surface acoustic wave (SAW) devices, such as SAW resonators and SAW filters, are used in many applications such as radio Frequency (RF) filters. For example, SAW filters are commonly used in second generation (2G), third generation (3G), and fourth generation (4G) wireless receiver front ends, duplexers, and receive filters. The widespread use of SAW filters is due to, at least in part, the fact that SAW filters exhibit low insertion loss with good rejection, can achieve broad bandwidths, and are a small fraction of the size of traditional cavity and ceramic filters. As the use of SAW filters in modern RF communication systems increase, there is a need for SAW filters with sharp transitions between desired passband frequencies and frequencies that are outside of desired passbands. 
     SUMMARY 
     The present disclosure relates to acoustic wave devices, and particularly to interdigital transducer (IDT) arrangements for surface acoustic wave (SAW) devices. Representative SAW devices are described herein that provide sharp transitions between passband frequencies and frequencies that are outside of desired passbands. In certain embodiments, a SAW device may include several IDTs arranged between reflective structures and one or more additional IDTs or electrode pairs that are configured to modify the influence of parasitic capacitance, or other internal device capacitance, thereby improving steepness on the upper side of a passband as well as improving rejection for frequencies outside of the passband. The one or more additional IDTs or electrode pairs may be configured as at least one of a capacitor, an IDT capacitor, an IDT with a floating electrode, or combinations thereof. 
     In one aspect, a SAW device, comprises a piezoelectric material, at least one input IDT on the piezoelectric material and electrically connected to an input signal and ground; at least one output IDT on the piezoelectric material and electrically connected to an output signal and ground; and an additional IDT on the piezoelectric material and electrically connected to the input signal and the output signal, wherein the additional IDT is arranged between the at least one input IDT and the at least one output IDT. In certain embodiments, the additional IDT comprises an IDT capacitor. The additional IDT may comprise a first electrode electrically connected to the input signal and a second electrode electrically connected to the output signal. The SAW device may further comprise a first reflective structure and a second reflective structure on the piezoelectric material, wherein the at least one input IDT, the at least one output IDT and the additional IDT are arranged between the first reflective structure and the second reflective structure. In certain embodiments, the at least one input IDT comprises a plurality of input IDTs and the at least one output IDT comprises a plurality of output IDTs. The plurality of input IDTs and the plurality of output IDTs may be configured in an alternating arrangement. In certain embodiments, at least one of the at least one input IDT, the at least one output IDT, and the additional IDT comprises an apodized IDT. 
     In another aspect, a SAW device comprises a piezoelectric material, at least one input IDT on the piezoelectric material and electrically connected to an input signal and ground; at least one output IDT on the piezoelectric material and electrically connected to an output signal and ground; a first additional electrode pair on the piezoelectric material and electrically connected to the input signal and the output signal; and a second additional electrode pair arranged between the at least one input IDT and the least one output second IDT, wherein the second additional electrode pair comprises at least one floating electrode. The SAW device may further comprise a first reflective structure and a second reflective structure on the piezoelectric material, wherein the at least one input IDT, the at least one output IDT, the first additional electrode pair, and the second additional electrode pair are arranged between the first reflective structure and the second reflective structure. In certain embodiments, the second additional electrode pair comprises a first electrode that is electrically connected to the input signal, a second electrode that is electrically connected to the output signal, and a floating electrode. In certain embodiments, the second additional electrode pair is not directly electrically connected to either of the input signal and the output signal. In certain embodiments, the second additional electrode pair comprises a first electrode and a second electrode and at least one of the first electrode and the second electrode is devoid of electrode fingers. In certain embodiments, at least one of the first additional electrode pair and the second additional electrode pair comprises an additional IDT. At least one of the input IDT and the output IDT may comprise an apodized IDT 
     In another aspect, a SAW device comprises a piezoelectric material; a first reflective structure and a second reflective structure on the piezoelectric material; a plurality of input IDTs and a plurality of output IDTs arranged on the piezoelectric material and between the first reflective structure and the second reflective structure, wherein the plurality of input IDTs and the plurality of output IDTs are configured in an alternating arrangement between the first reflective structure and the second reflective structure; and a plurality of additional IDTs arranged between corresponding ones of the plurality of input IDTs and corresponding ones of the plurality of output IDTs, wherein at least one additional IDT of the plurality of additional IDTs is electrically connected to an input signal and an output signal. In certain embodiments, the plurality of additional IDTs comprises at least two additional IDTs electrically connected to an input signal and an output signal. The at least one additional IDT of the plurality of additional IDTs may comprise a first electrode that is electrically connected to the input signal, a second electrode that is electrically connected to the output signal, and a floating electrode. In certain embodiments, at least one other additional IDT of the plurality of additional IDTs is not directly electrically connected to either of the input signal and the output signal. The at least one other additional IDT of the plurality of additional IDTs may comprise a first electrode electrically connected to ground and a second electrode that is a floating electrode. At least one of the plurality of input IDTs, the plurality of output IDTs, and the plurality of additional IDTs may comprise a metallization ratio in a range of about 0.2 to about 0.8. At least one of the plurality of input IDTs, the plurality of output IDTs, and the plurality of additional IDTs may comprise an apodized IDT. 
     In another aspect, 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 features and elements as disclosed herein may be combined with one or more other disclosed features and elements unless indicated to the contrary herein. 
     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 
       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. 
         FIG. 1  is a perspective view illustration of a representative surface acoustic wave (SAW) device  10 . 
         FIG. 2A  illustrates an example SAW couple resonated filter (CRF) structure that includes a plurality of interdigital transducers (IDTs) that are acoustically and longitudinally coupled between two reflective structures. 
         FIG. 2B  is an S-Parameters plot representing simulations of the SAW CRF structure of  FIG. 2A  with a variety of capacitor values. 
         FIG. 3  illustrates a SAW CRF structure that includes a plurality of IDTs that are longitudinally coupled between two reflective structures. 
         FIG. 4A  is a block diagram of a radio frequency (RF) duplexer that includes the SAW CRF structure of  FIG. 2A . 
         FIG. 4B  is a block diagram of an RF duplexer that includes the SAW CRF structure of  FIG. 3 . 
         FIG. 5A  is a top view of a device layout of the RF duplexer of  FIG. 4A . 
         FIG. 5B  is a top view of a device layout of the RF duplexer of  FIG. 4B . 
         FIG. 6A  is a comparison plot for isolation of RF duplexers with various SAW CRF structures as disclosed herein. 
         FIG. 6B  is a comparison plot for a passband of RF duplexers with various SAW CRF structures as disclosed herein. 
         FIG. 7  illustrates a SAW CRF structure that includes a plurality of IDTs that are longitudinally coupled between two reflective structures according to embodiments disclosed herein. 
         FIG. 8  illustrates a different SAW CRF structure that includes a plurality of IDTs that are longitudinally coupled between two reflective structures according to embodiments disclosed herein. 
         FIG. 9A  illustrates a different SAW CRF structure that includes a plurality of IDTs that are longitudinally coupled between two reflective structures according to embodiments disclosed herein. 
         FIG. 9B  illustrates an alternative configuration for the SAW CRF structure of  FIG. 9A . 
         FIG. 10  illustrates a different SAW CRF structure that includes a plurality of IDTs that are longitudinally coupled between two reflective structures according to embodiments disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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 departing from 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. 
     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. 
     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. 
     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 the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     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. 
     The present disclosure relates to acoustic wave devices, and particularly to interdigital transducer (IDT) arrangements for surface acoustic wave (SAW) devices. Representative SAW devices are described herein that provide sharp transitions between passband frequencies and frequencies that are outside of desired passbands. In certain embodiments, a SAW device may include several IDTs arranged between reflective structures and one or more additional IDTs or electrode pairs that are configured to modify the influence of parasitic capacitance, or other internal device capacitance, thereby improving steepness on the upper side of a passband as well as improving rejection for frequencies outside of the passband. The one or more additional IDTs or electrode pairs may be configured as at least one of a capacitor, an IDT capacitor, an IDT with a floating electrode, or combinations thereof. 
     Before describing particular embodiments of the present disclosure further, a general discussion of SAW devices is provided.  FIG. 1  is a perspective view illustration of a representative SAW device  10 . The SAW device  10  includes a substrate  12 , a piezoelectric layer  14  on the substrate  12 , an IDT  16  on a surface of the piezoelectric layer  14  opposite the substrate  12 , a first reflector structure  18 A on the surface of the piezoelectric layer  14  adjacent to the IDT  16 , and a second reflector structure  18 B on the surface of the piezoelectric layer  14  adjacent to the IDT 16  opposite the first reflector structure  18 A. 
     The IDT  16  includes a first electrode  20 A and a second electrode  20 B, each of which include a number of electrode fingers  22  that are interleaved with one another as shown. The first electrode  20 A and the second electrode  20 B may also be referred to as comb electrodes. A lateral distance between adjacent electrode fingers  22  of the first electrode  20 A and the second electrode  20 B defines an electrode pitch P of the IDT  16 . The electrode pitch P may at least partially define a center frequency wavelength λ of the SAW device  10 , where the center frequency is the primary frequency of mechanical waves generated in the piezoelectric layer  14  by the IDT  16 . For a single electrode IDT  16  such as the one shown in  FIG. 1 , the center frequency wavelength λ is equal to twice the electrode pitch P. For a double electrode IDT  16 , the center frequency wavelength λ is equal to four times the electrode pitch P. A finger width W of the adjacent electrode fingers  22  over the electrode pitch P may define a metallization ratio, or duty factor, of the IDT  16 , which may dictate certain operating characteristics of the SAW device  10 . 
     In operation, an alternating electrical input signal provided at the first electrode  20 A is transduced into a mechanical signal in the piezoelectric layer  14 , resulting in one or more acoustic waves therein. In the case of the SAW device  10 , the resulting acoustic waves are predominately surface acoustic waves. As discussed above, due to the electrode pitch P and the metallization ratio of the IDT  16 , the characteristics of the material of the piezoelectric layer  14 , and other factors, the magnitude and frequency of the acoustic waves transduced in the piezoelectric layer  14  are dependent on the frequency of the alternating electrical input signal. This frequency dependence is often described in terms of changes in the impedance and/or a phase shift between the first electrode  20 A and the second electrode  20 B with respect to the frequency of the alternating electrical input signal. An alternating electrical potential between the two electrodes  20 A and  20 B creates an electrical field in the piezoelectric material which generate acoustic waves. The acoustic waves travel at the surface and eventually are transferred back into an electrical signal between the electrodes  20 A and  20 B. The first reflector structure  18 A and the second reflector structure  18 B reflect the acoustic waves in the piezoelectric layer  14  back towards the IDT  16  to confine the acoustic waves in the area surrounding the IDT  16 . 
     The substrate  12  may comprise various materials including glass, sapphire, quartz, silicon (Si), or gallium arsenide (GaAs) among others, with Si being a common choice. The piezoelectric layer  14  may be formed of any suitable piezoelectric material(s). In certain embodiments described herein, the piezoelectric layer  14  is formed of lithium tantalate (LT), or lithium niobate (LiNbO 3 ), but is not limited thereto. In certain embodiments, the piezoelectric layer  14  is thick enough or rigid enough to function as a piezoelectric substrate. Accordingly, the substrate  12  in  FIG. 1  may be omitted. Those skilled in the art will appreciate that the principles of the present disclosure may apply to other materials for the substrate  12  and the piezoelectric layer  14 . The IDT  16 , the first reflector structure  18 A, and the second reflector structure  18 B may comprise aluminum (Al). While not shown to avoid obscuring the drawings, additional passivation layers, frequency trimming layers, or any other layers may be provided over all or a portion of the exposed surface of the piezoelectric layer  14 , the IDT  16 , the first reflector structure  18 A, and the second reflector structure  18 B. Further, one or more layers may be provided between the substrate  12  and the piezoelectric layer  14  in various embodiments. 
     SAW devices may be configured in so-called coupled resonator filter (CRF) or double mode SAW (DMS) filter arrangements. A typical CRF is designed by placing several IDTs between two reflective structures, or gratings.  FIG. 2A  illustrates an example SAW CRF structure  24  that includes a plurality of IDTs  26 - 1  to  26 - 5  that are longitudinally coupled between two reflective structures  28 - 1 ,  28 - 2 . A substrate (e.g.,  12  of  FIG. 1 ) and piezoelectric layer (e.g.,  14  of  FIG. 1 ) are not shown. The IDTs  26 - 1 ,  26 - 3 , and  26 - 5  are electrically connected to an input signal and ground, and may therefore be referred to as input IDTs. The IDTs  26 - 2 ,  26 - 4  are electrically connected to an output signal and ground, and may be referred to as output IDTs. The input IDTs  26 - 1 ,  26 - 3 , and  26 - 5  are configured in an alternating arrangement between the two reflective structures  28 - 1 ,  28 - 2 . In operation, surface acoustic waves are generated by the input IDTs  26 - 1 ,  26 - 3 , and  26 - 5  in response to an input signal and the surface acoustic waves are acoustically coupled to the output IDTs  26 - 2 ,  26 - 4  where they are converted back to an output signal. As discussed above, the electrode pitch and the metallization ratio of the plurality of IDTs  26 - 1  to  26 - 5 , the characteristics of the material of the underlying piezoelectric layer, and other factors influence the magnitude and frequency of the acoustic waves transduced and filtered by the SAW CRF structure  24 . It is desirable for the SAW CRF structure  24  to highly attenuate or reject frequencies outside of a desired passband. In practice, an internal or parasitic capacitance within the SAW CRF structure  24  may exist between the input IDTs  26 - 1 ,  26 - 3 , and  26 - 5  and the output IDTs  26 - 2 ,  26 - 4 . This capacitance can limit the rejection response of the SAW CRF structure  24 , particularly at frequencies above the passband, where a shoulder may be visible in plots of the frequency response. One way to reduce the influence of this capacitance is to place a capacitor  30  that is electrically coupled between the input signal and the output signal and outside of a cavity between the two reflective structures  28 - 1 ,  28 - 2 . In effect, this creates electrical zeros on either side of the passband while coupling adjacent resonance modes within the cavity between the two reflective structures  28 - 1 ,  28 - 2 . In some examples, the capacitor  30  can include an IDT connected between the input signal and the output signal outside of the reflective structures  28 - 1 ,  28 - 2 . 
     While providing improvement in the rejection response above a desired passband, the use of a capacitor as illustrated in  FIG. 2A  does have performance limitations.  FIG. 2B  is an S-Parameters plot representing simulations of the SAW CRF structure  24  of  FIG. 2A  with a variety of capacitor  30  values. The S-parameter magnitude is plotted in decibels (dB) across a megahertz (MHz) frequency range. Insertion loss, or S 2 ,  1 , is an indication of how much power is transferred. For frequencies where S 2 ,  1  is at or near 0 dB, then substantially all power from a signal is transferred. Accordingly, a passband is illustrated where the S 2 ,  1  values are at or near 0 dB. On either side of the passband, or the shoulder regions, the S 2 ,  1  values decrease rapidly. As the S 2 ,  1  value becomes farther away from 0 dB, more and more power is reflected and/or attenuated. For example, a value of −40 dB reflects more power than a value of −20 dB.  FIG. 2B  represents model simulations for the SAW CRF structure  24  of  FIG. 2A  with the capacitor  30  values starting at 0.0 picofarad (pF), or no capacitor, and progressively increasing to values of 0.2 pF, 0.4 pF, 0.6 pF, 0.8 pF, and 1.0 pF. As illustrated, a capacitor  30  value of 0.0 pF provides a shoulder above the passband that has a gradual slope. As the capacitor  30  values progressively increase from 0.2 pF to 1.0 pF, the gradual shoulder is steadily decreased to provide a steeper transition between passing frequencies and attenuated or filtered frequencies. Despite the improved shoulder steepness for the passband, a trade-off exists for increasing capacitor  30  values. As shown in the higher frequency ranges above the passband and shoulder region, such as between about 800 MHz and 850 MHz range, each increasing capacitor  30  value negatively impacts the rejection response. 
     According to embodiments disclosed herein, a SAW device may comprise a piezoelectric material, at least one input IDT on the piezoelectric material and electrically connected to an input signal and ground, at least one output IDT on the piezoelectric material and electrically connected to an output signal and ground, and an additional IDT on the piezoelectric material and located between an input IDT and an output IDT. The additional IDT may comprise corresponding electrodes that are respectively connected to the input signal and the output signal for the SAW device. In this manner, the additional IDT may alter an internal device capacitance to provide a sharper transition, or improved passband steepness, between frequencies in and out of a passband. Additionally, by placing the additional IDT between the input IDT and the output IDT, rejection of frequencies further above the passband may also be improved. 
       FIG. 3  illustrates a SAW CRF structure  32  that includes a plurality of IDTs  34 - 1  to  34 - 9  that are longitudinally coupled between two reflective structures  36 - 1 ,  36 - 2 . A substrate (e.g.,  12  of  FIG. 1 ) and a piezoelectric layer (e.g.,  14  of  FIG. 1 ) are not shown. Each of the IDTs  34 - 1 ,  34 - 5 ,  34 - 9  comprise corresponding electrodes that are electrically connected to an input signal and ground, and may be referred to as input IDTs. Each of the IDTs  34 - 3 ,  34 - 7  comprise corresponding electrodes that are electrically connected to an output signal and ground, and may be referred to as output IDTs. As illustrated in  FIG. 3 , the input IDTs  34 - 1 ,  34 - 5 ,  34 - 9  and the output IDTs  34 - 3 ,  34 - 7  may be configured in an alternating arrangement between the two reflective structures  36 - 1 ,  36 - 2 . The IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  are additional IDTs that are electrically connected to the input signal and the output signal. In this manner, the additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  are neither input IDTs nor output IDTs. In particular, each of the additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  comprises an electrode pair that includes a corresponding first electrode  38 - 2 ,  38 - 4 ,  38 - 6 ,  38 - 8  electrically connected to the input signal and a corresponding second electrode  40 - 2 ,  40 - 4 ,  40 - 6 ,  40 - 8  electrically connected to the output signal. Accordingly, the additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  comprise IDT capacitors that alter the internal device capacitance to provide a sharper transition, or improved passband steepness, between frequencies in and out of the passband. The additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  may be configured between each pair of alternating input IDTs  34 - 1 ,  34 - 5 ,  34 - 9  and output IDTs  34 - 3 ,  34 - 7 . For example, the additional IDT  34 - 2  is configured between the input IDT  34 - 1  and the output IDT  34 - 3 , the additional IDT  34 - 4  is configured between output IDT  34 - 3  and the input IDT  34 - 5 , and so on. In operation, surface acoustic waves that are generated by the input IDTs  34 - 1 ,  34 - 5 ,  34 - 9  in response to an input signal are acoustically coupled to the output IDTs  34 - 3 ,  34 - 7  through the additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8 . In this manner, the additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  also influence the passband of the device and may be configured to improve rejection performance outside of the passband. For example, the additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  may be designed to acoustically pass only frequencies within a desired passband, thereby altering the phase of frequencies outside of the passband. One or more of the additional IDTs  34 - 2 ,  34 - 4 ,  34 - 6 ,  34 - 8  may be placed in various positions between the reflective structures  36 - 1 ,  36 - 2  depending on the desired passband of a particular application. In this regard, the location of electrical zeros and/or poles within the SAW CRF structure  32  can be tailored for various applications. In certain embodiments, one or more of the plurality of IDTs  34 - 1  to  34 - 9  may comprise a metallization ratio, or duty factor, of any range between 0 and 1 of a center wavelength λ. In certain embodiments, the metallization ration is in a range of about 0.1 to about 0.9; or in a range of about 0.2 to about 0.8; or in a range of about 0.3 to about 0.7; or in a range of about 0.4 to about 0.5. In certain embodiments, the metallization ratio comprises a value of about 0.4, or a value of about 0.5. For simplicity,  FIG. 3  as well as figures for subsequent embodiments, illustrate the plurality of IDTs  34 - 1  to  34 - 9  as unapodized IDTs where each of the IDT electrode fingers has a uniform length. In certain embodiments, one or more of the plurality of IDTs  34 - 1  to  34 - 9  may comprise an apodized IDT where electrode fingers have different lengths at different positions along the apodized IDT that are configured for a particular response function. 
     SAW devices according to embodiments disclosed herein may be incorporated within larger devices and systems to provide simplified layouts or topologies.  FIGS. 4A, 4B, 5A, and 5B  illustrate representative radio frequency (RF) duplexing devices with various SAW CRF devices as disclosed herein. RF duplexing devices typically are configured to receive signals and transmit signals of a different band using a common antenna. One of the primary challenges of duplexing is that RF transmission signals and RF receive signals can interfere with one another and accordingly, RF duplexing devices may employ one or more filters to improve isolation. 
       FIG. 4A  is a block diagram of an RF duplexer  42  that includes the SAW CRF structure  24  of  FIG. 2A . The RF duplexer  42  includes a transmit (TX) port, a receive (RX) port, and an antenna (ANT) port. A TX filter  44  is positioned between the TX port and the antenna port and an RX filter  46  is positioned between the RX port and the antenna port. The TX filter  44  is configured as a ladder filter with series resonators TX 1 , TX 3 , TX 5  and shunt resonators TX 2 , TX 4 , TX 6 . The RX filter  46  includes series resonators RX 1 , RX 3 , and a shunt resonator RX 2  as well as the SAW CRF structure  24  as previously described for  FIG. 2A . The SAW CRF structure  24  includes five IDTs that alternate between input IDTs and output IDTs and a capacitor  48  is connected between the input and output of the SAW CRF structure  24 .  FIG. 4B  is a block diagram of an RF duplexer  50  that includes the SAW CRF structure  32  of  FIG. 3 . The RF duplexer  50  includes the TX filter  44  of  FIG. 4A  between the TX port and the antenna port, but an RX filter  52  that is different. The RX filter  52  includes the SAW CRF structure  32  where an additional IDT as previously described is positioned between each of the five total alternating input IDTs and output IDTs. In this regard, the capacitor  48  and the series resonator RX 3  of  FIG. 4A  may be omitted, thereby saving costs and die space in device layouts. 
       FIG. 5A  is a top view of a device layout of the RF duplexer  42  of  FIG. 4A . As illustrated, the RF duplexer  42  includes the resonators TX 1  to TX 6 , the resonators RX 1  to RX 3 , the SAW CRF structure  24 , and the capacitor  48  as previously described as well as areas for RX, TX, antenna, and various ground connections.  FIG. 5B  is a top view of a device layout of the RF duplexer  50  of  FIG. 4B . As illustrated, the RF duplexer  50  includes the resonators TX 1  to TX 6 , the resonators RX 1  to RX 2 , and the SAW CRF structure  32  as previously described as well as areas for RX, TX, antenna, and various ground connections. Due to the configuration of the SAW CRF structure  32 , the RF duplexer  50  does not include the capacitor  48  and the resonator RX 3  of  FIG. 5A . Additionally, the series resonator RX 1  may have a reduced size. In this regard, there is noticeably improved die space savings between the SAW CRF structure  32  and the resonator RX 2  as well as between the resonator RX 1  and the ground connection for the resonator RX 2 . 
       FIGS. 6A and 6B  are plots comparing the performance of RF duplexers with various SAW CRF structures as disclosed herein. In  FIGS. 6A and 6B , Duplexer  1  refers to the RF duplexer  50  of  FIG. 4B  that includes the SAW CRF structure  32  according to embodiments disclosed herein. Duplexer  2  refers to the RF duplexer  42  of  FIG. 4A  that includes the SAW CRF structure  24  and the capacitor  48 . For the sake of comparison, Duplexer  3  was included and refers to the RF duplexer  42  of  FIG. 4A , but with the series resonator RX 3  removed. In this regard, a comparison of Duplexer  1  and Duplexer  3  is useful to highlight the influence of just replacing the SAW CRF structure  24  and the capacitor  48  of  FIG. 4A  with the SAW CRF structure  32  of  FIG. 4B .  FIG. 6A  is a comparison plot for duplexer isolation in dB for Duplexers  1 ,  2 , and  3 , where a lower value indicates better isolation. As illustrated, Duplexer  1  has noticeably better isolation (e.g. 5 dB or more) than Duplexer  3 , particularly in the frequency range above 778 MHz. Additionally, Duplexers  1  and  2  show similar isolation values across the frequency range.  FIG. 6B  is a comparison plot for the passband of the Duplexers  1 ,  2 , and  3 . As described for  FIG. 2B , a passband is illustrated where the plot values are at or near 0 dB with shoulder regions on either side where the plot values noticeably decrease from 0 dB. As previously described, it is desirable to have a passband with sharp or steep shoulders that transition to high rejection performance on either side of the passband. As illustrated, Duplexer  1  shows a steep shoulder on the high frequency side of the passband and has noticeably improved rejection for regions above and below the passband. In particular, Duplexer  1  demonstrates an improvement of at least 2 dB or more for frequencies in the range of about 817 MHz to about 828 MHz. 
     As previously described, a SAW device may comprise at least one additional IDT located between an input IDT and an output IDT on a piezoelectric material. In certain embodiments, the at least one additional IDT may include an IDT capacitor with a floating electrode.  FIG. 7  illustrates a SAW CRF structure  54  that includes a plurality of IDTs  56 - 1  to  56 - 9  that are longitudinally coupled between two reflective structures  57 - 1 ,  57 - 2 . A substrate (e.g.  12  of  FIG. 1 ) and a piezoelectric layer (e.g.,  14  of  FIG. 1 ) are not shown. As previously described, each of the IDTs  56 - 1 ,  56 - 5 ,  56 - 9  comprise corresponding electrodes that are electrically connected to an input signal and ground, and may be referred to as input IDTs. Each of the IDTs  56 - 3 ,  56 - 7  comprise corresponding electrodes that are electrically connected to an output signal and ground, and may be referred to as output IDTs. As illustrated in  FIG. 7 , the input IDTs  56 - 1 ,  56 - 5 ,  56 - 9  and the output IDTs  56 - 3 ,  56 - 7  may be configured in an alternating arrangement between the two reflective structures  57 - 1 ,  57 - 2 . The IDTs  56 - 2 ,  56 - 4 ,  56 - 6 ,  56 - 8  are additional IDTs that are electrically connected to the input signal and the output signal. In this manner, the additional IDTs  56 - 2 ,  56 - 4 ,  56 - 6 ,  56 - 8  are neither input IDTS nor output IDTs. In particular, each of the additional IDTs  56 - 2 ,  56 - 4 ,  56 - 6 ,  56 - 8  comprises an electrode pair that includes a corresponding first electrode  58 - 2 ,  58 - 4 ,  58 - 6 ,  58 - 8  electrically connected to the input signal and a corresponding second electrode  60 - 2 ,  60 - 4 ,  60 - 6 ,  60 - 8  electrically connected to the output signal. Each of the additional IDTs  56 - 2 ,  56 - 4 ,  56 - 6 ,  56 - 8  also includes a corresponding floating electrode  62 - 2 ,  62 - 4 ,  62 - 6 ,  62 - 8  in between the first electrode  58 - 2 ,  58 - 4 ,  58 - 6 ,  58 - 8  and the second electrode  60 - 2 ,  60 - 4 ,  60 - 6 ,  60 - 8 . Each floating electrode  62 - 2 ,  62 - 4 ,  62 - 6 ,  62 - 8  is not directly connected to either of the input signal or the output signal, and may therefore be referred to as floating voltage electrodes. Accordingly, the additional IDTs  56 - 2 ,  56 - 4 ,  56 - 6 ,  56 - 8  comprise IDT capacitors with floating electrodes that may effectively form multiple capacitors in series within each of the additional IDTs  56 - 2 ,  56 - 4 ,  56 - 6 ,  56 - 8 . In this manner, the additional IDTs  56 - 2 ,  56 - 4 ,  56 - 6 ,  56 - 8  may alter the internal device capacitance to provide a sharper transition, or improved passband steepness, between frequencies in and out of the passband. 
     As previously described, a SAW device may comprise at least one additional IDT located between an input IDT and an output IDT on a piezoelectric material. In certain embodiments, the at least one additional IDT may include electrodes with alternative shapes.  FIG. 8  illustrates a SAW CRF structure  64  that includes a plurality of IDTs  66 - 1  to  66 - 9  that are acoustically and longitudinally coupled between two reflective structures  67 - 1 ,  67 - 2 . A substrate (e.g.  12  of  FIG. 1 ) and a piezoelectric layer (e.g.,  14  of  FIG. 1 ) are not shown. The IDTs  66 - 1 ,  66 - 5 ,  66 - 9  are input IDTs and the IDTs  66 - 3 ,  66 - 7  are output IDTs as previously described. The IDTs  66 - 2 ,  66 - 4 ,  66 - 6 ,  66 - 8  are additional IDTs that are electrically connected to the input signal and the output signal, or IDT capacitors as previously described. Each of the additional IDTs  66 - 2 ,  66 - 4 ,  66 - 6 ,  66 - 8  comprises an electrode pair that includes a corresponding first electrode  68 - 2 ,  68 - 4 ,  68 - 6 ,  68 - 8  electrically connected to the input signal and a corresponding second electrode  70 - 2 ,  70 - 4 ,  70 - 6 ,  70 - 8  electrically connected to the output signal. In  FIG. 8 , the first electrode  68 - 2 ,  68 - 4 ,  68 - 6 ,  68 - 8  and the second electrode  70 - 2 ,  70 - 4 ,  70 - 6 ,  70 - 8  comprise elongated fingers that are non-linear. For example, the first electrode  68 - 2  extends from an input signal line and includes two ninety-degree turns to form a U shape. In a similar manner, the second electrode  70 - 2  forms an inverted U shape that is interdigitated with the first electrode  68 - 2 . In this regard, the area where the first electrode  68 - 2  and the second electrode  70 - 2  are close to each other is increased. In certain embodiments, other electrode shapes are possible. For example, the first electrode  68 - 2  and the second electrode  70 - 2  may have curved turns to form a U shape. In certain embodiments, either the first electrode  68 - 2  or the second electrode  70 - 2  may comprise a linear shape and the other of first electrode  68 - 2  or the second electrode  70 - 2  comprises a non-linear shape. For example, the first electrode  68 - 2  may comprise a linear shape and the second electrode  70 - 2  may comprise a U shape that extends around the first electrode  68 - 2 . 
     In certain embodiments, a SAW device may comprise at least one additional IDT located between an input IDT and an output IDT on a piezoelectric material and the at least one additional IDT is not directly connected to either input or output signals. In certain embodiments, the at least one additional IDT may include a first electrode that is connected to ground and a second electrode that is floating.  FIG. 9A  illustrates a SAW CRF structure  72  that includes a plurality of IDTs  74 - 1  to  74 - 9  that are longitudinally coupled between two reflective structures  76 - 1 ,  76 - 2 . A substrate (e.g.,  12  of  FIG. 1 ) and a piezoelectric layer (e.g.  14  of  FIG. 1 ) are not shown. The IDTs  74 - 1 ,  74 - 5 ,  74 - 9  are input IDTs and the IDTs  74 - 3 ,  74 - 7  are output IDTs as previously described. In  FIG. 9A , the IDTs  74 - 2 ,  74 - 4 ,  74 - 6 ,  74 - 8  are additional IDTs that are not directly connected to the input signal or the output signal. Each of the additional IDTs  74 - 2 ,  74 - 4 ,  74 - 6 ,  74 - 8  comprises an electrode pair that includes a corresponding first electrode  78 - 2 ,  78 - 4 ,  78 - 6 ,  78 - 8  electrically connected to ground and a corresponding second electrode  80 - 2 ,  80 - 4 ,  80 - 6 ,  80 - 8  that is floating. An individual one of the additional IDTs  74 - 2 ,  74 - 4 ,  74 - 6 ,  74 - 8  is positioned between corresponding pairs of the input IDTs  74 - 1 ,  74 - 5 ,  74 - 9  and the output IDTs  74 - 3 ,  74 - 7 . Accordingly, a capacitance between the input IDTs  74 - 1 ,  74 - 5 ,  74 - 9  and the output IDTs  74 - 3 ,  74 - 7  is reduced, thereby altering the internal device capacitance to provide a sharper transition, or improved passband steepness, between frequencies in and out of the passband. In certain embodiments, each of the second electrodes  80 - 2 ,  80 - 4 ,  80 - 6 ,  80 - 8  are connected with each other. 
       FIG. 9B  illustrates an alternative configuration for the SAW CRF structure  72  of  FIG. 9A . In  FIG. 9B , common elements are numbered the same as in  FIG. 9A  and the description of the common elements provided above for  FIG. 9A  may also be applicable to  FIG. 9B . In  FIG. 9B , the first electrodes  78 - 2 ,  78 - 4 ,  78 - 6 ,  78 - 8  and the second electrodes  80 - 2 ,  80 - 4 ,  80 - 6 ,  80 - 8  comprise electrodes that are devoid of electrode fingers. In certain embodiments, the first electrodes  78 - 2 ,  78 - 4 ,  78 - 6 ,  78 - 8  and the second electrodes  80 - 2 ,  80 - 4 ,  80 - 6 ,  80 - 8  comprise rectangular, square, or other solid shapes. In certain embodiments, at least one of the first electrodes  78 - 2 ,  78 - 4 ,  78 - 6 ,  78 - 8  and a corresponding one of the second electrodes  80 - 2 ,  80 - 4 ,  80 - 6 ,  80 - 8  comprise L shapes. In this regard, the SAW CRF structure  72  includes an additional electrode pair  81 - 2 ,  81 - 4 ,  81 - 6 ,  81 - 8  arranged between the input IDTs  74 - 1 ,  74 - 5 ,  74 - 9  and the output IDTs  74 - 3 ,  74 - 7  that is configured to alter the internal or parasitic capacitance of the SAW CRF structure  72 . 
     In certain embodiments, a SAW device may comprise a first reflective structure and a second reflective structure on a piezoelectric material with a plurality of input IDTs and a plurality of output IDTs arranged between the first reflective structure and the second reflective structure. The plurality of input IDTs and the plurality of output IDTs may be configured in an alternating arrangement between the first reflective structure and the second reflective structure, and a plurality of additional IDTs is arranged between corresponding ones of the plurality of input IDTs and corresponding ones of the plurality of output IDTs. In certain embodiments, at least one additional IDT of the plurality of additional IDTs is electrically connected to an input signal and an output signal. In certain embodiments, at least one other additional IDT of the plurality of additional IDTs is not directly electrically connected to either of the input signal and the output signal. 
       FIG. 10  illustrates a SAW CRF structure  82  that includes a plurality of IDTs  84 - 1  to  84 - 9  that are acoustically and longitudinally coupled between two reflective structures  86 - 1 ,  86 - 2 . A substrate (e.g.,  12  of  FIG. 1 ) and a piezoelectric layer (e.g.,  14  of  FIG. 1 ) are not shown. The IDTs  84 - 1 ,  84 - 5 ,  84 - 9  are input IDTs and the IDTs  84 - 3 ,  84 - 7  are output IDTs as previously described. In  FIG. 10 , the IDTs  84 - 2 ,  84 - 4 ,  84 - 8  are additional IDTs that are not directly connected to the input signal or the output signal. In certain embodiments, each of the additional IDTs  84 - 2 ,  84 - 4 ,  84 - 6 ,  84 - 8  comprises an electrode pair that includes a corresponding first electrode  88 - 2 ,  88 - 4 ,  88 - 6 ,  88 - 8  and a corresponding second electrode  90 - 2 ,  90 - 4 ,  90 - 6 ,  90 - 8 . Notably, the additional IDTs  84 - 2 ,  84 - 4 ,  84 - 6 ,  84 - 8  may include different types of IDTs. For example, the additional IDT  84 - 6  is electrically connected to the input signal and the output signal. In this regard, the additional IDT  84 - 6  comprises a first electrode  88 - 6  that is electrically connected to the input signal and a second electrode  90 - 6  that is electrically connected to the output signal. Accordingly, the additional IDT  84 - 6  is an IDT capacitor that alters the internal device capacitance as previously described. In further embodiments, the additional IDT  84 - 6  may comprise a floating electrode between the first electrode  88 - 6  and the second electrode  90 - 6  as previously described. In contrast to the additional IDT  84 - 6 , the additional IDTs  84 - 2 ,  84 - 4 ,  84 - 8  are not directly electrically connected to either of the input signal and the output signal. The additional IDTs  84 - 2 ,  84 - 4 ,  84 - 8  instead comprise the corresponding first electrode  88 - 2 ,  88 - 4 ,  88 - 8  connected to ground and the corresponding second electrode  90 - 2 ,  90 - 4 ,  90 - 8  that is a floating electrode, or a floating voltage electrode. The additional IDTs  84 - 2 ,  84 - 4 ,  84 - 8  are positioned between corresponding ones of the input IDTs  84 - 1 ,  84 - 5 ,  84 - 9  and the output IDTs  84 - 3 ,  84 - 7 , thereby reducing a capacitance between the input IDTs  84 - 1 ,  84 - 5 ,  84 - 9  and the output IDTs  84 - 3 ,  84 - 7  as previously described. 
     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.