Patent Publication Number: US-2023133161-A1

Title: Surface acoustic wave (saw) structures with transverse mode suppression

Description:
BACKGROUND 
     I. Field of the Disclosure 
     The technology of the disclosure relates generally to surface acoustic waves and ways to reduce transverse modes spurious around resonance frequencies. 
     II. BACKGROUND 
     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 transceiver front ends, duplexers, and receive filters. The widespread use of SAW filters is due to, at least in part, that 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 with any electronic device, the performance of a SAW device is an important parameter that can impact the overall performance of a system. In this regard, there is a need for a high-performance SAW device. One such solution is a guided SAW device. 
     Guided SAW devices (i.e., SAW devices having a guided SAW structure) have a layered substrate where a layer of piezoelectric material, which is referred to here as a piezoelectric layer, is bonded or deposited on (e.g., directly on) the surface of a support, or carrier, substrate. As compared to conventional SAW devices, guided SAW devices have an improved quality factor (Q), an improved electromechanical coupling factor (K2), and an improved Temperature Coefficient of Frequency (TCF). However, unwanted spurious modes are typically generated in a guided SAW structure, which hinders a practical use of a guided SAW device. In particular, in a guided SAW device, spurious modes are generated above the resonance frequency of the guided SAW device and, as a result, out-of-band rejection specifications may not be satisfied. Accordingly, there is still a need for improved performance from SAW devices. 
     SUMMARY 
     Aspects disclosed in the detailed description include surface acoustic wave (SAW) structures with transverse mode suppression. In particular, exemplary aspects of the present disclosure provide digits or fingers with broad interior terminal end shapes. By providing such shapes, spurious modes above the resonance frequency of the SAW structure are suppressed thereby providing desired out-of-band rejection that helps satisfy design criteria such as keeping a higher quality factor (Q) value, a higher electromechanical coupling factor (K2) value, and better Temperature coefficient of Frequency (TCF). 
     In this regard in one aspect, a SAW filter is disclosed. The SAW filter comprises a first interdigitated electrode comprising a first finger. The first finger has a first width along a longitudinal axis of the first finger for a first length and a second width along the longitudinal axis of the first finger for a second length at a terminal end portion of the first finger. The SAW filter also comprises a second interdigitated electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a side elevational view of a guided surface acoustic wave (SAW) structure; 
         FIG.  1 B  is a top plan view of the guided SAW structure of  FIG.  1 B  with apodization of the interdigital structure; 
         FIG.  2    provides performance graphs for the guided SAW structure of  FIGS.  1 A and  1 B  with transverse modes above the resonant frequency highlighted; 
         FIG.  3 A  illustrates a guided SAW structure with transverse mode suppression according to an exemplary aspect of the present disclosure; 
         FIG.  3 B  illustrates an exemplary finger end used on the interdigital structure to facilitate transverse mode suppression; 
         FIG.  4    provides performance graphs for the guided SAW structure with wide finger ends of  FIGS.  3 A and  3 B  highlighting the suppressed transverse modes; 
         FIGS.  5 A and  5 B  illustrate how changing a width of a finger end relative to the finger shaft affects a magnitude of a transverse mode; 
         FIG.  5 C  illustrates how changing the area of a finger end affects a magnitude of a transverse mode; 
         FIGS.  6 A and  6 B  illustrate how changing a gap between finger ends affects a magnitude of a transverse mode; 
         FIGS.  7 A and  7 B  illustrate how changing a crossing gap between fingers affects a magnitude of a transverse mode; 
         FIG.  8    illustrates various alternate finger ends; 
         FIGS.  9 A and  9 B  illustrate how changing a substrate thickness may affect a magnitude of a transverse mode; and 
         FIG.  10    illustrates a top plan view of an alternate guided SAW structure with a chirped wave apodization. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to the drawing figures, several exemplary aspects of the present disclosure are described. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects. 
     Aspects disclosed in the detailed description include surface acoustic wave (SAW) structures with transverse mode suppression. In particular, exemplary aspects of the present disclosure provide digits or fingers with broad interior terminal end shapes. By providing such shapes, spurious modes above the resonance frequency of the SAW structure are suppressed thereby providing desired out-of-band rejection that helps satisfy design criteria such as keeping a higher quality factor (Q) value, a higher electromechanical coupling factor (K2) value, and better Temperature Coefficient of Frequency (TCF). 
       FIGS.  1 A and  1 B  illustrate a conventional guided SAW structure  100  having a support substrate  102  on which a piezoelectric layer  104  is positioned. Interdigitated electrodes  106 ,  108  are positioned on the piezoelectric layer  104  as better seen in  FIG.  1 B . The interdigitated electrodes  106 ,  108  form an interdigital transducer (IDT)  110  with optional reflectors  112 ,  114 . Further, one of the interdigitated electrodes  106 ,  108  (e.g., electrode  108 ) may be a dummy electrode. 
     With continued reference to  FIG.  1 B , each interdigitated electrode  106 ,  108  includes a respective plurality of fingers  106 A,  108 A that are aligned with one another and are separated by gaps  116 . The alignment and presence of the gaps  116  provide apodization of the IDT  110 . 
       FIG.  2    provides various graphs  200 A- 200 D showing performance of the guided SAW structure  100  against frequency, and particularly shows a resonance frequency f, with transverse modes proximate the resonance frequency highlighted. As is readily apparent, while the apodization does some work at suppressing the transverse modes, it is insufficient for some purposes. 
     As illustrated in  FIGS.  3 A and  3 B , exemplary aspects of the present disclosure provide a guided SAW filter  300  that includes a first interdigitated electrode  302  and a second interdigitated electrode  304  that are apodized. The first interdigitated electrode  302  has a plurality of fingers  306 ( 1 )- 306 (N) with an exemplary finger  306 (X) shown in  FIG.  3 B . Each of the fingers  306 ( 1 )- 306 (N) has a first width W 1  along a longitudinal axis  308  of the fingers  306 ( 1 )- 306 (N). The first width W 1  is provided along this longitudinal axis  308  for a first length defined by the distance between a lateral bar  310  and a terminal end portion  312 ( 1 )- 312 (N) of the finger  306 ( 1 )- 306 (N). The terminal end portion  312 ( 1 )- 312 (N) of the finger  306 ( 1 )- 306 (N) has a second width W 2  along the longitudinal axis  308  for a second length L (see also  FIG.  5 A ). The apodization is perpendicular to the longitudinal axis  308  and may be described as being along a length direction of the SAW structure  100 . Further, the apodization is formed in a wave pattern and may follow a sine or cosine wave pattern. 
     Similarly, the second interdigitated electrode  304  has a plurality of fingers  314 ( 1 )- 314 (N) with an exemplary finger  314 (X) shown in  FIG.  3 B . Each of the fingers  314 ( 1 )- 314 (N) has a first width W 1  along the longitudinal axis  308 . The first width W 1  is provided along this longitudinal axis  308  for a length defined by the distance between a lateral bar  316  and a terminal end portion  318 ( 1 )- 318 (N) of the finger  314 ( 1 )- 314 (N). The terminal end portion  318 ( 1 )- 318 (N) of the finger  314 ( 1 )- 314 (N) has a second width W 2  along the longitudinal axis  308  for a second length L (see also  FIG.  5 A ). 
     Note that as illustrated, both the fingers  306 ( 1 )- 306 (N) and the fingers  314 ( 1 )- 314 (N) include the wider terminal end portions  312 ( 1 )- 312 (N),  318 ( 1 )- 318 (N). However, the disclosure is not so limited and only one or the other of the fingers  306 ( 1 )- 306 (N) or  314 ( 1 )- 314 (N) may include the wider terminal end portions  312 ( 1 )- 312 (N),  318 ( 1 )- 318 (N). Note further, that as illustrated, the widths W 1  of the fingers  306 ( 1 )- 306 (N) are the same as the widths W 1  of the fingers  314 ( 1 )- 314 (N), but such identity is not required by the present disclosure and the W 1  of fingers  306 ( 1 )- 306 (N) may differ from the W 1  of the fingers  314 ( 1 )- 314 (N). Still further, even if both fingers  306 ( 1 )- 306 (N) and  314 ( 1 )- 314 (N) have the wider terminal end portions  312 ( 1 )- 312 (N),  318 ( 1 )- 318 (N), they do not necessarily have to have the same widths W 2  relative to each other. 
       FIG.  4    provides graphs  400 A- 400 C illustrating how the guided SAW filter  300  suppresses the transverse modes. For reference, the wave amplitude of the apodization is 2.4λ. The aperture  320 , length  322 , and IDT length/wave were set to 20λ, 100λ, and 1λ, respectively. By using the wide finger ends, the transverse modes can be significantly suppressed, and Q also improves. λ is an interdigital transducer (IDT) period of the SAW filter  300 . 
     There are a number of parameters associated with the finger ends which may be optimized to achieve desired transverse mode suppression.  FIG.  5 A  is provided which merely reillustrates an exemplary finger  306 (X).  FIGS.  5 B and  5 C  illustrate the ramifications of two parameter variations.  FIG.  5 B  illustrates how changing the ratio of W 2 /W 1  will affect how much the transverse mode is suppressed while  FIG.  5 C  illustrates how changing L and the area of the terminal end portion  312 (X) will affect how much the transverse mode is suppressed 
     In this regard,  FIG.  5 B  shows graphs  500 A and  500 B with L set to 0.1λ and various W 2 /W 1  ratios. Specifically, ratios of 1.2, 1.8, and 2.2 are illustrated, with the transverse mode being increasing suppressed as the W 2 /W 1  ratio increases. However, if the W 2 /W 1  ratio is too large (i.e., W 2  is too wide), there may be bridging to an adjacent finger, so the W 2 /W 1  ratio may have a practical limit of three (3) or less. 
       FIG.  5 C  illustrates graphs  500 C,  500 D showing what happens to transverse mode suppression with changes to L, while holding W 2 /W 1  constant. Specifically, L is varied between 0.1λ, 0.15λ, and 0.25λ. The data suggests that the greatest transverse mode suppression occurs around 0.1λ 2 . Areas larger than this show that the magnitude of the transverse mode slowly increases, and an area greater than approximately 0.7λ 2  will obviate any transverse mode suppression. Approximately as used herein means within five percent. 
     Similarly, a gap  600  illustrated in  FIG.  6 A  between the terminal end portions (e.g., end portions  312 (X) and  318 (X)) of the fingers  306 (X),  314 (X) may be varied to achieve varied results.  FIG.  6 B  shows the results of such variation in gap. Specifically, graphs  602 A,  602 B show how changing the gap from 0.1023λ, through 0.1136λ, to 0.1364λ while keeping L and W/W 2  constant affects the magnitude of the transverse mode. If the gap  600  is 0.24λ or less, the wider terminal end portions of the present disclosure provide more effective transverse mode suppression than narrowing the gap of a conventional finger. Note that if the gap is too narrow, it may be difficult to manufacture with current technologies, and accordingly, a gap  600  of more than approximately 0.1 micrometers may be used. 
     Still another parameter may be a crossing width  700  illustrated in  FIG.  7 A .  FIG.  7 B  illustrates graphs  702 A,  702 B, and  702 C showing the impact on the transverse mode suppression created by variations in the crossing width  700 . Specifically, a crossing width of 5λ is shown in graph  702 A and a crossing width  700  of 10λ is shown in graph  702 B. The effect of the wide terminal end portions  312 ( 1 )- 312 (N) and  318 ( 1 )- 318 (N) are greater as the crossing width  700  is smaller. However, if the crossing width  700  is too narrow, the distance between the upper and lower apodization regions are too close, so a crossing width  700  of at least approximately 1λ is suggested. 
     While  FIGS.  3 A,  5 A,  6 A, and  7 A  suggest that the terminal end portions  312 ( 1 )- 312 (N) and  318 ( 1 )- 318 (N) be rectilinear with right angles at the corners, the present disclosure is not so limited.  FIG.  8    provides exemplary alternate terminal end portions  800 A- 800 D where terminal end portion  800 A is rectilinear as previously illustrated, but terminal end portions  800 B- 800 D include rounded corner portions  802 B,  802 C,  802 D. The precise radius of curvature and the precise angle of slope  804 C,  804 D is not central to the present disclosure and may be varied without departing from the scope of the present disclosure. Likewise, minor variations that result from process variations during manufacture are intended to be included within the present disclosure. 
       FIGS.  9 A and  9 B  show the impact variations on the thickness (LT) of the piezoelectric material  900  that sits between the interdigitated electrodes  302 ,  304  and a silicon substrate  904  have on transverse mode suppression. It should be appreciated that the piezoelectric material  900  may be, for example, lithium tantalite or lithium niobite. Likewise, while described as a silicon substrate  904 , the substrate may alternatively be sapphire, spinel, quartz, or other ceramics. As shown by graphs  906 A and  906 B in  FIG.  9 B , the magnitude of a transverse mode may depend on the thickness of the piezoelectric material  900 . When the piezoelectric material  900  is less than 5λ, transverse modes are seen, and using the wide terminal end portions of the present disclosure may be useful when suppressing transverse modes when LT is less than approximately 5λ. 
     Still another possible variation is to chirp the wave apodization as illustrated in  FIG.  10   . That is, some portion  1000  of a wave  1002  has a different period than the rest of the wave  1002 . In an exemplary aspect, the chirped wave is symmetric about a vertical axis to a length direction  1004  of a SAW filter  1006 . 
     The surface acoustic wave structures with transverse mode suppression according to aspects disclosed herein may be provided in or integrated into any processor-based device. Examples, without limitation, include a set top box, an entertainment unit, a navigation device, a communications device, a fixed location data unit, a mobile location data unit, a global positioning system (GPS) device, a mobile phone, a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a tablet, a phablet, a server, a computer, a portable computer, a mobile computing device, a wearable computing device (e.g., a smart watch, a health or fitness tracker, eyewear, etc.), a desktop computer, a personal digital assistant (PDA), a monitor, a computer monitor, a television, a tuner, a radio, a satellite radio, a music player, a digital music player, a portable music player, a digital video player, a video player, a digital video disc (DVD) player, a portable digital video player, an automobile, a vehicle component, avionics systems, a drone, and a multicopter. 
     The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations. Thus, the disclosure is not intended to be limited to the examples and designs described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.