Abstract
An electro acoustic resonator is provided. The resonator has a gap short structure (GSS) to electrically short at least an area of the transversal gap to suppress transversal gap mode excitations. The gap short structure may be provided by a conductive stripe in the gap and parallel to or inclined with respect to the bus bar (BB) shorting adjacent IDT fingers. Additional connectors between the stripe and the bus bar may be provided. The connectors may have different pitch or metallization ratio with respect to the ID fingers. The connectors may be offset from the position of the fingers and my be inclined with respect to the bus bars. Multiple parallel stripes in the gap may provide a transversal reflector. By using a gap short structure a further improved transversal mode suppression of piston mode designs can be achieved.
Claims
1. An electro acoustic resonator with suppressed transversal gap mode excitation and reduced transversal modes, comprising: a piezoelectric material; an electrode structure arranged on or above the piezoelectric material; and a gap short structure arranged on or above the piezoelectric material; wherein: the electrode structure has two opposite busbars, two transversal gaps and electrode fingers, each electrode finger is alternately electrically connected to one of the busbars, the transversal gaps are arranged between the end of electrode fingers and the respective opposite busbar, and the gap short structure has conductor strips or patches and is arranged inside the transversal gaps.
2. The resonator of the claim 1, wherein the gap short structure electrically shorts an area of the gaps.
3. The resonator of claim 1, wherein the conductor strips of the gap short structure extend in a longitudinal direction.
4. The resonator of claim 1, wherein the conductor strips of the gap short structure and the electrode fingers share a same metal layer.
5. The resonator of claim 1, being selected from a SAW resonator, a TC-SAW resonator, a TF-SAW resonator, a GBAW resonator provided as a one-port resonator, a two-port resonator or a DMS resonator.
6. The resonator of claim 1, further comprising busbar connectors electrically connecting the gap short structure to the busbars.
7. The resonator of claim 1, wherein the busbar connectors comprise one or more structures selected from: electrode finger elements, electrode finger elements with an increased metallization ratio η, electrode finger elements with a reduced metallization ratio η, phase shifted electrode finger elements, phase shifted conductor patches, phase shifted electrode finger elements with an increased metallization ratio η, phase shifted electrode finger elements with a reduced metallization ratio η, asynchronous conductor patches with an increased pitch p, asynchronous conductor patches with a reduced pitch p, asynchronous conductor patches with an irregular pitch p, rotated conductor patches, trapezoid conductor patches, wherein in case of a TGR structure busbar connector conductor patches which are modified from one gap short conductor strip to the next.
8. The resonator of claim 1, wherein the gap short structures' conductor strips comprise one or more structures selected from rectangular conductor strips, rotated conductor strips, trapezoid conductor strips, a transversal gapshort reflector, TGR, including a plurality of conductor strips per side of the acoustic track.
9. The resonator of claim 1, wherein the resonator is part of an electro acoustic filter.
10. The resonator of claim 1, wherein the resonator is part of a multiplexer.
11. The resonator of claim 1, wherein the resonator is manufactured at least in part by creating the conductor strips of the gap short structure in the transversal gaps.
12. A method of reducing excitation of transversal gap modes in an electro acoustic resonator, comprising: electrically shorting an area of transversal gaps of the electro acoustic resonator.
13. The method of the claim 12, comprising: electrically shorting the area of the resonator's gaps via a gap short structure, providing the gap short structure as conductive strips, determining a width of the strips and distances between the strips and the electrode finger tips and between the strips and the busbars.
14. The method of claim 12, comprising: providing a TGR structure including a plurality of strips extending along a longitudinal direction; and selecting widths and distances of the plurality of strips towards electrode finger tips, the busbar and neighboring strips.
Description
[0053] In the figures:
[0054] FIG. 1 shows basic construction elements of an electro acoustic resonator;
[0055] FIG. 2 shows a possible implementation of the gap short structure;
[0056] FIG. 3 illustrates an enlarged view of a section of the acoustic track shown in FIG. 2;
[0057] FIGS. 4 to 13 show specific embodiments of the gap short structure;
[0058] FIG. 14a illustrates an enlarged view of a section of the acoustic track shown in FIG. 13;
[0059] FIG. 14b illustrates a possible modification of the busbar connector conductor patches;
[0060] FIG. 15 shows a comparison between the real part of the admittance (top portion), the imaginary part of the admittance (center portion) and the absolute value of the admittance (bottom portion) of a conventional resonator and of a resonator with a gap short structure according to FIG. 4;
[0061] FIG. 16 shows a zoomed view of FIG. 15;
[0062] FIG. 17 shows a further zoomed view of FIG. 16;
[0063] FIG. 18 shows a comparison between the transmission (top portion), input port reflection (center portion) and losses determined by unitarity violation (bottom portion) of a conventional bandpass filter and of an improved bandpass filter;
[0064] FIG. 19 shows a zoomed view of FIG. 18;
[0065] FIG. 20 shows a further zoomed view of FIG. 19;
[0066] FIGS. 21 to 26 show a comparison between properties of conventional and improved one resonating structure;
[0067] FIG. 27 shows a comparison between properties of conventional and improved one resonating structure active at its optimal working point;
[0068] FIGS. 28 to 31 show a comparison between properties of a conventional one-port resonator, a one-port resonator with a TGR structure and a resonator with a slanted IDT (inter digital transducer).
[0069] FIG. 1 shows general aspects of an electro acoustic resonator EAR. The resonator has a piezoelectric material PM on which an interdigitated structure IDS is arranged. The interdigitated structure IDS comprises interdigitating electrode fingers EF. Each electrode finger EF is connected to one of two busbars BB. The interdigitated structure IDS is arranged between acoustic reflectors R comprising reflector fingers. Mainly parallel to the busbars BB the electro acoustic resonator EAR has gap short structures GSS having the effect that the area between the gap short structure GSS and their corresponding busbars BB are essentially free of an electrical field. The gap short structure GSS comprises conductor strips that are essentially parallel to the conductor strips of the busbars BB and arranged in the transversal gap region G between the ends of the electrode fingers EF and the busbars BB of the opposite polarity.
[0070] By providing the gap short structure on the piezoelectric material PM the electric field strength in the portion of the transversal gap between BB and GSS is eliminated or at least strongly reduced, thus, reducing or eliminating the excitation of transversal gap modes
[0071] To that end, the conductor patches of the gap short structure GSS are kept at an essentially same potential as the accompanying busbar BB.
[0072] FIG. 2 shows a possible implementation of the gap short structure GSS which is established as a conductor strip extending along the longitudinal direction parallel to the busbars BB.
[0073] Further, the acoustic track shown in FIG. 2 shows conductor patches that are arranged at the ends of the electrode fingers and at corresponding lateral positions of the electrode fingers of the opposite polarity such that the acoustics of the resonator is improved by establishing a piston mode with reduced transversal modes.
[0074] Thus, the structure shown in FIG. 2 provides acoustically and electrically active means for suppressing the excitation of transversal gap modes and (further) reducing transversal modes at the same time.
[0075] FIG. 3 illustrates a zoomed section of FIG. 2 into the transversal gap region showing the definition of the finger pitch p in the central excitation area and the respective pitch pbbc of the conductor strips of the busbar connectors. The gap short structure GSS has a rectangular cross-section. The gap short structure extends along the longitudinal direction parallel to the busbar BB. The busbar connectors BBC electrically connect the gap short structure GSS to the busbar BB such that a field reduction in the transversal gap G can be obtained. The gap G is arranged between the central excitation area CEA and the busbar BB. The busbar connector is established in the example of FIG. 3 by the corresponding segments of the electrode fingers EF in the gap G between the gap short structure GSS and the busbar BB. Characteristic properties that determine the effect of the gap short structure GSS in this simplest embodiment are the structure's width W and the distances D.sub.1 and D.sub.2 to the electrode finger ends of the opposite electrode and to the busbar, respectively.
[0076] The electrode finger pitch p is defined as the distance of edges of adjacent electrode fingers pointing in the same direction, e.g. arranged at the upper side shown in FIG. 3.
[0077] Correspondingly, the pitch pbbc of the busbar connectors BBC is defined as the distance of edges of adjacent busbar connectors pointing in the same direction.
[0078] Similarly, the metallization ratio is defined as the finger width or busbar connector width divided by the corresponding finger pitch or busbar connector pitch.
[0079] Since the gap short structure should be applicable for various frequency bands all parameters characterizing the gap short structure should be defined in units of the pitch of the central excitation area. By this, scaling of the entire resonator including the gap short structure to different frequency bands is easily possible.
[0080] FIGS. 4 to 13 show various specific embodiments of the gap short structure.
[0081] FIG. 4 shows the easiest embodiment of the gap short structure according to FIGS. 2 and 3 with a design of the acoustic track where the gap short structure is realized as two rectangular strips (one per side of the acoustic track) inside the transversal gap region. The busbar connectors BBC are realized as the corresponding finger segments between the busbar BB and the gap short structure GSS. Finger ends and corresponding finger segments at the vertical position of the finger ends of the respective other polarity fingers have a locally increased width in order to increase the mass loading in order to establish a transversal acoustic waveguide structure with a piston mode.
[0082] FIG. 5 shows a design with phase shifted busbar connectors. With respect to the position along the longitudinal direction the strips of the busbar connectors are shifted. The shift along the longitudinal direction corresponds to a phase shift relative to the phase of the acoustic wave inside the central excitation area. In this specific example the phase shift is chosen to be 180°.
[0083] FIG. 6 shows a designs with a decreased metallization ratio for the busbar connectors. A reduced metallization ratio means that in the gap between the busbar BB and the gap short structure GSS the number of patches of the busbar connectors and/or the width (i.e. the extension along the longitudinal direction) is reduced compared to the corresponding electrode fingers of the same electrode.
[0084] FIG. 7 shows a design with an increased metallization ratio. Similar to the design of FIG. 6, the number of busbar connector conductor patches equals the number of electrode fingers of the corresponding electrode. However, while in FIG. 6 the busbar connector conductor patches have a reduced size compared to the width of the electrode fingers, the busbar connector conductor patches in FIG. 7 have an increased size corresponding to an increased metallization ratio 11.
[0085] FIG. 8 shows a design with an increased pitch of the busbar connector conductor patches. The number of conductor patches of the busbar connectors is reduced compared to the number of electrode fingers of the corresponding electrode. Additionally, the width of the busbar connector conductor patches (i.e. the extension along the longitudinal direction of propagation of acoustic waves) is increased compared to the width of the electrode fingers in order to restore the same electrical conductivity of the busbar connectors like in FIG. 4.
[0086] FIG. 9 shows a design where each busbar connector conductor patche of FIG. 4 is divided in three busbar connector conductor patches with reduced width. The width of the individual busbar connector conductor patches can be reduced with respect to the width of the electrode fingers to get the same electrical conductivity of the busbar connectors as before.
[0087] FIG. 10 shows the possibility of rotating the conductor patches of the busbar connectors. The absolute value of the rotation angle can be in the range between 1° and 60°, preferably between 5° and 15°. The rotation angle for both sides may have the same or different angle and may have the same or opposite sign resulting in a symmetric or antisymmetric structure.
[0088] FIG. 11 shows a design illustrating crossed conductor patches of the busbar connectors. For each electrode finger of the corresponding electrode there are two busbar connector conductor patches which cross each other inside the field free area of the transversal gap, preferably at the transversal center of the transversal gap region. Additionally, the form of the “X” is not limited to the design shown in FIG. 11. Other angles of the stripes are also possible.
[0089] FIG. 12 shows the possibility of rotating the conductor patch of the gap short structure. Essentially the gap short structure conductor patch runs parallel to the busbars. However, a rotation with respect to a pure parallel arrangement is obtained. It is possible to span—in a transversal direction—the gap along the longitudinal extension of the acoustic track. Thus, the width of the transversal gap and the length of the interdigital transducer determine the rotation of the conductor patch of the gap short structure.
[0090] FIG. 13 shows the possibility of providing the gap short structure as a plurality of parallel strips extending along the longitudinal direction. This gap short realization is called transversal gapshort reflector (TGR). The strips can be equally spaced in the transversal direction or change its distance, e.g. linearly increasing or decreasing. The same counts for the widths of the strips which may be constant or change from strip to strip, e.g. linearly increasing or decreasing. Another degree of freedom are the busbar connector conductor patches which may be modified from one gap short conductor strip to the next, e.g. stepwise phase-shifted, distorted or rotated.
[0091] FIG. 14a illustrates a zoomed section of FIG. 13 into the transversal gap region showing the definition of the geometry parameters of the special transversal gapshort reflector (TGR) structure design. The pitch pTGR and the width wTGR of the strips as well as the distance D.sub.tgr of the adjacent strip to the finger tips should be defined in units of the longitudinal finger pitch p for easy scaling to different frequency bands. The TGR structure can have a transversal pitch PTGR Moreover, the number of strips is a parameter which needs to be defined and offers an additional degree of freedom.
[0092] In all described realizations of the gap short structure the gap short structure may be exclusively applied in the interdigital transducer area or additionally expanded to the reflectors on one or both sides of the interdigital transducer. When expanded to the reflectors it must be considered that in case of shorted reflector grids the gap short structure needs to be separated in segments at both transitions from the interdigital transducer to each reflector in order to avoid electrical shorting of both electrodes.
[0093] FIG. 14b illustrates a possible modification of the busbar connector conductor patches from one gap short conductor strip to the next in form of a stepwise phase shift resulting in a stairs structure. In general the phase-shift of the busbar connector conductor patches may be changed arbitrarily from one gap short conductor strip to the next, e.g. positive (“+”) or negative (“−”) phase shift like a zigzag structure.
[0094] FIG. 15 shows a comparison between a conventional electro acoustic resonator and an electro acoustic resonator as described above having the gap short structure according to FIG. 4. Specifically, the top portion of FIG. 15 shows in curve 1 the real part of the admittance of a conventional electro acoustic resonator. In contrast, curve 2 shows the real part of the admittance of an improved electro acoustic resonator. Disturbances are strongly reduced in the admittance of the improved resonator, especially inside the stopband. Precisely, the excitation of the transversal gap mode at around 2020 MHz is suppressed by more than 20 dB and all peaks below this frequency and above resonance frequency caused by transversal modes are significantly reduced.
[0095] The center portion of FIG. 15 shows the corresponding curves for the imaginary part of the admittance. At approximately 2020 MHz the conventional resonator has spikes in the imaginary part of the admittance due to transversal gap mode excitation while the corresponding imaginary part of the admittance for the improved resonator is free from a disturbance in this frequency range.
[0096] The bottom part of FIG. 15 shows a comparison between the absolute values of the admittances of the conventional resonator (curve 1) and the improved resonator (curve 2). Similar to the curves of the imaginary part of the admittance, the conventional resonator shows a disturbance at 2020 MHz while the improved resonator does not.
[0097] FIG. 16 shows curves 1 and 2 of FIG. 15 for a zoomed frequency range.
[0098] Similarly, FIG. 17 shows a still further zoomed view of FIG. 16 with curves 1 and 2.
[0099] While FIGS. 15 to 17 show the differences in behaviour for a single one-port resonator based on admittances, FIG. 18 shows a comparison of filter characteristics synthesized from the single resonator admittances for a conventional filter and for a filter comprising the improved electro acoustic resonators with gap short structures in series and in parallel paths (5 series+4 parallel=9 total.fwdarw.SPSPSPSPS).
[0100] The top portion of FIG. 17 shows the transfer function |S21| of a filter in a broad frequency range. The center portion shows the reflection coefficient |S11| at the input port of the corresponding filter. The bottom portion of FIG. 18 shows the frequency dependent global losses determined by the unitarity violation 1−|S21|.sup.2−|S11|.sup.2.
[0101] In FIG. 18 curves 1 refer to a conventional filter while curves 2 correspond to a bandpass filter with improved resonators.
[0102] Especially within the passband ripples and insertion loss are reduced compared to the conventional filter. Also, losses corresponding to the ripples shown in the top portion of FIG. 18 are reduced.
[0103] Further, the passband skirts have an increased steepness supporting narrowband transitions between a passband and stopband.
[0104] FIG. 19 shows the curves shown in FIG. 18 in a zoomed frequency range clearly indicating that ripples and disturbances inside the passband are significantly reduced in the filter based on the improved resonators.
[0105] FIG. 20 shows a further zoomed view of FIG. 19 clearly indicating not only that ripples and disturbances are reduced in the filter based on the improved resonators, but also the overall insertion attenuation and global losses quantified by the unitarity violation inside the entire passband.
[0106] FIG. 21 shows Admittance (Y) Parameters (top: real part of Y; middle: imaginary part of Y; bottom: absolute value of Y) for a standard one port resonator (curve 1) and two improved resonators, a gapshort realization according to the easiest embodiment shown in FIG. 4 (curve 2) and a gapshort realization according to the TGR structure shown in FIG. 13 (curve 3).
[0107] FIG. 22 shows FIG. 21 in a zoomed frequency range.
[0108] FIG. 23 shows FIG. 22 in a still further zoomed frequency range.
[0109] FIG. 24 shows S-Parameters (S) (top: |S.sub.21|; middle: |S.sub.11|) and the unitary violation at input port 1 (bottom: UV.sub.1 [%]) of a basic section of a ladder type filter based on the standard resonator and the two improved resonators of FIGS. 21 to 23
[0110] FIG. 25 shows FIG. 24 in a zoomed frequency range.
[0111] FIG. 26 FIG. 25 in a still further zoomed frequency range.
[0112] FIGS. 21 to 26 refer to structures where in a first improved resonator structure one longitudinal strip is present as a gap short structure (curve 2). In the further improved structure a plurality of strips in a TGR structure is present (curve 3).
[0113] It can be seen that the TGR structure completely suppresses the transversal gap mode below approx. 2020 MHz in FIGS. 21 to 23. It has to be mentioned that the mode splitting of the resonance and the small peak slightly below 1950 MHz are just occurring because the piston mode working point is not at its optimum and both will disappear when the optimum piston mode working point is chosen.
[0114] In FIGS. 24 to 26 it can be seen that the basic section including the TGR completely suppresses the transversal gap modes at approx. 1940 MHz and 2020 MHz. The abovementioned mode splitting shows up in curve .sub.3 as a disturbance around 1850 MHz and a dip around 1920 MHz. As explained above, both can be avoided by chosing the optimum piston mode working point.
[0115] FIG. 27 shows Admittance (Y) Parameters (top: real part of Y; middle: imaginary part of Y; bottom: absolute value of Y) for a standard one port resonator (curve 1) and an improved resonator with a TGR structure according to FIG. 13 (curve 2) at its optimal piston mode working point. At the optimal piston mode working point even the abovementioned mode splitting of the resonance (e.g. indicated in FIGS. 21 to 23) is not present.
[0116] FIG. 28 shows Admittance (Y) Parameters (top: real part of Y; middle: imaginary part of Y; bottom: absolute value of Y) for a conventional one-port resonator (curve 1), an improved resonator with a TGR structure (curve 2) and a slanted resonator (curve 3).
[0117] FIG. 29 shows S-Parameters (S) (top: |S.sub.21|; middle: |S.sub.11|) and the unitary violation (bottom: UV.sub.1 [%]) of a basic section of a ladder type filter based on the standard resonator, the improved resonator with a TGR structure and the slanted resonator of FIG. 28.
[0118] FIG. 30 shows FIG. 29 in a zoomed frequency range.
[0119] FIG. 31 shows FIG. 30 in a still further zoomed frequency range.
[0120] The frequency dependent parameters shown in the figures providing the best electrical performance (e.g. by reduced ripple, reduced loss, . . . ) correspond to structures where some or all the above means have been applied.
[0121] Compared to the use of slanted IDTs (alternative for transversal mode suppression) the use of the gap short structure needs less space, does not restrict filter designers in terms of topology, and complies with the known methods for obtaining a piston mode and simulating the acoustic behavior of the structures. Further, the use of the gap short structure—when compared to designs with slanted IDTs—reduces losses, especially in the left half of a passband. Further, by applying a longitudinal piston mode (e.g. via chirping or via a variation of the IDT reflector pitch) pass band ripple can be further reduced than in solutions with slanted IDTs.
[0122] The electro acoustic resonator and the corresponding filter are not limited by the technical details described above and shown in the figures. Resonators comprising further structured elements such as reflector structures and resonators comprising further layers such as temperature compensation layers, passivation layers or trim layers are also comprised.
LIST OF REFERENCE SIGNS
[0123] 1: property of conventional resonator/filter
[0124] 2: property of improved resonator/filter
[0125] BB: busbar
[0126] BBC: busbar connector
[0127] CEA: central excitation area
[0128] D, DTGR: distance
[0129] EAR: electro acoustic resonator
[0130] EF: electrode finger
[0131] G: gap
[0132] GSS: gap short structure
[0133] IDS: interdigitated structure
[0134] P, PTGR: pitch
[0135] PBBC: pitch
[0136] PM: piezoelectric material
[0137] R: reflector
[0138] W: width
