Abstract
An electroacoustic transducer has reduced loss due to acoustic waves emitted in the transverse direction. For this purpose, a transducer comprises a central excitation area, inner edge areas flanking the central excitation area, outer edge areas flanking the inner edge areas, and areas of the busbar flanking the outer edge areas. The longitudinal speed of the areas can be set so that the excitation profile of a piston mode is obtained.
Claims
1. An electroacoustic transducer, comprising: a piezoelectric substrate; two busbars arranged on the substrate; two electrodes arranged on the substrate, each electrode having interdigital electrode fingers interconnected with a respective one of the two busbars, the electrodes configured for excitation of an acoustic wave; a first layer disposed on the electrode fingers of at least one of the electrodes, a material of the first layer comprising one of a first dielectric material and a conductive material, wherein the material of the first layer is different than a material of the two electrodes; and a plurality of regions running parallel to an acoustic track, wherein the acoustic wave experiences different longitudinal propagation velocity in each of the regions, the plurality of regions comprising: a central excitation region with a first longitudinal velocity; inner edge regions flanking the central excitation region on both sides, wherein a second longitudinal velocity in the inner edge regions is higher than the first longitudinal velocity of the central excitation region; outer edge regions flanking the inner edge regions, wherein third longitudinal velocity in the outer edge regions is higher than one of the first longitudinal velocity in the central excitation region or the second longitudinal velocity in the inner edge regions; regions of the busbars flanking the outer edge regions, wherein a fourth longitudinal velocity in the regions of the busbars is lower than the third longitudinal velocity in the outer edge regions.
2. The electroacoustic transducer of claim 1, the first layer is a mass loading layer.
3. The electroacoustic transducer of claim 1, wherein at least a portion of the first layer is disposed, in at least in lateral sections, in the inner edge regions.
4. The electroacoustic transducer of claim 1, wherein at least a portion of the first layer is arranged in the inner edge regions on, and between, the electrode fingers.
5. The electroacoustic transducer of claim 1, wherein and >1 when the third longitudinal velocity in the outer edge regions is higher than the second longitudinal velocity in the inner edge regions; wherein k.sub.x.sup.2+(1+)k.sub.y.sup.2=k.sub.0.sup.2 and <1 when the third longitudinal velocity in the outer edge regions is higher than the first longitudinal velocity in the central excitation region; and wherein k.sub.x is a component of a wave vector in a longitudinal direction, is an anisotropy factor, k.sub.y is a component of the wave vector in a transverse direction, and k.sub.o is the wave vector in a main propagation direction.
6. The electroacoustic transducer of claim 1, wherein the third longitudinal velocity in the outer edge regions is higher than the second longitudinal velocity in the inner edge regions; wherein k.sub.x.sup.2+(1+)k.sub.y.sup.2=k.sub.0.sup.2 and >1; and wherein k.sub.x is a component of a wave vector in a longitudinal direction, is an anisotropy factor, k.sub.y is a component of the wave vector in a transverse direction, and k.sub.o is the wave vector in a main propagation direction.
7. The electroacoustic transducer of claim 1, wherein the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the central excitation region and wherein k.sub.x.sup.2+(1+)k.sub.y.sup.2=k.sub.0.sup.2 and <1.
8. The electroacoustic transducer of claim 1, wherein the material of the first layer comprises one of hafnium oxide or tantalum oxide.
9. The electroacoustic transducer of claim 1, further comprising a second layer comprising a second dielectric material and covering at least one of the busbars or the electrode fingers of at least one of the two electrodes.
10. The electroacoustic transducer of claim 9, wherein the second dielectric material of the second layer is a silicon oxide (SiO.sub.2).
11. An electroacoustic transducer, comprising: a piezoelectric substrate; two busbars arranged on the substrate; two electrodes arranged on the substrate, each electrode having interdigital electrode fingers interconnected with a respective one of the two busbars, the electrodes configured for excitation of an acoustic wave; a dielectric layer covering at least the busbars; and a plurality of regions running parallel to an acoustic track, wherein the acoustic wave experiences different longitudinal propagation velocity in each of the regions, the plurality of regions comprising: a central excitation region with a first longitudinal velocity; inner edge regions flanking the central excitation region on both sides, wherein a second longitudinal velocity in the inner edge regions is higher than the first longitudinal velocity of the central excitation region; outer edge regions flanking the inner edge regions, wherein third longitudinal velocity in the outer edge regions is higher than one of the first longitudinal velocity in the central excitation region or the second longitudinal velocity in the inner edge regions; regions of the busbars flanking the outer edge regions, wherein a fourth longitudinal velocity in the regions of the busbars is lower than the third longitudinal velocity in the outer edge regions.
12. The electroacoustic transducer of claim 11, wherein the dielectric layer is a silicon oxide (SiO.sub.2) layer.
13. The electroacoustic transducer of claim 11, further comprising a first layer disposed on the electrode fingers of at least one of the electrodes, a material of the first layer comprising one of a dielectric material and a conductive material, wherein the material of the first layer is different than a material of the two electrodes.
14. The electroacoustic transducer of claim 13, wherein the material of the first layer comprises one of hafnium oxide or tantalum oxide.
15. The electroacoustic transducer of claim 11, wherein k.sub.x.sup.2+(1+)k.sub.y.sup.2=k.sub.0.sup.2 and >1 when the third longitudinal velocity in the outer edge regions is higher than the second longitudinal velocity in the inner edge regions; wherein k.sub.x.sup.2+(1+)k.sub.y.sup.2=k.sub.0.sup.2 and <1 when the third longitudinal velocity in the outer edge regions is higher than the first longitudinal velocity in the central excitation region; and wherein k.sub.x is a component of a wave vector in a longitudinal direction, is an anisotropy factor, k.sub.y is a component of the wave vector in a transverse direction, and k.sub.o is the wave vector in a main propagation direction.
16. An electroacoustic transducer, comprising: a piezoelectric substrate; two busbars arranged on the substrate; two electrodes arranged on the substrate, each electrode having interdigital electrode fingers interconnected with a respective one of the two busbars, the electrodes configured for excitation of an acoustic wave; a dielectric layer covering at least the electrode fingers of at least one of the electrodes; and a plurality of regions running parallel to an acoustic track, wherein the acoustic wave experiences different longitudinal propagation velocity in each of the regions, the plurality of regions comprising: a central excitation region with a first longitudinal velocity; inner edge regions flanking the central excitation region on both sides, wherein a second longitudinal velocity in the inner edge regions is higher than the first longitudinal velocity of the central excitation region; outer edge regions flanking the inner edge regions, wherein third longitudinal velocity in the outer edge regions is higher than one of the first longitudinal velocity in the central excitation region or the second longitudinal velocity in the inner edge regions; regions of the busbars flanking the outer edge regions, wherein a fourth longitudinal velocity in the regions of the busbars is lower than the third longitudinal velocity in the outer edge regions.
17. The electroacoustic transducer of claim 16, wherein the dielectric layer is a silicon oxide (SiO.sub.2) layer.
18. The electroacoustic transducer of claim 16, further comprising a first layer disposed on the electrode fingers of at least one of the electrodes, a material of the first layer comprising one of a dielectric material and a conductive material, wherein the material of the first layer is different than a material of the two electrodes.
19. The electroacoustic transducer of claim 18, wherein the material of the first layer comprises one of hafnium oxide or tantalum oxide.
20. The electroacoustic transducer of claim 16, wherein k.sub.x.sup.2+(1+)k.sub.y.sup.2=k.sub.0.sup.2 and >1 when the third longitudinal velocity in the outer edge regions is higher than the second longitudinal velocity in the inner edge regions; wherein k.sub.x.sup.2+(1+)k.sub.y.sup.2=k.sub.0.sup.2 and <1 when the third longitudinal velocity in the outer edge regions is higher than the first longitudinal velocity in the central excitation region; and wherein k.sub.t is a component of a wave vector in a longitudinal direction, is an anisotropy factor, k.sub.y is a component of the wave vector in a transverse direction, and k.sub.o is the wave vector in a main propagation direction.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Electroacoustic transducers according to the invention are explained in greater detail below on the basis of exemplary embodiments and associated schematic figures.
(2) FIG. 1 shows a transverse velocity profile in which the longitudinal velocity is lower in the inner edge region than in the central excitation region,
(3) FIG. 2 shows a transverse velocity profile in which the longitudinal velocity is higher in the inner edge region than in the central excitation region,
(4) FIG. 3 shows a schematic illustration of a conventional electroacoustic transducer,
(5) FIG. 4 shows a schematic illustration of a transducer according to the invention,
(6) FIG. 5 shows a schematic illustration of an alternative embodiment,
(7) FIG. 6 shows a configuration according to the invention of an electrode finger,
(8) FIG. 7 shows a further schematic illustration of an electrode finger according to the invention,
(9) FIGS. 8A, 8B show further schematic illustrations of an electrode finger,
(10) FIG. 8C shows the cross section of an electrode finger with local thickenings,
(11) FIG. 8D shows the cross section of an electrode finger with local thickenings increasing linearly,
(12) FIG. 9 shows a configuration of a transducer according to the invention with material deposited in rail-type fashion on the inner edge regions,
(13) FIG. 10 shows the illustration of the dependence of k.sub.y as a function of k.sub.x in the case of a substrate having concave slowness,
(14) FIG. 11 shows the admittance of an electroacoustic resonator according to the invention,
(15) FIG. 12 shows the principle of phase weighting,
(16) FIG. 13 shows a transducer having different partial regions of the outer excitation regions,
(17) FIG. 14A, B show a transducer having different longitudinal regions and different geometrical embodiments of an inner edge region,
(18) FIGS. 15A, B, C, D, show different transducers having different longitudinal regions and different embodiments of an inner edge region,
(19) FIGS. 16A, B, C, show different embodiments of transducers having different longitudinal regions,
(20) FIGS. 17A, B, C, D, E, show different embodiments of transducers having different longitudinal regions,
(21) FIGS. 18A, B, C, D, E, show different embodiments of transducers having different longitudinal regions,
(22) FIGS. 19A, B, C, D, E, F, show different embodiments of transducers having different longitudinal regions,
(23) FIGS. 20A, B, C, D, E, show different embodiments of transducers having different longitudinal regions,
(24) FIGS. 21A, B, C, D, E, show different embodiments of transducers having different longitudinal regions,
(25) FIGS. 22A, B, C, show different embodiments of transducers having different longitudinal regions,
(26) FIG. 23 shows the frequency-dependent insertion loss for transducers of different configurations,
(27) FIG. 24 shows the functioning of a piston mode component in the case of convex slowness, and
(28) FIG. 25 shows the functioning of a piston mode component in the case of concave slowness.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
(29) FIG. 1 shows a velocity profile of the longitudinal velocity of regions of the acoustic track which are arranged alongside one another in a transverse direction. The central excitation region ZAB is arranged in the interior of the acoustic track. The inner edge regions IRB flank the central excitation region ZAB, said inner edge regions being arranged directly alongside the central excitation region ZAB. In this case, the longitudinal velocities of the inner edge regions IRB are lower than the longitudinal velocity in the central excitation region ZAB. The inner edge regions IRB in turn are flanked by the outer edge regions ARB. In this case, the longitudinal velocity of the outer edge regions ARB is higher than the longitudinal velocity of the inner edge regions IRB. It can also be higher than the longitudinal velocity of the central excitation region ZAB. The outer edge regions ARB in turn are flanked by the regions of the busbars SB, in which the longitudinal velocity of the acoustic waves is lower than in the outer edge regions ARB.
(30) FIG. 2 shows a profile along the transverse direction in which the longitudinal propagation velocities of the different regions are plotted. The difference with respect to the configuration shown in FIG. 1 consists in the fact that the longitudinal velocity in the inner edge regions IRB is higher than the longitudinal velocity in the central excitation region ZAB. The longitudinal velocities in the central excitation region ZAB and in the inner edge regions IRB are lower, however, than in the outer edge regions ARB flanking the inner edge regions.
(31) FIG. 3 illustrates a conventional electroacoustic transducer, in which, in a central excitation region ZAB, in which electrode fingers of two different electrodes overlap, a conversion takes place between RF signals, on the one hand, and acoustic waves, on the other hand. The electrode fingers of an electrode must not touch the busbar of the other polarity, otherwise the transducer structure would be short-circuited. Therefore, an edge region RB, in which no electroacoustic conversion takes place, exists between the finger ends and the opposite busbar. Therefore, conventional transducers generally have only one edge region RB per side between the region of the busbar SB and the central excitation region ZAB.
(32) In contrast thereto, FIG. 4 shows an embodiment according to the present invention. In the central excitation region ZAB, the electrode fingers arranged in a comb-like manner convert between RF signals, on the one hand, and acoustic waves, on the other hand. Inner edge regions IRB flank the central excitation region ZAB. The electrode fingers are made wider in the inner edge region IRB, compared with the central excitation region ZAB. In the inner edge region IRB, too, a conversion takes place between RF signals and acoustic oscillations. As a result of the increased mass covering, for example on account of the increased finger thickness, the longitudinal velocity is reduced in the inner edge region IRB by comparison with the central excitation region ZAB.
(33) Outer edge regions ARB flank the inner edge regions IRB. The outer edge regions ARB do not actively participate in the conversion between RF signals and acoustic waves. However, acoustic waves are indeed capable of propagation in the outer edge regions ARB. On account of the reduced mass covering in the outer edge regions ARB, the longitudinal velocity in the outer edge regions ARB is increased compared with the longitudinal velocities of the inner edge regions, of the reflectors IRB and of the central excitation region ZAB.
(34) The regions of the busbars SB in turn flank the outer excitation regions. The mass covering is maximal here, compared with the rest of the transverse regions; the longitudinal velocity is minimal.
(35) The arising of transverse oscillation modes is a consequence of diffraction effects within the acoustic track having a finite width. The formation of a transverse profile according to the invention of the longitudinal velocity (piston mode) helps to reduce the arising of oscillation modes having a velocity in a transverse direction.
(36) FIG. 5 shows an alternative embodiment, which differs from the embodiment in FIG. 4 to the effect that the width of the electrode fingers is reduced in the inner edge regions IRB, compared with the finger width of the central excitation region ZAB. As a result, the longitudinal velocity in the inner edge regions IRB is increased relative to the velocity of the central excitation region ZAB. The inner edge region IRB is counted as part of the excitation region because electrode fingers of different polarities overlap in it.
(37) FIGS. 6, 7, 8a and 8b show possibilities for the configuration of electrode fingers in which the width of the fingers is reduced (FIG. 6) or increased (FIG. 7) in sections or in which the width of the electrode fingers decreases or increases linearly toward the finger end (FIGS. 8a and 8b).
(38) FIG. 8c shows the cross section parallel to the transverse direction through an electrode finger EF having local thickenings LA. FIG. 8D shows the cross section parallel to the transverse direction through an electrode finger EF having linearly increasing local thickenings LA. Suitable thickenings enable the mass covering and, if appropriate, the elastic parameters of the electrode finger to be set such that the desired velocity profile is obtained.
(39) FIG. 9 illustrates a configuration in which, in inner edge regions IRB flanking the central excitation region ZAB, dielectric material is arranged on the electrodes and the regions of the substrate between the finger electrodes. As a result, the longitudinal velocity is reduced in the inner edge regions IRB.
(40) FIG. 10 illustrates the relations between transverse and longitudinal wave numbers of the imaginary branch (dashed lines) and of the real branch (solid lines) for a concave slowness. In this case, .sub.min designates the smallest possible value of the values of k.sub.x for which guided oscillation modes can exist. .sub.max designates the largest possible value of the values of k.sub.x for which guided oscillation modes can exist. The curves A denote the wave vectors in the excitation region of the acoustic track; the curves B denote the wave vectors in the outer region, i.e., outside the acoustic track e.g., in the region of the busbars.
(41) FIG. 11 shows the real parts of three frequency dependent admittance profiles C, D, E in the case of convex slowness. Curve C shows the admittance of a conventional transducer having resonances at frequencies higher than the resonant frequency of the fundamental mode.
(42) Curve D shows the admittance profile of an electroacoustic transducer in which the longitudinal velocities in the central excitation region, in inner edge regions, in outer edge regions and in the regions of the busbars are adapted for achieving a piston mode. Resonances occur at the same frequencies as in curve C; however, their amplitudes increase greatly only starting from approximately 25 MHz above the resonant frequency.
(43) Curve E shows the calculated frequency-dependent admittance of an electroacoustic transducer whose longitudinal velocities in a central excitation region, in inner and outer edge regions and in regions of the busbar are adapted to a piston mode and in which the dispersion due to the anisotropy of =1 is excluded.
(44) FIG. 12 shows the principle of phase weighting on the basis of widening of the electrode fingers. The widenings in the inner edge regions IRB, which are not arranged at the finger ends, are not arranged symmetrically with respect to an axis running through the center of the fingers in a transverse direction. Rather, the widenings are displaced in a longitudinal direction relative to the respective center of the finger. By virtue of the fact that the center is displaced, the center between the finger edges is no longer exactly in phase with the acoustic wave, as a result of which the excitation intensity is reduced and adapted to the ideal flank of the deflection of the piston mode.
(45) A further option for adapting the acoustic wave is so-called stub or dummy fingers which are arranged in the region of the busbars and are substantially opposite the ends of the electrode fingers of the respective other polarity.
(46) FIG. 13 shows an embodiment of a transducer, wherein gap regions TG (TG=Transversal Gap) flank inner edge regions IRB. Outer edge regions flank the gap regions TG. The outer edge regions are themselves subdivided into different partial regions ARB1, ARB2, ARB3. The partial regions of the outer edge region themselves are in turn flanked by regions of the busbar SB. The longitudinal propagation velocity of a desired mode can be set in the different longitudinal regions in order to obtain a well-defined piston mode.
(47) FIG. 14A shows a configuration of a transducer, wherein a respective gap region TG is arranged between the inner edge regions and the outer edge regions. In this exemplary embodiment, the width of the gap region TG is defined by the distance between the electrode fingers and stub fingers connected to the opposite busbar. The electrode fingers have an increased metallization ratio in the inner edge regions IRB.
(48) FIG. 14B shows various embodiments of the finger widening which are possible for the inner edge regions. The finger width can increase or decrease linearly within the inner edge regions. A plurality of widened sections are possible between which the finger width is reduced. Moreover, it is possible to arrange elliptically shaped finger widenings.
(49) FIG. 15A shows a configuration of a transducer in which finger electrodes are widened in the inner edge regions and are covered by regions of a weighting layer.
(50) FIG. 15B shows a configuration of a transducer in which in addition to the finger electrodes in the inner edge regions rectangularly shaped weighting elements are arranged alongside the electrode fingers.
(51) FIG. 15C shows a configuration of a transducer in which the inner edge regions are completely covered by a dielectric weighting layer.
(52) FIG. 15D shows further design possibilities for increasing the mass covering in the inner edge regions. It is possible to arrange rectangularly shaped sections of a weighting layer on the electrode fingers in the inner edge regions. In this case, the widths of these rectangles can be greater than, less than or exactly equal to the width of the electrode fingers. It is furthermore possible to arrange rectangular elements in the inner edge regions which overlap the electrode finger edges. Elliptically shaped weighting sections can also be arranged in the inner edge regions on the electrode fingers.
(53) FIG. 16A shows a configuration of a transducer, wherein outer edge regions ARB comprise stub fingers. In addition, the electrode fingers are wider in the outer edge regions ARB than in the gap regions TG and in the central excitation region ZAB. The inner edge regions comprise rectangular or square sections of a weighting layer which are arranged on the electrode fingers.
(54) FIG. 16B shows a configuration of a transducer, wherein rectangular sections of a weighting layer are arranged on the electrode fingers in the outer edge regions.
(55) FIG. 16C shows a configuration of a transducer in which rectangularly configured sections of a weighting layer are arranged both in the inner edge regions and in the outer edge regions on the electrode fingers. In this case, the width of the sections of the weighting layer is less than the width of the electrode fingers.
(56) FIG. 17A shows a configuration of a transducer, wherein rectangularly configured sections of a weighting layer are arranged in the inner edge regions on the electrode fingers. In this case, the width of the sections is constant for all electrode fingers of one electrode and differs from the widths of the respective other electrode.
(57) FIG. 17B shows a configuration of a transducer in which rectangularly configured elements of a weighting layer are arranged in the inner edge regions and in the outer edge regions on the electrode fingers. In this case, the widths of the elements of the weighting layer are different in the outer edge regions and in the inner edge regions. Elements of the weighting layer which are arranged in the inner edge regions bring about a phase weighting.
(58) FIG. 17C shows a configuration of a transducer, wherein the electrode fingers have a smaller width in the inner edge regions than in the central excitation region. In addition, the inner edge regions are covered by a dielectric weighting layer.
(59) FIG. 17D shows a configuration of a transducer, wherein sections of a weighting layer which are arranged in the inner edge regions cause a phase weighting as a result of an asymmetrical arrangement on the electrode fingers.
(60) FIG. 17E shows a configuration of a transducer, wherein trapezoidal sections of a weighting layer are arranged in the inner edge regions on the electrode fingers. In this case, the mass covering increases linearly within the inner edge regions for one electrode, while the mass covering decreases from the inner area outward in the inner edge regions for the respective other electrode.
(61) FIG. 18A shows a configuration of a transducer, wherein the outer edge region is subdivided into two partial regions. In innermost sections of the outer edge region, the thickness of the electrode fingers corresponds to that thickness of the electrode fingers in the central excitation region. The electrode fingers are widened in inner partial regions of the outer edge regions. In outer partial regions of the outer edge regions, the thickness of the electrode fingers is equal to that thickness of the electrode fingers in the central excitation region.
(62) FIG. 18B shows a configuration of a transducer, wherein in inner partial regions of the outer edge regions all electrode fingers of an electrode are connected to a conductive or insulating material.
(63) FIG. 18C shows a configuration of a transducer, wherein in inner partial regions of the outer edge regions a weighting layer is arranged on the electrode fingers.
(64) FIG. 18D shows a configuration of a transducer, whereinfloatingrectangular sections of the electrode material, which are not connected to an electrode, are arranged in inner partial regions of the outer edge regions.
(65) FIG. 18E shows a configuration of a transducer, wherein rectangular elements of a weighting layer are arranged periodically along inner partial regions of the outer edge regions. In this case, the period of these elements deviates from the finger period by a factor of greater than or equal to 2.
(66) FIG. 19A shows an embodiment of a transducer in which a respective gap region TG is arranged between the inner edge regions IRB and the outer edge regions ARB. The electrode fingers are widened in the gap region. The outer edge regions each comprise an inner partial region ARB1 and an outer partial region ARB2. The inner partial regions ARB1 of the outer edge regions ARB are completely covered by electrode material. In the outer partial regions ARB2 of the outer edge regions ARB, the finger electrodes have the same width as in the central excitation region ZAB.
(67) FIG. 19B shows a configuration of a transducer in which gap regions TG are arranged between the outer edge regions ARB and the inner edge regions IRB. Within the gap regions, the finger width remains constant over a partial region of the gap regions. Between said partial region and the inner edge regions, the width of the electrode fingers decreases linearly in the gap region. At the ends of the electrode fingers, the width of the electrode fingers decreases linearly to zero in the gap region.
(68) FIG. 19C shows a configuration of a transducer in which the outer edge regions ARB are divided into inner partial regions ARB1 and outer partial regions ARB 2. The inner partial regions ARB1 are covered with a weighting layer. Between the outer edge regions and the inner edge regions, the width of the electrode fingers decreases substantially sinusoidally from the outer area inward. The finger ends of the electrode fingers are configured in substantially round fashion in the gap regions.
(69) FIG. 19D shows a configuration of a transducer in which in the gap regions TG between the outer edge regions and the inner edge regions at those ends of the electrode fingers which are remote from the busbar, rectangular sections of a dielectric material are arranged on the electrode fingers.
(70) FIG. 19E shows a configuration of a transducer in which the gap regions TG are covered by a dielectric layer. Furthermore, an inner partial region of the outer edge regions is covered with periodically arranged rectangular sections of a weighting layer. In this case, the period of said sections is less than of the wavelength .
(71) FIG. 19F shows a configuration of a transducer in which those sections of the electrode fingers of the gap regions are covered by a weighting layer.
(72) FIG. 20A shows a configuration of a transducer, wherein gap regions TG are arranged instead of inner edge regions between the outer edge regions ARB and the central excitation region ZAB. The electrode fingers are widened within the gap regions TG.
(73) FIG. 20B shows a configuration of a transducer in which the electrode fingers are widened in constant fashion in outer partial regions of the gap regions TG and in which in inner partial regions of the gap regions the thickness of the finger electrodes decreases linearly from the busbar side to the side of the finger ends.
(74) FIG. 20C shows a configuration of a transducer in which rectangular sections of a weighting layer are arranged in gap regions TG on the electrode fingers.
(75) FIG. 20D shows a configuration of a transducer in which in gap regions TG rectangular sections of a dielectric material are arranged periodically (in a longitudinal direction). The period length of these structures is less than one sixth of the wavelength of the acoustic wave.
(76) FIG. 20E shows a configuration of a transducer in which gap regions TG between the outer edge regions ARB and the central excitation region ZAB are completely covered with a dielectric material.
(77) FIG. 21A shows a configuration of a transducer in which the outer edge regions ARB comprise stub fingers between the electrode fingers. Furthermore, the electrode fingers are widened in the outer edge regions. The width of the stub fingers and the width of the widened electrode fingers are substantially identical.
(78) FIG. 21B shows a configuration of a transducer in which in each case three stub fingers per unit length in the unit of the wavelength are arranged in the outer edge regions ARB.
(79) FIG. 21C shows a configuration of a transducer in which outer edge regions are completely covered with a weighting layer.
(80) FIG. 21D shows a configuration of a transducer in which the electrode fingers are covered in the outer edge regions ARB by rectangular segments of a weighting layer. The segments of the weighting layer are in this case wider than the electrode fingers.
(81) FIG. 21E shows a configuration of a transducer, wherein rectangular segments of a weighting layer are arranged in the outer edge regions. The segments cover electrode fingers and the regions between the electrode fingers periodically. In this case, the segments are wider than the electrode fingers. In this case, the period length of the segments is less than one quarter of the period length of the acoustic waves .
(82) FIG. 22A shows a configuration of a transducer in which the outer edge regions ARB comprise stub fingers. The metallization ratios in the outer edge regions and in the central excitation region are identical. The metallization ratio in gap regions TG is lower than that in the outer edge regions.
(83) FIG. 22B shows a configuration of a transducer in which the electrode fingers are covered in the central excitation region by rectangular segments of a weighting layer.
(84) FIG. 22C shows a configuration of a transducer in which the central excitation region is covered by a dielectric layer. Furthermore, the outer edge regions ARB are covered by a weighting layer.
(85) FIG. 23 shows the frequency-dependent insertion loss curves of three transducers. Curve 1 shows the insertion loss of a conventional transducer. Curve 2 shows the insertion loss of a transducer with cosinusoidal weighting. Curve 3 shows the insertion loss of a transducer according to the invention. Curve 1 shows clearly discernible resonances above and below the passband. These resonances are significantly reduced in the case of the insertion loss in curve 2. However, the level of the curve is higher than minima of curve 1. In the case of the insertion loss in accordance with curve 3 of the resonator according to the invention, the manifestation of the resonances is significantly reduced and the curve is at the level of the minima of curve 1.
(86) FIG. 24 illustrates the functioning of a piston mode component in the case of convex slowness. The lower half of the figure shows an excerpt from the device structure having a plurality of transverse sections. In this case, there is an inner edge region IRB having widened fingers, an outer edge regions ARB corresponding to a gap region, and the region of the busbars SB. The upper half of the figure shows the associated transverse velocity profile v(y), and the amplitude of the deflection profile (y) of the piston mode.
(87) A reduced velocity by comparison with the central excitation region ZAB is present in the inner edge region IRB, and an increased velocity in the outer edge region ARB. The outer edge region ARB serves here as a decay region in which the mode decays exponentially outwardly. On substrates with high coupling or with heavy electrodes, the configuration of the outer edge region ARB as a gap region TG is particularly advantageous since a large difference in velocity with respect to the central excitation region ZAB is obtained here by the omission of every second finger. The utilization of the gap region as a decay region is novel. The outer edge region ARB should have a width at least such that, at its outer edge, the amplitude of the mode has decreased to 10% of the value in the central excitation region ZAB. The inner edge region IRB serves here for adapting a quasi-linear profile of (y) in the central excitation region ZAB to the exponential profile in the outer edge region ARB. For this purpose, the width W is chosen suitably.
(88) FIG. 25 illustrates the functioning of a piston mode component in the case of concave slowness. The lower half of the figure shows an excerpt from the device structure having a plurality of transverse sections. In this case, there is an inner edge region IRB having narrowed fingers, an outer edge region ARB corresponding to a gap region, and the region of the busbar SB. The upper half of the figure shows the associated transverse velocity profile v(y), and the amplitude of the deflection profile (y) of the piston mode. An increased velocity relative to the central excitation region ZAB is present in the inner edge region IRB, and an even higher velocity in the outer edge region ARB. The region of the busbar SB serves here as a decay region in which the mode decays exponentially outwardly. On account of the continuous metallization in the SB, a significant reduction of velocity by comparison with the central excitation region ZAB is achieved here primarily in the case of high layer thicknesses and high coupling on account of the increased mass loading. The width of the region of the busbar SB should be at least large enough that, at its outer edge, the amplitude of the mode has decreased to 10% of the value in the central excitation region ZAB. The inner edge region IRB and the outer edge region ARB serve here jointly for adapting a quasi-linear profile in the central excitation region ZAB to the exponential profile in the region of the busbar SB. For this purpose, the width W, corresponding to the sum of the widths of the inner edge region IRB and of the outer edge region ARB, has to be chosen suitably.
(89) An electroacoustic transducer is not restricted to one of the exemplary embodiments described. Variations comprising, for example, further velocity ranges arranged in lateral regions or correspondingly shaped electrode fingers, or combinations of different embodiments, likewise constitute exemplary embodiments according to the invention.
