Abstract
An improved electroacoustic transducer with an improved mode profile is provided. The transducer comprises a velocity profile with a periodic structure and an edge structure flanking the periodic structure. The velocity profile also allows to suppress the SH wave mode.
Claims
1. An electroacoustic transducer, comprising: a piezoelectric substrate; busbars arranged on the substrate; and interdigitated electrode fingers coupled to the busbars and arranged on the substrate such that acoustic waves propagating in the transducer have a velocity profile and an acoustically active region, wherein: the velocity profile has a periodic structure in the active region, the periodic structure has a plurality of minimal values and a plurality of maximal values larger than the minimal values, the periodic structure is flanked on both sides by an edge structure of the velocity profile, a velocity in the edge structure being lower than the maximal values of the periodic structure, and each edge structure is flanked by a gap structure of the velocity profile and arranged between the gap structure and the periodic structure, a velocity of the gap structure being larger than a maximal value of the velocity of the periodic structure.
2. The transducer of claim 1, where the edge structure comprises two stripes per unit cell, each of the two stripes being arranged directly next to a respective side of the periodic structure.
3. The transducer of claim 2, where the stripes of the edge structure have a length larger than 50% of a periodic length of the periodic structure.
4. The transducer of claim 2, where the stripes of the edge structure have a length smaller than 50% of a periodic length of the periodic structure.
5. The transducer of claim 1, wherein the gap structure comprises two stripes being arranged directly next to a respective side of the edge structure, and wherein the active region is arranged between the two stripes of the gap structure.
6. The transducer of claim 3, wherein the gap structure's stripes have a length from 0.5 to 10 or from 2 to 4.
7. The transducer of claim 3, wherein the gap structure has a metallization ratio from 0.2 to 0.8.
8. The transducer of claim 1, wherein: the busbars are aligned to a longitudinal direction of the transducer, and the interdigitated electrode fingers are aligned parallel to a transversal direction of the transducer.
9. The transducer of claim 1, wherein the velocity profile is adjusted via one or more measures selected from: reduced velocity by increased mass loading by increased finger width, reduced velocity by increased mass loading by increased finger thickness, reduced velocity by increased mass loading by additional material deposited on the electrode fingers, reduced velocity in the gap structures by increased mass loading by dummy patches in a gap region corresponding to the gap structure, reduced velocity by increased mass loading by material deposited in stripes on the electrode fingers, increased velocity by reduced mass loading by reduced finger width, increased velocity by reduced mass loading by reduced finger thickness, or increased velocity by reduced mass loading by material removed from the electrode fingers.
10. The transducer of claim 1, wherein the piezoelectric substrate comprises LiNbO.sub.3.
11. The transducer of claim 1, wherein the piezoelectric substrate comprises LiTaO.sub.3.
Description
BRIEF DESCRIPTION OF THE FIGURES
(1) The transducer is explained in greater detail on the basis of exemplary and not limiting embodiments and associated figures below.
(2) FIG. 1 shows the orientation of the longitudinal direction x with respect to the transversal direction y.
(3) FIG. 2 shows the transversal velocity profile v along the transversal direction y.
(4) FIG. 3 shows the orientation of the transducer with respect to the longitudinal direction x and the transversal direction y.
(5) FIG. 4 illustrates a possible causal connection between the velocity profile and a physical realization of the transducer's electrode structure.
(6) FIG. 5 shows the conductance of the transducer as described above compared to the conductance of conventional transducers.
(7) FIG. 6 shows the velocity profile and the profile of the fundamental mode of the improved transducer the conductance of which is shown in FIG. 5.
(8) FIG. 7 shows the velocity profile of the second symmetric wave mode of an embodiment of the transducer together with the mode profile and the absolute value of the mode profile.
(9) FIG. 8 shows the velocity profile and the respective mode profile and its absolute value of the third symmetric wave mode.
(10) FIG. 9 shows the velocity profile and the respective mode profile and its absolute value of the fourth symmetric wave mode.
(11) FIG. 10 shows the velocity profile and the mode profile and its respective absolute value of the fifth symmetric wave mode.
(12) FIG. 11 shows the frequency-dependent conductance of a transducer with a 2D periodic array with patches compared to the conductance of a conventional transducer.
(13) FIG. 12 shows the velocity profile and the wave mode and its respective absolute value of the transducer having the frequency characteristic shown in FIG. 11.
(14) FIG. 13 shows an embodiment of a transducer's electrode structure.
(15) FIG. 14 shows another embodiment of an electrode structure having an increased finger width in a region corresponding to the gap structure.
(16) FIG. 15 shows an embodiment of electrode structures having a reduced finger width in the region corresponding to the gap structure.
(17) FIG. 16 shows an embodiment of electrode structures comprising dummy finger patches arranged within the region corresponding to the gap structure.
(18) FIG. 17 shows stripes comprising a dielectric material arranged in the region corresponding to the gap structure.
(19) FIG. 18 shows an embodiment of electrode structures comprising metal bars arranged on a dielectric material arranged on the electrode fingers and the bus bars.
(20) FIG. 19 shows an embodiment of a transducer structure where material is removed in the active region to reduce the mass loading.
(21) FIG. 20 shows an embodiment of a transducer structure with individual additional masses arranged on each side of the electrode fingers to increase the mass loading.
(22) FIG. 21 shows electrode structures the material of which is partially removed to obtain the respective velocity profile.
(23) FIG. 22 shows an embodiment of electrode structures with additional material deposited on locally etched electrode structures.
(24) FIG. 23 shows electrode fingers with recesses.
(25) FIG. 24 shows electrode structures with additional metal bars in the region correlated to the gap structure and with recesses in the electrode fingers establishing the periodic structure.
DETAILED DESCRIPTION
(26) FIG. 1 shows a substrate SU which may comprise piezoelectric material such as lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3). x denotes the longitudinal direction. y denotes the transversal direction. Interdigital transducers are arranged in such a way that the main direction of propagation is parallel to the x-direction. Accordingly, a crystal cut of the substrate SU is chosen to obtain a high coupling coefficient.
(27) FIG. 2 illustrates the velocity profile VP of acoustic waves propagating in the substrate SU shown in FIG. 1. The velocity profile has a periodic structure PS which is flanked by edge structures ES in such a way that the periodic structure PS is arranged between the edge structures ES in the transversal direction y. The periodic structure PS comprises areas with a relative high velocity v and areas with a relative low velocity. The areas of high and low velocity alternate in such a way that a periodic velocity profile in the periodic structure is obtained. The velocity in the edge structure is lower than the maximum velocity in the periodic structure PS.
(28) The velocity in the edge structure may be equal to the lowest velocity in the periodic structure. However, the velocity in the edge structure ES may differ from the lowest velocity in the periodic structure. Also, the length of the edge structure is not limited. However, it may be preferred that the length of the respective stripe of the edge structure ES is larger than half of the periodic length of the periodic structure PS. Here the phrase length denotes the extension of the edge structure in the transversal direction.
(29) FIG. 3 shows the orientation of the transducer TD comprising bus bars BB and electrode fingers EF with respect to the longitudinal direction x and the transversal direction y. The area in which the electrode fingers of opposite electrodes overlap is called the acoustically active region AAR. The bus bars are oriented parallel to the longitudinal direction x. The electrode fingers EF are oriented parallel to the transversal direction y.
(30) When an RF signal is applied to the bus bars and the bus bars have opposite polarities, an acoustic wave is excited in the piezoelectric substrate SU.
(31) FIG. 4 shows the connection of the electrode structure and the velocity profile v. The velocity of acoustic waves at the surface or an interface of a piezoelectric material depends on the mass loading at the interface. A higher mass loading and/or a higher reflection decreases the velocity. However, higher elastic constants of a material deposited on the piezoelectric material increases the velocity. Thus, the shape of the velocity profile v along the transversal direction y can directly depend on geometric structures of the material arranged on the piezoelectric substrate, e.g. the electrode structures comprising the bus bars BB and the electrode fingers EF. To obtain the periodic structure of the velocity profile VP, the electrode fingers EF may have a shape with a corresponding periodic symmetry. To obtain minima in the velocity profile, the local finger width can be increased compared to sections where the velocity should be maximal and the according finger width is, thus, reduced. Further, in order to obtain the edge structure with a reduced velocity compared to the maximal velocity within the periodic structure, the finger width can be larger in the area corresponding to the edge structure compared to the area corresponding to the highest velocities in the periodic structure.
(32) FIG. 5 shows the conductance curves of two transducers. The frequency-dependent conductance of a conventional transducer is denoted as 1 while the conductance of an improved transducer is denoted as 2. Especially at a frequency of approximately 910 MHz, the conventional transducer offers a peak deriving from the SH mode. A resonance in the improved transducer is efficiently suppressed.
(33) The transducer, of which the frequency-dependent conductance is shown in FIG. 5, has a velocity profile shown in FIG. 6. The velocity profile VP has a periodic structure between two structures deviating from the periodic structure, namely the edge structure ES. In the velocity profile VP shown in FIG. 6, the fundamental mode FM is formed. The combination of the periodic structure and the edge structure ES allows to efficiently suppress the SH mode while also no or nearly no transversal modes are excited.
(34) FIG. 7 shows a velocity profile and the corresponding symmetric wave mode profile together with its absolute value. As can be seen, the amplitude of the mode profile with a positive displacement is mainly equal to the amplitude of the mode profile with a negative displacement in the periodic structure. Thus, the areas of positive and negative displacements, i.e. the value of the integral of the mode profile mainly vanishes and the excitation strength of the second symmetric mode is minimized.
(35) FIG. 8 shows, similar to the situation shown in FIG. 7, the mode profile and its absolute value of the velocity profile already shown in FIG. 7. Again, the amplitude of positive and negative displacements are mainly equal. Thus, the excitation strength of the third symmetric mode profile is minimized.
(36) FIG. 9 shows the mode profile of the fourth symmetric wave mode. Again, the amplitude of positive and negative displacements are mainly equal. Thus, the excitation strength of the fourth symmetric wave mode is minimized.
(37) FIG. 10 shows the mode profile and its absolute value of the fifth symmetric wave mode. Again, the excitation strength is minimized as the shape of the velocity structure with the edge structure flanking the periodic structure is an efficient means to suppress higher order modes.
(38) FIG. 11 shows conductance curves of a conventional transducer, curve 1 and of a transducer where the velocity in the edge structure is equal to the highest velocity in the periodic structure, curve 2. Further, the velocity in the periodic structure or in the active region (the velocity profile is shown in FIG. 12) is very slow so that the transversal mode, of which the wavelength is equal or similar to the transversal periodicity, is bounded. As a result, a plurality of resonances in the frequency-dependent conductanceas shown by curve 2are obtained.
(39) FIG. 12 shows the velocity profile corresponding to conductance curve 2 of FIG. 11. Since the transversal mode is bounded, the shape of the mode profile becomes cosine-like. A comparison between the mode profile shown in FIG. 12 and the absolute value of the mode profile reveals that the negative displacement has a smaller amplitude than the positive displacement. As a result, the signal is not cancelled out completely and the plurality of resonances shown in FIG. 11 are obtained.
(40) Thus, the depth of the velocity profile should not exceed critical values. The waveguide parameters have to be chosen in such a way that the highest bounded mode has a lower number than the mode responsible for the second resonances as shown in curve 2 of FIG. 11.
(41) The highest bounded modedenoted as n.sub.maxthat a wave guide can contain is a function of the aperture and the track and gap velocity:
n.sub.max=2A f.sub.0 sqrt[1/(v.sub.track).sup.21/(v.sub.gap).sup.2]
(42) Here, A denotes the aperture. f.sub.0 denotes the resonance frequency, V.sub.track is the lowest velocity in the active region. V.sub.gap is the velocity in the gap structure.
(43) FIG. 13 shows a basic embodiment where the periodic structure of the velocity profile is obtained by an according periodic finger width variation in the electrodes.
(44) FIG. 14 is an embodiment of a transducer where a reduced velocity in the gap structure is obtained by an increase of the finger width of the electrodes in the area corresponding to the gap structure.
(45) FIG. 15 shows an embodiment of a transducer structure where the velocity in the gap structure is increased by a reduced finger width in the corresponding section of the electrodes.
(46) FIG. 16 shows an embodiment where mass loading is increased to reduce the velocity in the gap structure by additional dummy fingers arranged in such a way that the distance between the bus bar and an electrode finger is reduced to a minimum.
(47) FIG. 17 shows an embodiment of a transducer where a dielectric material, e.g. comprising a silicon dioxide, is arranged in two stripes in the gap structure corresponding region.
(48) FIG. 18 shows an embodiment of a transducer where the electrode fingers are covered with a dielectric material, e.g. silicon dioxide. The left part of FIG. 18 shows a top view onto the transducer where two stripes of metal increase the mass loading to reduce the velocity in the gap structure corresponding region. The right part of FIG. 18 shows a cross-section through the transducer showing that the dielectric material is arranged between the metal stripes and the electrodes.
(49) As a dielectric material DS is arranged between the electrode fingers EF and the stripes for increased mass loading MS, the stripes MS can comprise a metal. This is in contrast to FIG. 17 where the stripes are directly arranged on the electrode fingers. Thus, an insulating material is preferred in the embodiment shown in FIG. 17.
(50) FIG. 19 shows on the left-hand side a top view onto a transducer structure and the thickness of the respective metallization of the electrode fingers along the transversal direction (right-hand side). In the active region, the thickness of the metallization is reduced. In the region corresponding to the gap structure, the thickness is larger than in the active region. Thus, a velocity profile in which the velocity in the gap structure is reduced compared to the periodic structure is obtained.
(51) FIG. 20 shows an embodiment of a transducer where the transducer structure is covered with a dielectric material DM. In order to increase the mass loading, individual segments of a material with a high density, e.g. a metal, are arranged on the respective electrode fingers of the edge structure. As the additional material is arranged directly on the electrode material and not isolated from the electrodes via the dielectric material DM, the segments of the additional material should not be in direct contact with each other.
(52) FIG. 21 shows an important aspect in forming the velocity profile shown in the upper right part of FIG. 21: The lower part of FIG. 21 shows that the height of the metallization establishing the electrode fingers can be varied along the transversal direction to adjust the velocity profile. A periodic profile can be etched into the electrode finger to obtain the periodic structure PS. The thicknesses in the regions corresponding to the edge structures and the gap structures can be adjusted accordingly.
(53) FIG. 22 shows a further embodiment of the transducer shown in FIG. 21 where further to the etching to obtain different thicknesses in the electrode structure, a dielectric, e.g. a silicon dioxide, is arranged on the electrode structures. Further, another material is deposited in stripes in the gap regions to adjust the velocity profile.
(54) FIG. 23 shows an embodiment where recesses are structured into the electrode fingers to provide a physical basis for the periodic structure of the velocity profile.
(55) FIG. 24 shows further to the recesses of FIG. 23 in the electrode fingers material bars, e.g. metal bars, in the areas corresponding to the gap structure as a physical realization of the velocity setting.
LIST OF REFERENCE SYMBOLS
(56) 1, 2: frequency-dependent conductance of a transducer AAR: acoustically active region BB: bus bar DM: dielectric material DS: dielectric stripe EF: electrode finger ES: edge structure FM: frequency modulation GS: gap structure MS: metallic stripe PS: periodic structure SU: substrate TD: transducer v: velocity VP: velocity profile x: longitudinal direction y: transversal direction
