Abstract
A transducer for SAW-type or PSAW-type acoustic waves is proposed in which the dielectric (DK) is applied onto the substrate so that the gap (GP) between the ends of the electrode fingers and the opposite bus electrode is completely filled with said dielectric (DK), but the active area of the transducer, thus transversal overlap area (UB) of the electrode fingers, is not covered by said dielectric.
Claims
1. A transducer for SAW-type or PSAW-type acoustic waves, constructed on a leaky wave substrate that has a crystal cut benefiting the generation of SAWs with two electrode combs arranged on the substrate, which electrode combs respectively have electrode fingers (EF) connected with on a bus electrode (BE), wherein the two electrode combs are arranged interleaved with one another so that their electrode fingers (EF) mutually overlap in a transversal overlap area (UB) in which the ends of overlapping electrode fingers (EF) of a first of the electrode combs and the opposite bus electrode (BE) of the respective second electrode comb, or the respective opposite ends of overlapping and non-overlapping short electrode fingers (SF), are spaced in the transversal direction so that a gap (GP) is formed between them in which a dielectric (DK) is applied onto the substrate so that the gap (GP) is completely filled with this, but the transversal overlap area (UB) of the electrode fingers is not covered by this in which the dielectric (DK) is chosen with regard to material and layer thickness so that an acoustic wave experiences approximately the same acoustic impedance in the transversal overlap area (UB) and in the gap area (GB), and the acoustic wave travels just as quickly in the gap area as within the overlap area.
2. The transducer according to claim 1, in which the dielectric (DK) is structured in the form of two parallel strips (DK.sub.S) that respectively travel parallel to the longitudinal direction of the transducer, covers the gap area (GB) with the gaps (GP) arranged at the same transversal height, and leaves the transversal overlap area (UB) uncovered.
3. The transducer according to claim 2, in which the strips of the dielectric (DKs) are so wide that they moreover extend beyond a border area (RB) of the transducer that comprises the non-overlapping stub fingers (SF), or up to the bus electrode (BE).
4. The transducer according to claim 1, in which the dielectric (DK.sub.F) is structured in the form of individual dots that extend the electrode fingers (EF) with the same or increased width beyond their ends.
5. The transducer according to one of the preceding claims, in which the dielectric (DK) comprises SiO.sub.2 or silicon nitride in which the metallization of the electrode fingers (EF) comprises Al, Cu or Ti, in which the metallization comprises a multi-layer structure made up of the different components in pure form, or in the form of alloys formed with one another in which the height of the dielectric layer corresponds to 10-500% of the height of the metallization.
6. The transducer according to one of the preceding claims, in which the dielectric (DK) comprises SiO.sub.2 or is composed of silicon nitride in which the height of the dielectric layer corresponds to 50-150% of the height of the metallization.
7. The transducer according to one of the preceding claims, constructed on a substrate that comprises lithium tantalate.
8. The transducer according to claim 7, in which the lithium tantalate has a crystal cut LT WI rot YX, wherein WI designates the cut angle, and wherein for WI it applies that 39°≦WI≦46, wherein WI is especially selected from 39°, 42° and 46°.
9. The transducer according to one of the preceding claims, in which the overlap area (UB) has a width of less than 20λ, wherein λ is the wavelength of the acoustic wave, wherein the aperture is preferably between 5λ and less than 20λ.
Description
[0031] The invention will be explained in greater detail below in reference to exemplary embodiments and the accompanying figures. The Figures serves solely for illustration of the invention, and therefore are schematic and not executed true to scale.
[0032] FIG. 1 shows, in schematic plan view, a transducer known per se and its distribution in the overlap area, gap areas and border areas,
[0033] FIG. 2 shows, in a schematic plan view, a strip-shaped dielectric applied in the gap area,
[0034] FIG. 3 shows, in the same plan view, as strip-shaped dielectric which also covers the border area,
[0035] FIG. 4 shows an applied strip-shaped dielectric which widens again, which dielectric also covers the bus electrode and the directly adjoining area,
[0036] FIG. 5 shows a strip-shaped dielectric which covers the gap area, the border area and the bus electrode of a resonator,
[0037] Similar to FIG. 5, FIG. 6 shows a resonator in which the dielectric moreover covers the entire reflector of the resonator,
[0038] FIG. 7 shows a transducer in plan view in which the dielectric is applied as dots, exclusively in the gap,
[0039] FIG. 8 shows the admittance and the quality of a resonator having a transducer according to the invention in comparison to conventional resonators,
[0040] FIG. 9 shows a section in the transversal direction through an electrode finger and the dielectric,
[0041] FIG. 10 shows three different sections in the transversal direction through an electrode finger whose metallization at the finger end has an edge that falls away in a non-vertical direction,
[0042] FIG. 11 shows dielectrics applied in dot form with different widths, in plan view,
[0043] FIG. 12 shows two different sections in the transversal direction through electrode fingers whose metallization at the finger ends has an edge that falls away with negative edge angle in a non-vertical direction.
[0044] FIG. 1 shows a transducer known per se in schematic plan view. The transducer comprises at least two bus electrodes BE from which respective electrode fingers EF extend in the transversal direction. The two bus electrodes with the electrode fingers attached thereon respectively form an electrode comb. In the transducer, two electrode combs are interleaved interdigitally so that their electrode fingers overlap in an overlap area UB. A gap GP, consequently a clearance between the two electrodes, is formed in the transversal direction between the ends of the electrode fingers and the bus electrode or the adjacent electrode comb. Another stub finger SF which has no overlap with the respective other electrode comb may be arranged between the gap GP and the nearest bus electrode BE. As shown in FIG. 1, the gap is then formed between the ends of the electrode fingers and the ends of the opposite stub fingers arranged at the same longitudinal position. The entire transducer is then subdivided into the bus electrode BE, the non-overlapping edge area RB, the gap area GB, and the overlap area UB. The gap area GB is then a rectangular area if all gaps are located at the same height transversally and have approximately the same transversal width. The drawn coordinate system shows that the transversal direction corresponds to the y-axis and that the longitudinal direction in the propagation direction of the acoustic surface wave corresponds to the x-axis.
[0045] FIG. 2 shows a first embodiment of the invention in which a respective strip of a dielectric precisely covers one of the two gap areas GB of the transducer. The overlap area is not covered by the dielectric. The border of the strip-shaped dielectric that faces toward the overlap area terminates flush with the finger end of the overlapping finger. The edge of the strip-shaped dielectric that faces toward the bus electrode BE here likewise terminates flush with the ends of the stub fingers; however, this may also partially overlap. It is thereby clear that a flush termination of finger ends and strip-shaped dielectric is thereby only achieved when at least the electrode fingers have steeply falling edges, in the ideal case even vertically falling edges. This is not achieved in practice with real structuring processes.
[0046] FIG. 3 shows a transducer according to the invention in plan view, in which the strip-shaped, structured dielectric DK also completely covers the border area of the transducer in addition to the gap area. Therefore, the entire transducer area is covered by the dielectric, with the exception of the bus electrode BE and the overlap area UB.
[0047] FIG. 4 shows in plan view a dielectric applied in strip shape, which dielectric additionally covers the bus electrode BE in addition to the gap area GB and border area RB, and optionally also additionally covers an adjoining area outside of the acoustic trace or outside of the transducer.
[0048] FIG. 5 shows a transducer that is part of an acoustic resonator. Given a resonator, acoustic reflectors are arranged in the longitudinal direction on both sides of the acoustic transducer. These comprise strip-shaped reflectors that exhibit finger width and finger spacing similar to the electrode fingers in the overlap area. The reflector is electrically insulated from the transducer, or is connected with only one of the potentials, preferably to ground. Given this embodiment in the resonator, the dielectric applied in strip form also additionally extends in the longitudinal direction across both reflectors. The transversal extent of the strip-shaped dielectric may vary as shown in FIGS. 2 through 4.
[0049] FIG. 6 shows a further embodiment having a resonator in schematic plan view, in which the entire resonator or resonators are also covered by the dielectric in addition to the surfaces shown in FIG. 5. Within the resonator, only the overlap area thereby remains uncovered, and there only the overlapping electrode fingers of the dielectric.
[0050] FIG. 7 shows an embodiment of a transducer in which the dielectric is structured in the form of dots and is arranged exclusively in the gaps. The dots are located in the gap area between finger ends of overlapping fingers and stub fingers, but not on the electrode fingers EF in the gap area. The width of the dots may vary, but approximately corresponds to the width of the electrode finger.
[0051] FIG. 8 shows three curves Q1, Q2 and A2, wherein Q1 shows the quality of a conventional resonator and Q2 shows the quality of a resonator according to the invention, coated with dielectric in the gap area, whereas A2 reflects the real part of the admittance of a transducer according to the invention. From the ratio of the two curves Q1 and Q2, it is clear that the quality of a transducer according to the invention, coated with dielectric in the gap area, significantly surpasses the quality of the conventional resonator. According to the presented example, at its peak the quality here is increased from 1160 to 1380, for example.
[0052] FIG. 9 shows three cross sections a through c in the transversal direction through electrode fingers, dielectric DK and stub finger SF. The z-axis shown in the Figure is the normal relative to the surface of the piezoelectric substrate. The three sections differ in the height of the applied dielectric DK. Whereas the layer thickness of the dielectric DK is smaller than the metallization height of the electrode finger EF in FIG. 9A, in FIG. 9B it approximately corresponds to the metallization height. In FIG. 9C, the dielectric DK has a significantly greater layer thickness than the metallization of the electrode finger EF.
[0053] FIG. 10 likewise shows three different cross sections through electrode finger EF, dielectric DK and stub finger SF. In this depiction, the cross section profiles of the electrode fingers is depicted closer to reality, meaning that the cross section profile of the electrode fingers does not fall away vertically relative to the substrate at the end of the finger but rather is rounded or beveled.
[0054] In the cross section depiction of FIG. 10A, the dielectric DK.sub.S, F fills the gap so that the edge profile of the DK.sub.S, F corresponds to the inverse of the edge profile at the ends of the electrode fingers EF. In plan view, a blurry region UBR results in which the diagonally trailing edges of electrode fingers EF and dielectric DK.sub.S, F overlap, such that in plan view no clear separation is to be drawn between dielectric and electrode fingers. In the instances in which a blurry region UBR exists, by definition both gap area and overlap area UB end “indistinctly” within the blurry region BR since the boundaries are effectively blurred across the blurry area UBR.
[0055] FIG. 10B shows an electrode finger likewise having diagonally trailing edge profile of the electrode fingers and a dielectric which is applied in the gap area and additionally covers the border area. The edge of the dielectric that faces toward the overlap area UB falls away with the same slope as the metallization at the end of the electrode finger, such that here as well a blurry region UBR is formed at the boundary between dielectric and electrode finger. The dielectric now extends across the upper edge of the electrode finger end, thus ends in a blurry region UBR.
[0056] FIG. 10C likewise shows a dielectric DK applied overlapping the electrode finger or electrode fingers EF in the blurry area UBR, with smaller layer thickness than the metallization of the electrode finger.
[0057] FIG. 11 shows in plan view three exemplary embodiments of dielectric DK applied in the form of dots. In FIG. 11A, the dielectric DK.sub.F is introduced into the gap with smaller width than the electrode fingers EF. In FIG. 11B, the outer edges of the dielectric DKF align with the outer edges of the electrode finger EF, whereas in FIG. 11C the dielectric DK.sub.F has a greater longitudinal width than the electrode fingers EF.
[0058] In FIG. 10A, with dielectric applied in dots or dielectric applied in the form of strips exclusively in the gap area, the boundary area between gap area GB and overlap area UB is a blurry area UBR in which the profiles of dielectric and metallization intersect. The blurry area UBR here is limited to a maximum transversal length of respectively 1 μm. In the exemplary embodiments according to FIGS. 10B and C, if the dielectric DK additionally extends beyond the border area, the bus electrode and the adjoining area outside of the transducer, the intersection of the dielectric with the overlap area UB is up to a maximum of 2 μm.
[0059] Given a DK.sub.F structured in the form of dots, as shown in plan view in FIG. 11, there may be a tradeoff between the maximum width of the blurry area UBR of the dielectric DK.sub.F with the edge of the electrode finger trailing off at the finger end and the longitudinal width of the dielectric dot DK.sub.F. A dot that is wider in the longitudinal direction may have a smaller blurry area UBR; by contrast to this, a narrower dot may have a larger blurry area UBR at the boundary with the overlap area UB.
[0060] Shown in FIG. 12, using schematic transversal sections directed through each electrode finger EF, is the embodiment in which the dielectric DK is applied chronologically before the metallization for the electrode fingers and the bus electrodes BE. Depending on manufacturing, the dielectric DK.sub.F, S, which may be applied as dots DK.sub.F or as strips DK.sub.S, has an edge profile that has a defined slope angle relative to the substrate. The metallization applied in a later step for electrode fingers EF and stub fingers SF adapts to the edge of the dielectric DK, and accordingly has an inverse edge profile matching this. Considered in plan view, here the boundary between gap area GB and overlap area UB can also not be clearly defined since an overlap between dielectric and metallization of the electrode finger EF is present in the blurry area UBR.
[0061] According to the invention, the value of this blurry area UBR is now set to a value that corresponds at most to the aforementioned limit values. A strip-shaped dielectric DK.sub.S which covers or fills the outer area and the gap area GB may thus cover the overlap area UB up to a blurry area UBR of at most 2 μm. A dielectric DK.sub.S which is applied in the form of strips exclusively in the gap area should cover the adjacent ends of stub fingers SF and electrode fingers EF with a blurry area of at most 1 μm each. If the dielectric DK.sub.F is applied in dot form exclusively in the gap, the two-sided overlap in the blurry area may thus likewise be set to a maximum of 1 to 2 μm.
[0062] The two different embodiments in FIGS. 12A and 12B indicate embodiments having different layer thickness ratios of dielectric DK and metallization for electrode fingers EF. It has been shown that a smaller blurry area EBR may be maintained with a smaller layer thickness of the metallization of the layer thickness of the electrode finger EF given unchanged layer thickness of the dielectric DK and unchanged edge angles. The smaller that the blurry area UBR is chosen to be, the more uniformly that the acoustic impedance of the entire structure may be set.
[0063] The invention could be explained only with reference to a few Figures and exemplary embodiments, but is not limited to these. The edge profile of the metallic and dielectric structures may especially deviate from the shown edge profiles depending on technology. Layer thickness ratios and other size ratios may be chosen differently than presented.
[0064] In all Figures, for better illustration the ratio of finger width to finger spacing is depicted larger than is typically chosen in transducers. Moreover, the invention is not limited to normal finger transducers in which electrode fingers EF alternately start from different bus electrodes BE in the overlap area UB. It is also possible to modify the connection sequence of the electrode fingers so that two or more different electrode fingers EF arranged in series start from the same bus electrode BE.
[0065] Moreover, it is possible to vary the transversal position of the gaps over the length of the transducer so that the gaps do not align in the longitudinal direction. In these instances, it is possible to structure a dielectric applied in a strip shape so that it follows the curve of the gaps. However, in this embodiment it may be especially advantageous to introduce the dielectric into the gaps exclusively in the form of dots. The structuring of the dielectric dots may then exactly follow the position of the respective gaps.
[0066] A transducer according to the invention, with dielectric applied in the gap area, is also not limited to the material combinations cited in the exemplary embodiments. For example, if a different conductive metal with deviating acoustic impedance is selected for the metallization, the dielectric is preferably also selected so that its acoustic impedance is adapted to that of the electrode material. For this, it may be necessary to choose a different dielectric than those specified.
LIST OF REFERENCE SIGNS
[0067] BE bus electrode
[0068] EF electrode finger
[0069] GP gap
[0070] SF non-overlapping electrode finger (stub finger)
[0071] DK dielectric
[0072] UB (transversal) overlap area
[0073] DK.sub.S strips (of dielectric)
[0074] DK.sub.F dots (of the dielectric)
[0075] GB (transversal) gap area
[0076] RB (transversal) border area
[0077] REF reflector
[0078] RF reflector finger
[0079] UBR blurry area
[0080] X, y, z spatial directions
