WO2006136282A1

WO2006136282A1 - Saw structure with stub fingers

Info

Publication number: WO2006136282A1
Application number: PCT/EP2006/005375
Authority: WO
Inventors: Markus Hauser; Ulrike RÖSLER; Werner Ruile
Original assignee: Epcos Ag
Priority date: 2005-06-23
Filing date: 2006-06-06
Publication date: 2006-12-28
Also published as: DE102005029249A1

Abstract

A SAW structure comprises, in the serial signal path between in input gate and output gate, a serial resonator (Rs) with stub fingers (SF), which have a stub finger length of four to eight times the period p of the converter. The SAW structure can have a parallel resonator (Rp), which is placed in a branching parallel path and which has stub fingers whose length is four to six times the period p of the converter.

Description

description

SAW structure with stubby fingers

The invention relates to a SAW structure, as it is part of an RF filter, for example. A SAW structure has at least one interdigital transducer. This comprises two telescoped sub-electrodes with electrode fingers. At the current busbar (busbar) of a partial electrode, electrode fingers are arranged in a transducer with a regular normal finger structure at longitudinal positions at intervals of one period p. In this case, each finger on a first subelectrode can correspond to a short stub finger hanging on the second subelectrode and relatively short, which is separated from the electrode finger by a gap. A transducer may be arranged in a resonator whose acoustic track is bounded on both sides by reflectors.

The general goal of SAW structures, in particular of SAW filters, is to provide components with high quality and thus high frequency accuracy, which show only small losses and occupy only a small chip area on the piezoelectric substrate.

This is achieved in resonator filters, in particular through the use of reflections that occur within the SAW structure. As a result of these reflections, the chip surface can be used several times by the acoustic signal and, at the same time, unwanted acoustic losses can be avoided. For transversal filters, on the other hand, the time signal is given by the length of the chip, so reflections here only as Disturbing factor occur, which is suppressed by appropriate measures, such as split fingers.

It is known that in SAW structures waveguides can be realized by suitable choice of the propagation conditions in the transverse direction, which guide the acoustic wave in a track. This spreadable guided mode thus avoids diffraction losses and therefore is desirable to achieve low losses.

A particular difficulty in the realization of a waveguide, however, consists in substrates with leaky-wave sections, since an acoustic wave which has too large an angle deviating from the main propagation direction suffers increased leakage wave losses. In the present invention, it will be shown how, by using certain stub fingers, a transverse design of the waveguide for leaky waves with particularly low propagation losses at the antiresonant frequency can be realized.

In general, the quality of a transducer with internal reflections, which serves for coupling or decoupling an acoustic surface wave, increases with its length or number of electrodes. In order to achieve a constant static capacity of the structure at the same time, the aperture is often reduced, which leads to increasing diffraction losses and thus to undesirable attenuation.

The diffraction in a transducer can be reduced by increasing its aperture, ie the transverse region in which overlaps occur. However, a larger aperture has the disadvantage that with increasing aperture higher propagation modes, so-called transversal modes with their own resonant frequency, can form, which Negative influence on transmission behavior and lead in particular to a bad passband with ripples With increasing aperture but also increase - regardless of transversal modes - additionally the electrical losses, and thus the insertion loss of a filter structure.

It is known to use stub finger to achieve a more homogeneous exposure in the edge region in the photolithographic generation of SAW transducers advantageous.

It is also known to use the above-mentioned stub fingers in transducers in order to improve the waveguide behavior of transducers. The stub fingers are arranged outside the actual excitation range, ie outside the aperture, conduct the wave leaking from the excitation region and thus reduce losses in the propagation of the SAW. It is known to perform such stub finger up to the length of about one acoustic wavelength. Longer stub fingers are avoided because they lead to additional ohmic losses in the converter.

From DE10135871A it is known to use stub fingers in a transducer having a periodic change in length as seen over the length of the transducer. Thus, a secondary weighting can be generated in the converter, which does not affect the actual transfer function, but can suppress disturbing suggestions.

The object of the present invention is to provide a SAW structure with a transducer which is improved in terms of low diffraction, high quality and low losses. This object is achieved by a SAW structure with the features of claim 1. Advantageous embodiments of the invention will become apparent from the dependent claims.

It is proposed to arrange a serial resonator with stub fingers in the serial signal path connecting the input and output ports of the SAW structure. The length of the stub finger corresponds to four to eight times the period p of the transducer. Period p is understood here and below to mean the distance between two adjacent electrode fingers.

Surprisingly, it has been shown that with such long stub fingers an improved overall behavior of the SAW structure is obtained. Considering, for example, a single resonator, a straightened admittance curve is obtained with the inventively extended stub fingers, in which an unevenness in the admittance curve has disappeared compared with an otherwise identically constructed reference resonator. Although the quality of the resonance of this series resonator is slightly worsened compared to the reference resonator, the quality of the antiresonance, which is of great importance for the filter behavior, in particular when used in a reactance filter, increases. With regard to the antiresonance, it is also not disadvantageous if the ohmic resistance of the transducer increases with longer stub fingers.

The SAW structure can, in addition to the series resonator, also have a parallel path branching off from the serial path and connected to ground, in which a parallel resonator is arranged. This too, in comparison to the prior art, can have long stub fingers whose length corresponds to four to six times the period p. The optimal The stub finger length of a parallel resonator is slightly shorter than that of a serial resonator, especially the upper limit, to which advantages for the overall behavior of the SAW structure can still be expected. It is thus advantageous, in a SAW structure, to make the stub fingers of the parallel resonator shorter than the stub finger length in the serial resonator. A serial resonator and a parallel resonator arranged in the immediate vicinity together form a basic element of a reactance filter which already has a passband and thus exhibits filter behavior. In such a reactance filter, the long stub fingers, which improve the overall filter behavior, can advantageously be used both in the serial and in the parallel resonator.

It has been found that the large stub finger lengths are particularly advantageous when the aperture of the transducer becomes smaller. It succeeds in reducing the negative effects of diffraction effects within the transducer. Although smaller apertures are possible, it is advantageous to set the aperture to a value at least equal to nineteen times the acoustic wavelength and thus 38 times the finger period.

It is now advantageous if in this mode, which results in this choice of stub finger lengths, can be coupled electrically well, on the one hand to avoid diffraction losses and on the other hand to realize the smallest possible static capacitance, which in terms of bandwidth and the adaptability of Advantage is.

In order to achieve a good coupling, the amplitude of the excitation in the transverse direction should be as good as possible cling to the amplitude of the propagating mode in the transverse direction. This can be achieved by various measures, such as by a variation of the fin width in the transverse direction, which leads, for example, to a high excitation in the center of the aperture and to a weakening of the excitation at the edge. At high frequencies, however, the finger widths are so small that such a solution can not be realized technologically well.

The solution proposed here achieves a weighting in the edge region in that a plurality of electrodes are viewed together, and by reducing (shortening or transversing) some fingers a transverse weighting is achieved, which approximates the propagating mode in the transverse direction.

A further improvement of the SAW structure is thus obtained if the transverse position of the gaps over the length of the transducer measured in the longitudinal direction is preferably varied regularly with a gap period gp different from twice the period p, so that the stub finger length is at least a partial electrode is changed according to the set variation.

The variation of the gap position is preferably carried out regularly over the entire transducer length, wherein the strength of the variation, ie the deviation from the average gap position, preferably also remains the same over the entire transducer length. Advantageously, the gap position varies at both sub-electrodes, wherein both gap periods at the two sub-electrodes are preferably the same, but may also be different. It is possible to perform the gap variation on both sub-electrodes with the same phase, whereby a constant overlap length results for each electrode finger with the respectively adjacent electrode finger associated with the other sub-electrode. It is advantageous, however, to make the period of gap variation on the two sub-electrodes of the transducer phase-shifted, so that at the same time a varying overlap length of the electrode fingers is obtained. The deviation of the overlap length also follows the gap period gp.

A further modified SAW structure has short-circuit structures, with each of which several adjacent stub fingers of the same sub-electrode are short-circuited. The short-circuit structure is formed, for example, as a metallization strip which runs substantially parallel or obliquely to the current busbar connecting the electrode fingers of a partial electrode. Such a short circuit structure may connect a number of stub fingers that is less than the number of a stub finger arranged within a gap period. These short-circuit structures reduce the ohmic losses of the converter.

The variation of the transverse gap position preferably takes place with a maximum amplitude with respect to the transverse positions a distance of approximately one to three periods p, which corresponds to the length of one to three half acoustic wavelengths.

The serial and parallel resonators may each comprise one or more transducers. The converters are advantageous arranged between two reflectors, which may consist of an open or electrically shorted stripe pattern. Preferably, the reflectors are connected to ground or one of the two transformer busbars.

In the following the invention will be explained in more detail by means of exemplary embodiments and the associated figures. These serve only to illustrate the invention and are therefore designed only schematically and not to scale. Identical or equivalent parts are designated by the same reference numerals.

FIG. 1 shows a transducer with long stub fingers,

FIG. 2 shows a SAW structure with a serial and a parallel resonator,

FIG. 3 shows a detail of a converter with a varied position of the gaps and the associated gap period.

FIG. 4 shows a detail of a converter with gap period and short-circuit structure,

FIG. 5 shows a converter with phase offset of the gap variation carried out on both partial electrodes,

FIG. 6 shows a SAW structure with a serial resonator,

FIG. 7 shows a SAW structure with three resonators designed according to the invention. Figure 1 shows a detail of an interdigital transducer which is part of a resonator of the SAW structure according to the invention. The transducer consists of two partial electrodes TE1, TE2, from which electrode fingers EF extend directed against each other and wherein both partial electrodes are pushed into each other like a comb. Each long electrode finger on a first subelectrode is associated with a relatively short stub finger SF on the opposite second subelectrode, and vice versa, with electrode fingers and associated stub fingers having the same longitudinal position along the x axis parallel to the length of the transducer. Electrode fingers and stub fingers are separated by a gap G. All gaps of the stub finger associated with a partial electrode TE are arranged at the same height in the transverse direction, so that the same length results for all stub fingers of a partial electrode. The same applies to the second subelectrode, so that the same electrode finger length EFL results for all electrode fingers with the same gap size and the same overlap length for two adjacent, different subelectrode electrode fingers. The maximum distance between two gaps in the transverse direction corresponds to the aperture Ap, which in the exemplary embodiment illustrated corresponds to the uniformly identical overlap length UL.

The transducer is designed as a normal finger transducer, so that the period p of the electrode fingers corresponds to the half-distance measured in the longitudinal direction of two stub fingers on a partial electrode.

A transducer as shown in Figure 1 can be used directly as a resonator in a SAW structure, but is preferably arranged between two reflectors (not shown in Figure 1).

FIG. 2 shows a SAW structure in which a serial resonator R _{3 is arranged} between two connections in the serial path associated with the input and output ports of the SAW structure, and a parallel resonator R p which is in a path branching from the serial path is arranged and connected here, for example, to ground. Both resonators have long stub fingers, wherein the stub finger length of the parallel resonator Rp is selected to be smaller than that of the serial resonator R ₃ .

Figure 3 shows a fragmentary view of a transducer which is part of a resonator in which the gap position varies over the length of the transducer. The lengths SFL of the stub finger SF also vary accordingly. A gap period gp here comprises three adjacent stub fingers, with the variation pattern repeated in each gap period.

The maximum gap variation GV _max , ie the maximum transverse distance between two gap positions on a subelectrode, was selected here for three finger periods p corresponding to one to three half acoustic wavelengths of the transducer. Advantageously, GV _max . In the range between 0.5 and 4 acoustic wavelengths of the converter selected (or to 1-8 periods p).

FIG. 4 shows a detail of a converter with a varying gap position, in which the stub fingers SF associated with a gap period gp are connected to one another via a short-circuit structure KS. This is here as a strip-shaped metallization parallel to the busbar SSl of the associated partial electrode aligned. It is also possible to guide the short-circuit structure KS at an acute angle to the busbar SS. The number of short-circuited with a short circuit stub finger corresponds here to the number of belonging to a gap period stub finger, but may also differ. Two short-circuit structures can have a lateral distance from one another, offset transversely from one another and also overlap one another. However, it is preferred to arrange the short-circuit structures regularly and preferably on both sub-electrodes.

FIG. 5 shows a detail of a converter with variation of the gap position made on both partial electrodes TE1, TE2. The variation is here carried out in opposite directions for both sub-electrodes, so that varying overlap lengths UL result, but which vary with the gap period gp. A gap period here comprises four electrode fingers EF or four neighboring stub fingers SF associated with a partial electrode TE.

FIG. 6 shows a SAW structure in which a serial resonator R _{s designed} according to the invention is connected in series with a filter F, which is embodied in an arbitrary filter technique. The filter can be, for example, a DMS filter, wherein the serial resonator in the serial path upstream or downstream of the filter. The filter may also consist of resonators connected together in a lattice or ladder-type arrangement. Filters or resonators can be implemented in SAW technology as well as in other techniques such as FBAR. The filter may also be a microwave ceramic filter or a dielectric filter. FIG. 7 shows a further variation in which a converter designed according to the invention is part of a DMS or MPR filter (multiport resonator filter) in which a plurality of resonators R 1, R 2, R 3 are arranged next to one another within the acoustic track. On both sides, the acoustic track of reflectors RFl, RF2 is limited. Each of the resonators can be designed in the same way with long stub fingers. However, it is also possible to make the stub finger lengths different in the different resonators. Preferably, the resonators are formed with gap variation.

The DMS structure shown as an example already represents a finished filter, but can be interconnected with further filter substructures, which are likewise designed as SAW structures. In particular, two DMS filters can be cascaded to form a two-track DMS filter. Further resonators or ladder-type basic elements can be upstream and downstream of the filter.

The invention has been described only with reference to a few embodiments and is therefore not limited to these. Deviating from the illustrated embodiments, many other variations are possible, the most general embodiment of which is indicated by the claims.

Claims

claims

1. SAW structure

- With an entrance gate and an exit gate, between which runs a serial signal path

- With a serial branch arranged in the serial resonator (R ₃ ) comprising a transducer with two telescoped sub-electrodes (TEl, TE2) with in a period p regularly arranged electrode fingers (EF), each electrode finger of a part electrode a relatively short stub finger (SF) is assigned to the opposite sub-electrode which is separated from the electrode finger by a gap (G),

- In which the stub finger length in the serial resonator corresponds to three to eight times the period p.

2. SAW structure according to claim 1, wherein a parallel resonator (R _P ) is arranged in a parallel path branched off from the signal path and connected to ground, having a stub finger length of four to six times the period p.

3. SAW structure according to claim 1 or 2, wherein the stub finger length of the parallel resonator (R _p ) is smaller than that of the serial resonator (R ₅ ).

4. SAW structure according to one of claims 1 to 3, wherein the transversal position of the gaps (G) over the length of the transducer measured in the longitudinal direction is regularly varied with a gap period (gp) different from the period p, such that the stub finger length varies correspondingly on at least one partial electrode (TE).

5. SAW structure according to claim 4, wherein the stub finger length is also varied at the second partial electrode (TE) with the gap period (gp).

6. SAW structure according to claim 5, wherein the gap period gp, with which the transverse position of the gaps (G) varies at the two sub-electrodes (TEl, TE2) in phase or out of phase is made so that differently long electrode fingers (EF ) whose lengths vary with the gap period.

7. SAW structure according to one of claims 1 to 6, wherein in each case a plurality of adjacent stub finger (SF) are short-circuited with a short-circuit structure (KS).

8. SAW structure according to claim 7, wherein the short-circuit structures (KS) are arranged regularly over the length of the transducer and in each case run obliquely or parallel to busbars (SS) of the sub-electrodes (TE).

9. SAW structure according to one of claims 1 to 8, wherein the aperture (Ap) of the resonator or resonators (R) is at least 19p, where p corresponds to the period of the electrode fingers on the corresponding transducer.

10. SAW structure according to one of claims 4 to 9, wherein a gap period comprises in each case 2 to 6 stub finger (SF).

11. SAW structure according to one of claims 4 to 10, in which the variation of the gap positions and the associated stub finger lengths comprises a change in length from Ip to 3p.

12. SAW structure according to one of claims 1 to 11, wherein at least one of the resonators (R) on both sides of an open reflector (REF) is limited, comprising electrically unconnected reflector strips.

13. SAW structure according to one of claims 2 to 12, wherein the series and the parallel resonators (R) of the SAW structure represent at least one basic element of a reactance filter.

14. SAW structure according to one of claims 1 to 13, wherein a parallel resonator is provided whose stub finger length corresponds to two to eight times the period p.

15. The SAW structure of claim 14, wherein the stub finger length of the parallel resonator is four to six times the period p.