JPH0423444B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is a surface acoustic wave, that is, a Rayleigh wave.
This article relates to the electrode structure of surface acoustic wave multimode filters that utilize shear waves (SSBW), which are SAW and SH waves.

(Prior Art) Several types of multimode filters using surface acoustic waves have been proposed in the past, but the most basic form is a pair of interdigital filters on a piezoelectric substrate 1, as shown in Figure 1a.・By arranging transducer (IDT) electrodes (resonators) 2 and 3 in parallel and close to each other at right angles to the propagation direction of the surface acoustic waves they excite, mutual interaction between the excited surface acoustic waves is achieved as shown in Figure b. A filter is constructed by utilizing two modes of vibration with different resonant frequencies (see figure c), called the symmetric mode (resonant frequency fs) and the antisymmetric mode (resonant frequency fs), which are generated as a result of acoustic coupling. It is something.

In this case, a filter is obtained in which fa is the center frequency and fs - fa = Δf is approximately half the width of the passband, and Δf is known to be controlled by the degree of acoustic coupling (Japanese Patent Laid-Open No. 59-131213). Publication, Special Publication No. 2-16613
(see publication). Further, reflectors are provided on both sides of the IDT electrodes 2 and 3 in order to increase the Q of the IDT electrodes (resonators) 2 and 3 arranged in parallel and close to each other and to more efficiently confine vibration energy within each IDT. It is also well known that this is realistic.

By the way, the inventors of the present application have conducted SAW regarding the acoustic coupling generation conditions between the resonators 2 and 3 arranged in close proximity.
According to an experimental study using a resonator, if a rotating Y-cut crystal substrate is used, the intersecting width w of the IDT electrode fingers of both resonators 2 and 3 is equal to the width of the IDT electrode sandwiching the common bus bar 4 between them. It depends on the finger non-crossing width g, and in particular, the value of this g is at most the excited SAW wavelength λ.
It turned out to be about 3 times or less. Under such conditions, assuming that the width of the common busper 4 is 100MHz at the center frequency and 1λ in the width of g,
It is necessary to set it to around 20 μm, and this means that the common bus bar 4 (this is always grounded).Since input/output terminals A and B are essential for the filter, the common bus bar 4 must be installed, and the ground terminal C must be connected somewhere on the common bus bar 4. (It is well known that the input and output terminals of the filter are interchangeable.) Not only is it not possible to directly bond the grounding lead wire to the SAW resonator 2, but also If the number of IDT electrode finger pairs is increased from 400 to 800 in order to increase the Q of , 3, or if a reflector is attached as described above, the common bus bar 4 will become long and its inherent loss will increase. It was clear that this resulted in an increase in the insertion loss of the filter itself.

(Objective of the Invention) The present invention has been made to solve the problems of surface acoustic wave multiplexing as described above, and it facilitates the connection between the common bus bar and the grounding lead wire, and also reduces the length of the common bus bar. The purpose is to reduce the inherent loss associated with this process.

(Summary of the Invention) In order to achieve the above-mentioned object, the surface acoustic wave multimode filter according to the present invention extends the common bus bar into the reflector along the propagation direction of the excited surface acoustic wave and widens its width. It is designed to gradually increase toward the reflector.

(Example) Hereinafter, the present invention will be explained in detail using examples shown in the drawings.

FIG. 2 is an electrode configuration diagram showing one embodiment of the SAW dual mode filter according to the present invention, in which a pair of electrodes formed on the voltage substrate 1 have a relatively small number of pairs.
The common bus bar 7 of IDT electrodes 5 and 6 is connected to both IDTs.
The reflectors 8, 9 provided on both sides of the electrodes 6, 7 are extended across the gratings (metal strips) of the reflectors 8, 9 without being connected to them, and their widths are gradually increased within these gratings. The grounding wire is to be bonded at the widest portions at both ends of the reflectors 8 and 9. In addition, both ends of the grating constituting the reflectors 8, 9, which face the extended portion of the IDT electrode common bus bar 7 in a non-connected manner and on the opposite side thereof, are connected to the non-common bus bars 10, 10 and 11, 11 (short-circuit type ).

By doing this, the only part where the width of the common bus bar 7 is extremely small is the part that connects the IDT electrodes 5 and 6, and the other parts have a large width even though the length is large. The ohmic loss of the bus bar 7 can be kept sufficiently small.

However, in the electrode configuration described above, the ground terminals C ₁ and C ₂ of this filter are located near the opposite sides of the chip and are connected to the pads for connecting to the external leads exposed inside the package that houses the chip. Although this is convenient, the input/output (hot) terminals A and B of this filter are also distributed on opposite sides of the legs, so the flow line of the head of the wire bonder becomes long.

The second embodiment shown in FIG. 3 eliminates this defect. That is, in order to alleviate this defect, an extension of the common bus bar of the IDT electrodes extending into the reflectors 8, 9 and non-common bus bars 10, 10 and 11, 11 of the grating aligned on both sides thereof are provided.
Both side ends are electrically opened. That is, non-common bus bars 10, 10 and 11, 11 as shown in FIG.
It is effective to disconnect the grating from the grating (open type).

Then, as shown in FIG. 3, the IDT electrodes 5, 6
One end of each of the non-common bus bars may be extended in the direction in which the reflector extends, and the ends may be used as input/output hot terminals A and B.

By doing this, the input/output hot terminal A and the ground terminal C ₂ and the other hot terminal B and the other ground terminal C ₁ form a pair and are concentrated at both ends of the chip, making it easier to wire bond the head. It is possible to shorten the flow line and increase production efficiency during mass production.

In the above two embodiments, one end of the reflector grating was separated from the non-common bus bars 10 and 11 as shown in FIG. 2. In the embodiment shown in FIG. 2, the grating was simply made of metal. This is because when forming the grating by etching the film, it is easier to form a desired pattern in this form than in the case where both ends of the grating are short-circuited by bus bars.

In addition, there are two types of reflectors consisting of metal gratings: open type and short type.
There is a type, but the difference between the two is, for example, the crystal substrate and
In the combination of Al electrodes, when the SAW excited by the IDT electrode hits the reflector and grating and is reflected, the phase of the wave is 90 degrees in the open type compared to that of the wave emitted by the IDT. The delayed shot type has the property of advancing 90 degrees.

Therefore, in a SAW resonator etc. that has a reflector, the gap between the IDT electrode and the reflector must be adjusted so that the phases of the waves are aligned (although adjustments may be made for other reasons, including these) ) is common;
As long as this adjustment is made, there is no electrical or acoustic difference between open and short reflectors.

Also, the presence of a common bus bar with a gradually increasing width in the reflector may have some effect on the SAW reflection characteristics of the reflector, but as a result of experiments, no particular effect was observed on the filter characteristics. Ta. Furthermore, the shape of the bus bar that crosses this reflector may be triangular, or it may have a profile that is part of an ellipse whose major axis is the SAW propagation direction and whose minor axis is the width direction of the reflector perpendicular to this. There won't be much difference.

The present invention has been explained above using a SAW multimode filter as an example, but the present invention also uses SAW as a surface acoustic wave.
It is not necessarily limited to those who use IDT.
Other waves that can be excited by the electrode, such as -50 degrees or +40 degrees, can be propagated inside near the substrate surface by attaching an Al electrode to the Y-cut crystal substrate surface.
SSBW, by attaching electrodes made of material such as Au to the Y-cut LiNbO ₃ substrate, Love waves propagate just below the substrate surface, and by attaching Al electrodes to the 36 degree Y-cut LiTaO ₃ substrate. It can be similarly applied to filters that utilize Brewstein-Gourier-Shimizu waves that propagate directly beneath the substrate surface, and in this case, the required acoustic coupling can be determined based on differences in characteristics such as the electromechanical coupling coefficient of the substrate. To obtain this, the parameters w, g, etc. shown in FIG. 1 may be adjusted.

(Effects of the Invention) Since the present invention is constructed as described above, it is possible to reduce the common loss caused by the extremely fine common bus bar in a surface acoustic wave multimode filter, and also to eliminate this loss. Since the lead terminal pad for grounding can be easily formed, there is a significant effect in reducing the insertion loss of the filter and facilitating its manufacture.
Furthermore, by connecting the non-common busbars of the IDT electrodes and reflectors, it is possible to form a pair of input/output terminals and ground terminals and concentrate them rotationally symmetrically on the two peripheries of the chip. Therefore, it is possible to mount this filter on a printed board. This also has the effect that there is no need to consider directionality when doing so.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the principle of the surface acoustic wave multimode filter that is the basis of the present invention, in which a is a diagram of its basic electrode configuration, b is a diagram explaining the state of acoustic coupling, and c is the result of acoustic coupling. FIG. 2 is an electrode configuration diagram showing one embodiment of the present invention, and FIG. 3 is an electrode configuration diagram showing a second embodiment. 1... Piezoelectric substrate, 2, 3, 5 and 6... IDT electrode, 4, 7... Common bus bar, 8, 9... Reflector.

Claims (1)

[Scope of Claims] 1. A plurality of interdigital transducer (IDT) electrodes are disposed on a piezoelectric substrate at right angles to the propagation direction of the surface acoustic waves excited by these electrodes and in parallel with each other, and A reflector that reflects the surface acoustic waves excited by the electrodes is arranged at both ends of the IDT electrode, and the width of the common bus bar of the plurality of IDT electrodes is set to reflect the surface acoustic waves excited by the electrodes. In the surface acoustic wave multimode filter, which is made small enough to produce the required acoustic coupling with each other, the common bus bar of each of the adjacent IDT electrodes is crossed without being connected to the grating of the reflector. The width of the grating is gradually increased to ground both ends of the grating, and both ends of the grating opposite to the extension of the IDT electrode common bus bar extending in the grating are electrically connected to each other. An electrode structure of a surface acoustic wave multimode filter characterized by short-circuiting. 2. A plurality of interdigital transducer (IDT) electrodes are arranged close to each other in parallel to each other at right angles to the propagation direction of the surface acoustic waves they excite on a piezoelectric substrate, and at both ends of the IDT electrodes. A reflector is arranged to reflect the surface acoustic waves excited by the electrodes, and the width of the common bus bar of the plurality of IDT electrodes is arranged so that the surface acoustic waves excited by the electrodes are coupled with each other as required. In a surface acoustic wave multimode filter that is small enough to produce During grating, the width is gradually increased, and the terminal end at the reflector end is electrically grounded, and both ends of the grating are opened on the side opposite to the side connected to the IDT electrode common bus bar extension. The electrode structure of a surface acoustic wave multimode filter is characterized by: 3 The surface acoustic wave excited by the IDT electrode is
SAW, SSBW, Love Wave, or Bluestein
The surface acoustic wave multiple mode according to claim 1 or 2, characterized in that it is a Gourier-Shimizu wave.
Filter electrode structure.