JPH0427727B2 - - 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, called symmetric mode (resonant frequency s) and antisymmetric mode (resonant frequency s), which are generated as a result of acoustic coupling (see figure c). It is something.

In this case, a filter is obtained in which a is the center frequency and s-a = Δ is approximately half the width of the passband, and Δ is known to be controlled by the degree of acoustic coupling (Japanese Patent Laid-Open No. 59-131213). Official Gazette, Special Publication 2-
(See Publication No. 16613). In addition, the above-mentioned parallel arrangement
In order to increase the Q of the IDT electrodes (resonators) 2 and 3 and confine vibration energy more efficiently within each IDT, the
It is also well known that it is practical to provide reflectors on both sides of the IDT electrodes 2,3.

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 bus bar 4 is 100MHz at the center frequency and 1λ in the width of g,
This means that the common bus bar 4 (this must be grounded).Since input/output terminals A and B are essential for the filter, the common bus bar 4 must be grounded, and the ground terminal C must be connected somewhere between the common bus bar 4 and the ground terminal C. (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.

(Object of the Invention) The present invention provides surface acoustic wave multiple mode as described above.
This was developed to solve the problems associated with filters, and its purpose is to facilitate the connection between the common bus bar and the grounding lead wire, and to reduce the inherent loss caused by the increase in the length of the common bus bar.

(Summary of the Invention) In order to achieve the above-mentioned object, the surface acoustic wave multimode filter according to the present invention has grooves or grooves constituting reflectors along the propagation direction of the surface acoustic waves that excited the common bus bar. The interior of the electrically independent metal strip extends without intersecting or connecting with these metal strips, and its width gradually increases toward both ends of 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 an embodiment of the SAW dual mode filter according to the present invention, in which a pair of electrodes formed on a piezoelectric substrate 1 has a relatively small number of pairs.
The common bus bar 7 of IDT electrodes 5 and 6 is connected to both IDTs.
Reflectors 8 and 9 provided on both sides of electrodes 6 and 7 are electrically independent metal strips 10 and 1, respectively.
0, ... and 11, 11, ... and extends without connecting thereto, and the reflectors 8,
9, with the width gradually increasing towards both ends.

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 inherent loss of the bus bar 7 can be kept sufficiently small, and one end of the bonding wire is directly connected to the wide parts C ₁ and C ₂ (ground terminals) at both ends of the common bus bar 7, and the other end is not shown in the figure. Since the common bus bar 7 can be bonded to the pad for connection with the external lead exposed on the inner surface of the package, it is now easier to ground the common bus bar 7, which has been difficult in the past. In addition,
In the figure, A and B are input/output hot terminals.

Although the presence of a common busbar with a gradually increasing width in the reflector may have some effect on the SAW reflection characteristics of the reflector, the results of the experiment did not show any particular effect on the filter characteristics. Furthermore, the shape of the bus bar that crosses this reflector may be triangular, or the profile may be a 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. But it probably won't make much of a difference.

By the way, in the above embodiment, the metal strips 10, 10, and 1 constituting the reflectors 8, 9
1, 11... are separated from the common bus bar 7 to make them completely independent (referred to as open reflectors). The reason for this is that when forming the reflectors 8, 9 by etching, the metal This is because the metal film between the strips 10, 10, . . . and 11, 11 . . . can be easily melted, and a reflector having a desired shape can be formed accurately. In addition, in the case of an open reflector as described above, for example, in the case of a combination of a crystal substrate and an Al strip, when the SAW excited by the IDT electrode is reflected, the phase of the wave is different from that of the SAW that emitted the IDT. It has the property of being delayed by 90 degrees. On the other hand, a so-called short-circuit reflector in which at least one end of the metal strips is connected to each other by a bus bar, the phase of the reflected wave is SAW that emitted IDT.
It exhibits different properties from open reflectors in that it travels 90 degrees compared to that of an open reflector.

The above is an example of a SAW device with an IDT electrode and a reflector made of a metal strip on a quartz substrate.
Although the present invention has been described with reference to a multi-mode filter of the type that excites It is clear that even if a groove is formed, the effect as a reflector is equivalent to that of the open metal strip described above. In this case, the reflector would obviously be open, since the groove is just a groove. Furthermore, the type of surface acoustic waves excited in the piezoelectric substrate is not limited to SAW only, but also other waves that can be excited by the IDT electrode, such as -50 degrees or +40 degrees Y cut Al on the surface of the crystal substrate. SSBW propagates inside near the substrate surface by attaching electrodes, Y-cut LiNbO ₃
By attaching electrodes made of material such as Au to the substrate, Love waves propagating just below the surface of the substrate and 36
By attaching an Al electrode to a Y-cut LiTaO ₃ substrate, Bluestein propagates just below the substrate surface.
It can be similarly applied to filters that utilize Gourier-Shimizu waves, and in that case, the method shown in Figure 1a is used to obtain the required acoustic coupling based on differences in characteristics such as the electromechanical coupling coefficient of the substrate. The parameters w, g, etc. 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.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the principle of a conventional surface acoustic wave multimode filter, which is the basis of the present invention.
2 is a diagram showing the basic electrode configuration, b is a diagram explaining the state of acoustic coupling, c is a diagram showing two wave modes generated as a result of acoustic coupling, and FIG. 2 is an electrode configuration diagram showing one embodiment of the present invention. It is. 1... Piezoelectric substrate, 2, 3, 5 and 6... IDT electrode,
4, 7... common bus bar, 8, 9... reflector, 10,
11...Metal strip or groove.

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 At both ends of the IDT electrode, grooves or reflectors of metal strips, each of which is electrically independent, are arranged to reflect the surface acoustic waves excited by the electrode, and the plurality of
In a surface acoustic wave multimode filter in which the width of the common bus bar of the IDT electrode is made small enough to cause the surface acoustic waves excited by each of the electrodes to mutually produce the required acoustic coupling, each of the adjacent
A common busbar of the IDT electrodes extends across the groove or metal strip of the reflector without intersecting or connecting it, and its width is extended in the groove or metal strip of the reflector. 1. An electrode structure for a surface acoustic wave multi-mode filter, characterized in that the electrode structure increases gradually toward both ends, and both ends are grounded. 2 The surface acoustic wave excited by the IDT electrode is
SAW, SSBW, Love Wave, or Bluestein
The electrode structure of a surface acoustic wave multimode filter according to claim 1, characterized in that the electrode structure is a Gourier-Shimizu wave.