JP3385169B2 - Surface acoustic wave multimode filter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interdigital transducer for exciting surface acoustic waves.
Further, the present invention relates to a surface acoustic wave multimode filter using this interdigital transducer.

[0002]

2. Description of the Related Art FIG. 4 shows the structure of a conventional interdigital transducer (hereinafter referred to as IDT). The IDT includes an excitation unit 101 that excites a surface acoustic wave and a signal terminal unit 102 that inputs or outputs an electric signal. Considering the propagation velocity of the surface acoustic wave propagating in each region, the velocity Vidt propagating in the excitation unit 101 is slower than the velocity Vbus propagating in the signal terminal unit 102. As a result, the excited surface acoustic wave is confined in the exciting unit 101. The amplitude distribution of the surface acoustic wave at this time is, as shown in FIG. 4, three modes (basic mode 104, higher order mode (first order) 105, higher order mode (2
Next) 106).

Next, the change of the propagation mode with respect to the width W of the excitation section 101 will be described. When the width W of the excitation unit 101 is sufficiently narrow, only the basic mode 104 among the modes shown in FIG. 4 appears. Here, when the width W of the excitation unit 101 is increased, the higher modes 105, 10 shown in FIG.
6 appear in order, and a plurality of modes are propagated together with the fundamental mode 104.

Since the propagation velocities of the higher-order propagation modes 105 and 106 are slightly different from the propagation velocities of the fundamental mode 104, when a surface acoustic wave filter is manufactured, unnecessary spurious will appear near the pass band. . Therefore,
In order to obtain a filter without unnecessary spurious, the width W of the exciting section 101 of the IDT had to be made sufficiently small. However, if the width W of the excitation unit 101 is reduced, the loss increases due to the diffraction effect, so it is very difficult to reduce the loss while suppressing spurious.

FIGS. 5 and 6 are block diagrams of a surface acoustic wave multimode filter in which the influence of higher-order propagation modes is remarkably exhibited. The surface acoustic wave multimode filter of FIG. 5 is referred to as a “laterally coupled multimode filter” because the IDTs of the surface acoustic wave resonators are arranged vertically with respect to the surface acoustic wave propagation direction. The surface acoustic wave multimode filter of FIG. 6 is a “longitudinal coupling type multimode filter” in which at least one input IDT and at least one output IDT are arranged in a direction parallel to the propagation direction of the surface acoustic wave. In both filters, the width W of the excitation unit 101 is designed to be relatively large in order to suppress the increase in loss due to the diffraction effect and to improve the impedance matching state with the external circuit. In this case, the spurious inevitably exists.

[0006]

Therefore, in the conventional surface acoustic wave multimode filter designed for the purpose of reducing the loss of the surface acoustic wave multimode filter and improving the impedance matching state with the external circuit, It is necessary to increase the width W of the excitation part of the IDT, and it is difficult to suppress spurious.

The present invention has been made in consideration of the above circumstances, and it is an object of the present invention to provide an interdigital transducer (IDT) capable of suppressing a higher-order propagation mode without reducing the width of an exciting part. And Another object of the present invention is to provide a laterally coupled multimode filter and a longitudinally coupled multimode filter in which spurious is suppressed by using this interdigital transducer (IDT).

[0008]

According to the present invention, there is provided an interdigital transducer formed on a piezoelectric substrate, the interdigital transducer including an exciting portion for exciting a surface acoustic wave and a signal terminal portion for inputting an electric signal, and a surface acoustic wave. Parallel to the direction of propagation
Close to both sides of the interdigital transducer
A surface acoustic wave resonator consisting of
Two close to each other in the direction perpendicular to the propagation direction of surface acoustic waves
Then, the two surface acoustic wave resonators are acoustically coupled by the coupling portion.
In a transversely coupled surface acoustic wave multimode filter for coupling
At least one of the two surface acoustic wave resonators
One of the interdigital transducers
However, the signal terminal portion is formed of a metal electrode film,
At least part of the metal electrode film is aligned in the propagation direction of the surface acoustic wave.
Has a plurality of slits, and propagates through the signal terminal
The velocity of the surface acoustic wave that propagates through the excitation part
An interdigital transformer slightly faster than the speed of the wave
Horizontally coupled elastic surface characterized by being a inducer
A wave multimode filter is provided.

The excitation portion [0009] of et al., Is composed of a plurality of electrode fingers which are arranged at regular intervals, said signal terminal portions, the ratio a / b of the width b of the distance a and the slit of the slit, It may be smaller than the ratio c / d of the width c of the electrode fingers of the excitation unit and the distance d.

Further, according to the present invention, the surface acoustic wave propagating through the coupling portion, the signal terminal portion and the excitation portion is expressed by the following equation: Vgap>Vbus> Vidt (wherein Vgap: propagation velocity propagating through the coupling portion, Vbus: (EN) Provided is a laterally coupled surface acoustic wave multimode filter having a propagation velocity satisfying the following relationship: Propagation velocity propagating in signal terminal part, Vidt: Propagation velocity propagating in excitation part).

Further, at least one input interface
For digital transducer and at least one output
Interdigital transducer and surface acoustic wave
Placed parallel to the propagation direction and placed reflectors on both sides
Longitudinal mode coupled surface acoustic wave multimode
A filter, said interdigital transformer
The transducer excites surface acoustic waves formed on the piezoelectric substrate.
The excitation section and the signal terminal section for inputting electrical signals.
, The input or output interdigital
At least one of the transducers has the signal terminal portion made of gold.
A metal electrode film, and at least one of the metal electrode films.
A plurality of slits lined up in the propagation direction of the surface acoustic wave
Having, the velocity of the surface acoustic wave propagating through the signal terminal portion,
Slightly lower than the velocity of the surface acoustic wave propagating in the excitation section
To be a fast interdigital transducer
A longitudinal coupling type surface acoustic wave multi-mode filter is provided.

FIG. 1 shows a block diagram of an interdigital transducer (IDT) according to the present invention. 1 in the figure
Is a piezoelectric substrate, 2 is an electrode finger that excites a surface acoustic wave,
3 is a signal terminal part. Here, a part of the signal terminal portion 3 has a grating structure having a plurality of slits in the propagation direction. When the signal terminal portion 3 has the grating structure as described above, the propagation velocity of the surface acoustic wave becomes slower than in the case of a uniform metal film. That is, the velocity difference between the surface acoustic waves propagating through the excitation section 4 and the signal terminal section 3 becomes small, and the propagation mode becomes difficult to be trapped. Therefore, the generation of the higher-order mode is suppressed. In order for the higher-order mode to exist, it is necessary to make the width of the excitation unit 4 sufficiently large.

2 and 3 are block diagrams of a "horizontal-coupling-type multimode filter" and a "longitudinal-coupling-type multimode filter", respectively, which are constructed by using the IDT. According to the surface acoustic wave multimode filter having such a configuration, it is possible to obtain filter characteristics in which spurious due to a higher order mode is suppressed.

The excitation section 4 and the signal terminal section 3 can be made of a metal material such as aluminum Al, gold Au, copper Cu, or titanium Ti. The excitation unit 4 of the IDT is composed of a plurality of electrode fingers 2 which are arranged at regular intervals, like the IDT which is normally used. The electrode finger 2 is a comb-shaped electrode electrically connected to the signal terminal portion 3, and these comb-shaped electrodes are alternately arranged at regular intervals as shown in FIG. 1 to generate surface acoustic waves. An exciting unit 4 for exciting is configured.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below based on the embodiments shown in the drawings. The present invention is not limited to this. First, consider the propagation mode of the surface acoustic wave in the conventional IDT shown in FIG. When the propagation velocity of the surface acoustic wave of the excitation unit 101 is slower than that of the signal terminal unit 102, the energy of the surface acoustic wave is confined in the excitation unit 101. At this time, when the mode of the vibration energy of the surface acoustic wave confined in the excitation unit 101 is approximated by the scalar potential φ, φ satisfies the following equation.

[0016]

[Equation 1]

Here, when the propagation velocity of the propagation mode represented by the scalar potential φ is Vmode, Vmo
de takes a value that satisfies the following equation.

[0018]

[Equation 2]

Using the above equation, the speed of the excitation part Vidt = 3
144m / s, signal terminal speed Vbus = 3156m
FIG. 7 shows the calculation result of the change in the velocity of the propagation mode with respect to the width of the excitation unit 101 when / s and the period λ of the IDT λ = 12.8 μm. In FIG. 7, the basic mode, the primary and the secondary
The calculation results for the following higher modes are shown. In the range where the width of the excitation unit 101 is sufficiently small as shown by a in FIG. 7,
Only the fundamental mode can exist, but it can be seen that higher-order modes appear as the width of the excitation part is increased. For example, in the range of b in FIG. 7, the first-order higher-order mode can propagate, and there are two propagation modes in total.

Next, FIG. 8 shows a change in the width of the excitation part 101 in which the first higher-order mode (first-order mode) appears when the surface acoustic wave propagation velocity of the signal terminal part 102 is changed. It can be seen that the width of the excitation portion 101 in which the first-order mode appears is increased by delaying the surface acoustic wave of the signal terminal portion 102. As a result, the propagation speed of the surface acoustic wave in the signal terminal portion 102 is slowed down to approach the propagation speed of the excitation portion 101, whereby the higher-order mode is suppressed.

Next, as in the present invention shown in FIG.
Consider the case where is a grating structure. Generally, the velocity of the surface acoustic wave propagating through the grating structure portion composed of the metal electrodes is slower than the velocity of the surface acoustic wave propagating through the uniform metal film portion. Therefore, if the velocity of the surface acoustic wave propagating through the signal terminal portion is reduced by using the grating structure for the signal terminal portion 3, the width of the exciting portion 4 in which the higher-order mode appears becomes large and the higher-order mode is suppressed. It will be.

That is, if the velocity of the surface acoustic wave propagating in the signal terminal portion 3 is made slightly higher than the velocity of the surface acoustic wave propagating in the excitation portion 4, the generation of the higher order mode surface acoustic wave in the IDT. Can be suppressed.

By the way, in order for the excited surface acoustic wave to be confined in the exciting portion 4, the velocity of the surface acoustic wave propagating in the exciting portion 4 must be slower than the velocity of the surface acoustic wave propagating in the signal terminal portion 3. is necessary. If the signal terminal portion 3 of the IDT has a grating structure and the metal material and film thickness of the signal terminal portion 3 are the same as the material and film thickness of the excitation portion 4, the surface acoustic wave can be confined in the excitation portion 4. , The ratio (a / b) between the distance a and the width b of the slits of the grating structure of the signal terminal portion 3 must be made smaller than the ratio (c / d) between the width c and the distance d of the electrode fingers of the excitation portion. . This is because the velocity of the surface acoustic wave propagating through the grating structure of the signal terminal portion 3 increases as the width b of the slit increases.

If a transversely coupled multimode filter and a longitudinally coupled multimode filter are manufactured using the interdigital transducer having such a structure, the generation of surface acoustic waves of higher modes can be suppressed, so that spurious A suppressed surface acoustic wave multimode filter is obtained. Examples of the surface acoustic wave multimode filter will be described below.

[0025]

【Example】

Example 1 FIG. 9 shows an example of a “laterally coupled multimode filter” manufactured by using an interdigital transducer (IDT) 11 in which a signal terminal portion 3 has a grating structure on an ST cut quartz substrate. This filter consists of two surface acoustic wave resonators. IDT11 electrode period λ: = 24 μm, excitation part 4 electrode finger line width: c = 6 μm, excitation part 4 electrode finger line interval: d = 6 μm, electrode film thickness: 0.76 μm, IDT electrode pair number: 150 pairs , The number of electrodes of the reflector: 120, the width of the coupling part: 0.3λ, the width of the excitation part of the IDT: 8.5λ, part of the signal terminal part 3 and the signal terminal part of the reflector 12 also have a grating structure , Slit spacing (a) / width (b)
Are 3.6 μm / 8.4 μm, respectively.

Such a surface acoustic wave resonator was cascade-connected in two stages at the coupling section 5 to produce a filter having a center frequency of about 130 MHz. Here, the electrode finger spacing d of the excitation unit 4
By increasing the width b of the slit, the surface acoustic wave is confined in the excitation section 4. The pass characteristic of the laterally coupled multimode filter of the first embodiment is
It is shown in FIG. For comparison, FIG. 10B shows characteristics when the signal terminal portion 3 is made of a uniform metal film with the same width of the excitation portion 4 of 8.5λ. Figure 10 (b)
It can be confirmed that the spurious circled with is suppressed in FIG. 10 (a).

By the way, in FIG. 9, the coupling portion 5 of the IDT has a uniform "solid film portion" and a "solid film portion".
Is composed of a portion where the electrode fingers in the vicinity of do not intersect.
In FIG. 9, the velocity (Vmetal) of the surface acoustic wave propagating through the “solid film” of metal is 3147 m / s, and the velocity (Vidt) of the surface acoustic wave propagating through the excitation unit 4 is 3082 m / s.
s, the velocity of the surface acoustic wave propagating through the signal terminal portion 3 (Vbu
s) is 3091 m / s. Further, the average velocity (Vgap) of the surface acoustic wave of the coupling portion 5 including the solid film portion and the portion where the electrode fingers do not intersect is 3114 m / s.

Therefore, in the embodiment of FIG. 9, Vmet
The relation of al>Vgap>Vbus> Vidt is established. That is, when such a speed relationship is established, it is possible to suppress the spurious as described above.

Further, as shown in FIG. 11, the coupling portion 5 may be electrically separated, and may be composed of the comb electrode portion 51, the solid film portion 52, and the free surface 53 as shown in FIG.
The effect of suppressing spurious is obtained. The free surface is a surface where the piezoelectric substrate is exposed without a metal film.
Here, the width of the comb electrode portion 51 is 2 μm and the width of the solid film portion 52 is 2 μm.
μm, and the width of the free surface 53 is 20 μm.

At this time, Vidt (= 3082 m /
s), Vbus (= 3091 m / s), Vmetal
(= 3147m / s) does not change, but 3 as above
The velocity Vgap of the surface acoustic wave of the coupling portion 5 including the three portions (51, 52, 53) is 3148 m / s. Therefore, the relationship of Vgap>Vmetal>Vbus> Vidt is established, but the same characteristics as in FIG. 10A are obtained, so that spurious can be suppressed in this case as well.

Example 2 FIG. 12 shows an example in which a "longitudinal coupling type multimode filter" is produced on an ST cut quartz substrate using an interdigital transducer having a signal terminal portion 3 having a grating structure. The filter configuration is a 2-input 1-output type longitudinally coupled multimode filter. IDT electrode period λ: 12.6 μm, number of electrode pairs of the first IDT 21 for input: 100 each
Pair, number of electrode pairs of second IDT 22 for output: 160 pairs, number of electrodes of reflector 23: 120, distance m between electrodes of input / output IDT: 0.25λ, opening length n: 700 μm, signal terminal portion 3 of IDT Slit width: 4.41 μm, IDT signal terminal portion 3 slit spacing: 1.89 μm IDT excitation portion 4 electrode finger width: 3.2 μm, IDT excitation portion 4 electrode finger spacing: 3.2 μm The width of the electrode fingers of the excitation unit 4 of the reflector is 3.2 μm, and the distance between the electrode fingers of the excitation unit 4 of the reflector is 3.2 μm.

The pass characteristic of the longitudinally coupled multimode filter at this time is shown in FIG. For comparison, FIG. 13B shows characteristics when the signal terminal portion 3 of the IDT is made of a uniform metal film. It can be seen that the spurious in the circled portion in FIG. 13 (b) is suppressed in FIG. 13 (a).

In the first and second embodiments described above, only the example using the ST quartz substrate is shown. However, regardless of the substrate material, if the speed difference between the signal terminal portion and the excitation portion of the interdigital transducer is reduced, Higher-order propagation modes are less likely to appear. Therefore, the same effect can be obtained by using a quartz substrate other than the ST-cut quartz substrate or a LiNbO3 or LiTaO3 substrate. Also, as a means for bringing the surface acoustic wave propagation velocity of the signal terminal portion closer to the surface acoustic wave propagation velocity of the excitation portion, it is not always necessary to use a grating structure, and it is not necessary to increase the signal terminal portion film thickness or the signal terminal portion. An insulating film may be stacked on the part.

[0034]

According to the present invention, an interdigital transducer in which higher-order modes are suppressed can be obtained without sufficiently narrowing the width of the excitation section. Further, by using this interdigital transducer, a laterally coupled multimode filter or a longitudinally coupled multimode filter in which spurious near the pass band is suppressed can be manufactured.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an interdigital transducer (IDT) of the present invention.

FIG. 2 is a configuration diagram of a laterally coupled surface acoustic wave multimode filter using the IDT of the present invention.

FIG. 3 is a configuration diagram of a longitudinally coupled surface acoustic wave multimode filter utilizing the IDT of the present invention.

FIG. 4 is a block diagram of a conventional IDT.

FIG. 5 is a configuration diagram of a conventional laterally coupled surface acoustic wave multimode filter.

FIG. 6 is a configuration diagram of a conventional longitudinally coupled surface acoustic wave multimode filter.

FIG. 7 is a graph showing a change in velocity of a propagation mode with respect to a width of an excitation part.

FIG. 8 is a graph showing a change in width of the excitation part in which a surface acoustic wave of a first mode appears.

FIG. 9 is a configuration diagram of a laterally coupled surface acoustic wave multimode filter according to the first embodiment of the present invention.

FIG. 10 is a bandpass characteristic diagram of the filter shown in FIG.

FIG. 11 is a configuration diagram when the coupling portion is electrically separated in the first embodiment of the present invention.

FIG. 12 is a configuration diagram of a longitudinally coupled surface acoustic wave multimode filter according to a second embodiment of the present invention.

13 is a bandpass characteristic diagram of the filter shown in FIG.

[Explanation of symbols]

1 Piezoelectric substrate 2 electrode fingers 3 signal terminals 4 Excitation section 5 connection 11 IDT 12 reflector 13 First surface acoustic wave resonator 14 Second surface acoustic wave resonator 21 IDT (for input) 22 IDT (for output) 23 reflector 51 Comb electrode part 52 Solid membrane part 53 Free surface

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeshi Endo 4-1-1 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Within Fujitsu Limited (72) Masanori Ueda 4-chome, Ueda-anaka, Nakahara-ku, Kawasaki, Kanagawa No. 1 within Fujitsu Limited (56) Reference JP-A-6-164297 (JP, A) JP-A-2-81511 (JP, A) JP-A-57-197908 (JP, A) JP-A-62- 47206 (JP, A) US Patent 4999535 (US, A) M.P. Tanaka, T .; morita, K .; ono, Y. Nakazawa, NARROW BANDPASS FILTER USING DOUBLE E-MODE SAW RESONAT ORS ON QUARTZ, 1984 f REQUENCY CONYROL S YMPOSIUM, IEEE, 1984, P. 286-293 (58) Fields investigated (Int.Cl. ⁷ , DB name) H03H 9/145 H03H 9/64

Claims (3)

(57) [Claims] 1. An interdigital transducer , which is formed on a piezoelectric substrate, and which includes an excitation unit for exciting a surface acoustic wave and a signal terminal unit for inputting an electric signal, and an elastic surface.
It is parallel to the wave propagation direction and
It consists of reflectors placed close to both sides of the transducer.
The surface acoustic wave resonator with respect to the propagation direction of the surface acoustic wave.
Two bullets are placed close to each other in the vertical direction, and two bullets are
Coupling type elastic table for acoustically coupling acoustic surface wave resonators
A surface-wave multimode filter comprising the two elastic surfaces
Of at least one of the wave resonators
In the digital transducer, the signal terminal is a metal electrode
Is formed from a film, and at least a part of the metal electrode film is
Having a plurality of slits arranged in the propagation direction of the surface acoustic wave,
The velocity of the surface acoustic wave propagating through the signal terminal portion is
The velocity is slightly higher than the velocity of the surface acoustic wave propagating in the vibration part.
It is an interdigital transducer
Horizontally coupled surface acoustic wave multimode filter. 2. A surface acoustic wave propagating through the coupling portion, the signal terminal portion and the excitation portion is expressed by the following equation: Vgap>Vbus> Vidt (wherein Vgap is a propagation velocity propagating through the coupling portion, and Vbus is a signal terminal portion). propagation velocity of propagation, Vidt: transversely coupled SAW multi-mode filter according to claim 1, characterized by having a propagation velocity which satisfies the relationship of propagation velocity) for propagating excitation portion. 3. At least one input interdigitation
Signal transducer and at least one output
Propagation of surface acoustic waves with a digital converter
Place parallel to the direction and place reflectors on both sides
Constructed longitudinal mode coupled surface acoustic wave multimode filter
The interdigital transducer.
The surface excites surface acoustic waves formed on the piezoelectric substrate.
It consists of an excitation section and a signal terminal section for inputting electrical signals.
Input or output interdigital transformer
At least one of the antennas has a metal electrode as the signal terminal portion.
Is formed from a film, and at least a part of the metal electrode film is
Has a plurality of slit'm parallel to the propagation direction of the sexual surface wave,
The velocity of the surface acoustic wave propagating through the signal terminal portion is
The velocity is slightly higher than the velocity of the surface acoustic wave propagating in the vibration part.
It is an interdigital transducer
Vertically coupled surface acoustic wave multimode filter.