JPH10145173A - Interdigital transducer and surface acoustic wave multiplex mode filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress generation of higher-order modes and to suppress a spurious signal near a pass band by making the speed of a surface acoustic wave transmitting a signal terminal part to be slightly faster than the speed of the surface acoustic wave transmitting an excitation part. SOLUTION: The speed of the surface acoustic wave, transmitting a signal terminal part 3 is set to be slightly faster than the speed of the surface acoustic wave transmitting an excitation part 4, and the excited surface acoustic wave is closed into the excitation part 4. Thus, the terminal part 3 is set to be a grating structure, and the ratio a/b of an interval (a) and width (b) in a slit and the ratio c/d of the width (c) and the interval (d) of an electrode finger in the excitation part 3 are decided. When the metal materials and film thickness of the terminal part 3 and those of the excitation part 4 are set to be equal, IDT with a/b<c/d is used, and a lateral/vertical connection-type multiplex mode filter is manufactured. Thus, the generation of the surface acoustic wave at the higher-order mode is suppressed, and spurious signal near the pass band can be suppressed.

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 a surface acoustic wave.
The present invention also relates to a surface acoustic wave multi-mode filter using the interdigital transducer.

[0002]

2. Description of the Related Art FIG. 4 shows a 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. Here, considering the propagation speed of the surface acoustic wave propagating in each region, the speed Vidt propagating in the excitation unit 101 is lower than the speed Vbus propagating in the signal terminal unit 102. As a result, the excited surface acoustic waves are confined in the excitation unit 101. As shown in FIG. 4, the amplitude distribution of the surface acoustic wave at this time includes three modes (basic mode 104, higher-order mode (first order) 105, and higher-order mode (2
Next) 106).

[0003] Next, a change in the propagation mode with respect to the width W of the excitation unit 101 will be described. When the width W of the excitation unit 101 is sufficiently small, only the basic mode 104 appears among the modes shown in FIG. Here, as the width W of the excitation unit 101 is increased, the higher-order modes 105, 10 shown in FIG.
6 appear in order, and a plurality of modes propagate along with the fundamental mode 104.

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

FIGS. 5 and 6 show the configuration of a surface acoustic wave multi-mode filter in which the influence of higher-order propagation modes appears remarkably. The surface acoustic wave multimode filter in FIG. 5 is called a “transversely coupled multimode filter” because the IDTs of the surface acoustic wave resonators are arranged in a direction perpendicular to the propagation direction of the surface acoustic wave. The surface acoustic wave multi-mode filter in FIG. 6 is a “longitudinal-coupling multi-mode filter” in which at least one input IDT and at least one output IDT are arranged in a direction parallel to the surface acoustic wave propagation direction. 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, there has been a problem that spuriousness exists inevitably.

[0006]

Therefore, in a conventional surface acoustic wave multimode filter designed to reduce the loss of a surface acoustic wave multimode filter and improve the impedance matching state with an external circuit, It is necessary to increase the width W of the excitation portion of the IDT, and it is difficult to suppress spurious.

The present invention has been made in view of the above circumstances, and has as its object to provide an interdigital transducer (IDT) that can suppress a higher-order propagation mode without reducing the width of an excitation section. And It is another object of the present invention to provide a horizontal coupling type multi-mode filter and a vertical coupling type multi-mode filter in which spurious is suppressed using the interdigital transducer (IDT).

[0008]

According to the present invention, there is provided an interdigital transducer formed on a piezoelectric substrate and comprising an excitation section for exciting surface acoustic waves and a signal terminal section for inputting an electric signal. It is an object of the present invention to provide an interdigital transducer characterized in that the speed of a surface acoustic wave propagating is slightly higher than the speed of a surface acoustic wave propagating in an excitation section.

Here, the signal terminal portion may be formed of a metal electrode film, and at least a part of the metal electrode film may have a plurality of slits arranged in the propagation direction of the surface acoustic wave. Further, the excitation section is composed of a plurality of electrode fingers arranged at regular intervals, and the signal terminal section has a ratio a / b of the interval a of each slit to the width b of the slit, and the electrode finger of the excitation section has a ratio a / b. May be smaller than the ratio c / d of the width c and the interval d.

Further, the present invention provides a surface acoustic wave resonator comprising an interdigital transducer and a reflector which is parallel to the propagation direction of the surface acoustic wave and which is disposed close to both sides of the interdigital transducer. A laterally-coupled surface acoustic wave multimode filter in which two two surface acoustic wave resonators are acoustically coupled by a coupling part by two adjacently disposed in a direction perpendicular to the propagation direction of the surface acoustic wave, A laterally coupled surface acoustic wave multi-mode filter characterized in that at least one of the two surface acoustic wave resonators is an interdigital transducer having the above-described configuration. is there.

[0011] Further, at least one input interdigital transducer and at least one output interdigital transducer are arranged in parallel to the propagation direction of the surface acoustic wave, and a reflector is arranged on both sides thereof. A longitudinally-coupled surface acoustic wave multimode filter, wherein at least one of the input and output interdigital transducers is an interdigital transducer having the above-described configuration. A wave multiplex mode filter is provided.

FIG. 1 shows a configuration diagram of an interdigital transducer (IDT) according to the present invention. 1 in the figure
Is a piezoelectric substrate, 2 is an electrode finger for exciting surface acoustic waves,
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 a grating structure in this manner, the propagation speed of the surface acoustic wave is lower than in the case of a uniform metal film. That is, the velocity difference between the surface acoustic waves propagating in the excitation section 4 and the signal terminal section 3 is reduced, and the propagation mode is hardly confined. Therefore, occurrence of a higher-order mode is suppressed. In order for a higher-order mode to exist, the width of the excitation unit 4 needs to be sufficiently large.

FIGS. 2 and 3 show the configuration of a "laterally-coupled multi-mode filter" and a "longitudinal-coupled multi-mode filter" constructed using the IDT, respectively. According to the surface acoustic wave multimode filter having such a configuration, a filter characteristic in which spurious due to a higher-order mode is suppressed can be obtained.

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, and titanium Ti. The excitation unit 4 of the IDT is composed of a plurality of electrode fingers 2 arranged at regular intervals, similarly to a commonly used IDT. The electrode fingers 2 are interdigital electrodes electrically connected to the signal terminal portion 3. These interdigital electrodes are alternately arranged at regular intervals as shown in FIG. An exciting unit 4 for exciting is configured.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings. Note that the present invention is not limited to this. First, the propagation mode of the surface acoustic wave in the conventional IDT shown in FIG. 4 will be considered. When the propagation speed of the surface acoustic wave of the excitation unit 101 is lower than the propagation speed of the signal terminal unit 102, the energy of the surface acoustic wave is confined in the excitation unit 101. At this time, if the mode of the vibration energy of the surface acoustic wave confined in the excitation unit 101 is approximated by a scalar potential φ, φ satisfies the following equation.

[0016]

(Equation 1)

Here, assuming that the propagation speed of the propagation mode represented by the scalar potential φ is Vmode, Vmode
de takes a value satisfying the following expression.

[0018]

(Equation 2)

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

Next, FIG. 8 shows a change in the width of the excitation section 101 where the first higher-order mode (first-order mode) appears when the surface acoustic wave propagation velocity of the signal terminal section 102 is changed. It can be seen that the width of the excitation unit 101 where the first-order mode appears increases by reducing the surface acoustic wave of the signal terminal unit 102. As a result, by lowering the propagation speed of the surface acoustic wave at the signal terminal unit 102 to approach the propagation speed of the excitation unit 101, higher-order modes are suppressed.

Next, as shown in FIG.
Is a grating structure. In general, the speed of a surface acoustic wave propagating through a grating structure portion formed of a metal electrode is lower than the speed of a surface acoustic wave propagating through a uniform metal film portion. Therefore, if the speed of the surface acoustic wave propagating through the signal terminal section is reduced by using the grating structure for the signal terminal section 3, the width of the excitation section 4 where the higher-order mode appears becomes larger, and the higher-order mode is suppressed. Will be.

That is, if the speed of the surface acoustic wave propagating in the signal terminal section 3 is made slightly higher than the speed of the surface acoustic wave propagating in the excitation section 4, the generation of the higher-order mode surface acoustic waves in the IDT can be achieved. Can be suppressed.

By the way, in order for the excited surface acoustic wave to be confined in the excitation section 4, the velocity of the surface acoustic wave propagating in the excitation section 4 must be lower than the velocity of the surface acoustic wave propagating in the signal terminal section 3. is required. Now, when the signal terminal 3 of the IDT has a grating structure and the metal material and the film thickness of the signal terminal 3 are the same as the material and the film thickness of the excitation unit 4, it is necessary to confine the surface acoustic wave to the excitation unit 4. The ratio (a / b) between the gap a and the width b of the slit of the grating structure of the signal terminal section 3 must be reduced (c / d) between the width c and the gap d of the electrode finger of the excitation section. . This is because the velocity of the surface acoustic wave propagating through the grating structure of the signal terminal section 3 increases as the width b of the slit increases.

If an interdigital transducer having such a structure is used to fabricate a laterally-coupled multimode filter and a vertically-coupled multimode filter, the generation of high-order mode surface acoustic waves can be suppressed. A suppressed surface acoustic wave multimode filter is obtained. Hereinafter, embodiments of the surface acoustic wave multi-mode filter will be described.

[0025]

【Example】

Embodiment 1 FIG. 9 shows an example of a “laterally coupled multimode filter” manufactured using an interdigital transducer (IDT) 11 in which a signal terminal 3 has a grating structure on an ST-cut quartz substrate. This filter consists of two surface acoustic wave resonators. Electrode period λ of IDT 11: = 24 μm, electrode finger line width of excitation unit 4: c = 6 μm, electrode finger line interval of excitation unit 4: d = 6 μm, electrode film thickness: 0.76 μm, number of IDT electrode pairs: 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, a 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 set to 3.6 μm / 8.4 μm, respectively.

Such a surface acoustic wave resonator is cascaded 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 unit 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. FIG. 10 (b)
It can be confirmed that spurious components circled by are suppressed in FIG.

In FIG. 9, the joining portion 5 of the IDT is composed of a uniform metal "solid film portion" and a uniform metal "solid film portion".
Are formed from portions where the electrode fingers in the vicinity do not intersect.
In FIG. 9, the velocity (Vmetal) of the surface acoustic wave propagating through the metal “solid film” 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 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 finger does not intersect is 3114 m / s.

Therefore, in the embodiment of FIG.
The relationship al>Vgap>Vbus> Vidt holds. 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 configured to be electrically separated, and may be composed of a comb electrode portion 51, a solid film portion 52, and a free surface 53 as shown in FIG.
The effect of suppressing spurious is obtained. The free surface is a surface on which 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
(= 3147 m / s) does not change, but 3
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 holds, but the same characteristics as those in FIG. 10A can be obtained. In this case, spurious can be suppressed.

Embodiment 2 FIG. 12 shows an example in which a "longitudinal coupling type multi-mode filter" is manufactured on an ST-cut quartz substrate using an interdigital transducer in which the signal terminal 3 has a grating structure. The filter configuration is a two-input, one-output type vertically coupled multi-mode filter. IDT electrode period λ: 12.6 μm, number of electrode pairs of first IDT 21 for input: 100 each
Pairs, 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 part 3 of IDT Slit width of the IDT signal terminal section: 1.89 μm Width of the electrode finger of the IDT excitation section: 3.2 μm, Interval of the electrode finger of the IDT excitation section: 3.2 μm The width of the electrode fingers of the excitation unit 4 of the reflector is 3.2 μm, and the interval between the electrode fingers of the excitation unit 4 of the reflector is 3.2 μm.

FIG. 13A shows the pass characteristics of the longitudinally coupled multimode filter at this time. For comparison, FIG. 13B shows the 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 part circled in FIG. 13B is suppressed in FIG.

In the first and second embodiments, only the example using the ST quartz substrate has been described. However, regardless of the substrate material, if the speed difference between the signal terminal portion of the interdigital transducer and the excitation portion 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 propagation speed of the surface acoustic wave of the excitation part, it is not always necessary to use a grating structure. An insulating film may be layered on the portion.

[0034]

According to the present invention, it is possible to obtain an interdigital transducer in which a higher-order mode is suppressed without making the width of the excitation section sufficiently small. Also, by using this interdigital transducer, a laterally coupled multimode filter or a vertically coupled multimode filter in which spurious components near the pass band are suppressed can be manufactured.

[Brief description of the 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 type surface acoustic wave multimode filter using an IDT of the present invention.

FIG. 3 is a configuration diagram of a longitudinally coupled surface acoustic wave multi-mode filter using an IDT of the present invention.

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

FIG. 5 is a configuration diagram of a conventional transversely 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 speed of a propagation mode with respect to a width of an excitation unit.

FIG. 8 is a graph showing a change in the width of the excitation unit where a first-order mode surface acoustic wave appears.

FIG. 9 is a configuration diagram of a laterally coupled surface acoustic wave multimode filter according to Embodiment 1 of the present invention.

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

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

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

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

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 piezoelectric substrate 2 electrode finger 3 signal terminal unit 4 excitation unit 5 coupling unit 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 film part 53 Free surface

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Yoshio Sato, Inventor 4-1-1 Kamikadanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Tsuyoshi Endo 4-1-1, Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture No. 1 Fujitsu Limited (72) Inventor Masanori Ueda 4-1-1 Uedanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Within Fujitsu Limited

Claims (7)

[Claims] In an interdigital transducer formed on a piezoelectric substrate and comprising an excitation section for exciting a surface acoustic wave and a signal terminal for inputting an electric signal, the speed of the surface acoustic wave propagating through the signal terminal is determined. An interdigital transducer characterized in that the speed is slightly higher than the speed of the surface acoustic wave propagating in the excitation section. 2. The surface acoustic wave propagating through the signal terminal portion and the excitation portion is expressed by the following formula: Vmetal>Vbus> Vidt (where Vmetal: a propagation speed propagating through a uniform metal solid film structure portion, Vbus: 2. The interdigital transducer according to claim 1, wherein the interdigital transducer has a propagation speed that satisfies the following relationship: Propagation speed at the signal terminal, Vidt: propagation speed at the excitation unit. 3. The signal terminal part is formed of a metal electrode film, and at least a part of the metal electrode film has a plurality of slits arranged in the direction of propagation of the surface acoustic wave. Interdigital transducer. 4. The excitation unit includes a plurality of electrode fingers arranged at regular intervals, and the signal terminal unit is configured such that a ratio a / b of an interval a of each slit to a width b of the slit is equal to the excitation. 4. The interdigital transducer according to claim 3, wherein the ratio is smaller than the ratio c / d between the width c of the electrode finger of the portion and the interval d. 5. A surface acoustic wave resonator comprising an interdigital transducer and reflectors which are parallel to the direction of propagation of the surface acoustic wave and are disposed close to both sides of the interdigital transducer. A laterally coupled surface acoustic wave multimode filter in which two adjacent surface acoustic wave resonators are acoustically coupled by a coupling portion by two adjacently arranged in a direction perpendicular to the direction.
2. The interdigital transducer of at least one of the two surface acoustic wave resonators according to claim 1.
5. A laterally coupled surface acoustic wave multi-mode filter, which is the interdigital transducer according to any one of 4. to 4. 6. The surface acoustic wave propagating through the coupling section, the signal terminal section, and the excitation section is represented by the following equation: Vgap>Vbus> Vidt (where Vgap is the propagation speed propagating through the coupling section, and Vbus is the signal terminal section. 6. The laterally-coupled surface acoustic wave multimode filter according to claim 5, wherein the filter has a propagation speed that satisfies the following relationship: Propagation propagation speed, Vidt: Propagation speed propagating through the excitation unit. 7. An at least one interdigital transducer for input and at least one interdigital transducer for output are arranged in parallel to a surface acoustic wave propagation direction, and reflectors are arranged on both sides thereof. A longitudinal mode coupling type surface acoustic wave multi-mode filter, wherein at least one of the input and output interdigital transducers is the interdigital transducer according to any one of claims 1 to 4. Vertical coupled surface acoustic wave multimode filter.