JPS63260213A - Resonator using high coupling love wave type saw substrate - Google Patents

Abstract

PURPOSE:To obtain a small-sized resonator with a very small capacitance ratio by adding a heavy metal interdigital transducer electrode to a prescribed rotation Y-cut LiNbO3 substrate surface and exciting and propagating a surface acoustic wave in a direction of the substrate. CONSTITUTION:The resonator is constituted by adding an IDT (heavy metal interdigital transducer) electrode to the surface of a rotation Y-cut LiNbO3 substrate and exciting and propagating a SAW (surface acoustic wave) in a direction X of the substrate. As the rotation Y-cut LiNbO3 substrate, the rotation Y-cut LiNbO3 substrate is used, whose rotation angle is selected in a way that the electromechanical coupling coefficient as to the Love wave stimulated in the substrate is large and that of the SAW stimulated at the same time is small. Moreover, as the IDT electrode, an electrode made of Au, Ag or Pt is used. Then the frequency-temperature characteristic is improved by coating SiO2 of a prescribed thickness onto the surface of the substrate.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a Love wave type SAW resonator, particularly a LiNbO
IDT electrodes made of heavy materials such as gold are attached to the substrate and SAW is performed.
Concerning a resonator that excites.

(Prior Art) SAW resonators that have been put into practical use generally have IDT electrodes of a considerable number of logarithms attached on a piezoelectric substrate, and both sides of the IDT electrodes are used to excite the SAW and confine the excited SAW energy as much as possible within the IDT electrodes. A large number of grating-like reflectors are attached to the resonator, which not only increases the size in the SAW propagation direction, but also increases the capacity ratio of the resonator because the configuration for increasing Q as described above inevitably increases the capacitance ratio. The frequency band is extremely narrow, and although it is very convenient as a resonator for an oscillator, it is not suitable for constructing filters, VCOs, etc.

On the other hand, if a crystal with good temperature characteristics is used, the logarithm of the IDT is reduced, and the reflector is also removed, there is a problem that the resonance phenomenon does not occur because the electromechanical coupling coefficient of the crystal is originally extremely small.

(Objective of the Invention) The present invention has been made in view of the problems in the characteristics of the conventional general SAW resonator as described above, and is small and has an extremely small capacitance ratio suitable for constructing a filter or a VCO. The purpose of the present invention is to provide a Love wave type SAW resonator.

(Summary of the Invention) In order to achieve the above-mentioned objects, a love wave type SAW according to the present invention is provided.
The resonator is basically LiNb with a large electromechanical coupling coefficient for Love waves and a small rotation angle for SAW.
An O5 rotating Y plate is used and a relatively small number of pairs of IDTs are attached to the main surface of the plate using a heavy material such as gold.

(Example) The following nine inventions will be described in detail based on the results of theoretical calculations and experiments shown in the drawings.

Prior to describing embodiments, the nature of waves propagating on or just below the surface of a piezoelectric substrate will be briefly explained in order to aid understanding of the present invention.

As mentioned above, conventional surface acoustic wave (SAW) resonators are made of Li.
It uses Rayleigh wave type waves (longitudinal SV waves) excited on the surface of Nboa rotating Y plate, 8T cut crystal, etc., but the electromechanical coupling coefficient for this type of wave is 8. Since the resonator is small, only a few inches at most, the capacitance ratio of the resonator is extremely large, ranging from 50 to 1,000.

- In addition to Rayleigh waves, the surface of the piezoelectric substrate as described above has S
It is known that pseudo surface acoustic waves, which are H waves (piezoelectric surface shear waves), exist.

As shown in FIG. 2, the electromechanical coupling coefficient for this wave motion exceeds 201 for the LiNbO5 rotating Y plate.

However, this pseudo surface acoustic wave is a leaky wave that propagates while radiating a bulk wave into the substrate, and if left as it is, the propagation attenuation is large.

To deal with this problem, the inventors of the present invention have already confirmed that if a heavy substance with low sound velocity is attached to the surface of the LiNbO3 substrate, the pseudo-elastic wave becomes a Love wave type surface wave with no propagation attenuation. .

The present invention further develops the knowledge based on the research previously conducted by the inventors as described above, and basically uses LiNbO
5 Heavy metal directly on the rotating Y plate - Due to the large baldness coefficient, I
The idea is that the number of DT electrode pairs is extremely small and that resonance can be achieved even if the reflector is removed.

Therefore, first of all, if we look at the rotation angle at which the electromechanical coupling coefficient for pseudo surface acoustic waves (Leaky waves) has a maximum of 2 from the wJ2 diagram, it is almost zero, so we decided to simply use a LiNbO3 Y plate, as shown in Figure 1. A without a reflector as shown
Attach the IDT electrode by u.

In this case, the logarithmic N of IDT is 15.20 and 25, and the electrode film thickness H is 0.8, 1.6, 2, and 2.2 mm.
3 types of IDT electrodes, 6fi and 1.8
The experiment was conducted with two types: m.

As a result, as is clear from FIGS. 3(a) and 3(b), resonance appears even when the number of electrode finger pairs N is 15, and it is understood that the Q of resonance increases as the electrode film thickness H increases. Good morning. In Figure (b), resonance cracking occurs near the anti-resonance, but this leakage disappears when sound absorbing material is applied from both sides of the IDT toward the inside of the IDT, and when the crossover width is reduced. Since the number of responses decreases, it is thought that this is caused by transverse mode vibration, so it may be possible to eliminate it even if weighting is applied to the crossover width A of the IDT.

Next, when we examine the relationship between the capacitance ratio γ of such a resonator and the electrode film thickness H, we understand that the larger the electrode film thickness H becomes, the smaller it becomes, as shown in Figure 4, and its value is extremely small, about 3. It will be.

Furthermore, as shown in FIG. 5, the relationship between the Q of the resonator and the electrode film thickness H shows that as the film thickness H increases, the Q also increases.

Finally, in the resonator of the present invention with extremely thick electrodes, the resonant frequency cannot be uniquely determined only by the IDT electrode pitch, so the resonant frequency depends on the electrode film thickness. When we investigated this, we obtained the results shown in Figure 6.

As is clear from this figure, as the electrode film thickness H increases, the resonance frequency decreases by about 16% per 1 μm of film thickness. Therefore, when manufacturing an actual resonator, it is necessary to take this phenomenon into consideration when designing the IDT wt pole.

By the way, as is clear from the experimental results shown in FIG. 3, not only is the resonance Q not very large, but also the anti-resonance Q is extremely small. The reason for this seems to be that the electrode fingers near both ends of the IDT radiate surface waves to the outside of the IDT. I! At the poles, weighting is applied so that the cross width is large in the center and becomes small toward both ends, or as shown in FIG.
The DTs may be arranged in a distributed manner, with the T logarithm being larger near the center and smaller near both ends.

Above, we have only explained the case of using a LiNb03Y plate that exhibits the maximum electromechanical coupling coefficient for love waves (Leaky), but as is clear from FIG.
Since the LiNb03Y board has a considerably large electromechanical coupling coefficient for SAW (Rayleigh waves), this may appear as spurious. It is therefore obvious that a LiNbO5 rotating Y-plate may be used which has a smaller rotation angle, although the electromechanical coupling coefficient for the Love wave is somewhat lower than that for the SAW. The rotation angle at this time will be approximately in the range of 110° to 40°.

By the way, the highly coupled love wave type SA according to the present invention as described above
Since a resonator using a W substrate has a small capacitance ratio, it is possible to widen the variable range of the resonant frequency, and is suitable as a resonator for a filter or a voltage controlled oscillator (VCO).

However, since LiNbO5 originally does not have good frequency-temperature characteristics compared to artificial quartz, it is expected that temperature compensation will be troublesome.

To solve this problem, the frequency-temperature characteristics should be changed to LiNb.
By applying SiOz, which is the opposite of O3, to a predetermined thickness on the surface of the resonator as shown in FIG. 1 or FIG.
(See figure) In this way, the frequency-temperature characteristics of the filter will be greatly improved, and it will be possible to avoid complicating the temperature compensation circuit for vCO.

It should be noted that since the IDT ml pole of the resonator according to the present invention requires a heavy metal such as Au to be adhered to a considerable thickness, it is effective to use a process such as a lift-off method or a chipping method for manufacturing it. Dew.

(Effects of the Invention) Since the resonator according to the present invention is constructed as described above, it is not only smaller than conventional SAW resonators, etc., but also allows its capacitance ratio to be extremely small. Therefore, it exhibits a remarkable effect in satisfying the strict demands for miniaturization and widening of the band for filters or VCO resonators.

[Brief explanation of drawings]

! Figure 1 is a plan view showing the basic configuration of the resonator according to the present invention, and Figure 2 is a plan view showing the Love wave (L) excited in a LiNbO5 rotating Y plate.
Figure 3 (
al and (bl are diagrams of experimental results showing the resonance characteristics of prototype resonators with different electrode film thicknesses according to the present invention, respectively, and Figures 4 to 6 are diagrams showing the parameters (IDT) of the resonator according to the present invention, respectively.
Figures 7 and 8 are diagrams of experimental results showing the IDT electrode film thickness dependence of the active capacitance ratio r, the resonance Q, and the resonance frequency fr when changing the logarithm and crossover width, respectively. FIG. 2 is a plan view and a sectional view showing an example.

Claims (5)

[Claims] (1) Comparison with a rotated Y-cut LiNbO_3 substrate surface in which a rotation angle was selected such that the electromechanical coupling coefficient for Love waves excited in the substrate is large and that for surface acoustic waves (SAW) excited at the same time is small. A small number of Au, Ag
Alternatively, a resonator using a highly coupled Love wave type SAW substrate, characterized in that a heavy metal interdigital transducer (IDT) electrode such as Pt is attached to excite and propagate a Love wave type surface wave in the X direction of the substrate. (2) A resonator using a highly coupled Love wave type SAW substrate according to claim (1), wherein the rotation angle is from -10° to +40°. (3) The frequency-temperature characteristics are improved and the thickness of the IDT electrode is reduced by coating the surface of the LiNbO_3 substrate with the IDT electrode to a required thickness. 1) or (2
A resonator using the highly coupled Love wave type SAW substrate described in ). (4) Claims (1) and (2) characterized in that radiation of vibrational energy to the outside of the IDT is reduced by arranging a grating constituting an acoustic cavity between the IDT electrodes and at both ends thereof. ) or a resonator using the highly coupled Love wave type SAW substrate described in (3). (5) Claims (1), (2), or (3) characterized in that the crossing width of the IDT electrode is continuously weighted such that it becomes larger at the center and smaller at both ends. A resonator using the highly coupled Love wave type SAW substrate described in ).