JPS60143016A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To obtain a stable surface acoustic wave resonator where no resonance frequency is decreased even at the use of a long period by constituting a metallic strip constituting a reflector aluminum including silicon as an impurity. CONSTITUTION:An interdigital electrode 32 where a couple of comb tooth electrodes 32a, 32b are meshed with each other is formed on a piezoelctric substrate 31. The electrode 32 converts an input electric signal applied to an input terminal IN into a surface acoustic wave propagating on the surface of the substrate 31. Grating reflectors 33, 34 for reflecting the surface acoustic wave are formed on the substrate 31 at both sides of the electrode 32. Many metallic strips having strip width of lambda/4 are arranged in the reflectors 33, 34 at a period of lambda/2 pitch and all the reflected waves are applied in the same phase. No deterioration is caused to the strips even if the resonator is used for a long period by constituting the metallic strips of the reflectors 33, 34 with aluminum including silicon as an impurity and the surface acoustic wave resonator with stable and less change of resonance frequency is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave resonator having a grating reflector.

In recent years, surface acoustic wave resonators for use in oscillators, filters, etc. have been actively developed. The basic structure of a surface acoustic wave resonator is already known, as described in, for example, Japanese Patent Laid-Open No. 51-244. That is, interdigital electrodes for exciting surface acoustic waves are formed on the piezoelectric substrate, and on both sides of the interdigital electrodes, the width of the IJ tube is λ/4.
(λ: surface acoustic wave wavelength)
It is constructed by forming grating reflectors arranged periodically at a pitch of /2. Surface acoustic waves excited by the interdigital electrodes propagate on the piezoelectric substrate, but are reflected toward the center by grating reflectors provided on both sides. This amount of reflection is small per IJ tube, but since reflections occur with each strip having a period length of 1/2 wavelength, these reflections are added and synthesized. The amount of reflection is approximately 1. At this time, a strong standing wave of surface acoustic waves is generated on the piezoelectric substrate. This phenomenon corresponds exactly to the resonance of a crystal resonator. By appropriately setting the position of the grating reflector in this way, a resonator similar to a crystal resonator can be realized in the surface acoustic wave mode.

Incidentally, in such a surface acoustic wave resonator, aluminum is normally used for the metal strip of the grating reflector due to advantages such as good workability, but it is difficult to construct the grating reflector with such an aluminum film and operate it. As a result, a phenomenon was observed in which the resonant frequency significantly decreased over time, and at the same time, the resonant resistance increased and the Q decreased. At this time, when the grating reflector was observed using an electron microscope (2000x magnification), the aluminum film was undamaged before use, as seen in the micrograph in Figure 1 (,), but after long-term use, the grating reflector was not damaged. As is clear from the micrograph in FIG. 1(b), the central portion was damaged and cracked, and it was found that this caused the reduction in the resonance frequency and Q.

Although there have been no reports on the observation of such a phenomenon, various studies have revealed that it is due to the following phenomenon unique to surface acoustic wave resonators. That is, in a surface acoustic wave resonator, a large standing wave of surface acoustic waves is generated on the piezoelectric substrate as described above. Therefore, stress due to this surface acoustic wave energy is applied to the aluminum film of the grating reflector. Moreover, this stress is extremely repetitive stress corresponding to the frequency of surface acoustic waves. When we investigated the relationship between this standing wave and the deteriorated portion of the aluminum film, we found that there was a relationship as shown in Figure 2. That is, the shaded area 21 in FIG. 2(, ) is the degraded portion of the aluminum film, and deterioration was observed in the shaded area 24 in the figure excluding the interdigital electrode portion 22 and the surrounding area of the grating reflector 230. Also, if we compare this with the stress of the standing wave in FIG. It was found that the part corresponding to the antinode of the standing wave stress, that is, the part corresponding to the large stress part, was degraded. From this, it was confirmed that the cause of the above-mentioned deterioration of the aluminum film was due to standing wave stress.

The present invention has been made in view of these points, and an object of the present invention is to provide a stable surface acoustic wave resonator in which the metal strip of the grating reflector does not deteriorate and the resonant frequency does not decrease even when used for a long time. It is something to do.

Another object of the present invention is to provide a surface acoustic wave resonator whose Q is very high and whose Q does not decrease even after long-term use.

The present invention aims to reduce the resonant frequency of a surface acoustic wave resonator and
Since the decrease in IJ is due to the deterioration of the grating reflector due to stress, the metal
This method was developed based on the results of a study on tube materials, and the metal strip was made of aluminum containing silicon as an impurity.

However, according to the surface acoustic wave resonator of the present invention in which the grating reflector is formed of such a metal material, it is possible to obtain stable characteristics without a decrease in the resonance frequency or a decrease in Q even when used for a long time. can.

The present invention will be explained in detail below with reference to the drawings.

FIG. 3 shows a surface acoustic wave resonator according to an embodiment of the present invention. This surface acoustic wave resonator is made of, for example, lithium tantalate (L i Ta0B ), lithium niobate (
A transducer for converting an input electrical signal into a surface acoustic wave propagating on the piezoelectric substrate 31 is mounted on a piezoelectric substrate 31 such as L 1Nb03 ), for example, an interface formed by interlocking a pair of comb-like electrodes 32 a and 32 b. A digital electrode 32 is formed.

This interdigital electrode 32 converts the input electrical signal supplied to the input terminal IN into a surface acoustic wave propagating on the surface of the piezoelectric substrate 31. Furthermore, grating reflectors 33 and 34 for reflecting the surface acoustic waves excited by the interdigital electrode 2 are formed on the piezoelectric substrate 31 on both sides of the interdigital electrode 32, respectively. These grating reflectors 33 and 34 are made by arranging a large number of metal strips with a strip width of λ/4 at a pitch of λ/2, so that all the reflected waves reflected from each metal strip are added in the same phase. ing. The ends of these metal strips are also electrically shorted together.

In the surface acoustic wave resonator having such a configuration, in the present invention, the metal strip of the grating reflector 32 is made of aluminum containing silicon as an impurity. There is no particular limit to the silicon content, but considering that even pure aluminum contains about 0.01% silicon, it is more than that, and if it exceeds 50%, these can no longer be considered impurities. It disappears. Practically speaking, it is considered that about 10% or less of the total amount is desirable in terms of the performance and processing of the resonator.

According to the surface acoustic wave resonator of the present invention having a grating reflector made of aluminum containing silicon as an impurity, the following remarkable effects were observed.

Figure 4 shows the resonant frequency of the resonator over time when the surface acoustic wave resonator of the present invention was designed under the following conditions and operated with an excitation power of 2 mW in an atmosphere at a temperature of 65°C. This shows the rate of change. In other words, the design conditions for the surface acoustic wave resonator are X as a piezoelectric substrate) Li
Using TaO3, the propagation direction of surface acoustic waves is set 11 from the Y axis.
It was set at a 2° tilt. Interdigital electrode 2
consists of 11 pairs of electrode fingers (-1, and each of the grating reflectors 33 and 34 consists of 200 strips.Similarly, the spacing between the strips is 9.0μ.
It was set as m.

Further, the opening lengths of the interdigital electrode 2 and the grating were each 0.7 rmn. The grating reflectors 33 and 34 were fabricated by mixing aluminum with 2% silicon and depositing it on a LiTaO3 substrate to a thickness of 1.3 μm. In addition, for comparison with the surface acoustic wave resonator of the present invention, grating reflectors 33 and 34 formed of pure aluminum film were similarly manufactured.

In FIG. 4, a curve 41 shows the characteristics of the resonator when the grating reflector is formed of the above-mentioned pure aluminum film, and a curve 43 shows the characteristics of the resonator of the above-mentioned invention in which the grating reflector is made of aluminum mixed with 2% silicon. It shows the characteristics of As is clear from this figure, when the grating reflector is made of pure aluminum film, the resonant frequency decreases significantly over time, whereas with the surface acoustic wave resonator of the present invention, the resonant frequency decreases. It can be seen that the amount has been significantly reduced. i.e. 1
After 0,000 hours, the rate of change in the resonance frequency is -0,045% when pure aluminum is used, while it is -0,045% in the case of the present invention in which silicon is mixed as an impurity.
17%, and the reduction in resonance frequency can be suppressed to about 1/3. In addition, a slight decrease in the resonant frequency was observed in the surface acoustic wave resonator of the present invention in which silicon was mixed with aluminum as described above, but this was due to the reflector strip due to the standing wave stress of the surface acoustic wave. In order to investigate whether this is due to deterioration of the resonator, two types of resonators, one made of pure aluminum and one made of aluminum mixed with silicon, were left in a non-operating state, and the changes in the resonant frequency after each period of time were measured. Also, almost the same characteristics as curve 43 in FIG. 4 were obtained.

This indicates that the decrease in the resonant frequency in the resonator of the present invention is not due to deterioration of the grating reflector due to standing wave stress, but is due to other causes. In other words, it has been revealed that in the surface acoustic wave resonator of the present invention, the grating reflector hardly deteriorates due to the standing wave stress of the surface acoustic wave.

In fact, when the grating reflector of the surface acoustic wave resonator according to the present invention was observed under a microscope, it was found that the strip film showed almost no deterioration even after being operated for a long time.

Figure 5 is a photograph of aluminum mixed with silicon as an impurity and exposed for 1000 hours without applying any force, but there was almost no change even when full power was applied. Note that a horizontal white line can be seen at the bottom of the strip film in the figure, but this is silicon residue and α
It is not a damaged part.

This silicon residue can be removed by etching or the like if necessary.

Figure 6 shows the rate of change in resonance frequency when the surface wave excitation level is changed. As shown in curves 61 to 64, when the excitation level is increased, the resonance frequency increases in a reflector made of pure aluminum. It can be seen that, while the frequency changes greatly, according to the present invention, the resonant frequency hardly changes even when the excitation level changes, as shown by a curve 65.

In particular, with pure aluminum reflector strips, the higher the excitation level, the greater the deterioration and the greater the change in the resonant frequency, whereas in the present invention, the resonant frequency does not change even when the excitation level is large. The higher the excitation level is used, the more pronounced the effect will be.

In this way, the deterioration of a pure aluminum reflector strip changes depending on the magnitude of the excitation level, but in general, deterioration becomes a problem when the excitation level exceeds several milliwatts.
It is difficult to clearly state whether the present invention is effective against this problem. This is because in this experimental example, the excitation level is 0.5mW.
Deterioration of the γ-luminicum reflector (IJ) was observed below this level, but this excitation level does not necessarily correspond to changes in the substrate material, resonant frequency, electrode shape, etc. However, since the cause of reflector strip deterioration is surface wave stress, reflector strip deterioration becomes a problem when the stress on the resonator surface is about 105 (NaWtm/m'1) or more. It is thought that
The present invention is effective because the reflector strip does not deteriorate even when such stress is applied. Although the exact reason why aluminum films with impurities degrade less than pure aluminum is not clear,
It is thought that impurities precipitate at the grain boundaries of aluminum and serve as nuclei to form boundaries, which prevent deterioration due to metal fatigue.

As described above, according to the present invention, it is possible to obtain a surface acoustic wave resonator whose resonance frequency does not change much even when used for a long time.

On the other hand, it has been found that the surface acoustic wave resonator of the present invention provides the following new and effective effects. In other words, a surface acoustic wave resonator whose grating reflector is made of a pure aluminum film has a resonant resistance of 24 ohms and a Q of about 12,000, whereas a surface acoustic wave resonator whose grating reflector is made of a pure aluminum film has a resonant resistance of 12,000 ohms, whereas a surface acoustic wave resonator whose grating reflector is made of a pure aluminum film has a resonant resistance of 12,000 ohms, whereas a surface acoustic wave resonator in which 2% silicon is mixed into aluminum has a resonant resistance of 17 ohms. Q is about 1
7000 were obtained. Although this is an average value of a large number of prototype samples, and there is a variation of about 20% in all of them, a significant increase in Q is recognized compared to the case where the grating reflector is made of pure aluminum.

As mentioned above, surface acoustic wave resonators are applied to oscillators and filters, and a resonator with a high Q can construct a stable oscillator, and a resonator with a high Q can be used as a filter with low insertion loss. Therefore, the surface acoustic wave resonator of the present invention is extremely effective from this point of view as well.

As described above, according to the present invention, a surface acoustic wave resonator that is stable and has good characteristics can be obtained.

In addition, in the above embodiment, the piezoelectric substrate is
Although the case using i T *Os has been described, it is equally applicable and effective to piezoelectric substrates such as quartz crystal and L i Nb Os. Further, the present invention can be applied to any surface acoustic wave resonator having a grating reflector, and is not limited to the patterns of the above embodiments.

Furthermore, it is thought to be effective to mix not only silicon but also copper, teeth, Cr, Mg, etc. as impurities in aluminum.

In addition, to form an aluminum film mixed with these impurities, using aluminum mixed with an appropriate weight of impurities in advance as a target, and performing sputter deposition, heating with a heater, or deposition using an electron beam, a surface acoustic wave resonator can be created using a pure aluminum film. It can be carried out in exactly the same process as the manufacturing process.

Furthermore, although the present invention has been described in the case where it is applied to a surface acoustic wave resonator, it can be applied to all surface acoustic wave devices such as surface acoustic wave filters.

[Brief explanation of the drawing]

Figure 1 is a micrograph showing the surface condition of a pure aluminum reflector strip before use and after long-term operation. Figure 2 shows the deteriorated part of the grating reflector strip and the stress distribution of the standing waves of surface acoustic waves. FIG. 3 is a diagram showing an embodiment of the surface acoustic wave resonator of the present invention, FIG. The figure is a micrograph of a reflector strip according to the invention, and FIG. 6 is a diagram showing the change in resonant wave number with respect to the change in surface wave excitation level. 31... Piezoelectric substrate, 32... Interdigital electrode, 33, 34... Grating reflector. Agent Patent attorney Noriyuki Chika (1 other person) Figure 1 (α) (ra) Figure 2 Figure 3 AI Figure 4 3θ /ρρ 3ρρ lρθρ Octopus direction D%] Figure 5

Claims (2)

[Claims] (1) A piezoelectric substrate, a surface acoustic wave transducer provided on the piezoelectric substrate for converting an input electric signal into a surface acoustic wave propagating on the piezoelectric substrate, and this converter and a reflector formed by periodically arranging a large number of metal strips for reflecting the surface acoustic waves, which are provided on the piezoelectric substrate to face each other, and the metal strips constituting the reflector are A surface acoustic wave resonator characterized by being constructed of aluminum containing silicon as an impurity. (2) The surface acoustic wave resonator according to claim 1, wherein the surface acoustic wave transducer is an interdigital electrode made of the same material as the metal of the reflector.