JPH02153612A - Surface acoustic wave oscillator - Google Patents

Abstract

PURPOSE:To obtain a stable characteristic even in long use by setting the size of stress by means of the surface acoustic wave of a surface acoustic wave resonator, on the surface of a piezo-electric substrate to more than a specified value. CONSTITUTION:An inter-digital electrode 32 where a pair of comb line electrodes 32a and 32b are mutually engaged is generated on the piezo-electric substrate 31, and grating reflectors 33 and 34 are generated on both sides of the electrode 32. The metallic strips of the reflectors 33 and 34 are composed of aluminium in which copper is included as impurity. An oscillation circuit in which the size of stress by the surface acoustic wave on the surface of the piezo-electric substrate 31 in the surface acoustic wave resonator is set to >=10<5>N/m<2> is constituted. The deterioration of the metallic strips in the grating reflectors 33 and 34 is prevented and the stable characteristic can be obtained in a condition that the resonator is excited in a considerably high excitation level for a long time.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave oscillator using 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 zero surface acoustic wave resonator is described, for example, in Japanese Patent Application Laid-Open No. 51-244. Already known. In other words, an interdigital electrode for exciting surface acoustic waves is formed on a piezoelectric substrate, and a strip width λ/4 (λ
:Surface acoustic wave wavelength) of multiple metal strips at λ/2
It is constructed by forming grating reflectors arranged periodically at pitches. 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 for each strip, but with multiple strips, the amount of reflection is reduced by 1/2.
Since this is the periodic length of the wavelength, these reflections are added, and the combined reflection amount becomes 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. Where I let it happen.

A phenomenon was observed in which the resonant frequency decreased significantly 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 (magnification: 2000x), it was found that the aluminum film was not damaged in any way before use, as seen in the micrograph in Figure 1 (a), but after long-term use, the grating reflector was not damaged. As is clear from the micrograph in Figure 1 (b), the central part was damaged,
It was found that the structure was in a cracked state, which caused the above-mentioned reduction in the resonant 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 oblique part 21 in FIG. 2(, ) is a deteriorated part of the aluminum film, and deterioration was observed in the hatched part 24 in the figure excluding the interdigital electrode part 22 and the peripheral part of the grating reflector 23. 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. Interdigital electrode 22
At this point, the surface acoustic wave energy is at its maximum intensity, but the aluminum film on the electrode finger does not deteriorate. This is because the node of the standing wave is located exactly at the electrode finger. Therefore, in conventional grating reflectors composed of groups (grooves), this phenomenon could only be observed when the resonator was excited with considerable intensity.

The present invention has been made in view of these points, and is designed to provide stability in which the metal strip of the grating reflector does not deteriorate and the resonant frequency does not decrease even when the resonator is used under conditions where the resonator is excited at a sufficiently high excitation level for a long period of time. The object of the present invention is to provide a surface acoustic wave oscillator using a surface acoustic wave resonator.

Another object of the present invention is to provide a surface acoustic wave oscillator using a surface acoustic wave resonator which has a large Q 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
This was done based on the results of a study on the metal strip material for the grating reflector, since the decrease in the grating reflector is caused by stress-induced deterioration of the grating reflector.The metal strip is made of aluminum containing copper as an impurity. However, the magnitude of the stress due to the surface acoustic wave on the surface of the piezoelectric substrate of this surface acoustic wave resonator is 10'N.
The oscillation circuit is constructed such that the durability is greater than 1/2. However, according to the surface acoustic wave resonator used in the present invention in which the grating reflector is formed of such a metal material, the resonant frequency decreases and the Q
It is possible to obtain stable characteristics without any deterioration.

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 (LiTaO), lithium niobate (Li
A transducer for converting an input electrical signal into a surface acoustic wave propagating on the piezoelectric substrate 31 on a piezoelectric substrate 31 such as N b Ox .

For example, an interdigital electrode 32 is formed by interlocking a pair of comb-shaped electrodes 32a and 32b with each other. This interdigital electrode 32 is connected to the input terminal IN
The input electrical signal supplied to the piezoelectric substrate 31 is converted into a surface acoustic wave that propagates 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 constructed 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 a 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 copper as an impurity. There is no particular limit to the copper content, but considering that even pure aluminum contains about 0.01% copper and silicon, it is higher than that, and 50%.
Once this is reached, these can no longer be called impurities.

Practically speaking, it is considered desirable for the resonator to account for 10% or less of the entire resonator in terms of performance and processing.

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

Figure 4 shows a surface acoustic wave resonator according to the present invention designed under the following conditions, and a power output of 2 mW in an atmosphere at a temperature of 65°C.
This figure shows the rate of change in the resonant frequency of the resonator over the course of one hour when the oscillator is operated so that the excitation power is applied to the resonator.

That is, the design conditions for the surface acoustic wave resonator were such that an X-cut LiTaO was used as the piezoelectric substrate, and the propagation direction of one surface acoustic wave was set in a direction inclined by 112 degrees from the Y axis.

The interdigital electrode 2 consists of 11 pairs of electrode fingers, and each grating reflector 33, 34 has 20 pairs of electrode fingers.
It consisted of 0 strips. Furthermore, the strip line widths of the interdigital electrode 32 and the grating reflectors 33 and 34 are both 9.0. Similarly, the distance between the strips is 9. It was set as OIm.

Furthermore, the distance between the ends of the interdigital electrode 2 and the grating reflector 33, 34 was set to 22.5p, and the aperture length of each of these was set to 0.7 mm. The grating reflectors 33 and 34 are made of aluminum mixed with 4% copper to a thickness of 1.3 μs and made of LiTa0. It was manufactured by vapor deposition on a substrate. 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 42 shows the characteristics of the resonator of the above-mentioned invention in which the grating reflector is made of aluminum mixed with 4% copper. 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. 10
After 00 hours, the rate of change in resonance frequency is -0,045% when pure aluminum is used;
In the present invention in which copper is mixed as an impurity, it is -0,017%, and the reduction in resonance frequency can be suppressed to about 1/3. It should be noted that a slight decrease in the resonant frequency was also observed in the surface acoustic wave resonator according to the present invention in which copper 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 was due to deterioration of the resonator, we left two types of resonators, one made of pure aluminum and one made of aluminum mixed with copper, in a non-operating state, and measured the change in the resonant frequency after each time elapsed. In both cases, characteristics approximately equal to curve 42 in FIG. 4 were obtained.

From this, it was found that the resonator of the present invention had the above-mentioned resonance. In other words, it has been revealed that in the surface acoustic wave resonator according to 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, as shown in Figure 5. Ta. However, the figure (a) shows the state before use, and the figure (b) shows the state after 2000 hours of operation.

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 a reflector strip made of pure aluminum, the higher the excitation level, the greater the deterioration and the change in the resonant frequency. 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, when the excitation level exceeds some ff1W, deterioration becomes a problem, and it is clear whether the present invention is effective against this problem. It is difficult to say. This is because in this practical example, deterioration was observed in the aluminum reflector strip when the excitation level was below about 0.57W.

If the substrate material, resonant frequency, electrode shape, etc. change, this excitation level will not necessarily correspond. however,
Since the cause of reflector strip deterioration is surface wave stress, the magnitude of stress on the resonator surface is 10'
(Newton/m) or higher, deterioration of the reflector strip is considered to be a problem, and the present invention is effective without deterioration of the reflector strip even when such stress is applied. The reason why an aluminum film mixed with impurities deteriorates less than pure aluminum is not precisely clarified, but impurities precipitate at the grain boundaries of aluminum, and these act as nuclei, forming boundaries. This is thought to prevent deterioration due to metal fatigue.

As described above, according to the present invention, it is possible to obtain a surface acoustic wave oscillator using a surface acoustic wave resonator whose resonance frequency hardly changes even when used for a long time.

- It has been found that the surface acoustic wave resonator invented by Rikiki can provide the following new and effective effects. In other words, in a surface acoustic wave resonator in which the grating reflector is made of pure aluminum film, the resonance resistance is 24 ohms and the Q is about 12,000.
%, the resonance resistance is 14 ohms and the Q is about 20.
000 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 according to 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, X-cut LiT was used as the piezoelectric substrate.
a0. Although we have explained the case using crystal Li
It is also applicable and effective to piezoelectric substrates such as N b 03. 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, impurities mixed into aluminum include not only copper but also silicon, N,
It is also considered effective to mix i, Cr, Mg, etc. together.

In addition, to form an aluminum film mixed with these impurities, sputter deposition is performed using an aluminum film that has been mixed with an appropriate amount of impurities as a target.

If heating with a heater or vapor deposition with an electron beam is performed, it can be performed in exactly the same process as the manufacturing process of a surface acoustic wave resonator using a pure aluminum film.

[Brief explanation of the drawing]

Figures 1(a) and (b) are micrographs showing the surface condition of a pure aluminum reflector strip before use and after long-term operation; Figures 2(a) and (b) are of a grating reflector strip. FIG. 3 is a diagram showing an embodiment of the surface acoustic wave resonator according to the present invention. FIG. 4 is a diagram showing the stress distribution of the deteriorated portion and the standing wave of the surface acoustic wave. FIG. 4 is a diagram showing the resonator according to the embodiment of the present invention. 5(a) and (b) are micrographs of a reflector strip according to the invention; FIG. 6 shows the change in resonant frequency as a function of the surface wave excitation level. It is a diagram. 31... Piezoelectric substrate, 32... Interdigital electrode, 33, 35... Grating reflector. Figure 2b Figure -0,51 Mutual) Tatsuriheru 1.52 [mWl Figure 6 Heisei year month and year

Claims (2)

[Claims] (1) a piezoelectric substrate; a surface acoustic wave transducer provided on the piezoelectric substrate for converting an input electrical signal into a surface acoustic wave propagating on the piezoelectric substrate; and a reflector formed by periodically arranging a plurality of metal strips for reflecting the surface acoustic waves provided on the piezoelectric substrate, and the metal strips constituting the reflector, A surface acoustic wave resonator made of aluminum containing copper as an impurity is provided, and the magnitude of stress due to the surface acoustic wave on the surface of the piezoelectric substrate of the surface acoustic wave resonator is 10^5 N/m^2.
A surface acoustic wave oscillator characterized by comprising an oscillation circuit as described above. (2) The surface acoustic wave oscillator according to claim 1, wherein the surface acoustic wave transducer is an interdigital electrode made of the same material as the metal strip of the reflector.