JPH02153613A - Manufacture of surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To obtain a stable characteristic in long use by generating the metallic strips of grating reflectors by means of aluminium in which copper is included as impurity. CONSTITUTION:An inter-digital electrode 32 in which a pair of comb line electrodes 32a and 32b are mutually engaged is generated on a piezo-electric substrate 31. The grating reflectors 33 and 34 for reflecting a surface acoustic wave excited by the inter-digital electrode are generated on the piezo-electric substrates 31 on both sides of the electrode 32. The metallic strips constituting the reflectors 33 and 34 set aluminium in which copper is included as impurity to be a target, and an aluminium film is generated by sputter evapor deposition. Then, the strips are generated from the aluminium films by etching and the like. Thus, the surface acoustic wave resonator in which the deterioration of the metallic strips of the grating reflectors 33 and 34 and a resonant frequency is prevented in long use can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing 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. 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 reflection is reduced by 1/2 each.
Because of the periodic length of the wavelength, these reflections will be additive.

The combined reflection amount 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 (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. The micrograph in Figure 1(b) clearly shows that the central part was damaged and cracked, which caused the 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 a phenomenon unique to first-order 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(a) is the 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.

The present invention has been made in view of the above points, and an object of the present invention is to provide a method for manufacturing 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. The purpose is to

Another object of the present invention is to provide a method for manufacturing a surface acoustic wave resonator that has a large Q and does not decrease in Q during 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. It was designed to do so. In other words.

The basic configuration of the present invention is that the metal strip constituting one reflector targets aluminum containing copper as an impurity, and forms an aluminum film by sputter deposition.
This aluminum film is then formed by etching or the like. At this time, the interdigital electrodes may also be formed of the same material and in the same process as the reflector. However, according to the manufacturing method of the surface acoustic wave resonator of the present invention in which the grating reflector is formed of such a metal material, stable characteristics can be maintained without a decrease in the resonant frequency or a decrease in Q even during long-term use. Obtainable.

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

FIG. 3 shows a surface acoustic wave resonator manufactured by a method for manufacturing 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 (LiTaOi), lithium niobate (LiNb○,)
A transducer for converting an input electric signal into a surface acoustic wave propagating on the piezoelectric substrate 31, such as a pair of comb-shaped electrodes 32a and 32b, is installed on the piezoelectric substrate 31, such as
interdigital electrodes 32 formed by interlocking with each other.
is formed. This interdigital electrode 32 transfers the input electric signal supplied to the input terminal IN to the piezoelectric substrate 3.
1 into a surface acoustic wave propagating on the surface. Further, on the piezoelectric substrate 31 on both sides of the interdigital electrode 32, there are grating reflectors 33 for reflecting the surface acoustic waves excited by the interdigital electrode 2.
．． 34 is formed. This grating reflector 3
3.34 is a structure in which a large number of metal strips each having a strip width of λ/4 are arranged at a pitch of λ/2, so that the reflected waves reflected by each metal strip are all added in the same phase.

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 performance and processing of the resonator to be approximately lθ% or less of the total.

According to the method of manufacturing a 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 the results when a surface acoustic wave resonator manufactured by the method of manufacturing a 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. , which shows the resonant frequency change rate of the resonator over time. That is, the design conditions for the surface acoustic wave resonator were as follows: an X-cut LiTa○3 was used as the piezoelectric substrate, and the propagation direction of the surface acoustic wave was set to be inclined at 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. Further, the strip line widths of the interdigital electrode 32 and the grating reflectors 33 and 34 were all 9.0 μs, and the interval between the strips was also 9.0 μs. Further, the interval between the ends of the interdigital electrode 2 and the grating reflector 33, 34 was set to 22.5 μs, and the aperture length of each of these was set to 0.7 mn. Grating reflector 33.
No. 34 was manufactured by using aluminum mixed with 4% copper as a target and depositing an aluminum film to a thickness of 1.3 μs on a Li Ta 03 substrate by sputter deposition. 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, the resonant frequency of the grating reflector formed from a pure aluminum film significantly decreases over time, whereas According to the surface wave resonator, it can be seen that the drop in the resonant frequency is significantly reduced. In other words, after 1000 hours, the rate of change in the resonant frequency is -0,045% when pure aluminum is used, whereas it is -0,017% in the case of the present invention in which copper is mixed as an impurity. The decrease can be suppressed to about 1/3. It should be noted that a slight decrease in the resonant frequency was observed in the surface acoustic wave resonator manufactured by the method of manufacturing a surface acoustic wave resonator of the present invention in which copper was mixed with aluminum as described above, but the cause of this was due to the elasticity of the surface acoustic wave resonator. In order to examine whether the deterioration of the reflector strip is due to the standing wave stress of surface waves, two types of resonators, one made of pure aluminum and one made of aluminum mixed with copper, were left in a non-operating state, and the time elapsed. When the changes in the resonant frequency were measured later, characteristics almost equal to those of the curve 42 in FIG. 4 were obtained in all cases.

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 manufactured by the surface acoustic wave resonator manufacturing method 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, as shown in Figure 5. . 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, in a reflector made of pure aluminum, when the excitation level is increased, resonance occurs. 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.However, in the present invention, the resonant frequency does not change even if 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.
In contrast, it is difficult to clearly state whether the present invention is effective. This is because in this experimental example, the excitation level is 0.5 mW.
Deterioration was observed in the aluminum reflector strip at a lower level, but this excitation level does not necessarily correspond to changes in 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 (105 (Ne/m) or more). The present invention is effective without causing any deterioration of the reflector strip even when such stress is applied.The exact reason why the deterioration of the aluminum film containing impurities is less than that of pure aluminum is as follows. Although not fully understood, it is thought that impurities precipitate at the grain boundaries of aluminum, and these 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 method for manufacturing a surface acoustic wave resonator according to the present invention provides the following new and effective effects. In other words, in a surface acoustic wave resonator whose grating reflector is made of pure aluminum film, the resonance resistance is 2.
At 4 ohms, the Q is about 12,000, whereas with aluminum mixed with 4% copper, a resonance resistance of 14 ohms and a Q of 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. So you can configure
The method of manufacturing a 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, X-cut LiT was used as the piezoelectric substrate.
Although the case using aO3 has been explained, crystal LiNb
0. It is also applicable and effective to piezoelectric substrates such as the following. 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, Ni, Cr1
It is also thought to be effective to mix Mg or the like together.

In addition, in order to form an aluminum film mixed with these impurities, if an aluminum film mixed with an appropriate weight of impurities is used as a target and sputter deposition is performed, the manufacturing process is exactly the same as the manufacturing process of a surface acoustic wave resonator using a pure aluminum film. It can be done in the process.

[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. A diagram showing the stress distribution of a deteriorated part and a standing wave of a surface acoustic wave, FIG. 3 is a diagram showing an example of the method for manufacturing a surface acoustic wave resonator of the present invention, and FIG. 4 is a diagram showing an example of the method of manufacturing a surface acoustic wave resonator of the present invention. Figures 5(a) and (b) are micrographs of a reflector strip according to the present invention, and Figure 6 shows the change in resonance frequency as a function of the surface wave excitation level. It is a figure showing a change. 31...Piezoelectric substrate. 32... Interdigital electrode, 33, 35... Grating reflector. Agent Patent Attorney Nori Ken Yudo Takehana Kikuo Figure 2 Figure 3 oO0 Figure

Claims (3)

[Claims] (1) forming a surface acoustic wave transducer for converting an input electric signal into a surface acoustic wave on a piezoelectric substrate; forming a reflector formed by periodically arranging a plurality of metal strips for reflecting the surface acoustic waves on the surface of the surface acoustic wave; A method of manufacturing a surface acoustic wave resonator, characterized in that the process comprises forming an aluminum film by sputter deposition using aluminum containing copper as an impurity as a target. (2) The step of forming the surface acoustic wave transducer targets aluminum containing copper as an impurity,
2. The method of manufacturing a surface acoustic wave resonator according to claim 1, wherein the step comprises forming an aluminum film by sputter deposition. (3) Manufacturing the surface acoustic wave resonator according to claim 2, wherein the step of forming the surface acoustic wave transducer and the step of forming the reflector are performed simultaneously. Method.