JPS6147011B2 - - Google Patents

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, for example, disclosed in Japanese Patent Application Laid-Open No. 1986-
Already known as described in No. 244. That is, an interdigital electrode for exciting surface acoustic waves is formed on a piezoelectric substrate, and a large number of metal strips with a strip width of λ/4 (λ: surface acoustic wave wavelength) are arranged on both sides of the interdigital electrode.
It is constructed by forming grating reflectors arranged periodically in pitches. The 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 since multiple strips each produce reflections with a period length of 1/2 wavelength, these reflections are added, and the combined amount of reflection is is almost close to 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 oscillator. By appropriately setting the position of the crating reflector in this manner, 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 its advantages such as good workability. When the device was operated, it was observed that the resonant frequency decreased significantly over time, and at the same time, the resonant resistance increased and the Q factor decreased. At this time, the grating reflector was observed using an electron microscope (magnification
(2000x) Before use, the aluminum film had no damage as seen in the micrograph in Figure 1a, but after long-term use, the center part was damaged as seen in the micrograph in Figure 1b. 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 a relationship as shown in Figure 2. That is, the shaded area 21 in FIG.
Deterioration was observed in the shaded area 24 in the figure, excluding the peripheral area. Also, if we compare this with the stress of the standing wave in Figure 2b, we can see that the central part of the interdigital electrode 22 and the outer end 25 of the reflective strip are stress nodes, and the inner end 26 of the reflective strip is the stress antinode. It was found that the antinode part 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. That is.

Another object of the present invention is to provide a surface acoustic wave resonator which has a large Q and whose Q does not decrease even when used for a long time.

The present invention was developed based on the results of a study on the metal strip material of the grating reflector, since a decrease in the resonant frequency and a decrease in Q of a surface acoustic wave resonator are caused by deterioration of the grating reflector due to stress. The metal strip is 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, even when used for a long time, 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 converts an input electric signal onto a piezoelectric substrate 31 such as lithium tantalate (LiTaO ₃ ) or lithium niobate (LiNbO ₃ ) into a surface acoustic wave that propagates on the piezoelectric substrate 31 . For example, an interdigital electrode 32 is formed by interdigitating a pair of comb-like electrodes 32a and 32b. 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. Further, grating reflectors 33 and 34 are formed on the piezoelectric substrates 31 on both sides of the interdigital electrode 32, respectively, for reflecting the surface acoustic waves excited by the interdigital electrode 2. This grating reflector 33,3
Reference numeral 4 has a large number of metal strips each having a strip width of λ/4 arranged at a period of λ/2 pitches, so that the reflected waves reflected from 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 with such a configuration,
In the present invention, the metal strip of the grating reflector 32 is made of aluminum doped with silicon. There is no particular limit to the silicon content, but given that even pure aluminum contains about 0.01% silicon, it should be higher than that, and if it exceeds 50%, it can no longer be considered an impurity. Practically speaking, it is thought that a ratio of about 10% or less of the total 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. That is, the design conditions for the surface acoustic wave resonator were as follows: X-cut LiTaO 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 the grating reflector 3
3 and 34 each consisted of 200 strips. The strip line widths of the interdigital electrode 32 and grating reflectors 33 and 34 were all 9.0 μm, and the spacing between the strips was also 9.0 μm. Furthermore, the distance between the ends of the interdigital electrode 2 and the grating reflectors 33 and 34 was set to 22.5 μm, and the aperture length of each of these was set to 0.7 mm. Grating reflector 3
Nos. 3 and 34 were fabricated by mixing 2% silicon into aluminum and depositing the mixture to a thickness of 1.3 μm on a LiTaO ₃ substrate. Further, in order to compare with the surface acoustic wave resonator of the present invention, one in which grating reflectors 33 and 34 were formed of pure aluminum film was similarly manufactured.

In FIG. 4, a curve 41 shows the characteristics of the resonator when the grating reflector is made of the pure aluminum film, and a curve 43 shows the characteristics of the resonator when the grating reflector is made of aluminum mixed with 2% silicon. It shows the characteristics of the resonator. As is clear from this figure, in the case where the grating reflector is made of pure aluminum film, the resonant frequency decreases significantly over time, whereas according to the surface acoustic wave resonator of the present invention, the resonant frequency decreases significantly over time. It can be seen that the drop has been significantly reduced. In other words, after 1000 hours, if pure aluminum is used, the rate of change of the resonant frequency is −
While it is 0.045%, in the present invention in which silicon 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 of the present invention in which silicon was mixed with aluminum, as described above, but this was due to the stress of the reflector strip due to the standing wave stress of the surface acoustic wave. In order to investigate whether this was due to deterioration in the Also, almost the same characteristics as curve 43 in FIG. 4 were obtained.

From this, it was found that the decrease in the resonant frequency in the resonator of the present invention was not due to deterioration of the grating reflector due to standing wave stress, but was 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 hardly deteriorated even after being operated for a long time.

Figure 5 is a photograph of aluminum mixed with silicon as an impurity and left for 1000 hours without applying power, but there was almost no change even when power was applied. It should be noted that although a horizontal white linear part is seen at the lower side of the strip film in the figure, this is silicon residue and is not a damaged part of the electrode.
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, and curve 61
As shown in curve 64, when the excitation level is increased in a reflector made of pure aluminum, the resonant frequency changes significantly, whereas according to the present invention, as shown in curve 65, the resonant frequency changes significantly when the excitation level changes. It can be seen that the resonant frequency hardly changes. 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.In contrast, 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 becomes.

In this way, the deterioration of pure aluminum reflector strips varies depending on the magnitude of the excitation level, but in general, deterioration becomes a problem when the excitation level exceeds several milliwatts, and the present invention is effective against this. It is difficult to say clearly whether there is. This is because in this experimental example, deterioration was observed in the aluminum reflector strip when the excitation level was below about 0.5 mW, but this excitation level does not necessarily correspond if the substrate material, resonant frequency, electrode shape, etc. change. 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 10 ⁵ (NaWtm/m ² ) or more. The present invention is effective because the reflector strip does not deteriorate even when such stress is applied.
The reason why an aluminum film mixed with impurities deteriorates less than pure aluminum has not been precisely elucidated, but impurities precipitate at the grain boundaries of aluminum, which become nuclei and form 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 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 24 ohms and a Q of approximately 12,000, whereas 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 approximately 12,000. A Q of approximately 17,000 was obtained. This is the average value of many prototype samples, and there is a variation of about 20% in each case.
A significant increase in Q is observed compared to when the grating reflector is made of pure aluminum. As mentioned above, surface acoustic wave resonators are applied to oscillators and filters, and the larger the Q of the resonator, the more stable the oscillator can be constructed, and the larger the Q of the resonator, the lower the insertion loss of the filter. 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 the above embodiment, the case where X-cut LiTaO ₃ was used as the piezoelectric substrate was explained, but the present invention can be similarly applied to piezoelectric substrates such as quartz or LiNbO ₃ and is effective. 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, Ni, Cr, Mg, etc. as impurities in aluminum.

In addition, to form an aluminum film mixed with these impurities, sputter deposition is performed using aluminum mixed with an appropriate weight 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.

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 elastic surface 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 is a 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. FIG. 5 is a micrograph of a reflector strip according to the present invention, and FIG. 6 is a diagram showing the change in resonant wave number with respect to change in surface wave excitation level. 31...Piezoelectric substrate, 32...Interdigital electrode, 33, 34...Grating reflector.

Claims (1)

[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 a reflector formed by periodically arranging a large number of metal strips for reflecting the surface acoustic waves provided on the piezoelectric substrate facing the transducer, the metal constituting the reflector; A surface acoustic wave resonator characterized in that the strip is made 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 strip of the reflector.