JPH02131609A - Surface acoustic wave oscillator - Google Patents

Abstract

PURPOSE:To obtain a stable resonator whose Q is not deteriorated by forming an oscillation circuit in a way such that the stress by a surface acoustic wave on the surface of a piezoelectric substrate of a surface acoustic wave resonator is selected to be a specific value. CONSTITUTION:The metallic strip of a grating reflector 32 is made of aluminum including silicon as its impurity and an oscillation circuit is formed in such a way that the stress by a surface acoustic wave on the surface of a piezoelectric substrate of the surface acoustic wave resonator is 10<5>N/m<2> or over. The strip line width of the interdigital electrode 32 and the grating reflectors 33, 34 is selected to be, e.g., 9.0mum and the interval between strip lines is selected to be 9.0mum. Moreover, the interval between the interdigital electrode 2 and the grating reflectors 33, 34 is selected to be 22.5mum and the aperture length is selected to be 0.7mm respectively. The grating reflectors 33, 34 are manufactured by mixing 2% of silicon to aluminum and vapor-depositing the result onto an LiTaO3 substrate in the thickness of 1.3mum.

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 surface acoustic wave resonator is already known, for example, as described in Japanese Patent Application Laid-open No. 51-244. In other words, an interdigital electrode for exciting surface acoustic waves is formed on a piezoelectric substrate, and multiple metal strips with a strip width of λ/4 (λ: surface acoustic wave wavelength) are arranged on both sides of the interdigital electrode in a pitch of λ/2. It is constructed by forming grating reflectors arranged periodically. The surface acoustic waves excited by the interdigital electrodes propagate on the piezoelectric substrate, but are reflected toward the center by grating reflectors installed on both sides. This amount of reflection is small for each strip, but since multiple strips each produce reflections with a period length of 172 wavelengths, these reflections are added together.
The combined reflection amount is approximately 1. At this time, a strong standing wave of surface acoustic waves is created on the piezoelectric substrate. This phenomenon corresponds exactly to the resonance of a crystal oscillator. 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 its advantages such as good workability. When operated, it was observed that 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 and cracked, which caused the reduction in the resonant frequency and Q. Although there have been no reports of such a phenomenon, various studies have revealed that it is due to the following phenomenon unique to surface acoustic wave resonators. In other words, in a surface acoustic wave resonator, a large standing wave of surface acoustic waves is created on the piezoelectric substrate as mentioned above. As a result, stress is applied to the aluminum film of the grating reflector due to this surface acoustic wave energy. 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 the relationship shown in Figure 2. That is, the shaded area 21 in FIG. 2(a) is the deteriorated part of the aluminum film, and deterioration was observed in the shaded area 24 in the figure, excluding the interdigital electrode part 22 and the peripheral area of the grating reflector 23. This is also shown in Figure 2 (b
), the center part of the interdigital electrode 22 and the outer end part 25 of the reflective strip
It was found that the stress node corresponds to the stress node and the inner end 26 of the reflective strip corresponds to the stress antinode, and that the antinode part of the standing wave stress, that is, the part corresponding to the large stress part has deteriorated. This confirmed that the cause of the aluminum film deterioration mentioned above was due to standing wave stress. At the interdigital electrode 22, the surface acoustic wave energy is at its maximum intensity, but the aluminum film of the electrode finger does not deteriorate. This is because the node of the standing wave is located exactly at the electrode finger. Therefore, if the conventional grating reflector is composed of groups (grooves), etc., the resonator must be excited with a considerable intensity. This was an undetected phenomenon. The present invention has been made in view of these points, and even when the resonator is used under conditions where the resonator is excited at a high excitation level for a sufficient period of time, the metal strip of the grating reflector does not deteriorate and the resonant frequency does not decrease. The purpose is to provide a surface acoustic wave oscillator using a stable 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 silicon as an impurity. However, the magnitude of stress due to surface acoustic waves on the piezoelectric substrate surface of this elastic surface resonator is 10
5N/Il! The oscillation circuit is constructed as described above. 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, stable characteristics can be obtained without a decrease in the resonance frequency or a decrease in Q even during long-term use. Can be done.

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) or lithium niobate (Li).
A transducer for converting an input electric signal into a surface acoustic wave propagating on the piezoelectric substrate 31 is mounted on a piezoelectric substrate 31 such as NbO3, for example, a pair of comb-shaped electrodes 32a,
An interdigital electrode 32 is formed by interlocking the electrodes 32b with each other. 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 3l. Further, on the piezoelectric substrates 3l on both sides of the interdigital electrode 32, there are reflecting gratings 1}33. for reflecting the surface acoustic waves excited by the interdigital electrode 2, respectively. 34 is formed. This grating reflector 33. 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 all the reflected waves reflected from each metal strip are added in the same phase. The ends of these metal strips are also electrically shorted to each other. 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 tricone 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 anything over 50% can no longer be considered an impurity. 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.

The surface acoustic wave resonator of the present invention, which has a grating reflector made of aluminum containing silicon as an impurity, has the following remarkable effects. Figure 4 shows the surface acoustic wave resonator according to the present invention designed under the following conditions, and the oscillator is operated in an atmosphere at a temperature of 65°C so that an excitation power of 2 mV is applied to the resonator. This figure shows the rate of change in the resonant frequency of the resonator over time. In other words, the design conditions for the surface acoustic wave resonator are to use X-cut LiTa03 as the piezoelectric substrate,
The propagation direction of the surface acoustic wave was set to a direction inclined by 112a from the Y axis. The interdigital electrode 2 consists of 11 pairs of electrode fingers, and a grating reflector 33. 34 each consisted of 200 strips. Furthermore, the strip line widths of the interdigital t-pole 32 and the grating reflectors 33 and 34 are both 9.0. year,
Similarly, the spacing between the strips was 9.0-. Furthermore, an interdigital electrode 2 and a grating reflector 33. The end spacing of 34 was 22.5-mm, and each opening length was 0.7 am. Grating reflector 33. 34 is aluminum with 2% silicon
It was fabricated by mixing LiTaO with a thickness of 1.3 μs and depositing it on a substrate. Also, for comparison with the surface acoustic wave resonator according to the present invention, a grating reflector 33. 34 was similarly fabricated using a pure aluminum film. In FIG. 4, a curve 41 shows the characteristics of the resonator when the grating reflector is formed of the pure aluminum film,
Curve 43 shows the characteristics of the resonator of the present invention in which the grating reflector is made of aluminum mixed with 2% silicon. As is clear from this figure, in the case where the grating reflector is made of pure aluminum film, the resonant frequency significantly decreases over time, whereas in the surface acoustic wave resonator according to the present invention, the resonant frequency decreases significantly. It can be seen that the amount has been 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 silicon is mixed as an impurity. The decrease is about 173
It can be suppressed to Note that a slight decrease in the resonant frequency was also observed in the surface acoustic wave resonator according to the present invention in which silicon was mixed with aluminum, as described above.
In order to investigate whether the cause of this is due to deterioration of the reflector strip due to the standing wave stress of surface acoustic waves, two types of resonators, one made of pure aluminum and one made of aluminum mixed with silicon, were left in a non-operating state. When the changes in the resonant frequency after each time period were measured, characteristics almost equal to curve 43 in FIG. 4 were obtained in all cases. This indicates that the decrease in the resonant frequency of 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 become clear 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 there was almost no deterioration in the strip film even after long-term operation. Figure 5 is a photograph of aluminum mixed with silicon as an impurity and left for 100 hours without applying power, but there was almost no change even when 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 is not a damaged part of the electrode. This silicon residue can be removed by etching, etc., 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 reflector is made of pure aluminum, as 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, 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 becomes.

In this way, the deterioration of pure aluminum reflector strips varies depending on the magnitude of the excitation level, but in general, the deterioration becomes a problem when the excitation level is higher than Hefeng V.
It is difficult to clearly state whether the present invention is effective in this regard. This is because in this experimental example, the excitation level is 0.5mV.
Deterioration was observed in the aluminum reflector strip at a lower level, 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, the magnitude of stress on the resonator surface is 10
It is thought that deterioration of the reflector strip becomes a problem when the stress exceeds 1000 m (Newton/m), and the present invention is effective because the reflector strip does not deteriorate even when such stress is applied. The reason why the aluminum film deteriorates less than that of pure aluminum has not been elucidated exactly, but impurities precipitate at the grain boundaries of aluminum, and these serve as nuclei to form boundaries, which are caused by metal fatigue. Thus, according to the present invention, it is possible to obtain a surface acoustic wave oscillator using a surface acoustic wave resonator whose resonant frequency does not change much even when used for a long time. It was also found that the following new effective effects can be obtained using a surface acoustic wave resonator: In a surface acoustic wave resonator whose grating reflector is made of pure aluminum film, the resonance resistance is 24 ohms and the Q is 12,000, whereas aluminum mixed with 2% silicon had a resonance resistance of 17 ohms and a Q of about 17,000.This is the average value of many prototype samples, and all of them had a Although there is a variation of about %, 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. , 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 can be constructed. is also extremely effective.

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 embodiments, an X-cut LiTaO was used as the piezoelectric substrate, but quartz, LiNbO. It is also applicable and effective to piezoelectric substrates such as 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 silicon but also copper, Ni. Cr. It is also thought to be effective to mix Mg etc. together. 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, it is possible to create a surface acoustic wave resonator in a pure aluminum film. This can be done in exactly the same process as the manufacturing 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. 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 part and the standing wave of the surface acoustic wave. FIG. 5 is a micrograph of a reflector strip according to the present invention, and FIG. 6 is a diagram showing changes in resonant wave number as a function of surface wave excitation level. 3l...Piezoelectric substrate, 32...Interdigital electrode, 33. 34...Grating reflector. Agent Patent Attorney Yudo Noriyuki Chika Kikuo Takehana Figure 2 32b Figure 3 B#f Alternative (Written Procedural Amendment (Method))

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 silicon 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 I10^5N/m^
A surface acoustic wave oscillator comprising two or more oscillation circuits. (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.