JPH05267976A - Surface acoustic wave element - Google Patents

Abstract

PURPOSE:To form the surface acoustic wave element which prevents damage of a surface acoustic wave reflector and a surface acoustic wave converter even when it is used by large power for a long period, and in which characteristic variations such as deterioration of Q, an increase of insertion loss, a variation of a passing band, etc., are small. CONSTITUTION:As for an electrode of a surface acoustic wave reflector 14 and a surface acoustic wave converter 13, an aluminum electrode in which 0.01weight%-5weight% palladium is contained and 0.01weight% to 5weight% scandium is contained is used, and generation of electromigration and stress migration is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave resonator and a surface acoustic wave filter, and more particularly to a surface acoustic wave device that handles high frequencies and high power.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave devices have been actively researched for application to oscillators and filters. In particular, due to the recent development of mobile communication and higher frequencies, higher frequencies of surface acoustic wave devices are required.

The basic structure of a resonator of a surface acoustic wave element is, as is well known, that a comb-shaped electrode (interdigital transducer) is formed on a piezoelectric substrate to excite a surface acoustic wave, and strips are formed on both sides thereof. Width λ /
4 (λ: wavelength of a surface acoustic wave), a plurality of metal strips are periodically arranged at a λ / 2 pitch to form a grating reflector. The surface acoustic wave excited by the interdigital transducer propagates on the piezoelectric substrate, but is reflected to the interdigital transducer side by the reflectors provided on both sides. The amount of reflection per wire is small, but the number of strips reduces the reflection to 1
These reflections are superposed because they occur at 1/2 wavelength interval,
As a result, the reflection amount becomes almost 1. In such a situation, a strong standing wave of surface acoustic waves stands on the piezoelectric substrate. This phenomenon is exactly the same as the resonance phenomenon of a crystal unit. In this way, a resonator similar to a crystal oscillator can be realized by appropriately adjusting the position and the number of reflectors.

As is well known, the basic structure of a surface acoustic wave filter is such that at least one set of surface acoustic wave converters is formed on a piezoelectric substrate to excite surface acoustic waves. The device is used as an input and the surface acoustic wave converter on the opposite side is used as an output, and is used as a filter by utilizing the frequency characteristic of conversion of electric signal-surface acoustic wave-electric signal.

[0005]

By the way, in such a surface acoustic wave element, aluminum is usually used as an electrode. This is because it is advantageous over other metals in terms of good workability, small density and light weight.
When aluminum was used as the electrode metal and the above-mentioned resonator was prepared and durability was confirmed, changes in the resonance frequency and a decrease in Q were confirmed with time. When the electrode of this resonator was observed with a microscope, the electrode of the reflector was damaged.

Further, when a surface acoustic wave filter was similarly formed and durability was confirmed, a change in pass band and an increase in insertion loss were observed as in the resonator. This was also due to the deterioration of the electrodes as in the resonator.

The reason why such a phenomenon occurs is that the surface acoustic wave converter is an electromechanical converter. It is presumed that the electrode deterioration is caused by electromigration and stress migration. In the case of a normal small signal input, the current flowing through the electrode and the mechanical displacement of the surface acoustic wave are small and the electrode is less likely to deteriorate. However, when a large signal is input, or when the energy is concentrated on one part like a resonator, electromigration easily occurs due to the current flowing through the electrodes. In addition, the mechanical displacement of the surface acoustic wave also increases, and stress migration occurs. Particularly in recent years, a surface acoustic wave filter is often used as a high frequency filter for mobile communication, especially for downsizing of mobile phones. However, the high frequency circuit section of this telephone system uses the 800-900 MHz band, and the electrode width of the surface acoustic wave converter is about 1 μm. Therefore, electromigration and stress migration are likely to occur, and in particular, as a filter where a large signal passes, a dielectric filter having a large volume is currently used.

[0008]

[Means for Solving the Problems] In view of the above points,
In the present invention, the electrodes of the surface acoustic wave reflector and the surface acoustic wave converter contain palladium in an amount of 0.01% by weight or more and 5% by weight or less, and scandium in an amount of 0.01% by weight or more and 5% by weight or less.
Aluminum contained below is used to prevent the occurrence of electromigration and stress migration.

[0009]

[Function] The electrode of the surface acoustic wave element is 0.01% palladium.
By using aluminum containing at least 5% by weight and not more than 0.01% by weight and not more than 5% by weight of scandium, deterioration of the surface acoustic wave element can be suppressed even when a high power is used for a long period of time. It is a thing.

[0010]

(Embodiment 1) Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. 1 shows a surface acoustic wave resonator according to a first embodiment of the present invention. This surface acoustic wave resonator forms an interdigital transducer 13 as a surface acoustic wave converter for converting an electric signal input from an electric terminal 12 into a surface acoustic wave on a piezoelectric substrate 11 made of lithium tantalate or the like. The surface acoustic wave reflector 1 for reflecting surface acoustic waves to the interdigital transducers on both sides of the interdigital transducer 13
4 is formed in the configuration of a grating reflector. This grating reflector is formed by arranging a large number of metal strips each having a strip width of λ / 4 at intervals of λ / 2, and the reflections from the respective metal strips constituting the reflector are superimposed in phase. In the surface acoustic wave resonator having such a structure, the electrodes of the surface acoustic wave converter and the surface acoustic wave reflector are formed of aluminum containing palladium and scandium.

As a piezoelectric substrate, 36 ° Y-cut X-propagation
Interdigital transducer 1 using LiTaO3
3 is composed of 20 pairs of electrode fingers, and the grating reflector 1
4 was composed of 100 strips. As the electrodes, aluminum containing 0.2% by weight of palladium and 0.2% by weight of scandium was used. In order to confirm the effect of the electrode with the same structure, the one using pure aluminum for the electrode was also prepared and compared. This surface acoustic wave resonator has 10
When a power of mW was applied and left for 1000 hours, the resonance frequency changed with the characteristics shown in FIG. As can be seen from this, it can be clearly seen that the durability is better when palladium and scandium are mixed than when pure aluminum is used for the electrode. Although the reason for this has not been clarified exactly, it is considered that impurities such as palladium and scandium are precipitated at the crystal grain boundaries of aluminum, and these serve as nuclei to prevent deterioration due to metal fatigue. In this way, it is possible to obtain a surface acoustic wave resonator that can be used for a long period of time, that is, the characteristic of the resonator is small.

In order to confirm the effect of the added amounts of palladium and scandium, various compositions were prepared by changing the added amount of palladium and the added amount of scandium from 0.01% by weight to 5% by weight in the above constitution. Although the effect was confirmed, the durability was confirmed to be improved in this range although the effect was large and small. Further, among the amounts of scandium added from 0.01% by weight to 5% by weight, the effects of improving durability were particularly remarkable when the amount of scandium added was from 0.05% by weight to 0.3% by weight.

With the same structure, an electrode having a palladium content and a scandium content of not more than 0.01% by weight was prepared and tested. No significance was seen by comparison. On the contrary, an electrode made of aluminum having a palladium content and a scandium content of 5% by weight or more was prepared and tested, but the electrode resistance was increased as compared with the case where pure aluminum was used as the electrode. It was not possible to put it into practical use due to the deterioration of the characteristics, which is thought to be due to the above, and problems during the manufacturing process.

(Embodiment 2) Next, with the configuration shown in FIG.
A 0MHz filter was constructed. The electrodes have a multi-electrode configuration of 4 inputs 21 and 5 outputs 22 to reduce insertion loss, and a 36 ° Y-cut X-propagation lithium tantalate 23 substrate is used.
It was used. Each of the input and output surface acoustic wave converters consisted of 20 pairs of interdigital transducers. Here again, in order to confirm the effect due to the difference in the electrodes, two types were created: one using 0.2% by weight of palladium and scandium mixed as the electrode and one using pure aluminum as the electrode. When a power of 2 W was applied to this surface acoustic wave filter and the time change of the insertion loss was measured, the time change as shown in FIG. 4 was shown. As is clear from this figure, when pure aluminum is used as the electrode, the deterioration of insertion loss is severely destroyed, but when palladium and scandium are mixed, there is almost no deterioration and it can withstand sufficient use. I understand. This is also due to electromigration due to printing of high power as described above and stress migration due to large displacement applied to the electrode in proportion to the applied power, and the electrode mixed with palladium and scandium. When is used, it is considered that palladium and scandium, which are impurities, are deposited at the crystal grain boundaries of aluminum and serve as nuclei to prevent deterioration due to metal fatigue.

In order to confirm the effect of the added amount of scandium, various effects were prepared by changing the added amount of palladium and the added amount of scandium from 0.01% by weight to 5% by weight in the above constitution. As a result, it was confirmed that the durability was improved in this range although the effect was large and small.
Further, among the amounts of scandium added from 0.01% by weight to 5% by weight, the effects of improving durability were particularly remarkable when the amount of scandium added was from 0.05% by weight to 0.3% by weight.

With the same constitution, an electrode having a palladium content and a scandium content of not more than 0.01% by weight was used for the electrode, and the test was conducted. No significance was seen by comparison. On the contrary, an electrode made of aluminum having a palladium content and a scandium content of 5% by weight or more was prepared and tested, but the electrode resistance was increased as compared with the case where pure aluminum was used as the electrode. It was not possible to put it into practical use due to the deterioration of the characteristics, which is thought to be due to the above, and problems during the manufacturing process.

Example 3 Next, a surface acoustic wave resonator having the structure shown in FIG. 1 was prepared in the same manner as in Example 1. However, in order to confirm the effect of inclusion of palladium in the electrode, aluminum containing scandium mixed with 0.15% of scandium and aluminum containing scandium containing 0.15% of scandium and palladium of 0.15% were used. When the durability of the surface acoustic wave resonator was measured by using the same material as in Example 1, the effect of palladium addition was confirmed as shown in FIG. Furthermore, various measurements were made by changing the amount of scandium added from 0.01% by weight to 5% by weight and the amount of palladium added from 0.01% by weight to 5% by weight. It was confirmed that the durability was improved. Further, among the amounts of scandium added from 0.01% by weight to 5% by weight, the effects of improving durability were particularly remarkable when the amount of scandium added was from 0.05% by weight to 0.3% by weight.

(Embodiment 4) Next, in order to confirm the effect of addition of scandium, aluminum containing aluminum mixed with palladium and aluminum containing scandium or palladium were used as electrodes. A surface acoustic wave resonator was created. The durability of these surface acoustic wave resonators was measured by the same method as in Example 1. As a result, 0.15% by weight of scandium and 0.1% of palladium were added to aluminum as electrodes.
In order to obtain the same effect as when using the one mixed with 2% by weight, it was necessary to use aluminum mixed with 3% by weight of palladium as an electrode. When comparing the characteristics of these two resonators, a difference due to the difference in electrode resistance was confirmed. As the electrode, aluminum scandium 0.15 wt% and palladium 0.2 wt% were used. The surface acoustic wave resonator had better characteristics. Experiments were carried out with various amounts of scandium added. In all cases, when scandium was added and when no scandium was added, the same durability was obtained when scandium was not added, and when scandium was added. Need to have more palladium content than
Therefore, it was confirmed that the characteristics were deteriorated due to the increase of the electrode resistance.

In these embodiments, the piezoelectric substrate is 36
Although Y-cut X-propagation LiTaO3 was used, it is clear that similar effects can be obtained with other cut-angle LiTaO3 and other piezoelectric substrates such as LiNbO3.

[0020]

According to the present invention, the surface acoustic wave reflector and the surface acoustic wave converter are prevented from being damaged even when they are used for a long period of time at a high electric power. It is possible to form a surface acoustic wave element with a small change in characteristics.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a surface acoustic wave resonator according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a change with time of a resonance frequency in the first embodiment.

FIG. 3 is a configuration diagram of a surface acoustic wave filter according to the second embodiment.

FIG. 4 is a diagram showing a change over time of insertion loss in the second embodiment.

FIG. 5 is a diagram showing a change over time of insertion loss in the third embodiment.

[Explanation of symbols]

 11 Piezoelectric Substrate 36 ° Y-cut X Propagation LiTaO3 12 Electrical Terminal 13 Interdigital Transducer 14 Surface Acoustic Wave Reflector 21 Input Terminal 22 Output Terminal 23 Piezoelectric Substrate 36 ° Y-cut X Propagation LiTaO3

Front page continuation (72) Keiji Onishi Keiji Onishi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (4)

[Claims] 1. A piezoelectric substrate and a surface acoustic wave converter for converting an input electric signal provided on the piezoelectric substrate into a surface acoustic wave propagating on the piezoelectric substrate. A surface acoustic wave converter comprising: a reflector formed by periodically arranging a plurality of metal strips facing the surface acoustic wave converter and reflecting the surface acoustic wave provided on the piezoelectric substrate. Surface acoustic wave reflector with palladium 0.01
A surface acoustic wave device comprising aluminum containing at least 5% by weight but not more than 0.01% by weight and not more than 5% by weight of scandium. 2. The surface acoustic wave device according to claim 1, wherein the content of scandium in aluminum is set to be 0.05 to 0.3% by weight. 3. A piezoelectric substrate, and at least one set of surface acoustic wave transducers for converting an input electric signal provided on the piezoelectric substrate into surface acoustic waves propagating on the piezoelectric substrate. In a filter having a structure in which an electric signal propagates as a surface acoustic wave between the surface acoustic wave converters, the surface acoustic wave converter contains 0.01% by weight of palladium.
5% by weight or less, and scandium of 0.1%.
A surface acoustic wave element, which is composed of aluminum in an amount of 05 wt% or more and 5 wt% or less. 4. The surface acoustic wave device according to claim 3, wherein the content of scandium in aluminum is particularly set to 0.05 to 0.3% by weight.