JPH0314309A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To improve stress migration resisting characteristics by forming electrodes by an aluminum film oriented in the fixed direction like crystal orientation. CONSTITUTION:In the surface acoustic wave device utilizing leaked elastic surface waves propagated on a crystal base 1 from 103 deg. rotation Y cut up to 107 deg. rotation Y cut (LST cut), the electrodes 2a, 2b are formed by the aluminium film oriented in the fixed direction like crystal orientation. In such a case, the fine addition of an additive such as Cu, Ti, Ni, Mg, and Pd excellent in stress migration resisting characteristics is effective for the formation of the aluminium film and the proper quantity of its addition is preferably 0.1 to 10wt.%. Consequently, resistance to stress migration due to intergranular diffusion can be increased and the effective temperature characteristics of the surface acoustic wave device using the crystal base can be utilized.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is applicable to a 103° rotation Y cut to a 107° rotation Y cut.
This invention relates to a surface acoustic wave device in which electrodes are provided on the surface of a cut (referred to as LST cut) crystal substrate. [Background Art] In recent years, surface acoustic waves (hereinafter sometimes referred to as SAW) have been developed.
) surface acoustic wave devices such as filters, resonators, and oscillators have become widely used.

These surface acoustic wave devices generally have interdigital electrodes (interdigital interdigital electrodes), metal strip grating electrodes, etc. formed on the surface of a piezoelectric substrate. Aluminum is generally used as this electrode metal because it is easy to photolithography, has low specific gravity, has little electrode load mass effect, and has high conductivity. It is. In addition, recently, approximately 10
5° rotated Y cut (LST cut) crystal substrates are attracting attention. This surface acoustic wave device using an LSAW-cut crystal substrate allows leaky surface acoustic waves (LSAW) to propagate on the substrate surface, and has better temperature characteristics than a surface acoustic wave device using a conventional ST-cut crystal substrate. It has the characteristic of having [Problem to be solved by the invention] However, when a high voltage level signal is applied to such a SAW filter or SAW resonator, the aluminum electrode is subjected to strong stress due to surface acoustic waves, causing migration. I understand. This migration is caused by stress, so it is called stress migration. When this occurs, electrical short circuits, increased insertion loss, and a decrease in the Q of the resonator occur. and,
This stress migration occurs more easily as the frequency increases, so it has become a major problem when increasing the frequency of surface acoustic wave devices. Conventional countermeasures to this problem include adding trace amounts of Cu, Ti, Ni, Mg, Pd, etc. to the aluminum electrode material to improve stress migration resistance, as in the case of electromigration. However, the improvement of its characteristics was still insufficient. Therefore, the inventors of the present invention further investigated the cause of this stress migration. According to the research results, conventional aluminum electrodes made by electron beam evaporation, sputtering, etc. are not crystallographically oriented in a fixed direction and are amorphous polycrystalline films. It was thought that this property is weak against stress migration due to field diffusion. In addition, in the case of conventional ST-cut crystal substrates, no particular problem occurred even if the thickness of the aluminum electrode was about 2% of the surface acoustic wave wavelength, whereas in the case of LST-cut crystal substrates, aluminum It is known that if the thickness of the electrode is 1% or more of the wavelength, the Q of the resonator will drop sharply and the characteristics will deteriorate. On the other hand, in order to avoid such deterioration of characteristics, when the film thickness of the aluminum electrode on the LST-cut crystal substrate is made thinner, the grain size is large in the conventional randomly oriented aluminum film, so the apparent resistance is reduced. This results in an increase in insertion loss and a decrease in Q. This drawback was particularly noticeable in the high frequency range where the wavelength becomes short. However, the present invention has been made based on the above-mentioned drawbacks of the conventional example and the knowledge reached by the inventors, and its purpose is to have excellent stress migration resistance characteristics and increase resistivity even with a thin film thickness. It is an object of the present invention to provide a surface acoustic wave device which is equipped with aluminum electrodes having a small amount of heat and has good temperature characteristics using an LST cut crystal substrate. [Means for Solving the Problems] Therefore, the surface acoustic wave device of the present invention has a concave Y of 103゜.
In a surface acoustic wave device that uses leaky surface acoustic waves propagating on a Y-cut crystal substrate rotated 107 degrees from the cut, we have demonstrated that an electrode with a migration prevention function is formed using an aluminum film oriented in a certain crystal orientation. It is a feature. Further, the aluminum film includes Cu,
It is preferable to add a trace amount of an additive with excellent migration resistance such as Ti, Ni, Mg, or Pd, and the amount added is preferably 0.1 wt% to 10 wt%. [Function] As mentioned above, conventional aluminum electrodes are amorphous polycrystalline films that are not oriented in a fixed crystal orientation, and are therefore vulnerable to stress migration. In addition, since conventional aluminum electrodes are polycrystalline films made up of grains (microcrystals), as the film thickness of the aluminum electrode becomes thinner, it becomes an island-like structure, and as the film thickness becomes thinner, the electrical resistivity increases. Eventually, there was no electrical continuity.

This resulted in an increase in insertion loss and a decrease in the Q of the resonator. In contrast, the surface acoustic wave device of the present invention uses aluminum film electrodes with crystal axes oriented in a constant direction. Such crystallographically oriented aluminum T4 poles are not an aggregation of grains, but are thought to exhibit properties close to single crystal films, and are extremely resistant to stress migration due to grain boundary diffusion. Furthermore, since it is not a collection of grains, electrical continuity can be obtained even when the aluminum electrode has a relatively thin film thickness, and electrical resistivity can be kept low. Therefore, according to the surface acoustic wave device of the present invention, the occurrence of stress migration can be suppressed, electrical short circuits and insertion losses caused by stress migration can be reduced, and the Q of the resonator can be maintained satisfactorily. In particular, in the past, stress migration was more pronounced as the frequency increased, so according to the present invention, the high frequency characteristics of the surface acoustic wave device can be improved. Furthermore, since the occurrence of stress migration can be suppressed even when high-level signals are applied, it can be used in circuits with large signal levels, and the product life can be extended. In addition, until now, even if the film thickness of the aluminum electrode was made thicker or thinner, the characteristics deteriorated, so it was not possible to take advantage of the good temperature characteristics of the LST power-cut crystal substrate. By using an aluminum film oriented in a certain direction in terms of crystal orientation, it has become possible to obtain an electrode with good electrical conductivity even if the film thickness is made relatively thin. The conventional problem can be solved by making it thinner.
This makes it possible to take advantage of the good temperature characteristics of surface acoustic wave devices using LST-cut crystal substrates. In particular, this significance is remarkable in a high frequency region, and it is possible to increase the frequency of a surface acoustic wave device using an LST cut crystal substrate. [Example] Hereinafter, an example of the present invention will be described in detail based on the attached drawings. What is shown in FIG. 1 is a two-boat surface acoustic wave resonator 3, in which two interdigital electrodes 2a are provided on the surface of a piezoelectric substrate 1, and grating electrodes 2b (reflectors) are provided on both sides of this electrode fi 2a. A lead terminal 4 is drawn out from the interdigital electrode 2a.This two-boat SAW resonator 3 will be taken as an example and will be described below according to the manufacturing order.The piezoelectric substrate 1 is made of mirror-polished 105. Using a rotated Y-cut crystal substrate, an aluminum film is deposited on the surface of the piezoelectric substrate 1 by electron beam evaporation to a thickness of approximately 400 mm and approximately 0.7% of the wavelength by appropriately controlling the deposition rate and substrate temperature.
) was formed. For example, the deposition rate and substrate temperature were conventionally evaporated at 10 people/sec and +160°C, but in the range of experiments conducted by the inventors, the deposition rate and substrate temperature were changed to 40 people/sec and +80°C, which was faster and lower temperature. An AQ epitaxial film was obtained. It was confirmed by RHEED (reflection high-speed electron diffraction) that this aluminum film was growing epitaxially (Figure 5(a) shows this RHEED photograph. Figure 5(b) is the same as Figure 5(a). ) is an explanatory diagram of the photograph, where A is the electron beam and what can be seen within the mouth area is the reflected light.
). The epitaxial relationship is currently under consideration, but according to the results of experiments using the RHEED method, there is no doubt that it is an epitaxial relationship. On the other hand, under conventional vapor deposition conditions, no epitaxial growth of the aluminum film was observed, and the aluminum film became randomly oriented amorphous (a RHEED photograph of this is shown in Figure 6(a).
Figure (b) is an explanatory diagram of the photograph in Figure 6 (a), where C is the electron beam and what can be seen in the area 2 is reflected light. ). This anoreminium film was processed by photolithography to form two interdigital electrodes 2a and a grating electrode 2b on the surface of the piezoelectric substrate 1, thereby producing the two-boat SAW resonator 3 as described above. In the SAW resonator 3 actually fabricated in this manner, the wavelength of the surface acoustic wave is approximately 5.9 am (electrode finger width approximately 1.47 am), the aperture length is approximately 100 wavelengths, and the interdigital electrodes are each There are 50 pairs of grating electrodes and 500 metal strip grating electrodes each. These two boats S
The 50Ω transmission characteristics of the AW resonator are shown in Figure 2. As shown in Figure 2, the peak frequency is approximately 6
74 Ml-1z, and the insertion loss was about 6 dB.

This is compared to the conventional amorphous aluminum electrode.
The characteristics are almost the same as those of the T-cut SAW resonator.

Here, in order to evaluate the power resistance characteristics (stress migration resistance characteristics), a system as shown in FIG. 3 was used.

This is configured such that a power amplifier 6 is connected to the output of the oscillator 5 to amplify the power of the output signal of the oscillator 5, and the output of the power amplifier 6 is applied to the SAW resonator 3. on the other hand,
The output p (t) of the SAW resonator 3 is input to a power meter 7 and its level is measured. Further, the output of the power meter 7 is fed back to the oscillator 5 via the computer 8, and the frequency of the oscillator 5 is controlled so that the frequency of the applied signal always becomes the peak frequency of the transmission characteristics. Further, the SAW resonator 3 was housed in a constant temperature bath 9, and was subjected to accelerated deterioration by increasing the ambient temperature to 85°C. Therefore, the output of power amplifier 6 is set to IW (50
Ω system) and measure the initial output level P(t)=PO, and the output P(L) after a certain time t has passed as p(t
)≦P o 1.0 (dB), the SAW
The lifespan of resonator 3 was set as td. This means that p (t) in a
The curve is as shown in Figure 4, so we thought it would be appropriate to estimate the lifespan td with a 1 dB decrease. Each of the evaluated samples A, B, C, and D used the four types of electrode metals shown below. A: Randomly oriented pure AQ electrode B: (Randomly oriented AQ + 1wt% Cu) @pole C: Ebitaxial pure AQ electrode D: (Ebitaxial AQ + 1wt% Cu) electrode
Samples A and B are SA using a normal ST-cut crystal substrate.
It is a W resonator, and the AQ film thickness is about 1000. In B, Cu is added to the electrode metal as a stress migration countermeasure. C and D are LS according to the present invention using an LST-cut crystal substrate rotated by 105 degrees in Y as described above.
It is an AW resonator, and D also has Cu added to the electrode metal. As a result of the experiment, the lifespan of each sample was t. a is A:
5 minutes or less B: Approximately 150 minutes C: 800 minutes or more D: 7,000 minutes or more (for 2.5W). Comparing Samples A and B, the addition of Cu achieves a lifespan of more than 30 times longer, but by making the aluminum film ebitaxial, the effect is further increased by more than 5.3 times. . That is, when comparing Samples A and C using electrodes made of pure aluminum, the lifespan is actually 160 times longer.

Next, we will introduce Cu, which is effective in improving migration resistance characteristics.
In the case of sample D, in which the electrode was formed with an AQ epitaxial film containing -1 wt% of
When the lifespan was measured by applying the output of 7,000
A lifespan of more than 1 minute was obtained. Here, an output of 2.5 W was applied because an output of 1 W had too long a life and was inappropriate for conducting experiments. Therefore, when Cu is added, it has a longer life at higher power than the pure AQ epitaxial film. In general, it is said that the acceleration coefficient due to electric power is the third to fourth power, so the acceleration coefficient in the case of 2.5W is l5 to 39 (ξ2.53 to 2.53) in the case of IW.
5') times 7,000 for an output of 2.5W.
Lifespan of 105,000 to 27 minutes is converted to IW.
3. This corresponds to a lifespan of more than 1,000 minutes. In this way, when Cu is added to the AQ epitaxial film, compared to the pure AQ epitaxial film, the
It has achieved a lifespan of 0 to 340 times longer than Ti, Ni
The use of additives other than Cu, which are said to be used to prevent migration, such as , Mg, and Pd, has the same effect on extending life. If the amount of each of the above additives is too small, it will not be effective, so it is usually necessary to add 0.1 wt% or more, and if it is too large, the resistivity of the aluminum film will increase.
Normally, 10 wt% or less is desirable. Therefore, Cu,
The amount of additives such as Ti, Ni, Mg, and Pd is preferably in the range of 0.1 wt% to 10 wt%. In addition, for comparison with the examples of the present invention, an LSAW resonator was fabricated by forming conventional randomly oriented aluminum electrodes (film thickness: 400 mm) on the surface of an LST-cut crystal substrate. Big, 2-boat SAW
The characteristics of the resonator could not be obtained at all. Therefore, AQ
After investigating the relationship between the film thickness and electrical resistivity of the aluminum film for epitaxial films and AQ randomly oriented films,
The results shown in Figure 7 were obtained. In Figure 7, the horizontal axis is the thickness of the aluminum film (in humans), and the vertical axis is the specific resistivity (Ω・
cm), solid line A indicates the AQ epitaxial film,
The dashed line indicates the AQ randomly oriented film. As shown in the graph in Figure 7, the resistivity remains low even with the film thickness of 400 people in the AQ epitaxial exhibition, whereas the resistivity remains low even with the film thickness of 400 people in the case of the conventional AQ random orientation film. It was found that the resistivity had become extremely large, and as a result, the characteristics of the SAW resonator had deteriorated. The reason for this is thought to be that the conventional AQ randomly oriented film is made up of a collection of grains, so if the film is thin, it becomes an island-like structure and there is no electrical conduction. Note that as a base for the aluminum alignment film, a very thin Ti film, Cr film, or the like may be provided to the extent that it does not interfere with the alignment. Further, in the above embodiment, a two-boat SAW resonator has been described, but it goes without saying that the present invention can also be applied to a one-boat SAW resonator, a SAW filter, and the like. It is also possible to use one without a reflector. [Effects of the Invention] As described above, according to the present invention, the stress migration resistance of an aluminum electrode can be improved. In particular, stress migration can be suppressed even when a high-level signal is applied. By improving stress migration resistance in this way, electrical short circuits and deterioration of insertion loss can be reduced, and the Q of the resonator can be maintained at a good level. Furthermore, high frequency characteristics can also be improved. Additionally, it can be used in circuits with high signal levels (for example, in the transmitting stage). Furthermore, the lifetime at a constant signal level is extended, resulting in high reliability. In addition, it is possible to obtain aluminum electrodes with low resistivity even in the case of thin film thickness, and in order to obtain good characteristics, thin aluminum electrodes are required for leaky surface acoustic wave devices using LST-cut crystal substrates. It can enable higher frequencies.

[Brief explanation of the drawing]

Figure 1 is a schematic plan view of a two-boat surface acoustic wave resonator;
The figures show the same 50Ω system transmission characteristics as above, Figure 3 (i) is a schematic diagram of the stress-resistant migration evaluation system, Figure 4 is a curve showing the lifespan judgment based on the stress-resistant migration characteristic number, and Figures 5 (a), (b) and t. Figures 6(a) and (b) are X-ray photographs of the AQ epitaxial film on the rotating Y-cut quartz crystal substrate of the present invention and their explanatory diagrams; Figures 6(a) and (b); Fig. 7 is a graph showing the relationship between the film thickness of the aluminum electrode and its specific resistivity. 1... Piezoelectric substrate 2a... Interdigital electrode 2b... Grating electrode Fig. 3 8 Fig. 4 1A single culm

Claims (3)

[Claims] (1) 103° rotated Y-cut to 107° rotated Y-cut A surface acoustic wave device that uses leaky surface acoustic waves propagating on a quartz crystal substrate has a migration prevention function using an aluminum film oriented in a certain crystal orientation. A surface acoustic wave device characterized by forming electrodes. (2) Cu, Ti, Ni, Mg on the aluminum film
2. The surface acoustic wave device according to claim 1, further comprising a trace amount of an additive having excellent anti-migration properties, such as Pd or the like. (3) The amount of the additive added is 0.1 wt% to 10 wt.
%, the surface acoustic wave device according to claim 2.