JP2936228B2 - Surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to quartz, LiTaO ₃ (lithium tantalate), Li
Single crystal such as NbO ₃ (lithium niobate), Li ₂ B ₄ O ₇ (lithium tetraborate), ZnO / Al ₂ O ₃ with zinc oxide film formed on sapphire (Al ₂ O ₃ ) substrate The present invention relates to a surface acoustic wave filter provided with electrodes on the surface of a piezoelectric substrate.

[Background Art] In recent years, a surface acoustic wave filter (including a trap) using a surface acoustic wave (hereinafter, sometimes referred to as SAW) has been widely used.

These surface acoustic wave filters generally have an interdigital electrode (interdigital electrode) or a metal strip grating electrode formed on the surface of a substrate having piezoelectricity. Aluminum is generally used as the electrode metal because of its features such as easy photolithography, low specific gravity, low electrode load mass effect, and high conductivity. is there.

[Problems to be Solved by the Invention] However, it has been found that when a signal of a high voltage level is applied to such a SAW filter, a strong stress is applied to the aluminum electrode by a surface acoustic wave to cause migration. Since this is migration due to stress, it is called stress migration.
When this occurs, an electric short circuit, an increase in insertion loss, a decrease in Q of the resonator, and the like occur. Since this stress migration is more likely to occur at higher frequencies, there has been a serious problem in increasing the frequency of the surface acoustic wave filter. In particular, since a high voltage level signal is applied to a transmission filter, stress migration is likely to occur, and improvement thereof has been desired.

As a conventional countermeasure against this, as in the case of electromigration, a small amount of Cu, Ti, Ni, Mg, Pd, etc. is added to aluminum as an electrode material to improve stress migration resistance. However, the improvement of the characteristics was still insufficient.

Therefore, the inventors of the present invention have further investigated the cause of the stress migration. According to the research results, conventional aluminum electrodes formed by electron beam evaporation, sputtering, etc., are not crystallographically oriented in a certain direction, but are amorphous polycrystalline films, and therefore, grain boundaries It was considered that the material exhibited a weak property against stress migration due to diffusion.

The present invention has been made based on the above-mentioned drawbacks of the conventional example and the knowledge obtained by the inventors, and an object of the present invention is to provide a surface acoustic wave filter having an aluminum electrode having excellent resistance to stress migration. Is to provide.

[Means for Solving the Problems] For this reason, the surface acoustic wave filter of the present invention uses LiTaO ₃ ,
In a surface acoustic wave filter using a piezoelectric substrate made of a substrate material selected from LiNbO ₃ , Li ₂ B ₄ O ₇ and ZnO, Cu, T
It is characterized in that an electrode having an anti-migration function is formed by an epitaxially grown aluminum film having a small amount of an additive having excellent anti-migration properties, such as i, Ni, Mg, and Pd, which is oriented in a certain direction in the crystal orientation.

Further, it is appropriate to use the additive having excellent migration resistance in the range of 0.1 wt% to 10 wt%.

[Operation] As described above, the conventional aluminum electrode is an amorphous polycrystalline film that is not crystallographically oriented in a certain direction, and is therefore weak against stress migration.

On the other hand, in the surface acoustic wave filter of the present invention, an electrode of an epitaxially grown aluminum film whose crystal axis is oriented in a certain direction is used. Such an epitaxially grown aluminum electrode which is crystallographically oriented in a certain direction is considered to exhibit properties close to a single crystal film, and is extremely resistant to stress migration due to grain boundary diffusion.

Therefore, according to the surface acoustic wave filter of the present invention,
The occurrence of stress migration can be suppressed, an electrical short circuit and insertion loss due to the stress migration can be reduced, and the Q of the resonator can be favorably maintained. Especially,
Conventionally, since the stress migration becomes more remarkable as the frequency increases, according to the present invention, the high frequency characteristics of the surface acoustic wave filter can be improved.

Further, since a high-voltage signal is applied to the transmission filter, stress migration is particularly likely to occur. However, according to the present invention, the occurrence of stress migration can be effectively suppressed even in such a case. Thus, the surface acoustic wave filter can be put to practical use for a transmission stage.

Moreover, in the present invention, the addition of a very small amount of Cu, Ti, Ni, Mg, Pd, etc. having excellent migration resistance to the above-mentioned epitaxially grown aluminum film improves the effect of improving the migration resistance of the electrode. Thus, a surface acoustic wave filter having an excellent migration preventing function can be manufactured. Further, by adding a trace amount of an additive having excellent migration resistance such as Cu, Ti, Ni, Mg, and Pd to the epitaxially grown aluminum film, it is possible to extend the life of the surface acoustic wave filter even at high power.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows a two-port surface acoustic wave filter 3
Two interdigital electrodes 2a are provided on the surface of the piezoelectric substrate 1, and grating electrodes are provided on both sides of the electrodes 2a.
2b (reflector) is provided and interdigital electrode 2a
Lead terminal 4 is drawn out from the terminal. These two ports
The SAW filter 3 is taken as an example, and will be described below according to the manufacturing order.

As the piezoelectric substrate 1, a mirror-polished 33.5 ° rotated Y-cut quartz substrate is used, and an aluminum film is formed on the surface of the piezoelectric substrate 1 by electron beam evaporation at a thickness of about 1000 ° by appropriately controlling the deposition rate and the substrate temperature. It was formed to a film thickness.

For example, the deposition rate and the substrate temperature are conventionally 10 ° / sec, +
The (311) orientation film was obtained by vapor deposition at 160 ° C., but at a high speed and low temperature of 40 ° C./sec and + 80 ° C. within the range of the experiments performed by the inventors. (3) of this aluminum film
11) The epitaxial growth of the surface was confirmed by the RHEED (reflection high-energy electron diffraction) method (FIG. 5 (a) shows this RHEED photograph. FIG. 5 (b) is the same as FIG. 5 (a)). It is an explanatory view of a photograph, in which a is an electron beam and what is seen in the area of b is reflected light.) Under the conventional deposition conditions, no epitaxial growth of the aluminum film was observed,
FIG. 6 (a) shows this RHEED photograph. FIG. 6 (b) is an explanatory view of the photograph of FIG. 6 (a). What is visible in the area is the reflected light.)

This aluminum film is processed by photolithography to form two interdigital electrodes 2a and a grating electrode 2b on the surface of the piezoelectric substrate 1.
A port SAW filter 3 was manufactured.

In the SAW filter 3 actually manufactured in this manner, the surface acoustic wave has a wavelength of about 4.7 μm (electrode finger width of about 1.1 μm).
7 μm), the aperture length is about 100 wavelengths, 50 pairs of interdigital electrodes each, and 300 grating electrodes each made of metal strip. FIG. 2 shows the 50Ω transmission characteristics of the two-port SAW filter. As shown in FIG. 2, the peak frequency was about 674 MHz, and the insertion loss was about 6 dB. This is almost the same as the characteristics of the conventional amorphous aluminum electrode.

Here, a system as shown in FIG. 3 was used in order to evaluate the power resistance characteristics (stress migration resistance characteristics). In this configuration, a power amplifier 6 is connected to the output of the oscillator 5 to amplify the output signal of the oscillator 5 with power, and the output of the power amplifier 6 is applied to the SAW filter 3. On the other hand, the output P (t) of the SAW filter 3 is input to the power meter 7 and the level is measured. 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. The SAW filter 3 was housed in a thermostat 9, and was accelerated and degraded by increasing the ambient temperature to 85 ° C.

Thus, the output of the power amplifier 6 is set to 1 W (50Ω system), the initial output level P (t) = P ₀ is measured, and the output P (t) after a lapse of a certain time t becomes P (t) ≦ P ₀ -1.0 when became (dB) was life t _d of the SAW filter 3. This curve generally P (t) is, since as FIG. 4, in order that considered appropriate by performing the estimation of the lifetime t _d in reduction of 1 dB.

Each of the evaluated samples A, B, C, and D was obtained by forming electrodes of the same shape on the same cut angle quartz substrate using the following four types of electrode metals.

A: Randomly oriented pure Al electrode B: (Randomly oriented Al + 1wt% Cu) electrode C: Epitaxial pure Al electrode D: (Epitaxial Al + 1wt% Cu) electrode A and B are ordinary SAW filters, B is stress migration resistant As a countermeasure, Cu is added to the electrode metal. C and D are S using epitaxially grown aluminum film
A is an AW filter, and D is a SAW filter according to the present invention in which Cu is added to the electrode metal.

The results of the experiments, the lifetime t _d of each sample, respectively A: 5 minutes or less B: about 150 minutes C: 900 minutes or more D: became 8,000 minutes or more (in the case of 2.5 W).

Comparing Samples A and B, the addition of Cu has attained a longer life of 30 times or more, but epitaxial growth of the aluminum film has an effect of 6 times or more. That is, sample A using pure aluminum electrodes,
In comparison between C, the life is actually 180 times longer.

Next, Cu which is effective in improving the migration resistance
In the case of sample D in which an electrode was formed of an Al epitaxial film to which 1 wt% was added, the life was measured by applying an output of 2.5 W from a power amplifier, and a life of 8,000 minutes or more was obtained. Here, the output of 2.5 W was applied because the life was too long at 1 W, which was inappropriate for conducting experiments. Therefore, when Cu is added, the life is longer at a higher power than that of the pure Al epitaxial film. Generally, it is said that the acceleration coefficient by electric power is a power of 3 to 4. Therefore, the acceleration coefficient in the case of 2.5 W is 15 to 39 in the case of 1 W.
(≒ 2.5 ³ ~2.5 ⁴⁾ becomes doubled, 8,000 for the output of 2.5W
A life of more than 1 minute is equivalent to a life of 120,000 to 312,000 minutes or more when converted to 1 W.

Thus, when Cu is added to the Al epitaxial film, 130 to 340 compared to the case of the pure Al epitaxial film.
Although the life is doubled, the use of Ti, Ni, Mg, Pd and other additives other than Cu, which are said to be used for countermeasures against migration, also has the effect of extending the life. The amount of each of the above additives is too small to be effective, so usually 0.1 wt%
This is necessary, and if it is too large, the resistivity of the aluminum film increases. Therefore, as an additive amount of additives such as Cu, Ti, Ni, Mg, Pd, etc., 0.
A range from 1 wt% to 10 wt% is preferred.

Further, as a base of the aluminum alignment film, an extremely thin Ti film, Cr film, or the like that does not hinder the alignment may be provided.

The Al epitaxial film has a (311) orientation on a quartz substrate in the range of 25 ° to 39 ° rotation Y-cut, but may be oriented at other cut angles. Generally, in order to perform epitaxial growth, it is necessary to match the crystal lattice of the substrate and the Al thin film.
Since the (311) plane of the Al epitaxial film is aligned in a crystal lattice, a 25 ° to 39 ° rotated Y-cut quartz substrate
The (311) plane of the Al thin film was grown epitaxially. However, the (311) plane of the Al epitaxial film does not necessarily need to be parallel to the surface of the quartz substrate, and if the cut surface of the quartz substrate deviates from the above angle,
Since the epitaxial film is oriented so that the (311) plane follows the cut surface of the quartz substrate, the orientation direction of the Al thin film corresponding to other rotation cut angles is determined, especially limited to the rotation Y cut. There is no need to be done. For example, 2
Even in the case of a rotation cut quartz substrate, epitaxial growth can be performed in a direction in which the (311) plane of the Al thin film substantially satisfies the lattice matching condition.

In the above description, the case where a quartz substrate is used has been described, but a LiTaO ₃ substrate, a LiNbO ₃ substrate, a Li ₂ B
_Even when a ZnO thin film is formed on a ₄ O ₇ substrate or a sapphire (Al ₂ O ₃ ) substrate, the film formation conditions of Al (eg, ion beam sputtering, ion plating, etc.)
Can be formed to form an aluminum alignment film. In these cases, the Al epitaxial film is not limited to the (311) plane, but in any case, the crystal orientation of the Al epitaxial film is determined so as to satisfy the crystal lattice matching condition between the Al thin film and the substrate material.

In the above embodiment, a two-port SAW having a reflector is used.
Although the explanation has been made with the filter, the one without the reflector may be used.

[Effects of the Invention] As described above, according to the present invention, the stress migration resistance characteristics of an aluminum electrode can be improved. In particular, even when a high-level signal is applied, occurrence of stress migration can be suppressed.

By improving the resistance to stress migration in this way, it is possible to reduce the deterioration of electrical short circuit and insertion loss,
In addition, the Q of the filter can be favorably maintained. Further, high frequency characteristics can be improved.

Further, since stress migration hardly occurs even when a signal of a high voltage level is applied, the surface acoustic wave filter can be used for the transmission stage, and the use of the surface acoustic wave filter can be expanded.

Moreover, in the present invention, since a small amount of an additive having excellent migration resistance such as Cu, Ti, Ni, Mg, or Pd is added to the epitaxially grown aluminum film, the effect of improving the migration resistance of the electrode is improved. Thus, a surface acoustic wave filter having an excellent migration preventing function can be manufactured. Further, by adding a trace amount of an additive having excellent migration resistance, such as Cu, Ti, Ni, Mg, and Pd, to the epitaxially grown aluminum film, it is possible to extend the life of the surface acoustic wave filter even at high power.

[Brief description of the drawings]

FIG. 1 is a schematic plan view of a two-port surface acoustic wave filter, FIG. 2 is a schematic diagram of a 50Ω transmission characteristic of the above, FIG. 3 is a schematic view of a stress migration resistance evaluation system, and FIG. A curve indicating
FIGS. 5 (a) and 5 (b) are RHEED photographs of an Al epitaxial film on a rotating Y-cut quartz substrate of the present invention and explanatory views thereof.
FIGS. 6 (a) and 6 (b) show RHEED of a normal aluminum electrode.
It is a photograph and its explanatory view. 1 ... Piezoelectric substrate 2a ... Interdigital electrode 2b ... Grating electrode

Continuation of the front page (72) Inventor Koji Kimura 2-26-10 Tenjin, Nagaokakyo-shi, Kyoto Prefecture Murata Manufacturing Co., Ltd. (56) References JP-A-55-49014 (JP, A) JP-A-62-98812 (JP, A) JP-A-62-163408 (JP, A)

Claims (2)

(57) [Claims] 1. A surface acoustic wave filter using a piezoelectric substrate made of a substrate material selected from the group consisting of LiTaO ₃ , LiNbO ₃ , Li ₂ B ₄ O ₇ , and ZnO, wherein Cu, Ti, Ni, Mg, Pd, etc. A surface acoustic wave filter for a transmission stage, characterized in that an electrode having a migration preventing function is formed by an epitaxially grown aluminum film having a small amount of an additive having excellent migration resistance and being crystallographically oriented in one direction. 2. The surface acoustic wave filter according to claim 1, wherein the amount of the additive is 0.1 wt% to 10 wt%.