JPH0640613B2 - Surface acoustic wave resonator - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a surface acoustic wave resonator, and more particularly to a structure of a grating reflector.

[Technical background of the invention and its problems]

The surface acoustic wave resonator is a resonance element formed by forming an interdigital converter and a grating reflector on a piezoelectric substrate. Since the characteristics of such a surface acoustic wave resonator are mainly determined by the grating reflector, improvement of the characteristics of the grating reflector is a major issue.

The grating reflector in the conventional surface acoustic wave resonator is configured by arranging a large number of surface acoustic wave reflectors such as strips, ridges, and grooves at half wavelength intervals of the surface acoustic wave. Generally, when the reflectance ε per one of these reflectors is large, the number of reflectors may be small, but as ε becomes smaller, a large number of reflectors are required. Peter Cross, Reference 1: IEEE Trans.on Sonice &
Ultrason. (Vol su-23, No.4 pp.255-262) 1976 stated that the number of reflectors in a grating reflector must be N≈3 / ε or more to obtain a sufficient reflection amount. ing. This means that a large number of reflectors with ε≈0.01, for example, 300 or more are required. Generally, considering that two or more grating reflectors are used in the resonator, the entire resonator is very large. There is a problem that it becomes.

On the other hand, if the strip thickness, ridge height, and groove depth are increased in order to increase ε per reflector, the mode conversion from surface acoustic wave to bulk wave increases accordingly. The conversion loss causes the Q of the resonator to decrease.

As a means for solving these problems, the present inventors have proposed the Institute of Electronics and Communication Engineers Ultrasonic Research Group (Reference 2: Technical Report of the Institute of Communication Engineers US).
84-30 September 1984), a grating in which two types of reflectors having a λ / 8 width and a reflection coefficient opposite to each other with respect to the surface wave wavelength λ of the used frequency are alternately arranged in a λ / 4 period. Proposing a reflector. In the grating reflector having this structure, the reflected wave at the reflector having a positive reflection coefficient and the reflected wave at the reflector having a negative reflection coefficient are added, so that the reflectors are arranged at a λ / 2 cycle. The amount of reflection per unit length is almost doubled as compared with the conventional general grating reflector. On the other hand, the loss of mode conversion to bulk wave is only 1.4 times. Therefore, when the amount of reflection is the same as that of the conventional general one, the mode conversion loss to the bulk wave is reduced.

FIGS. 7 and 8 show the structure of the specific grating reflector disclosed in the above-mentioned document. In FIG. 7, a 128 ° Y piezoelectric substrate having a very large electromechanical coupling coefficient is used. On the -X LiNbO ₃ substrate 21, the reflectors 22 electrically short-circuited with a λ / 8 width and the reflectors 23 electrically isolated with a λ / 8 width are alternately arranged and arranged at a λ / 4 cycle. ing. Since this structure can form the interdigital converter and the grating reflector at the same time and only by the etching process of the aluminum thin film or the like, the productivity is extremely good. However, as is well known, the LiNbO ₃ substrate has a large temperature relationship, and there is a problem that it is difficult to be practically used in a field applied as a resonator.

In FIG. 8, ridges 32 having a width of λ / 8 and groups 33 are alternately arranged on the ST quartz substrate 31 at a λ / 4 period. According to this structure, since the temperature coefficient of the quartz substrate 31 is small, the problem as shown in FIG. 7 can be avoided, but
It is extremely difficult to form the convex portion (ridge) 32 and the concave portion (groove) 33 on the same crystal substrate 31, and there is a problem in terms of productivity.

[Object of the Invention]

The present invention has been made in view of the above problems, and has a basic structure in which two kinds of surface acoustic wave reflectors having positive and negative reflection coefficients are alternately arranged at a cycle of ¼ of the surface acoustic wave wavelength,
Despite being a small shape, it has excellent reflection efficiency, and we have further improved the grating reflector with reduced mode conversion loss to bulk waves.
Moreover, it is an object of the present invention to provide a surface acoustic wave resonator including a grating reflector having excellent productivity.

[Outline of Invention]

The surface acoustic wave resonator according to the present invention is mainly composed of gold having a negative reflection coefficient on a predetermined piezoelectric substrate in relation to the surface acoustic wave propagating on the substrate and the substrate material. A first surface acoustic wave reflection strip made of a thin film and a second surface acoustic wave reflection strip made of a thin film containing aluminum as a main component having a positive reflection coefficient with respect to the surface wave in relation to the substrate material. It is characterized in that it comprises grating reflectors arranged alternately and at a period of ¼ of the wavelength of the surface acoustic wave.

Here, as the piezoelectric substrate, for example, a quartz substrate or L
An iTaO ₃ substrate or the like can be used. On these piezoelectric substrates, the surface acoustic wave reflection strip made of a gold thin film has a negative reflection coefficient, and the surface acoustic wave reflection strip made of an aluminum thin film, on the contrary, shows a positive reflection coefficient.

〔The invention's effect〕

In the grating reflector of the present invention, the first and second negative and positive reflection coefficients are provided on the piezoelectric substrate.
Since it has a basic structure in which the surface acoustic wave reflection strips of 1 are alternately arranged at a period of 1/4 of the surface acoustic wave wavelength, high reflection efficiency is obtained by adding the reflected waves of both strips. That is, the reflection amount of the surface acoustic wave per unit length increases, and the mode conversion loss from the surface acoustic wave to the bulk wave is relatively reduced. As a result, the Q of the resonator is lowered, and further, the deviation of the resonance frequency due to the energy storage effect at the end of the surface acoustic wave reflection strip is suppressed.

Further, according to the present invention, in addition to the characteristic improvement in terms of the basic performance of the surface acoustic wave resonator as described above, the temperature characteristic or the problem of productivity observed in the conventional grating reflector for improving the reflection efficiency. Is eliminated. That is, in the grating reflector in the surface acoustic wave resonator according to the present invention, it is possible to use a quartz substrate having a very small temperature coefficient, a LiTaO ₃ substrate, or the like as the piezoelectric substrate, and a resonance operation that is extremely stable against temperature changes. In addition, since the structure can be obtained by forming two kinds of surface acoustic wave reflection strips of a gold thin film and an aluminum thin film on a piezoelectric substrate, the manufacturing process is the conventional aluminum thin film of 1/4 wavelength line width. Compared with the surface acoustic wave resonator using the surface acoustic wave reflection strip by ED, only the process of forming the surface acoustic wave reflection strip by the gold thin film is newly added, and the grating that forms the ridge and the groove on the piezoelectric substrate is added. Productivity is significantly improved compared to a reflector.

Example of Invention

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a portion of a grating reflector in a surface acoustic wave resonator according to an embodiment of the present invention, in which a quartz substrate 1 made of a 32 ° rotating Y plate as a piezoelectric substrate is provided on the substrate. 1 where λ is the wavelength of the surface acoustic wave propagating on
Film thickness λ / 200 at 8 (equivalent to 1600Å at 100 MHz)
A first surface acoustic wave reflection strip 2 of a gold thin film of
The second surface acoustic wave reflection strips 3 made of an aluminum thin film having a line width of λ / 8 and a film thickness of λ / 50 (corresponding to 6400Å in the case of 100 MHz) are alternately arranged in a λ / 4 cycle.

The thickness dependence of the reflectance of the surface acoustic wave reflection strip formed by the gold thin film and the aluminum thin film formed on the quartz substrate is shown by the solid line and the broken line in FIG. As can be seen, the reflectance is square for a reflection strip made of a gold thin film, and positive and reverse for a reflection strip made of an aluminum thin film, and the film thickness dependence is 4 for a gold thin film compared to that for an aluminum thin film. Twice as big. Therefore, in order to obtain the same reflectance in absolute value, the thickness of the gold thin film may be 1/4 of the thickness of the aluminum thin film.

Further, in the above description, the cut angle of the quartz substrate 1 is selected so that the apex temperature is near room temperature with the gold thin film and the aluminum thin film formed on the surface. It is necessary to make a corresponding decision.

Next, an example of the manufacturing process of the grating reflector in this embodiment will be described with reference to FIG.

First, as shown in FIG. 3 (a), on the entire surface of the quartz substrate 1,
A gold thin film 2'is attached by vapor deposition or sputtering. At this time, the gold thin film 2'for the purpose of improving the adhesiveness.
An underlayer consisting of an extremely thin layer of chrome or titanium may be attached underneath.

Next, as shown in FIG. 3B, a first surface acoustic wave reflection strip 2 made of a gold thin film having a line width λ / 8 and a period λ / 2 is formed by photolithography. If there are underlying layers, they are also removed by etching at the same time as the gold thin film.

Then, as shown in FIG. 3 (c), an aluminum thin film 3'is deposited on the entire surface by vapor deposition, sputtering, or the like so as to have a thickness approximately four times that of the gold thin film 2 '. At this time, the surface of the aluminum thin film 3'is sufficiently flat because the film thickness of the first surface acoustic wave reflection strip 2 made of the gold thin film located therebelow is 1/20 or less of the line width, which is extremely small.

Then, as shown in FIG. 3 (d), the second surface acoustic wave reflection strip 3 made of an aluminum thin film having a line width λ / 8 and a period λ / 2 is formed by a photolithography technique on the first surface acoustic wave made of a gold thin film. The reflective strips 2 are formed so as to be located substantially in the middle thereof. When the aluminum thin film 3'is formed, the first surface acoustic wave reflection strip 2 made of the gold thin film therebelow cannot be seen.
For the purpose of facilitating mask alignment in the step of FIG. 3 (d), a mask alignment mark 4 is formed outside the area where the grating reflector is formed in the step of FIG. 3 (b). In the step (c), it is necessary to prevent at least the area where the mark 4 is formed on the crystal substrate 1 from being covered with the aluminum thin film.

It is desirable that the step of forming the grating reflector includes a step of forming an interdigital converter for converting electric-surface acoustic waves, which is another component of the surface acoustic wave resonator. Particularly when the interdigital converter is formed of at least one of gold and aluminum, the pattern of the interdigital converter may be formed in the mask pattern in the step (b) or (d). The interdigital converter is formed simultaneously with the formation of the grating reflector, and the relative positional relationship with the grating reflector can be set with high accuracy.

FIG. 4 shows an embodiment obtained by modifying the embodiment shown in FIG.
The second surface acoustic wave reflection strips 2 and 3 are electrically short-circuited and also electrically connected to each other. When the piezoelectric substrate is the quartz substrate 1, since the electromechanical coupling coefficient is very small, even if the surface acoustic wave reflection strips 2 and 3 are electrically short-circuited in this way, they are electrically opened in operation. It is almost the same as the case of. The same applies when only one of the first and second surface acoustic wave reflection strips 2 and 3 is electrically short-circuited, or when both are short-circuited but not electrically connected to each other. Is.

In the above embodiment, the crystal substrate 1 is rotated by 32 °
Although it has been described that a plate is used, it is not necessarily limited to this angle, and a quartz substrate having another cutting angle can achieve the intended object of the present invention by slightly changing the film thickness ratio of the gold thin film and the aluminum thin film. You can In addition, when it is called a surface acoustic wave, it generally refers to a Rayleigh wave, but it is not necessarily limited to this vibration mode, for example, −50.5 of a quartz substrate.
Pseudo-surface acoustic waves propagating on a ゜ rotating Y-plate at right angles to the X-axis, and 105 ° ± 10 ° rotating Y-plate, grating reflector of a surface-acoustic-wave resonator using a pseudo-surface acoustic wave propagating on the X-axis. The objective can be achieved similarly by simply changing the film thickness ratio of the gold and aluminum thin films.

Furthermore, although a quartz substrate is used in all of the above explanations, the same effect can be obtained by using a LiTaO ₃ substrate, for example.
FIG. 5 shows an embodiment thereof, in which a first surface acoustic wave reflection strip 12 made of a gold thin film and a second surface acoustic wave reflection strip 13 made of an aluminum thin film are formed on a LiTaO ₃ substrate 11. . Basically, it may be similar to the case of the quartz substrate, but the LiTaO ₃ substrate has an electro-mechanical coupling coefficient larger than that of the quartz substrate, and as shown in the figure, the first surface acoustic wave reflection strip 12 made of a gold thin film is Although the shape is electrically isolated, it is more desirable to electrically short-circuit the second surface acoustic wave reflection strip 12 made of an aluminum thin film in order to increase the reflectance as much as possible.

The reflectance per strip of the surface acoustic wave reflection strips 12 and 13 having the line width λ / 8 by the gold and aluminum thin films on the LiTaO ₃ substrate 11 (X-cut 112 ° Y plate) is shown in FIG. As described above, the reflection coefficients are in opposite phases, and the film thickness of the gold thin film is about twice as large as that of the aluminum thin film. However, this characteristic is obtained by electrically opening the first surface acoustic wave reflection strip 12 made of a gold thin film and electrically shorting the second surface acoustic wave reflection strip 13 made of an aluminum thin film as shown in FIG. Is. According to this, the film thickness of the first surface acoustic wave reflection strip 12 made of a gold thin film may be about 1/2 of that of the second surface acoustic wave reflection strip 13 made of an aluminum thin film.

The present invention is not limited to the above-described embodiments. For example, the materials of gold and aluminum used for the surface acoustic wave reflection strip in the embodiments may include a trace amount of additive of several% or less. Absent. For example, when aluminum is doped with copper or silicon in an amount of 0.1 to 4% by weight, metal migration can be suppressed, which can contribute to improvement in power withstand characteristics (excitation strength) of the surface acoustic wave resonator.

Also, regarding the line width of the surface acoustic wave reflection strip, λ /
8 does not need to be specified precisely, and generally, the amount of surface acoustic wave reflection varies by about 8% with a deviation of 10% with respect to the line width of the reflecting strip. Line width deviation is allowed.

[Brief description of drawings]

FIG. 1 is a perspective view showing a structure of a grating reflector in a surface acoustic wave resonator according to an embodiment of the present invention, and FIG. 2 is an elastic surface made of a gold and aluminum thin film formed on a quartz substrate in FIG. The figure which shows the film thickness dependence of the reflectance of a wave reflection strip, FIG. 3 is a process drawing for explaining the manufacturing process of the grating reflector in the same Example, and FIG. 4 and FIG. FIG. 6 is a perspective view showing a structure of a grating reflector in the surface acoustic wave resonator according to the embodiment, and FIG. 6 is a graph showing the reflectance of the surface acoustic wave reflection strip formed by the gold and aluminum thin films formed on the LiTaO ₃ substrate in FIG. view for explaining a film thickness dependency, FIG. 7 is a perspective view showing a structure of a conventional grating reflectors which attained using LiNbO ₃ piezoelectric substrate reflection efficiency, 8 (a) and 8 (b) are perspective views showing a structure of another conventional grating reflector in which a quartz substrate is used as a piezoelectric substrate and a surface acoustic wave reflector is composed of a ridge and a groove to improve reflection efficiency. And FIG. 1 ...... quartz substrate, a first surface acoustic wave reflecting strips according to 2 ...... gold thin film, 3 ...... aluminum thin film according to the second surface acoustic wave reflecting strips, 4 ...... mask alignment mark, 11 ...... LiTaO ₃ substrate , 12 ... a first surface acoustic wave reflection strip made of a gold thin film, 13 ... a second surface acoustic wave reflection strip made of an aluminum thin film.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroaki Sato 1 Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa Inside the Toshiba Research Institute Co., Ltd. (56) References IEICE Technical Report US84-30 Surface acoustic wave grating reflectors and resonators consisting of reflective elements with coefficients "

Claims (5)

[Claims] 1. A first elastic surface made of a thin film containing gold as a main component and having a negative reflection coefficient for a surface acoustic wave propagating on a piezoelectric substrate made of a quartz substrate or a LiTaO ₃ substrate. The wave reflection strips and the second surface acoustic wave reflection strips made of a thin film containing aluminum as a main component and having a positive reflection coefficient for the surface acoustic waves are alternately arranged and ¼ of the wavelength of the surface acoustic waves. A surface acoustic wave resonator comprising: a grating reflector configured to be arranged at a period of. 2. A surface acoustic wave resonator according to claim 1, wherein a rotating Y plate is used as a quartz substrate which constitutes the piezoelectric substrate. 3. A quartz substrate that constitutes the piezoelectric substrate
The surface acoustic wave resonator according to claim 1, wherein a 105 ° ± 10 ° rotating Y plate is used and a pseudo surface acoustic wave is used as a vibration mode. 4. The surface acoustic wave according to claim 1, wherein the film thickness of the first surface acoustic wave reflecting strip is smaller than the film thickness of the second surface acoustic wave reflecting strip. Resonator. 5. The second surface acoustic wave reflecting strip comprises:
The surface acoustic wave resonator according to claim 1 or 4, wherein the surface acoustic wave resonator comprises a thin film in which aluminum is doped with at least one of copper and silicon.