JPS6352486B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave (hereinafter simply referred to as surface wave) resonator.

A surface wave device having interdigital electrodes on the surface of a piezoelectric material is an excellent telecommunication circuit device in the VHE UHF region because of its simple structure, small size, and low loss. Furthermore, since the surface wave device can be fabricated in large quantities by batch processing using photolithographic technology, it is possible to reduce the cost, and its practical application is highly promising.

One of the uses of such surface wave elements is as a resonator. In addition to conventional resonators that combine inductance and capacitance, there are also known resonators that utilize the mechanical resonance phenomenon of piezoelectric ceramics and crystal plates. However, the applicable frequency range that is effective for these resonators is usually several tens of megahertz (M
Hz), and at higher frequencies, loss increases and design, manufacturing, handling, etc. become difficult. In contrast, surface wave resonators can be easily used at high frequencies of several tens of MHz or higher.

The well-known basic configuration of a surface wave resonator is the first
As shown in the figure, input/output interdigital transducers 14, 15 connected to input/output terminals 12, 13, respectively, on the surface of the piezoelectric material 11 are connected to distributed reflectors 16, 17.
(hereinafter simply referred to as reflectors). The energy of the surface wave excited by the transducer 14 is stored in the resonator while reciprocating between the left and right reflectors 16 and 17 as shown by the arrow, and a portion is taken out from the transducer 15 to the outside. At this time, the transfer characteristics between the input and output terminals 12 and 13 exhibit sharp frequency dependence determined by the resonator length and the reflection characteristics of the reflector. Therefore, it can be used as a narrow band filter with a fractional band of less than 0.1%.

The above-mentioned reflector is usually obtained by applying a surface acoustic impedance disturbance to the substrate surface at a period equal to one-half of a specific wavelength of the surface wave, by some physical means. That is, when the surface waves reflected at each discontinuous portion of the surface acoustic impedance overlap in the same phase, strong reflection is caused. At this time, if the reflection coefficient for each period of the reflector is uniform within the total reflector, side ropes will appear on both sides of the main passband in the amplitude characteristics of the above filter, ensuring sufficient stopband attenuation. Not recommended. Therefore, it is desirable to gradually change the reflection coefficient for each cycle within the reflector, that is, to apply so-called weighting to the reflection coefficient.

A typical conventional reflector is one in which a metal film having a periodic pattern is attached to the surface of a substrate. The pattern is usually a simple checkered pattern.

However, in the case of checkered patterns, the above-mentioned weighting is often difficult. In such cases, instead of checkered stripes, a periodic pattern in which minute circles (hereinafter referred to as dots) are arranged at striped positions is used. A reflector made of such a dot-shaped array has the feature that by changing the number of dots in each row, the reflection coefficient can be weighted as described above. Either a checkered or dot array metal film reflector can be easily fabricated in the same process as the transducer using photoetching techniques well known in IC fabrication processes. On the other hand, since the lattice-stripe metal film reflector utilizes the above-mentioned surface acoustic impedance discontinuity occurring through piezoelectric reaction, the material used for the resonator is often lithium niobate (LiNbO ₃ ) single crystal. The drawback is that it is limited to highly piezoelectric materials such as . Furthermore, since the dot-type metal film reflector utilizes the discontinuity of surface acoustic impedance based on the mass addition effect, the applicable material is not limited to piezoelectric materials. However, since the metal film for the reflector is usually manufactured in the same process as the interdigital electrode, there is a limitation on the thickness, and therefore there is a drawback that there are considerable limitations on increasing the reflection coefficient.

Another conventionally known reflector is one in which grooves having a periodic pattern are formed in the surface of a substrate. Such a reflector utilizes the fact that the surface acoustic impedance changes discontinuously due to mass loss in the groove.

The aforementioned weighting of the reflection coefficients can be achieved by changing the depth of the grooves. Such grooves are typically realized by microfabrication techniques such as selectively removing the surface of the substrate using ion beam etching techniques. In this case, changing the depth of the grooves is achieved by placing a shadow mask near the surface of the substrate in the ion etching process and moving the substrate in parallel to gradually change the etching time of each groove. However, since the width of the groove is usually minute, ranging from several μm (microns) to several tens of μm, it is necessary to mechanically move the substrate in parallel with the above-mentioned shadow mask with a positional accuracy of several μm to several tens of μm. It's not easy to force someone to do something. In this way, the groove-type reflector with a depth-weighted structure has the advantage, unlike the metal film reflector described above, that it can be applied to all materials, regardless of their piezoelectric properties. Nevertheless, it has had the disadvantage of manufacturing difficulties in varying the depth of the grooves according to the desired weighting function.

To summarize the drawbacks of conventional reflectors, metal film reflectors have restrictions on the materials they can be used with and restrictions on increasing the reflection coefficient, and groove reflectors are manufactured when weighting of the reflection coefficient is required. The disadvantage is that it is difficult to

SUMMARY OF THE INVENTION The object of the present invention is to provide a surface wave resonator with a reflector of a new structure that eliminates the drawbacks of the prior art.

According to the present invention, in a surface wave resonator comprising an interdigital electrode provided on the surface of a piezoelectric substrate and a reflector, the reflector is filled with a material different from that of the substrate, or is filled with a raised pit. A surface acoustic wave resonator characterized by being composed of an aggregate is obtained.

Next, details of the present invention will be explained using the drawings.

FIG. 2 is a structural sectional view of a reflector section in an embodiment of the present invention. In the figure, pits 22 are formed on the surface of an ST-cut crystal substrate 21 using an ion beam etching device, and a gold vapor deposited film is embedded in the bottom. The straight lines 25 are arranged such that each interval is equal to one-half the wavelength of the surface wave. Since each pit 22 has less mass than its surroundings, the surface waves incident from the direction of the arrow 23 are reflected at each pit. Since only the reflected waves with overlapping phases are combined and reflected in the direction of arrow 24, such a structure can serve as an effective surface wave reflector.

FIG. 3 is a plan view of the reflector section in the embodiment of the invention shown in FIG. 2. As described in FIG. 2, pits 31 are formed on the surface of the substrate along linear rows 32 using ion beam etching technology. In this embodiment, since the depth of each pit is uniform, the surface wave reflection efficiency of each pit is constant. However, the number of pits arranged in each straight line 32 differs slightly for each arrangement. For this reason,
A surface wave incident from the direction of arrow 33 is reflected in the direction of arrow 34 with a different average reflection coefficient for each pit arrangement. In this embodiment, a surface wave resonator is constructed in which the reflector is weighted by changing the linear density of the pits.

FIG. 4 is a partial structural sectional view of the reflector section in the embodiment of the present invention shown above. In the same figure, a indicates that the pit 41 is covered with a gold (Au) vapor deposited film 4.
2, which was buried to about 1/2 the depth. In addition, in FIG. 1B, the pits 41 are filled with a similar metal vapor deposited film 42 and further raised. The pit shown in FIG. 4a uses the fact that its mass is smaller than its surroundings to cause a discontinuity in surface acoustic impedance, and serves as a surface wave reflector. On the other hand, the fourth
In the structure shown in Figure b, depending on the embedding material,
Often the mass of the pit is greater than the surrounding area.
However, in any of the above cases, the surface acoustic impedance of the pit region is disturbed to constitute a surface wave reflector.

In the above two embodiments, the center frequency wavelength of the surface waves is 200 MHz and 16 μm, respectively. For this reason, the diameter and depth of the pits and the repetition period of the pit row are selected to be 4 μm, 0.2 μm, and 8 μm, respectively. A surface acoustic wave resonator having such a pit can be obtained by a method similar to or similar to the method of manufacturing a resonator having a groove-type reflector known in the art. A conventional example of a typical manufacturing method is the 1976 Ultrasonic Symposium Proceedings (1976 Ultrasonic Symposium Proceedings).
Symposium Proceedings), pages 389 to 390. To summarize, an aluminum (Al) pattern lacking interdigital electrode portions and reflector portions is formed on the substrate surface, and using this as a mask, the electrodes and pit portions are etched using an ion beam etching device or a reverse sputtering device. Thereafter, the groove in the electrode portion is filled with a vapor-deposited aluminum film to form an electrode using a backside exposure photoetching method and a lift-off method.

By using a manufacturing method completely similar to that of the above-mentioned document, a reflector having a structure in which the pit portion is filled with a gold vapor deposited film, as in the embodiment shown in FIG. 4, can be obtained. Since the interdigital electrode and reflector patterns are formed in the same process using the same photomask, the parallelism between the two is not impaired.

Next, the advantages of the present invention will be described. In the reflector of the present invention, the above-mentioned weighting can be applied to the reflection coefficient by changing the areal density of the etching pits according to a desired distribution function. At this time, the etch pit pattern can be formed with a photomask pattern together with the interdigital electrodes. Furthermore, the reflectance of each pit can be finely adjusted by selecting the material and thickness of the embedded film. Therefore, the present invention has the advantage that the weighting of the reflectors can be adjusted with high precision.

Furthermore, as mentioned above, in conventional groove-type reflectors, the depth of the grooves is controlled to perform the weighting, but this requires expensive ion beam etching equipment and sample movement control equipment. Ta. On the other hand, in the present invention, the depth of the pit or the thickness of the buried film may be uniform, so it is sufficient to use a reverse sputtering device, which simplifies the device and facilitates manufacturing. This is the second advantage of the present invention.

The third advantage of the present invention is that, unlike metal film reflectors, it is effective for materials with low piezoelectricity, such as ST-cut quartz, which is well known for its excellent temperature stability. It could be a reflector.

The basic gist of the present invention is that the reflector portion of the surface wave resonator is constructed with a structure in which the aforementioned pits are filled with another material instead of the checkered grooves. Therefore, the pit manufacturing method, pit filling material, etc. are not limited to those shown in the embodiments, and the patent rights of the present invention extend to all surface acoustic wave resonators shown in the above-mentioned claims.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of a conventionally known surface wave resonator, FIG. 2 is a partial structural sectional view of a reflector in an embodiment of the present invention, and FIG. 3 is a diagram of a reflector in the embodiment of FIG. The plan view, FIG. 4, is a partial structural sectional view of the reflector in the embodiment of FIG. In the figure, 11 and 21 are piezoelectric substrates, 16 and 17 are reflectors, 22, 31 and 41 are pits, and 42 is a gold vapor deposited film.

Claims (1)

[Claims] 1. An interdigital electrode provided on the surface of a piezoelectric substrate;
A surface acoustic wave resonator consisting of a distributed reflector, in which the distributed reflector is filled with a material that changes the propagation velocity of the surface acoustic wave, or is filled with a pit so as to protrude from the surface. A surface acoustic wave resonator characterized by being composed of an aggregate of.