JPS5915564B2 - surface acoustic wave resonator - Google Patents

Description

[Detailed description of the invention] The present invention relates to reflector structures for surface acoustic wave resonators.

Conventionally, it is well known that a surface acoustic wave resonator is constituted by an interdigital transducer and a reflector consisting of a large number of stripes that reflect surface acoustic waves.

This basic invention was filed by Clinton Silpelter Hartman et al. (Japanese Patent Application Laid-open No. 51-244).
The fact that good characteristics can be obtained has been confirmed by the present inventor and from reports of follow-up tests at numerous academic conferences, and its application and practical use in high-frequency filters, oscillators, etc. is expanding.

However, the structure of the stripes for surface acoustic wave reflection mentioned above is greatly restricted by the material of the substrate of the surface acoustic wave element, and it is not necessarily possible to select it freely due to manufacturing process considerations. .

The structure of a conventionally reported reflector will be shown and the reflection mechanism will be explained.

In the reflector having the structure shown in Fig. 1, a thin aluminum film is deposited on a lithium niobate substrate 1, and a stripe 2 having a line width of 1/4 of the surface wave wavelength at the resonant frequency is formed by photoetching. It has a structure in which many pieces are arranged repeatedly.

The reflection mechanism of this structure is based on the fact that reflection occurs due to the difference in acoustic impedance between parts of the substrate surface covered with a conductive film and parts not covered with a conductive film.

However, this surface acoustic wave resonator with an aluminum reflector formed on a lithium niobate substrate has a Q of only 80.
00, which is low and has not been put into practical use.

On the other hand, the reflector having the structure shown in FIG. 2 has shallow grooves 4 on the surface of a piezoelectric substrate 3 made of crystal, lithium niobate, lithium tartrate, etc. having a width of about 1/30 or less of the surface wave wavelength at the resonance frequency.
A large number of lines with the same line width as in FIG. 1 are arranged repeatedly.

This reflection mechanism applies acoustic reflection at the step at the end of the groove by applying geometrical perturbations on the substrate surface.

The relationship between groove depth and reflectance has been widely published both theoretically and experimentally, and quartz lithium niobate has been used as the substrate material. Although chemical methods such as plasma etching can also be used (since Si02), no suitable chemical method has been found for substrates such as lithium niobate and lithium quantalate, so ion beam milling, which is a physical processing method, has to be relied upon. I have no choice but to.

Ion milling requires expensive equipment and poor mass production.
It has not yet been widely produced.

The reflector with the structure shown in Fig. 3 has the same concept as the groove, but instead of using the groove, convex stripes 6 are formed on the surface of the substrate 5 using dielectric material, metal film, etc., and the reflector has the same structure as the groove method. It is something that causes a reflex.

It is known that this mechanism is similar to the groove system.

Examples of experimental reports using this method include a structure in which aluminum stripes with a thickness of about 1/30 of the surface acoustic wave wavelength are attached on a quartz substrate, and a structure in which zinc oxide stripes are attached on a quartz substrate. Things are being tested.

Although this result shows that a good reflector is obtained, there are other problems in the combination of the quartz plate and the above materials.

This is because the surface wave propagation speed within the reflector changes greatly depending on the thickness of the stripe material, and the resonant frequency changes.

This means that the thickness of the stripe material must be formed with extremely high precision in order to manufacture a surface acoustic wave resonator having a desired resonance frequency.

This becomes a very big problem during mass production, and each
Therefore, it becomes necessary to trim the two resonators using a certain method.

The present invention eliminates the above-mentioned drawbacks, and provides a surface acoustic wave resonator with a reflector that has good reflection characteristics and a combination of a substrate and a stripe material that has a small change in resonance frequency with respect to the film thickness of the stripe material. It is something that provides children.

Hereinafter, the present invention will be explained with reference to the drawings.

FIG. 4 shows an embodiment of the present invention, in which a mekat plate of lithium quantalate is used for the substrate 7, and the input/output quadrature transducer 8 is configured so that surface acoustic waves propagate in the 112@Y force direction with less bulk spurious. are formed at appropriate intervals, and convex stripes 9 are formed between the input and output transducers 8 by photo-etching an aluminum film.
This is provided.

The stripes 9 are 1/4 of the surface acoustic wave wavelength λR, the repetition interval is 1/2 of λR, and only the central portion is separated by 3/4 of λR.

A total of 400 stripes 9 are provided.

If reflection occurs at the end of each stripe 9, the reflected waves from each part will be added, resulting in a standing wave of surface acoustic waves at the stripe part.

At this time, when looking at the transmission characteristics between the input and output quadrature transducers 8, the characteristics are as shown in FIG.

The dip portion in FIG. 5 and 11 is determined by the reflectance of the stripe reflector, and the greater the reflectance, the deeper the dip, and the reflectance can be estimated from the shape of this dip.

Further, a peak portion 12 in the figure indicates that a resonance state is occurring, and the Q of resonance can be calculated from the half-width of this peak.

FIG. 6 shows the results of measuring the reflectance per stripe when the height h of the convex stripes was varied using this element.

This result shows almost the same good result as the result of providing a convex aluminum stripe reflector on a quartz crystal plate as described in the conventional example.

The Q value also shows a similar value, and the Q of a resonator with an aluminum reflector formed on a conventional lithium niobate substrate.
I was able to obtain a Q of about 15,000, which is much larger than that.

Next, FIG. 7 shows the results of measuring the rate of change in resonance frequency with respect to change in film thickness of the stripe material.

This measurement can be performed by measuring the 12 frequencies in FIG. 5 for each film thickness.

The solid line 13 in FIG. 7 is for the reflector of the present invention, and the broken line 14
1 is a case where a reflector is provided with aluminum stripes on a quartz substrate, and a chain line 15 is a case where an aluminum stripe reflector is provided on a lithium niobate substrate.

Looking at the thickness of around 1 μm, in order to produce a resonant frequency with an accuracy of about 0.1%, in the conventional example, the film thickness must be formed with an accuracy of about 500A, but with the reflector of the present invention, the film thickness has an accuracy of about 3000λ. That's fine.

Figure 10 shows the measurement of the figure of merit of the surface acoustic wave resonator with respect to the change in the film thickness of the aluminum reflector at the center frequency of the surface acoustic wave of 100 MHz.
It can be seen that a large figure of merit is shown with ~1.2μ.

Generalizing this to all frequencies, the surface wave wavelength λR and the aluminum film thickness are h/λ□-0,006 to 0.04.
A large figure of merit will be obtained if the value is within the range of .

Next, an example will be shown in which a filter is constructed of a resonator using a reflector of the present invention.

Figure 8 shows the characteristics of a filter in which two boat-shaped resonators are connected in a four-stage cascade, with two reflectors made of 1 μm thick aluminum stripes on a lithium tantalate X plate, installed at both ends of the input and output quadratic transducers. show.

This shows that a good filter with a center frequency of 1.00 MHz, a specific bandwidth of 1/1000, and an insertion loss of 3 dB can be provided at low cost through a simple manufacturing process.

Further, the reflective stripes do not necessarily have to be as in the embodiment, but may be stripes 18 with predetermined steps as shown in FIG. 9.

As described above, the surface acoustic wave resonator constituted by a reflector made of a combination of a substrate material and a convex reflective stripe material according to the present invention, and a filter to which the same is applied, have a resonant frequency,
Manufacturing variations in center frequency are reduced, making it possible to provide high-quality resonators and filters.

On the other hand, the manufacturing process is simplified, yield is improved, and costs are reduced.

Although the embodiment shows the case where the aluminum reflector is deposited directly on the lithium tankate substrate, it may be deposited via a substance that improves adhesion, such as chromium.

In this case, it becomes strong against high-power surface acoustic waves.

[Brief explanation of the drawing]

Figures 1 to 3 are diagrams showing conventional surface acoustic wave resonators;
FIG. 4 is a diagram showing an embodiment of the surface acoustic wave resonator of the present invention, FIG. 5 is a diagram showing the transmission characteristics of the element of FIG. 4, and FIG. 6 is a diagram showing the aluminum film thickness of the reflector of the present invention. 7 is a diagram showing the measurement results of the change rate of the resonant frequency with respect to the aluminum film thickness of the conventional example and the reflector of the present invention. FIG. A frequency characteristic diagram of a filter configured by cascading four stages, FIG. 9 is a diagram showing a variation example of the present invention, and FIG. 10 is a figure of merit for changes in the aluminum reflector film thickness of the surface acoustic wave resonator of the present invention. FIG. 7... Lithium cuncurate substrate, 8...
・Input/output transducer, 9...Reflection stripe.

Claims (1)

[Claims] 1. A lithium tantalate substrate, a transducer provided on the substrate for converting an electric signal into a surface acoustic wave, and a transducer for reflecting the surface acoustic wave excited by the transducer. A surface acoustic wave resonator comprising: a reflector made of an aluminum film provided on the surface acoustic wave propagation path of the lithium tantalate substrate to generate a resonance state. 2. The surface acoustic wave resonator according to claim 1, wherein the reflector is formed by forming a plurality of striped aluminum films in parallel. 3. The surface acoustic wave resonator according to claim 1, wherein the reflector is an aluminum film in which striped thick film portions are periodically provided. 4. The surface acoustic wave resonator according to claim 1, wherein a substance that increases adhesion is deposited between the lithium tantalate substrate and the aluminum film.