JPH0584089B2 - - Google Patents

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a surface acoustic wave resonator.

(Prior Art) Conventionally, it has been known that a surface acoustic wave resonator is configured by an interdigital transducer and a reflector consisting of a large number of stripes that reflect surface acoustic waves. This basic invention was made by Clinton, Silpelter, and Hartman (Japanese Patent Application Laid-Open No. 51-244), and is now being applied to high-frequency filters, oscillators, etc., and its practical use 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 with 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 in which an aluminum reflector is formed on a lithium niobate substrate has a low Q of about 8000 at most, and has not been put into practical use.

On the other hand, the reflector with the structure shown in Figure 2 uses a piezoelectric substrate 3 made of quartz, lithium niobate, lithium tartrate, etc.
A large number of shallow grooves 4 having a width of about 1/30 or less of the surface wave wavelength at the resonant frequency are repeatedly arranged on the surface of the groove 4 and have the same line width as that shown in FIG. This reflection mechanism applies acoustic reflection at the step at the end of the groove by applying geometrical vibrations 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 (because SiO ₂ ), lithium niobate,
Currently, no suitable chemical method has been found for substrates such as lithium tantalate, so we have no choice but to rely on physical processing, ion beam milling. Ion milling requires expensive equipment and is not suitable for mass production, so it has not 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 causes a reflex.

This mechanism is known to be similar to the groove system.

As an example of an experimental report using this method, on a crystal substrate,
One structure with an aluminum stripe about 1/30 the thickness of the surface acoustic wave wavelength, and the other with a zinc oxide stripe on a quartz substrate 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.
This is because 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 it becomes necessary to trim each resonator by some method.

(Problems to be Solved by the Invention) As described above, conventional surface acoustic wave resonators have problems such as not being able to obtain good reflection characteristics and large changes in the resonant frequency due to changes in stripe film thickness. However, it had not yet been put into practical use.

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

[Structure of the invention]

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a lithium tantalate substrate and a transducer provided on the substrate for converting an electric signal into a surface acoustic wave. , provided in the surface acoustic wave propagation path of the lithium tantalate substrate to reflect the surface acoustic waves excited by the transducer and generate a resonance state, and having a film thickness of 0.015 normalized by the surface acoustic wave wavelength. The present invention provides a surface acoustic wave resonator characterized by comprising a reflector made of an aluminum film with a thickness of 0.05 or less.

(Function) According to the surface acoustic wave resonator of the present invention in which an aluminum reflector is provided on such a lithium tantalate substrate, compared to a surface acoustic wave resonator in which an aluminum reflector is provided on a crystal or lithium niobate substrate, the aluminum reflector is provided on a lithium tantalate substrate. The change in the surface acoustic wave propagation velocity with respect to the change in film thickness is small, and as a result, the variation in the resonance frequency of the surface acoustic wave resonator is small, and the effect is that the frequency reproducibility for mass production is excellent.

In addition, in order to obtain a large surface acoustic wave reflectance, it is necessary that the electro-mechanical coupling coefficient k ² of the piezoelectric substrate be large and that the weight of the reflector be large. In this respect, the k ² of lithium tantalate is It is smaller than that of lithium niobate, and the specific gravity of lithium tantalate is approximately 7.5, while the specific gravity of aluminum is approximately 2.4, so initially it was sufficient to provide an aluminum reflector on the lithium tantalate substrate. There was a concern that it would not be possible to obtain a good reflector, but in reality, good reflection characteristics can be obtained by setting the aluminum film thickness to 0.015 or more and 0.05 or less, which is normalized by the surface acoustic wave wavelength. It has been found that a much larger Q can be obtained compared to conventional surface acoustic wave resonators.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Figure 4 shows an embodiment of the present invention, in which an X-cut plate of lithium tantalate is used for the substrate 7, and the input/output transducer is configured so that the surface acoustic waves propagate in the 112°Y direction with less bulk bias. Users 8 are formed at appropriate intervals, and convex stripes 9 are provided between the input and output transducers 8 by photoetching an aluminum film.

In this stripe 9, the repetition interval of 1/4 of the surface acoustic wave wavelength λ _R is 1/2 of λ _R , and only the central portion is separated by 3/4 of λ _R. 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, the transmission characteristics between the input and output transducers 8 are as shown in FIG. 5.

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. In addition, the 12 peaks in the figure are
This indicates that a resonance state is occurring, and the resonance Q can be calculated from the half-width of this peak.

In this device, the film thickness (height) of the convex stripe is h
FIG. 6 shows the results of measuring the reflectance per stripe when changing the reflectance. As is clear from the figure, a reflectance of 1% can be obtained with a film thickness of 0.5 μm, and a reflectance of about 3% with a film thickness of 1.5 μm. As is clear from Figure 5, this measurement value was obtained at a center frequency of 100MHz, so the film thickness normalized by the surface acoustic wave wavelength λ _R is
0.5μm corresponds to 0.015, and 1.5μm corresponds to 0.05. 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. Furthermore, the Q value exhibited a similar value, and it was possible to obtain a Q of about 15,000, which is much larger than that of a conventional resonator in which an aluminum reflector is formed on a lithium niobate substrate. 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 is for the reflector with aluminum stripes on a quartz substrate.
A dashed line 15 shows the case where an aluminum stripe reflector is provided on a lithium niobate substrate. Looking at the thickness of around 1 μm, if you want 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 500 Å, but with the reflector of the present invention, the film thickness can be formed with an accuracy of about 3000 Å. .

Next, an example will be shown in which a filter is constructed of a resonator using the reflector of the present invention. Lithium tantalate
Figure 8 shows the characteristics of a filter in which a 2-port resonator is connected in a 4-stage cascade, with two reflectors made of 1 μm thick aluminum stripes on a plate at both ends of the input/output transducer. This shows that a good filter with a center frequency of 100 MHz and a relative bandwidth of 1/1000 and an insertion loss of 3 dB can be provided at low cost using 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.

〔Effect of the invention〕

As described above, the surface acoustic wave resonator configured with a reflector made of a combination of a substrate material and a convex reflective stripe material according to the present invention, and a filter using the same, have small manufacturing variations in the resonant frequency and center frequency. This means that we can 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 tantalate substrate, it may be deposited through 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]

1 to 3 are diagrams showing a conventional surface acoustic wave resonator, FIG. 4 is a diagram showing an embodiment of the surface acoustic wave resonator of the present invention, and FIG. 5 is a diagram showing a surface acoustic wave resonator according to an embodiment of the present invention. Figure 6 is a diagram showing the transmission characteristics, Figure 6 is a diagram showing the measurement results of reflectance with respect to aluminum film thickness of the reflector of the present invention, and Figure 7 is a diagram showing the change in resonance frequency with respect to aluminum film thickness of the conventional example and the reflector of the present invention. Fig. 8 is a frequency characteristic diagram of a filter constructed by cascading four resonators shown in Fig. 6.
FIG. 9 is a diagram showing a modification of the present invention. 7...Lithium tantalate substrate, 8...I/O 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 plurality of stripes are provided in the propagation path of the surface acoustic wave of the lithium tantalate substrate to generate a resonance state, and are made of an aluminum film having a film thickness of 0.015 or more and 0.05 or less normalized by the surface acoustic wave wavelength. A surface acoustic wave resonator characterized by comprising a reflector made of