Surface Acoustic Wave Device

The present invention relates to a surface acoustic wave device which is applicable to high frequencies.

In general, an interdigital transducer (IDT) is employed as an electromechanical transducer of a surface acoustic wave device, and the width W of the finger of a comb-shaped electrode in the IDT is proportionate to the wavelength .lambda. (generally the width W is 1/4 or 1/8 of the wavelength .lambda.), that is, inversely proportional to the frequency f, and thus the width W is narrowed in a high frequency device, wherefore formation of the IDT by, e.g., photoetching is made difficult.

However, since it is well known in the art that the relation between the frequency f, the wavelength .lambda. and SAW (surface acoustic wave) velocity v is represented by the equation .lambda.=v/f, the wavelength .lambda. is increased by raising the SAW velocity v, wherefore formation of the IDT is simplified even if the device is applied to high frequencies.

Further, electromechanical conversion efficiency of the IDT depends on the square value k.sup.2 of an electromechanical coupling coefficient k of a substrate employed therein, and the conversion efficiency is improved as the value k is increased.

Thus, there is required a substrate of high SAW velocity having a high coupling coefficient as the substrate to be employed in a high frequency surface acoustic wave device.

With respect to a substrate generally employed in a conventional surface acoustic wave device, lithium niobate (LiNbO.sub.3) is selected as the material applicable to high frequencies with the SAW velocity v being about 4000 m/s the square value k.sup.2 of the coupling coefficient k being equal to 5.6%. On the other hand, there has been proposed a substrate member utilizing so-called Sezawa wav or higher mode by forming zinc oxide (ZnO) film on a sapphire substrate or an alumina ceramic substrate. In such structure, the SAW velocity v is 5500 to 6000 m/s, which is over 1.4 times as large as that of the LiNbO.sub.3 substrate though the value k.sup.2 of about 5% is similar to that of the LiNbO.sub.3 substrate, and in view of the high SAW velocity value v, ZnO on the sapphire substrate is more suitable as the material for the substrate applicable to a high frequency device. However, sapphire is considerably expensive in comparison with LiNbO.sub.3. Further, with respect to the alumina ceramic substrate, it is difficult to obtain a mirror face suitable for formation of a fine IDT pattern while the propagation loss is increased in a high frequency range since the same is a sintered substance having large grain sizes, wherefore it is difficult to apply the alumina ceramic substrate to a high frequency device.

An object of the present invention is to overcome the aforementioned disadvantages of the prior art by providing a surface acoustic wave device in which a substrate of high SAW velocity and high coupling coefficient are implemented while maintaining practically applicable manufacturing cost and workability, for facilitating application to high frequencies and high efficiency simply at a low cost.

According to the present invention, briefly stating, there is employed a surface acoustic wave substrate member of three-layer structure consisting of a dielectric substrate, an oxide film and a piezoelectric thin film, with thickness h.sub.1 of the oxide film and thickness h.sub.2 of the piezoelectric thin film being within the ranges of 0.1.ltoreq.h.sub.1 /.lambda..ltoreq.10 and 0.05.ltoreq.h.sub.2 /.lambda..ltoreq.0.35 respectively, in which .lambda. is representative of the wavelength of Sezawa wave, thereby to utilize Sezawa wave (or similar mode wave) on the surface acoustic wave substrate member. Although Sezawa wave is generally considered as a sort of the surface acoustic wave on a substrate member of two-layer structure, a similar surface acoustic wave may be transmitted on the substrate member of three-layer structure as in the present invention, and such surface acoustic wave is hereinafter referred to as "Sezawa wave" in this specification.

The above and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIG. 1 is a cross-sectional view showing an embodiment of a surface acoustic wave device according to the present invention; and

FIGS. 2 and 3 are characteristic diagrams showing values v and k.sup.2 of a surface acoustic wave substrate member employed in the embodiment of the present invention.

FIG. 4 is a top view of the embodiment of the invention shown in cross-section in FIG. 1.

An embodiment of the present invention is now described with reference to the attached drawings.

In FIG. 1, a dielectric substrate 1 is formed by borosilicate glass material, which is low in cost and ready for forming a mirror face. On the glass dielectric substrate 1, there is formed a thin film 2 of aluminum oxide (Al.sub.2 O.sub.3) by, e.g., RF sputtering. According to the present embodiment, the film thickness h.sub.1 of the Al.sub.2 O.sub.3 thin film 2 is about 30 .mu.m. On the Al.sub.2 O.sub.3 thin film 2, there is formed an interdigital transducer (IDT) 3 consisting of comb-shaped electrodes having wavelength .lambda. of 100 .mu.m, on which a piezoelectric thin film 4 of zinc oxide (ZnO) is formed by sputtering. Such an interdigital transducer may be formed on the dielectric substrate 1 or on the piezoelectric thin film 4 instead of on the Al.sub.2 O.sub.3 thin film 2.

FIG. 4 is a top view of the embodiment of the invention shown in cross-section in FIG. 1. In this view, two interdigital transducers 3,3' are shown. The transducer 3 comprises interleaved comb-shaped electrodes 3a and 3b. The transducer 3' comprises two interleaved comb-shaped electrodes 3'a and 3'b.

In the three-layer structure of the ZnO/Al.sub.2 O.sub.3 /glass substrate member formed in the aforementioned manner, measurement is performed with respect to the SAW velocity v and the square value k.sup.2 of the electromechanical coupling coefficient k of Sezawa wave and Rayleigh wave upon change in the film thickness h.sub.2 of the ZnO thin film 4, the results of which are shown in FIGS. 2 and 3. Further, a short-circuit electrode 5 (counter electrode shown --in FIG. 1 by dotted lines) is formed over the uppermost ZnO thin film 4 to effect similar measurement, the results of which are also shown in FIGS. 2 and 3.

In FIG. 2, the characteristic curve A represents the SAW velocity v of the Sezawa wave. The characteristic A is not substantially influenced by provision of the short-circuit electrode.

FIG. 3 shows the square value k.sup.2 of the electromechanical coupling coefficient k, and the characteristic curve B represents the Sezawa wave utilized in the embodiment a shown in FIG. 1 in which no short-circuit electrode is provided and the characteristic curve C also represents the Sezawa wave transmitted in a case where the short-circuit electrode 5, as shown in FIG. 1 by dotted lines, is further provided, while the characteristic curve D represents the Rayleigh wave transmitted in a case where no short-circuit electrode is provided and the characteristic curve E represents the Rayleigh wave transmitted in a case where the short-circuit electrode is provided.

As clearly seen from FIG. 2, the SAW velocity v in the Sezawa wave (characteristic curve A) is over 4000 m/s, and particularly around h.sub.2 /.lambda.=0.25, high SAW velocity of over 6000 m/s is obtained. Further, with respect to the value k.sup.2, there is the relation of k.sup.2 .perspectiveto.2.5% around h.sub.2 /.lambda.=0.22 as seen from the characteristic curve B in FIG. 3, which value of about 2.5% is sufficient since the practically applicable minimum value is about 0.5 to 0.6%. Particularly when the short-circuit electrode is provided, there is obtained extremely large value, i.e., k.sup.2 .perspectiveto.5% and over (characteristic curve C).

The Rayleigh wave is rendered spurious when the Sezawa wave is utilized as in the present invention, though, the value k.sup.2 of the Rayleigh wave is smaller than the value k.sup.2 of the Sezawa wave in a case where the value h.sub.2 /.lambda. is smaller than 0.35 as shown in FIG. 3 and the value k.sup.2 of the Rayleigh wave is substantially zero particularly when the value h.sub.2 /.lambda. is smaller than 0.2, whereby such spuriousness can be disregarded. Therefore, the value h.sub.2 /.lambda. is preferably not more than 0.35. With respect to the Sezawa wave, further, a state of mode cut-off is induced when the value h.sub.2 /.lambda. is around 0.05 and the propagation loss is increased when the subject value becomes smaller than 0.05, and hence the preferable range of the value h.sub.2 /.lambda. is 0.05.ltoreq.h.sub.2 /.lambda..ltoreq.0.35.

With respect to the film thickness h.sub.1 of the Al.sub.2 O.sub.3 thin film 2, the Sezawa wave is generated when the value h.sub.1 /.lambda. equals or exceeds 0.1, and in consideration of irregularity in the surface of the film 2 and stress caused in the same when the film thickness h.sub.1 is too large, the practically applicable range of the value h.sub.1 /.lambda. is 0.1.ltoreq.h.sub.1 /.lambda..ltoreq.10. More preferably, the applicable range of the value h.sub.1 /.lambda. is 0.1.ltoreq.h.sub.1 /.lambda..ltoreq.1.0 particularly when the employed wavelength .lambda. is about 100 .mu.m.

According to the present embodiment, there is readily obtained a surface acoustic wave device provided with a substrate member of high SAW velocity having a relatively high coupling coefficient.

Since, according to the present embodiment, the dielectric substrate is made of glass, the surface acoustic wave device can be manufactured at an extremely low cost in comparison with that employing a sapphire or LiNbO.sub.3 substrate. Further, the characteristic in the high frequency range is improved according to the present embodiment since surface flatness of the glass substrate is excellent and the propagation loss thereof is decreased in comparison with an alumina ceramic substrate. Although the dielectric substrate may be made of plastic or metal material, glass is optimum as the material therefor in view of excellence in surface flatness, i.e., readiness in formation of the IDT and reasonableness in cost.

With respect to the oxide film, Al.sub.2 O.sub.3 is preferable in view of readiness in formation of the film, although beryllium oxide (BeO) may be employed as the material for the film other than Al.sub.2 O.sub.3 The Al.sub.2 O.sub.3 film may be formed by processes such as the CVD process, the MO-CVD process or RF ion plating other than RF sputtering process targetting Al.sub.2 O.sub.3.

Although zinc selenide (ZnSe), cadmium sulfide (CdS) etc. may be employed as the material for the piezoelectric thin film other than ZnO, the piezoelectric thin film is most preferably made of ZnO in consideration of the coupling coefficient and readiness in formation of the film. Similarly to the case of the oxide film, the piezoelectric thin film may be formed by processes such as sputtering, the CVD process and ion plating.

Further, the surface acoustic wave to be utilized is not restricted to the fundamental wave of the IDT, and the response of the third harmonic wave or other higher harmonics may be utilized. Assuming that, in this case, the fundamental period of the IDT is represented by .lambda..sub.0, it is clear that the wavelength .lambda..sub.n of the n-order high harmonic wave is represented by the relation .lambda..sub.n =.lambda..sub.0 /n.

Further, although the above description of the present embodiment has been made with respect to the substrate member of three-layer structure, the present invention may be applied to a device comprising four or more layers in entire structure with a dielectric substrate consisting of two or more layers.

Since, as hereinabove described, a surface acoustic wave substrate member in three-layer structure of piezoelectric thin film/oxide film/dielectric substrate is employed in the present invention, there can be implemented a surface acoustic wave device having a substrate member of high SAW velocity having a relatively high coupling coefficient which is suitable for high frequencies at a low cost, leading to extensive usefulness in the industrial field.