Surface acoustic wave device with an electrode insulating film a

Details Surface acoustic wave device with an electrode insulating film a Surface acoustic wave device with an electrode insulating film a

1996-09-13

1999-07-13

Pascal, Robert

Wave transmission lines and networks

Coupling networks

Electromechanical filter

310313A, 310313B, 310360, 29 2535, H03H 9145, H03H 925, H03H 964

Patent

active

059232315

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a surface acoustic wave device using, as a propagation mode, surface acoustic waves, such as leaky waves, longitudinal leaky waves, etc., which propagate radiating bulk waves in the direction of depth of a piezoelectric substrate, and a method for fabricating the same.


BACKGROUND ART

Surface acoustic wave devices are circuit devices which conduct signal processing of converting electric signals to surface acoustic waves, and they are used in filters, resonators, delay lines, etc. Usually electrodes of conducting film of, e.g., metal, which are called inter-digital transducers (IDTs) are formed on a piezoelectric substrate for converting electric signals to surface acoustic waves, and vice versa.
Characteristics of the surface acoustic waves depend on propagation characteristics thereof on the piezoelectric substrates. Especially for higher frequencies of the surface acoustic wave devices, the piezoelectric substrates must have high propagation velocities for the surface acoustic waves.
As substrate materials for surface acoustic waves, quartz, lithium tantalate (LiTaO.sub.3), lithium niobate (LiNbO.sub.3), lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7), etc., are conventionally known. As surface acoustic waves to be used in the surface acoustic wave devices, Rayleigh waves and leaky surface waves are mainly known.
Rayleigh waves are surface acoustic waves which propagate on the surfaces of elastic bodies, and propagate without radiating their energy into the elastic bodies, i.e., without theoretical propagation losses. As substrate materials to be used in the surface acoustic wave devices using Rayleigh waves, an ST-cut quartz with a 3100 m/sec propagation velocity, an X-cut 112.degree.-Y propagating LiTaO.sub.3 of a 3300 m/sec propagation velocity, 128.degree.-rotated Y-cut X-propagating LiNbO.sub.3 with a 4000 m/sec propagation velocity, a 45.degree.-rotated X-cut Z-propagating Li.sub.2 B.sub.4 O.sub.7 with a 3400 propagation velocity, etc., are known.
On the other hand, the use of leaky surface waves in the surface acoustic wave devices have been studied. The leaky surface waves are surface acoustic waves which propagate on the surfaces of elastic bodies, radiating one transverse component thereof as bulk waves in the direction of depth of the elastic bodies.
Generally, leaky surface waves have such large propagation losses due to the above radiation that it is difficult to use leaky surface waves in the surface acoustic wave devices. At a certain cut angle and in a certain propagation direction, however, usually the propagation losses are relatively small, thus leaky surface waves could be used in the surface acoustic wave devices.
Leaky surface waves, which have higher propagation velocity than Rayleigh waves, are widely used in surface acoustic wave devices for relatively high frequencies (above the UHF band).
As substrate materials for the surface acoustic wave devices using leaky surface waves, an LST-cut quartz with a 3900 m/sec propagation velocity, a 36.degree.-rotated Y-cut X-propagating LiTaO.sub.3 with a 4200 m/sec propagation velocity, a 41.degree.-rotated Y-cut X-propagating LiNbO.sub.3 with a 4500 m/sec propagation velocity, a 64.degree.-rotated Y-cut X-propagating LiNbO.sub.3 with a 4500 m/sec propagation velocity, etc., are known.
The inventors of the present application recently further developed the theory of leaky surface waves and ascertained the presence of surface waves (hereinafter called longitudinal leaky waves) which propagate with the longitudinal waves as the main component, radiating two transverse wave components as bulk waves into substrates.
They ascertained the presence of surface acoustic waves of a propagation velocity as high as 5000-7500 m/sec on lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7) whose cut angles and propagation directions are an Eulerian angle representation of (0.degree.-45.degree., 30.degree.-90.degree., 40.degree.-90.degree.) and a direction equivalent to the representation (see Japanese Patent Lai

REFERENCES:
A copy of the International Search Report issued by the Japanese Patent Office for PCT/JP95/01554 (2 pages).

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