A surface acoustic wave device comprises a silicon substrate, a conductive layer provided on the silicon substrate, a silicon dioxide layer provided on the conductive layer, input and output electrodes provided on the silicon dioxide layer for input and output of a surface acoustic wave, and a zinc oxide layer provided on the electrodes. The silicon substrate is cut by a crystalline surface substantially equivalent to the (111)-surface, and the zinc oxide layer is such that its crystalline surface substantially equivalent to the (0001)-surface is parallel to the cut-surface of the silicon substrate, so that the surface acoustic wave entered by the input electrode travels to the output electrode in a direction substantially equivalent to the [112]-axis of the silicon substrate.

FIELD OF THE INVENTION 
This invention relates to a surface acoustic wave device. 
BACKGROUND OF THE INVENTION 
There are a great demand and progress of surface-acoustic-wave devices of 
various types which use a surface acoustic wave propagated near the 
surface of an elastic solid. One of the reasons of the development is that 
a surface acoustic wave travels so slowly as 10.sup.-5 times the speed of 
an electromagnetic wave and hence enables an extreme reduction in size of 
the device. Another reason is that a surface acoustic wave which travels 
near the surface of a solid can be readily picked up from any point of the 
propagation path. A further reason is that since energies are concentrated 
near the surface of a solid, the device can be used as a device which 
utilizes an interaction between light and a carrier of a semiconductor or 
a nonlinearity due to the high energy concentration. A still further 
reason is that the device can be fabricated by a circuit integration 
technology and hence readily combined with integrated circuits to provide 
a new device. 
FIGS. 1 and 2 show structures of the prior art surface-acoustic-wave 
devices. Reference numeral 1 denotes a piezoelectric substrate made from 
lithium niobate (LiNbO.sub.3), 2 is a semiconductor substrate made of 
silicon which is cut by a crystalline surface substantially equivalent to 
the (100)-surface, 3 is a piezoelectric layer made from zinc oxide (ZnO) 
whose crystalline surface substantially equivalent to the (0001)-surface 
is parallel to the said cut surface of the silicon substrate 2, and 4 and 
5 are comb-shaped electrodes which are provided on the lithium niobate 
substrate 1 or on the silicon substrate 2, with their splits 
interdigitating each other. For example, the electrode 4 is used as input 
electrode and the electrode 5 is used as output electrode. 
A surface acoustic wave excited and entered by the input electrode 4 
travels along the surface of the lithium niobate substrate 1 or of the 
silicon substrate 2 and is picked up from the output electrode 5. 
If a Rayleigh wave is used as said surface acoustic wave, the device of 
FIG. 1 provides a large value in square K.sup.2 of the electromechanical 
coupling coefficient K which is one of the most important factors of the 
device's nature. This advantage increases the demand of the device in 
various technical fields. However, since the substrare is made from a 
single material, the device of FIG. 1 involves such a drawback that the 
electromechanical coupling coefficient K is fixed by the crystalline axis 
direction of the substrate and the travelling direction of a surface 
acoustic wave. 
In FIG. 2, however, if a Rayleigh wave is propagated in the [112]-axis 
direction of the silicon substrate 2, the device may have a flexible 
K.sup.2 characteristic and a larger electromechanical coupling coefficient 
K by selecting a thickness h.sub.1 of the zinc oxide layer 3 which is 
obtained by an analysis. For example, if the thickness h.sub.1 is selected 
so as to satisfy the relation .omega.h.sub.1 =8500 (where .omega. is the 
angular frequency of the surface acoustic wave), K.sup.2 becomes 3.05% 
approximately. The device of FIG. 2, however, is expensive because it 
needs an increased thickness of the zinc oxide layer which is normally 
fabricated by a sputtering technology. 
OBJECT OF THE INVENTION 
It is therefore an object of the invention to provide a surface acoustic 
wave device which has a reduced thickness of the zinc oxide layer and 
represents a larger electromechanical coupling coefficient K. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a surface 
acoustic wave device comprising: 
a silicon substrate which is cut with a crystalline surface substantially 
equivalent to the (111)-surface; 
a conductive layer provided on said silicon substrate; 
a silicon dioxide layer provided on said conductive layer; 
electrodes provided on said silicon dioxide layer for input and output of a 
surface acoustic wave; and 
a zinc oxide layer provided on said electrodes so that a crystalline 
surface thereof substantially equivalent to the (0001)-surface is parallel 
to said (111)-oriented surface of said silicon substrate, said surface 
acoustic wave entered from said input electrode travelling in a direction 
substantially equivalent to the [112]-axis of said silicon substrate up to 
said output electrode. 
The invention will be better understood from the description given below, 
referring to some embodiments illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a sectional view of a surface acoustic wave device embodying the 
invention. Reference numeral 11 denotes a silicon substrate having a 
(111)-oriented major surface. Reference numeral 12 designates a silicon 
dioxide (SiO.sub.2) layer deposited on the major surface of the silicon 
substrate 11 and having a thickness h.sub.2. Reference numeral 13 denotes 
a zinc oxide layer with a thickness h.sub.1 which is deposited on the 
silicon dioxide layer 12 so that a substantially (0001)-oriented major 
surface thereof extends parallel to the major surface of the silicon 
substrate 11. Reference numerals 14 and 15 denote input and output 
electrodes for input and output of a surface acoustic wave such as a 
Rayleigh wave. The respective electrodes 14 and 15 are interdigitating, 
comb-shaped electrodes provided on the zinc oxide layer 12. Reference 
numeral 16 denotes a conductive layer interposed between the silicon 
substrate 11 and the silicon dioxide layer 12 and is prefereably so thin 
as possible. The conductive layer is not restricted to a metal layer but 
may be a superficial part of a substrate significantly increased in 
conductivity. 
The conductive layer 16 or the zinc oxide layer 13 is preferably provided 
just above or below the interdigitating part P of the comb-shaped 
electrodes 14 and 15 as shown in FIG. 6. 
With this arrangement (hereinafter called "ZnO(0001)/ SiO.sub.2 
/Si(111)[112]), when a Rayleigh wave is propagated in the [112]-axis 
direction of the silicon substrate 11, the device represents the K.sup.2 
characteristics as shown in FIG. 4. 
In FIG. 4, the abscissa expresses the thickness h.sub.1 of the zinc oxide 
layer 13 by .omega.h.sub.1 (where .omega. is the angular frequency), and 
the ordinate shows the square K.sup.2 of the electromechanical coupling 
coefficient K by percentage. The Figure shows variations of K.sup.2 where 
.omega.h.sub.1 is varied (preferably in the range between 4000-12000) 
under .omega.h.sub.2 =2600. The thickness h.sub.2 of the silicon dioxide 
layer 12 may be varied in the range 0.ltoreq..omega.h.sub.2 .ltoreq.10000. 
As seen from FIG. 4, by selecting the thickness h.sub.1 and h.sub.2 of the 
zinc oxide layer 13 and the silicon dioxide layer 12 so that 
.omega.h.sub.1 =7600 and .omega.h.sub.2 =2600, the maximum value K.sup.2 
=3.49% is obtained at a point A. 
The above value is larger than the value (K.sup.2 =3.05%, .omega.h.sub.1 
=8500) obtained by the prior art construction of FIG. 2. Additionally, the 
thickness h.sub.1 of the zinc oxide layer 13 is decreased from the prior 
art value .omega.h.sub.1 =8500 to .omega.h.sub.1 =7600 by interposing the 
silicon dioxide layer 12. This contributes to a cost reduction in 
fabrication of the device. 
Other variations of the thicknesses h.sub.1 and h.sub.2 of the zinc oxide 
layer 13 and the silicon dioxide layer 12 within the above-mentioned 
ranges also give improved characteristics and flexibility to the surface 
acoustic wave devcie as compared to the prior art construction. 
No substantial difference is found in characteristics of the device when 
the orientations of the major surface of the silicon substrate 11 and the 
zinc oxide layer 13 deviate within 10 degrees from (111) and (0001) 
respectively and the propagation axis of the silicon substrate 11 deviates 
within 10 degrees from [112]. 
Referring now to FIG. 5 which shows a convolver device having the inventive 
construction, reference numeral 17 designates a gate electrode provided on 
the zinc oxide layer 13 in a central portion between the input and output 
electrodes 14 and 15. This also gives the substantially same excellent 
K.sup.2 characteristics as in the former embodiment. 
It is also expected to provide a device which uses an electrical potential 
generated within the silicon substrate 11, the silicon dioxide layer 12 
and the zinc oxide layer 13, without using the comb-shaped electrodes. 
As described above, the inventive device comprises the silicon substrate 
with the (111)-oriented major surface, the silicon dioxide layer provided 
on the silicon substrate, the zinc oxide layer provided on the silicon 
dioxide layer with its (0001)-oriented major surface disposed parallel to 
the major surface of the silicon substrate, the electrodes provided on the 
silicon dioxide layer, and further includes the conductive layer 
interposed between the silicon substrate and the silicon dioxide layer, so 
that a surface acoustic wave travels in the [112]-axis direction of the 
silicon substrate. Therefore, it is possible to increase the 
electromechanical coupling coefficient and hence to ensure an effective 
operation of the surface acoustic wave device. 
Further, as shown in FIG. 7, by using a single silicon substrate as the 
substrate 11 of the inventive surface acoustic wave device SAW and also as 
a substrate 11' of an integrated circuit IC connected to the input and 
output electrodes of the surface acoustic wave device, it is possible to 
unite a functional device and a semiconductive device by the circuit 
integration technology to provide a more compact circuit system with much 
more circuit elements combined.