Surface acoustic wave device including IDT electrode having solid electrode portion and split electrode portion

A surface acoustic wave device achieving improved insertion loss and fraction defective without lowering the TTE level includes an IDT electrode located on a piezoelectric substrate. The IDT electrode includes a solid electrode portion and a split electrode portion continuously formed at an end of the solid electrode portion in the propagating direction of a surface acoustic wave and having an electrode finger width of about .lambda./8 wherein .lambda. is the wavelength of the surface acoustic wave.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a surface acoustic wave device which 
receives and excites a surface acoustic wave. 
2. Description of the Related Arts 
A general type of surface acoustic wave device has a surface acoustic wave 
transducer in which an interdigital transducer, hereinafter referred to as 
IDT electrode, is formed on a piezoelectric substrate. Normally, a pair of 
surface acoustic wave transducers are respectively provided on the 
excitation side and the receiving side, and arranged in parallel on a 
single piezoelectric substrate in the propagating direction of the surface 
acoustic wave. 
The IDT electrode is, as shown in FIGS. 4(a) and 4(b), formed by meshing 
electrode fingers 21 of a pair of comb-shaped electrodes 20A and 20B 
disposed opposite to each other. FIG. 4(a) illustrates a split IDT 
electrode 22 in which the electrode fingers 21 are meshed two by two (a 
width j10 of each of the electrode fingers 21 is .lambda./8, wherein 
.lambda. indicates the wavelength of the surface acoustic wave), and FIG. 
4(b) illustrates a solid IDT electrode 23 in which the electrode fingers 
21 are meshed one, by one wherein a width j11 of each of the electrode 
fingers 21 is .lambda./4. 
In a conventional surface acoustic wave device like a transversal filter, 
in order to reduce Triple Transit Echo (abbreviated to "TTE" hereinafter), 
the surface acoustic wave transducer on one of the excitation and 
receiving sides is a split IDT electrode 22, and the surface acoustic wave 
transducer on the other side is a solid IDT electrode 23. 
However, in such a conventional surface acoustic wave device using the 
split IDT electrode 22 as one of the surface acoustic wave transducers, 
short circuits and cutoffs are likely to occur during production. Although 
the IDT electrodes are formed on the piezoelectric substrate by 
photolithography, since the electrode finger width j10 of the split IDT 
electrode 22 is .lambda./8, which is half the electrode finger width j11 
(.lambda./4) of the solid IDT electrode 23, the production of the split 
IDT electrode 22 is extremely difficult, and there is a high probability 
that a production failure will be caused by a short circuit, a cutoff and 
the like. 
Furthermore, static electricity produced in the surface acoustic wave 
device during or after production is likely to cause electrostatic 
breakdown. In the IDT electrode formed by the comb-shaped electrodes 20A 
and 20B meshed with each other, static electricity produced on the 
piezoelectric substrate is likely to be discharged between the opposed 
comb-shaped electrodes 20A and 20B due to the structure thereof. If static 
discharge occurs, the electrode fingers 21 in the discharge portion are 
damaged. Such discharge tends to occur more frequently as the clearance 
between the opposed comb electrodes 20A and 20B becomes narrower. In the 
split IDT electrode 22, the electrode finger width j10 is small and the 
interval between the meshing electrode fingers 21 of the comb-shaped 
electrodes 20A and 20B is also narrow, with the result that a discharge is 
likely to occur between the comb-shaped electrodes 20A and 20B. In 
addition, since each electrode finger 21 increases the resistance to 
discharge breakdown as the width thereof increases, and the split IDT 
electrode 22, whose electrode finger width is half that of the solid IDT 
electrode 23, cannot be expected to be sufficiently resistant to discharge 
breakdown. 
A large insertion loss is a further problem. It is well known that the 
insertion loss of the surface acoustic wave device increases as the 
electromechanical coupling coefficient k.sup.2 of the IDT electrode 
decreases. The electromechanical coupling coefficient k.sup.2 of the split 
IDT electrode 22 is, as well known, 75% of that of the solid IDT electrode 
23, and the insertion loss is thereby increased. 
SUMMARY OF THE INVENTION 
The preferred embodiments of the present invention overcome the problems of 
the prior art described above by decreasing the insertion loss of a 
surface acoustic wave device and the number of defective devices without 
lowering the TTE level. 
More specifically, the preferred embodiments of the present invention 
provide a surface acoustic wave device comprising an IDT electrode formed 
on a piezoelectric substrate and including a solid electrode portion and a 
split electrode portion, the split electrode portion being continuously 
formed at an end of the solid electrode portion in the propagating 
direction of a surface acoustic wave and having an electrode finger width 
of about .lambda./8 (.lambda.=the wavelength of the surface acoustic 
wave). 
It is preferable that the clearance between the split electrode portion and 
the solid electrode portion be substantially equal to .lambda./8 and that 
an electrode finger at an end of the solid electrode portion closest to 
the electrode clearance has a width of about 5.lambda./16. Furthermore, it 
is preferable that the electrode finger width of the solid electrode 
portion is substantially equal to about 3.lambda./8. 
Other features and advantages of the present invention will become apparent 
from the following description of the preferred embodiments of the 
invention which refers to the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will be described in detail 
below with reference to the attached drawings. 
FIG. 1 is a plan view of a surface acoustic wave device A according to a 
first preferred embodiment of the present invention. In this surface 
acoustic wave device A, a pair of surface acoustic wave transducers 2 and 
3 are provided on the surface of a piezoelectric substrate 1. The 
piezoelectric substrate 1 preferably comprises, for example, a 
borosilicate glass substrate with a zinc oxide (ZnO) film of 25 .mu.m in 
thickness formed on the surface thereof. The surface acoustic wave 
transducers 2 and 3 are respectively used for excitation and receiving of 
a surface acoustic wave, and arranged in parallel on the piezoelectric 
substrate 1 along the propagating direction of the surface acoustic wave. 
An IDT electrode 4, forming the surface acoustic wave transducer 2, is in 
the form of a normal-type solid electrode. The IDT electrode 4 includes 
comb-shaped electrodes 5A and 5B disposed opposite to each other, and 
electrode fingers 6 of the comb-shaped electrodes 5A and 5B are meshed one 
by one. Furthermore, a mesh depth h1 of the electrode fingers 6 is 
preferably fixed (for example, 1000 .mu.m). A width j1 and an interval i1 
of the electrode fingers 6 are preferably set at .lambda./4 (.lambda. 
indicates the wavelength of the surface acoustic wave). 
An IDT electrode 7 forming the other surface acoustic wave transducer 3 is 
preferably formed by connecting a normal-type solid electrode portion 7a 
and a normal-type split electrode portion 7b. The IDT electrode 7 includes 
comb-shaped electrodes 8A and 8B disposed opposite to each other, and 
electrode fingers thereof are meshed alternately. Electrode fingers 9a, 
which are positioned on the end portion of the IDT electrode 7 located 
farthest away from the above-mentioned surface acoustic wave transducer 2, 
are meshed one by one to form the solid electrode portion 7a. A mesh depth 
h2 of the electrode fingers 9a in the solid electrode portion 7a is 
preferably fixed at the same value as the above mesh depth h1 (for 
example, 1000 .mu.m). Furthermore, a width j2 and an interval i2 of the 
electrode fingers 9a are preferably set at .lambda./4 (.lambda.=the 
wavelength of the surface acoustic wave). 
The other electrode fingers, that is, electrode fingers 9b positioned 
adjacent to the above-mentioned surface acoustic wave transducer 2 are 
meshed alternately two by two to form the split electrode portion 7b. A 
mesh depth h3 of the electrode fingers 9b in the split electrode portion 
7b is preferably fixed at the same value as the above mesh depth h1 (for 
example, 1000 .mu.m). Furthermore, a width j3 and an interval i3 of the 
electrode fingers 9b are preferably set at .lambda./8 (.lambda.=the 
wavelength of the surface acoustic wave). 
The solid electrode portion 7a and the split electrode portion 7b are 
formed on the piezoelectric substrate 1 at an area ratio of about 1 to 1, 
and an electrode finger interval i4 on the border therebetween is 
preferably set at 3.lambda./16. This setting is made to maintain the 
continuity of the surface acoustic wave between the solid electrode 
portion 7a and the split electrode portion 7b. 
FIG. 2 is a plan view of a surface acoustic wave device B according to a 
second preferred embodiment of the present invention. This surface 
acoustic wave device B basically has a similar structure to the first 
preferred embodiment. The components which are the same as and similar to 
those in the first preferred embodiment are denoted by the same numerals, 
and a detailed description thereof is omitted. The surface acoustic wave 
device B differs from the first preferred embodiment in the following 
structure. Although an electrode finger width j2 of a solid electrode 
portion 7a in one surface acoustic wave transducer 3 is preferably set to 
be .lambda./4, only an electrode finger 9a' at the end of the IDT 
electrode 7 closest to a split electrode portion 7b has an electrode 
finger width j5 of 5.lambda./16. Furthermore, an electrode finger interval 
i5 on the border between the solid electrode portion 7a and the split 
electrode portion 7b is preferably set at .lambda./8 in the same manner as 
an electrode finger interval i3 of the split electrode portion 7b. 
Only the electrode finger width j5 of the electrode finger 9a' is set at 
5.lambda./16 for the following reason. If it is assumed that the electrode 
finger width j5 of the electrode finger 9a' is taken as .lambda./4, the 
midpoint of the electrode finger interval i5 between the split electrode 
portion 7b and the solid electrode portion 7a deviates by .lambda./32 from 
the position where it should be, that is, the position best-suited for 
excitation and receiving of surface acoustic waves, toward the solid 
electrode portion 7a, which lowers excitation and receiving efficiencies 
and increases the insertion loss. In order to compensate for the 
deviation, the electrode finger width j5 of the electrode finger 9a' is 
widened to 5.lambda./16. 
FIG. 3 is a plan view of a surface acoustic wave device C according to a 
third preferred embodiment of the present invention. This surface acoustic 
wave device C basically has a similar structure to the first preferred 
embodiment. The components that are the same as and similar to those in 
the first preferred embodiment are denoted by the same numerals, and a 
detailed description thereof is omitted. The surface acoustic wave device 
C differs from the first preferred embodiment in the following structure. 
Each of the electrode fingers 9a" in a solid electrode portion 7a 
preferably has an electrode finger width j6 of 3.lambda./8. An electrode 
finger interval i6 between the electrode fingers 9a" and an electrode 
finger interval i7 on the border between the solid electrode portion 7a 
and a split electrode portion 7b are preferably .lambda./8 which is the 
same as an electrode finger interval i3 of the split electrode portion 7b. 
The electrode finger width j6 is set at 3.lambda./8 for the following 
reason. It is well known that a surface acoustic wave produced between 
electrode fingers at different potentials is made proportional to the 
strength of an electric field added between the electrodes by the 
piezoelectric action and reaction. The same is true for a received surface 
acoustic wave. Furthermore, it is also well known that the strength of an 
electric field added between electrode fingers at different potentials is 
proportional to the potential difference and inversely proportional to the 
clearance between the electrode fingers. 
Since the solid electrode portion 7a and the split electrode portion 7b are 
both components of comb-shaped electrodes 8A and 8B and connected to each 
other, the potential difference between the comb-shaped electrodes 8A and 
8B is constant wherever the solid electrode portion 7a and the split 
electrode portion 7b are formed. However, since the solid electrode 
portion 7a and the split electrode portion 7b have different electrode 
finger intervals (i6.noteq.i3), the strength of the electric field added 
between the comb-shaped electrodes 8A and 8B differs between the forming 
position of the solid electrode portion 7a and the forming position of the 
split electrode portion 7b. Such nonuniformity of the electric field 
strength causes distortion of the waveform and increase of the insertion 
loss. 
Accordingly, this preferred embodiment compensates for the above 
nonuniformity of the electric field strength by setting the electrode 
finger width j6 of the electrode fingers 9a" in the solid electrode 
portion 7a at 3.lambda./8 which is wider than other electrode fingers. 
Table 1 gives a comparison of examples made in accordance with the surface 
acoustic wave devices A, B and C described above and a conventional 
surface acoustic wave device F on the basis of the TTE level, the 
insertion loss and the fraction defective (the ratio of defectives, which 
result from short circuits, cutoffs and the like in the photolithographic 
process and electrostatic breakdown before and after production, to all 
pieces). The examples of the surface acoustic wave devices A, B, C and F 
were each comprised of a piezoelectric substrate 1 made of a borosilicate 
glass substrate with a zinc oxide (ZnO) film of 25 .mu.m in thickness 
formed thereon, and IDT electrodes 4 and 7 formed on the piezoelectric 
substrate 1 which have a center frequency of 41 MHz and IDT electrode mesh 
depths h1, h2 and h3 (1000 .mu.m) and include 37 pairs of electrode 
fingers. These surface acoustic wave devices A, B, C and F were tested 
with a circuit having a resistance of 330 .OMEGA. and an inductance of 1.2 
.mu.H on the input side and a resistance of 330 .OMEGA. and a capacitance 
of 5.1 pF on the output side. 
TABLE 1 
______________________________________ 
Defective Insertion Loss 
TTE Level Fraction 
Prior Art F 
13.2 dB 41 dB Datum 
______________________________________ 
Embodiment A 
13.0 dB 41 dB 27% Decreased 
Embodiment B 
12.8 dB 41 dB 27% Decreased 
Embodiment C 
12.4 dB 41 dB 10% Decreased 
Embodiment D 
13.0 dB 41 dB 21% Decreased 
Embodiment E 
13.0 dB 39 dB 27% Decreased 
______________________________________ 
This table reveals that the surface acoustic wave devices A, B and C 
improved the characteristics without making the TTE level lower than the 
prior art F. Specifically, the surface acoustic wave device A decreased 
the insertion loss by 0.2 dB and the fraction defective by 27%. This 
result is achieved because the area ratio of the split electrode portion 
to the entire IDT electrode is half of that of the prior art F. 
The surface acoustic wave device B decreased the insertion loss by 0.4 dB 
and the fraction defective by 27%. This also is because the area ratio of 
the split electrode portion to the entire IDT electrode is half of that of 
the prior art F. The insertion loss is 0.2 dB lower than that of the 
surface acoustic wave device A because the midpoint of the electrode 
finger interval i5 between the split electrode portion 7b and the solid 
electrode portion 7a is set in a desired location by widening the 
electrode finger width j5 to 5.lambda./16. 
The surface acoustic wave device C decreased the insertion loss by 0.8 dB 
and the fraction defective by 10%. Such a substantial decrease of the 
insertion loss by 0.8 dB results for the following reason. Since all the 
electrode finger intervals are the same, the strength of the electric 
field added between the comb-shaped electrodes 8A and 8B does not differ 
between the forming position of the solid electrode portion 7a and the 
forming position of the split electrode portion 7b, and the entire surface 
acoustic wave device C is in balance. 
Although the electrode finger interval i6 of the solid electrode portion 7a 
in the surface acoustic wave device C is the same narrow value of 
.lambda./8 as the electrode finger interval i3 of the split electrode 
portion 7b, the decrease rate of the fraction defective is 10% which is 
relatively high. This result is achieved because of the wide electrode 
finger width j6 of the solid electrode portion 7a and the high resistance 
to discharge breakdown. 
Furthermore, although not shown in Table 1, the above surface acoustic wave 
device A was constructed to have a center frequency of 36 MHz and include 
8 pairs of electrode fingers, and tested with a circuit having a 
resistance of 330 .OMEGA. and an inductance of 1.2 .mu.H on the input side 
and a resistance of 330 .OMEGA. and a capacitance of 5.1 pF on the output 
side. The test confirmed that the insertion loss can be decreased by 0.9 
dB without lowering the TTE level. 
Although the area ratio of the solid electrode portion 7a to the split 
electrode portion 7b is about 1 to 1 in the above description of the 
preferred embodiments, the preferred embodiments of the present invention 
are not limited to such a structure. In short, it is only necessary that 
one of the surface acoustic wave transducers partially includes the split 
electrode portion 7b. Accordingly, a surface acoustic wave device D, whose 
area ratio of the solid electrode portion 7a to the split electrode 
portion 7b is set at about 2 to 1, is also given as an example, and the 
measurement result of characteristics thereof is shown in Table 1. 
Although Table 1 shows only the measurement result of the surface acoustic 
wave device D which is different from the surface acoustic wave device A 
in the 2:1 area ratio of the solid electrode portion 7a to the split 
electrode portion 7b, it is needless to say that similar results can be 
obtained even when the area ratio of the solid electrode portion 7a to the 
split electrode portion 7b in the other surface acoustic wave devices B 
and C is changed to about 2 to 1. 
As Table 1 reveals, the surface acoustic wave device D, whose area ratio of 
the solid electrode portion 7a to the split electrode portion 7b is 2 to 
1, decreased the insertion loss by 0.2 dB and the fraction defective by 
21% as compared with the prior art F. 
Furthermore, in the above preferred embodiments, the split electrode 
portion 7b is located on the end portion of the IDT electrode 7 that is 
closer to the surface acoustic wave transducer 4, and the solid electrode 
portion 7a is located on the opposite end portion. However, to the 
contrary, the solid electrode portion 7a may be located on the end portion 
of the IDT electrode 7 that is closer to the surface acoustic wave 
transducer 4 and the split electrode portion 7b may be located on the 
opposite end portion. The measurement result of the characteristics of a 
surface acoustic wave device E having such a structure is also shown in 
Table 1. Although the solid electrode portion 7a and the split electrode 
portion 7b in the surface acoustic wave device E are placed in the 
opposite order to the surface acoustic wave device A, it is needless to 
say that similar measurement results can be obtained by structuring the 
other surface acoustic wave devices B and C in the same manner as above. 
As Table 1 reveals, although the TTE level of the surface acoustic wave 
device E is a little lower than that of the prior art F, the insertion 
loss and the fraction defective thereof were improved by 0.2 dB and 27%, 
respectively. 
Although both the IDT electrodes 4 and 7 are of the normal type in the 
above-mentioned preferred embodiments, it is needless to say that the 
preferred embodiments of the present invention are also applicable to a 
surface acoustic wave device equipped with an IDT electrode whose mesh 
depth is weighted, and a surface acoustic wave device in which the 
electrode period is changed and the characteristics are weighted. 
As described above, according to the preferred embodiments of the present 
invention, it is possible to decrease the insertion loss and the fraction 
defective without lowering the TTE level. 
While preferred embodiments of the invention have been disclosed, various 
modes of carrying out the principles disclosed herein are contemplated as 
being within the scope of the following claims. Therefore, it is 
understood that the scope of the invention is not to be limited except as 
otherwise set forth in the claims.