Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-03-31
2002-11-26
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R
Reexamination Certificate
active
06486752
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electrode pattern structure in a surface acoustic wave (hereafter referred to SAW) filter which is utilized in the wireless portion of a mobile communication device such as a cellular phone and a SAW filter manufacturing method.
FIG.
3
(
a
) is a circuit diagram of a ladder-type SAW filter in the prior art. Between input terminals
33
and
34
and output terminals
35
and
36
, SAW resonators
31
are provided in a series arm and SAW resonators
32
are provided in respective parallel arms. This is a ladder-type resonator structure achieving so-called duplex mode resonance characteristics. FIG.
3
(
b
) shows the electrode pattern in the SAW resonators
31
and
32
. The SAW resonators
31
and
32
both adopt a resonator structure achieved by arraying reflection elements (electrode fingers) constituting two grating reflectors
37
and
38
and an IDT (inter-digital transducer)
39
provided between them over a half wavelength cycle. The IDT
39
is constituted of crossed or interleaved electrodes achieved by crossing or interleaving two tooth comb-type electrodes as shown in the figure.
A method for manufacturing this ladder-type SAW filter may be summarized as below. Using a substrate constituted of a piezoelectric material, a wafer processing step is performed several times through photolithography. The electrode pattern that includes the reflectors, the IDTs and the like mentioned above is formed with a metal film. A number of ladder-type SAW filter chips are manufactured on the wafer. Then, the wafer is cut into individual chips.
During the photolithography step, which is performed several times as described above, hard contact exposure and soft contact exposure are implemented. In addition, each SAW resonator has several hundred comb shape pairs of mutually opposed primary electrode fingers and secondary electrode fingers. Thus, the SAW resonators become electrically charged readily during the manufacturing process and a great deal of static electricity is generated between the wafer and the manufacturing equipment. Furthermore, changes in temperature tend to be caused by the thermal load imparted during the assembly step, which often results in occurrence of an electrical discharge. A discharge occurring between crossed electrodes destroys them. In order to prevent this, a discharge prevention pattern that equalizes the potentials at the areas of the metal film corresponding to one of the plurality of crossed electrodes formed thereupon is provided as disclosed in Japanese Publication No. 58-43607. By providing the discharge prevention pattern, it is possible to prevent electrostatic breakdown from occurring during the wafer process and the SAW filter assembly step. The discharge prevention pattern includes a connection pattern extending from the electrodes to a dicing line (the line used as a guide when cutting the wafer into individual chips). This connection pattern is formed concurrently with the formation of the crossed electrodes implemented in the wafer process. Thus, the potentials at the plurality of crossed electrodes are equalized in the first place to ensure that no discharge occurs due to a potential difference. This discharge prevention pattern is required in all the electrode patterns formed through the photolithography step implemented during a first stage, to ensure that the potentials of the electrode patterns are equalized.
A discharge is prevented by providing the discharge prevention pattern described above in a SAW filter pattern with standard ladder-type resonators. However, it is not always possible to provide a discharge prevention pattern in a pattern adopting a power withstanding structure, which may be employed on the transmission side of a SAW wave-splitter, for the reasons given below.
In a pattern adopting a power withstanding structure, the crossing length at the crossed electrodes in the individual SAW resonators may be set to a large value to increase the resistance component. This makes it possible to reduce the electrical load on each unit so that the SAW resonator is prevented from being destroyed by intense power such as that imparted by a power amplifier or the like. However, if the crossed electrodes are simply lengthened to achieve a greater crossing length, impedance matching for the filter is not achieved, and thus, this method is not desirable. Accordingly, the original length of crossed electrodes is doubled and two identical SAW resonators are connected in series. In other words, a resonator having crossed electrodes over two levels is prepared. The original SAW resonator is replaced by the unit constituted by connecting two identical SAW resonators in series.
FIG. 4
shows the structure of the SAW filter electrode pattern formed on a chip in this manner. Reflectors
43
,
44
,
45
,
46
,
47
,
48
,
49
and
50
are provided on both sides of IDTs
41
and
42
, each having tooth comb-type crossed electrodes provided over two levels. In addition, an output pad
52
and wire bonding pads
53
and
54
are connected with the individual electrodes via a wiring pattern
51
. At the periphery, a dicing line
55
which is used as a guide when cutting the wafer into individual chips is provided.
FIG. 5
is a circuit diagram of a SAW filter adopting the electrode pattern shown in FIG.
4
. SAW resonators
31
′ provided in the series arm in
FIG. 5
are each constituted of the IDT
41
and the reflectors
43
,
44
,
45
and
46
in FIG.
4
. Likewise, SAW resonators
32
′ provided in the parallel arms in
FIG. 5
are each constituted of the IDT
42
and the reflectors
47
,
48
,
49
and
50
shown in FIG.
4
.
In
FIG. 6
, connection patterns
61
,
62
,
63
,
64
,
65
and
66
for discharge prevention are provided in the pattern shown in FIG.
4
. As illustrated in the figure, it is not possible to provide a discharge prevention pattern extending from the common center electrodes of the IDTs
41
and
42
to the dicing line
55
due to the blocking presence of the reflectors. If a discharge prevention pattern is formed extending from the common center electrode of IDT
42
along the direction in which no reflector is present, i.e., over the area indicated by the dotted lines in the figure, the common electrode of IDT
42
can be connected to the dicing line and thus becomes grounded. However, this formation method is not desirable since some of the electrodes of the IDT
42
may become longer than the rest or changes in the characteristics may occur when the wafer is divided into individual chips along the dicing line. Ultimately, a discharge prevention pattern that connects the common electrode of IDT
42
with the dicing line cannot be provided, which makes the common electrode of IDT
42
an ungrounded floating electrode posing a risk of electrical discharge occurring during the process. Likewise, the IDT
41
becomes a floating electrode, posing a risk of discharge. It is necessary to ground all the electrodes to prevent discharge. However, in the pattern having electrodes with tooth comb-type crossed electrodes provided over two levels as described above, there is a problem in that the entire electrode pattern cannot always be grounded.
SUMMARY OF THE INVENTION
An object of the present invention, which has been completed by addressing the problem of the prior art discussed above, is to provide a surface acoustic wave filter pattern that makes it possible to prevent an electrostatic breakdown from occurring due to a discharge by grounding all the electrode patterns even in a SAW filter having electrodes achieved by providing tooth comb-type crossed electrodes over a plurality of levels and a method for manufacturing a surface acoustic wave filter.
In order to, achieve the object described above, in a first aspect of the present invention, a surface acoustic wave filter pattern achieved by connecting in a ladder pattern surface acoustic wave resonators each having tooth comb-type crossed electrodes provided over a plurality of levels and refle
Kunitz Norman N.
Oki Electric Industry Co, Ltd.
Pascal Robert
Summons Barbara
Venable
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