Metal working – Piezoelectric device making
Reexamination Certificate
2001-04-12
2003-05-06
Arbes, Carl J. (Department: 3729)
Metal working
Piezoelectric device making
C029S025350, C029S846000, C029S848000, C216S049000, C216S013000, C216S055000, C310S31300R, C310S365000
Reexamination Certificate
active
06557225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing a surface acoustic wave device such as a surface acoustic wave filter or other device, and more particularly, to a method of producing a surface acoustic wave device having bumps formed by a flip chip process.
2. Description of the Related Art
In recent years, mobile communications equipment has been greatly reduced in size and weight. Also, it has been required to reduce the sizes of electronic parts for use in the mobile communications equipment. Thus, for surface acoustic wave devices used as band filters in mobile communication devices, miniaturization has been urgently required.
In conventional surface acoustic wave devices, surface acoustic wave elements are contained in a package. Electrode pads of the surface acoustic wave elements are electrically connected to electrode patterns provided for the package via wires. Accordingly, miniaturization of the surface acoustic wave devices has had a limit.
In recent years, the flip chip process has been utilized more frequently in which bumps are formed on electrode pads on a surface acoustic wave element, and the bumps are arranged to contact an electrode pattern on a package and are bonded to the pattern, e.g., by application of supersonic waves.
Japanese Unexamined Patent Application Publication No. 9-172341 discloses an example of a method of producing a surface acoustic wave device, using the above-mentioned flip chip process. In the surface acoustic wave device described as the conventional technique, a surface acoustic wave element is covered with a dielectric layer in order to enhance the anti-dust property and the corrosion resistance.
In particular, first, interdigital electrodes and planar electrodes defining electrode pads are formed on a piezoelectric substrate. Then, a dielectric layer defining a protection layer is formed on the piezoelectric substrate. Moreover, for electrical connection to the external upper electrodes each having the same areas as those of the planar electrodes when seen in plan view, are arranged such that the upper electrodes overlap the planar electrodes via the dielectric layer. Moreover, by bonding metal bumps to the upper electrodes, the dielectric layer is broken by the upper electrodes, so that the planar electrodes and the upper electrodes are electrically connected directly to each other. The metal bumps, after they are connected to the planer electrodes as described above, are connected to a package substrate in the above-mentioned flip chip process.
On the other hand, Japanese Unexamined Patent Application Publication No. 10-163789 discloses the structure in which surface acoustic wave filter elements can be mounted onto a circuit substrate without the element being contained in a package. In particular, as shown in
FIG. 8
, a surface acoustic wave element
52
is mounted directly to a circuit substrate
51
. For the purpose of realizing the above-mentioned mounting, the surface acoustic wave element
52
is formed as follows.
That is, first, interdigital electrodes, a reflector, an input electrode pad, and an output electrode are formed on a piezoelectric substrate. Then, a protection film
53
made of SiO
2
and having a film thickness of 200 Å to 15 &mgr;m is formed. Furthermore, the central portions of input-output electrode pads
52
a
are exposed from the protection film
53
. An electro-conductive bonding agent
54
is made to adhere thereto. As the conductive bonding agent
54
, a solder ball, a gold bump, or an electro-conductive resin is used. The conductive bonding agent
54
is arranged to contact a corresponding electrode pattern
51
a
of the circuit substrate
51
. In the state in which the surface acoustic wave element
52
is placed onto a circuit substrate, the conductive bonding agent such as the solder ball is heated and melted, whereby the surface acoustic wave filter element is mounted directly to the circuit substrate
51
.
Japanese Unexamined Patent Publication No. 4-371009 discloses a problem that a surface acoustic wave device likely experiences a pyroelectric breakdown due to a potential difference between electrodes which is caused by deposition of a passivation film on a piezoelectric substrate and heat applied during the formation of a window in the passivation film. To this end, the Publication discloses a method to connect between an input terminal and an output terminal via a connection wire before formation of a passivation film and disconnect the connection wire after formation of the passivation film, whereby the short-circuit between the input and output terminal prevents the pyroelectric breakdown.
In the case in which a surface acoustic wave element is bonded to a package substrate or a circuit substrate by use of a bump, it is necessary to heat a piezoelectric substrate during the process of forming the bump in order to promote the interdiffusion of metal, so that the bump shear strength can be increased.
However, in the case of a piezoelectric substrate having pyroelectric properties used, a potential difference is generated between the electrodes of the interdigital transducers as the surface acoustic wave element, due to changes in the temperature caused by the above heating is carried out. Thus, discharge occurs. Problematically, this discharge causes the interdigital transducer to be pyroelectrically broken, so that the acceptance ratio is reduced.
In addition, it is impossible to measure frequency characteristics of a device before formation of a passivation film if the input and output terminals are short-circuited before formation of the passivation film to prevent a pyroelectric breakdown. As a result, it is impossible to adjust an operation frequency of a device by changing a thickness of a passivation film. Even if it is possible to adjust an operation frequency of a device by disconnection a connection wire after formation of a passivation film, measuring an operation frequency of a device and changing a thickness of a passivation film, thereafter, there sill remains a problem of a pyroelectric breakdown as the connection wire has been cut during a formation of bumps.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of producing a surface acoustic wave device which can be mounted onto a package substrate or a circuit substrate, using a flip chip process, in which pyroelectric breaking of the interdigital transducer, caused by heating during formation of a bump, is prevented, and variations in characteristics are prevented, and the acceptance ratio is greatly improved.
According to a preferred embodiment of the present invention, a method of producing a surface acoustic wave device by use of a flip chip process, includes the steps of forming on a piezoelectric substrate, at least one interdigital transducer and a plurality of electrode pads electrically connected to the interdigital transducer, forming bumps on the respective electrode pads and providing an insulating film at a region other than a region where the bumps are formed.
In the present preferred embodiment, the insulating film greatly improves the dust resistance and erosion resistance in a surface acoustic wave device. Further, it is possible to miniaturize an electronic apparatus such as a communication apparatus in which a surface acoustic wave device is used as a surface acoustic wave device can be mounted on a circuit board by a flip-chip bonding method using the bumps.
The method may also include the step of forming an insulating film on an entire surface of the piezoelectric substrate, and removing a portion of the insulating film at least on the bumps.
In the case, since the portions of the insulating film on the bumps are removed, it is possible to easily mount the surface acoustic wave device on a circuit board by a flip-chip bonding method.
The method may further includes the step of etching a surface of the electrodes after the step of forming an insulatin
Takada Norihiko
Takata Toshiaki
Yamato Shuji
Arbes Carl J.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Nguyen Tai
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