Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2003-03-07
2004-10-19
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S370000, C029S025350, C073S504160
Reexamination Certificate
active
06806797
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to piezoelectric devices, such as piezoelectric resonators and piezoelectric oscillators for use in various electronic devices, and piezoelectric vibrating gyroscopes for use as angular speed sensors; More particularly, the invention relates to a tuning-fork piezoelectric resonator element used in those devices, and a production method of the tuning-fork piezoelectric resonator element.
2. Description of Related Art
In related art consumer and industrial electronic devices such as timepieces, household electrical appliances, various information and communication devices, and office automation devices, piezoelectric devices, such as a piezoelectric resonator, an oscillator and a real time clock module in which a piezoelectric resonator and an IC chip are sealed in the same package, can be used as a clock source of an electronic circuit. Furthermore, piezoelectric vibrating gyroscopes can be used as rotation angular speed sensors to control the attitude and navigation of ships, aircrafts, automobiles, and the like, and to prevent and detect the camera shaking of video cameras and the like, and can also be applied to rotating direction sensors, such as three-dimensional mice. Such a piezoelectric vibrating gyroscope is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-55479 (JP 479), and Japanese Unexamined Patent Application Publication No. 10-170272 (JP 272).
In particular, with a reduction in the related art in size and thickness of electronic devices in which these piezoelectric devices are mounted, the piezoelectric devices are required to further reduce the size and thickness thereof. The piezoelectric devices are also required to ensure a low CI (crystal impedance) value and to achieve high quality and high stability. In order to keep the CI value low, for example, a tuning-fork piezoelectric resonator element provided with resonating arms each having a groove has been developed, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 56-65517 (JP 517), and the specification of Japanese Patent Application No. 2000-595424 (JP 424).
In this tuning-fork piezoelectric resonator element provided with resonating arms each having a groove, upper and lower principal surfaces
3
a
and
3
b
of a pair of resonating arms
2
a
and
2
b
extending in parallel from a base portion
1
are provided with linear grooves
4
a
and
4
b
extending along the lengthwise direction thereof, as illustrated in FIG.
8
. As shown in FIGS.
9
(A) and
9
(B), first electrodes
5
a
and
5
b
are formed on side faces and bottom faces of the grooves
4
a
and
4
b
, and second electrodes
6
a
and
6
b
are formed on side faces of the resonating arms
2
a
and
2
b
. The first electrodes
5
a
(
5
b
) of one of the resonating arms are electrically connected to the second electrodes
6
b
(
6
a
) of the other resonating arm, thereby constituting driving electrodes to vibrate the tuning-fork quartz resonator element. When an alternating voltage is applied from connecting terminals
7
to the driving electrodes, electric fields E
1
and E
2
parallel to the principal surfaces are produced between the first electrodes
5
a
and
5
b
and the second electrodes
6
a
and
6
b
adjoining each other, and as a result, field efficiency is substantially enhanced, and the CI value can be reduced.
Usually, by processing a wafer made of a piezoelectric single-crystal material, such as quartz crystal, by wet etching using photolithography to form a desired outline of the resonator element and the grooves
4
a
and
4
b
, electrode films are formed on the surfaces of the resonator element and the grooves
4
a
and
4
b
by patterning. More specifically, corrosion-resistant films are formed on both surfaces of a quartz wafer, and photoresists are applied thereon and are dried to form resist films. In a state in which a pair of upper and lower first photomasks having the same etching pattern corresponding to the desired outline of the resonator element are placed thereon, the surfaces of the corrosion-resistant films are exposed by exposure and development, and are removed with etchant to expose the surfaces of the quartz wafer. After the remaining resist films are stripped off, photoresists are applied again on the remaining corrosion-resistant films, and are dried to form new resist films. Then, a pair of upper and lower second photomasks having an etching pattern corresponding to the shape of the grooves of the resonating arms are placed thereon, and the surfaces of the corrosion-resistant films are exposed by exposure and development.
Subsequently, the exposed surfaces of the quartz wafer are etched with a quartz etchant, thereby forming the outline of the resonator element including the resonating arms. Furthermore, the exposed surfaces of the corrosion-resistant films are removed with etchant to expose the surfaces of the quartz wafer. By half-etching the exposed surfaces of the quartz wafer to a predetermined depth with a quartz etchant, grooves are formed on the upper and lower principal surfaces of the resonating arms. An electrode material is deposited on all the surfaces of the quartz element thus formed, including the inner faces of the grooves of the resonator element, by evaporation, sputtering, and the like, and is polarized by photoetching, thereby forming desired driving electrodes, extraction electrodes, and lines.
For example, the used quartz wafer is formed by cutting out quartz crystal around the X-axis at a cutting angle &thgr; ranging, for example, from approximately 30 minutes to 2 degrees from the Z-axis. As shown in
FIG. 8
, the lengthwise, widthwise, thickness directions of the resonating arms
2
a
and
2
b
of the tuning-fork piezoelectric resonator element are oriented corresponding to the Y-axis called a mechanical axis of the quartz crystal structure, the X-axis called an electric axis, and the Z-axis called an optical axis, respectively. Therefore, the widthwise direction of the resonating arms
2
a
and
2
b
coincides with the X-axis direction, the lengthwise direction corresponds to the Y′-direction inclined at the angle &thgr; to the Y-axis direction, and the thickness direction corresponds to the Z′-direction inclined at the angle &thgr; to the Z-axis direction.
SUMMARY OF THE INVENTION
As shown in FIG.
9
(A), the grooves
4
a
(
4
b
) of the resonating arm
2
a
(
2
b
) are placed so that a center line C
1
thereof coincides with a center line C
2
of the resonating arm. However, since most piezoelectric resonator elements are made of a piezoelectric single-crystal material having etching anisotropy, such as quartz crystal, the cross-sections of the resonating arm and the grooves formed by wet etching are often asymmetrical with respect to the center lines C
1
and C
2
because of the crystal orientation. In particular, the etching rate of the quartz crystal has a crystal-axis dependence, and the quartz crystal is prone to be etched in widthwise direction of the resonating arm, that is, in the +X direction in the example shown in FIG.
8
. Therefore, the cross-sections of the grooves
4
a
(
4
b
) are not shaped like an ideal rectangle that is shown by imaginary lines
8
in FIG.
9
(B), but are asymmetrical, that is, the left side faces thereof are inclined rightward in the figure, and a projection
9
is formed on the right side face of the resonating arm
2
a
(
2
b
).
For this reason, a non-negligible difference in stiffness is formed between the right and left sides of the center line C
2
of the resonating arm, and bending of the resonating arm is unbalanced between the right and left sides, that is, inside and outside of the tuning fork during excitation. Consequently, vibrations are not confined in the resonating arm, but leak from a mount portion of the piezoelectric resonator element toward the package, which may cause a loss of strain energy. When the amount of unbalance in bending between the right and left si
Oliff & Berridg,e PLC
Seiko Epson Corporation
Summons Barbara
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