Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2001-07-03
2003-10-14
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514160, C073S504120
Reexamination Certificate
active
06631642
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an external force detecting sensor formed by using a semiconductor micro-processing technique or the like.
2. Description of the Related Art
Generally, acceleration sensors and angular velocity sensors are known as external force detecting sensors. Each of these external force detecting sensors is provided with a movable portion which is displaced in accordance with an external force, such as acceleration, angular velocity, or the like applied to the external force detecting sensor. The displacement is electrically detected to obtain an acceleration signal or angular velocity signal. For example, as shown in
FIG. 5
, an acceleration sensor using a piezoelectric element described in Japanese Unexamined Patent Application Publication No. 10-104263 has a movable portion
1
, which includes a weight portion
4
supported on a supporter
2
by beams
3
in the central portion thereof. A supporting substrate
5
and a cap substrate
6
having recesses
5
a
and
6
a
, respectively, are mounted to the supporter
2
so as to sandwich the supporter
2
from the top and bottom. In addition, a cavity is formed at the central portion thereof using the recesses
5
a
and
6
a
of the supporting substrate
5
and the cap substrate
6
, respectively, such that the movable portion
1
can be displaced. In addition, piezoelectric elements
7
are provided on the beams
3
, and when acceleration is applied to the weight portion
4
to cause a stress on the beams
3
, the piezoelectric elements
7
generate acceleration signals.
However, when the recesses
5
a
and
6
a
respectively provided on the supporting substrate
5
and cap substrate
6
are shallow, gaps between the weight portion
4
and a top surface
6
b
and bottom surface
5
b
become narrower, and when the weight portion
4
is rapidly displaced, a large phase lag or an output signal occurs. This is due to air damping due to the viscosity of the air sealed in the cavity. As a result, responsiveness of the acceleration sensor deteriorates. Therefore, in the above-described acceleration sensor, in order to eliminate the influence of the air damping, the recesses
5
a
and
6
a
of the supporting substrate
5
and cap substrate
6
, respectively, are made higher (deeper), and thereby the vertical space in the cavity for the weight portion
4
is increased, thus improving the responsiveness of the acceleration sensor.
The influence of the air damping is the same in an external force detecting sensor which electrostatically detects an external force. Such an external force detecting sensor described in Japanese Unexamined Patent Application Publication No. 2000-22170 is described with reference to
FIGS. 6 and 7
. By processing a silicon substrate, two weight portions
8
and
9
are combined with supporters
11
and
12
via beams
11
a
and
12
a
, respectively, to compose a movable portion
10
. The two weight portions
8
and
9
respectively have a plurality of plate-shaped movable interdigitated electrodes
8
a
and
9
a
outwardly provided thereon. Fixed portions
13
and
14
are provided at positions respectively opposing the weight portions
8
and
9
. The fixed portions
13
and
14
have a plurality of plate-shaped fixed interdigitated electrodes
13
a
and
14
a
provided thereon which protruded toward the weight portions
8
and
9
, respectively, and are interdigitated with the movable electrodes
8
a
and
9
a
, respectively. A frame
15
is provided so as to surround the movable portion
10
and the fixed portions
13
and
14
. A functional element composed as described above is supported by a supporting substrate
18
and cap substrate
19
made of Pyrex glass so as to sandwich it from the top and the bottom. In addition, inside the functional element, a cavity is formed by recesses
18
a
and
19
a
respectively provided on the supporting substrate
18
and the cap substrate
19
, so as to enable displacement of the movable portion
10
. On the bottom surface of the recess
18
a
of the supporting substrate
18
, detecting electrodes
16
and
17
are provided beneath the weight portions
8
and
9
, respectively, via gaps.
Now, an operation of the external force detecting sensor of the configuration is described when it is used as an angular velocity sensor. When a voltage is applied across the supporters
11
and
12
and the fixed portions
13
and
14
, the two weight portions
8
and
9
vibrate in mutually opposing directions due to electrostatic forces exerted between the movable interdigitated electrodes
8
a
and
9
a
and the fixed interdigitated electrodes
13
a
and
14
a
. In such a vibrating state, when a rotational force is applied to the external force detecting sensor about an axis in a direction connecting the supporters
11
and
12
, the two weight portions
8
and
9
experience inverse Coriolis forces in the perpendicular direction. For example, when the weight portion
8
of one side receives a downward Coriolis force, the weight portion
9
of the other side receives an upward Coriolis force, and the two weight portions
8
and
9
vibrate in vector directions respectively determined by the electrostatic force and the Coriolis forces. Due to the vibrations, electrostatic capacitances between the two weight portions
8
and
9
and the detecting electrodes
16
and
17
are differentially altered, and outputs of the two detecting electrodes
16
and
17
are converted into voltages, which are differentially amplified by a differential amplifier to obtain an angular velocity signal.
Now, an operation is described of the external force detecting sensor of the above configuration when it is used as an acceleration sensor. In a state where a D.C. voltage is applied across the supporters
11
and
12
, the fixed portions
13
and
14
, and the detecting electrodes
16
and
17
, when an acceleration is applied to the weight portions
8
and
9
, namely from a vector component in a direction connecting the two fixed electrodes, directly opposite acceleration signals are obtained from the two fixed portions
13
and
14
. In other words, one of the acceleration signals increases the electrostatic capacitance and the other decreases the electrostatic capacitance. From a vector component in the vertical direction, acceleration signals are obtained from the detecting electrodes
16
and
17
. Accordingly, accelerations in two directions can be detected.
In the above-described external force detecting sensor, since the movable portion
10
is displaced in a sealed cavity, the acceleration sensor is strongly influenced by air damping when the movable portion
10
is vertically displaced. In addition, in such a case, when the movable portion
10
is driven to continuously vibrate at a fixed vibration frequency, such as in the angular velocity sensor, air damping exerts an undesirable influence on the operation of the movable portion
10
, such as deterioration of the mechanical quality factor of the driving vibration of the movable portion
10
, or the like.
Furthermore, when the cap substrate
19
having the recess
19
a
formed thereon is mounted on the movable portion
10
in a manufacturing process of the external force detecting sensor, a frame
15
, the supporters
11
and
12
, the fixed portions
13
and
14
, and the supporting substrate
18
and the cap substrate
19
are bonded together by an anodic bonding method using a high voltage; this, however, can cause the movable portion
10
to be drawn by a strong electrostatic attraction to the bottom surface of the supporting substrate
18
or the top surface of the cap substrate
19
, thus rendering the movable portion
10
inoperable. Accordingly, to avoid this problem, the recesses
18
a
and
19
a
of the supporting substrate
18
and the cap substrate
19
, respectively, comprising the cavity accommodating the movable portion
10
are preferably formed deep.
However, if the recesses
18
a
and
19
a
of the supporting substrate
18
and the cap substrate
Konaka Yoshihiro
Oguchi Takahiro
Shibahara Teruhisa
Keating & Bennett LLP
Kwok Helen
Murata Manufacturing Co. Ltd.
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