Capacitance type sensor

Details Capacitance type sensor Capacitance type sensor

2000-02-09

2002-06-25

Le, N. (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Lumped type parameters

C324S660000, C073S862626

Reexamination Certificate

active

06411107

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an electrostatic capacitance type sensor for measuring physical and chemical amounts on the basis of an electrostatic capacitance between a plurality of electrodes arranged to face each other and, more particularly, to an electrostatic capacitance type sensor having a means for compensating for an error in electrostatic capacitance based on positioning of the plurality of electrodes.
BACKGROUND ART
An electrostatic capacitance type sensor is known as a device for measuring various types of physical and chemical amounts such as a pressure, temperature, humidity, displacement, variate, and acceleration. The arrangement of a conventional electrostatic capacitance type sensor will be described by way of a pressure sensor for measuring a pressure.
FIG. 13
is a perspective view showing the arrangement of a conventional electrostatic capacitance type sensor.
A recess is formed in one surface of a base substrate
101
. The base substrate
101
is bonded to a diaphragm substrate
102
with its rim
101
a
around the recess. The recess is accordingly closed by the diaphragm substrate
102
to form a capacitance chamber
103
.
In the capacitance chamber
103
, a stationary electrode
110
is arranged on the base substrate
101
side, and a movable electrode
120
is arranged on the diaphragm substrate
102
side, i.e., on a movable portion
102
a
of the diaphragm substrate. The electrodes
110
and
120
are connected to the input of a signal processor
104
arranged on the lower surface of the base substrate
101
.
When a pressure P is applied to the diaphragm substrate
102
, the movable portion
102
a
displaces in accordance with the pressure P. Since the movable electrode
120
displaces in an interlocked manner to the movable portion
102
a,
a gap between the stationary electrode
110
and movable electrode
120
changes accordingly, so that an electrostatic capacitance between the two electrodes
110
and
120
changes. On the basis of the capacitance obtained at this time, the signal processor
104
calculates the pressure P.
FIG. 14
is a sectional view showing a section taken along the line XIV—XIV′ in the conventional electrostatic capacitance type sensor shown in FIG.
13
.
The stationary electrode
110
is made up of an electrode portion
111
and electrode extraction portion
112
. An electrode pad
113
is formed in the capacitance chamber
103
on the base substrate
101
side to extract the stationary electrode
110
and connect it to the signal processor
104
shown in FIG.
13
. The electrode extraction portion
112
of the stationary electrode
110
is connected to the electrode pad
113
.
Similarly, the movable electrode
120
is made up of an electrode portion
121
and electrode extraction portion
122
, and is connected to the signal processor
104
through an electrode pad
123
formed in the capacitance chamber
103
on the diaphragm substrate
102
side.
In a process of manufacturing the electrostatic capacitance type sensor shown in
FIG. 13
, first, the stationary electrode
110
is formed in the recess of the base substrate
101
in accordance with known film formation and photoetching. Similarly, the movable electrode
120
is formed on one surface of the diaphragm substrate
102
. The base substrate
101
to which the stationary electrode
110
is attached and the diaphragm substrate
102
to which the movable electrode
120
is attached are bonded to each other. Thus, a capacitor structure comprised of the stationary electrode
110
and movable electrode
120
is formed.
When bonding the substrates
101
and
102
to each other, if they are positioned precisely, the facing area of the two electrodes
110
and
120
becomes as designed, and a desired capacitance can be obtained.
In practice, however, it is difficult to assemble the base substrate
101
and diaphragm substrate
102
as designed, and a positioning error occurs between the electrodes
110
and
120
. If the electrodes
110
and
120
are made to have completely the same size, the facing area of the two electrodes
110
and
120
changes largely. Then, a desired capacitance cannot be obtained, and an offset occurs in the sensor.
For this reason, in the electrostatic capacitance type sensor shown in
FIG. 13
, the stationary electrode
110
is entirely formed smaller than the movable electrode
120
, as shown in FIG.
14
. Even if a positioning error occurs, the stationary electrode
110
does not move outside a region facing the movable electrode
120
, and an offset occurring in the sensor can be suppressed.
In the conventional electrostatic capacitance type sensor shown in
FIG. 13
, the electrode pad
113
is formed outside the region facing the movable electrode
120
, as shown in FIG.
14
. If the electrode pad
113
is formed within the region facing the movable electrode
120
, the detection precision of the pressure P degrades, and an inconvenience occurs in electrode extraction. For this reason, the electrode extraction portion
112
of the stationary electrode
110
is formed to extend from the region facing the movable electrode
120
.
FIGS.
15
(
a
) and
15
(
b
) are schematic diagrams showing a positional relationship between the stationary electrode
110
and movable electrode
120
when a positioning error occurs. Referring to FIGS.
15
(
a
) and
15
(
b
), a direction from the electrode pad
113
toward the electrode portion
111
is defined as an x direction, and a direction from the electrode portion
111
toward the electrode pad
113
is defined as a -x direction.
As shown in FIG.
15
(
a
), when the stationary electrode
110
(i.e., the electrode extraction portion
112
) displaces in the direction of arrow x, part
112
a
of the electrode extraction portion
112
enters the region facing the movable electrode
120
. Inversely, as shown in FIG.
15
(
b
), when the stationary electrode
110
displaces in the direction of arrow -x, part
112
b
of the electrode extraction portion
112
moves outside the region facing the movable electrode
120
. The electrode extraction portion
112
also forms an electrostatic capacitance with its portion facing the movable electrode
120
. Hence, the electrode extraction portion
112
can produce an offset when a positioning error occurs.
To downsize the electrostatic capacitance type sensor, the electrodes
110
and
120
must be downsized. In the conventional electrostatic capacitance type sensor, when a positioning error occurs, the electrode extraction portion
112
changes the electrostatic capacitance between the two electrodes
110
and
120
, as described above. Hence, if the electrodes
110
and
120
are downsized, the offset increases accordingly. As a result, a high-precision compact sensor cannot be obtained.
The present invention has been made in order to solve this problem, and has as its object to decrease an offset in a compact electrostatic capacitance type sensor.
DISCLOSURE OF INVENTION
In order to achieve the above object, the present invention is characterized by comprising first and second electrodes which are arranged to face each other so as not to come into contact with each other and a gap between which changes in accordance with a change in an amount to be detected, and a signal processor connected to the first and second electrodes to calculate the amount to be detected on the basis of an electrostatic capacitance formed between the first and second electrodes, the first electrode having those portions, areas of which that face the second electrode increases and decreases, respectively, upon a positioning error that occurs in a direction parallel to the first electrode, and which portions have the same area. Even if a positioning error occurs, the facing area of the two electrodes does not change, so that the electrostatic capacitance between the electrodes is constant. As a result, an offset in a compact electrostatic capacitance type sensor can be decreased.
In particular, the first electrode may be smaller than the second electrode as a whole.
In th

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