Electrostatic capacity-type pressure sensor

An electrostatic capacity-type pressure sensor includes a diaphragm support body, first and second lids, a fixed electrode, a coupling member, an electrode support member, and a movable electrode. The diaphragm support body is formed into a tube shape. The first and second lids have thin diaphragm portions and thick fixing portions formed integrally with peripheral edge portions of the diaphragm portions and are arranged to face each other by bringing the fixing portions in tight contact with the two end portions of the diaphragm support body. The first and second lids separate the interior of the diaphragm support body from the outside by closing the two end portions of the diaphragm support body. The fixed electrode is formed on the inner surface of the fixing portion of at least one of the first and second lids. The coupling member has two ends respectively connected to the first and second lids to couple the diaphragm portions facing each other. The electrode support member is supported by the coupling member and arranged between the first and second lids at a predetermined interval. The movable electrode is formed on the electrode support member so as to face the fixed electrode. The movable electrode and the fixed electrodes are arranged parallel to each other at a predetermined interval to constitute a capacitor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electrostatic capacity-type pressure 
sensor with a diaphragm structure for detecting a change in pressure to be 
measured in an electrostatic capacity manner. 
2. Description of the Prior Art 
FIG. 4 shows the structure of a conventional electrostatic capacity-type 
pressure sensor. In FIG. 4, reference numeral 1 denotes a frame body as a 
diaphragm support body; 2 and 3, a pair of thin diaphragms formed to close 
opening portions at the two ends of the frame body 1; 4, a support for 
connecting and fixing the pair of diaphragms 2 and 3 in the frame body 1; 
5, a movable electrode support plate fixed to the support 4 so as to face 
the diaphragms 2 and 3; and 6a and 6b, movable electrodes respectively 
consisting of thin conductive films and formed on the two surfaces of the 
movable electrode support plate 5. 
Reference numerals 7a and 7b denote fixed electrode support plates which 
face each other via the movable electrode support plate 5, allow the 
support 4 to be inserted therethrough, and are fixed to the inner wall 
surfaces of the frame body 1; and 8a and 8b, fixed electrodes respectively 
consisting of thin conductive films and formed on the surfaces of the 
fixed electrode support plates 7a and 7b so as to face the corresponding 
movable electrodes 6a and 6b of the movable electrode support plate 5. The 
frame body 1, the diaphragms 2 and 3, the support 4, the movable 
electrodes 6a and 6b, and the fixed electrode support plates 7a and 7b 
consist of a material such as sapphire glass. 
The movable electrodes 6a and 6b and the fixed electrodes 8a and 8b are 
arranged via intra-electrode gaps G (about 1 .mu.m) to face each other and 
form capacitors C1 and C2. The capacitors C1 and C2 constitute a 
capacity-type sensor element. The interior of this sensor element is 
separated from the outer surroundings and completely sealed to keep a 
vacuum state or sealed with a gas. Reference symbols P1 and P2 denote 
measurement pressures to be applied to the diaphragms 2 and 3. 
In the electrostatic capacity-type pressure sensor with the above 
structure, when the measurement pressures P1 and P2 are applied to the 
diaphragms 2 and 3, the diaphragms 2 and 3 are deformed to complementarily 
change the capacitance value of the capacitor C1 constituted by the fixed 
electrode 8a and the movable electrode 6a and the capacitance value of the 
capacitor C2 constituted by the fixed electrode 8b and the movable 
electrode 6b. Therefore, a pressure difference P1-P2 (P1&gt;P2) can be 
detected by measuring the changes in capacitance values of the capacitors 
C1 and C2. Note that the electrostatic capacity-type pressure sensor of 
this type is disclosed in, e.g., Japanese Patent Laid-Open No. 6-186106. 
In the electrostatic capacity-type pressure sensor with the above 
arrangement, however, the mechanical strength of the fixed electrode 
support plates 7a and 7b must be stabilized. That is, the intra-electrode 
gap G between the movable electrode 6a and the fixed electrode 8a or the 
movable electrode 6b and the fixed electrode 8b is normally about 1 .mu.m. 
For this reason, a slight change in positions of the fixed electrode 
support plates 7a and 7b changes the capacitance values of the capacitors 
C1 and C2 regardless of the displacement of the diaphragms, directly 
affecting the measurement accuracy. Therefore, highly accurate size 
management is required in the manufacture, and such a manufacturing method 
is also required which can reliably prevent an internal stress produced 
with a deterioration over time. 
The assembly structure of the fixed electrode support plates 7a and 7b and 
the movable electrode support plate 5 is complicated, resulting in an 
increase in the number of processes during manufacture. In addition, the 
propositions of an increase in strength and downsizing of the fixed 
electrode support plates 7a and 7b contradict each other. For example, if 
the fixed electrode support plates 7a and 7b are made thin to realize 
downsizing, they are more easily deformed and the capacitance values of 
the capacitors C1 and C2 change regardless of the displacement of the 
diaphragms, and a degradation in performance such as a zero shift, occurs. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electrostatic 
capacity-type pressure sensor whose manufacturing process can be greatly 
decreased due to a simple structure and which can be easily downsized. 
It is another object of the present invention to provide a high-performance 
electrostatic capacity-type pressure sensor free from a performance 
degradation caused by downsizing. 
In order to achieve the above objects, according to the present invention, 
there is provided an electrostatic capacity-type pressure sensor embodying 
a diaphragm support body formed into a tube shape, first and second lids 
having thin diaphragm portions and thick fixing portions formed integrally 
with peripheral edge portions of the diaphragm portions and arranged to 
face each other by bringing the fixing portions in tight contact with two 
end portions of the diaphragm support body, the first and second lids 
separating an interior of the diaphragm support body from the outside by 
closing the two end portions of the diaphragm support body, a fixed 
electrode formed on an inner surface of the fixing portion of at least one 
of the first and second lids, a coupling member having two ends 
respectively connected to the first and second lids to couple the 
diaphragm portions facing each other, an electrode support member 
supported by the coupling member and arranged between the first and second 
lids at a predetermined interval, and a movable electrode formed on the 
electrode support member so as to face the fixed electrode, the movable 
electrode and the fixed electrodes being arranged parallel to each other 
at a predetermined interval to constitute a capacitor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will be described in detail below with reference to 
the accompanying drawings. 
FIG. 1 shows a cross-section of an electrostatic capacity-type pressure 
sensor 10 according to the preferred embodiment of the present invention. 
In FIG. 1, reference numeral 11 denotes a frame body as a diaphragm 
support body formed into an almost square tube shape; and reference 
numerals 12 and 13 denote first and second lids respectively provided 
opposite to each other so as to close opening portions at the two ends of 
the frame body 11. The interior of the frame body 11 is separated from the 
outside and sealed by the first and second lids 12 and 13. 
The first and second lids 12 and 13 are constituted by diaphragm portions 
12a and 13a respectively formed thinly at the central portions and fixing 
portions 12b and 13b formed thickly at the peripheral edge portions, 
respectively. The diaphragm portions 12a and 13a and the fixing portions 
12b and 13b are preferably formed integrally and unitarily. The inner 
periphery of the fixing portion 12b or 13b or the outer periphery of the 
diaphragm portion 12a or 13a, i.e., the connection portion between the 
diaphragm portion 12a or 13a and the fixing portion 12b or 13b, 
respectively each has a size smaller than the inner size of the frame body 
11. 
The diaphragm portions 12a and 13a of the first and second lids 12 and 13, 
respectively are arranged to face each other at the central portion of the 
frame body 11 by bringing the fixing portions 12b and 13b in tight contact 
with the opening portions at the two ends of the frame body 11 to arrange 
the first and second lids 12 and 13 to face each other. The first and 
second lids 12 and 13 are fixed and held in this state to constitute a 
housing 14. 
Reference numeral 15 denotes a support connected and fixed between the 
opposite surfaces of the diaphragm portions 12a and 13a of the first and 
second lids 12 and 13 in the frame body 11 to couple the diaphragm 
portions 12a and 13a; and reference numeral 16 denotes an electrode 
support plate supported by and fixed to the support 15 so as to face the 
first and second diaphragm portions 12a and 13a and is preferably formed 
integrally and unitarily with the support 15. The electrode support plate 
16 preferably has a size smaller than the inner size of the frame body 11 
and larger than that of the connection portion between the diaphragm 
portion 12a or 13a and the fixing portion 12b or 13b. 
Reference numeral 17 denotes a first movable electrode consisting of a thin 
conductive film and formed at a peripheral edge portion, overlapping with 
the fixing portion 12b, on one surface side of the electrode support plate 
16; and reference numeral 18 denotes a first fixed electrode consisting of 
a thin conductive film and formed on the inner surface side of the fixing 
portion 12b of the first lid 12 forming an intra-electrode gap G (about 1 
.mu.m) with the facing first movable electrode 17 of the electrode support 
plate 16. The first movable electrode 17 and the first fixed electrode 18 
are arranged to face each other, and define the intra-electrode gap G to 
constitute a capacitor C1. 
Reference numeral 19 denotes a second movable electrode consisting of a 
thin conductive film and formed at a peripheral edge portion, overlapping 
with the fixing portion 13b, on the other surface side of the electrode 
support plate 16; and reference numeral 20 denotes a second fixed 
electrode consisting of a thin conductive film and formed on the inner 
surface side of the fixing portion 13b of the second lid 13 so as to face 
the second movable electrode 19 of the electrode support plate 16. The 
second movable electrode 19 and the second fixed electrode 20 are arranged 
to face each other and define the intra-electrode gap G to and constitute 
a capacitor C2. The capacitors C1 and C2 preferably have the same base 
capacitance value with the same structure (base capacitance value obtained 
when measurement pressures P1 and P2 are not applied). The interior of the 
frame body 11 having the capacitors C1 and C2 formed therein is separated 
from the outer surroundings and is completely sealed. The capacity-type 
sensor element 10 is constituted by keeping the sealed interior of the 
frame body 11 a vacuum or sealing the interior with a gas. 
Note that the first and second lids 12 and 13 consist of an insulating 
material such as glass, silica, or sapphire, whereas the frame body 11, 
the support 15, and the movable electrode support plate 16 consist of, 
e.g., silicon. 
In the electrostatic capacity-type pressure sensor 10 according to the 
present invention, when the measurement pressures P1 and P2 are 
respectively applied to the diaphragm portions 12a and 13a of the first 
and second lids 12 and 13, respectively, to produce a pressure difference 
(P1-P2), the diaphragm portions 12a and 13a and the support 15 are 
integrally displaced. The capacitance value of the capacitor C1 
constituted by the first movable electrode 17 and the first fixed 
electrode 18 and the capacitance value of the capacitor C2 constituted by 
the second movable electrode 19 and the second fixed electrode 20 
increase/decrease upon the displacement of the diaphragm portions 12a and 
13a. Changes in the two capacitance values C1 and C2 can be measured to 
detect the measurement pressure. 
With this arrangement, the base capacitance values of the capacitors C1 and 
C2 are canceled to obtain only changes in capacitance values in accordance 
with the pressure difference (P1-P2). All the factors of errors caused by 
a change in atmospheric surroundings can be completely removed. As a 
result, a pressure range from a very low pressure up to a high pressure 
can be measured with a wide dynamic range, and at the same time the 
pressure can be measured with a high accuracy and a high reliability. 
FIGS. 2A through 2E show the electrostatic capacity-type pressure sensor 10 
in FIG. 1 detailing the respective steps in a method of manufacturing the 
pressure sensor 10. First, as shown in FIG. 2A, an insulating substrate 21 
is bonded by a direct bonding method to a thick insulating ring 22 having 
an opening formed by an ultrasonic process or a laser process to have a 
size equal to that of the diaphragm portions 12a and 13a. In this manner, 
a lid structure 23 corresponding to the first or second lid 12 or 13 is 
formed. 
Next, a thin metal film consisting of an electrode material such as Pt or 
Au is formed on the insulating substrate 21 of the lid structure 23 by 
deposition, sputtering, ion plating, or the like. The thin metal film is 
patterned by photolithography, etching, or a lift-off method to form a 
fixed electrode 24 corresponding to the first or second fixed electrode 18 
or 20 in a predetermined region, corresponding to the insulating ring 22, 
on the insulating substrate 21, as shown in FIG. 2B. In this manner, a lid 
structure 25 with an electrode is formed. 
As shown in FIG. 2C, an oxide film (not shown) is patterned in regions, 
corresponding to the frame body 11 and the support 15, on the two surfaces 
of a silicon wafer. Then, the silicon wafer is dipped in an etching 
solution such as KOH or TMAH to form a block body 26. As will be described 
below, the block body 26 is a silicon block on which the frame body 11, 
the support 15, and the electrode support plate 16 are integrated. In 
forming the block body 26, the distance between the first movable 
electrode 17 and the first fixed electrode 18 and the distance between the 
second movable electrode 19 and the second fixed electrode 20, i.e., the 
capacitance values of the capacitors C1 and C2, are controlled by the 
etching time of the silicon wafer. The first and second movable electrodes 
17 and 19 are formed by growing polysilicon in predetermined regions on 
the two surfaces of the electrode support plate 16 consisting of a silicon 
material so as to face the first and second fixed electrodes 18 and 20, 
respectively. 
As shown in FIG. 2D, the block body 26 in FIG. 2C from which the oxide film 
is removed is bonded to the lid structure 25 with the electrodes aligned 
in their respective predetermined directions. Then, the joint portion 
between the electrode support plate 16 and the frame body 11 is etched and 
removed by dry etching to form a space therethrough. The block body 26 is 
divided in this manner, and the unit of the frame body 11, the support 15, 
and the electrode support plate 16 supported by and fixed to the support 
15 is formed. As a method of bonding the block body 26 to the lid 
structure 25 with the electrode, an anode bonding method, a direct bonding 
method, an anode bonding method via thin pyrex glass, or the like is 
properly selected in accordance with the material. 
As shown in FIG. 2E, another lid structure 25 with an electrode which has 
been formed in the step of FIG. 2B is bonded to the frame body 11 and the 
support 15 in a predetermined direction. As a result, the capacity-type 
sensor element shown in FIG. 1 is completed. 
FIG. 3 shows a step to explain the main part of an alternative method of 
manufacturing the electrostatic capacity-type pressure sensor 10 in FIG. 
1. The step shown in FIG. 3 corresponds to the step of FIG. 2C when the 
frame body 11, the support 15, and the electrode support plate 16 are 
formed of a glass material. In this example, resists or thin metal films 
(not shown) are patterned on the two surfaces of a glass substrate. Then, 
the resultant structure is dipped in, e.g., an HF etching solution or 
dry-etched to form a block body 30 constituting the frame body 11, the 
support 15, and the electrode support plate 16. 
An electrode material such as Pt or Au on the block body 30 is patterned 
into a predetermined shape by deposition, sputtering, ion plating, or the 
like. Movable electrodes 31 corresponding to the first and second movable 
electrodes 17 and 19 are formed to form a block body 32 with electrodes. 
The subsequent steps are the same as shown in FIGS. 2D and 2E. 
In the case of the block body 32, the distance between the first movable 
electrode 17 and the first fixed electrode 18 and the distance between the 
second movable electrode 19 and the second fixed electrode 20, i.e., the 
capacitance values of the capacitors C1 and C2, are controlled by the 
etching time of a glass substrate and the thickness of the movable 
electrode 31. 
According to the above-described embodiments, the first and second fixed 
electrodes 18 and 20 are formed at the fixing portions 12b and 13b to have 
sufficient mechanical strengths on the first and second lids 12 and 13. 
With this arrangement, a degradation such as a zero shift caused by 
deformation of the fixing portions 12b and 13b is prevented. 
In addition, no fixed electrode support plate for supporting the first and 
second fixed electrodes 18 and 20 is formed on the frame body 11, so that 
the structure of the electrode support plate 16 is simplified because it 
need not be complicatedly assembled with the fixed electrode support 
plate. As a result, the manufacturing steps are decreased in number, and 
the manufacture is facilitated. 
Note that the above-described embodiment exemplifies the case in which the 
first and second diaphragm portions 12a and 13a have a square shape. 
However, they can be similarly constituted even with a circular or another 
shape. In addition, the shape of the frame body 11 is not limited to a 
square tube, and it may be similarly constituted even with a tube or 
another shape. 
The above-described embodiment exemplifies the case in which the housing 14 
embodies the capacitor C1 constituted by the first movable electrode 17 
and the first fixed electrode 18 and the capacitor C2 constituted by the 
second movable electrode 19 and the second fixed electrode 20 therein. 
However, the present invention is not limited to this, and only either one 
of the capacitors C1 and C2 may be constituted. 
As has been described above, according to the present invention, a thick 
fixing portion is formed on at least one of the first and second lids 12 
and 13 which seal the two end portions of the frame body 11, and a fixed 
electrode is formed on the thick fixing portion. A fixed electrode 
mounting portion is strengthened to prevent deformation. Therefore, a 
performance degradation such as a zero shift does not occur. 
A fixed electrode support plate for supporting a fixed electrode can be 
eliminated. The arrangement is simplified, the manufacturing steps are 
decreased in number, the internal stress generated in the manufacture is 
reduced, and size reduction and a decrease in thickness are realized. 
Further, according to the present invention, fixed and movable electrodes 
are hardly degraded because the fixed and movable electrodes are separated 
from a fluid having a pressure to be measured. When two pairs of counter 
fixed and movable electrodes are formed, original capacitance values can 
be canceled because one capacitance value increases while the other 
decreases. Therefore, the measurement accuracy can be greatly increased. 
While the invention has been described in terms of a preferred embodiment, 
it is apparent that other forms could be adopted by one skilled in the 
art. Accordingly, the scope of the invention is to be limited only by the 
following claims.