Semiconductor pressure sensor including reference capacitor on the same substrate

A semiconductor pressure sensor utilizing electrostatic capacitance has a plurality of pressure sensing electrostatic capacitances and a reference electrostatic capacitance formed on one side of a silicon chip. As a movable electrode, the pressure sensing electrostatic capacitances each have a diaphragm, which may have a displacement portion composed of a central area thereof, and a peripheral portion which is more deformable than the central portion.

BACKGROUND AND SUMMARY OF THE INVENTION 
This application claims the priority of Japanese Application No. 8-262748, 
filed Oct. 3, 1996, and Japanese Application No. 8-283670, filed Oct. 26, 
1996, the disclosures of which are expressly incorporated by reference 
herein. 
The present invention relates to a semiconductor type sensor for detecting 
a pressure as a change in electrostatic capacitance and, more 
particularly, to a surface device type complex sensor which has sensors 
formed entirely on the same surface of a silicon chip. 
As disclosed in Japanese patent document JP A No. 1-256177, a conventional 
complex sensor is a bulk type piezo-resistance sensor in which a 
differential pressure sensor, a static pressure sensor and a temperature 
sensor are formed on a single substrate, which is worked from the backside 
surface into a diaphragm. 
In the conventional complex sensor of bulk type, since the silicon 
substrate is worked from the backside surface into a diaphragm, the 
position of the diaphragm is apt to deviate slightly from the sensing 
elements formed on the front surface of the silicon substrate. The 
sensitivity of the sensor would be substantially reduced even if there is 
a small deviation in the relative position. Therefore, in order to avoid 
this problem, it has been necessary to design the diaphragm on a 
relatively large scale. This is because positioning accuracy of the 
process for forming the sensors on both sides of the silicon wafer is 
lower than positioning accuracy for the formation of sensors on only one 
side of the substrate. 
In addition, the conventional complex sensor has a high manufacturing cost 
because the silicon wafer must be mirror-finished on the both surfaces. 
Further, the piezo-resistance type sensor has a non-linear characteristic, 
and accordingly varies significantly with change in environmental 
temperature. Therefore, characteristic compensation of the sensor becomes 
more difficult, and the cost increases. 
Japanese patent document JP A 4-143628 discloses electrostatic capacitance 
type pressure sensors each utilizing a silicon tip. However, it does not 
disclose any pressure sensors in which multi-function sensor portions are 
formed on only one side. 
Japanese patent document JP A 5-187947 also discloses electrostatic 
capacitance type pressure sensors each utilizing a silicon chip. However, 
it does not disclose any pressure sensors each of which has pressure 
portions formed on only one side. 
Further, conventional electrostatic capacitances or sensors are known which 
comprises a movable electrode of a flat member, a fixed electrode, an 
insulating film and a support for supporting the movable electrode to 
provide a gap between the movable electrode and fixed electrode. When a 
pressure is applied on the movable electrode, the movable electrode 
deflects so that displacement of the movable electrode is greatest at the 
center and is smaller toward the periphery. Therefore, the relationship of 
the electrostatic capacitance to the pressure is non-linear, and the 
non-linearity causes output errors. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a complex sensor which is 
small in size and low in cost. 
Another object of the present invention is to provide such a sensor which 
is simple in structure and in manufacturing method, suitable for 
mass-production and economical. 
Still another object of the present invention is to provide a 
multi-function pressure sensor having a plurality of sensors and a 
plurality of signal processing circuits on a single chip. 
A further object of the present invention is to provide an IC pressure 
sensor which is highly sensitive and less-affected by environmental 
temperature. 
A still further object of the present invention is to provide a 
high-pressure-resistant complex sensor. 
A still further object of the present invention is to provide a 
high-precision sensor of reduced non-linearity. 
The objects of small size and low cost can be achieved by an electrostatic 
capacitance type sensor which is simple in structure and in manufacturing 
method and is manufactured by forming a multi-layered film on only one 
side of a silicon wafer, by means of a surface processing technology. 
Initially, a conductive film which serves as a sensor diaphragm and an 
insulator film (sacrifice film) are formed, and the sacrifice film is then 
removed to form a space, so that an electrostatic capacitance is formed 
between the diaphragm and a fixed electrode formed in the substrate side 
of a silicon wafer. In order to attain the object of mass-production, a 
silicon wafer of the standard specification for an LSI is used, and then 
sensors and signal processing circuits are formed on one side of the 
silicon wafer, by means of a semiconductor fabrication process. 
The object of multi-function can be attained by providing a multi-function 
pressure sensor which is manufactured by forming two absolute pressure 
sensing electrostatic capacitances as a complex on a single chip. One of 
the pressure sensing electrostatic capacitances is then used as a sensor 
for measuring absolute pressure, while the other is used as a sensor for 
measuring atmospheric pressure. Together these two sensors serve as a 
relative pressure sensor, by taking the difference of their outputs. 
In order to attain the object of high sensitivity and reduced temperature 
effect, floating capacitance produced by signal lead wires is reduced 
substantially by forming the signal processing circuits for calculating 
output signals of the complex sensor near the sensor capacitances. 
Further, a reference capacitance, which is formed at the same time as the 
two electrostatic capacitance type sensors that are formed on one side of 
the silicon substrate, has a temperature coefficient nearly the same value 
as the sensors. Thus, it is possible to obtain a pressure sensor having 
less temperature effect by differencing the both signals. Furthermore, 
since a signal from the reference capacitance does not vary with pressure, 
the reference capacitance can function as a temperature sensor by itself. 
In order to attain the ability to withstand high-pressure, a multi-layered 
film is formed on the substrate surface of the silicon wafer, with a gap 
of several microns under the surface layer. With such an arrangement, the 
diaphragm is deformed and brought in contact with the substrate surface of 
the silicon wafer, thus restricting excessive deformation of the diaphragm 
and preventing rupture of the diaphragm when an over-load pressure is 
applied. Therefore, the sensor can withstand a pressure which is several 
tens of times as high as measuring range pressure. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Firstly, a semiconductor pressure sensor according to the present invention 
will be described hereunder. 
The diaphragm formed on one side of the silicon chip is an electrically 
conductive film which acts as the movable film, and is moved according to 
a pressure loaded thereon. The space formed by removing the insulator film 
(sacrifice film) gives rise to the pressure sensing electrostatic 
capacitance between the diaphragm and the fixed electrode formed in the 
substrate side of the wafer. The space is hermetically sealed and 
evacuated to provide a vacuum, and functions as a reference pressure 
chamber. Thus, the space creates an electrostatic capacitance type 
pressure sensor which senses pressure on the basis of absolute pressure. 
The diaphragm is displaced downward and becomes concave when it is exposed 
to atmospheric pressure, and is returned to a flat-shape as the load 
pressure approaches vacuum pressure. When the load pressure is a positive 
pressure relative to atmospheric pressure, the diaphragm is displaced 
farther downward and accordingly the width of the space between the 
diaphragm and the fixed electrode becomes smaller. 
As described above, the electrostatic capacitance formed with the fixed 
electrode is varied by the load pressure. This electrostatic capacitance 
change is converted into a standardized output signal within a range of, 
for example, 1V to 5V by the signal processing circuit, and the loaded 
pressure is detected from this value. 
In order to provide a relative pressure signal, two pressure sensing 
electrostatic capacitances having the structure of absolute pressure 
reference can be formed as a complex on the single chip. A pressure to be 
measured is then introduced onto one of the pressure sensing electrostatic 
capacitances, and atmospheric pressure is introduced onto the other. By 
all differencing the outputs of the two sensors using a signal processing 
circuit, the difference is output as a relative pressure signal. By 
differencing the two pressure sensing capacitances and the reference 
capacitance, the differences are output as signals of an absolute pressure 
sensor and an atmospheric pressure sensor. 
A gap of several microns is formed under the surface layer, and the 
diaphragm is deformed and brought in contact with the substrate surface of 
the silicon wafer to prevent excessive deformation of the diaphragm, and 
to prevent rupture of the diaphragm when an over-load pressure is applied. 
Therefore, the sensor can withstand a pressure which is several ten of 
times as high as the measurement range pressure. 
Referring to FIGS. 1A to 1C, embodiments of the present invention will now 
be explained. 
FIG. 1A is a block circuit diagram showing an embodiment of a semiconductor 
pressure sensor in accordance with the present invention. 
In FIG. 1A, the pressure sensor comprises three capacitances 11, 12 and 13, 
and a signal processing circuit 15, which are formed as a complex on a 
sensor substrate. Two of the three capacitances are pressure sensing 
capacitances 11, 12 to detect a pressure by means of electrostatic 
capacitance changes, and generate output signals indicative thereof; one 
of them is a reference or temperature sensing capacitance 13 which can 
detect a temperature as an electrostatic capacitance change, and output a 
signal. The signals from the pressure sensing capacitances 11, 12 are 
processed by the signal processing circuit 15 including signal amplifiers 
151 with adjustable resistors 152 and 153 to output a signal on the basis 
of absolute pressure from a terminal 142 and a signal of relative pressure 
from a terminal 143. 
The temperature sensing capacitance 13, which can detect a temperature as 
mentioned above, also can be used as a reference capacitance for removing 
an influence of temperature on the pressure sensing capacitances 11 and 
12. 
The pressure sensing capacitance 12 has a construction to sense a pressure 
to be measured as an absolute pressure. A signal (capacitance change 
value) from the pressure sensing capacitance 12 is subjected to a 
calculation of a difference between that signal and a signal from the 
reference (temperature sensing) capacitance 13 by closing a switching 
device 155. The difference is processed and output from the terminal 142 
as an absolute pressure. 
The pressure sensing capacitance 11 is constructed to sense atmospheric 
pressure. When a relative pressure output is desired, a difference in 
capacitance between this atmospheric pressure sensing capacitance 11 and 
the pressure sensing capacitance 12 is taken by switching the switching 
device 155; the difference is then processed by the signal processing 
circuit and output as a relative pressure from the terminal 143. 
FIG. 1B shows a modification of FIG. 1A. This is, the switching device 155 
is arranged so that signals from the capacitances 11, 12 and 13 are 
subjected to calculation to obtain a difference or differences after the 
signals are processed by the signal processing circuit. 
If necessary, by constructing the sensor as shown in FIG. 1C, an 
atmospheric pressure signal is output from a terminal 144 by calculating 
the difference between sensed values from the pressure sensing capacitance 
11 and a reference capacitance 13'. Further, a temperature signal is 
output from the terminal 141 by signal-processing of variations in the 
reference capacitance 13 due to temperature changes. 
In this case, the two pressure sensing capacitances 11, 12 and the 
reference capacitance 13 have essentially the same temperature 
characteristic since they are formed on a single substrate adjacent to one 
another, and in the same process. Therefore, even if the ambient or 
environmental temperature is changed, errors due to temperature change can 
be compensated by calculating the difference between them using the signal 
processing circuit. Although the temperature coefficient of the 
capacitance type pressure sensor of the present invention is smaller than 
that of a conventional piezo-resistance type pressure sensor, the effect 
of temperature can be further decreased by such calculation processing. 
The embodiment of FIG. 1C has two pressure sensing capacitances 11, 12 and 
two reference capacitances 13, 13'. A temperature signal can be obtained 
from the reference capacitance 13; an absolute pressure signal can be 
obtained from the difference in capacitance between the pressure sensing 
capacitance 12 and the (temperature sensing) reference capacitance 13; a 
relative pressure signal can be obtained from the difference in 
capacitance between the pressure sensing capacitance 11 and the pressure 
sensing capacitance 12; and an atmospheric pressure signal can be obtained 
from the difference in capacitance between the pressure sensing 
capacitance 11 and the reference capacitance 13'. In this embodiment, a 
temperature signal, as well as an absolute pressure signal, a relative 
pressure signal and an atmospheric pressure signal which are not affected 
by environmental temperature change, can be accurately obtained by the 
four electrostatic capacitances and the signal processing circuit 15 
formed on a single chip. 
An embodiment of the sensor portion shown in FIGS. 1A to 1C will be 
described in detail hereunder, referring to FIG. 2A, which is a 
cross-sectional view showing an assembled stand-alone type sensor. FIG. 2B 
is an enlarged cross-sectional view showing the sensor portion and FIGS. 
1A to 1C of the block circuit diagrams, including structural details which 
are omitted from FIG. 2 for simplicity. 
A sensor substrate 1 is made of silicon, and has two major sides (upper and 
lower sides in FIG. 2B). A casing 2, which is made of a material such as 
plastic, holds and surrounds the sensor substrate 1. A signal lead 
connector 3 is connected to the casing 2. An organic adhesive 4 is 
provided for air-tightly adhering the sensor substrate 1 to the casing 2. 
A hole 21 is formed in a portion of the casing 2 for introducing 
atmospheric pressure. A pressure introducing pipe 5 is connected to the 
casing 2 at the opposite side to the hole 21 for introducing a pressure to 
be measured. A connecting wire 7 connects between a sensor portion and a 
connector 31. Silicone gel 6 is filled in a spacing formed in the casing 2 
for protecting the sensor substrate 1 and the connecting wire 7. 
As shown in FIG. 2B, two pressure sensing capacitances 11, 12 and a 
reference capacitance or temperature sensing capacitance 13 are formed as 
a complex on a single chip (the sensing substrate 1). One pressure sensing 
capacitance 12 is used for an absolute pressure reference sensor by 
introducing on it a pressure to be measured, and the other pressure 
sensing capacitance 11 is used to detect variation of atmospheric pressure 
by introducing atmospheric pressure thereon. An absolute pressure is 
obtained from difference in capacitance between the pressure sensing 
capacitance 12 and the reference capacitance 13. Further, as shown in 
FIGS. 1A to 1C, an output as a relative pressure sensor is output by 
taking the difference of the output signals from both capacitances 11, 12, 
using the signal processing circuit 15. 
The structure of the sensor portion will be described in detail hereunder, 
referring to FIG. 2B. The two pressure sensing capacitances 11, 12 each 
having an absolute pressure reference structure in which pressure is 
sensed on the basis of absolute pressure, the reference capacitance 13 and 
the signal processing circuits 15 are formed as a complex on the sensor 
substrate 1. The two pressure sensing capacitances 11, 12 have exactly the 
same structure and are formed at the same time in a surface device process 
to be described later. 
A fixed electrode 124 is formed on the sensor substrate 1, and an etching 
stopper film 123 is formed on the fixed electrode 124 for limiting a gap 
of a space 122. A diaphragm 121 is supported on an insulator film 125 
formed on the sensor substrate to have a suitable gap between the etching 
stopper film 123 and the diaphragm 121. A seal 126 is provided in a hole 
formed in the diaphragm 121 for sealing the chamber 122 to provide a 
vacuum. 
Further, the construction of the sensing portion of this embodiment 
includes the following. That is, on the diaphragm 121, a glass plate 20 is 
mounted, and a reinforcing plate 50 thereon. The glass plate 20 and the 
reinforcing plate 50 have holes 22 and 52 for introducing a pressure to be 
measured to the pressure sensing capacitance 12. A pressure to be measured 
is introduced onto the diaphragm 121 through the pressure introducing pipe 
5, the holes 52 and 22. 
The glass plate 20 and the reinforcing plate 50 further have holes 23 and 
51, respectively, and silicone gel is filled in the holes 23 and 51. The 
holes 23 and 51 communicate with the hole 21 of the casing 2 (FIG. 2A), 
and atmospheric pressure is introduced onto the diaphragm of the pressure 
sensing capacitance 11 through the holes 21, 51 and 23. 
The reference capacitance 13 also has the same construction and size as the 
pressure sensing capacitance 12 except that a vacuum chamber 132 defined 
by the diaphragm 121 and the glass plate 20 is provided. 
An electrode from each of these sensors is connected to the adjacent signal 
processing circuit 15 in a minimum distance to reduce the floating 
capacitance as much as possible. A capacitance is converted into a 
standardized signal within a range of, for example, 1V to 5V by the signal 
processing circuit 15, and is led to a bonding pad or terminal 14 in the 
outside. The signal processing circuit 15 is air-tightly bonded in the 
periphery portion 127 with the glass plate 20, and is protected from 
moisture and contamination from the outside by sealing and maintaining the 
formed space 150 under vacuum or dry gas environment. 
The reference capacitance 13 detects neither the pressure to be measured 
nor the atmospheric pressure, since the vacuum in chamber 132 of the 
reference capacitance 13 is air-tightly sealed with the glass plate 20. 
That is, the reference capacitance 13 detects only changes of the 
capacitance due to change of the environmental temperature. Therefore, the 
reference capacitance 13 functions as a kind of temperature sensor. The 
capacitance change due to temperature variation is caused also by change 
in the dielectric constant of the dielectric material, change in the gap 
due to thermal strain and so on. 
The material of the sensor substrate 1 and the reinforcing plate 50 is a 
silicone resin, and the glass plate 20 is made of boron-silicate glass 
having a thermal expansion coefficient near that of silicone resin. The 
sensor substrate 1 and the glass plate 20, and the reinforcing plate 50 
and the glass plate 20 are bonded to each other by an electrostatic method 
without using any adhesive. 
As shown in FIG. 2A, since it is bonded and fixed to the casing 2 made of a 
plastic material, the silicone resin reinforcing plate 50 is designed to 
be thicker than the glass plate 20 so as to have a high rigidity. 
Irreversible thermal strain due to differences of thermal expansion 
coefficients of the both materials is therefore avoided. 
The pressure to be measured is introduced through the pressure introducing 
pipe 5 onto the pressure sensing capacitance 12 of the sensor plate 1. The 
diaphragm 121 of the pressure sensing capacitance 12 is moved 
corresponding to this pressure, so that the gap of the chamber 122 
changes, changing the electrostatic capacitance. In processing the 
electrostatic capacitance using the signal processing circuit 15, its 
characteristic change due to the environmental temperature change is 
corrected using the reference capacitance 13 and the electrostatic 
capacitance change is converted to a standardized electric signal within a 
range of, for example, 1V to 5V. The signal proportional to the pressure 
to be measured is then output to the external through the connector 3. 
Atmospheric pressure introduced through the hole 21 formed in the casing 2 
is received on the diaphragm 121 through the reinforcing plate 50 and the 
hole 21 in the glass plate 20. Although there is silicone gel 6 having a 
viscoelasticity in the middle of the passage introducing the atmospheric 
pressure, the pressure sensing capacitance 11 can measure the atmospheric 
pressure relative to the absolute pressure since the silicone gel is soft 
enough to transmit pressure. 
Although the above description has been made on a case where the processing 
calculation is performed in the stage of electrostatic capacitance, the 
calculation may be performed in the stage after the capacitance change is 
converted into a voltage signal as shown in FIG. 1B. Selection of the 
calculation stage is determined depending on the design of the sensor such 
as magnitude of the signal change, SN ratio and so on. 
FIG. 3 shows a mounting structure of another embodiment of a sensor in 
accordance with the present invention. The structure of the sensor 
substrate 1 is the same as that described above. The different points are 
the casing 2 mounting the sensor substrate 1 and the structure of signal 
lead-out portion. In order to cope with the so-called surface mount where 
the signal lead-out is connected to a wiring pattern 91 on a mounting 
board 9 using solder 8, the sensor substrate 1 is fixed inside a connector 
3 having a plated wiring 31 with adhesive 4, and the wiring 31 and the pad 
of the sensor substrate 1 are connected with a connecting wire 7. Pressure 
to be measured is introduced through a pressure introducing pipe 50' fixed 
to the casing 2 and applied onto the absolute pressure or reference 
sensing capacitance 12 (not shown in FIG. 3) on the sensor substrate 1; 
and atmospheric pressure is applied to the pressure sensing capacitance 11 
(not shown in FIG. 3) through an atmospheric pressure introducing portion 
21. The signal processing circuit 15 (not shown in FIG. 3) executes the 
calculation described in FIGS. 1A, 1B or 1C, and outputs a signal relative 
to absolute pressure and a sensor signal proportional to a relative 
pressure, if required. 
FIG. 4 shows an embodiment of an atmospheric pressure sensor in accordance 
with the present invention. 
In this embodiment, the atmospheric pressure sensor comprises a pressure 
sensing capacitance 12 as mentioned in the above embodiment and a 
reference capacitance 13. The reference capacitance 13 has the same 
construction as in FIG. 2B except that a support 139 instead of the vacuum 
space 132 of FIG. 2B is formed in the central portion of the diaphragm so 
that the reference capacitance is not changed by atmospheric pressure. By 
calculating the difference between the signals of the pressure sensing 
capacitance 12 detecting atmospheric pressure and the reference 
capacitance 13, a signal proportional to the atmospheric pressure is 
output. Since the pressure to be measured is applied directly to the 
sensor 1, the casing 2 and the pressure introducing pipe 50' fixed to the 
casing can be removed thereby simplifying the structure. 
A manufacturing process of the sensor portion as shown in FIG. 2B will be 
described below, referring to FIGS. 5A to 5F. 
FIG. 5A shows a signal processing circuit forming process which forms the 
signal processing circuits 15, using, for example, a standard CMOS 
(complementary metal oxide semiconductor) circuit forming process. 
FIG. 5B shows a process of forming a sacrifice film 122' to be removed 
later, wherein the film 122' of, for example, SiO.sub.2 is formed so as to 
have a thickness necessary for a designed value of electrostatic 
capacitance of the sensor. 
FIG. 5C shows a process of forming a polysilicone film to provide a 
diaphragm 121 which is formed corresponding to a measured pressure range. 
Insulating grooves 129 each are formed by removing a part of the 
polysilicone film in the peripheral portion of the adjacent diaphragm. 
Further, a polysilicone film 127 formed in the peripheral portion of the 
diaphragm and in the peripheral portion of the signal processing circuit 
15 acts as an adhesive for a glass substrate to be bonded later. 
FIG. 5D shows a process for etching removal of the sacrifice film 122', in 
which the SiO.sub.2 is removed (by introducing fluoric acid through holes 
1211 each bored in a part of the polysilicone film) to form a space 122. 
Since an Si.sub.3 N.sub.4 film has been formed on the fixed electrode 124 
in the process (A), the electrode 124 is not etched, and the space 122 
having a gap equal to the thickness of the oxide film 122' of the 
sacrifice film is accurately formed. 
A diameter of the diaphragm and a gap of the space are determined by the 
thick oxide film 125 formed in the peripheral portion and the oxide film 
122' as the sacrifice film. The electrostatic capacitance (that is, the 
sensitivity) is determined accordingly. 
FIG. 5E shows a process of sealing the holes 1211 of the polysilicone film 
121. Seals 126 are formed by forming the same films of polysilicone and 
SiO.sub.2 and removing unnecessary portions. 
FIG. 5F shows a process for forming circuits 15 and aluminum terminals 14 
of bonding pads. 
According to the present invention, it is possible to provide a pressure 
sensor in which the effect of temperature variations is small, and which 
is high in sensitivity, small in size and low in cost, by forming an 
absolute pressure sensor, a relative pressure sensor and an atmospheric 
pressure sensor as a complex on a single chip. 
Further, according to the present invention, when an over-load pressure 
(exceeding the measuring range) is applied to the diaphragm, the diaphragm 
is deformed over the gap, but is brought into contact with the substrate 
surface, thereby avoiding excessive deformation. Therefore, the sensor can 
withstand a pressure which is several tens of times as high as the 
measuring range. 
Another embodiment is described hereunder, referring to FIGS. 6 to 10. 
However, before description of the embodiment, a conventional 
semiconductor pressure sensor is described referring to FIGS. 11-15. 
An example of a conventional sensor is shown in FIG. 11. The sensor 
comprises a movable electrode 201 having a radius (a) and a uniform 
thickness (h), a support 202 for supporting the movable electrode 201 to 
provide a gap 205 of width (d), a fixed electrode 203 disposed inside or 
on a substrate 206 and an insulator film 204. When a pressure acts on the 
sensor, the movable electrode 201 deforms close to the fixed electrode 
203, changing the electrostatic capacitance between them. By detecting the 
change, the pressure is detected. 
As shown in FIG. 11, the periphery of the movable a electrode 201 is fixed 
and is not deformed by the application of pressure, while the central 
portion, which is free, is bent as shown in FIG. 12. If displacement of 
the central portion of the movable electrode 201 is designated as .delta., 
electrostatic capacitance between the movable electrode and the fixed 
electrode at zero displacement is represented by C.sub.O, and specific 
displacement is .alpha. (=.delta./d), the electrostatic capacitance 
C(.delta.) of the sensor can be expressed by Equation 1. 
##EQU1## 
Since the relationship between electrostatic capacitance C(.delta.) and 
displacement .delta. or .alpha. is non-linear as shown by the equation, 
the relationship of electrostatic capacitance C(.delta.) to pressure is 
also non-linear, as shown in FIG. 13. Such non-linearity causes an output 
error. On the other hand, since the sensor has a structure in which the 
support 202 is interposed between the electrodes 201, 203, a parasitic 
capacitance is produced in this portion. This parasitic capacitance 
further increases the non-linearity and decreases sensitivity of the 
sensor at the same time. 
The embodiment described hereinafter,which relates to an electrostatic 
capacitance type physical sensor, is designed to solve the problem 
described above, as explained below. 
In FIG. 6, the sensor comprises a fixed electrode 1240 placed in an inside 
position of a substrate or an upper position of the substrate, a movable 
electrode 1210 which is opposite to the fixed electrode 1240, supported by 
a support 1250 in the periphery so that it can be deformed by an external 
force, and a signal processing unit for calculating the reciprocal of an 
electrostatic capacitance. (The signal processing unit is not shown in 
FIG. 6, but may be incorporated in the signal processing circuits as 
formed in FIGS. 1A to 1C.) 
The movable electrode 1210 has a displacement portion composed of a central 
portion 1210A and a peripheral portion which is more deformable than the 
central portion. The central portion 1210A has a larger elasticity or a 
thicker thickness compared to the peripheral portion, and the fixed 
electrode 1240 has a dimension not larger than a range of the high elastic 
central portion 1210A of the movable electrode 1210 excluding an electrode 
lead portion when the fixed electrode is seen from a direction normal to 
the movable electrode. 
By forming the central portion 1210A so as to have a larger elasticity or a 
thicker thickness compared to the peripheral portion, the central portion 
can be deformed when pressure is applied, and is flat when it is without 
strain. Therefore, the movable electrode 1210 is moved toward and away 
from the fixed electrode 1240, parallel to itself. On the other hand, by 
limiting the dimension of the fixed electrode 1240 within the flat portion 
211 of the movable electrode 1210, the electrostatic capacitance varies 
according to Equation 2. By calculating the reciprocal of the 
electrostatic capacitance, an output can be made linear to displacement 
.delta. of the movable electrode 1210 as expressed by Equation 3. 
##EQU2## 
Further, a highly accurate electrostatic capacitance type physical sensor 
can be provided by making a magnitude of electrostatic capacitance of the 
parallel component to the sensor portion of a parasitic electrostatic 
capacitance produced in the periphery of the sensor not larger than a base 
capacitance of the sensor portion. 
The movable electrode 1210 has a higher elastic portion 1210A or a thicker 
thickness portion arranged in the central portion. The higher elastic 
portion is manufactured by vapor-depositing a polycrystalline silicon or 
SIC or by thickening with a raw material of the electrode itself. By 
forming it in a chemical vapor deposition method or a gas phase epitaxial 
growth method and then by shaping through etching, the movable electrode 
1210 can be manufactured in a desired shape without being restricted by 
properties of the material used. By a conventional manufacturing method of 
anisotropic etching of single crystal silicon, it is difficult to form the 
movable electrode in a desired shape except for a square shape. Further, 
there is a difficulty to independently determine its thickness and its 
area since there is a correlation between them. On the other hand, it is 
preferable that an area of the central portion 1210A of the movable 
electrode is between 15% and 70% of the total displacement area of the 
movable electrode, preferably about 70%. The acceptable plane shape of the 
movable electrode may be square, circular and polygonal. 
The fixed electrode 1240 has a dimension not larger than a range of the 
high elastic central portion (excluding an electrode lead portion), when 
seen from a direction normal to the movable electrode, as well as a shape 
which conforms to that of the high elastic central portion of the movable 
electrode. By applying this limitation to the fixed electrode 1240, it is 
possible to obtain an electrostatic capacitance which conforms faithfully 
to Equation 1. The support 1250 may have any value of electric 
conductivity since there is the insulator film 1211 on the fixed electrode 
1240. However, it is preferable that the support is made of an insulator 
in order to suppress parasitic capacitance, as described later. 
FIG. 7 shows the sensor with the movable electrode 1210 displaced. A region 
A (1210A) is a central region which remains flat even after displacement 
by the high elastic central portion 1210A of the movable electrode 1210. 
The movable electrode 1210 is moved toward and away from the fixed 
electrode 1240 parallel to itself. By limiting movement of the fixed 
electrode 1240 within this range, it is possible to reduce the effect of 
the deformed portion B (1210B) of the movable electrode 210, which 
produces non-linearity. 
The sensor of this embodiment is a circular pressure sensor formed on a 
silicon wafer, in which full-scale pressure is 1 kPa. The movable 
electrode 1210 is made of poly-crystal silicon; the radius (a) of the 
movable electrode 1210 is 85 .mu.m; the thickness (h) of the movable 
electrode is 5 .mu.m; and the distance (d) between the movable electrode 
210 and the fixed electrode 1240 is 0.5 .mu.m. Capacitance when the 
movable electrode 1240 is not displaced (base capacitance C.sub.0) is 
0.486 pF. The radius of the elastic central portion 1210A of the movable 
electrode is 68 .mu.m. 
FIG. 9 shows non-linearity 220 of the sensor of FIG. 8 in accordance with 
the present invention and non-linearity 221 of the conventional sensor of 
FIG. 14. The maximum non-linearity of the conventional sensor is 0.57% at 
pressure of 0.505 kPa, whereas the maximum non-linearity of the sensor in 
accordance with the present invention is 0.12% at pressure of 0.501 kPa. 
Therefore, by employing the sensor in accordance with the present 
invention, the non-linearity can be reduced to less than one fifth that of 
the conventional sensor. 
On the other hand, by limiting the fixed electrode 1240 to a dimension not 
larger than a range of the high elastic central portion (excluding an 
electrode lead portion) when seen from a direction normal to the movable 
electrode 1210, the parallel component of the parasitic capacitance can be 
reduced. The parallel parasitic capacitance Cp decreases intensity of the 
sensor signal and accordingly reduces its sensitivity. In the structure of 
the conventional sensor shown in FIG. 15, since the fixed electrode 203 
also exists under the support 202, parallel parasitic capacitance is 
produced in the supporting portion. The support is made of silicon dioxide 
and has a width 207 of 50 .mu.m, a thickness 208 of 0.5 .mu.m, and thereby 
the parasitic capacitance Cp becomes 2.33 pF which is equivalent to 478% 
of the base capacitance of the sensor. On the other hand, in the sensor in 
accordance with the present invention as shown in FIG. 6, the parallel 
parasitic capacitance is produced only by that portion 1241 where the 
supporting portion overlaps with the lead wire portion of the fixed 
electrode. The width of the lead wire 1241 is 2 .mu.m and the width 1242 
of the supporting body is 50 .mu.m, and accordingly the produced parasitic 
capacitance Cp can be suppressed to 6.7 fF which is equivalent to 1.4% of 
the sensor base capacitance C.sub.0. 
FIG. 10 shows the change in non-linearity NL with respect to parallel 
parasitic capacitance Cp/C.sub.0 =0.7 under specific displacement 
.delta./d of the movable electrode. NL(C) indicates non-linearity when 
reciprocal processing is not performed. NL(1/C) indicates non-linearity 
when reciprocal processing is performed. It can be understood that the 
parasitic capacitance is reduced by performing the reciprocal processing, 
whereby non-linearity can be reduced and high accuracy can be attained. In 
a range of the specific parallel capacitance Cp/C.sub.0 below 1 (one), the 
non-linearity becomes smaller than 10% and high accuracy can be attained. 
The present invention is not limited to the application of an external 
force in the form of pressure as such, but can be applied to any external 
forces to displace the movable electrode. 
As described above, the present invention can provide a highly accurate 
electrostatic capacitance type physical sensor, with reduced non-linearity 
in its output, in which the movable electrode is composed of a central 
portion and a peripheral portion, the central portion having a larger 
elasticity or a thicker thickness than the peripheral portion, and the 
fixed electrode has a dimension not larger than a range of the high 
elastic central portion of the movable electrode (excluding an electrode 
lead portion) when the fixed electrode is seen from a direction normal to 
the movable electrode. Further, the magnitude of the electrostatic 
capacitance of the parallel component to the sensor portion relative to a 
parasitic electrostatic capacitance produced in the periphery of the 
sensor, is no larger than the base capacitance of the sensor portion. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.