Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1999-08-26
2002-04-23
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S548000
Reexamination Certificate
active
06377056
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electrostatic capacitance type dynamic quantity sensor for detecting a dynamic quantity such as pressure, acceleration, etc.
As an example of conventional pressure sensors, there is an electrostatic capacitance type pressure sensor of surface device processing type as disclosed in JP B 7-50789, for instance. This pressure sensor is formed by forming a first electrode (fixed electrode) by diffusing impurities on a monocrystal silicon substrate, and arranging a second electrode (movable electrode) of diaphragm shape formed of poycrystal silicon having conductivity on the monocrystal silicon substrate so as to oppose the first electrode with an air gap therebetween, whereby pressure induced displacement of the second electrode changes the electrostatic capacitance, thereby allowing detection of the pressure.
FIG. 7
shows a section of a pressure detecting portion of the above-mentioned electrostatic capacitance type pressure sensor.
As shown in
FIG. 7
, a fixed electrode
122
is formed as a diffusion layer on a monocrystal silicon substrate
121
, and a movable electrode (diaphragm)
131
is arranged above a protective film
123
and an air gap
125
. The movable electrode
131
is composed of protective films
126
,
129
of the nitrified film or the like and a conductive layer
128
of polycrystal silicon.
Signals (electric signals) of electrostatic capacitance between the movable electrode
131
and the fixed electrode
122
are taken out through aluminum wiring or the like. Assuming that an opposing surface area between the movable electrode
131
and the fixed electrode
122
is S and a distance of air gap between the fixed and movable electrodes
122
,
131
is d, an electrostatic capacitance value C
s
of this capacitance conversion element is expressed by the following equation (1)
C
s
=∈
0
·S/d
(1)
where ∈
0
is dielectric constant in vacuum.
When pressure is applied onto the movable electrode
131
, the air gap d between the fixed electrode
122
and the movable electrode
131
changes and the sensor capacitance C
s
changes. However, in the case where the capacitance C
s
is converted into an electric signal without modification of the above equation (1), the larger the sensitivity is made (the larger the distance d is changed) the larger is the degree of non-linearity of characteristics of change in the capacitance C
s
to the pressure inputted. Therefore, even if one tries to make a larger change in electrostatic capacitance to input pressure in order to achieve both of the electrostatic capacitance sensor small in size and improving a S/N ratio, the above-mentioned non-linear characteristic becomes a bar to realizing it.
Further, since the electrode is formed on the silicon substrate (in this case, it is formed by diffusion), the floating capacitance between the fixed electrode
122
and the silicon substrate
121
and between the movable electrode
131
or the wiring
130
and the silicon substrate
121
becomes large relative to the sensor capacitance C
s
.
As a method of solving the non-linear characteristic problem of the electric signal, a system of obtaining an output proportional to a reciprocal of the electrostatic capacitance C
s
has been already proposed, for example, it is a circuit disclosed in Sensors and Actuators A 60 (1997) page 32 to 36. This is constructed so that a circuit for converting electrostatic capacitance changing by pressure is constructed by integration feedback capacitance of an operation amplifier, whereby an electric charge charged onto the integration feedback capacitance is converted into a pressure signal.
FIG. 8
shows the conventional circuit. In
FIG. 8
, a reference (standard) number
1
denotes a constant voltage source,
2
a
and
3
a
each denote a changeover switch,
4
denotes a reference electrostatic capacitance element the capacitance C
R
of which is fixed,
5
denotes a dynamical quantity detecting electrostatic capacitance element which is formed by a movable electrode and a fixed electrode and in which the electrostatic capacitance, that is, C
s
changes according to a dynamic quantity and
7
denotes an operational amplifier.
The reference electrostatic capacitance element
4
is connected to a reverse input terminal of the operational amplifier
7
, and the dynamic quantity detecting electrostatic capacitance element
5
(electrostatic capacitance C
s
) is provided in a feedback circuit between the reverse input terminal and an output terminal of the operational amplifier. The switches
2
a
,
3
a
are elements of a charge discharge circuit of the electrostatic capacitance C
R
, C
s
, and positioned at places of solid lines (in
FIG. 8
) when timing is &phgr;
1
and positioned at places of broken lines (in
FIG. 8
) when timing is &phgr;
1
B.
According to this circuit, when timing is &phgr;
1
, a voltage value Vcc of the constant voltage source
1
is applied to the reference electrostatic capacitance element
4
through the switch
2
a
, and charged charges are integrated in the dynamic quantity detecting electrostatic capacitance element
5
. When timing is &phgr;
1
B, the charges charged in the reference electrostatic capacitance element
4
are discharged through the switch
2
a
and the charges charged in the dynamic quantity detecting electrostatic capacitance are discharged through the switch
3
a
. By repeating the above-mentioned two modes, pulse like output signals are obtained at the output terminal
9
.
Output Vout of the circuit is expressed by the following equation (2)
Vout=(
C
R
/C
s
)Vcc=−(
S
R
d
s/
S
S
d
R
)Vcc (2)
where S
S
denotes an area of the dynamic quantity detecting electrostatic capacitance element, d
s
denotes a distance (air gap) between the electrodes of the dynamic quantity detecting electrostatic capacitance element, S
R
denotes an area of the reference electrostatic capacitance element and d
R
denotes a distance (air gap) between the electrodes of the reference electrostatic capacitance element.
Accordingly, since it is constructed so that output voltage changes proportionally to a reciprocal of capacitance C
s
of the dynamic quantity detecting electrostatic capacitance element, that is, proportionally to a change of an air gap d
s
, the output becomes an excellent characteristic without non-linearity in principal. Such a circuit is disclosed in JP A 4-329371 and JP A 5-18990, for example.
In this case, electrostatic capacitance C
s
for dynamic quantity detection has turned to be integration feedback capacitance of the operational amplifier, so that driving frequency for detection is restricted according to response speed of the operational amplifier. In order to precisely convert very small capacitance (1 pF or less) into an electric signal, it is necessary to detect dynamic quantity by high speed detection driving frequency (several hundreds kHz or higher). However, in the event that the detection driving frequency is restricted according to response speed of the operational amplifier, as mentioned above, a high speed operational amplifier is needed to detect capacitance at high frequency and with high precision, which is increases the cost and makes large in size.
Further, where an element of large floating capacitance (as shown in the first prior art) is driven, since the floating capacitance becomes a bar to improving on the response of the operational amplifier and stability, finally, the conventional sensor is not suitable for detection of very small capacitance with high precision. Further, in order to obtain D.C. output, it is necessary to add a sample and hold circuit to a rear stage.
Further, JP A 6-507723 (Laid-open PCT application) discloses a pressure measurement apparatus in which a ratio of a difference between sensor capacitance C
s
and reference capacitance C
R
is taken by reference capacitance C
f
divided of an electrode displaced by pressure. Transmission function F deriving pressure by capacitance measurement in this case is e
Hanzawa Keiji
Horie Junichi
Ichikawa Norio
Kuryu Seiji
Matsumoto Masahiro
Crowell & Moring LLP
Hitachi , Ltd.
Kerveros J.
Metjahic Safet
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