Silicon integrated accelerometer

Details Silicon integrated accelerometer Silicon integrated accelerometer

2000-05-10

2004-01-27

Eckert, George (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Physical deformation

C257S254000, C257S415000, C257S419000

Reexamination Certificate

active

06683358

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an accelerometer for detecting acceleration by bending a beam, especially, relates to a silicon integrated accelerometer wherein a signal processing circuit with sensor elements in a mixed manner is formed on a structure by using micro machine technique, thereby detecting acceleration components in multi-axial directions.
BACKGROUND ART
In recent years, in place of accelerometers capable of detecting the acceleration in only one axis, multi-axial accelerometers capable of detecting the acceleration components in biaxial directions or in triaxial directions have been known (see Japanese Patent Application Laid-open No. 9-113534 (1997) etc.).
First of all, as a first example according to the prior art, a structure of an accelerometer for detecting acceleration components in triaxial directions will be described with reference to
FIGS. 1
to
3
.
FIG. 1
is a sectional view showing an accelerometer
102
for detecting acceleration components in triaxial directions. This accelerometer
102
has a hollow cylindrical support
104
and a circular silicon substrate
105
bonded on the support
104
. A columnar seismic mass
107
consisting of the Pyrex glass is bonded to a center of a bottom surface of the silicon substrate
105
.
In the silicon substrate
105
, a diaphragm
108
formed into a thin thickness is structured between a peripheral portion
105
a
bonded to the support
104
and a central portion
105
b
bonded to the seismic mass
107
.
FIG. 2
is a top view showing the accelerometer
102
. As the center of the silicon substrate
102
is regarded as the origin, piezo-resistance elements Rx
1
, Rx
2
, Rx
3
, and Rx
4
are formed on a top surface of the portion
108
, and each two of them positioned in a positive or negative portion on the X axis, respectively.
Samely, piezo-resistance elements Ry
1
, Ry
2
, Ry
3
, and Ry
4
are formed on the portion
108
, each two of them positioned in a positive or negative portion on the Y axis, respectively. Further, piezo-resistance elements Rz
1
, Rz
2
, Rz
3
, and Rz
4
are arranged at position closely adjacent to the piezo-resistance elements Rx
1
, Rx
2
, Rx
3
, and Rx
4
in parallel to each of them on the X axis, respectively.
Each group of piezo-resistance elements Rx
1
-Rx
4
, piezo-resistance elements Ry
1
-Ry
4
, and piezo-resistance elements Rz
1
-Rz
4
composes a bridge circuit, respectively. Before the measurement of the acceleration, each of these three bridge circuits maintains a null balance condition indicating zero output.
As described the above structure, when the acceleration is applied to the accelerometer
102
, a stress is applied at the diaphragm
108
by weight of the seismic mass
107
, so that the diaphragm
108
is deformed mechanically.
This mechanical deformation causes the change in the resistance of the piezo-resistance elements disposed in the direction to which the stress is applied on the flexible part
108
, and the bridge circuit is unbalanced to generate an electric output. Because the resistance of the piezo-resistance elements composing the bridge circuits are changed according to the magnitude and direction of the acceleration, the acceleration components in the three axis (namely, the X, Y, and Z axis) directions can be measured by measuring the changes in the resistance of respective bridge circuits.
FIG. 3
shows an example of a bridge circuit composed of the piezo-resistance elements Rz
1
-Rz
4
for detecting the acceleration component in the Z axis (hereinafter referred to as “Z axis acceleration component”). In
FIG. 1
, when the acceleration is applied to an upward direction, the piezo-resistance elements Rz
1
and Rz
4
are applied a negative (minus) stress and the Rz
2
and Rz
3
are applied a positive (plus) stress. Hence, the bridge circuits induce imbalance, and a potential difference Vab between bridge terminals occurs, thus detecting the Z axis acceleration component.
Furthermore, as a second example according to the prior art, a current detection-type accelerometer having a differential amplifier circuit therein will be described with reference to
FIGS. 4A and 4
b
(see Japanese Patent Application Laid-open No. 6-207948 (1994)).
In
FIG. 4A
, diffusion layers
151
and
152
having electroconductivity opposite to that of a semiconductor substrate
150
are formed on the top surface of the semiconductor substrate
150
. Moreover, an electrode
153
is provided above the semiconductor substrate
150
with a given separation, being between the diffusion layers
151
and
152
. This electrode
153
is defined as a movable electrode of beam structure. In this way, a MIS (Metal Insulator Semiconductor) transistor is structured with the use of air as an insulator film.
Then, by the application of an appropriate voltage on the electrode
153
, the inversion layer
154
is formed just below the electrode
153
, and the inversion layer
154
conducts electric current between the diffusion layer
151
and the diffusion layer
152
, thus flowing a current proportional to a capacitance between the electrode
153
and the semiconductor substrate
150
.
Then, if the acceleration is exerted in the X direction perpendicular to the semiconductor substrate
150
, the electrode
153
deforms in a direction perpendicular to the substrate surface and the distance between the electrode
153
and the substrate surface changes. As a result, the capacitance changes, and then the amount of current flowing in the inversion layer
154
change, thereby measuring the acceleration proportional to the change in the amount of current.
Further, in
FIG. 4B
, when a differential amplifier circuit is structured by using two MOS transistors, the acceleration in the X axis effects both of the two MOS transistors
160
,
161
equally, so that the amount of currents flowing in the MOS transistors
160
,
161
do not have a difference. However, with respect to the displacement of the electrodes in the Z direction, an overlapping width Wa of the MOS transistor
160
and an overlapping width Wb in the MOS transistor
161
are in such a relationship that when the one increases, the other decreases. As a result of this relationship, the currents flowing in the two MOS transistors
160
,
161
also vary according to respective overlapping widths Wa, Wb, thus detecting only the acceleration in the Z direction.
In various systems having pressure sensors or accelerometers, miniaturizing the sensors and making these power consumption lower have been in progress. Recently, there is a sensor module in which a peripheral circuit as well as sensor bodies are integrated on a sensor substrate.
In addition, in consideration of the connection of this kind of modules to computers, a sensor equipped with an A/D (analog to digital) conversion function that directly gives a digital output has been developed. As an A/D converter circuit for integrating this kind of sensor, there is a simplified A/D converter in which a plurality of CMOS inverters each of which has a different logic threshold are parallel-connected.
In the first example according to the prior art, as shown in
FIG. 1
above, semiconductor diffusion resistance layers as piezo-resistance elements are formed on the flexible part
108
and a bridge circuit is structured by the four piezo-resistance elements, as a unit, thus measuring the acceleration by means of the piezo-resistance effect.
However, in the case of a structure with the use of the semiconductor diffusion resistance layers, there is no function for adjusting the fabrication imbalance in the voltage of an operating point of the bridge circuit and hence offset voltage cannot be reduced sufficiently. Therefore, it is necessary to provide separately a compensation circuit for compensating an offset voltage due to the fabrication imbalance, thus increasing a fabrication cost.
Furthermore, in a configuration with such a bridge circuit, an amplifying function for amplifying a detected signal cannot be provided and hence the output signal level is low. Especially in ac

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