Semiconductor capacitive acceleration sensor

A semiconductor capacitive acceleration sensor is configured so that it can be constructed in a small size, it has a high detection sensitivity, and the detection output characteristic is linear. The sensor comprises a semiconductor substrate 1; supporters 21A to 21D each of which is made of a conductive semiconductor, and which are disposed on the upper face of the semiconductor substrate 1 through an insulating layer 1A so as to be placed at positions corresponding to corners of a quadrilateral; beams 26A to 26D which respectively have ends connected with the supporters, which coincide with each other when the beams are rotated, and which elongate in the side directions of the quadrilateral; a movable electrode 23 which has a quadrilateral shape, and which is disposed so as to be separated from the beams by a predetermined distance; connectors 27A to 27D which respectively connect the movable electrode 23 with the beams; and a stationary electrode 31 which is disposed so as to be separated from the lower face of the movable electrode 23 by a predetermined distance.

BACKGROUND OF THE INVENTION 
This invention relates to a microminiature semiconductor capacitive 
acceleration sensor which detects, for example, an acceleration state, a 
joggling state of an automobile and processes a detected signal so as to 
be used in various controls. 
A technique has been developed in which a silicon layer made of, for 
example, polysilicon, and a sacrifice layer made of, for example, PSG 
(Phospho Silicate Glass) are formed so as to constitute a multilayer 
structure, the multilayer structure is processed by a micromachinning 
technique, and then the sacrifice layer is removed away by hydrofluoric 
acid or the like. Hereinafter, this technique is referred to as 
"multilayer micromachinning technique". A microminiature semiconductor 
capacitive acceleration sensor has been developed with using such a 
technique. 
FIG. 5 is a perspective view showing an example of a conventional 
semiconductor capacitive acceleration sensor which is produced with using 
the multilayer micromachinning technique. The sensor comprises: a 
semiconductor substrate 1; a supporter 21 which is made of polysilicon and 
disposed on the upper face of the semiconductor substrate 1 through an 
insulating layer 1A made of silicon oxide or the like; beams 22A and 22B 
each of which has one end perpendicularly connected with the supporter 21, 
which horizontally elongate in parallel, and which have the same length; a 
movable electrode 23 which is connected with the other ends of the beams 
22A and 22B, and which horizontally elongate; a stationary electrode 31 
which is disposed on the upper face of the silicon substrate 1 through the 
insulating layer 1A, so as to be separated from the lower face of the 
movable electrode 23 by a predetermined distance; supporters 42A and 42B 
which are disposed on the upper face of the silicon substrate 1 through 
the insulating layer 1A; and a stationary electrode 41 which is connected 
at the periphery with the supporters 42A and 42B, and which is disposed so 
as to be separated from the upper face of the movable electrode 23 by a 
predetermined distance. A terminal M is drawn out from the movable 
electrode 23 through the beams 22A and 22B and the supporter 21, a 
terminal S.sub.1 from the stationary electrode 31, and a terminal S.sub.2 
from the stationary electrode 41 through the supporter 42B. 
The polysilicon is doped with an impurity so that the specific resistance 
is reduced to, for example, about 1 .OMEGA.cm or the polysilicon is 
conductive. It is a matter of course that single crystal silicon which is 
doped with an impurity so as to become conductive may be used in place of 
polysilicon. However, a sensor using polysilicon can be produced at a 
lower cost than that using single crystal silicon (this is also applicable 
to the sensors described below). 
The semiconductor capacitive acceleration sensor operates in the following 
manner: When acceleration is applied to the movable electrode 23 in the 
vertical direction, the movable electrode 23 receives a force in the 
vertical direction so that the beams 22A and 22B is bent while their ends 
connected with the supporter 21 function as fulcrums, whereby the movable 
electrode 23 is rotated in one of the directions of arrows P. For example, 
the rotation of the movable electrode 23 causes the distance between the 
movable electrode 23 and the stationary electrode 31 to be reduced, 
thereby increasing the electrostatic capacity between the electrodes, and, 
in contrast, the distance between the movable electrode 23 and the 
stationary electrode 41 to be increased, thereby decreasing the 
electrostatic capacity between the electrodes. The values of these 
electrostatic capacities are respectively obtained through the terminals M 
and S.sub.1, and M and S.sub.2, and then subjected to a signal processing 
by a differential amplifier or the like, thereby detecting the applied 
acceleration. 
FIG. 6 is a perspective view showing another example of a conventional 
semiconductor capacitive acceleration sensor which is similarly produced 
with using the multilayer micromachinning technique. The sensor comprises: 
a semiconductor substrate 1; a supporter 21 which is made of polysilicon 
and disposed on the upper face of the semiconductor substrate 1 through an 
insulating layer 1A; a beam 24A which has one end connected with the 
supporter 21, and which horizontally elongates; a beam 24B which has one 
end connected with the supporter 21, which has the same length as the beam 
24A, and which is oppositely directed; and a movable electrode 23 which 
horizontally elongates. The movable electrode 23 has a rectangular window 
which is sifted from the center of gravity of the electrode in one lateral 
direction, for example, the rightward direction. The other ends of the 
beams 22A and 22B are respectively connected with the sides of the window 
which are in the longitudinal direction of the window. The sensor further 
comprises: a stationary electrode 31 which is disposed on the upper face 
of the silicon substrate 1 through the insulating layer 1A, so as to be 
separated from one side portion of the lower face of the movable electrode 
23 with respect to the beams 24A and 24B (in FIG. 6, the left portion of 
the lower face) by a predetermined distance; and a stationary electrode 41 
which is disposed on the upper face of the silicon substrate 1 through the 
insulating layer 1A, so as to be separated from the other portion of the 
lower face of the movable electrode 23 (i.e., the right portion of the 
lower face) by a predetermined distance. A terminal M is drawn out from 
the movable electrode 23 through the beam 24A (24B) and the supporter 21, 
a terminal S.sub.1 from the stationary electrode 31, and a terminal 
S.sub.2 from the stationary electrode 41. 
The semiconductor capacitive acceleration sensor operates in the following 
manner: When acceleration is applied to the movable electrode 23 in the 
vertical direction, the right and left portions of the movable electrode 
23 receive forces in the vertical direction, respectively. Since the left 
portion is heavier than the right portion, the beams 24A and 24B are 
twisted while their ends connected with the supporter 21 function as 
fulcrums, whereby the movable electrode 23 is rotated in one of the 
directions of arrows Q. For example, the rotation of the movable electrode 
23 causes the distance between the movable electrode 23 and the stationary 
electrode 31 to be reduced, thereby increasing the electrostatic capacity 
between the electrodes, and, in contrast, the distance between the movable 
electrode 23 and the stationary electrode 41 to be increased, thereby 
decreasing the electrostatic capacity between the electrodes. The values 
of these electrostatic capacities are respectively obtained through the 
terminals M and S.sub.1, and M and S.sub.2, and then subjected to a signal 
processing by a differential amplifier or the like, thereby detecting the 
applied acceleration. 
FIG. 7 is a perspective view showing a further example of a semiconductor 
capacitive acceleration sensor of the prior art which is similarly 
produced with using the multilayer micromachinning technique. The sensor 
comprises: a semiconductor substrate 1; supporters 21A and 21B each of 
which is made of polysilicon and disposed on the upper face of the 
semiconductor substrate 1 through an insulating layer 1A; beams 24A and 
24B which respectively have ends connected with the supporters 21A and 
21B, and which elongate horizontally opposingly; a movable electrode 23 
which is connected between the other ends of the beams 24A and 24B, and 
which horizontally elongates; a stationary electrode 31 which is disposed 
on the upper face of the silicon substrate 1 through the insulating layer 
1A, so as to be separated from one side portion of the lower face of the 
movable electrode 23 with respect to the beams 24A and 24B (in FIG. 7, the 
rear portion of the lower face) by a predetermined distance; and a 
stationary electrode 41 which is disposed on the upper face of the silicon 
substrate 23 through the insulating layer 1A, so as to be separated from 
the other portion of the lower face of the movable electrode 23 (i.e., the 
front portion of the lower face) by a predetermined distance. A terminal M 
is drawn out from the movable electrode 23 through the beam 24A and the 
supporter 21A, a terminal S.sub.1 from the stationary electrode 31, and a 
terminal S.sub.2 from the stationary electrode 41. The reference numeral 
25 designates a weight connected with the rear portion of the movable 
electrode 23. 
The semiconductor capacitive acceleration sensor operates in the following 
manner: When acceleration is applied to the movable electrode 23 in the 
vertical direction, the rear and front portions of the movable electrode 
23 receive forces in the vertical direction, respectively. Since the rear 
portion is heavier than the front portion, the beams 24A and 24B are 
twisted while their ends connected with the supporters 21A and 21B 
function as fulcrums, whereby the movable electrode 23 is rotated in one 
of the directions of arrows R. For example, the rotation of the movable 
electrode 23 causes the distance between the movable electrode 23 and the 
stationary electrode 31 to be reduced, thereby increasing the 
electrostatic capacity between the electrodes, and, in contrast, the 
distance between the movable electrode 23 and the stationary electrode 41 
to be increased, thereby decreasing the electrostatic capacity between the 
electrodes. The values of these electrostatic capacities are respectively 
obtained through the terminals M and S.sub.1, and M and S.sub.2, and then 
subjected to a signal processing by a differential amplifier, etc., 
thereby detecting the applied acceleration. 
FIGS. 8A and 8B show a still further example of a semiconductor capacitive 
acceleration sensor of the prior art which is similarly produced with 
using the multilayer micromachinning technique. FIG. 8A is a perspective 
view, and FIG. 8B is a sectional view along the line C--C of FIG. 8A. The 
sensor comprises: a semiconductor substrate 1; supporters 21A to 21D each 
of which is made of polysilicon, and which are disposed on the upper face 
of the semiconductor substrate 1 through an insulating layer 1A so as to 
be placed at positions corresponding to corners of a quadrilateral; beams 
22A to 22D which respectively have ends connected with the respective 
supporters 21A to 21D, which coincide with each other when the beams are 
rotated by 90.degree., and which elongate in the diagonal directions of 
the quadrilateral; a movable electrode 23 which is connected with the 
other ends of the beams 22A to 22D; a stationary electrode 31 which is 
disposed on the upper face of the silicon substrate 1 through the 
insulating layer 1A, so as to be separated from the lower face of the 
movable electrode 23; supporters 42A and 42B which is disposed on the 
upper face of the silicon substrate 1 through the insulating layer 1A; and 
a stationary electrode 41 which is connected at a periphery with the 
supporters 42A and 42B, so as to be separated from one portion of the 
upper face of the movable electrode 23 (in FIGS. 8A and 8B, the left 
portion of the upper face) by a predetermined distance. A terminal M is 
drawn out from the movable electrode 23 through the beam 22A and the 
supporter 21B, a terminal S.sub.1 from the stationary electrode 31, and a 
terminal S.sub.2 from the stationary electrode 41 through the supporter 
42A. 
The semiconductor capacitive acceleration sensor operates in the following 
manner: When acceleration is applied to the movable electrode 23 in the 
vertical direction, the movable electrode 23 receives a force in the 
vertical direction to be moved vertically. For example, the vertical 
movement of the movable electrode 23 causes the distance between the 
movable electrode 23 and the stationary electrode 31 to be reduced, 
thereby increasing the electrostatic capacity between the electrodes, and, 
in contrast, the distance between the movable electrode 23 and the 
stationary electrode 41 to be increased, thereby decreasing the 
electrostatic capacity between the electrodes. The values of these 
electrostatic capacities are respectively obtained through the terminals M 
and S.sub.1, and M and S.sub.2, and then subjected to a signal processing 
by a differential amplifier, etc., thereby detecting the applied 
acceleration. 
In the semiconductor capacitive acceleration sensors shown in FIGS. 5 to 7, 
when acceleration is applied, the movable electrode is rotated with 
respect to the stationary electrodes, and the changes of the electrode 
gaps caused by the rotation of the movable electrodes are detected as the 
changes of the electrostatic capacities. The amount of the rotation of the 
movable electrode is not proportional to that of each change of the 
electrode gap, thereby producing a problem in that the detection output 
characteristic is not linear. In the semiconductor capacitive acceleration 
sensor shown in FIGS. 8A and 8B, when acceleration is applied, the movable 
electrode is vertically moved with respect to the stationary electrode, 
and the changes of the electrode gaps caused by the vertical movement of 
the movable electrode are detected as the changes of the electrostatic 
capacities. The amount of the vertical movement of the movable electrode 
is proportional to that of each change of the electrode gap, so that 
detection output shows a liner characteristic. Since the movable electrode 
is supported by four short beams, however, there is a problem in that the 
amount of the vertical movement of the movable electrode is so small that 
the detection sensitivity is low (if the beams are lengthened as they are 
so as to solve this problem, the device size is increased). In the 
semiconductor acceleration sensor shown in FIGS. 8A and 8B, moreover, the 
one stationary electrode (designated by 31 in FIGS. 8A and 8B) and the 
other stationary electrode (designated by 41 in FIGS. 8A and 8B) which 
respectively produce electrostatic capacities changing in the manners 
contradictory to each other have different areas, and hence the values 
(absolute values) of the electrostatic capacities are different from each 
other. This causes the circuit for processing signals, for example, the 
differential amplifier to be complicated, thereby increasing the 
production cost. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a semiconductor capacitive 
acceleration sensor which can be constructed in a small size, which has a 
high detection sensitivity, and in which the detection output 
characteristic is linear. In a sensor wherein electrostatic capacities 
respectively change in manners contradictory to each other, moreover, one 
stationary electrode and the other stationary electrode which respectively 
produce the electrostatic capacities changing in the manners contradictory 
to each other are configured so as to have a substantially same area. 
In order to attain the object, the semiconductor capacitive acceleration 
sensor of the invention comprises: a semiconductor substrate; a plurality 
of supporters which are made of a conductive semiconductor and disposed on 
an upper face of the semiconductor substrate through an insulating layer, 
the supporters being placed at positions which correspond to corners of a 
regular polygon, respectively; beams which are respectively connected with 
the supporters at one end, and which elongate along sides of the regular 
polygon, the beams coinciding with each other when the beams are moved by 
rotation; a movable electrode which is disposed with being separated from 
the beams by a predetermined distance, the electrode having a shape of the 
regular polygon; connectors which connect the movable electrode with the 
other ends of the beams, respectively; and a stationary electrode which is 
disposed on the upper face of the semiconductor substrate through the 
insulating layer, the stationary electrode being separated from a lower 
face of the movable electrode by a predetermined distance. 
Alternatively, the semiconductor capacitive acceleration sensor comprises: 
a semiconductor substrate; a plurality of supporters which are made of a 
conductive semiconductor and disposed on an upper face of the 
semiconductor substrate through an insulating layer, the supporters being 
placed at positions which correspond to corners of a regular polygon, 
respectively; beams which are respectively connected with the supporters 
at one end, and which elongate along sides of the regular polygon, the 
beams coinciding with each other when the beams are moved by rotation; a 
movable electrode which is disposed with being separated from the beams by 
a predetermined distance, the electrode having a shape of the regular 
polygon; connectors which connect the movable electrode with the other 
ends of the beams, respectively; a first stationary electrode which is 
disposed on the upper face of the semiconductor substrate through the 
insulating layer, the stationary electrode being separated from a lower 
face of the movable electrode by a predetermined distance; a plurality of 
supporters which are disposed on the upper face of the semiconductor 
substrate through the insulating layer; and a second stationary electrode 
which is connected at a periphery with the supporters, and which is 
separated from an upper face of the movable electrode by a predetermined 
distance. 
It is more preferable that, in the semiconductor capacitive acceleration 
sensors, a metal film is attached to the movable electrode. 
In the semiconductor capacitive acceleration sensor of the invention, the 
movable electrode has a regular polygonal shape, and is uniformly 
supported at the periphery by the plural beams which are respectively 
disposed in parallel to the sides of the regular polygon with being 
separated therefrom by a predetermined distance. When acceleration is 
applied in the vertical direction, the movable electrode is vertically 
moved with respect to the stationary electrode. The other ends of the 
beams are connected through the connectors with end portions of the 
regular polygon of the movable electrode that are remote from the 
supporters to which the ends of the beams are connected, respectively, 
whereby the lengths of the beams can be lengthened in accordance with the 
length of the sides of the regular polygon. Therefore, the sensor of the 
invention can be constructed as a semiconductor capacitive acceleration 
sensor which is miniaturized, which has a high detection sensitivity, and 
in which the detection output characteristic is linear. 
In a sensor wherein electrostatic capacities respectively change in manners 
contradictory to each other, one stationary electrode is disposed so as to 
be separated from the lower face of the movable electrode by a 
predetermined distance, and another stationary electrode is similarly 
disposed so as to be separated from the whole of the upper face of the 
movable electrode by a predetermined distance. Consequently, the area of 
the one stationary electrode can be made substantially equal to that of 
the other stationary electrode. 
In the semiconductor capacitive acceleration sensors, moreover, the movable 
electrode is provided with a weight. Accordingly, the amount of the 
movement of the movable electrode due to acceleration is increased so that 
the detection sensitivity is further improved.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
First Embodiment: 
FIGS. 1A to 1C shows an embodiment of the semiconductor capacitive 
acceleration sensor of the invention. FIG. 1A is a plan view, FIG. 1B is a 
sectional view along the line A--A of FIG. 1A, and FIG. 1C is a sectional 
view along the line B--B of FIG. 1A. The sensor comprises: a semiconductor 
substrate 1; supporters 21A to 21D each of which is made of polysilicon, 
and which are disposed on the upper face of the semiconductor substrate 1 
through an insulating layer 1A so as to be placed at positions 
corresponding to corners of a quadrilateral; beams 26A to 26D which 
respectively have ends connected with the supporters 21A to 21D, which 
coincide with each other when the beams are rotated by 90.degree., and 
which respectively elongate in the side directions of the quadrilateral; a 
movable electrode 23 which is disposed so as to be separated from the 
beams 26A to 26D by a predetermined distance .iota.; connectors 27A to 27D 
which respectively connect the movable electrode 23 with the other ends of 
the beams 26A to 26D; and a stationary electrode 31 which is disposed on 
the upper face of the silicon substrate 1 through the insulating layer 1A, 
so as to be separated from the lower face of the movable electrode 23 by a 
predetermined distance. A terminal M is drawn out from the movable 
electrode 23 through the connector 27A, the beam 26A and the supporter 
21A, and a terminal S.sub.1 from the stationary electrode 31. 
The semiconductor capacitive acceleration sensor operates in the following 
manner: When acceleration is applied to the movable electrode 23 in the 
vertical direction, the movable electrode 23 receives a force in the 
vertical direction to be moved vertically. For example, the vertical 
movement of the movable electrode 23 causes the distance between the 
movable electrode 23 and the stationary electrode 31 to be reduced, 
thereby increasing the electrostatic capacity between the electrodes. The 
value of the electrostatic capacity is obtained through the terminals M 
and S.sub.1, and then subjected to a signal processing by a differential 
amplifier, etc., thereby detecting the applied acceleration. 
In the semiconductor capacitive acceleration sensor, the movable electrode 
23 has a quadrilateral shape, and is uniformly supported at the periphery 
by the beams 26A to 26D which are respectively disposed in parallel to the 
sides of the quadrilateral with being separated therefrom by the 
predetermined distance .iota.. When acceleration is applied in the 
vertical direction, the movable electrode 23 is vertically moved with 
respect to the stationary electrode 31. Since the other ends of the beams 
are respectively connected through the connectors with end portions of the 
regular polygon of the movable electrode 23 that are remote from the ends 
of the beams with which the supporters are connected (the state shown in 
FIG. 1), the lengths of the beams can be lengthened in accordance with the 
length of the sides of the regular polygon. Therefore, the sensor can be 
constructed as a semiconductor capacitive acceleration sensor which is 
miniaturized, which has a high detection sensitivity, and in which the 
detection output characteristic is linear. 
Second Embodiment: 
FIGS. 2A to 2C show a second embodiment of the semiconductor capacitive 
acceleration sensor of the invention. FIG. 2A is a plan view, FIG. 2B is a 
sectional view along the line A--A of FIG. 2A, and FIG. 2C is a sectional 
view along the line B--B of FIG. 2A. In addition to the components shown 
in FIG. 1, the sensor of FIG. 2 further comprises supporters 42A to 42D 
which are disposed on the upper face of the silicon substrate 1 through 
the insulating layer 1A, and a stationary electrode 41 which is connected 
at the periphery with the supporters 42A to 42D, and which is disposed so 
as to be separated from the upper face of the movable electrode 23 by a 
predetermined distance. A terminal S.sub.2 is drawn out from the 
stationary electrode 41 through the supporter 42C. 
The semiconductor capacitive acceleration sensor operates in the following 
manner: When acceleration is applied to the movable electrode 23 in the 
vertical direction, for example, the distance between the movable 
electrode 23 and the stationary electrode 31 to be reduced, thereby 
increasing the electrostatic capacity between the electrodes, and, in 
contrast, the distance between the movable electrode 23 and the stationary 
electrode 41 to be increased, thereby decreasing the electrostatic 
capacity between the electrodes. The values of these electrostatic 
capacities are respectively obtained through the terminals M and S.sub.1, 
and M and S.sub.2, and then subjected to a signal processing by a 
differential amplifier or the like, thereby detecting the applied 
acceleration. 
The semiconductor capacitive acceleration sensor detects the electrostatic 
capacities which are respectively produced between the movable electrode 
23 and the stationary electrode 31, and the movable electrode 23 and the 
stationary electrode 41, and which respectively change in manners 
contradictory to each other. Therefore, the detection sensitivity is 
further improved. Since the stationary electrode 41 is disposed so as to 
oppose the whole of the upper face of the movable electrode, the areas of 
the stationary electrodes 31 and 41 can be made substantially equal to 
each other. Accordingly, the values (absolute values) of the electrostatic 
capacities between the stationary electrodes 31 and 41 and the movable 
electrode 23 are substantially equal to each other, so that, for example, 
the circuit of the differential amplifier can be simplified to reduce the 
production cost. 
FIGS. 3A to 3C and 4A to 4C show further embodiments of the semiconductor 
capacitive acceleration sensor of the invention. In the figures, FIGS. 3A 
and 4A are plan views, FIGS. 3B and 4B are sectional views along the line 
A--A of FIGS. 3A and 4A, and FIGS. 3C and 4C are sectional views along the 
line B--B of FIGS. 3A and 4A. These embodiments are respectively obtained 
by modifying the sensors of FIGS. 1 and 2 so that a metal film 29 is 
attached to the movable electrode 23. The provision of the metal film 29 
increases the weight of the movable electrode 23, and hence the detection 
sensitivity of each sensor is further improved. Since a metal has a large 
specific gravity, the provision of the metal film 29 hardly causes the 
movable electrode portion to be enlarged. However, it is a matter of 
course that the gap between the movable electrode 23 and the stationary 
electrode 41 is corrected considering the thickness of the metal film 29. 
The semiconductor capacitive acceleration sensor of the invention has a 
small size, and a high detection sensitivity, and a linear detection 
output characteristic. Therefore, the sensor is preferably used in various 
purposes including an automobile. In a sensor wherein electrostatic 
capacities for detecting acceleration respectively change in manners 
contradictory to each other, moreover, the values (absolute values) of the 
electrostatic capacities changing in manners contradictory to each other 
are equal to each other. Therefore, a circuit for processing signals, for 
example, a differential amplifier can be simplified so that the production 
cost is reduced.