Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2003-02-04
2003-12-16
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06662659
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acceleration sensor for detecting acceleration, which is used for toys, automobiles, aircrafts, portable terminals and the like, and particularly to an acceleration sensor that can be produced using a semiconductor technology.
2. Description of the Related Art
Acceleration sensors utilizing a change in physical quantity such as a piezo resistance effect and a change in electrostatic capacity have been developed and commercialized. These acceleration sensors can be widely used in various fields, but recently, such small-sized acceleration sensors as can detect the acceleration in multi-axial directions at one time with high sensitivity are demanded.
Since silicon single crystal becomes an ideal elastic body due to the extreme paucity of lattice defect and since a semiconductor process technology can be applied for it without large modification, much attention is paid to a piezo resistance effect type semiconductor acceleration sensor in which a thin elastic support portion is provided at a silicon single crystal substrate, and the stress applied to the thin elastic support portion is converted into an electric signal by a strain gauge, for example, a piezo resistance effect element, to be an output.
As a conventional triaxial acceleration sensor, there is the one disclosed in, for example, Japanese Laid-Open Patent No. 63-169078, and its plan view is shown in
FIG. 11
, and a sectional view taken along the XII—XII line in
FIG. 11
is shown in
FIG. 12
, and a perspective view is shown in FIG.
13
. The acceleration sensor
200
has elastic support arms
230
each of a beam structure, constituted by a thin portion of a silicon single crystal substrate. A mass portion
220
in a center, which is constituted by a thick portion of a silicon single crystal substrate, and a frame
210
in a periphery thereof are connected by the elastic support arms
230
. A plurality of strain gauges
240
are formed in each axial direction on the elastic support arms
230
.
An entire structure will be explained, referring to
FIG. 11
, FIG.
12
and FIG.
13
. The sensor
200
has the mass portion
220
constituted by the thick portion of the silicon single crystal substrate, a frame
210
placed to surround the mass portion
220
, and two pairs of elastic support arms
230
in a beam form, which are perpendicular to each other and each constituted by the thin portion of the silicon single crystal substrate to bridge the mass portion
220
and the frame
210
. When the acceleration works, the mass portion moves in the frame to deform the elastic support arms, and thus the deformation is detected by the strain gauges provided on the elastic support arms to obtain the acceleration that works. The acceleration in an X-axis direction in
FIG. 11
is measured by the four strain gauges
240
provided on the elastic support arms extending in the X-axis direction, and the acceleration in a Y-axis direction is measured by the four strain gauges
240
provided on the elastic support arms extending in the Y-axis direction. The acceleration in a Z-axis direction is measured by means of all the strain gauges
240
. By making four L-shaped through-holes
250
in the silicon single crystal substrate having the size of the frame
210
, the mass portion
220
in the center, the frame
210
in the periphery and the support arms
230
bridging them are formed, and by making the support arm portions thin, the acceleration sensor is constructed to be deformable and highly sensitive.
In the above-described acceleration sensor, to enhance sensitivity, it is effective to increase the volume of the mass portion
220
to increase the mass, or to increase the length of the elastic support arms
230
, and as is generally well-known, the sensitivity increases substantially in proportion to the mass of the mass portion and the length of the support arms. That is, the volume of the mass portion
220
is increased, or the length of the elastic support arms
230
are increased, whereby the elastic support arm
230
becomes more deformable and the stress can be effectively transmitted to the strain gauges, thus enhancing sensitivity. However, increasing the mass portion
220
and increasing the length of the elastic support arms
230
are mutually contradictory, and both of them are not mutually compatible especially under the condition that the chip size is kept constant, or when reduction in size is planned. That is, if the mass portion
220
is made larger, the length of the elastic support arms
230
becomes smaller, and a great deal of improvement in sensitivity cannot be improved. Thus, glass pieces and the like are bonded on the back surface of the mass portion
220
in assembly process to increase the volume (that is, weight) of the mass portion
220
, whereby the sensitivity is enhanced. The length of the elastic support arms
230
cannot be made large, while the chip is made larger in the thickness direction (the thickness direction of the silicon single crystal substrate), whereby the mass of the mass portion
220
is increased to enhance the sensitivity. Accordingly, it is conventionally impossible to realize a compact and thin acceleration sensor with high sensitivity.
SUMMARY OF THE INVENTION
The present invention is accomplished in view of the above-described circumstances, and its object is to solve the above-described problem and provide a compact and thin acceleration sensor capable of enhancing sensitivity.
In order to solve the above-described problem, the present invention adopts an acceleration sensor as follows. That is, the acceleration sensor of the present invention comprises a mass portion provided in a center; a thick frame surrounding the mass portion with a predetermined distance from the mass portion; a plurality of elastic support arms extending from an edge of a top surface of the mass portion, bridging the top surface edge of the mass portion and an inside edge of a top surface of the thick frame and hanging the mass portion inside of the thick frame; and a plurality of strain gauges disposed on the elastic support arms. The mass portion has the top surface, a bottom surface opposite to the top surface and a plurality of side walls surrounding the mass portion between the top surface and the bottom surface of the mass portion. The thick frame has the top surface, a bottom surface opposite to the top surface and inside walls on inside surfaces, facing the mass portion, of the thick frame between the top surface and the bottom surface of the thick frame.
The acceleration sensor is made of a silicon single crystal wafer, preferably a SOI (Silicon-on-insulator) wafer, and can be constructed to have a thick-walled frame with its plane shape being substantially a square, a mass portion which is provided at a center of the thick-walled frame and is formed to be substantially a square, and four elastic support arms which connect centers of sides of the square on the top surface of the mass portion and centers of inner sides of the thick-walled frame in a square shape which is on the top surface of the thick frame. In the acceleration sensor made of a silicon single crystal wafer or a SOI wafer, the top surface of the thick-walled frame, the top surface of the mass portion and the top surfaces of the four elastic support arms are formed by using the surface of one side of the wafer, and therefore they are on substantially the same surface. A bottom surface of the thick-walled frame and a bottom surface of the mass portion are formed by using a surface of the other side of the wafer, and therefore they are on substantially the same surface. The elastic support arm is formed to be thin by being cut by etching or the like from the surface of the other side of the wafer, and therefore when it is made of the SOI wafer, it is formed of a remaining SiO
2
layer, or a laminated product of the SiO
2
layer and a silicon layer.
In the acceleration sensor of the present invention, at least either of the mass portion or the thic
Hitachi Metals Ltd.
Moller Richard A.
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