Acceleration sensor

Details Acceleration sensor Acceleration sensor

2003-03-11

2004-07-20

Kwok, Helen (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

Reexamination Certificate

active

06763719

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. 13
, and a sectional view taken along the line XIV—XIV in
FIG. 13
is shown in
FIG. 14
, and a perspective view is shown in FIG.
15
. The acceleration sensor
500
has elastic support arms
530
each of a beam structure, constituted by a thin portion of a silicon single crystal substrate. A mass portion
520
in a center, which is constituted by a thick portion of a silicon single crystal substrate, and a frame
510
in a periphery thereof are connected by the elastic support arms
530
. A plurality of strain gauges
560
are formed in each axial direction on the elastic support arms
530
.
An entire structure will be explained, referring to
FIG. 13
, FIG.
14
and FIG.
15
. The sensor
500
has the mass portion
520
constituted by the thick portion of the silicon single crystal substrate, a frame
510
placed to surround the mass portion
520
, and two pairs of elastic support arms
530
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
520
and the frame
510
. 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. 13
is measured by the four strain gauges
560
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
560
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
560
. By making four L-shaped through-holes
550
in the silicon single crystal substrate having the size of the frame
510
, the mass portion
520
in the center, the frame
510
in the periphery and the support arms
530
bridging them are formed, and by making the support arm portions thin, the acceleration sensor is constructed to be deformable and highly sensitive.
Although the acceleration in the Z-axis direction is detected or measured by both the strain gauges
560
that detect X-axis acceleration and the strain gauges
560
that detect Y-axis acceleration in the acceleration sensor
500
shown in
FIGS. 13 through 15
, it is preferable that a circuit detecting Z-axis acceleration is separated from a circuit detecting X-axis/Y-axis acceleration. In the co-pending patent application, Chinese Patent Application N/A (Feb. 12, 2003), European Patent Application 03002164.6 (Feb. 3, 2003), Korean Patent Application 10-2003-008738 (Feb. 12, 2003) and U.S. Ser. No. 10/357,408 (Feb. 4, 2003) filed by the same assignee based on Japanese Patent Application 2002-33696 of Feb. 12, 2002, strain gauges for detecting Z-axis acceleration are different from strain gauges for detecting X-axis acceleration, while the Z-axis strain gauges are located on elastic support arms in X-axis direction in the same way as X-axis strain gauges.
In
FIG. 16
, an acceleration sensor
600
has a mass portion
620
in a center, a thick frame
610
around it, and elastic support arms
631
,
632
,
633
and
634
for bridging the mass portion
620
and the thick frame
610
. Since the elastic support arms
631
,
632
,
633
and
634
are thin, the mass portion deforms the elastic support arms when acceleration acts on the mass portion
620
. Large deformation of each of the elastic support arms occurs to end portions of the elastic support arms, that is, connecting portions of an edge of a top surface of the mass portion and the elastic support arms, and connecting portions of inside edges of a top surface of the thick frame and the elastic support arms. In order to enhance the sensitivity of the acceleration sensor, strain gauges are attached at the portions of the elastic support arms, which are deformed most by the acceleration.
In the acceleration sensor
600
in
FIG. 16
, strain gauges
661
,
662
,
663
and
664
for detecting acceleration in the X-axis direction, and strain gauges
681
,
682
,
683
and
684
for detecting acceleration in the Z-axis direction are placed on the elastic support arms
631
and
633
. It is generally known that there exists the relationship as shown in
FIG. 17
between sensitivities of the X-axis strain gauge and the Z-axis strain gauge (output with respect to acceleration 1 G, and drive voltage 1 V). When the acceleration of 1 G in the X-axis direction acts on the mass portion, bending moment applied to the elastic support arm is proportional to a product of a distance from the top surface of the mass portion to a center of gravity of the mass portion by a mass of the mass portion. Since the bending moment is proportional to the distance and the mass, the sensitivity in the X-axis direction changes as a quadric function with respect to the thickness of the mass portion. On the other hand, when the acceleration of 1 G acts in the Z-axis direction, the bending moment applied to the elastic support arm is proportional to a product of length of the elastic support arm and mass of the mass portion. When the thickness of the mass portion is changed, the length of the elastic support arm does not change, but only the mass of the mass portion changes, and therefore the sensitivity of the Z-axis becomes a linear function.
When the acceleration sensor
600
shown in
FIG. 16
is produced with use of a Si single crystal substrate which is generally used in semiconductor fabrication, thickness of the Si single crystal substrate is 625 &mgr;m or 525 &mgr;m, and therefore as can be seen from FIG.
17
, the sensitivity of the Z-axis strain gauge becomes larger than that of the X-axis strain gauge. If the sensitivities of the Z-axis strain gauge and the X-axis strain gauge are about the same, the amplifiers having about the same output amplification factors can be used for the Z-axis strain gauge and the X-axis strain gauge. In order to make the sensitivity of the Z-axis strain gauge the same as that of the X-axis strain gauge, it is suitable to make the acceleration sensor
600
with use of the Si single crystal substrate of thickness of about 800 &mgr;m, but such a thick Si single crystal substrate as this has to be especially prepared only for this acceleration sensor, and this increases the cost of the acceleration sensor.
Alternatively, it is theoretically possible to change piezo-properties by changing impurity concentrations of the piezoresistors used for the Z-axis strain gauge and X(Y)-axis strain gauge. Howe

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