Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1998-10-20
2001-09-25
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514320, C073S514340, C438S052000
Reexamination Certificate
active
06293149
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flexure transducer element and a method of producing the same which is used for a semiconductor acceleration sensor having a both end supported beam structure and used for an automobile, an aircraft or a domestic electric appliance, and also relates to an acceleration sensor including such an element. For example, such a sensor can be used for sensing an acceleration by separately obtaining an X-axis component, Y-axis component and Z-axis component of the acceleration applied thereto with respect to an X-Y-Z coordinate rectangular system having the three axes.
2. Description of Background Information
The acceleration sensor as described above is disclosed in U.S. Pat. No. 5,485,749. The sensor is a piezoresistor-type acceleration sensor which converts a mechanical flexure (or a strain) of a member generated by an acceleration into an electric signal, and one example of such a sensor is shown in
FIGS. 18
(a schematic perspective view) and
19
(a cross sectional view taken along a line A-A′ in FIG.
18
).
The acceleration sensor
500
includes a flexure transducer element
502
and a bottom cover
504
. The flexure transducer element
502
includes a frame
506
and a sheet member
508
. The frame
506
has an upper surface
510
and a lower surface
512
which is supported by a support member
514
. The sheet member
508
includes flexible parts
515
and a center part
516
(a portion surrounded by the dash and dot lines in FIG.
18
). The flexible part
515
extends outward from the center part
516
and integrally connects with an inner edge
518
of the frame
502
. A weight
520
connects with the center part
516
of the sheet member
508
below the center part
516
.
An inward side surface
524
of the support member
514
is facing to an outward side surface
526
of the weight
520
through a first space
528
. Further, a second space
530
is present between the flexible parts
515
and the weight
520
, and connects with the first space
528
. In addition, there is a third space
532
which is surrounded by the frame
506
and the flexible parts
515
. The flexible parts
515
include a plurality of piezoresistors
534
and wirings (not shown) connected thereto on their surfaces.
The bottom cover
504
includes a periphery part
541
which defines a recess part
540
corresponding to and surrounding the weight
520
, and the support member
514
is bonded to the periphery part of the bottom cover
504
by an appropriate means such as anodic bonding. The bottom cover
504
functions as a stopper which prevents the sheet member
508
from being broken due to over-displacement of the weight when an excessive acceleration is applied.
When the acceleration sensor
500
as described above includes the plurality of the piezoresistors
534
, it can be used as the acceleration sensor which detects the acceleration by obtaining separately the X, Y and Z axis components of the acceleration applied to the sensor with respect to the X-Y-Z three axis regular coordinate defined by the X, Y and Z axes which regularly intersecting with one another (the X axis and the Y axis extend over the upper surface defined by the sheet member
508
and the frame
506
).
Interconnections between the frame
506
and the sheet member
508
as well as between the sheet member
508
and the weight
520
are such that when the acceleration is applied to the sensor
500
, concretely to the element
502
, at least a portion of the flexible part
515
which portion has the piezoresistor
534
is elastically deformed by the displacement of the weight
520
relative to the frame
506
(it is noted that the center part
516
of the sheet member
508
which is connected to the neck part
522
is substantially not deformed), and thereby a resistance change of the piezoresistor
534
is converted to an electric signal. By detecting the signal, the acceleration applied to the sensor is determined.
The production of the acceleration sensor as described above can be carried out based on a method disclosed in the U.S. Pat. No. 5,485,749, and concretely carried out as follows using a production sequence as shown in
FIG. 20
which shows schematic cross sectional views similar to FIG.
19
:
FIG.
20
(
a
): First, a silicone nitride films
602
and
604
are formed on the both surfaces of a first silicon substrate
600
from which the support member
514
and the weight
520
are to be formed.
FIG.
20
(
b
): Then, an opening
606
is formed by removing a portion of the silicon nitride film
602
which corresponds to the second space
530
, and an opening
608
is formed by removing a portion of the silicon nitride film
604
which corresponds to the first space
528
.
FIG.
20
(
c
): By digging from the openings
606
and
608
to form recess parts
610
and
612
respectively, and then remaining silicon film
602
is removed so that one surface of the first silicon substrate
600
is exposed, on which a second silicon substrate
616
is laminated so that a portion of the recess part
610
is formed into the second space
530
and the rest part is formed into the neck part
522
of the weight and the upper surface of the support member
514
.
FIG.
20
(
d
): In order that the flexible part
515
is deformed upon the application of the predetermined acceleration when finally completed as the sensor, the second silicon substrate
616
is thinned to a thickness (t) by grinding or etching, whereby the second silicon substrate is formed into the sheet member
508
and the frame
506
.
FIG.
20
(
e
): Then, the piezoresistors
618
are formed on the sheet member
508
of the thinned second silicon substrate
616
using diffusion of an impurity of which conductivity type is different from that of the second silicon substrate
616
.
FIG.
20
(
f
): Then, after wirings (not shown) connected to the piezoresistors
618
are formed, a first space
528
reaching the third space
530
is formed by anisotropic etching from the recess part
612
so that the weight
520
is connected to and supported integrally by the center part
516
of the second silicon substrate
616
through the neck part
522
.
Finally, the predetermined portion of the second silicon substrate
616
is etched so that the third space
532
(not shown) is formed, whereby the flexure transducer element
502
is obtained. It is noted that the silicon nitride film
604
on the bottom surface of the first silicon substrate may be optionally removed.
The element
502
thus obtained is bonded to a bottom cover
504
(not shown in FIG.
20
), which results in the piezoresistor-type acceleration sensor.
Alternatively, the following method is also known: the second space
530
is not formed directly from the substrate, but a portion which corresponds to the second space is once formed as a sacrificial layer of a polysilicon, and then the sacrificial layer is removed by supplying an etchant through the first space
528
after the first space
528
has been formed (see Japanese Patent Kokai Publication No. 7-234242 and its counterpart foreign patent applications if any and U.S. Pat. No. 5,395,802).
In such an acceleration sensor, the acceleration to be detected is converted to a flexure of the flexible part as at least a portion of the sheet member, so that the resistance of the piezoresitor formed on the flexible part is changed by means of the flexure, whereby finally the acceleration is converted to the electric signal.
Therefore, the sensitivity of the semiconductor acceleration sensor is controlled by particularly the thickness of the flexible part of the sheet member which is elastically deformed (or flexed). That is, when the flexible part becomes thicker, the sensitivity becomes worse, and the sensitivity is affected by scattering of the thickness of the flexible part. Thus, the uniform and precise control of the thickness of the sheet member is important in the production process of the semiconductor acceleration sensor.
As another type of the sensor, an electrostat
Ishida Takuro
Kamakura Masanao
Kasano Fumihiro
Nakamura Takuro
Oka Naomasa
Greenblum & Bernstein P.L.C.
Matsushita Electric & Works Ltd.
Moller Richard A.
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