Acceleration sensor

A small-sized, reliable, low-cost acceleration sensor includes a working mechanism having a large joint surface. In this respect, an acceleration sensor 1 is arranged so that a diaphragm portion 2D is formed on a (100) face of a plate of single crystal silicon. The four corner portions of the joint surface 2A of a square frame are expanded in the center direction of the plate of single crystal silicon so as to form expanded portions 2B. Moreover, the (100) face of the square plate of single crystal silicon is covered with a mask having at least an octagonal opening 10E before being subjected to anisotropic etching, so that the expanded portions 2B directed to the center portion are formed in the respective four corner portions of the plate of single crystal silicon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an acceleration sensor which uses 
resistance elements and to a method for producing such an acceleration 
sensor. More particularly, the present invention relates to an 
acceleration sensor for detecting mechanical deformation in terms of 
variations in electrical resistance introduced by resistance elements 
formed in a working mechanism. In such a configuration, deformation is 
caused by an acceleration applied to the working mechanism which, in a 
preferred embodiment, is formed of a semiconductor substrate material. 
2. Discussion of the Related Art 
Sensors are known for detecting acceleration, force and the like by 
obtaining variations in electrical resistance that resistance elements 
undergo when working mechanisms in the sensors are subjected to elastic 
deformation due to the action of acceleration, force or the like exerted 
on the working mechanisms. In such sensors, a thin-walled portion capable 
of elastic deformation is formed as the working mechanism on a 
semiconductor substrate in the form of a plate of single crystal silicon. 
The resistance elements are also formed in the working mechanism. 
FIG. 8 is a plan view of a conventional acceleration sensor having a 
circular diaphragm portion. FIG. 9 is a sectional view of the sensor shown 
in FIG. 8. 
In FIGS. 8 and 9, an acceleration sensor 50 has a working mechanism 52 in 
the form of a plate of single crystal silicon as well as a pedestal 51. 
The working mechanism 52 has a thin-walled diaphragm 52D in its center. A 
weight portion 53 is coupled to the center of the diaphragm 52D and, upon 
undergoing acceleration, is displaced vertically and bilaterally in 
relation to the view shown in FIG. 9. With the displacement of the weight 
portion 53, the thin-walled diaphragm 52D is also displaced vertically and 
bilaterally. Resistance elements provided for the diaphragm 52D detect the 
displacement in terms of variations in electrical resistance, whereby 
acceleration is detected. 
The diaphragm portion is often designed so that its outer periphery is made 
circular. This is intended not only to attain highly sensitive properties, 
but also to secure reliability and mechanical strength of the sensor of 
the sort described above. 
Although the thin-walled diaphragm portion is formed by etching from a 
plate of single crystal silicon, various techniques have been devised to 
implement circular etching because the etching rate differs according to 
the crystalline orientation of the plate of single crystal silicon. 
For example, the thickness of the diaphragm portion has been regulated by 
first applying isotropic etching and then skillfully combining anisotropic 
etching and an etch stop technique. Therefore, the method of forming the 
diaphragm portion tends to become complicated and the problem is that such 
a method makes it difficult to improve the yield and tends to increase 
processing time. In other words, the conventional method requires 
increased levels of skill on the part of the worker. 
Consequently, one technique has been introduced in which the diaphragm 
portion is formed by anisotropic etching only. FIG. 10 is a plan view of a 
working mechanism 60 of an acceleration sensor having a square diaphragm 
portion 62 and a pedestal 61 which is positioned in the lower part 
thereof. 
Another technique that has been proposed is to subject the (110) crystal 
face of a plate of single crystal silicon to anisotropic etching using an 
octagonal mask pattern. 
FIG. 11 is a diagram illustrating stress distribution in the acceleration 
sensor of FIG. 10 based on the finite element method. Analysis under the 
finite element method (FEM analysis) is one of the simulation techniques 
used for analyzing structural properties, such as stress distribution, in 
each part in accordance with the solution of convergence obtained through 
numerical analysis, such as successive calculation and the like, by 
dividing an object to be examined into extremely small elements, creating 
mathematical models element by element, and assigning each model a 
space-temporal boundary condition together with an initial condition. 
FIG. 11 shows the results of calculation of the distribution of stresses 
applied to the square diaphragm portion 62 of FIG. 10 formed with a wafer 
of single crystal silicon. 
As shown in FIG. 11, when acceleration is exerted in the direction of G, 
stresses concentrate on parts perpendicular to the direction G out of the 
outer periphery of the diaphragm portion 62 and appear as stresses st1, 
st2. Other stresses concentrate on parts perpendicular thereto out of the 
periphery of the central square of the diaphragm portion 62 and appear as 
stresses st3, st4. 
FIG. 12 is a plan view illustrative of the structure of the working 
mechanism 70 of an acceleration sensor prepared by employing the 
aforementioned technique. As shown in FIG. 12, an octagonal diaphragm 
portion 72 is formed on the working mechanism 70 in the form of a plate of 
single crystal silicon having a rectangular plane. 
In an attempt to implement the aforementioned techniques of producing 
acceleration sensors and, more specifically, where such a working 
mechanism has the square diaphragm portion 62 of FIG. 10, for example, it 
is essential to make the area of the square diaphragm portion 62 equal to 
or greater than that of the aforementioned circular diaphragm portion (52D 
of FIG. 8) in order to secure sensitivity equal to that which is available 
from the circular diaphragm portion. Therefore, the area of the square 
diaphragm portion 62 is maximized to the extent possible within the 
working mechanism of limited dimensions. However, this can result in 
connection failures as the pedestal-to-diaphragm coupling area decreases. 
On the other hand, an anisotropic etching interface S71 in the working 
mechanism 70 having the octagonal diaphragm portion 72 of FIG. 12 forms an 
angle .theta.2 as small as approximately 35.degree. as shown in FIG. 13; 
this is also the case with the interface S73. Therefore, the dimensions of 
the working mechanism 70 have to be reduced to secure the 
pedestal-to-diaphragm coupling area. Actually, the top of a working 
mechanism measuring 8 mm by 6.8 mm respectively representing the 
horizontal length L2 and the vertical length L3 as shown in FIG. 12 is not 
reducible. This is disadvantageous and still poses a problem in that the 
L2 and L3 constitute an obstacle to reducing the size of the acceleration 
sensor. Moreover, the number of acceleration sensors to be cut out of a 
wafer tends to decrease and the disadvantage again is that such 
acceleration sensors become costly. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to an acceleration sensor 
and method for producing the same which substantially obviate one or more 
problems due to limitations and disadvantages of the related art. 
An object of the present invention is to solve and overcome the foregoing 
problems and disadvantages by providing a small-sized, reliable, low-cost 
acceleration sensor including a working mechanism having a large joint 
surface. 
Another object of the present invention is to provide a method for simply 
and easily producing an acceleration sensor. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure particularly pointed out in the written 
description and claims hereof as well as the appended drawings. 
To achieve these and other advantages and in accordance with the purpose of 
the present invention, as embodied and broadly described, an acceleration 
sensor comprises a working mechanism including a square plate of single 
crystal silicon, the working mechanism having a thin-walled diaphragm 
portion at a central portion of the working mechanism, and an edge portion 
of the working mechanism forming a joint surface of a square frame; a 
weight portion disposed at a center of the diaphragm portion; and a 
plurality of resistance elements, each of the resistance elements being 
one of incorporated in the diaphragm portion and fastened onto a surface 
of the diaphragm portion, and each of the resistance elements having a 
resistance which varies in response to telescopic displacement of the 
diaphragm portion when acceleration and force are applied to the working 
mechanism such that acceleration and force are detected according to 
variations in the resistance values of the resistance elements; wherein 
the diaphragm portion is formed on a (100) face of the plate of single 
crystal silicon and multiple corner portions on the joint surface of the 
square frame comprise expanded portions directed in a center direction of 
the plate of single crystal silicon. 
In another aspect, a method of producing an acceleration sensor comprises 
the steps of: covering a (100) face of a square plate of single crystal 
silicon with a mask having at least an octagonal opening; and subjecting 
the plate of single crystal silicon to anisotropic etching so as to form 
an expanded portion in each of four corner portions of the plate of single 
crystal silicon, the expanded portions being directed in a center 
direction of the plate of single crystal silicon. 
Under the method for producing the acceleration sensor according to the 
present invention, the coupling of the square diaphragm portion to a 
pedestal is strengthened by the expanded portions on the joint surface 
formed in the four corner portions of the outer periphery of the diaphragm 
portion. Consequently, any defective joint occurring during processing is 
eliminated and the processability is improved to ensure mass-producibility 
with a high yield at low cost. Further, a long life and stable operation 
can be achieved for sensors in use. 
Under the method of producing the acceleration sensor according to the 
present invention, moreover, the (100) face of the plate of single crystal 
silicon is covered with the mask having the octagonal opening before being 
subjected to anisotropic etching. Consequently, the expanded portions 
directed in the center direction are readily formed in the respective four 
corner portions of the plate of single crystal silicon, whereby the 
manufacture of the acceleration sensor thus structured is readily 
accomplished. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF PREED EMBODIMENT(S) 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
FIG. 1 is a plan view showing the structure of an acceleration sensor 
according to the present invention. FIG. 2 is a sectional view of the 
acceleration sensor of FIG. 1. 
As shown in FIGS. 1 and 2, an acceleration sensor 1 comprises a square 
plate-like working mechanism 2 made of a semiconductor pellet cut out of a 
wafer of single crystal silicon; a frame 4 which is a square and 
substantially similar in dimension to the working mechanism 2 and which 
has a recess in its central portion and holds the working mechanism 2 via 
a joint surface 2A with its four-sided frame; and a weight portion 3. 
The working mechanism 2 is provided with a thin-walled flexible diaphragm 
portion 2D having an outer periphery whose four corners of the square are 
folded inwardly. The diaphragm portion 2D is formed on the face (100) of 
the plate of single crystal silicon. 
The weight portion 3 is coupled to the central portion of the diaphragm 
portion 2D in a suspended condition. An area outside the outer periphery 
(the linear portion of FIG. 1) of the diaphragm portion 2D forms the joint 
surface 2A, the linear portion being in parallel to the edge of the working 
mechanism 2, and expanded portions 2B are each formed in areas outside the 
respective four corners of the outer periphery thereof. 
Both the joint surface 2A and the expanded portions 2B are coupled to the 
frame 4 and, especially because the expanded portions 2B are coupled to 
the frame 4, the working mechanism 2 is rigidly and securely supported by 
the frame 4. 
In the diaphragm portion 2D are four resistance elements RX, four 
resistance elements RY and four resistance elements RZ, which are formed 
in the respective predetermined directions. The resistance value of each 
resistance element varies with the displacement of and the stress applied 
to the diaphragm portion 2D. Thus, the resistance values of the resistance 
elements vary according to the deformation of the diaphragm portion 2D. 
Acceleration and force are then detected by detecting variations in the 
resistance values of these 12 resistance elements RX, RY, RZ. 
The resistance elements RX, RY, RZ are directly incorporated in the 
diaphragm portion 2D or otherwise provided on the surface of the diaphragm 
portion 2D by joining their separate bodies onto the surface of the 
diaphragm portion. 
In operation, while the weight portion 3 is in such a state that it is 
displaceable in the recess of the frame 4, external force acts on the 
weight portion 3 when acceleration and vibration are applied thereto, 
whereby the weight portion 3 is vertically and bilaterally displaced 
accordingly. 
The displacement of the weight portion 3 is transmitted to the diaphragm 
portion 2D and the mechanical deformation of the diaphragm portion 2D 
occurs. Therefore, the electrical resistance of the resistance elements 
RX, RY, RZ is caused to vary and the variations therein can be sensed or 
measured outside. 
Although greater force acts on the diaphragm portion 2D in the course of 
application of such acceleration and vibration, since the expanded 
portions 2B in addition to the joint surface 2A have been coupled to the 
frame 4 as described above, and since the working mechanism 2 is rigidly 
and securely supported by the frame 4 due to the expanded portions 2B 
coupled to the frame 4, the working mechanism 2 having the diaphragm 
portion is prevented from peeling off the frame 4. 
FIG. 3 is a plan view of an etching mask for use in the process of 
producing an acceleration sensor according to the present invention. FIG. 
4 is a sectional view illustrative of an etching process using the etching 
mask of FIG. 3. 
The following description is for a method for producing the acceleration 
sensor according to the present invention by reference to both FIGS. 3 and 
4. During the step of forming a diaphragm portion by etching from a square 
plate of single crystal silicon, a (100) crystal face is, as shown in 
FIGS. 3 and 4, covered with a mask 10 having an octagonal opening 10E so 
as to apply only anisotropic etching to the (100) face of the plate of 
single crystal silicon. Then, the portions 2B directed in the center 
direction are formed in the four corner portions of the plate of single 
crystal silicon by anisotropic etching using the (100) crystal face and 
the mask 10 (see FIG. 1). 
FIG. 5 is a plan view illustrative of the structure of the working 
mechanism 12 of an acceleration sensor 11 which is produced on the (100) 
crystal face of the plate of single crystal silicon by anisotropic etching 
with an octagonal mask pattern. In FIG. 5, an octagonal diaphragm portion 
is formed on the working mechanism 12 of the square plate of single 
crystal silicon. 
FIG. 6 is a sectional view showing an angular configuration of the 
interface of FIG. 5. 
In the case of the working mechanism 12 having the octagonal diaphragm 
portion in FIG. 5, an anisotropic etching interface S1 as shown in FIG. 6 
forms an angle .theta. as large as approximately 55.degree.; this is also 
the case with other interfaces S2-S8. The working mechanism consequently 
becomes reducible in size to about 75% of the prior art example shown in 
FIG. 12, whereby the size of such an acceleration sensor can be reduced 
with the added effect of attaining lower cost. 
FIG. 7 is a diagram illustrative of stress distribution in the acceleration 
sensor under the finite element method according to the present invention. 
The distribution of stresses applied to an octagonal diaphragm portion 12D 
formed of the wafer of single crystal silicon shown in FIG. 5 is calculated 
by analysis under the finite element method, the result of which is shown 
in FIG. 7. 
As shown in FIG. 7, when acceleration is exerted in the direction of G, the 
stresses concentrate on parts perpendicular to the direction G out of the 
outer periphery of the diaphragm portion 12D and appear as stresses st1, 
st2. The stresses concentrate on parts perpendicular thereto out of the 
periphery of the central square of the diaphragm portion 12D and appear as 
stresses st3, st4. This is also the case where the stress is generated when 
the acceleration is applied in a direction perpendicular to the direction 
G. 
As is clear from the result of the numerical value analysis above, the 
formation of the expanded portions of the joint surface in the four corner 
portions of the outer periphery of the square diaphragm portion according 
to the present invention is intended to achieve the effect of securing a 
rigid junction without affecting the sensitivity of the acceleration 
sensor, which effect has been proved by the simulation technique. 
Moreover, the results of characteristic tests using actual products have 
been shown to be similar to the results of the simulation mentioned above. 
As set forth above, the expanded portions on the joint surface are formed 
in the respective four corner portions of the outer periphery of the 
square diaphragm portion in the acceleration sensor according to the 
present invention, so that the coupling of the diaphragm portion to the 
pedestal is strengthened. Further, any defective joint occurring during 
the processing is eliminated and the yield is improved to allow 
mass-producibility with efficiency. Further, a long life and stable 
operation of the sensor in use make achievable the effect of securing 
reliability. 
Under the method of producing the acceleration sensor according to the 
present invention, moreover, the (100) face of the plate of single crystal 
silicon is covered with the mask having the octagonal opening before being 
subjected to anisotropic etching. Consequently, the expanded portions 
directed in the center direction are readily formed in the respective four 
corner portions of the plate of single crystal silicon with the effect of 
readily accomplishing the manufacture of the acceleration sensor as 
described above. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the acceleration sensor of the present 
invention without departing from the spirit or scope of the invention. 
Thus, it is intended that the present invention cover the modifications 
and variations of this invention provided they come within the scope of 
the appended claims and their equivalents.