Accelerometer and method for making same

An accelerometer based on monocrystalline silicon. A moving mass is connected to a fixed frame by suspension arms. The sensor is produced in the stack of two silicon wafers in which stop and counter-stop functions are produced so as to limit the amplitude of the movements of the moving mass.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of the invention is that of deformable microstructures with 
two-directional operation, particularly that of accelerometers comprising 
a moving mass connected to a fixed frame via slender suspension arms. 
2. Discussion of the Background 
The accelerations experienced by the moving mass may typically be detected 
thanks to the presence of piezoresistive gauges on the slender suspension 
arms. Such piezoresistive gauges may also control an electromagnetic 
device for controlling the moving mass, in order to compensate for its 
displacements. 
In this case, the accelerometer strain gauges are intended to generate, by 
means of an electronic circuit, a supply current for a coil which, using 
phenomena of electromagnetic induction, compensates for a displacement of 
the moving mass under the effect of external conditions, for example 
seismic movements. 
At the present time, such accelerometers can be built using silicon wafers, 
using silicon etching techniques developed in the context of the 
manufacture of semiconductor electronic components. The use of such 
technologies allows the moving mass and the elements associated with it to 
be produced by a collective process on slices of silicon and thus allows a 
series of acceleration sensors to be defined, using a limited number of 
technological steps. 
Typically, the manufacture of this type of sensor comprises assembling 
several silicon wafers, at least one central wafer in which the moving 
mass is produced, and two outer wafers having stop elements for the moving 
mass, thus making it possible to limit the amplitude of the movements 
which the said moving mass can be made to experience, and thus to optimize 
the protection of the moving mass and of the suspension arms. An example 
of an accelerometer made up of at least three silicon wafers according to 
the known art is illustrated in FIG. 1. 
More specifically, FIG. 1 illustrates a cross-section through this type of 
accelerometer the manufacture of which is, in particular, the result of 
assembling 4 silicon wafers, a so-called upper wafer constituting part of 
the fixed frame and comprising upper stops Bs, two central wafers 
constituting the moving mass 1 and part of the fixed frame and a fourth 
wafer, called the lower wafer, comprising part of the fixed frame and a 
stop plane Bi for the lower face of the moving mass. 
To simplify the architecture of this type of accelerometer and make the 
method of manufacturing it easier, accelerometers are known which result 
from assembling two wafers in which all the elements and functions are 
defined (moving mass, suspension arms, frame) as disclosed in Application 
WO 95/04284 or U.S. Pat. No. 5,121, 633. 
SUMMARY OF THE INVENTION 
The application also proposes a simplified architecture in which the stops 
and counter-stops functions are cunningly produced. 
More specifically, the subject of the invention is an accelerometer 
comprising a moving mass connected to a fixed frame, the moving mass and 
the fixed frame being defined in a stack of two monocrystalline silicon 
wafers known as the upper wafer and the lower wafer, characterized in 
that: 
the upper wafer comprising [sic]: 
the upper part of the moving mass, including a first and a second 
counter-stop along an axis Y defined in the plane of the moving mass; 
the upper part of the fixed frame, including a first and a second stop 
along an axis X perpendicular to the axis Y and defined in the plane of 
the moving mass; 
the upper part of the moving mass and the upper part of the fixed frame 
being connected by suspension arms; 
the lower wafer comprising [sic]: 
the lower part of the moving mass, including 
a third and a fourth counter-stop along the axis X; 
the lower part of the fixed frame, including a third and a fourth stop 
along the axis Y; 
the first and second counter-stops being opposite the third and fourth 
stops respectively; 
the third and fourth counter-stops being opposite the first and second 
stops respectively. 
By convention, the stops are defined as fixed elements of the 
accelerometer, against which counter-stops, which are moving elements of 
the accelerometer, come to rest. 
According to an alternative form of the invention, the accelerometer is 
characterized in that: 
the upper part of the moving mass comprises a first central part and first 
and second immobilizing arms lying one on either side of the said first 
central part, along the axis Y; 
the upper part of the fixed frame comprises a peripheral part and first and 
second arms lying inside the peripheral part, along the axis X, the upper 
wafer comprising first and second suspension arms connecting the central 
part of the upper part of the moving mass to the first and second arms of 
the peripheral part; 
the lower part of the moving mass comprises a central part and third and 
fourth immobilizing arms lying one on either side of the said second 
central part, along the axis X; 
the lower part of the fixed frame comprises a peripheral part and third and 
fourth arms lying inside the peripheral part along the axis Y; 
the first and second counter-stops being defined in the first and second 
immobilizing arms; 
the first and second stops being defined in the first and second arms; 
the third and fourth counter-stops being defined in the third and fourth 
immobilizing arms; 
the third and fourth stops being defined in the third and fourth arms. 
According to an alternative form of the invention, the wafers 6 and 7 of 
the accelerometer comprise slender guide arms connecting the moving mass 1 
to the frame 3 in such a way that only movements of the moving mass along 
an axis Z perpendicular to the plane of the said mass are possible. 
Advantageously, these arms may lie along two axes oriented at 45.degree. 
to the axes X and Y and lying towards the top of the wafer 6 and towards 
the bottom of the lower wafer 7, that is to say in planes which correspond 
to the outer plane of the wafer 6 and to the outer plane of the wafer 7, 
the inner planes of the said wafers corresponding to the plane of contact 
of these said wafers.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
One embodiment of the accelerometer according to the invention is 
illustrated by FIGS. 2 and 3 together. FIGS. 2a and 2b relate to the 
wafers 6 and 7 the stacking of which allows the moving mass 1 connected to 
a fixed frame 3 to be defined. 
FIG. 2a relates to the upper wafer 6. This wafer 6 comprises the upper part 
11 of the moving mass consisting of a central part 13 and of two 
immobilizing arms 14 and 15 lying one on either side of the central part 
and, in FIG. 2a, oriented along the axis Y. 
The upper wafer 6 also comprises the upper part 31 of the fixed frame, 
consisting of a peripheral part 33 and of two arms 34 and 35, which in 
FIG. 2a are oriented along the axis X. 
Part 42 of the immobilizing arm 15, and part 41 of the immobilizing arm 14 
constitute counter-stops on the part 11 of the moving mass. 
Part 43 of the arm 34 and part 44 of the arm 35 constitute stops on the 
part 31 of the fixed frame. 
This wafer 6 also comprises two suspension arms 21 and 22 connecting the 
central part 13 of the moving mass to the arms 34 and 35 secured to the 
fixed frame. 
FIG. 2b relates to the lower wafer 7. This wafer 7 comprises the lower part 
12 of the moving mass consisting of a central part 16 and of two 
immobilizing arms 17 and 18 which in the figure are oriented along the 
axis X. 
The wafer 7 also comprises the lower part 32 of the fixed frame, consisting 
of a peripheral part 36 and. of two arms 37 and 38 which in the figure are 
oriented along the axis Y. 
Part 42 of the immobilizing arm 15 and part 41 of the immobilizing arm 14 
constitute counter-stops facing the stop 48 part of the arm 38 and the 
stop 47 part of the arm 37, respectively. 
Likewise, part 45 of the immobilizing arm 17 and part 46 of the 
immobilizing arm 18 constitute counter-stops facing the stop 43 of the arm 
44 and the stop 44 of the arm 45, respectively. 
FIGS. 3a and 3b illustrate how the various stops and the various 
counter-stops can be produced. 
The arms 17, 18, 37 and 38 of the lower wafer 7 have the same architecture. 
They consist of a second element of thickness e.sub.1 less than the 
thickness e.sub.0 of a silicon wafer. The difference in thickness between 
these arms and the other parts of the silicon wafer makes it possible to 
form stops or counter-stops for the counter-stops or stops which 
respectively face them. 
The arms 14, 15, 34 and 35 of the upper wafer 6 all have the same 
architecture. They consist of a single element of thickness e.sub.0, part 
of each arm facing the thinned part which constitutes an arm in the lower 
wafer 7. 
We are going to describe one example of the method for producing an 
accelerometer according to the invention, built from a stack of two 
silicon wafers. 
FIGS. 5 and 6 respectively illustrate the steps in the method for producing 
the lower wafer, without the piezoresistive gauge, and the method for 
producing the upper wafer with piezoresistive gauges. 
More precisely, FIG. 5 as a whole describes the production of the elements 
36, 17, 16 depicted in FIG. 3a, and FIG. 6 as a whole describes the 
production of the elements 33, 43, 21, 13 depicted in FIG. 3a. 
To give the accelerometer a certain degree of robustness, the upper and 
lower wafers may comprise slender guide arms, which hold the moving mass 
in place relative to the fixed frame. 
FIGS. 4a and 4b illustrate one example of a configuration in which the 
upper wafer comprises four guide arms 81, 82, 83, 84 (FIG. 4a) and the 
lower wafer also comprises four guide arms 85, 86, 87, 88 (FIG. 4b). 
Typically, the guide arms may have a thickness similar to that of the 
suspension arms 21 and 22 and be situated, on the one hand, in the upper 
part of the wafer 6 (in the same plane as the suspension arms) and, on the 
other hand, be situated in the lower part of the wafer 7. 
In order to construct the various elements of the lower wafer 7, use may be 
made of a substrate of the SIMOX type which corresponds to a 
monocrystalline silicon substrate in which a layer of oxide 90 is embedded 
(FIG. 5a). 
Two layers of oxide or of nitride 91 and 92 are then produced on the two 
faces of the substrate of the SIMOX type (FIG. 5b). 
A mask is produced by photolithography and etching at the layer 91 (FIG. 
5c). 
This mask is refined by a second etching step, as illustrated in FIG. 5d. 
The silicon is chemically etched in the conventional way in the regions 
which have no protective layer (FIG. 5e). 
The layer of oxide or of nitride 91 is then thinned down as illustrated in 
FIG. 5f. 
A second step of etching of the silicon is then carried out in order to 
define the counter-stop 45 in the immobilizing arm 17 (FIG. 5g). 
Finally, a laser etching step is carried out, which leads to the 
immobilizing arm 17 being separated from the peripheral part 36 of the 
fixed frame (FIG. 5h). 
In order to achieve the desired functions on the upper water 6, it may be 
particularly advantageous to use a silicon wafer in which two insulating 
layers 93 and 94 have been made, the said wafer moreover being covered 
with an insulating layer 95 on the opposite face. 
The two embedded layers 93 and 94 may be produced in various ways. 
Use may be made of a wafer of the SIMOX type, in which the layer 93 is 
embedded in the conventional way. In the conventional way, a layer 4000 
.ANG. thick may be embedded at a depth of about 4000 .ANG.. It is then 
possible to grow the layer of monocrystalline silicon by epitaxy. The 
layer of oxide 94 can then be embedded in this epitaxially grown layer 
(FIG. 6a). 
Another method consists in the high-temperature welding of two silicon 
wafers one of which is oxidized at its surface. This yields an assembly of 
two silicon wafers joined together at the oxide layer which then 
constitutes the layer 93. One of the wafers can then be machined so as to 
define the desired thickness of silicon in which oxygen can be embedded 
ionically to define the layer 94. 
The thin layer 96 makes it possible to define silicon elements which are 
isolated from the rest of the wafers and may constitute piezoelectric 
gauges which are better defined and calibrated than those which result 
from implantation within a silicon substrate in a conventional way (FIG. 
6b). 
The piezoelectric gauges are protected by a layer 97 of nitride, for 
example, or of oxide, or of both and openings are produced locally at the 
layer 97 to allow for the electrical connections (FIG. 6c). 
A conductive layer can then be deposited and then etched in order, in the 
known way, to form tracks for connecting each gauge, these not being 
depicted in FIG. 6c. 
A mask is produced by etching through the layer 95 as illustrated in FIG. 
5d. 
The manufacture of this mask is followed by a step of etching the silicon 
chemically, making it possible to define the suspension arm 21, the arm 34 
with its stop 43, and the upper central part of the moving mass 13. 
The two silicon wafers 6 and 7 thus machined may be welded by heating 
typically to a temperature of the order of 1000.degree. C. 
The two wafers may be joined together before the laser cutting illustrated 
in FIG. 5h is performed, thus making it possible to detach the immobilzing 
arm 17 from the peripheral part of the frame 36, so as to make-the said 
assembly easier. 
All the steps described hereinabove are also used to produce the arms 14, 
15, 37 and 38 along the axis Y. 
The slender guide arms 81, 82, 83, 84 can also be produced in the upper 
wafer in the same way as the slender suspension arms and in the same 
plane, thanks to the etching arresting layer. 
The slender guide arms 85, 86, 87, 88 can also be produced in the lower 
wafer thanks to the layer of silicon which lies between the layers of 
oxide 90 and 92.