Method of manufacturing sensor

A method for manufacturing sensors from a multilayer plate with upper and lower monocrystalline silicon layers and an etching layer between them. The upper silicon layer is structured by the introduction of troughs therein extending down to the etching layer. Sensor structures, such as a bending beam that is used in an acceleration sensor, are created by etching the etching layer beneath a part of the silicon layer structured in this manner.

FIELD OF THE INVENTION 
The present invention relates to sensors and methods relating to their 
manufacture. More specifically, the present invention relates to methods 
of manufacturing sensors in a multilayer plate of silicon and the sensors 
thus manufactured. 
BACKGROUND OF THE INVENTION 
A method for manufacturing acceleration sensors, in which troughs are 
etched into a two-layer plate of monocrystalline silicon, is described in 
German Patent No. DE 40 00 903. The troughs delineate the structure of an 
acceleration sensor--with a bending blade suspended from a mount and a 
counterelectrode--in the upper layer of the two-layer silicon plate. The 
bending blade and counterelectrode are both fastened onto the second layer 
which forms a tabular substrate. The bending blade and the 
counterelectrode are insulated from this tabular substrate by a p-n 
junction. Etching of the tabular substrate beneath the bending blade makes 
the bending blade movable. 
SUMMARY OF THE INVENTION 
The present invention provides a method of manufacturing a sensor in a 
multilayer plate of silicon. The use of an etching layer between two 
silicon layers makes possible a manufacturing process with etching steps 
which can be controlled with particular accuracy. Because the etching 
steps are easily controllable, high dimensional accuracy can be achieved 
in the production of sensors, while process management is simple and 
uncomplicated. 
A multilayer plate, for use in manufacturing a sensor in accordance with 
the present invention, can be fabricated with little need for complex 
equipment by joining two silicon plates. In the alternative, a multilayer 
plate can also be fabricated using only one silicon plate by implanting 
impurity atoms into the silicon plate. 
A sensor according to the present invention exhibits particularly good 
insulation among the individual sensor components, thus allowing the use 
of a particularly simple measurement configuration to measure the 
capacitance between a bending blade and a counterelectrode of the sensor. 
Additional aspects of the present invention include the use of insulating 
materials, which are also used for the etching layer, to insulate sensor 
components. Furthermore, insulating materials can be etched with 
particularly good selectivity with respect to silicon. By introducing 
troughs or diffusion zones, individual regions can be insulated from one 
another particularly well and with simple means. If the structure is 
etched away by using troughs, the multilayer plate need only be processed 
on one side. If, however, the etching layer is etched through an etching 
opening in the lower silicon layer, it is possible to arrange structures 
on the top surface that will also be affected when etching of the top 
surface occurs.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIGS. 1 and 2 depict a process for manufacturing a multilayer plate 5. As 
shown in FIG. 1, impurity atoms are implanted into a silicon plate 23. The 
impurity atoms are implanted by means of a beam of accelerated impurity 
atoms, as depicted by the arrows. An impurity atom layer 24 forms in 
silicon plate 23 as a function of the energy of the impurity atom beam. 
Atoms suitable for such an implantation process include, for example, 
oxygen, which is intercalated into the silicon plate 23 with an energy of 
200 keV. Reaction between the impurity atoms and the silicon of plate 23 
is assisted by a baking process. Reaction between the implanted layer 24 
and the silicon thus forms an etching layer 3, as shown in FIG. 2. With 
the implantation of oxygen, for example, heat treatment at more than 
1200.degree. C. for several hours forms a silicon oxide layer. This 
silicon oxide layer is suitable as an etching layer 3, and is embedded 
between two monocrystalline silicon layers 1 and 2. The silicon layer 2 
consists of the remaining thickness of the silicon plate 23, and is 
particularly large compared to the two layers 1 and 3. Typical thicknesses 
for the layers 1 and 3, which are formed by the implantation of oxygen 
into a silicon wafer, are on the order of less than 1 micrometer. The 
thickness of the layer 1 can be increased by an epitactic growth process. 
FIGS. 3 to 5 depict a second method for manufacturing a multilayer plate 5. 
This method begins with two silicon plates 20 and 21, as shown in FIG. 3. 
The two silicon plates 20 and 21 are joined to one another by means of a 
"bonding" process. One such bonding process consists of placing two 
silicon plates with smooth surfaces against one another, possibly after a 
chemical pretreatment of the surfaces, and then insolubly joining them to 
one another by means of a heat treatment. The joining surfaces of the 
silicon wafers can also be provided with an auxiliary layer that is not 
made of silicon. Suitable auxiliary layers are made, for example, of 
silicon oxide, silicon nitride, or glass. Auxiliary layers can be provided 
on only one joining surface or on both. In FIG. 3, the silicon plate 21 is 
provided with a silicon oxide layer 22. 
The two silicon plates 21 and 22 are placed against one another, as 
indicated by the arrows in FIG. 3. Silicon plates 20 and 21 are joined to 
one another, as shown in FIG. 4, by means of a baking process, for 
example, by heating to more than 400.degree. C. for several hours. The 
silicon plates 20 and 21 are permanently joined to one another by the 
silicon oxide layer 22. As shown in FIG. 5, the multilayer plate 5 is then 
formed, by subsequent processing, from the plate stack shown in FIG. 4. 
In the subsequent processing, the thickness of the upper silicon plate 20 
is reduced so the thickness of the upper silicon layer 1 of the multilayer 
plate 5 can be adjusted in a defined manner. The thickness of the upper 
silicon plate 20 can be reduced by mechanical surface machining, in which 
much of the thickness is first removed by grinding. The thickness of the 
silicon plate 20 can also be reduced by chemical etching methods. The 
thickness and surface finish of the upper silicon layer 1 are then 
adjusted with a polishing process, possibly also including a chemical 
polishing process. The thickness of the silicon layer 1 can also be 
adjusted by means of a prior doping of the silicon plate 20, for example, 
by embedding an etch-stopping layer into the silicon plate 20. 
When a multilayer plate 5 is fabricated by implantation of impurity atoms, 
only a single silicon plate need be processed. This method therefore 
requires little starting material. However, suitable devices for 
implanting impurity atoms such as oxygen, are not used in standard 
semiconductor production, and therefore must be procured additionally for 
this process. Conversely, there is no need for such an implanting device 
when forming a multilayer plate 5 by joining two silicon plates to one 
another in a bonding process, although, in this case, two silicon plates 
are required. 
FIG. 6 shows a multilayer plate 5 that has been divided into mutually 
isolated regions 10, 11, and 12 by an etching layer 3 and a trough 4 or an 
insulating diffusion zone 14. The silicon oxide etching layer 3 insulates 
upper silicon layer 1 from lower silicon layer 2. By introducing one or 
more troughs 4, or an insulating diffusion zone 14, as is used, for 
example, in bipolar technology, it is possible to divide the upper layer 1 
into individual regions 10, 11, 12 that are insulated from one another. 
This type of insulation allows the manufacture of sensors in which 
individual components are particularly well insulated from one another. 
FIG. 7 shows the manufacture, and FIGS. 8 and 9 show a first exemplary 
embodiment of a sensor in accordance with the present invention. FIG. 7 
shows a multilayer plate 5 comprising an upper silicon layer 1, a lower 
silicon layer 2, and a silicon oxide etching layer 3 located between the 
upper and lower silicon layers 1 and 2. Troughs 4, which extend down to 
the etching layer 3, are introduced into the upper silicon layer 1. 
FIG. 8 shows a cross-section of an acceleration sensor manufactured from 
the multilayer plate 5 of FIG. 7. The sensor comprises a bending blade 30 
attached to a mount 32. The mount 32 is fastened onto a substrate 33 by 
means of an insulation layer 34. The sensor also has a frame 35 that is 
separated from the mount 32 and the bending blade 30 by a trough 4. The 
frame 35 is also anchored onto substrate 33 by means of an insulating 
layer 34. 
FIG. 9 shows a plan view of the sensor whose cross-section is shown in FIG. 
8 (with line I--I corresponding to the cross-section shown in FIG. 8.) In 
addition to the mount 32 and the bending blade 30 suspended therefrom, two 
counterelectrodes 31 are visible in the plan view. The counterelectrodes 
31 are arranged on either side of the bending blade 30 and are again 
anchored to the substrate 33 by means of the insulating layers 34 (not 
visible in FIG. 9). Also evident in the plan view is the geometrical shape 
of the troughs 4, which are introduced into the upper silicon layer 1 and 
thus delineate the frame 35, the mount 32, the bending blade 30, and the 
counterelectrodes 31, all in the upper silicon layer 1. 
The steps by which a sensor is manufactured, in accordance with the present 
invention, are illustrated in FIGS. 7 and 8. First, as shown in FIG. 7, 
troughs 4 are introduced into a multilayer plate 5 (which is formed in 
accordance with either FIGS. 1 and 2, or FIGS. 3 to 5.) The troughs 4 
create, within the upper silicon layer 1, the structure of the mount 32, 
the bending blade 30, the counterelectrodes 31, and the frame 35. Because 
the troughs extend down to the silicon oxide layer 3, the individual 
sensor components are thus electrically insulated from one another. 
In a further etching step, the silicon oxide layer beneath the bending 
blade 30 is then removed. This etching step is evident from FIG. 8, in 
which individual insulating layer regions 34 are formed, from the silicon 
oxide layer 3, beneath the mount 32 and the frame 35. The 
counterelectrodes 31 are also anchored onto the substrate 33 by means of 
individual insulating layer regions. 
The bending blade 30 is configured so that it exhibits its lowest flexural 
strength in an axis parallel to the surface of the substrate 33. The 
bending blade 30 is thus particularly easily deflected by accelerations in 
the directions indicated by the double-headed arrow shown in FIG. 9. This 
deflection causes a change in the electrical capacitance between the 
bending blade 30 and the counterelectrodes 31. Acceleration can thus be 
measured by measuring the capacitance between the bending blade 30 and the 
counterelectrodes 31. 
The troughs 4 are etched in using etching processes that produce edges as 
close to vertical as possible. This can be done, for example, with an 
anisotropic plasma etching process, such as reactive ion etching. Another 
possibility is to utilize the anisotropic etching properties of silicon 
with respect to fluid etching solutions, for example, aqueous KOH 
solutions. Vertical walls can, for example, be etched into silicon plates 
with a (110) surface orientation. Etching layer 3, beneath the bending 
blade 30, can also be etched with an etching fluid. Suitable etching 
fluids include hydrofluoric acid, which etches silicon oxide without 
attacking silicon structures. 
The manufacturing method presented here is not confined to the manufacture 
of acceleration sensors according to FIG. 9. Other geometries for 
acceleration sensors, or other sensors such as those for pressure, force, 
or the like, can just as easily be implemented. 
FIG. 10 depicts a further exemplary embodiment of an acceleration sensor in 
accordance with the present invention. This sensor comprises a bending 
blade 30 suspended from a mount 32. The bending blade 30 and the mount 32 
are separated from the frame 35 by troughs 4. In plan view, the sensor of 
FIG. 10 corresponds to an acceleration sensor as shown in FIG. 9. The 
frame 35 and the mount 32 are once again joined to a substrate 33 by means 
of insulating layer regions 34. An opening 6 is introduced into the 
substrate 33 beneath the bending blade 30. 
The sensor of FIG. 9 is manufactured from a multilayer plate 5, as depicted 
in FIG. 7. The troughs 4 once again structure the upper silicon layer 1 in 
such a way that the mount 32, the bending blade 30, and the 
counterelectrodes 31 are formed from the upper silicon layer 1. In a 
further etching step, the etching opening 6 is then introduced into the 
lower silicon layer 2, beneath the bending blade 30. The etching opening 6 
is formed so that it is situated only immediately beneath the bending 
blade 30. The etching layer 3 is then removed, in a subsequent etching 
step, from beneath the bending blade 30 through the etching opening 6. 
Because the etching opening 6 is confined to the region immediately 
beneath the bending blade 30, the mechanical attachment between the mount 
32 and the counterelectrodes 31 provided by the silicon oxide layer, is 
not impaired. 
Each of the two manufacturing processes has specific advantages. Etching of 
the etching layer 3 from above by means of the troughs 4, eliminates the 
structuring of the lower silicon layer 2. As a result, two-sided 
processing of silicon plates is not necessary, thereby reducing production 
costs. However, when the etching layer 3 is etched from the underside 
through the etching opening 6 in the lower silicon layer 2, the etching 
layer 3 can be removed from beneath even large-area structures without 
thereby endangering retaining regions, such as the mount 32, due to 
undercutting. This makes it possible, for example, to provide the bending 
blade with a spatially large seismic mass, which increases the sensitivity 
of the sensor. Moreover, with this etching process, circuits in the upper 
silicon layer 1--used, for example, for initial evaluation of the sensor 
signals--can be better protected from attack by the etching medium used 
for etching layer 3. Also, because of the closer spatial proximity of such 
circuits to the sensor, made possible with such a process, any 
interference signals are thus minimized.