FR2885410A1 - Physical energy e.g. force, measuring device for e.g. tire, has rigid bar connected to deformable membrane and integrated to support, where electrodes disposed on membrane and support are moved when bar is stressed by force to be measured - Google Patents

Abstract

The device has an electrode (15) disposed on a rigid support (10`), and another electrode (16) disposed on a deformable membrane (12) with respect to the electrode (15). A rigid bar (10) is connected to the membrane and is integrated to the rigid support, where the electrodes are displaced when the bar is stressed by a force to be measured. A hole in a central zone of the membrane and in the bar is adapted to receive transmission units of the force to be measured. The bar and the support are enclosed in a cavity formed by the membrane.

Description

The present invention relates to a device for measuring force by

capacitive sensing.

  It generally relates to force sensors that can be integrated into any system where it is desired to measure a force (pneumatic, joystick or joystick for video games, ...) or an acceleration (trigger air bag in an automobile, pacemaker, ...), a pressure, a moment.

  Capacitive detection in a force measuring device is generally achieved through a rigid assembly of two parts for facing electrodes thus forming a variable impedance capacitance.

  Such a sensor is described for example in US 5,392,658.

  The measuring device comprises a rigid rod intended to be stressed by a force or an acceleration and a head comprising a deformable membrane intended to be connected to the rod so as to be deformed when the rod is urged.

  Electrodes are arranged on this deformable membrane. Opposite this deformable membrane, and at a distance called air gap, is a fixed part on which are also arranged electrodes so that each of these electrodes are in opposition with another electrode of the deformable membrane. Variable capacities are thus produced as soon as the electrodes of the deformable membrane are displaced under the action of the force transmitted by the rod.

  The measurement of the capacitance variation makes it possible to know the force or acceleration due to a law obtained by calibrating the force measuring device.

  Advantageously, reference is made to US Pat. No. 5,392,658 for measuring the force applied as a function of the variation measured across the capacitors.

  Such a sensor is however difficult to achieve given the difficulties encountered in controlling the value of the air gap between the electrodes.

  The present invention aims to solve the aforementioned drawbacks and to provide a device for measuring manufacturing force and simplified calibration.

  For this purpose, the present invention aims at a capacitive sensing force measuring device comprising at least a first electrode disposed on a rigid support and at least a second electrode disposed on a deformable membrane, vis-à-vis said first electrode , and a rod connected to the deformable membrane.

  According to the invention, the rod is secured to the rigid support.

  Thus, thanks to the invention and the presence of the rod connecting both the membrane and the rigid support, the electrodes both on the deformable part and on the rigid support are moved when the rod is stressed. The air gap at each capacity is thus modified only by the relative deformation of the membrane relative to the rigid support.

  In practice, the rod extends between the rigid support and the deformable membrane, and more particularly between the central zone of the deformable membrane and the central zone of the rigid support.

  Advantageously, the rod comprises passage means of the electrical conductors connected to said at least first electrode of the rigid support, for moving the electrical connection of the first electrode or electrodes of the rigid support. Advantageously, the connection pads of said first and second electrodes are arranged on a non-deformable portion of the membrane.

  Other features and advantages of the invention will become apparent in the description below.

  In the accompanying drawings, given by way of nonlimiting examples: FIG. 1 is a schematic view illustrating a force measuring device according to an embodiment of the invention; - Figure 2 is a view similar to Figure 1 wherein the force measuring device is biased by a normal force; - Figure 3 is a view similar to Figure 1 wherein the force measuring device is biased by a tangential force - Figure 4 is a top view in section along the line IV-IV in Figure 1; FIG. 5 is a view from above of the force measuring device of FIG. 1; FIGS. 6, 7 and 8 illustrate various embodiments of the protection of the capacitive detection force measuring device vis-à-vis the external medium; Figs. 9A to 9P illustrate the steps of a method of manufacturing a force measuring device according to a first embodiment of the invention; FIGS. 9K 'to 9P' illustrate the modified steps of the method described with reference to FIGS. 9A to 9P according to a second embodiment of the invention; and FIGS. 10A to 10M illustrate the steps of a method of manufacturing a force measuring device according to a third embodiment of the invention.

  We will first describe with reference to Figure 1 a force measuring device according to one embodiment of the invention.

  The force measuring device of the invention is a capacitive type detection force sensor.

  It comprises a rigid rod 10 intended to be biased by a force to be measured or to be moved under the action of an acceleration. This rod 10 is fixed to a head 11 which comprises a deformable membrane 12 connected to the rod 10. In this embodiment, the rod 10 is connected in a central zone 13 of the deformable membrane 12.

  This deformable membrane may be a solid membrane or may consist of several radiant arms, extending between the central zone 13 and a peripheral zone 14.

  In this peripheral zone 14, the membrane 12 comprises anchoring points making it possible to maintain a stable position at the periphery of the membrane, whether or not the membrane is subjected to deformation via the rod 10.

  The rod 10 extends between the central zone 13 of the deformable membrane 12 and a central zone of a rigid support 10 'disposed opposite the membrane 12.

  The rigid support 10 'has a face 10a which extends opposite the deformable membrane 12.

  Preferably, the rigid support 10 'comprises at least a first electrode 15, and in this embodiment as illustrated in detail in FIG. 4, three electrodes 15 on the face 10a of the rigid support 10'.

  Vis-à-vis this face 10a is disposed at least a second electrode 16 on the deformable membrane 12.

  Thus, the second electrode 16 is in opposition with a first electrode 15 in order to achieve a capacitance.

  In this embodiment, the membrane 12 is disk-shaped and the rod 10 is cylindrical. In addition, the rigid support 10 'is cylindrical so that the upper face 10a disposed opposite the membrane 12 also has a disc shape.

  As is well illustrated in FIG. 4, the three electrodes are here disposed at 120 on this face 10a of the rigid support 10 '.

  As illustrated in FIG. 5, a common electrode is disposed on the membrane 12, this common electrode 16 comprising electrode portions 16a arranged opposite the first electrodes 15.

  The impedance of each capacitor thus constituted by the pairs of electrodes 15, 16a depends on the distance separating these electrodes, noted gap 17 in FIG.

  Note that in this embodiment, the rod 10 and the rigid support 10 'are monobloc and thus made in one piece.

  When a force measuring device thus constituted is used as a force sensor, the force is applied to the rod 10 and deforms the membrane 12 to which this rod 10 is attached.

  Due to the deformation of the membrane, as illustrated in Figures 2 and 3, the gap 17 between the capacitors varies. The measurement of the capacities makes it possible to know the components of the applied force.

  Thus, as illustrated in FIG. 2, when a normal force FZ, that is perpendicular to the membrane 12, is applied to the sensors, the air gap 17 varies in the same way for the three capacitors formed respectively of a first electrode 15 and an electrode portion 16a of the common electrode 16.

  On the other hand, as illustrated in FIG. 3, when the applied force is a tangential force FX, the gap 17 is decreased for one or the other of the capacities and increased for the other capacities.

  The operation is identical when the force measuring device is used as an accelerometer, the rod 10 and the rigid support 10 'constituting a seismic mass to which the acceleration to be measured is applied.

  This seismic mass is then subjected to a force as described previously with reference to FIGS. 2 or 3.

  As well illustrated in Figures 4 and 5, the rod 10 comprises means 18 for passing electrical conductors connected to each first electrode 15 disposed on the rigid support 10 '.

  As well illustrated in Figure 5, these passage means 18 are flush with the surface of the deformable membrane 12; the connection pads 19 of these first electrodes 15 are generally offset to the non-deformable portion of the membrane and, for example, the peripheral zone 14 of the membrane.

  Similarly, the common electrode 16, each portion 16a extends opposite the first electrodes 15, is connected by a common connection pad 20 also disposed on the non-deformable portion 14 of the membrane.

  Thus, by arranging the connection pads 19, 20 of the electrodes on one and the same surface, in this example the deformable membrane 12, it facilitates the interconnection of the sensor with the signal processing electronics measured across the capacitors.

  6 to 8 different embodiments of a force sensor according to the invention will now be described with reference to FIGS.

  It will be noted first of all that in the embodiment described above, the rigid support 10 'has a free end 10b of large surface area with respect to the anchoring zone of the rod 10 at the central zone 13 of the the membrane.

  This large surface 10b can be possibly troublesome if a normal force FZ applied does not have a uniform field in the plane 15 of this end 10b of the rod 10.

  Indeed, if the normal force FZ is not uniform, the deformation under the action of this normal force can be likened to a deformation as generated by a tangential force Fx and illustrated in FIG.

  In order to solve this drawback, it is possible to structure the rigid support 10 'in a different manner, and in particular to reduce this end-end face 10b by producing a shoulder. Such a force sensor is illustrated for example in FIG. 7.

  In the case where the rigid support 10 'must cooperate with an external element to be measured, it may be advantageous to structure one or more faces of the rigid support by slots of various shapes etched on the different faces (parallel or perpendicular to the membrane, circular shape or streaks), so as to improve the anchoring of the support in the outer member.

  An example of crenellations made in the end face 10b of the rigid support 10 'is illustrated in FIG. 6.

  When the force measuring device as described above is used as an accelerometer, the protection of the capacitances of the external medium can be realized simply as illustrated in FIG. 6, while relating to the rear face of the sensor, at the level of the peripheral zone 14. anchoring the membrane 12 a bonded or sealed substrate 21.

  Thus, the rod 10 and the rigid support 10 'are enclosed in a cavity 22 closed by the membrane.

  In micro-technology manufacturing techniques, the sealing of this substrate 21 can be achieved by conventional techniques such as anodic silicon glass sealing or direct sealing by molecular adhesion. A gas trapped inside the cavity 22 and present in particular at the air gap 17 may be an inert gas at a controlled pressure. Sealing can also be performed under vacuum.

  As illustrated in FIG. 7, when the force measuring device is used as a force sensor, the rod 10, and in particular its free end 10b, must be in contact with the zone of application of the force that it is desired to measure. . To ensure the protection of the capacitors, it is also possible to relate a substrate 23 in which an opening, here circular, is formed to allow the passage to the end 10b of the rod 10.

  The seal can be achieved by means of a flexible seal 24 disposed between a longitudinal wall of the rod 10 and the protective substrate 23.

  Note that the additional stiffness provided by the seal 24 must be low compared to the stiffness of the structure which must be sensitive to a force applied in the three directions.

  The sealing may also be performed under vacuum or in the presence of an inert gas at a controlled pressure.

  Alternatively, there is illustrated in Figure 8 a solution also for sealing a substrate 25 on the periphery 14 of the head 11 of the sensor. In this embodiment, the central zone 13 of the membrane 12 and the rod 10 have a hole 26 adapted to receive means for transmitting the force to be measured and, for example, a second rod (not shown) which will itself be even in contact with the application area of the force to be measured.

  FIGS. 9A to 9P will now illustrate a first method of manufacturing by micro-technology a force sensor as described previously.

  This method of manufacture uses a silicon on insulator (SOI) substrate.

  From an SOI substrate as illustrated in FIG. 9A, the ion implantation of the lower electrodes 15 is carried out as illustrated in FIG. 9B.

  It should be noted that the type (P or N) of the dopant must be different from the type of the substrate in which the electrodes are implanted so as to form a PIN junction which isolates the electrodes from the substrate.

  Thus, for example, the electrodes are obtained by implanting P-type ions for an N-type substrate to form a P / N junction.

  Then, as illustrated in FIG. 9C, the opening of holes 30 in the surface of the substrate is made to allow the epitaxial resumption.

  Figure 9D illustrates the epitaxial step. This epitaxial step consists in growing a monocrystalline silicon layer from the monocrystalline superficial silicon of the SOI substrate. The thickness of epitaxy regulates the thickness of the membrane 12 at the end of the manufacturing process.

  As illustrated in FIG. 9E, a mask 31, for example based on Si3N4, is then produced for wet anisotropic etching of the silicon surface layer.

  The step illustrated in FIG. 9F shows the result of anisotropic etching of the superficial silicon layer.

  As illustrated in FIG. 9G, the Si3N4 mask is then eliminated.

  Then, as illustrated in FIG. 9H, a dielectric layer 32 is deposited, making it possible to isolate the substrate with a view to subsequent metal etching.

  Then, as shown in FIG. 91, openings 33 are made for connection to the lower electrodes 15 through dielectric layers 32.

  A step of deposition and etching of a metal 34 is illustrated in FIG. 9J, to allow the creation of the contacts at the lower electrodes 15 and the realization of the conductive tracks to the connection pads 19 as previously described with reference to FIG. Figure 5.

  The following steps illustrate a deep etching of the rear face of the substrate to make the rod and the rigid support of the lower electrodes 15.

  In practice, in the step as illustrated in FIG. 9K, a deep etching mask 35, 36 is made on the rear face of the substrate.

  A first deep etching is illustrated in FIG. 9L to initiate the shape of the rigid support 10 '.

  The first deep etch mask 35 is then removed as shown in FIG. 9M.

  A second deep etching is carried out up to the SOI substrate oxide.

  The second mask 36 is also removed as shown in FIG. 9N.

  Before etching the sacrificial oxide layer, the surface layers are first protected by a resin mask 37, for example on the upper face of the substrate as shown in FIG.

  Wet etching of the sacrificial oxide layer is then carried out as illustrated in FIG. 9P to produce the air gap 17 of the capacitors.

  The upper protective layer 37 is also removed.

  It will be noted that, thanks to this method, the dimension of the gap 17 is very precise since it corresponds to the thickness of the sacrificed oxide layer in the basic SOI substrate.

  It is thus possible to obtain an air gap as small as possible to have a maximum sensitivity of the capacity.

  Unlike manufacturing methods in which the different parts are assembled, and for which the control of the gap depends on the machining precision of the parts and the method of assembly, the manufacturing process reproducibly allows to achieve sensors with a small air gap.

  The steps as described with reference to FIGS. 9K to 9P may possibly be modified if the end end 10c of the rod 10 must have a reduced application surface in order to reliably capture a force perpendicular to the plane of the membrane even if it is not uniform in its application plan.

  For this, in the modified step as illustrated in FIG. 9K ', a deep etching double mask 38, 39 is produced whose dimensions of the second mask 39 are smaller than those of the first external mask 38.

  As previously, FIGS. 9L 'and 9M' illustrate the first deep etching and the removal of the first deep etching mask 38.

  The second mask 39 having only a reduced portion in the center of the rod, the second deep etching step as illustrated in FIG. 9N 'makes it possible to carry out an etching until the etching stop under the FIG. oxide and a shoulder in the free end 10c of the rod.

  FIGS. 90 'and 9P' illustrate, in a manner similar to FIGS. 90 and 9P, the protection of the surface layers for the wet etching of the sacrificial oxide layer and the realization of the gap 17.

  In both cases, the value of the air gap can be precisely controlled up to a few hundred nanometers.

  This gap is determined here by the thickness of the buried oxide SOI substrate used initially.

  In addition, this manufacturing method allows to postpone the electrical contacts on the same face of the substrate without the need for the passage of conductors from one substrate to another as would be the case if the force measuring device was made in two parts assembled .

  A second method of manufacture will now be described in which two SOI substrates are glued to each other.

  As illustrated in FIG. 10A, a silicon substrate S is first used.

  A thermal oxidation step is illustrated in Figure 10B.

  The lower electrodes 15 are then made by polysilicon deposition and etching, as illustrated in FIG. 10C.

  An oxide deposit 40 on the upper face of the substrate is then produced, as illustrated in FIG. 10D, then the assembly is subjected to a planarization step in order to obtain a flat surface 40 as illustrated in FIG. 10E.

  The oxide is then partially opened at the lower electrodes 15 as shown in FIG. 10F.

  An SOI substrate is then glued, as illustrated in FIG. 10G, in order to produce the deformable membrane.

  As well illustrated in Figure 10H, this substrate is then thinned. With the use of an SOI substrate, the control of the remaining thickness is easier, especially if the membrane is very thin (less than 2 or 3 m). An anisotropic etching step of the superficial silicon is then carried out. 10I to allow the realization of the electrical connections.

  A layer 41 of a dielectric material is then deposited as illustrated in FIG. 10J, then, as illustrated in FIG. 10K, openings 42 are made at the level of the lower electrodes 15 to allow the making of the electrical contacts through the superficial dielectric layers.

  As illustrated in FIG. 10L, a deposit and a metal etching 43 make it possible to make the contacts at the level of the lower electrodes 15 and the various conductive tracks.

  Finally, a method of etching the rear face is implemented in the same manner as that described above with reference to FIGS. 9K to 9N or 9K 'to 9N' to allow the realization of the rear face.

  Note that in this embodiment, the thickness of the gap 17 depends on the oxide thickness after polishing during the planarization step as described above with reference to Figure 10E.

  In this case also, one can achieve precise control of the air gap, with values of the order of a few hundred nanometers.

  Of course, many modifications can be made to the embodiment described above without departing from the scope of the invention.

  In particular, in all the examples described above, the second electrode 16 is located on the outer face of the membrane 12, opposite the face 10a of the rigid support 10 ', the membrane itself being conductive. Of course, the arrangement of the second electrode 16 could be internal, on the face of the membrane directly opposite the face 10a of the rigid support 10 '.

Claims (11)

  A capacitive sensing force measuring device comprising at least a first electrode (15) disposed on a rigid support (10 ') and at least a second electrode (16) disposed on a deformable membrane (12), facing -vis of said first electrode (15), and a rod (10) connected to said deformable membrane (12), characterized in that said rod (10) is integral with the rigid support (10 ').   2. Force measuring device according to claim 1, characterized in that the rod (10) extends between the rigid support (10 ') and the deformable membrane (12).   3. Force measuring device according to one of claims 1 or 2, characterized in that the rod (10) extends between the central zone (13) of the deformable membrane (12) and the central zone of the support rigid (10 ') placed opposite the diaphragm (12).   4. Force measuring device according to one of claims 1 to 3, characterized in that it comprises a hole (26) in a central zone (13) of the membrane and in the rod (10), this hole (26) being adapted to receive transmission means of the force to be measured.   Force measuring device according to one of Claims 1 to 4, characterized in that the rigid support (10 ') is structured.   6. force measuring device according to one of claims 1 to 5, characterized in that the rigid support (10 ') comprises a plurality of first electrodes (15).   7. force measuring device according to claim 6, characterized in that the deformable membrane (12) comprises a common electrode (16) comprising electrode portions (16a) disposed respectively vis-à-vis said first electrodes (15).   8. force measuring device according to one of claims 1 to 7, characterized in that the rod (10) comprises means for passage (18) of the electrical conductors connected to said at least first electrode (15) of the support rigid (10 ').   9. Force measuring device according to claim 8, characterized in that the connection pads (19, 20) of said first and second electrodes (15, 16) are arranged on a non-deformable portion (14) of the membrane.   10. Force measuring device according to one of claims 1 to 9, characterized in that the rod (10) and the rigid support (10 ') are monobloc.   11. Force measuring device according to one of claims 1 to 10, characterized in that the rod (10) and the rigid support (10 ') 10 are enclosed in a cavity closed by the membrane.