Abstract:
A mechanical microstructure including a deformable first layer overhanging a second layer and defining a cavity set back from an external face of the deformable first layer and having an abutment stud projecting into the cavity, in which a wire is connected to a portion of an internal face of the deformable first layer. The portion of the first layer is opposite a bottom area of the cavity into which the abutment stud projects, but the abutment stud remains at a distance from the deformable first layer. A method of producing the mechanical microstructure is also disclosed.

Description:
This application claims priority under 35 U.S.C. §119 to French patent application No. 0117014, filed Dec. 28, 2001. 

   TECHNICAL FIELD 
   The present invention relates to method of producing a mechanical microstructure and to a mechanical microstructure having an electrically active portion. This kind of microstructure can advantageously constitute sensor, for example a pressure sensor, or an actuator, for example an optical microsystem (MOEMS). It has a preferred, but not exclusive, application in the field of inertial components, such as accelerometers. 
   BACKGROUND 
   The commonly assigned French patent FR 2,558,263 discloses the fabrication of micromechanical devices by microelectronic techniques. These methods produce a microstructure, which is defined as a mechanical structure produced using microelectronic techniques, that is to say collective processing of substrates by deposition, etching, photolithography, etc. 
   In the case of accelerometers, the electrically active portion can be partly carried by a mobile member, and in this case acceleration is measured by measuring a variation in the electrical capacitance between the mobile member and a fixed portion of the mechanical microstructure, for example. 
   The above kind of microstructure can include a cap to confine the active portion, as disclosed in particular in the commonly assigned French patent FR 2,770,339. 
   During this development process, the inventors were first confronted with the problem of stray capacitances appearing between the active portion and the cap. It is necessary to reduce the stray capacitances, especially if a highly sensitive sensor is required. 
   To this end, and in accordance with a first feature of the invention disclosed in FR 2,770,339, the inventors formed a recess in the cap, vertically aligned with and at a predetermined distance from the active portion. 
   The predetermined distance is in particular sufficient to limit or even prevent the appearance of the stray capacitance previously cited. 
   Furthermore, and as explained in the French patent FR 2,770,339, the electrically active portion can be produced in a layer formed on a substrate including one or more holes opening onto an internal face of the layer, through which an electrical contact is made with the electrically active portion on the internal face of the layer. 
   The electrical contact can be made with a metal deposit, for example using an ultrasound welding technique known to the person skilled in the art. 
   The inventors of the subject matter disclosed in FR 2,770,339 were confronted with problems related to the small thickness of the layer in which the active portion is produced (for example 60 μm); and more particularly to its great fragility. 
   During their research, they found that deterioration could occur, especially at the moment of producing the electrical contact with the internal face of the layer, as, if the external face of the layer is facing the cavity, the ultrasound vibration combined with the pressure of the welding device is likely to lead to rupturing of the thin layer, because it is very fragile. 
   They further found that providing too great a predetermined distance, in accordance with the first feature of the invention referred to above, was likely to encourage such rupturing, as it is difficult if not impossible to arrive at a compromise between minimizing the stray capacitances and avoiding rupture. 
   BRIEF SUMMARY 
   To address the foregoing problem, the present invention is directed to a method of producing a mechanical microstructure including a deformable first layer overhanging a second layer including a cavity set back from an external face of the deformable first layer, in which method a wire is connected to a portion of an internal face of the deformable first layer. 
   According to one aspect of the invention, the aforementioned portion faces a bottom area in the cavity into which an abutment stud projects and is at a distance from the deformable first layer. 
   Thanks to these features, the invention overcomes the drawbacks previously cited. This is because:
         first, thanks to the localized character of the abutment stud, the recess in vertical alignment with the first layer is at a predetermined distance sufficient to prevent the formation of stray capacitances between the first layer and the second layer, and   second, because of the vertical alignment with the connecting portion and because the distance between the first layer and the second layer is reduced, without being reduced to zero, the abutment stud limits deformation of the thin first layer during welding, which prevents local damage to the layer.       

   According to a first feature, the microstructure further includes a support layer for said first layer and in the course of the method according to invention a well is formed in the support opening opposite the portion of the internal face of the first layer and the wire is connected to the first layer via the well. 
   According to another feature, after the step of forming the well, a localized contact layer is deposited on the bottom of well and said connection between the connecting wire and said first layer is made by means of the contact layer. 
   Thus the connection can be obtained using the Ball Bonding technique. 
   The present invention also provides a mechanical microstructure including a deformable first layer overhanging a second layer including a cavity set back from an external face of the deformable first layer and includes a portion to which is connected a wire. 
   According to another aspect of the invention, the aforementioned portion is opposite a bottom area of the cavity into which an abutment stud projects remains at a distance from the deformable first layer. 
   According to one particular feature the microstructure includes an insulative layer between the first layer and the support layer and the insulative layer includes an undercut under the edges of the well. 
   This feature is particularly advantageous because it guarantees that there is an electrical discontinuity between any deposit on a wall of the well and the localized contact layer. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other aspects and advantages of the present invention will become more clearly apparent on reading the following description of particular embodiments, which description is given by way of nonlimiting example only and with reference to the accompanying drawings, in which: 
       FIG. 1  illustrates a geophone microstructure in accordance with the invention nearing completion, 
       FIG. 2  illustrates a bottom view of a different embodiment in accordance with the invention of the microstructure shown in  FIG. 1 , 
       FIG. 3  illustrates a plan view of a second substrate in accordance with the invention after freeing a sensitive portion, 
       FIG. 4  illustrates an enlarged view of the microstructure from  FIG. 1  in accordance with the invention, after turning it over for fixing an electrical connection, and 
       FIG. 5  illustrates a diagrammatic general view in accordance with the invention of a substrate supporting a thin layer to which an electrical connection is made in an overhanging area. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a geophone microstructure approaching completion and having at least the following three main portions: a central layer  20  including a mobile mass  24  sensitive to the accelerations to be detected by the geophone, a support layer  10  that carries the central portion, and a top layer  30 , which covers the central layer and defines with the support layer  10 , an enclosure that can be evacuated if necessary. 
   In a preferred embodiment, each layer is made from a silicon substrate. 
   One or more of the substrates can instead be made from a different material. 
   Each substrate can initially have the same characteristics and in particular the same thickness, for example 450 μm. 
   In a preferred embodiment, the top layer  30  corresponding to the top portion is 300 μm thick. 
   However, since, in a geophone, the central layer must in principle have a much smaller final thickness (it is referred to as a thin layer), it is necessary, prior to the specific steps that produce the required microstructure, to reduce the thickness of the central layer  20  to obtain a layer a few tens of microns thick, for example. 
   In a preferred embodiment of the invention, the final thickness of the central layer  20  is of the order of about 60 microns. 
   The central layer can be obtained by any appropriate means known in the art, for example by etching the surplus portion of the substrate. 
   The production of the above kind of microstructure is described briefly next. 
   The support layer  10  is first coated with a discontinuous oxide layer  12  which is applied to the portions of the face of the support layer  10  intended to be attached to the central layer  20 . 
   The oxide layer  12  is preferably made from the same material as the support layer  10 , for example, both can be silicon oxide. 
   The central layer  20  is then attached to the support layer  10 , in which a cavity  15  has been formed, for example by etching through a mask (not shown). 
   The thin layer is preferably transferred to the support layer  10  using a vacuum Si/SiO 2  molecular bonding technique. Here, the oxide layer  12  provides insulation between the central layer and the support layer  10 . In this bonding technique, subsequent annealing at a high temperature, for example about 1100° C. (see below), strengthens the attachment. 
   After the bonding and annealing operations, the central layer  20  is thinned by mechanical-chemical machining and/or by chemical etching to obtain a layer  20  of the required thickness. 
   The support layer  10  is etched completely through its thickness at the location of an opening  11 A to form a well  17  for making an electrical contact. 
   In this example the etching is anisotropic etching using KOH, producing a well  17  with a section that decreases in size toward the top or, more specifically, toward the central layer  20 . 
   The etching, in practice using a chemical etching technique (for example employing hydrofluoric acid HF), continues through the oxide layer  12 , which produces an undercut  18  in the oxide layer  12  under the edges of the well  17 , and therefore vertically aligned therewith vis a vis the thin layer. 
   As can be seen in  FIG. 2 , there is advantageously a plurality of contact wells  17 , the number of wells being defined as a function of the number of connections to be made to the thin layer (there is thus an equal number of openings). In this example the wells are identical and have a square section. 
   Using a mask, not shown, a portion  23  (structure) is then cut into the central layer  20 , for instance by dry etching (for example Deep Reactive Dry Etching (DRIE)), so as to mechanically and electrically trim the structure, which after this etching is freed with respect to the remainder of the thin layer and the remainder of the whole. 
   As shown in  FIG. 3 , the structure obtained in this example has a central layer  24  connected to the edges of the cavity by supporting beams  25  which act as return springs and which are connected to two anchor studs  26 . 
   Cantilever beams  24 A near the fixed cantilever beams  27  are fastened to this central mass by virtue of a capacitive effect, the cantilevered beams measure the amplitude of movement of the central portion in response to acceleration. For more information on this, the reader may refer to FR 2,558,263 referred to above. 
   Top layer  30 , forming a cap, is then placed over the central layer  20 , which is itself placed on the support layer  10 . 
   The top layer  30  can be attached to, the combination of the central layer  20  and the support layer  10  using, for example, a eutectric attachment technique known in the art and not described here. 
   After the three layer  10 ,  20  and  30  shown in  FIG. 1  have been fastened together, provision is made for connecting the active portion to the outside world. 
   Thus,  FIG. 1  shows the above combination after fixing connecting wires  51  and  52 , one of which is connected to the exterior surface of the support layer  10  and the other of which is fixed at the bottom of a well  17 . 
   As can be seen in  FIG. 4 , the connection at the bottom of the well  17  is advantageously made after depositing a localized contact layer  53 . 
   This layer can be produced by using the walls of the well as a mask, for example, in which case the localized contact layer shown in this Figure has the same section as the bottom portion of the well. 
   The existence of a deposit on the walls of this well is not a problem because the undercut  18  in the oxide layer  12  guarantees that there is a discontinuity between any deposit on this wall and the localized layer. 
   In practice, electrical contacts are made to the thin layer in an area outside the opening within which the central portion moves. 
   However, the thin layer, which is built-in but overhanging the location concerned, may not be rigid enough to resist the thrust forces deployed in welding a wire to the localized area. In the  FIG. 4  configuration, these thrust forces are directed downward. 
   Preferably, a stud  37  is included in the top layer  30  specifically to prevent these thrust forces from damaging the central layer  20 . A stud  37  is located in the third substrate  30  opposite to each area of the central layer in which a weld must be made. 
   Thus, the stud  37  constitutes an abutment that limits deformation of the central layer during the welding operation to minimize damage to the central layer. The welding operation can therefore be carried out in total safety, for example using a ball bonding technique. 
     FIG. 5  shows the general principle of making this connection: a connecting wire  52 ′ is advantageously connected to a deformable layer portion B overhanging a support A including a cavity opposite a bottom area of the cavity into which an abutment stud  37 ′ projects, although the stud remains at a distance from the layer. 
   The distance of the abutment stud  37  or  37 ′ from the central layer depends on the build-in distance and, in this example, is advantageously of the order of few microns. 
   The foregoing description applies to geophones and to various other systems or microstructures in which there is a need to connect a thin layer overhanging a substrate. The connection to the contact studs is advantageously made by means of wells and abutment studs as described hereinabove.