Abstract:
The invention relates to a method of fabricating a composite micromechanical component, particularly for timepiece movements, including steps: a) providing a substrate including a horizontal top layer and a horizontal bottom layer made of electrically conductive, micromachinable material, and secured to each other by an electrically insulating, horizontal, intermediate layer; b) etching a pattern in the top layer through to the intermediate layer, thereby forming at least one cavity in the substrate; c) coating the top part of the substrate with an electrically insulating coating; d) directionally etching the coating and the intermediate layer to limit the presence thereof exclusively at each vertical wall; e) performing an electrodeposition by connecting the electrode to the conductive bottom layer of the substrate to form at least one metal part of the component; g) releasing the composite component from the substrate.

Description:
This application claims priority from European Patent Application No. 09162292.8 filed Jun. 9, 2009, the entire disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a composite micromechanical component, at least one surface of which has a low friction coefficient, and a method of fabricating the same. 
     BACKGROUND OF THE INVENTION 
     EP Patent No. 2 060 534 discloses a method of fabricating a silicon-metal composite micromechanical component, obtained from photolithography using photosensitive resins, silicon etching and galvanic growth. However, this method is complex to implement for metal parts over several levels and final coating steps have to be provided to improve the tribological properties of silicon. 
     Moreover, a method of this type is not suitable for micromechanical components with a high slenderness ratio where a material such as nickel-phosphorus with, for example 12% phosphorus, tends to peel off. The galvanic depositions of this type of component delaminate because of inner stresses in the deposited nickel-phosphorus. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome all or part of the aforementioned drawbacks by proposing a composite micromechanical component whose useful parts are coated with a better tribological material than the micromachinable material used, and a method of fabricating such components that includes fewer steps. 
     The invention therefore relates to a method of fabricating a composite micromechanical component that includes the following steps:
         a) providing a substrate that includes a horizontal top layer and a horizontal bottom layer made of electrically conductive, micromachinable material, secured to each other by an electrically insulating, horizontal, intermediate layer;   b) etching at least one pattern in the top layer through to the intermediate layer, so as to form at least one cavity in the substrate that will form at least one part, made of micromachinable material, of the composite component;   c) coating the top part of said substrate with an electrically insulating coating;   d) directionally etching said coating and said intermediate layer so as to limit the presence thereof exclusively at each vertical wall formed in the top layer;   e) performing an electrodeposition by connecting the electrode to the conductive bottom layer of the substrate to form at least one metallic part of the composite component;   f) releasing the composite component from the substrate.       

     Thus, advantageously according to the invention, deposition of the layer that is tribologically better than the micromachinable material is entirely integrated within the fabrication method and not performed subsequent to fabrication of said component. 
     According to other advantageous features of the invention:
         after step d) a part is assembled above the top layer to form at least one recess that communicates with said at least one cavity, so as to form a second level of said component;   prior to step e), the method includes step g): assembling a pin in said at least one cavity so as to form a hole in the future composite component;   step b) includes phase h): structuring at least one protective mask on the conductive top layer, phase i): performing an anisotropic etch of said top layer over the parts that are not protected by said at least one protective mask, and phase j): removing said at least one protective mask;   prior to step e), the method includes step g): assembling a pin in said at least one cavity so as to form a hole in the future composite component;   prior to step f), the method includes step b′): etching a pattern in the bottom layer though to said metal part so as to form at least a second cavity in said substrate, step c′): coating the bottom part of said substrate with a second electrically insulating coating, and step e′): performing an electrodeposition by connecting the electrode to the conductive bottom layer of the substrate so as to finish forming the metal parts of said component;   after step c′), the method includes step d′): directionally etching said second coating so as to reveal exclusively the bottom of the bottom layer;   a part is assembled after step d′) so as to form at least one recess that communicates with said at least one second cavity, offering a second additional layer to said component;   prior to step e′) the method includes step g′): assembling a pin in said at least one second cavity of the bottom layer so as to form a hole in the future composite component;   step b′) includes phase h′): structuring at least one protective mask on the conductive bottom layer, phase i′): performing an anisotropic etch of said bottom layer in the parts that are not protected by said at least one protective mask, and phase j′): removing the protective mask;   several composite micromechanical components are fabricated on the same substrate;   the conductive layers include a doped silicon-based material.       

     The invention also relates to a composite micromechanical component that includes a horizontal silicon part, which includes a hole receiving a metal part, characterized in that the silicon part is formed by doped silicon and includes at least one vertical part for transmitting a mechanical force that is coated with silicon dioxide to improve the tribological qualities of said doped silicon. This component can advantageously be used for transmitting forces via its silicon-dioxide coated silicon part, for example by being driven in at the metal part. 
     In accordance with other advantageous features of the invention:
         the metal part includes a portion that projects from said silicon part so as to form a uniquely metal level above the silicon part;   said silicon part cooperates, via a silicon dioxide layer, with a second silicon part;   the second silicon part is formed by doped silicon and has vertical, silicon-dioxide walls to improve the tribological qualities of said doped silicon;   the second silicon part includes at least one hole for receiving a second metal part;   said second metal part includes a portion that projects from said second silicon part so as to form a uniquely metal level below the second silicon part;   each metal part includes a hole for driving said component against a pivot.       

     Finally, the invention also relates to a timepiece that includes a composite micromechanical component in accordance with one of the preceding variants, wherein at least one silicon part forms a wheel or escapement pallets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages will appear clearly from the following detailed description, given by way of non-limiting indication, with reference to the annexed drawings, in which: 
         FIGS. 1 to 6  are diagrams of the successive steps of a method of fabricating a micromechanical component in accordance with a first embodiment of the invention; 
         FIG. 7  is a micromechanical component according to a first embodiment of the invention; 
         FIGS. 8 to 13  are diagrams of the successive steps of a method of fabricating a micromechanical component in accordance with a second embodiment of the invention; 
         FIG. 14  is a flow chart of a method of fabricating a micromechanical component according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As  FIG. 14  shows, the invention relates to a method  1  of fabricating a composite micromechanical component  41 ,  41 ′. Method  1  preferably includes a preparation method  3  followed by galvanoplasty step  5  and step  7  of releasing the composite component  41 ,  41 ′ thereby formed. 
     Preparation method  3  includes a series of steps for preparing a substrate  9 ,  9 ′ made of at least partially micromachinable material, such as, preferably, silicon-based materials. Preparation method  3  is for facilitating the reception and growth of galvanic deposition step  5 . 
     A first step  10  of method  3  consists in taking a substrate  9  that includes a top layer  21  and a bottom layer  23 , made of electrically conductive micromachinable material, and secured to each other by an electrically insulating, intermediate layer  22 , as illustrated in  FIG. 1 . 
     Preferably, substrate  9  is a Silicon On Insulator (S.O.I). Thus, intermediate layer  22  is preferably made of silicon dioxide. Moreover, top layer  21  and bottom layer  23  are made of crystalline silicon, sufficiently doped for said layers to be electrically conductive. 
     According to the invention, method  3  includes a second step  11 , consisting in structuring at least one protective mask  24  on conductive top layer  21  as illustrated in  FIG. 1 . As  FIG. 1  also shows, mask  24  has at least one pattern  26  which does not cover top layer  21 . This mask  24  may, for example, be obtained by photolithography using a positive or negative photosensitive resin. 
     In a third step  12 , top layer  21  is etched until intermediate layer  22  is revealed. According to the invention, etching step  12  preferably includes a DRIE type anisotropic dry etch. The anisotropic etch is performed in top layer  21  in accordance with pattern  26  of mask  24 . 
     In a fourth step  14 , mask  24  is removed. Thus, as visible in  FIG. 2 , at the end of fourth step  14 , top layer  21  is etched over the entire thickness thereof with at least one cavity  25 , thus forming a silicon part of at least one final composite component  41 ,  41 ′. 
     In a fifth step  16 , an electrically insulating coating  30  is deposited, coating the entire top of substrate  9  as shown in  FIG. 3 . Preferably, coating  30  is obtained by oxidising the top of the etched top layer  21 , and intermediate layer  22 . As  FIG. 3  shows, a silicon-dioxide layer is thus obtained both on the top of top layer  21  and intermediate layer  22  and on the vertical walls  51 ,  52  of top layer  21 . 
     In accordance with a sixth step  18 , a directional etch is performed on coating  30  and intermediate layer  22 . Step  18  is for limiting the presence of insulating layers exclusively on each vertical wall formed in top layer  21 , i.e. walls  51 ,  52  respectively of the exterior of the future composite component  41 ,  41 ′ and said at least one cavity  25 . According to the invention, during a directional or anisotropic etch, the vertical component of the etch phenomenon is favoured relative to the horizontal component, by modulating, for example the chamber pressure (very low pressure work), in a reactive ion type etch reactor. This etching may, for example, be “ion milling” or “sputter etching”. As this step  18  is carried out and as shown in  FIG. 4 , it is clear that the bottom of cavity  25  is formed by electrically conductive bottom layer  23  and the top of substrate  9  is formed by top layer  21 , which is also conductive. 
     In order to improve the adhesion of the subsequent galvanoplasty of step  5 , an adhesion layer can be provided on the bottom of each cavity  25  and/or on the top of top layer  21 . The adhesion layer could then consist of a metal such as the alloy CrAu. 
     Preferably, in sixth step  18 , a pin  29  can also be assembled so as to form an arbour hole  42 ,  42 ′ straight away for the composite micromechanical component  41 ,  41 ′ in galvanoplasty step  5 . This not only has the advantage of meaning that component  41 ,  41 ′ does not require machining once the galvanoplasty has finished, but also means that an inner section of any shape can be made, whether uniform or not, over the entire top of hole  42 ,  42 ′. Pin  29  is preferably obtained, for example, via a photolithographic method using a photosensitive resin. 
     After step  18 , preparation method  3  is finished and method  1  of fabricating the composite micromechanical component continues with galvanoplasty step  5  and step  7  of releasing said component. 
     Galvanoplasty step  5  is performed by connecting the deposition electrode to bottom layer  23  so as to grow an electrolytic deposition  33  in cavity  25 . 
     Fabrication method  1  ends with step  7  in which the component formed by top layer  21  and the metal part deposited in cavity  25  is released from the rest of substrate  9 , i.e. from bottom layer  23  and pin  29 . According to this embodiment, it is clear that the micromechanical component obtained has a single level of identical shape over the entire thickness thereof which may include an arbour hole. 
     This micromechanical component could, for example, be an escape wheel, escapement pallets or even a pinion including a metal part at the arbour hole thereof which allows said micromechanical component to be driven in. Moreover, the external wall of the silicon part has a silicon dioxide layer with more advantageous features than those of silicon part  21 , offering geometrical precision of the order of a micrometer. 
     According to an alternative to this embodiment, illustrated by a double line in  FIG. 14 , after step  18 , preparation method  3  includes an extra step  20  for forming at least a second level  45 ,  45 ′ of the metal part  43   43 ′, as illustrated in  FIG. 5 . Thus, second level  45 ,  45 ′ is made by mounting a part  27 , with electrically insulating walls  32 , on top layer  21 , which was not etched during step  12 . 
     Preferably the added part  27  forms at least one recess  28  of larger section than the parts removed in accordance with pattern  26 , for example via a photolithographic method using a photosensitive resin. However, part  27  could also include a silicon-based material that is pre-etched, and then secured to conductive layer  21 . 
     Consequently, according to the alternative to the embodiment above, after step  20 , preparation method  3  is finished and method  1  of fabricating the composite micromechanical component  41  continues with galvanoplastic step  5  and step  7  of releasing said component from substrate  9 . 
     Galvanoplasty step  5  is performed by connecting the deposition electrode to bottom layer  23  so as to grow, firstly, an electrolytic deposition in cavity  25 , and then, only secondly, in recess  28 , as illustrated in  FIG. 5 . 
     Indeed, advantageously according to the invention, when the electrolytic deposition is flush with the top part of cavity  25 , it electrically connects top layer  21  (or, possibly, its adhesion layer), which allows homogenous horizontal growth of the deposition in the whole of recess  28 . Thus, the invention provides composite components  41  that have a first metal part  43  over the same thickness as top layer  21  and a second, projecting, metal part  45 . 
     Advantageously, the second metal part  45  can have a high slenderness ratio, i.e. the section of cavity  25  can be much smaller than that of recess  28 . Indeed, because of method  1 , part  45  is fabricated avoiding any peeling off problems, even with a deposited metal such as nickel-phosphorus, containing for example, 12% phosphorus. This advantageous effect is due in part to the use of silicon as conductive layers  21 ,  23  (and possibly their adhesion layer), which decreases delamination phenomena at the interfaces. 
     According to the alternative of the above embodiment, fabrication method  1  ends with step  7 , in which the formed component  41  is released, i.e. part  27  and pin  29  are removed and component  41  is removed from layers  22 ,  23  of substrate  9 . 
     It is clear that, as illustrated in  FIGS. 6 and 7 , the composite micromechanical component  41  obtained has two levels, each of different shape over a perfectly independent thickness and able to include a single arbour hole  42 . The first level thus includes top layer  21 , whose vertical walls  51 ,  52  are coated with silicon dioxide and whose inner cavity  25  receives a first part  43  of the galvanic deposition. The second level is formed exclusively by the second metal part  45 , which extends as an extension of the first part  43  and projects from top layer  21 . In the example illustrated in  FIGS. 6 and 7 , it will be noted that second part  45  also partially overlaps with top layer  21 . 
     As  FIGS. 6 and 7  show, micromechanical component  41  can consequently have the same first level as that obtained by the embodiment without step  20  and thus have geometrical precision of the order of a micrometer but also ideal referencing, i.e. perfect positioning between the two levels. Micromechanical component  41  can then form a wheel set including a toothed wheel  2  and a pinion  4  such as, for example, an escape wheel. According to the invention, the micromechanical component obtained is not limited to a wheel set. In a variant, it is perfectly possible to envisage obtaining pallets  2  with single block pallet stones coated with silicon dioxide and including a dart  4 . 
     According to a second embodiment of method  1  (illustrated in double dotted lines in  FIG. 14 ) partly representing a continuation of the embodiment already explained. As illustrated in  FIGS. 8 to 13 , it is thus possible to apply method  3  to bottom layer  23  as well, so as to add at least one or two other additional levels to said micromechanical component. To avoid overloading the Figures, a single example is detailed above, but it is clear that bottom layer  23  can also be transformed in accordance with the embodiment explained above with or without the alternative. 
     The steps of the second embodiment remain identical or similar to method  1  described above as far as step  18  or  20 . In the example illustrated in  FIGS. 8 to 13 , we will take the embodiment example with the alternative step  20 , illustrated in  FIG. 5 , as the starting point for method  1 . 
     Preferably, according to this second embodiment, bottom layer  23  will be etched so as to form at least one second cavity  35 . As can be seen, preferably between  FIG. 5  and  FIG. 8 , a deposition  33  has been performed in one part of the first cavity  25  so as to provide a start layer for the second galvanoplasty. Preferably, this deposition  33  is carried out by starting step  5  up to a predetermined thickness. However, this deposition  33  can be performed in accordance with another method. 
     As illustrated by double dotted lines in  FIG. 14  and  FIGS. 8 to 13 , the second embodiment of method  1  applies steps  11 ,  12 ,  14 ,  16  and  18  of the first embodiment of method  3  explained above to bottom layer  23 . 
     Thus, according to the second embodiment, method  3  includes a new step  11 , consisting in structuring at least one mask  34  on conductive bottom layer  23  of substrate  9 ′ as illustrated in  FIG. 9 . As  FIG. 9  also shows, mask  34  has at least one pattern  36  and  31 , which does not cover bottom layer  23 . This mask  34  can, for example, be obtained by photolithography using a photosensitive resin. 
     Next, in new step  12 , layer  23  is etched in accordance with patterns  36  and  31  until electrically conductive deposition  33  and intermediate layer  22  are revealed. Then protective mask  34  is removed in a new step  14 . Thus, as  FIG. 10  shows, at the end of step  14 , bottom layer  23  is etched over its entire thickness with at least one cavity  35  and  39 . 
     In a new step  16 , an electrically insulating coating  38  is deposited, covering the whole of the bottom of substrate  9 ′, as illustrated in  FIG. 11 . Preferably, coating  38  is obtained by depositing a silicon dioxide on the bottom of the bottom layer  23 , for example using vapour phase deposition. 
     Preferably, in our example of  FIGS. 8 to 13 , a new step  18  is only performed for removing the oxide layer present in the bottom of said at least one cavity  35 . However, if a second level is desired, a directional etch is performed on all the horizontal parts of coating  38 . The new step  18  would then be for limiting the presence of the insulating layer exclusively at each vertical wall  53 ,  54  formed in bottom layer  23 , i.e. the walls of the outside of the future component  41 ′ and said at least one cavity  35 . 
     In new step  18 , as previously explained, a pin  37  can be assembled so as to form arbour hole  42 ′ in micromechanical component  41 ′ immediately in galvanoplasty step  5  with the same advantages mentioned above. 
     In the second embodiment of method  1 , after step  18 , preparation method  3  is finished and method  1  of fabricating the micromechanical component continues with galvanoplasty step  5  and step  7  of releasing composite component  41 ′. Preferably, if pins  29  and  37  are respectively formed in cavities  25  and  35 , they are aligned. Moreover, pin  37  is, preferably, obtained via a photolithography method using a photosensitive resin. 
     After new step  18  (or  20 ), galvanoplasty step  5  is performed by connecting the deposition electrode to bottom layer  23  so as to grow an electrolytic deposition in cavity  35  but also to continue the growth of the deposition in cavity  25 , and then, only secondly, in recess  28 , as illustrated in  FIG. 12 . In the case of the example illustrated in  FIG. 12 , to connect said electrode, it is thus, for example, possible to etch one part of the silicon dioxide layer  38  contained below top surface  23  in order to access it. One could also envisage directly connecting deposition  33 . 
     Fabrication method  1  according to the second embodiment ends with step  7 , in which component  41 ′ is released, i.e. part  27  and pins  29 ,  37  are removed and component  41 ′ is withdrawn from substrate  9 ′. 
     According to this second embodiment, it is clear, as illustrated in  FIG. 13 , that the composite micromechanical component  41 ′ obtained has at least three levels, each of different shape over a perfectly independent thickness, and with a single arbour hole  42 ′. The first level thus includes top layer  21 , whose internal walls  51 ,  52  are coated with silicon dioxide and whose inner cavity  25  receives a first part  43 ′ of the galvanic deposition. The second level is formed exclusively by the second metal part  45 ′, which extends as an extension of first part  43 ′ and as a projecting portion from top layer  21 . Finally, the third layer is formed by bottom layer  23 , whose vertical walls  53 ,  54  are coated with silicon dioxide and whose inner cavity  35  receives a third part  47 ′ of the galvanic deposition. 
     Micromechanical component  41 ′ can consequently have the same first two levels as that obtained via the first embodiment with step  20 . This micromechanical component  41 ′ could, for example, be a coaxial escape wheel  21 - 52 ,  23 - 54  with its pinion  45 ′ or a wheel set with three layers of teeth  21 - 52 ,  23 - 54 ,  45 ′ with geometrical precision of the order of a micrometer, but also ideal referencing, i.e. perfect positioning between said levels. 
     Of course, this invention is not limited to the illustrated example but can have various variants and modifications, which will appear to those skilled in the art. Thus, several composite micromechanical components  41 ,  41 ′ can be fabricated on the same substrate  9 ,  9 ′ so as to achieve mass production of micromechanical components  41 ,  41 ′, which are not necessarily identical to each other. Likewise, one could also envisage changing the silicon-based materials for crystallised alumina or crystallised silica or silicon carbide. One could also envisage insulating depositions  30  and/or  38  being different in nature and/or being each deposited via different methods from those explained above.