Abstract:
The invention relates to a method ( 3 ) of fabricating a mould ( 39, 39′ ) including the following steps:
       a) depositing ( 9 ) an electrically conductive layer on the top ( 20 ) and bottom ( 22 ) of a wafer ( 21 ) made of silicon-based material;   b) securing ( 13 ) said wafer to a substrate ( 23 ) using an adhesive layer;   c) removing ( 15 ) one part ( 26 ) of said conductive layer from the top of the wafer ( 21 );   d) etching ( 17 ) said wafer as far as the bottom conductive layer ( 22 ) thereof in the shape ( 26 ) of said part removed from the top conductive layer ( 22 ) to form at least one cavity ( 25 ) in said mould.       
 
     The invention concerns the field of micromechanical parts, particularly for timepiece movements.

Description:
[0001]    This application claims priority from European Patent Application No. 09155123.4 filed Mar. 13, 2009, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a mould for fabricating a micromechanical part using galvanoplasty and the method of fabricating said mould. 
       BACKGROUND OF THE INVENTION 
       [0003]    Galvanoplasty has been used and known for a long time. LIGA type methods (a well know abbreviation for the German term “rontgenLIthographie, Galvanoformung &amp; Abformung”) are more recent. They consist in forming a mould by photolithography using a photosensitive resin, and then, by galvanoplasty, growing a metal deposition, such as nickel, therein. The precision of LIGA techniques is much better than that of a conventional mould, obtained, for example, by machining. This precision thus allows the fabrication of micromechanical parts, particularly for timepiece movements, which could not have been envisaged before. 
         [0004]    However, these methods are not suitable for micromechanical parts with a high slenderness ratio, such as a coaxial escape wheel made of nickel-phosphorus containing, for example 12% phosphorus. Electrolytic depositions of this type of part delaminate during plating, because of internal stresses in the plated nickel-phosphorus, which cause it to split away at the interface with the substrate. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to overcome all or part of the aforementioned drawbacks, by proposing an alternative mould that offers at least the same fabrication precision and allows fabrication of parts with several levels and/or a high slenderness ratio. 
         [0006]    The invention therefore concerns a method of fabricating a mould that includes the following steps:
       a) depositing an electrically conductive layer on the top and bottom of a wafer made of silicon-based material;   b) securing said wafer to a substrate using an adhesive layer;   c) removing one part of said conductive layer from the top of the wafer;   d) etching said wafer as far as the conductive layer on the bottom thereof in the shape of said part removed from the top conductive layer to form at least one cavity in said mould.       
 
         [0011]    According to other advantageous features of the invention:
       after step d), the method includes step e): mounting a part on the conductive layer the top of said wafer to form a second level in said mould;   step e) is obtained by structuring a photosensitive resin by photolithography or by securing a pre-etched part made of silicon-based material;   after step d), the method includes step f): mounting a rod in said at least one cavity to form a shaft hole in said part;   the adhesive layer and the conductive layer on the bottom are inverted;   the adhesive layer includes a photosensitive resin;   the substrate includes a silicon-based material;   the method includes step d′): etching the substrate as far as the conductive top layer to form at least one recess in the mould.   after step d′), the method includes step e′): mounting a part on a conductive layer deposited on the top of the substrate to form an additional level in the mould;   after step d′), the method includes step f′): mounting a rod in said at least one hollow to form a shaft hole in the part;   step d) includes the following phases g): structuring a protective mask by photolithography using a photosensitive resin on the portion of the top conductive layer that has not been removed, h): performing an anisotropic etch of the wafer along the parts that are not covered by said protective mask, and i): removing the protective mask;   step d) includes phase h′): performing an anisotropic etch of the wafer using the top conductive layer as a mask to etch the wafer in the parts removed from said conductive layer;   several moulds are fabricated on the same substrate.       
 
         [0024]    The invention also relates to a method of fabricating a micromechanical part by galvanoplasty, characterized in that it includes the following steps:
       j) fabricating a mould in accordance with the method of one of the preceding variants;   k) performing an electrodeposition by connecting the electrode to the conductive layer on the bottom of the wafer made of silicon-based material, to form said part in said mould;   l) releasing the part from said mould.       
 
         [0028]    Finally, the invention advantageously relates to a mould for the fabricating of a micromechanical part by galvanoplasty, characterized in that it includes a substrate, a part made of silicon-based material mounted on said substrate and comprising at least one cavity that reveals an electrically conductive surface of said substrate, allowing an electrolytic deposition to be grown in said at least one cavity. 
         [0029]    According to other advantageous features of the invention:
       the mould has a second part, which is mounted on the first and includes at least one recess that reveals an electrically conductive surface and at least one cavity of said first part for continuing the electrolytic deposition in said at least one recess after said at least one cavity has been filled;   the substrate is formed of a silicon-based material and includes at least one hollow that reveals an electrically conductive surface of said substrate, allowing an electrolytic deposition to be grown in said at least one hollow;   the mould includes an additional part, which is mounted on the substrate and includes at least one recess revealing an electrically conductive surface and at least one hollow in said substrate for continuing the electrolytic deposition in said at least one recess after said at least one hollow has been filled.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Other features and advantages will appear more clearly from the following description, given by way of non-limiting illustration, with reference to the annexed drawings, in which: 
           [0034]      FIGS. 1 to 7  are diagrams of the successive steps of a method of fabricating a micromechanical part in accordance with the invention; 
           [0035]      FIG. 8  is a flow chart of a method of fabricating a micromechanical part in accordance with the invention; 
           [0036]      FIGS. 9 to 13  are diagrams of the final successive steps of a method of fabricating a micromechanical part in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    As  FIG. 8  shows, the invention relates to a method  1  of fabricating a micromechanical part  41 ,  41 ′ by galvanoplasty. Method  1  preferably includes a method  3  of fabricating a mould  39 ,  39 ′ followed by galvanoplasty step  5  and step  7  of releasing part  41 ,  41 ′ from said mould. 
         [0038]    Mould fabrication method  3  includes a series of steps for fabricating a mould  39 ,  39 ′ that includes at least one part  21  made of silicon-based material. In a first step  9  of method  3 , a wafer  21  made of silicon-based material is coated on the top and bottom thereof with electrically conductive layers, respectively referenced  20  and  22  as illustrated in  FIG. 1 . Conductive layers  20 ,  22  may include, for example, gold or copper. 
         [0039]    In a second step  11 , a substrate  23 , which may also be silicon-based, is coated on the top thereof with a layer  24  of adhesive material, as illustrated in  FIG. 2 . This material may, for example, be a non-activated photosensitive resin or more generally an easily removable photosensitive resin. In the third step  13 , adhesive layer  24  is used for at least temporarily securing wafer  21 , coated with substrate  23 , as illustrated in  FIG. 3 . 
         [0040]    According to an alternative of the invention, the adhesive layer  24  and bottom conductive layer  22  are inverted, as explained below. 
         [0041]    In a fourth step  15 , one part  26  of the conductive layer  20  on the top of wafer  21  is removed to reveal part of wafer  21  as illustrated in  FIG. 3 . In a fifth step  17 , wafer  21  is etched until the bottom conductive layer  22  is revealed. According to the invention, etching step  17  is preferably made in the same pattern as part  26  which was removed from conductive layer  20  in step  15 . 
         [0042]    Etching step  17  preferably includes an anisotropic dry attack of the deep reactive ion etching type (DRIE). 
         [0043]    According to a first variant of step  17 , the material of the conductive layer  20  on the top of wafer  21  is chosen to act as a protective mask. Thus, when the assembly of mask  20 -wafer  21  is subjected to the anisotropic etch, only the unprotected parts  26  of the wafer are etched. At the end of step  17 , at least one cavity  25  is thus obtained in wafer  21 , the bottom of which partially reveals bottom conductive layer  22  as illustrated in  FIG. 4 . 
         [0044]    According to a second variant of step  17 , firstly, a protective mask is coated on wafer  21 , preferably in the same shape as removed parts  26  for example, via a photolithographic method using a photosensitive resin. Secondly, when the mask-wafer assembly is subjected to the anisotropic etch, only the unprotected parts of the wafer are etched. Finally, in a third phase, the protective mask is removed. At the end of step  17 , at least one cavity  25  is thus obtained in wafer  21 , the bottom of which partially reveals the bottom conductive layer  22  as illustrated in  FIG. 4 . 
         [0045]    In the case of the aforecited alternative illustrated in triple lines in  FIG. 8 , in which adhesion layer  24  and bottom conductive layer  22  are inverted, it is no longer necessary, in a step  18 , to continue said cavity  25  into adhesive layer  24  to reveal said bottom conductive layer  22 . Preferably, the material used in this alternative is then a photosensitive resin which is exposed to radiation in order to continue cavity  25 . 
         [0046]    After step  17 , the invention provides two embodiments. In a first embodiment, illustrated in a single line in  FIG. 8 , after step  17 , mould fabricating method  3  is finished and micromechanical part fabricating method  1  continues immediately with galvanoplasty step  5  and step  7  of releasing the part from said mould. Galvanoplasty step  5  is achieved by connecting the deposition electrode to bottom conductive layer  22  of wafer  21  so as to grow, firstly, an electrolytic deposition in cavity  25  of said mould, and then in step  7 , the part formed in cavity  25  is released from said mould. 
         [0047]    According to this first embodiment, it is clear that the micromechanical part obtained has a single level whose shape is identical throughout the entire thickness thereof. 
         [0048]    According to a second embodiment of the invention, illustrated in double lines in  FIG. 8 , step  17  is followed by step  19  for forming at least one second level in mould  39 . Thus, the second level is achieved by mounting a part  27  on one part of the top conductive layer  20 , which was not removed in step  15 . 
         [0049]    Part  27  is preferably formed on conductive layer  20  in a recess  28  of larger section than the removed parts  26 , for example, via a photolithographic method using a photosensitive resin. 
         [0050]    Moreover, as illustrated in  FIG. 5 , in step  19 , a rod  29  is preferably mounted to form shaft hole  42  for micromechanical part  41  straight away during the galvanoplasty. This not only has the advantage of meaning that part  41  does not need to be machined once the galvanoplasty has finished, but also means that an internal section of any shape can be formed, whether uniform or not, over the entire height of hole  42 . Rod  29  is preferably obtained in step  19  at the same time as part  27 , for example, via a photolithographic method using a photosensitive resin. 
         [0051]    In the second embodiment, mould  39  fabrication method  3  ends after step  19 , and the micromechanical part fabrication method  1  continues with galvanoplasty step  5  and step  7  of releasing the part from said mould. 
         [0052]    Galvanoplasty step  5  is achieved by connecting the deposition electrode to conductive layer  22  on the bottom of wafer  21 , firstly, to grow an electrolytic deposition in cavity  25  of said mould, and then, exclusively in a second phase, in recess  28 , as illustrated in  FIG. 6 . 
         [0053]    Indeed, advantageously according to the invention, when the electrolytic deposition is flush with the top part of cavity  25 , it electrically connects conductive layer  20 , which enables the deposition to continue to grow over the whole of recess  28 . Advantageously, the invention allows fabrication of a part with a high slenderness ratio, i.e. wherein the section of cavity  25  is much smaller than that of recess  28 , avoiding delamination problems even with a nickel-phosphorus material containing, for example, 12% phosphorus. 
         [0054]    Owing to the use of silicon under conductive layer  20 , delamination phenomena at the interfaces decrease, which avoids splitting, caused by internal stresses in the electrodeposited material. 
         [0055]    According to the second embodiment, fabrication method  1  ends with step  7 , in which the part  41  formed in cavity  25  and then in recess  28  is released from mould  39 . Release step  7  could, for example be achieved by delaminating layer  24  or by etching substrate  23  and wafer  21 . According to this second embodiment, it is clear, as illustrated in  FIG. 7 , that the micromechanical part  41  obtained has two levels  43 ,  45 , each of different shape and perfectly independent thickness. 
         [0056]    This micromechanical part  41  could, for example, be a coaxial escape wheel, or escape wheel  43 -pinion  45  assembly with geometrical precision of the order of a micrometer, but also ideal referencing, i.e. perfect positioning between said levels. 
         [0057]    According to second variant of method  1  illustrated by a double dotted lines in  FIGS. 1 to 5  and  8  to  13 , it is possible to add at least a third level to mould  39 . The second variant remains identical to method  1  described above as far as step  17 ,  18  or  19 , depending upon the alternative or variant used. In the example illustrated in  FIGS. 9 to 13 , we will take the second embodiment as illustrated in double lines in  FIG. 8 , as the starting point. 
         [0058]    Preferably, according to this second variant, substrate  23  is formed from a silicon-based material and is etched to form a hollow  35  in mould  39 ′. 
         [0059]    As can be seen, preferably between  FIG. 5  and  FIG. 9 , a deposition  33  has been performed in one part of the first cavity  25  to provide a conductive layer that is thicker than layer  22  alone, for the purpose of mechanically withstanding the steps of the second variant of method  1 . Preferably, this deposition  33  is performed by starting step  5  up to a predetermined thickness. However, this deposition can be performed in accordance with a different method. 
         [0060]    As illustrated in double dotted lines in  FIG. 8 , the second variant of method  1  applies steps  17 ,  18  and/or  19  of the end of method  3  to substrate  23 . Thus, in the new step  17 , substrate  23  is etched until conductive layer  22  is revealed. Etch step  17  preferably includes deep reactive ion etching (DRIE). 
         [0061]    Preferably, firstly, as illustrated in  FIG. 9 , a protective mask  30  is coated on substrate  23 , comprising pierced parts  36  for example, via a photolithographic method using a photosensitive resin. Secondly, the mask  30 -substrate  23  assembly is subjected to the anisotropic etch, with only the unprotected parts of the substrate being etched. 
         [0062]    Thirdly, protective mask  30  is removed. At least one hollow  35  is thus obtained in substrate  23 , the bottom of which partially reveals adhesive layer  24 , as illustrated in  FIG. 10 . Finally, fourthly, hollow  35  is extended into layer  24  and, possibly, also into layer  22 . The material used for adhesive layer  24  is preferably a photosensitive resin which is exposed to radiation to continue hollow  35 . At the end of step  17 , at least one hollow  35  is thus obtained in substrate  23 , the bottom of which partially reveals conductive layer  22  or, possibly, deposition  33 . 
         [0063]    Of course, in a similar way to that explained above, a conductive layer can also be deposited on substrate  23  instead of photostructured resin mask  30 , the material of which is chosen so that it can act as protective mask. 
         [0064]    Likewise, in the case of the aforecited alternative in which adhesive layer  24  and bottom conductive layer  22  are inverted, it is no longer necessary to continue said hollow  35  into adhesive layer  24  to reveal conductive layer  22  or, possibly, deposition  33 . 
         [0065]    After step  17  of the second variant of method  1 , the invention can also provide the two aforecited embodiments, i.e. continuing with galvanoplasty step  5  and release step  7 , or continuing with a step  19  to form at least one additional level on substrate  23 . To simplify the Figures,  FIGS. 11 to 13  are realised from the first embodiment. 
         [0066]    Preferably, whichever embodiment is chosen, as illustrated in  FIG. 11 , a rod  37  is mounted to form hole  42 ′ for micromechanical part  41 ′ immediately during the galvanoplasty. Preferably, if rods  29  and  37  are formed respectively in cavity  25  and hollow  35 , they are aligned. Preferably, rod  37  is obtained, for example, via a photolithographic method using a photosensitive resin. 
         [0067]    After the new steps  17  or  19 , galvanoplasty step  5  is performed by connecting the deposition electrode to conductive layer  22  to grow an electrolytic deposition in hollow  35 , but also to continue the growth of deposition  33  in cavity  25 , and then, exclusively in a second phase, in recess  28 , as illustrated in  FIG. 12 . Fabrication method  1  ends with step  7 , in which part  41 ′ is released from mould  39 ′ as explained above. 
         [0068]    According to this second variant, it is clear, as illustrated in  FIG. 13 , that the micromechanical part  41 ′ obtained has at least three levels  43 ′,  45 ′ and  47 ′, each of different shape and perfectly independent thickness, with a single shaft hole  42 ′. 
         [0069]    This micromechanical part  41 ′ could, for example, be a coaxial escape wheel  43 ′,  45 ′ with its pinion  47 ′, or a wheel set with three levels of teeth  43 ′,  45 ′,  47 ′ with geometrical precision of the order of a micrometer, but also ideal referencing, i.e. perfect positioning between said levels. 
         [0070]    Of course, the present invention is not limited to the example illustrated, but is open to various alterations and variants, which will be clear to those skilled in the art. In particular, part  27  could include a pre-etched silicon-based material, and then be secured to conductive layer  20 . 
         [0071]    Moreover, several moulds  39 ,  39 ′ are fabricated from the same substrate  23  to achieve series fabrication of micromechanical parts  41 ,  41 ′, which are not necessarily identical to each other. 
         [0072]    Likewise, a rod  29  can be formed in cavity  25  to form a shaft hole  42  for the future part  41 , even within the scope of the first, single level embodiment. One could also envisage changing silicon-based materials for crystallised alumina or crystallised silica or silicon carbide. 
         [0073]    Finally, layer  20  formed in step  9 , and then partially pierced in step  15 , can also be obtained via a single, selective, deposition step  15 . This step  15  could then consist, firstly, in depositing a sacrificial layer in the same shape as section  26 , prior to deposition of conductive layer  20 . Secondly, a conductive layer  20  is deposited on top of the assembly. Finally, in a third phase, the sacrificial layer is removed and, incidentally, the conductive layer part deposited thereon, which provides the same layer  20  as that visible in  FIG. 3 . This step  15  is known as “lift-off”.