Abstract:
The invention concerns the field of micromechanical parts, in particular, for timepiece movements. The invention relates to a method of fabricating a mould that includes the following steps: (a) providing a substrate that has a top layer and a bottom layer made of electrically conductive, micromachinable material, and secured to each other by an electrically insulating, intermediate layer; (b) etching at least one pattern in the top layer as far as the intermediate layer to form at least one cavity in the mould; (c) coating the top part of the substrate with an electrically insulating coating; and (d) directionally etching the coating and the intermediate layer to limit the presence thereof exclusively at each vertical wall formed in the top layer.

Description:
[0001]    This application is a divisional of U.S. patent application Ser. No. 12/723,191, filed Mar. 12, 2010, which claims priority from European Patent Application No. 09155125.9 filed Mar. 13, 2009, the entire disclosures of which are 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 “röntgenLlthographie, 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) providing a substrate that has a top layer and a bottom layer made of electrically conductive, micromachinable material, and secured to each other by an electrically insulating, intermediate layer;   b) etching at least one pattern in the top layer as far as the intermediate layer so as to form at least one cavity in said mould;   c) coating the top part of said substrate with an electrically insulating coating;   d) directionally etching said coating and said intermediate layer to limit their presence exclusively at each vertical wall formed in said top layer.       
 
         [0011]    According to other advantageous features of the invention:
       a second pattern is etched in step b) to form at least one recess that communicates with said at least one cavity, providing said top layer with a second level;   after step d), a part is mounted to form at least one recess that communicates with said at least one cavity, providing said mould with a second level;   the method includes the final step e): mounting a rod in said at least one cavity to form a hole in the future part made in said mould;   step b) includes phase f): structuring at least one protective mask on the conductive top layer, phase g): performing an anisotropic etch of said top layer on the parts that are not coated by said at least one protective mask and phase h): removing the protective mask;   after the preceding steps, the method includes step a′): depositing an electrically conductive material in the bottom of said at least one cavity, b′): etching a pattern in the bottom layer as far as the deposition of said conductive material, to form a least one cavity in said mould and c′): coating the whole assembly with a second, electrically insulting coating;   after step c′), the method includes step d′): directionally etching said second coating to limit the presence thereof exclusively at each vertical wall formed in said bottom layer;   a second pattern is etched during step b′) to form at least one recess that communicates with said at least one cavity, providing said bottom layer with a second level;   a part is mounted after step d′) to form at least one recess that communicates with said at least one cavity, providing said mould with a second level;   the method includes the final step e′): mounting a rod in said at least one cavity in the bottom layer to form a hole in the future part made in said mould;   step b′) includes phase f′): structuring at least one protective mask on the conductive top layer, g′): performing an anisotropic etch of said top layer on the parts that are not covered by said at least one protective mask and h′): removing the protective mask;   several masks are fabricated on the same substrate;   the conductive layers include a doped, silicon-based material.       
 
         [0024]    The invention also relates to a method of fabricating a micromechanical part using galvanoplasty, characterized in that it includes the following steps:
       i) fabricating a mould in accordance with the method of one of the preceding variants;   j) performing an electrodeposition by connecting the electrode to the conductive bottom layer of the substrate to form said part in said mould;   k) releasing the part from said mould.       
 
         [0028]    Finally, the invention relates to a mould for fabricating a micromechanical part using galvanoplasty, characterized in that it includes a substrate that has a top layer and a bottom layer, which are electrically conductive and secured to each other by an electrically insulating, intermediate layer, wherein the top layer has at least one cavity, which reveals part of the bottom layer of said substrate and has electrically insulating walls, allowing an electrolytic deposition to be grown in said at least one cavity. 
         [0029]    According to other advantageous features of the invention:
       the top layer also has at least one recess that communicates with said at least one cavity and has electrically insulating walls for continuing the electrolytic deposition in said at least one recess after said at least one cavity has been filled;   the bottom layer includes at least one cavity that reveals part of the electrically conductive layer of said substrate and has electrically insulating walls, allowing an electrolytic deposition to be grown in said at least one cavity in the bottom layer;   the bottom layer also has at least one recess that communicates with said at least one cavity in the bottom layer and has electrically insulating walls for continuing the electrolytic deposition in said at least one recess, after said at least one cavity in the bottom layer 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 a first embodiment of the invention; 
           [0035]      FIGS. 8 to 12  are diagrams of the successive steps of a method of fabricating a micromechanical part in accordance with a second embodiment of the invention; 
           [0036]      FIG. 13  is a flow chart of a method of fabricating a micromechanical part in accordance with the invention; 
           [0037]      FIGS. 14 to 19  are diagrams of the successive steps of a method of fabricating a micromechanical part in accordance with a variant of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0038]    As  FIG. 13  shows, the invention relates to a method  1  of fabricating a micromechanical part  41 ,  41 ′,  41 ″ using galvanoplasty. Method  1  preferably includes a method  3  of fabricating a mould  39 ,  39 ′,  39 ″ followed by galvanoplasty step  5  and step  7  of releasing part  41 ,  41 ′,  41 ″ from said mould. 
         [0039]    Mould fabrication method  3  includes a series of steps for fabricating a mould  39 ,  39 ′,  39 ″ that preferably includes silicon-based materials. 
         [0040]    A first step  10  of method  3  consists in taking a substrate  9 ,  9 ′ that includes a top layer  21 ,  21 ′ and a bottom layer  23 ,  23 ′, which are made of electrically conductive, micromachinable material and secured to each other by an electrically conductive, intermediate layer  22 ,  22 ′, as illustrated in  FIGS. 1 to 8 . 
         [0041]    Preferably, substrate  9 ,  9 ′ is a S.O.I. (Silicon On Insulator). Moreover, top and bottom layers  21 ,  21 ′ and  23 ,  23 ′ are made of crystalline silicon, sufficiently doped to be electrically conductive and the intermediate layer is made of silicon dioxide. 
         [0042]    According to the invention, method  3  includes two distinct embodiments after step  11 , respectively represented by a triple line and a single line in  FIG. 13 . 
         [0043]    According to a first embodiment, in step  11 , protective masks  15 , then  24 , are structured on conductive top layer  21  as illustrated in  FIG. 2 . As  FIG. 2  also shows, mask  15  has at least one pattern  27  which does not cover top layer  21 . Moreover, mask  24 , which preferably totally covers mask  15 , has at least one pattern  26 , which does not cover top layer  21 . 
         [0044]    By way of example, mask  15  can be made by depositing a silicon oxide layer to form said mask to a predetermined depth. Next, mask  24  can, for example, be obtained by photolithography, using a photosensitive resin to cover mask  15 . 
         [0045]    According to the first embodiment shown in a triple line in  FIG. 13 , in a third step  2 , top layer  21  is etched to reveal intermediate layer  22 . According to the invention, etching step  2  preferably includes an anisotropic dry attack of the Deep Reactive Ion Etching type (DRIE). 
         [0046]    First of all in step  2 , an anisotropic etch is performed in top layer  21  in pattern  26  of mask  24 . This etch is the start of the etching of at least one cavity  25  in top layer  21  over one part of the thickness thereof. Secondly, mask  24  is removed, then a second anisotropic etch is performed in pattern  27  of mask  15  that is still present on top layer  21 . The second etch continues the etching of said at least one cavity  25 , but also starts the etching of at least one recess  28 , which communicates with said at least one cavity  25 , but has a larger section. 
         [0047]    In a fourth step  4 , mask  15  is removed. Thus, as  FIG. 3  shows, at the end of fourth step  4 , the entire thickness of top layer  21  is etched with said at least one cavity  25  and a part of the thickness thereof is etched with said at least one recess  28 . 
         [0048]    In a fifth step  6 , an electrically insulating coating  30  is deposited, covering the entire top of substrate  9 , as illustrated in  FIG. 4 . Coating  30  is preferably obtained by oxidising the top of the etched top layer  21  and intermediate layer  22 . 
         [0049]    In a sixth step  8 , a directional etch of coating  30  and intermediate layer  22  is performed. Step  8  is for limiting the presence of the insulating layers exclusively at each vertical wall formed in top layer  21 , i.e. walls  31  and  32  respectively of said at least one cavity  25  and said at least one recess  28 . 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 working pressure), in a RIE reactor. This etch may be, by way of example, ion milling or sputter etching. 
         [0050]    By performing step  8 , as illustrated in  FIG. 5 , it is clear that the bottom of cavity  25  reveals the electrically conductive, bottom layer  23  and that the bottom of recess  28  reveals top layer  21 , which is also conductive. 
         [0051]    In order to improve the adhesion of the future galvanoplasty, an adhesion layer can be provided on the bottom of each cavity  25  and/or on the bottom of each recess  28 . The adhesion layer could thus consist of a metal, such as the alloy CrAu. 
         [0052]    Preferably, during sixth step  8 , as illustrated in  FIG. 5 , a rod  29  is mounted to form the shaft hole  42  for micromechanical part  41  immediately during galvanoplasty step  5 . This not only has the advantage of avoiding the need to machine part  41  once the galvanoplasty has finished, but also means that an inner section of any shape, whether uniform or not, can be made over the entire height of hole  42 . Preferably, rod  29  is obtained, for example, via a photolithographic method using a photosensitive resin. 
         [0053]    In the first embodiment, after step  8 , method  3  of fabricating mould  39  is finished and method  1  of fabricating the micromechanical part continues with galvanoplasty step  5  and step  7  of releasing part  41  from mould  39 . 
         [0054]    Galvanoplasty step  5  is achieved by connecting the deposition electrode to bottom layer  23  of mould  39  to grow, firstly, an electrolytic deposition in cavity  25  of said mould, and then exclusively in a second phase, in recess  28 , as illustrated in  FIG. 6 . 
         [0055]    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 , possibly by the adhesion layer thereof, which enables the deposition to continue growing over the whole of recess  28 . Advantageously, the invention enables parts  41  with a high slenderness ratio to be made, i.e. wherein the section of cavity  25  is much smaller than that of recess  28 . This avoids delamination problems, even with a nickel-phosphorus material, containing, for example, 12% phosphorus. 
         [0056]    Owing to the use of silicon for conductive layers  21 ,  23 , and possibly for their adhesion layer, delamination phenomena at the interfaces decreases, which avoids splitting caused by internal stresses in the electrodeposited material. 
         [0057]    According to the first embodiment, fabrication method  1  ends with step  7 , in which part  41 , formed in cavity  25  and then in recess  28 , is released from mould  39 . Release step  7  can, for example, be achieved by etching layers  23  and  21 . According to this first 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 and including a single shaft hole  42 . 
         [0058]    This micromechanical part  41  could, for example, be a coaxial escape wheel or an 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. 
         [0059]    According to a second embodiment of the invention, method  3  has a second step  11 , consisting in structuring at least one protective mask  24 ′ on the conductive top layer  21 ′ as illustrated in  FIG. 8 . As  FIG. 8  also shows, mask  24 ′ includes at least one pattern  26 ′, which does not cover top part  21 ′. This mask  24 ′ can, for example, be obtained by photolithography using a photosensitive resin. 
         [0060]    In a third step  12 , top layer  21 ′ is etched until it reveals intermediate layer  22 ′. According to the invention, etching step  12  preferably includes a dry anisotropic attack of the deep reactive ion etching type (DRIE). The anisotropic etch is performed on top layer  21 ′ in pattern  26 ′ of mask  24 ′. 
         [0061]    In a fourth step  14 , mask  24 ′ is removed. Thus, as  FIG. 9  shows, at the end of fourth step  14 , the entire thickness of top layer  21 ′ is etched with at least one cavity  25 ′. 
         [0062]    In a fifth step  16 , an electrically insulating coating  30 ′ is deposited, covering the whole top of substrate  9 ′ as illustrated in  FIG. 10 . Coating  30 ′ is preferably obtained by oxidising the top of the etched top layer  21 ′ and intermediate layer  22 ′. 
         [0063]    According to a sixth step  18 , coating  30 ′ and intermediate layer  22 ′ are directionally etched. Step  18  is for limiting the presence of insulating layers exclusively at each vertical wall formed in top layer  21 ′, i.e. walls  31 ′ of said at least one cavity  25 ′. By performing this step  18  and as illustrated in  FIG. 11 , it is clear that the bottom of cavity  25 ′ reveals the electrically conductive bottom layer  23 ′ and the top of top layer  21 ′, which is also conductive. 
         [0064]    As in the first embodiment, in order to improve the adhesion of the future galvanoplasty, 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. 
         [0065]    During sixth step  18 , as explained for the first embodiment of  FIGS. 1 to 7 , a rod can be mounted to form the shaft hole for the micromechanical part straight away in galvanoplasty step  5 , with the same advantages indicated above. 
         [0066]    In the second embodiment, after step  18 , method  3  of fabricating mould  39 ′ ends and method  1  of fabricating the micromechanical part continues with galvanoplasty step  5  and step  7  of releasing the part from mould  39 ′. 
         [0067]    Galvanoplasty step  5  is performed by connecting the deposition electrode to bottom layer  23 ′ of mould  39 ′ to grow an electrolytic deposition in cavity  25 ′ of mould  39 ′. 
         [0068]    According to the second embodiment, fabrication method  1  ends with step  7 , which is similar to that explained in the first embodiment, and in which the part formed in cavity  25 ′ is released from mould  39 ′. According to this second embodiment, it is clear that the micromechanical part obtained has a single level of identical shape throughout the entire thickness thereof and it may contain a shaft hole. 
         [0069]    This micromechanical part could, for example, be an escape wheel, or escape pallets or even a pinion with geometrical precision of the order of a micrometer. 
         [0070]    According to an alternative of this second embodiment illustrated by a double line in  FIG. 13 , after step  18 , method  3  of fabricating mould  39 ′ includes an additional step  20  for forming at least a second level in mould  39 ′ as illustrated in  FIG. 12 . Thus, the second level is made by mounting a part  27 ′, which includes electrically insulating walls  32 ′, on top layer  21 ′, which was not removed during step  12 . 
         [0071]    Preferably, the added part  27 ′ forms at least one recess  28 ′ of larger section than the removed parts  25 ′, for example, via a photolithographic method using a photosensitive resin. However, part  27 ′ could also include an insulating, silicon-based material that is pre-etched and then secured to conductive layer  21 ′. 
         [0072]    Consequently, according to the alternative of the second embodiment, after step  20 , method  3  of fabricating mould  39 ′ ends and method  1  of fabricating the micromechanical part continues with galvanoplasty step  5  and step  7  of releasing part  41 ′ from mould  39 ′. 
         [0073]    Galvanoplasty step  5  is performed by connecting the deposition electrode to bottom layer  23 ′ of mould  39 ′ in order, firstly, to grow an electrolytic deposition in cavity  25 ′ of said mould, then, exclusively in a second phase, in recess  28 ′, as illustrated in  FIG. 12 . 
         [0074]    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 ′, possibly by the adhesion layer thereof, which enables the deposition to continue growing over the whole of recess  28 ′. Advantageously, the invention enables parts  41 ′ with a high slenderness ratio to be made, i.e. wherein the section of cavity  25 ′ is much smaller than that of recess  28 ′. This avoids delamination problems even with a nickel-phosphorus material, containing, for example, 12% phosphorus. 
         [0075]    Owing to the use of silicon for conductive layers  21 ′,  23 ′, and possibly for their adhesion layer, delamination phenomena at the interfaces decreases, which avoids splitting caused by internal stresses in the electrodeposited material. 
         [0076]    According to the second embodiment alternative, fabrication method  1  ends with step  7 , as explained in the first embodiment, in which part  41 ′ formed in mould  39 ′ is released. It is clear, as illustrated in  FIG. 12 , that the micromechanical part  41 ′ obtained has two levels, each of different shape and perfectly independent thickness and they may include a single shaft hole. This micromechanical part  41 ′ can consequently have the same shape as part  41  obtained with the first embodiment and it can therefore have geometrical precision of the order of a micrometer, but also ideal referencing, i.e. perfect positioning between said levels. 
         [0077]    According to a variant (illustrated in double dotted lines in  FIG. 13 ) of the two embodiments of method  1  seen in  FIGS. 14 to 19 , it is also possible to apply method  3  to bottom layer  23 ,  23 ′, to add one or two other levels to mould  39 ,  39 ′. To avoid overloading the Figures, a single example is detailed below, but it is clear that bottom layer  23 ,  23 ′ can also be transformed in accordance with the first and second embodiments (with or without the variant) explained above. 
         [0078]    The variant remains identical to method  1  described above until step  8 ,  18  or  20 , depending upon the embodiment used. In the example illustrated in  FIGS. 14 to 19 , we will take the example of the first embodiment, illustrated in triple lines in  FIG. 13 , as the starting point of the method  1 . 
         [0079]    Preferably, according to this variant, bottom layer  23  will be etched to form at least a second cavity  35  in mould  39 ″. As can be seen, preferably between  FIG. 5  and  FIG. 14 , a deposition  33  has been made in one part of the first cavity  25  to provide a galvanoplastic start layer. Preferably, this deposition  33  starts at step  5  up to a predetermined thickness. However, this deposition can be performed in accordance with a different method. 
         [0080]    As illustrated in double dotted lines in  FIG. 13  and  FIGS. 14 to 19 , the variant of method  1  applies steps  11 ,  12 ,  14 ,  16  and  18  of the second embodiment of method  3  to bottom layer  23 . 
         [0081]    Thus, according to the variant, method  3  includes a new step  11 , consisting in structuring at least one mask  34  on the conductive bottom layer  23 , as illustrated in  FIG. 15 . As  FIG. 15  also shows, mask  34  includes at least one pattern  36 , which does not cover bottom layer  23 . This mask  34  can, for example, be obtained by photolithography using a photosensitive resin. 
         [0082]    Next, in the new step  12 , layer  23  is etched in pattern  36  until the electrically conductive deposition  33  is revealed. Then, protective mask  34  is removed in a new step  14 . Thus, as  FIG. 16  shows, at the end of step  14 , the entire thickness of bottom layer  23  is etched with at least one cavity  35 . 
         [0083]    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. 17 . Coating  38  is preferably obtained by depositing a silicon oxide on the top of bottom layer  23 , for example, using a vapour phase deposition. 
         [0084]    A new step  18  is preferably unnecessary if a single level is added to mould  39 ″. Otherwise, directional etching of coating  38  is performed. The new step  18  would be for limiting the presence of the insulating layer exclusively at each vertical wall  39  formed in bottom layer  23 , i.e. the walls of said at least one cavity  35 . In our example of  FIGS. 14 to 19 , a new step  18  is only carried out to remove the oxide layer present in the bottom of said at least one cavity  35 . 
         [0085]    In the new step  18 , as explained previously, a rod  37  can be mounted to form shaft hole  42 ″ in the micromechanical part  41 ″ immediately during galvanoplasty step  5 , with the same aforecited advantages. 
         [0086]    In the variant of method  1 , after step  18 , method  3  of fabricating mould  39 ″ ends and method  1  of fabricating the micromechanical part continues with galvanoplasty step  5  and step  7  for releasing part  41 ″ from mould  39 ″. Preferably, if rods  29  and  37  are respectively formed in cavities  25  and  35 , they are aligned. Rod  37  is preferably obtained, for example, via a photolithographic method using a photosensitive resin. 
         [0087]    After new steps  8 ,  18  or  20 , galvanoplasty step  5  is performed by connecting the deposition electrode to bottom layer  23  to grow an electrolytic deposition in cavity  35 , but also to continue the growth of the deposition in cavity  25 , then, exclusively in a second phase, in recess  28 , as illustrated in  FIG. 18 . Fabrication method  1  ends with step  7 , in which part  41 ″ is released from mould  39 ″, as explained above. 
         [0088]    According to this variant, it is clear, as illustrated in  FIG. 19 , 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 ″. 
         [0089]    This micromechanical part 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. 
         [0090]    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. Thus, several moulds  39 ,  39 ′,  39 ″ are fabricated on the same substrate  9 ,  9 ′,  9 ″ to achieve series fabrication of micromechanical parts  41 ,  41 ′,  41 ″, which are not necessarily identical to each other. Likewise, one could envisage changing silicon-based materials for crystallised alumina or crystallised silica or silicon carbide.