Abstract:
The invention relates to a method ( 1 ) of manufacturing a silicon-metal composite micromechanical component ( 51 ) combining DRIE and LIGA processes. The invention also relates to a micromechanical component ( 51 ) including a layer wherein one part ( 53 ) is made of silicon and another part ( 41 ) of metal so as to form a composite micromechanical component ( 51 ). The invention concerns the field of timepiece movements.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a silicon-metal composite micromechanical component and a method of manufacturing the same. 
       BACKGROUND OF THE INVENTION 
       [0002]    Silicon is known in tribology for its low friction coefficient. The application of silicon in the field of mechanical watchmaking is advantageous particularly for escape systems and more specifically for the impulse pinions of escape wheels. However, silicon is also known in mechanics for its low plastic deformation zone. The brittle nature of silicon means that it is difficult to adapt to the usual techniques of driving parts onto arbours. 
       SUMMARY OF THE INVENTION 
       [0003]    It is an object of the present invention to overcome all or part of the aforementioned drawbacks by proposing a manufacturing method that can advantageously produce a composite micromechanical component that can be easily adapted to most horological applications. 
         [0004]    The invention thus relates to a method of manufacturing a silicon-metal composite micromechanical component including the following steps:
       a) taking a substrate including a top and bottom silicon layer between which an intermediate silicon oxide layer extends,   b) selectively etching at least one cavity in the top layer to define the pattern of a silicon part of said component;   c) continuing the etch of said at least one cavity in the intermediate layer;       characterized in that it further includes the following steps:
       d) growing a metal layer from at least one portion of said at least one cavity in order to form a metal part in the thickness of said component in order to insulate the silicon part of said micromechanical component from destructive stress;   e) releasing the silicon-metal composite micromechanical component from the substrate.   
       
 
         [0011]    The method advantageously provides a monoblock component, which enjoys the tribological properties of silicon and the mechanical properties of metal. 
         [0012]    According to other advantageous features of the invention: step d) includes the following steps:
       covering the top of the substrate with photosensitive resin,   selectively performing photolithography on the photosensitive resin in order to photostructure said resin in accordance with the predetermined pattern of the metal part;   depositing a metal layer by electroplating, starting from the conductive top surface of the bottom layer, which is vertical to said at least one cavity, and growing therein, from the bottom, the layer between respectively the photostructured resin and the intermediate or top layer for forming the metal part in accordance with said pattern;   and in that step e) is achieved by removing the photostructured resin.   said conductive top surface of the bottom layers, which is vertical to said at least one cavity is made conductive by doping the bottom layer and/or by depositing a conductive layer;   during the photolithography step, the photostructured resin projects from the top layer of the substrate so that the layer can continue to grow by electroplating at least between said projecting portions of the photostructured resin in order to form a second metal part of the micromechanical component above the silicon part;   the method includes, after step d), a step of machining the top surface of the substrate in order to make the metal layer the same height as the top end of said photostructured resin;   the metal layer includes nickel;   the method includes, before the release step, steps of machining and etching at least one cavity in the bottom layer of the substrate to form a second silicon part of the micromechanical component in accordance with a determined thickness and shape;   the method includes, between the steps of machining and etching the bottom substrate layer and the release step, a step of growing a second metal layer by electroplating in at least one portion of said at least one cavity of the bottom layer in order to form at least one additional metal part in the thickness of the bottom layer;       
 
         [0023]    the growth step includes the following steps:
       covering the bottom of the substrate with photosensitive resin;   selectively performing photolithography on the photosensitive resin to photostructure the resin in accordance with the predetermined pattern of the metal part;   depositing a metal layer by electroplating from the bottom of said at least one cavity by growing, from the bottom, the layer for forming the metal part in accordance with said pattern;   during the photolithography step the photostructured resin projects from the bottom layer of the substrate so that the layer can continue to grow by electroplating in order to form a second additional metal part of the micromechanical component below the second silicon part;   prior to the release step, the method includes a step of machining the bottom surface of the substrate in order to make the metal part the same height as the bottom end of said photostructured resin;   the second electroplated metal layer includes nickel;   several micromechanical components are produced on the same substrate.       
 
         [0031]    The invention also relates to a silicon-metal composite micromechanical component comprising one part formed in a silicon layer, characterized in that said silicon part includes a toothing for forming a wheel or a pinion and, at least over a portion of the thickness thereof, a metal part with a thickness of more than 6 microns, which insulates the silicon part from destructive stress. 
         [0032]    The monoblock component thus enjoys the tribological features of silicon and the mechanical features of metal. 
         [0033]    According to other advantageous features of the invention:
       the metal part forms a sleeve that covers the peripheral wall of said silicon part;   the metal part forms a sleeve in a cavity made in the silicon part for receiving a pivoting arbour that is driven therein;   each sleeve is connected to the wall of the silicon part by bridges of material;   each metal part includes in the extension thereof a second metal part projecting from the silicon part;   the second metal part includes a toothing for forming a wheel or a pinion;   each metal part includes nickel;   the component comprises a second silicon part formed from a second layer;   the second silicon part includes, at least over one portion of the thickness thereof, an additional metal part for insulating the second silicon part from destructive stress;   the additional metal part forms a sleeve in a cavity made in the second silicon part for receiving a pivoting arbour that is driven therein;   said sleeve is connected to the wall of said cavity by bridges of material;   the additional metal part includes, in the extension thereof, a second additional metal part projecting from the second silicon part;   the second additional metal part includes a toothing for forming a wheel or pinion;   the second silicon part is mounted on the silicon part via an intermediate silicon oxide layer;   the second silicon part includes a toothing for forming a wheel or pinion.       
 
         [0048]    Finally, the invention relates to a timepiece, characterized in that it includes at least one composite micromechanical component in accordance with one of the preceding variants. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]    Other features and advantages will appear clearly from the following description, given by way of non-limiting illustration, with reference to the annexed drawings, in which: 
           [0050]      FIGS. 1 to 7  are cross-sections of a composite micromechanical component at different phases of the manufacturing method according to the invention; 
           [0051]      FIG. 1   b  is a perspective diagram of  FIG. 1 ; 
           [0052]      FIG. 8  is a diagram of a first example of the final step according to the method of the invention; 
           [0053]      FIG. 9  is a diagram of a second example of the final step according to the invention; 
           [0054]      FIG. 10  is a flow diagram of the manufacturing method according to the invention; 
           [0055]      FIG. 11  is a perspective diagram of a gear train according to the invention; 
           [0056]      FIG. 12  is a top view of a hand obtained in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0057]    The invention relates to a method of manufacturing  1  a silicon-metal composite micromechanical component  51 . As  FIGS. 1 to 7  show, method  1  includes a series of steps for forming at least one components  51 ,  51 ′,  51 ″,  51 ′″, which may be complex and/or made over several layers and/or with several materials. The object of method  1  consists in offering a minimum of one component including at least one silicon part and at least one metal part. 
         [0058]    The first step  11  consists in taking a silicon on insulator (SOI) substrate  3 . Substrate  3  includes a top layer  5  and a bottom layer  7  made of mono or polycrystalline silicon. An intermediate layer  9 , formed of amorphous silicon oxide (SiO 2 ) extends between top and bottom layers  5  and  7 . 
         [0059]    In step  11 , substrate  3  is preferably chosen such that, as seen in 
         [0060]      FIG. 1 , the height of top and intermediate layers  5  and  9  match the final height of one portion  53  of the final micromechanical component  51 . 
         [0061]    In a second step  13 , as  FIGS. 1 and 1   b  show, cavities  37 ,  45  are selectively etched, for example by a deep reactive ion etching (DRIE) method in top silicon layer  5 . These two cavities  37  and  45  form the pattern defining the inner and outer contours of silicon part  53  of micromechanical component  51 . 
         [0062]    In the example illustrated in  FIG. 1 , cavity  37  is present on each side of cavity  45 , since, as  FIG. 1   b  shows, it is substantially annular and surrounds cavity  45 . The proximal wall of cavity  37  is preferably selectively etched to form a toothing  55  on the peripheral edge of portion  53 . Cavity  45  is substantially cylindrical with a disk-shaped section and it is coaxial to annular cavity  37 . 
         [0063]    In a third step  15 , wet or dry chemical etching is carried out to extend cavities  37  and  45  in intermediate layer  9 , so that part  53  is formed in accordance with the same pattern in intermediate layer  9  until bottom layer  7  is partially exposed. 
         [0064]    Method  1  according to the invention then comprises implementation of a LIGA process  19 , comprising a series of steps ( 17 ,  21  and  23 ) for electroplating a metal in a particular shape on the top surface of substrate  3  using a photostructured resin. 
         [0065]    In a fourth step  17 , a layer of photosensitive resin  57  is deposited on the top surface of substrate  3 , as shown in  FIG. 2 . Step  17  can be achieved using a mould casting method. Photosensitive resin  57  is preferably of the Su- 8  type, for example the Microchem Corp “nano™Su-8” product. 
         [0066]    In a fifth step  21 , photolithography is performed, i.e. an impression is made in said resin by selective exposure to radiation R, using, for example, a partially pierced mask M, as shown in  FIG. 2 . Next, resin  57  is developed, i.e. all of the portions of resin  57  that have not been exposed to radiation R are removed. The resin thereby photostructured  71 ,  73  and  75  forms the metal layer, in accordance with the predetermined shape. 
         [0067]    In the example of  FIG. 3 , the photostructured resin includes a bottom ring  71 , a top ring  73  and a cylinder  75 . In the example of  FIG. 3 , the shape of bottom ring  71  matches that of cavity  37 . Top ring  73  covers bottom ring  71  and partially covers top layer  5  of substrate  3 . Finally, the height of cylinder  75  is substantially equivalent to the thickness of the stack of rings  71  and  73 , and the cylinder is centred in cavity  45 . In the example illustrated in  FIGS. 1 to 7 , the inner diameter of top ring  73  includes a toothing  59 . 
         [0068]    In a sixth step  29 , a conductive anchoring layer  61  is preferably deposited on the top surface of substrate  3 , as shown in  FIG. 4 . This step may be achieved, for example, via a conventional metallizing method using vacuum cathodic sputtering. Preferably, layer  61  includes gold, i.e. pure gold or a gold alloy. The thickness of layer  61  can be comprised between 10 and 100 nm. 
         [0069]    In a seventh step  23 , the start of a metal layer is made by electroplating the top surface of substrate  3 , i.e. a metal layer  63  is grown to form at least one metal part  41  on micromechanical component  51 . In the example illustrated in  FIG. 5 , layer  63  starts substantially on the top surface of bottom layer  7  exposed by cavity  45 . 
         [0070]    The presence of photostructured resin cylinder  75  forces the metal layer  63  to grow in successive annular rings between cylinder  75  and intermediate layer  9 , then between cylinder  75  and top layer  5  of substrate  3 . This first electroplating phase forms a first metal part  41  in at least one portion of cavity  45 . 
         [0071]    It is thus clear from this first phase  23  that micromechanical component  51  is now formed on a layer that includes a silicon and silicon oxide portion  53  wherein at least one portion of one of cavities  45  includes a metal part  41 . 
         [0072]    In the example illustrated in  FIG. 5 , the electroplating is continued, forming layers not in cavity  45 , but above and also on one part of top layer  5  of substrate  3 . The successive layers are then formed exclusively between top ring  73  and photostructured resin cylinder  75 . It is thus clear that the layer  39  formed in the second phase is substantially annular in shape and that the external diameter of said layer includes a toothing that is the reverse of toothing  59  of photostructured resin top ring  73 . At the end of step  23 , it may happen that a metal layer  63  is obtained over the entire top surface of substrate  3 , as seen in  FIG. 5 . 
         [0073]    Layer  63 , i.e. particularly metal parts  39  and  41  preferably include nickel, i.e. nickel or a nickel alloy. The potential difference of substrate  3  necessary at electroplating step  23  is preferably achieved via contact on the bottom and/or top surface thereof. 
         [0074]    In an eighth step  25 , the top surface of substrate  3  is machined, for example by lapping, so as to make the height of metal part  39 , obtained during said second phase of step  23 , level with the thickness of top ring  73  and cylinder  75  in the photostructured resin, as shown in  FIG. 6 . This allows the second metal part  39  that includes the reverse toothing (hereafter referenced  59 ) to be delimited correctly. 
         [0075]    It is thus clear, at this step  25 , that micromechanical component  51  is now formed over two layers. The first layer includes a silicon and silicon oxide part  53  wherein at least one portion of one of cavities  45  includes a metal part  41 . The second layer formed above the first comprises a second metal part  39 . 
         [0076]    In the ninth and tenth steps  27  and  31 , as illustrated by the double lines in  FIG. 10 , a second silicon part  65  is formed in bottom layer  7  of substrate  3 . During step  27 , the bottom surface of substrate  3  is machined to reduce the thickness thereof to the value of the desired final bottom layer  7 . During step  31 , cavities  47 ,  49  are etched, for example by a DRIE method, in bottom silicon layer  7 . As in steps  13  and  15 , these two cavities  47  and  49  form the pattern defining the contour of second silicon part  65  of micromechanical component  51 . 
         [0077]    In the example illustrated in  FIG. 7 , cavity  49  is present on both sides of cavity  47 , since it is substantially annular and surrounds cavity  47 . 
         [0078]    The proximal wall of cavity  49  is preferably etched selectively to form a toothing  67  on the peripheral edge of second portion  65 . Cavity  47  is substantially cylindrical with a disk-shaped section and it is coaxial to annular cavity  49 . 
         [0079]    There is no preferred sequence for steps  27  to  31  and these steps can therefore occur in any order. Machining step  27  preferably consists of mechanochemical polishing such as lapping by chemical abrasion. 
         [0080]    It is thus clear that, after steps  27  and  31 , micromechanical component  51  is now formed over three layers. The first layer includes one silicon and silicon oxide part  53  wherein at least one portion of one of cavities  45  includes a metal part  41 . The second layer above the first is formed by a second metal part  39 . The third layer below the first is formed by a second silicon part  65 . 
         [0081]    Method  1  according to the invention then includes, as illustrated by the triple lines in  FIG. 10 , implementation of a new LIGA process  19 ′ comprising a series of steps ( 17 ′,  21 ′ and  23 ′) for electroplating a metal in a particular shape on the bottom surface of substrate  3  using a photostructured resin. 
         [0082]    In an eleventh step  17 ′, a layer of photosensitive resin is deposited on the bottom surface of substrate  3 , for example using a mould casting method. In a twelfth step  21 ′, photolithography is performed to make the growth pattern for the future metal electroplating. 
         [0083]    In a thirteenth step  29 ′ an anchoring layer is preferably deposited on substrate  3 . This step can be achieved, for example, by vacuum deposition, as mentioned above, of a pure gold or gold alloy layer. 
         [0084]    In a fourteenth step  23 ′, a metal layer is electroplated on the bottom surface of substrate  3  to form at least one additional metal part  41 ′ of micromechanical component  51  in at least one portion of cavity  47 . 
         [0085]    It is thus clear at this stage that micromechanical component  51  is still formed over three layers. The first layer includes a silicon and silicon oxide part  53  wherein at least one portion of one of cavities  45  includes a metal part  41 . The second layer above the first is formed by a second metal part  39 . The third layer below the first is formed by a second silicon part  65  wherein at least one portion of one of cavities  47  includes an additional metal part  41 ′. 
         [0086]    Electroplating step  23 ′ can be continued to form layers that are not in cavity  47  but below it and, possibly, over part of the bottom surface  7  of substrate  3 . The successive layers are then formed exclusively between the resin, photostructured during step  21 ′, in a second additional metal part  39 ′. 
         [0087]    The electroplated metal layer, i.e. the additional metal parts  39 ′ and  41 ′ preferably include nickel, i.e. pure nickel or a nickel alloy. 
         [0088]    In a fifteenth step  25 ′, the bottom surface of substrate  3  is machined, for example by lapping, so as to delimit the second additional metal part  39 ′ correctly. In a similar manner to second portion  39 , second additional metal part  39 ′ can also include a toothing  59 ′. 
         [0089]    It is thus clear at this stage that micromechanical component  51  is now formed over four layers. The first layer includes a silicon and silicon oxide part  53  wherein at least one portion of one of cavities  45  includes a metal part  41 . The second layer above the first is formed by a second metal part  39 . The third layer below the first is formed by a second silicon part  65  wherein at least one portion of one of cavities  47  includes an additional metal part  41 ′. The fourth layer below the third is formed by a second additional metal layer  39 ′. 
         [0090]    Of course, the advantage of this method is that it also advantageously allows several micromechanical components  51  to be made on the same substrate  3 . Moreover, with the help of the above explanation and the single, double and triple lines in  FIG. 10 , it should be understood that method  1  does not have to be not carried out in its entirety, i.e. depending upon the complexity of the micromechanical component  51  to be manufactured, the component may be completely constructed, for example, after step  25 ,  31  or  25 ′. However, for every construction variant, method  1  includes a last step  33  that consists in releasing micromechanical component  51  from substrate  3 . By way of example, several embodiments of method  1  and/or micromechanical component  51  are explained below. 
         [0091]    In a first embodiment, release step  33  of method  1  occurs after step  25  of constructing metal part  41  in the silicon part  53  as represented by single lines in  FIG. 10 . Release step  33  then consists in removing photostructured resin and bottom layer  7  or bottom and intermediate layers  7  and  9 . The micromechanical component  51 ″ thereby manufactured is free relative to the rest of substrate  3 . The component includes, as shown in 
         [0092]      FIG. 12 , over a single layer including a silicon or silicon and silicon oxide part  53 ″ shaping the body of a hand wherein at least one portion of one of cavities  45 ″ has a metal part  41 ″ forming a sleeve. In a second embodiment illustrated in  FIG. 8 , it is clear that, for example, release step  33  of method  1  is performed after step  31  of constructing the second silicon part  65  as represented by the double lines in  FIG. 10 . Release step  33  then consists in removing photostructured resin parts  71 ,  73  and  75  for example by means of etching and/or stripping. The micromechanical component  51  thereby manufactured is free relative to the rest of substrate  3 . It therefore includes, over three stacked layers, a second metal part  39 , a silicon and silicon oxide part  53  wherein at least one portion of one of cavities  45  includes a metal part  41  and a second silicon part  65 . 
         [0093]    The micromechanical component  51  obtained in accordance with the second embodiment of method  1  explained above and with reference to  FIGS. 1 to 8  and  10  substantially comprises a consecutive stack of three wheel parts respectively  39 ,  53  and  65  including a toothing  59 ,  55  and  67 . 
         [0094]    This component  51  is preferably adapted to form an escape wheel for cooperating with a coaxial escape pallet. Silicon toothings  55  and  67  are then advantageously used for forming impulse toothings for cooperating with the pallet stones of said pallet. Metal toothing  59  is thus used as an escape pinion for regulating the timepiece movement to which micromechanical component  51  belongs. 
         [0095]    In a third embodiment illustrated in  FIG. 9 , it can be seen, for example, that release step  33  of method  1  occurs after step  25 ′ of constructing the second additional metal part  39 ′ represented by the triple lines in  FIG. 10 . Release step  33  then consists in removing not only photostructured resin parts  71 ,  73  and  75  from across the top part of substrate  3 , but also those present on the bottom layer of said substrate. The micromechanical component  51 ′ thereby manufactured is free relative to the rest of substrate  3 . 
         [0096]    Micromechanical component  51 ′ thus includes, as shown in  FIG. 11 , over four stacked layers, a second metal part  39 , a silicon and silicon oxide part  53  wherein at least one portion of one of cavities  45  includes a metal part  41 , a second silicon part  65  wherein at least one portion of one of cavities  47  includes an additional metal part  41 ′ and a second additional metal part  39 ′. The micromechanical component  51 ′ obtained in accordance with the third embodiment of method  1  explained above and with reference to  FIGS. 1 to 7  and  9  to  11  substantially comprises a consecutive stack of four wheel layers respectively  39 ,  53 ,  65  and  39 ′ including a toothing  59 ,  55 ,  67  and  59 . 
         [0097]    In all of these embodiments, micromechanical components  51 ,  51 ′,  51 ″ are advantageously driven not directly onto a silicon part  53  and  65 , but onto metal parts  39 ,  39 ′,  41  and  41 ′. The thickness of metal part  41  is more than  6  microns, in order, preferably, for metal part  41  to sufficiently insulate silicon part  53 . In fact, beyond this thickness and ideally starting from  10  microns, a metal, such as for example nickel, is capable of absorbing stress elastically or plastically without passing it onto the silicon. 
         [0098]    It should be understood upon reading the above explanation that micromechanical components  51 ,  51 ′,  51 ″ of the Figures are simply example embodiments, which demonstrate that method  1  can form a stack of up to four layers (two comprising metal and two comprising silicon and metal) without any excessive complications. The configuration of the first embodiment could thus constitute the simplest micromechanical component and the third embodiment a highly complex component. 
         [0099]    In a variant represented in dotted lines in  FIG. 10 , step  29 , occurring between steps  21  and  23 , and consisting in depositing anchoring layer  61 , could be moved to between steps  15  and  17 , i.e. between the etch of intermediate layer  9  and deposition of the photosensitive resin layer  57 . In this first variant, the two silicon layers  5  and  7  will preferably be doped. 
         [0100]    Of course, the present invention is not limited to the example illustrated, but could be subject to various variants and alterations which will be clear to those skilled in the art. In particular, other metal layers  63  could be envisaged, for example gold, aluminium, chromium or any of their alloys. Likewise, other anchoring layers  61  could be envisaged if they are conductive and adhere perfectly to the metal selected for the galvanic growth layer  63 . However, it should be noted that step  29  of depositing layer  61  is not essential for galvanic growth to take place properly if both silicon layers  5  and  7  are doped. 
         [0101]    Likewise, patterns that are different from toothings  55 ,  59 ,  59 ′ and  67  could be etched, such as hooks or clicks. It should also be noted that layer  63  could be made against said etched patterns such as toothings  55 ,  59 ,  59 ′ and  67  for example. 
         [0102]    Likewise, the photolithography could, of course, form a negative or positive structure depending upon the photostructured resin employed or the application envisaged. A spray coating method could also deposit resin layer  57 . 
         [0103]    Finally, metal layer  63  could equally well be made on an inner wall portion of a cavity  45  as on the peripheral wall of at least one of silicon parts  53  and  65 . Layer  63  could also be structured such that it is connected to said silicon wall via bridges of material.