Abstract:
The invention relates to a silicon-based component with at least one reduced contact surface which, formed from a method combining at least one oblique side wall etching step with a “Bosch” etch of vertical side walls, improves, in particular, the tribology of components formed by micromachining a silicon-based wafer.

Description:
This application claims priority from European Patent Application No. 15173825.9 filed on Jun. 25, 2015, the entire disclosure of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to micromechanical component with a reduced contact surface and a method for fabrication of the same. More specifically, the invention relates to such a component formed by micromachining a wafer of material. 
     BACKGROUND OF THE INVENTION 
     CH Patent 698837 discloses the fabrication of a timepiece component by micromachining a water of amorphous or crystalline material, such as crystalline or polycrystalline silicon. 
     Such micromachining is generally obtained by deep reactive ion etching (also known by the abbreviation “DRIE”). As illustrated in  FIGS. 1 to 3 , a known micromachining method consists in structuring a mask  1  on a substrate  3  (cf.  FIG. 1 , step A) followed by a “Bosch” deep reactive ion etching combining in succession an etching phase (cf.  FIG. 1 , steps B, D, E) followed by a passivation phase (cf.  FIG. 1 , step C, layer  4 ) in order to obtain from the pattern of mask  1 , an anisotropic, i.e. substantially vertical, etch  5 , in the wafer (cf.  FIG. 2 ). 
     As illustrated in  FIG. 3 , an example of a “Bosch” deep reactive ion etching is shown with, in solid lines, the flow of SF 6  in sccm as a function of time in seconds, for etching a silicon wafer and, in dotted lines, the flow of C 4 F 8  in sccm as a function of time in seconds, for the passivation, i.e. the protection, of the silicon wafer. It is thus clearly seen that the phases are strictly consecutive and each have a specific flow and time. 
     In the example of  FIG. 3 , there is shown a first etching phase G 1 , with a flow of SF 6  at 300 sccm for 7 seconds, followed by a first passivation phase P 1  with a flow of C 4 F 8  at 200 sccm for 2 seconds, followed by a second etching phase G 2  with a flow of SF 6  at 300 sccm for 7 seconds again, and finally, followed by a second passivation phase P 2  with a flow of C 4 F 8  at 200 sccm for 2 seconds again, and so on. It is thus noted that a certain number of parameters enable the “Bosch” deep reactive ion etch process to be varied to obtain more or less marked scalloping in the wall of vertical etch  5 . 
     After several years of fabrication, it was found that these vertical etches  5  were not entirely satisfactory, particularly as regards tribology. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome all or part of the aforecited drawbacks by proposing a new type of micromechanical component and a new type of fabrication method making it possible to improve the tribology of components formed by micromachining a wafer of material. 
     The invention therefore relates to a method for fabricating a silicon-based micromechanical component including the following steps:
         a) taking a silicon-based substrate;   b) forming a mask pierced with holes on a horizontal portion of the substrate;   c) etching, in an etching chamber, substantially vertical walls, in a part of the thickness of the substrate from holes of the mask, in order to form peripheral walls of the micromechanical component;   d) forming a protective layer on the vertical walls, leaving the bottom of the etch made in step c) without any protective layer;   e) etching, in the etching chamber, predetermined oblique walls, in the remaining thickness of the substrate from the bottom, which has no protective layer, in order to form oblique lower surfaces beneath the peripheral walls of the micromechanical component;   f) releasing the micromechanical component from the mask and from the substrate.       

     It is understood that two distinct types of etch are obtained in the same etching chamber. It is immediately clear that the oblique etch of step e) can form a substantially oblique second surface and form several micromechanical components on the same substrate having a peripheral wall with a reduced contact surface. It can also be observed that, as a result of the protective layer provided only on the vertical walls, the oblique etch of step e) allows for a considerably more open angle and a substantially rectilinear direction of etching, which avoids being limited by the parameters of a Bosch deep reactive ion etching which, conversely, is used in step c) with its optimised vertical etching parameters. 
     In accordance with other advantageous variants of the invention:
         step c) is achieved by alternating an etching gas flow and a passivation gas flow in the etching chamber in order to form substantially vertical walls;   step d) includes phase d1): oxidising the etch obtained in step c) to form the silicon oxide protective layer; and phase d2): directionally etching the protective layer in order to selectively remove only the part of the protective layer at the bottom of the etch made in step c);   step e) is achieved by mixing the etching gas and the passivation gas in the etching chamber in order to form oblique walls;   in step e), the continuous etching and passivation gas flows are pulsed to enhance the etch of the cavity bottom.       

     Moreover, the invention relates to a micromechanical component obtained from the method according to any of the preceding variants, wherein the component includes a silicon-based body whose peripheral wall includes a first substantially vertical surface and a second oblique surface thereby decreasing the contact surface of the peripheral wall. 
     Advantageously according to the invention, it is understood that the peripheral or internal vertical wall of the micromechanical component offers a reduced contact surface or, on insertion of a member along an internal wall of the micromechanical component, can provide improved tribological contact with another component. 
     In accordance with other advantageous variants of the invention:
         the micromechanical component further includes at least one cavity comprising an internal wall also including a first substantially vertical surface and a second substantially oblique surface;   the micromechanical component forms all or part of an element in the movement or external parts of a timepiece.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIGS. 1 to 3  are diagrams intended to explain the “Bosch” deep reactive ion etching process used within the scope of the invention; 
         FIGS. 4 to 10  are views of steps of a method for fabricating a micromechanical component according to the invention; 
         FIG. 11  is a view of a micromechanical component according to the invention; 
         FIG. 12  is a flow diagram of the fabrication method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention relates to a method  11  for fabricating a silicon-based micromechanical component. As illustrated in  FIG. 12 , method  11  includes a first step  13  of taking a silicon-based substrate. 
     The term “silicon-based” means a material including single crystal silicon, doped single crystal silicon, polycrystalline silicon, doped polycrystalline silicon, porous silicon, silicon oxide, quartz, silica, silicon nitride or silicon carbide. Of course, when the silicon-based material is in crystalline phase, any crystalline orientation may be used. 
     Typically, as illustrated in  FIG. 4 , the silicon-based substrate  31  may be a silicon-on-insulator substrate (also known by the abbreviation “SOI”) comprising an upper silicon layer  30  and a lower silicon layer  34  joined by an intermediate silicon oxide layer  32 . However, alternatively, the substrate could comprise a silicon layer added to another type of base such as, for example, a metal base. 
     The method continues with step  15  of forming a mask  33  pierced with holes  35  on a horizontal portion of substrate  31 . In the example of  FIG. 4 , mask  33  is formed on the upper portion of upper silicon layer  30 . Mask  33  is formed from a material capable of withstanding the future etching steps of method  11 . Thus, mask  33  may be formed from silicon nitride or from silicon oxide. In the example of  FIG. 4 , mask  33  is formed from silicon oxide. 
     Advantageously according to the invention, method  11  continues with a step  17  of etching, in an etching chamber, substantially vertical walls  36 , in at least part of the thickness of substrate  31  from the pierced holes  35  in mask  33 , in order to form peripheral or internal walls of the micromechanical component. 
     The substantially vertical etching step  17  is typically a “Bosch” deep reactive ion etching described above, i.e. alternating an etching gas flow and a passivation gas flow in an etching chamber so as to form substantially vertical walls  36 . 
     Indeed, step  17  allows for a substantially vertical etching direction relative to mask  33 , as seen in  FIG. 5 . There is thus obtained an etch  39  whose section, visible in  FIG. 5 , is substantially in the form of a right-angled quadrilateral. Of course, depending on the shape of holes  35 , the shape of the volume removed during the etching varies. Thus, a circular hole will give a cylindrical etch, and, a square hole, a cube or rectangular parallelepiped. 
     Method  11  continues with step  19  of forming a protective layer  42  on vertical walls  36 , leaving the bottom  38  of etch  39  without any protective layer, as seen in  FIG. 7 . 
     Preferably, protective layer  42  is formed of silicon oxide. Indeed, as seen in  FIGS. 6 and 7 , step  19  may then comprise a first phase  18  intended to oxidise the entire top of substrate  31 , i.e. mask  33  (if made of silicon oxide), walls  36  and bottom  38  formed by etch  39 , to form an added thickness on mask  33  and a thickness on vertical walls  36  and bottom  38  of etch  39 , to form a protective layer  42  made of silicon oxide. 
     The second phase  20  could then consist in directionally etching protective layer  42  in order to selectively remove the horizontal silicon oxide surfaces from a part of mask  33  and from the entire part of protective layer  42  only on bottom  38  of etch  39  as seen in  FIG. 7 . 
     Method  11  may then continue with step  21  of etching, in the same etching chamber, but according to predetermined oblique walls  37 , in the remaining thickness of substrate  31  from bottom  38  without any protective layer  42 , in order to form oblique lower surfaces beneath the peripheral walls of the micromechanical component. 
     Oblique etching step  21  is not a “Bosch” deep reactive ion etching described above. Indeed, as a result of protective layer  42 , step  21  allows for a much more open angle and a substantially rectilinear etching direction, which avoids being limited by the parameters of a “Bosch” deep reactive ion etching. Indeed, it is generally considered that, even by modifying the parameters of a “Bosch” deep reactive ion etch, the opening angle cannot exceed 10 degrees with a curved etching direction. 
     Advantageously according to the invention, step  21  is preferably achieved by mixing the SF 6  etching gas and the C 4 F 8  passivation gas in the etching chamber in order to form oblique walls  37 . More specifically, the continuous SF 6  etching and C 4 F 8  passivation gas flows are pulsed to enhance the etch at the bottom of the cavity. 
     It is thus understood that step  21  allows for a much more open angle, typically around 45 degrees in the  FIG. 8  example, instead of the maximum of 10 degrees obtained using a “Bosch” deep reactive ion etching with optimised parameter modification. Advantageously according to the invention, step  21  can thus give a precise opening angle without modifying the surfaces of vertical walls  36 . The angle between vertical walls  36  and oblique walls  37  is highly reproducible and can advantageously be comprised between substantially 0° and substantially 45°. As explained above, it is particularly the possibility of etching at an angle of more than 10° which is remarkable compared to a “Bosch” deep reactive ion etching. Preferably, the angle according to the invention between vertical walls  36  and oblique walls  37  is greater than 10° and less than 45°, and even more preferably, greater than 20° and less than 40°. 
     Further, the continuous flow pulsation allows for improved etching directivity, and can even provide substantially truncated cone-shaped walls and not spherical walls (sometimes called isotropic etches) as with a wet etch or a dry etch, for example, using only SF 6  gas. 
     To obtain the shape of walls  37  in  FIG. 8 , it is possible, for example, to apply a sequence that may include a first phase with a flow of SF 6  mixed with a flow of C 4 F 8  for a first duration, followed by a second phase with an increased flow of SF 6  mixed with a reduced flow of C 4 F 8  for a second duration, and then the first and second phases again and so on. 
     By way of example, this sequence could include a first phase with a flow of SF 6  at 500 sccm mixed with a flow of C 4 F 8  at 150 sccm for 1.2 second, followed by a second phase shown with a flow of SF 6  at 600 sccm mixed with a flow of C 4 F 8  at 100 sccm for 0.8 second, followed by a third phase again with, a flow of SF 6  at 500 sccm mixed with a flow of C 4 F 8  at 150 sccm for 1.2 second and followed by a fourth phase with a flow of SF 6  at 600 sccm mixed with a flow of C 4 F 8  at 100 sccm for 0.8 second and so on. 
     It is thus noted that the continuous flow pulsation enhances the etching at the bottom level of the cavity which will gradually widen, during step  21 , the possible opening of etch  41  as a function of its depth and, incidentally, a wider etch opening  41  in the lower portion of upper layer  30  until there is obtained an etch opening  41  wider than hole  35  in the mask  33  or than the section of the bottom  38  of etch  39  at the start of step  21 , as seen in the change from  FIG. 7  to  FIG. 8 . 
     Finally, method  11  finishes with step  23  of releasing the micromechanical component from substrate  31  and from mask  33 . More specifically, in the example shown in  FIGS. 9 and 12 , step  23  may include a deoxidation phase  24  to remove the silicon oxide mask  33  and, possibly, all or part of the intermediate silicon oxide layer  32 , and then a release phase  25  from substrate  31  with the aid, for example, of a selective chemical etch. 
     The method  11  illustrated in single lines in  FIG. 12  allows for two different types of etching in the same etching chamber. It can also be observed that the oblique etching of step  21  allows for a considerably more open angle and a substantially rectilinear direction of etching, which avoids being limited by the parameters of a “Bosch” deep reactive ion etching and using the latter in step  17  with optimised vertical etching parameters. 
     Advantageously according to the invention, the micromechanical component  51  that forms a wheel in the example of  FIG. 11  comprises a peripheral wall  54  forming a toothing which includes a reduced contact surface. 
     As seen more clearly in  FIG. 10 , which is an enlarged view of a portion of component  51 , micromechanical component  51  thus includes a silicon-based body  61  whose peripheral wall  54  borders a horizontal upper surface  53  and a horizontal lower surface  55  and includes a first substantially vertical surface  56  and a second oblique surface  57 . 
     It is thus clear that the second oblique, substantially rectilinear surface  57  provides peripheral wall  54  forming a toothing, with a decreased contact surface allowing for improved tribological contact with another component. It is also clear that inner wall of a hole  60  may also more easily receive a member. 
     Of course, the present invention is not limited to the illustrated example but is capable of various variants and modifications which will appear to those skilled in the art. In particular, an oxidizing step  22 , intended to smooth the silicon walls, may be performed between steps  21  and  23 . 
     Further, a metal or metallic alloy part could be deposited in etch  41 , in an optional step between phases  24  and  25 , so as to form a sleeve  59  in the hole  60  of micromechanical component  51 , as illustrated in  FIG. 11 . 
     This metal or metallic alloy part could even overlap over etch  41  to form an additional functional level of composite micromechanical component  51  formed only of metal. 
     Thus, after step  24  of deoxidizing substrate  31 , method  11  could continue with a step of selectively filling a cavity formed during etches  17  and  21 , with a metal or metallic alloy in order to provide an attachment to the micromechanical component. 
     By way of example, lower layer  34  of substrate  31  could then preferably be highly doped and used as the direct or indirect base for filling by electroplating. Thus a first phase could be intended to form a mould, for example made of photosensitive resin, on top of mask  33  and in a part of etch  41 . A second phase could consist in electroplating a metallic part, from lower layer  34 , at least between the micromechanical silicon component and a part of the mould formed in etch  41 . Finally, a third phase could consist in removing the mould formed in the first phase. The method would finish with phase  25  of releasing the composite micromechanical component from substrate  31  by a selective chemical etch. 
     Advantageously according to the invention, it is thus understood that galvanic deposition  59  is, because of the shapes of first substantially vertical surface  56  and a second oblique surface  57 , more difficult to remove than an essentially vertical surface and enjoys improved shearing resistance. 
     Further, said at least one hole  60 , which is at least partially filled with a metal or a metal alloy  59  can provide an attachment to composite micromechanical component  51 . Thus, in the example of  FIG. 11 , hole  60  could leave a cylindrical recess  62  allowing composite micromechanical component  51  to be driven onto an arbor with good mechanical strength when the metal or metal alloy part  59  expands as a result of the shapes of peripheral wall  54 . 
     Finally, micromechanical component  51  is not limited to the application of a wheel as seen in  FIG. 11 . Thus, micromechanical component  51  can form all or part of an element in the movement or external parts of a timepiece. 
     By way of non-limiting example, micromechanical component  51  may thus form all or part of a balance spring, an impulse pin, a balance wheel, an arbor, a roller, a pallets such as a pallet-staff, pallet-lever, pallet-fork, pallet-stone or guard pin, a wheel set such as a wheel, arbor or pinion, a bar, a plate, an oscillating weight, a winding stem, a bearing, a case such as the case middle or horns, a dial, a flange, a bezel, a push-piece, a crown, a case back, a hand, a bracelet such as a link, a decoration, an applique, a crystal, a clasp, a dial foot, a setting stem or a push-piece shaft.