METHOD FOR CREATING UNREST

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a method of manufacturing a composite balance wheel for a balance wheel resonator - spiral and more particularly a balance wheel made of at least two materials such as silicon and a metal. TECHNICAL BACKGROUND OF THE INVENTION

[0002] The regulating organ, sometimes called the regulating organ, of a timepiece is generally formed by an inertia-elasticity type resonator. The quality of the regulating organ is crucial to the accuracy of a timepiece, notably by controlling the frequency of the watch movement. A typical inertia-elasticity type resonator used in watchmaking is a balance wheel and hairspring assembly. Such a balance wheel and hairspring assembly has evolved considerably in recent years, leading to a significant improvement in the accuracy of mechanical timepieces.

[0003] The balance wheel, in particular, has thus been manufactured using a wide variety of materials. However, manufacturing methods that improve geometric precision, manufacturing consistency, or the absence of influence from changes in operating conditions such as changes in temperature, magnetic field, or position are still being sought.

[0004] Document EP 3 822 709 A1 presents a method for manufacturing a balance wheel. SUMMARY OF THE INVENTION

[0005] The invention aims to propose a new manufacturing process for a balance wheel that provides high production precision and high reproducibility between manufactured parts while giving the balance wheel reduced sensitivity to the operating conditions of the watch movement.

[0006] To this end, the invention relates to a method for manufacturing a composite balance wheel, characterized in that it comprises the following steps: a. Obtain a silicon-based wafer; b. Cover one face of the wafer with at least one electrically conductive layer; c. Form openings through the wafer and said at least one electrically conductive layer so as to outline the shape of the composite balance wheel comprising a peripheral rim connected, by at least one arm, to a central hub; d. Form a mold over the wafer-at least one electrically conductive layer assembly with said openings in order to create, at the level of the peripheral rim, at least one cavity of the mold, at least part of the bottom of which is formed by said at least one electrically conductive layer; e. Form, by electroplating, at least one layer of metal in said at least one cavity of the mold in order to thicken at least part of the peripheral rim and thus form the composite balance wheel; f. Release the composite balance wheel.

[0007] Advantageously, the process according to the invention makes it possible to manufacture a balance wheel formed from a first layer of silicon-based material (forming the basic shape of the balance wheel), a second layer of intermediate thickness formed by said at least one electrically conductive layer (present at least in the web), and a third layer of thickness formed by said at least one metal layer (present only in the web). It is noted that the process according to the invention comprises simple steps that avoid the need for, for example, highly complex selective deposition steps or deposition steps in several different types of materials.

[0008] The three levels thus form the total thickness of the composite balance wheel with a very pronounced radius of gyration, optimizing the mass-to-moment-of-inertia ratio through a significant difference between the density of the first level (much lower) and the density of the third level (much higher only at the rim). It follows that the watch movement, in which a composite balance wheel obtained by the process according to the invention is mounted, is therefore less sensitive to changes in horizontal and vertical positions.

[0009] The process according to the invention advantageously utilizes techniques such as silicon-based material etching, widely proven in the field of electronics, to leverage their nanometer-scale precision in order to achieve high manufacturing accuracy. Furthermore, etching on a wafer allows for the simultaneous production of multiple composite balance wheels with high reproducibility (products manufactured with high uniformity over time).

[0010] The invention may also include one or more of the following optional features, taken alone or in combination.

[0011] The wafer can be made of monocrystalline silicon, that is, entirely of monocrystalline silicon. Of course, the wafer can also be made of polycrystalline silicon without departing from the scope of the invention. Step a of the process according to the invention may or may not involve attaching the wafer to a support substrate for increased mechanical strength.

[0012] By way of non-limiting example, the wafer could be a working layer (sometimes called a "device") of a silicon-on-insulator (SOI) stack, further comprising an intermediate layer of silicon dioxide and a second silicon-based support layer (sometimes called a "handle"). The thickness of the working layer is typically between 20 µm and 300 µm and, preferably, between 100 µm and 150 µm. Consequently, during the process according to the invention, the silicon-on-insulator stack allows the working wafer to be held in a simple and robust manner to carry out the various steps. Furthermore, during step c, the intermediate layer could be used as a stop layer for the formation of the openings, i.e., the maximum depth limit.Finally, in this particular example, at step f, we understand that it is necessary to remove the intermediate layer of silicon dioxide and the second silicon-based support layer.

[0013] Said at least one electrically conductive layer is preferably based on a metal such as gold (Au) and / or platinum (Pt) and / or tantalum (Ta) and / or aluminum (Al) and / or chromium (Cr) and / or copper (Cu) or one of their alloys. These materials can be deposited in a stacked manner, that is, in successive layers that at least partially overlap, such as, for example, by using stacked chromium- and gold-based materials. It is therefore understood that one (or more) layer(s) of material(s) (identical or different) can be used to coat the wafer face. Typically, step b can be achieved by physical vapor deposition. Of course, other types of deposition can be used without departing from the scope of the invention, such as electron beam evaporation.

[0014] Step c may include a deep reactive ion etching (DRIE) step of the openings in the wafer. However, any etching method that allows the formation of openings in the wafer and at least one electrically conductive layer may be used. By way of non-limiting example, step c may include a first phase of selective chemical etching, such as using a masking technique, to begin forming the openings in the thickness of at least one electrically conductive layer, followed by a second phase of selective deep reactive ion etching (with the at least one electrically conductive layer forming a masking technique) to continue and complete the openings throughout the entire thickness of the wafer.

[0015] Step d can be carried out by photolithography. Of course, any other method of mold formation is possible without departing from the scope of the invention. However, photolithography is preferred because, being widely used in the field of electronics, it is highly reproducible and allows for high precision in the geometry of each cavity obtained. It is immaterial whether a positive or negative photosensitive resin is used. Typically, the bottom of all or part of said at least one mold cavity is at least partially formed by said at least one electrically conductive layer in order to facilitate the subsequent deposition in step e.

[0016] By way of non-limiting example, step d could include a first phase of deposition of a photosensitive resin such as SU-8, for example, carried out by spin coating. A second phase could consist of selectively illuminating the resin according to said at least one desired cavity (for example, using ultraviolet radiation through a mask with corresponding perforations), and then a third phase intended to develop the photosensitive resin in order to obtain the resin mold with said at least one cavity formed in its thickness.

[0017] Step e can be carried out by electrically connecting the cathode to said at least one electrically conductive layer in a galvanic bath in order to begin electroplating from the bottom of each mold cavity. Electroplating is preferred because it allows for a large aspect ratio, that is to say, particularly within the framework of the invention, a very large deposition height in said at least one cavity (within the thickness of the mold) relative to the dimensions of the bottom of said at least one cavity. Said at least one metal layer can be based on gold (Au), nickel (Ni), tungsten (W), copper (Cu), or an alloy of these metals.The material of said at least one layer of metal is selectively chosen to adjust the moment of inertia of the future composite balance wheel, that is to say, the density of the material of said at least one layer of metal, higher than that of the material of the plate, is chosen to obtain the desired moment of inertia of the future composite balance wheel as a function of the predetermined volume deposited of material of said at least one layer of metal.

[0018] The process may include a phase of removing at least one electrically conductive layer from at least one arm and the hub in order to distribute the mass as much as possible around the periphery of the composite balance wheel, thereby reducing viscous friction. As a non-limiting example, the removal phase could be carried out during step f after a prior phase of chemical dissolution of the mold formed in step d. By leaving at least one electrically conductive layer only at the rim of the composite balance wheel, it is understood that the radius of gyration and, consequently, the moment of inertia of the composite balance wheel are further reduced.Step f thus essentially consists of removing the composite rocker arm from all other non-useful parts (remains of the plate from step a, the possible support substrate from step a, said at least one electrically conductive layer at said at least one arm and hub, the mold from step d, etc.) used during the process.

[0019] The process preferably includes a machining phase of the mold assembly—at least one layer of metal—in order to selectively adjust the thickness of the balance wheel, particularly the thickness of the balance rim. Typically, this phase can be carried out between steps e and f or during step f. For example, the presence of the optional support substrate and the mold can be used to mill the top of the assembly obtained at the end of step e without risk of delamination of the three layers of the composite balance wheel. This allows for the removal of excess metal from said at least one layer and, at the same time, the precise adjustment of the thickness (dimensions, flatness, roughness, etc.) of the third layer formed by said at least one layer of metal, and, incidentally, the total thickness of the composite balance wheel before its release from the other non-useful parts used during the process.

[0020] The process may include a final finishing step (g) intended to clean or correct the composite balance wheel so that it is ready for mounting in a watch movement. This finishing step (g) may include a mechanical cleaning phase and / or a chemical cleaning phase to remove any remaining unwanted metallic film residue and / or to correct the composite balance wheel, such as improving its aesthetic appearance or adjusting its moment of inertia by removing material. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Other features and advantages of the invention will become clear from the description given below, which is by way of example and in no way limiting, with reference to the attached drawings, in which: there figure 1 is a schematic view of an example of a timepiece according to the invention; the figure 2is a perspective view of an example of a resonator with a composite pendulum according to the invention; the figures 3 to 8 are schematic cross-sectional views of example steps in a process according to the invention; the figure 9 is a schematic cross-sectional view of a first example of a composite balance wheel obtained by the process according to the invention; the Figure 10 is a schematic cross-sectional view of a second example of a composite balance wheel obtained by the process according to the invention. DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION

[0022] In the various figures, identical or similar elements bear the same references, possibly with an additional subscript. Therefore, a description of their structure and function is not systematically repeated.

[0023] Throughout this text, orientations refer to the orientations of the figures. In particular, the terms "upper," "lower," "left," "right," "above," "below," "forward," and "backward" are generally understood in relation to the direction in which the figures are represented. The term "thickness E" (or "height") below is generally used for the direction of the smallest dimension of the balance wheel 7, as illustrated in particular in the figure 2 .

[0024] By "horological component 2", we mean all types of timekeeping or measuring instruments such as clocks, small clocks, watches, etc...

[0025] By "watch movement 3", we mean all types of mechanisms capable of counting time and powered by mechanical energy (for example, a barrel).

[0026] By "based on", we mean a material or alloy constituting at least 50% by total mass or weight of a given element such as in particular 51%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% by total mass.

[0027] By "pure material," we mean a material theoretically composed of 100% by total mass or weight of a given metal, that is, without any other alloying elements. In practice, depending on the manufacturing process, the resulting material may contain so-called contaminants whose proportion by weight does not exceed 0.2% of the total mass. This generally prevents obtaining 100% material by total mass, resulting in a proportion between 97% and 100%.

[0028] In what follows, unless otherwise stated, all percentages (%) shown are percentages by total mass or weight (in English "weight").

[0029] The term "photosensitive resin" refers to any polymeric material composed of at least one polymer chain, sometimes called a fiber, of varying length, which may be of natural or synthetic origin and whose physical state changes upon exposure to a given electromagnetic radiation. Within the scope of this invention, the term can therefore refer to a photosensitive resin of either the positive or negative type. Depending on its type, the photosensitive resin, after exposure, can be developed by chemical exposure with at least one developing agent to remove either the exposed or unexposed portion of the resin. By way of non-limiting example, a negative-type photosensitive resin such as SU-8 can, for instance, be used in step d of the process according to the invention.

[0030] The invention relates to a method for manufacturing a composite balance wheel 7 for a watch movement 3 intended to be integrated into a timepiece 2 such as, for example, a wristwatch illustrated in the figure 1 The watch movement 3 preferably comprises an oscillator with a resonator 1 of the balance wheel 7 - balance spring 9 type and an escapement device such as, for example, a Swiss lever escapement. The resonator 1 of the balance wheel 7 - balance spring 9 type has, in a known manner, a pivoting shaft 11 on which a balance wheel 7 and a balance spring 9 made of metal or silicon are fitted in order to mount the pivoting resonator between a mainplate and a bridge (not shown). Since such an oscillator is known in itself, it will not be described further below.

[0031] Advantageously, according to the invention, the balance wheel 7 comprises a hub 13 peripherally connected to a rim 17 that is at least partially annular in order to form a flywheel. In the example illustrated in the figure 2 The balance wheel 7 comprises a hub 13 with a central hole for fitting onto a pivoting shaft 11 in a known manner. The balance wheel 7 preferably comprises at least two arms 15 (three in the case of figure 2 and four to the figure 5 ) each of which has a first end fixed peripherally to the hub 13 and a second end fixed to the internal diameter of a serge 17 at least partially annular.

[0032] Of course, the dimensions and geometries of the hub 13, the central hole, the arms 15, and the serge 17 could differ without departing from the scope of the invention. Thus, without limitation, the number of arms 15 could be greater (five, six, etc.) or smaller (one or two) without losing the benefits of the invention. As another example, the serge 17 could be continuous, as in the example of the figures 2 And 5or discontinuous without departing from the scope of the invention. Furthermore, the balance wheel 7 could include adjustable weights (not shown) fixed to the rim which, by changing their position, allow correction of the moment of inertia of the balance wheel 7 and, incidentally, the daily rate of the clockwork movement 3. Finally, the central hole of the hub 13 does not necessarily have to conform to a discoidal cross-section as illustrated in the figure 5 but could have an elliptical or polygonal cross-section without losing the advantages of the invention.

[0033] As is known, a resonator 1 of the balance wheel 7 - spiral 9 type follows the following relationships: T = 2 π I M in which: T is the period of resonator 1; I is the moment of inertia of the pendulum 7; M is the elastic moment of spiral 9. EtI=m⋅r2 in which: I is the moment of inertia of the pendulum 7; mis the mass of the balance wheel 7; r is the radius of gyration of the pendulum 7.

[0034] It is therefore understood that the process according to the invention seeks in particular to manufacture a pendulum 7 whose radius of gyration r is improved and adaptable in particular by the arrangement of a main proportion of the total mass of the balance wheel 7 privileged at the level of the rim 17, the rest of the mass being distributed over the other parts of the balance wheel 7. More generally, the invention aims to propose a new manufacturing process for a balance wheel 7 providing high production precision and high reproducibility between the balance wheels 7 manufactured while giving the balance wheel 7 a reduced sensitivity to the operating conditions of the watch movement 3.

[0035] To this end, the manufacturing process includes a first step a for obtaining a silicon-based wafer 19. The wafer 19 may be made of monocrystalline silicon. Of course, the wafer 19 may also be made of polycrystalline silicon without departing from the scope of the invention. Step a of the process according to the invention may or may not provide for the wafer 19 to be attached to a support substrate 4, 5 for increased mechanical strength of the wafer 19. The wafer 19 may have a thickness (vertical direction to the figure 3 ) between 20 µm and 300 µm and, preferably, between 100 µm and 150 µm.

[0036] By way of non-limiting example, the wafer 19 could thus be a working layer (sometimes called a "device") of a silicon-on-insulator stack (sometimes abbreviated as "SOI") further comprising an intermediate silicon dioxide layer 4 and a second silicon-based support layer 5 (sometimes called a "handle"). Consequently, during the process according to the invention, the silicon-on-insulator stack allows the working wafer 19 to be held in a simple and robust manner to carry out the various steps. Furthermore, during a step c of opening formation 20 described below, the intermediate layer 4 could be used as a stop layer for opening formation, i.e., the maximum depth limit.Finally, in this particular example, at a step f of release of the balance wheel 7 from the plate 19 described below, it is necessary to remove the intermediate layer 4 made of silicon dioxide and the second layer 5 of silicon-based support.

[0037] The process continues with a second step b intended to cover a face, such as the top face to the figure 3of the wafer 19 using at least one electrically conductive layer 21. Said at least one electrically conductive layer 21 is preferably based on a metal such as gold (Au) and / or platinum (Pt) and / or tantalum (Ta) and / or aluminum (Al) and / or chromium (Cr) and / or copper (Cu) or one of their alloys. These materials can be deposited in a stacking fashion, that is to say, in successive layers that at least partially overlap, such as, for example, by using stacked chromium- and gold-based materials. It is therefore understood that one (or more) layer(s) of material(s) (identical or different) can be used to coat the face of the wafer 19. Said at least one electrically conductive layer 21 may have a thickness (vertical direction to the figure 3) between 50 nm and 400 nm. Typically, step b can be achieved by physical vapor deposition such as electron beam evaporation. Of course, other types of deposition can be used without departing from the scope of the invention. As explained below, said at least one electrically conductive layer 21 is, advantageously according to the invention, used as an electrical energy transmission layer for step e of forming at least one metal layer 25.

[0038] Next, the process includes a third step c designed to form openings 20 through the wafer 19 and said at least one electrically conductive layer 21 so as to outline the shape of the composite balance wheel 7. Thus, in a first silicon-based layer as illustrated in the example of the figure 5The composite balance wheel 7 comprises a first level of the peripheral rim 17 connected, by at least one arm 15, to a central hub 13. Typically, in the example illustrated in the figure 4 The cross-section shows three openings 20. The central opening 20 corresponds to the central hole of the hub 13 of the composite balance wheel 7, and between the peripheral openings 20 (from the center to the periphery) are the hub 13, the arms 15, and the rim 17. It is understood that to form the arms 15 illustrated in the example of the figure 5 It is also necessary to create four openings 20 in the shape of roughly a quarter of a disc.

[0039] Finally, for practical reasons, at least two retaining bars 22 are left to keep the composite balance 7 attached to the plate 19 as the steps of the process according to the invention progress, i.e. the bars are broken during step f to release the composite balance 7 from the plate 19. It is therefore understood that the bars have a shape designed to facilitate step f such as, typically, a reduced section at the level of the serge 17.

[0040] By way of non-limiting example, step c may include a first sub-step of selective engraving to begin the formation of the openings 20 in the thickness (vertical direction to the figure 4) of said at least one electrically conductive layer 21. The first etching substep can thus include a first photolithography phase to form a resist mask with perforations corresponding to the future openings 20. A second phase could then be a chemical etching phase, according to the perforations of the resist mask, of the electrically conductive layer 21 in order to form the openings 20.

[0041] Step c can continue with a second sub-step of selective engraving to continue and finish the 20 openings throughout the entire thickness (vertical direction to the figure 4) of the wafer 19. It is therefore understood that the masking material formed in the first phase of the first substep of step c is also used for the second substep with the same perforations, i.e., etching the wafer 19 according to each perforation pattern. The second substep of step c may include a deep reactive ion etching (sometimes abbreviated "DRIE") phase of the openings 20 in the wafer 19. Deep reactive ion etching is preferred for its directional etching. However, any etching method that allows the formation of openings 20 in the wafer 19 and / or at least one electrically conductive layer 21 may be used. The process advantageously according to the invention uses a silicon-based material etching technique widely proven in the field of electronics to leverage its nanometer-scale precision to achieve high manufacturing accuracy.Furthermore, it is understood that engraving on a plate 19 allows several composite balance wheels 7 to be manufactured at the same time and with high reproducibility (very uniform manufacturing over time).

[0042] After step c, the process includes a fourth step d for forming a mold 23 over the wafer assembly 19 – at least one electrically conductive layer 21 with said openings 20 – in order to create, at the level of the peripheral serge 17, at least one cavity 24 of the mold 23, at least part of the bottom of which is formed by said at least one electrically conductive layer 21. Step d can be carried out by photolithography. Of course, any other method of forming a mold 23 is possible without departing from the scope of the invention. The mold 23 may have a thickness (vertical direction to the figure 3 ) between 180 µm and 1 mm (not including the height of the openings 20).

[0043] However, photolithography is preferred because, being widely used in electronics, it is highly reproducible and allows for high precision in the geometry of each cavity 24 obtained. It is irrelevant whether a positive or negative photosensitive resin 18 is used. Advantageously, according to the invention, the bottom of all or part of said at least one cavity 24 of the mold 23 is at least partially formed by said at least one electrically conductive layer 21 in order to facilitate the subsequent deposition in step e.

[0044] Here again, the process, advantageously according to the invention, uses a photolithography technique widely proven in the field of electronics to leverage its nanometer-scale precision to achieve high manufacturing accuracy. Furthermore, it is understood that photolithography on the upper part of the wafer assembly 19 – at least one electrically conductive layer 21 with the aforementioned apertures 20 – also allows for easy adaptation if several composite balancers 7 have been trimmed simultaneously during step c, and with high reproducibility (very uniform manufacturing over time).

[0045] By way of non-limiting example, step d could include a first phase of deposition of a photosensitive resin 18, such as SU-8, for example, carried out by spin coating. A second phase of alignment of a mask 6 (having suitable perforations) with the shape of the balance wheel 7, obtained in step c, is then carried out. A third phase may consist of selectively illuminating (for example, using ultraviolet radiation) the photosensitive resin 18 according to said at least one cavity 24 desired using the mask 6 as illustrated in the figure. Finally, a fourth phase intended to reveal the photosensitive resin 18 is carried out in order to obtain the resin mold 23 with said at least one cavity 24 (a single circular cavity in the example of the figure 7 ) formed in its thickness (vertical direction to the figure 7). It is noted that the use of a photosensitive resin 18 advantageously allows the openings 20 to be sealed in order to improve the mechanical resistance of the assembly, particularly for the steps of the process planned after step d as explained below.

[0046] According to the invention, the process continues with the fifth step e, intended to form, by electroplating, at least one layer 25 of metal in said at least one cavity 24 of the mold 23 in order to thicken at least a portion of the peripheral band 17 and thus form the composite balance wheel 7. Said at least one layer 25 of metal is preferably deposited to fill said at least one cavity 24, that is to say, along at least the thickness (vertical direction to the figure 8 ) of mold 23.

[0047] Step e can be carried out by electrically connecting the cathode to said at least one electrically conductive layer 21 in a galvanic bath in order to begin electroplating from the bottom of each cavity 24 of the mold 23. Electroplating is preferred because it allows a large aspect ratio, that is to say, particularly within the framework of the invention, a very large height (vertical direction to the figure 8 ) of deposit in said at least one cavity 24 (in the thickness of the mold 23) with respect to the substantially horizontal dimensions (horizontal direction to figures 7 and 8) of the bottom of said at least one cavity 24. Said at least one layer 25 of metal may be based on gold (Au), nickel (Ni), tungsten (W), copper (Cu) or an alloy of these metals. The material of said at least one layer 25 of metal is selectively chosen to adjust the moment of inertia of the future composite balance wheel 7, i.e., the density of the material of said at least one layer 25 of metal, higher than that of the material of the plate, is chosen to obtain the desired moment of inertia of the future composite balance wheel 7 as a function of the predetermined volume of material deposited in said at least one layer 25 of metal.

[0048] Finally, the process can conclude with the sixth step f, designed to release the composite balance wheel 7. Step f essentially consists of removing the composite balance wheel 7 from all the other non-essential parts (remains of the plate 19 from step a, the optional support substrate 4, 5 from step a, the mold 23 from step d, etc.) used during the process. As explained above, since several composite balance wheels 7 are manufactured simultaneously, step f thus allows all the composite balance wheels 7 to be detached. By way of non-limiting example, step f may thus include a phase of breaking the retaining bars 22 to free each composite rocker 7 from the plate 19. If a support substrate 4, 5 is used, the phase of breaking the retaining bars 22 may be preceded by a phase of detachment of the support substrate 4, 5 using, for example, a selective chemical attack of the intermediate layer 4.

[0049] Thus, advantageously according to the invention, the process makes it possible to manufacture a composite balance wheel 7 formed from a first layer of silicon-based thickness (forming the basic shape of the composite balance wheel 7), a second intermediate layer of thickness formed by said at least one electrically conductive layer 21 (present at least at the level of the band 17), and a third layer of thickness formed by said at least one metal layer 25 (present only at the level of the band 17). It is noted that the process according to the invention comprises simple steps that avoid the need to use, for example, very complex selective deposition steps or deposition steps in several different types of materials.

[0050] The three levels thus form the total thickness E1, E2 of the composite balance wheel 7 examples of the Figures 9 and 10with a very pronounced radius of gyration so as to optimize the ratio between mass and moment of inertia by a significant difference between the density of the first level (much lower) compared to the density of the third level (much higher only at the level of the serge 17). It is understood that the watch movement 3, in which a composite balance wheel 7 obtained by the process according to the invention is mounted, is therefore less sensitive to changes in horizontal and vertical positions.

[0051] The process may include a phase of removing at least one electrically conductive layer 21 from at least one arm 15 and / or the hub 13 in order to optimize the mass distribution around the periphery of the composite balance wheel 7, thereby reducing viscous friction. By way of non-limiting example, the removal phase could be carried out during step f after a prior phase of chemical dissolution of the mold 23 formed in step d, leaving at least one electrically conductive layer only at the level of the serge 17 of the composite balance wheel 7, as illustrated in the example of the Figure 10 , we understand that we are moving the radius of gyration even further and, incidentally, the moment of inertia of the composite balance 7.

[0052] The process preferably includes a machining phase of the mold assembly 23 - at least one layer 25 of metal in order to selectively adjust the thickness (vertical direction to the figure 8 ) of the composite balance 7. Typically, this phase can be carried out between steps e and f or during step f. For example, the presence of the possible support substrate 4, 5 and / or the mold 23 can be used to mill the top of the assembly obtained at the end of step e without risk of delamination of the three levels of the composite balance 7 to remove the excess of the deposit of said at least one layer 25 of metal and at the same time precisely adjust the thickness (dimension, flatness, roughness, etc.) of the third level formed by said at least one layer 25 of metal and, incidentally, that of the total thickness E1, E2 of the composite balance 7 before its release from the other non-useful parts used during the process.

[0053] The process may finally include a final finishing step g intended to clean - correct the composite balance wheel 7 so that it is ready to be mounted in a watch movement 3. The finishing step g may include a mechanical cleaning phase and / or a chemical cleaning phase to remove any remaining unwanted pieces and / or rectify the composite balance wheel 7 such as giving it a better aesthetic appearance or adjusting its moment of inertia by removing material.

[0054] The invention is not limited to the embodiments and variations shown, and other embodiments and variations will be obvious to those skilled in the art. Thus, the above embodiments are examples. The scope of the invention is defined by the appended claims.

[0055] Furthermore, it is also possible to apply the manufacturing process to other types of components such as an oscillating weight of an automatic winding mechanism comprising a first silicon-based level in the form of a partial disc and a second metal-based level at the periphery of said partial disc or a mobile for a gear train or escapement device comprising a first silicon-based level with a first peripheral toothing and a second metal-based level with a second peripheral toothing coaxial with the first peripheral toothing. LIST OF REFERENCES

[0056] 1 - resonator 2 - watch part 3 - watch movement 4 - intermediate layer 5 - support layer or substrate 6 - openwork exposure mask 7 - composite balance wheel 9 - balance spring 11 - balance shaft 13 - balance hub 15 - balance arm 17 - balance rim 18 - photosensitive resin 19 - plate 20 - opening 21 - electrically conductive layer 22 - retaining bar 23 - mold 24 - cavity 25 - metal layer

Claims

1. Method for manufacturing a composite balance wheel (7), characterised in that it comprises the following steps: a. procuring a silicon based plate (19); b. covering one side of the plate (19) with at least one electrically conductive layer (21); c. forming openings (20) through the plate (19) and said at least one electrically conductive layer (21) so as to cut out the shape of the composite balance wheel (7) comprising a peripheral felly (17) connected, by at least one arm (15), to a central hub (13); d. forming a mould (23) above the plate (19) - at least one electrically conductive layer (21) assembly with said openings (20) to create, at the peripheral felly (17), at least one cavity (24) of the mould (23) of which at least a part of the bottom is formed by said at least one electrically conductive layer (21); e. forming, by galvanoplasty, at least one metal layer (25) in said at least one cavity (24) of the mould (23) to thicken at least a part of the peripheral felly (17) and thus form the composite balance wheel (7); f. releasing the composite balance wheel (7).

2. Method according to the preceding claim, wherein the plate (19) is made of monocrystalline silicon.

3. Method according to claim 1 or 2, wherein step b is obtained by a physical vapour deposition.

4. Method according to any one of the preceding claims, wherein said at least one electrically conductive layer (21) is based on gold, platinum, tantalum, aluminium, chromium or copper.

5. Method according to any one of the preceding claims, wherein step c comprises a step of deep reactive-ion etching of the openings in the plate.

6. Method according to any one of the preceding claims, wherein step d is carried out by photolithography.

7. Method according to any one of the preceding claims, wherein step e is carried out by electrically connecting the cathode to said at least one electrically conductive layer (21).

8. Method according to any one of the preceding claims, wherein said at least one metal layer (25) is based on gold, nickel, tungsten or copper.

9. Method according to any one of the preceding claims, comprising a phase of removing said at least one electrically conductive layer (21) from said at least one arm (15) and the hub (13) to optimise the weight distribution around the composite balance wheel (7).

10. Method according to any one of the preceding claims, comprising a phase of machining the mould (23) - at least one metal layer (25) assembly to selectively adjust the thickness (E1, E2) of the composite balance wheel (7).

11. Method according to any one of the preceding claims, comprising a final finishing step g intended to correct the composite balance wheel (7).