Method for manufacturing a silicon hairspring

Abstract

The invention relates to a method for manufacturing a watch component (1) in silicon having a functional external profile.

Description

Technical field of the invention

[0001] The invention relates to a method for manufacturing a silicon watch component, and more specifically a silicon watch component having a functional external profile. Technological background

[0002] Silicon watch components are generally manufactured by deep reactive ion etching – a technique also known by the English abbreviation DRIE – of a silicon-based wafer. The wafer can be a silicon wafer etched through its entire thickness (see, for example, patent applications EP 1722281, EP 2145857 and EP 3181938) or a silicon-on-insulator (SOI) substrate comprising a top silicon layer and a bottom silicon layer bonded by an intermediate layer of silicon oxide, the top silicon layer being the one in which the etching is performed (see, for example, patent applications WO 2019 / 180177 and WO 2019 / 180596).Compared to a simple silicon wafer, the silicon-on-insulator substrate has the advantage of having a rigid support (the lower silicon layer, which is thicker than the upper layer) making it easier to handle and maintain, and a stop layer (the intermediate silicon oxide layer) to stop the etching.

[0003] Regardless of the wafer type used, multiple components are etched simultaneously onto the same wafer, and clips or bridges left in place during etching hold the components attached to the wafer for subsequent manufacturing steps. The components are then released from the wafer by breaking or removing the clips.

[0004] Such fasteners, which connect the periphery of each component to the wafer, can pose problems, particularly when the component's periphery is a functional surface whose function must not be disrupted by fastener residue, or when the component's external surface must have a particularly neat appearance, as is the case with a needle. In some cases, especially for components with micro-serrations, the external functional surface does not have a sufficiently large free space to accommodate a sufficiently robust fastener.

[0005] Patent application WO 2019 / 166922 proposes a method for manufacturing a watch balance spring in which a silicon substrate bearing a layer of silicon oxide is used, through holes are formed in the silicon oxide layer, a layer of silicon is grown by epitaxy on the silicon oxide layer, this silicon layer filling the through holes to form attachments or bridges of material, spirals are engraved in the silicon layer, the silicon oxide layer is removed, the spirals remaining attached to the silicon substrate by said attachments, the spirals are subjected to heat treatments and finally the spirals are detached from the silicon substrate.

[0006] With this method, the spirals remain attached to the substrate after etching by attachments extending beyond the plane of the spirals, rather than between the outer surface of the last spiral and the etching silicon layer, as is generally the case. However, this method does not allow the use of commercially available silicon-on-insulator substrates, and growing the silicon layer in which the spirals will be formed by epitaxy is a complex operation.

[0007] Patent application WO 2019 / 166922 aims to solve this problem and proposes a method for manufacturing a silicon watch component from a SOI wafer in which the component is attached to an internal anchor within its contour. Thus, the attachment does not encroach on the component's functional external contour. However, this method does not allow for machining the component's back face, particularly for components requiring surface treatment or even decoration.

[0008] Patent application WO 2021 / 224804 A1 also aims to solve this problem with a method for manufacturing a watch component in silicon comprising the following steps: a) taking a wafer comprising a first layer of silicon, a second layer of silicon and, between the two, an intermediate layer of silicon oxide; b) etching the first layer of silicon to form the watch component, at least one connecting element located in a recess of the watch component and at least one attachment connecting this connecting element to an internal surface of the watch component; c) removing the intermediate layer of silicon oxide between the watch component and the second layer of silicon and leaving it at least partially between the connecting element and the second layer of silicon; d) applying at least one treatment to the watch component; e) detaching the watch component from the wafer by breaking or removing at least one attachment. Summary of the invention

[0009] The invention solves the aforementioned disadvantages by proposing a solution allowing the components to remain attached to the wafer, including those which do not allow for attachment on their external contour, while freeing up the back face to allow for working and / or decorating.

[0010] To this end, the present invention relates to a method for manufacturing a silicon watch component comprising the following steps: a) to obtain an SOI wafer comprising successively a silicon layer called "device", a silicon oxide bonding layer, and a silicon layer called "handle"; b) to grow a silicon oxide layer on the surface of the wafer; c) to etch the silicon oxide layer (etching mask) on the front, then the "device" layer by DRIE, to form the silicon watch component as well as an internal anchoring element and a material bridge connecting said anchoring element to an internal wall of a hole arranged to receive a shaft or the wall of an internal opening in the watch component; d) to etch the silicon oxide layer (etching mask) on the back, then the "handle" layer by DRIE, to form at least one narrow bridge as well as at least one rear anchoring integral with the at least one narrow bridge, the rear anchoring being connected to the anchoring element of the "device" layer by the silicon oxide layer bonding "device" and "handle";e) freeing the watch component from the silicon oxide layers by means of wet etching, the watch component being held to the wafer by the anchoring element via the material bridge, the bonding oxide layer being present only where both the "device" layer and the "handle" layer have not been attacked by the wet etching, the whole resting on at least one rear anchoring connected to at least one narrow bridge itself connected to the "handle" layer. ;

[0011] In accordance with other advantageous variants of the invention: the silicon oxide bonding layer is partially present between the anchoring element and the rear anchor at the end of step e); the narrow bridge and the material bridge are not superimposed; during step d), an integrated shadow mask is also formed during the DRIE etching of the "handle" layer so as to create a deliberate and intentional pattern of openings on the rear face, said openings allowing for elaborate decoration by CVD or PVD deposition; the process includes a step f) of wafer finishing of the front and / or rear faces of the watch component, the finishing step consisting of a deposition of layers, structuring and / or decoration for example; the watch component is a wheel, a cam, a hand, a rocker, a snail, an index or an applique.

[0012] It is therefore understood that the process allows access to the back face of the watch components attached to the wafer in order to work and / or decorate it. Brief description of the figures

[0013] Other features and advantages of the invention will become apparent from the following detailed description, given by way of non-limiting example, with reference to the accompanying drawings in which: THE figures 1a and 1b represent respectively a perspective view of a silicon component attached to the "device" layer and a perspective view of the "handle" layer; figures 2a and 2b represent respectively a top and bottom perspective view of a silicon component formed in an SOI wafer. Detailed description of the invention

[0014] The invention relates to a method for manufacturing a silicon watch component, and more particularly a silicon watch component whose external profile is functional.

[0015] By functional external profile, we mean a watch component whose outer perimeter forms a functional surface arranged to cooperate with other watch parts and / or components.

[0016] The use of a silicon-based material for the manufacture of a watch component offers the advantage of being precise with existing engraving methods and of possessing good mechanical and chemical properties, notably being little or not at all sensitive to magnetic fields.

[0017] Preferably, the silicon-based material used can be single-crystal silicon, regardless of its crystal orientation. Of course, other silicon-based compounds or other materials can be considered, such as glass, ceramics, cermets, metals, or metal alloys. For simplicity, the explanation below will focus on a silicon-based material.

[0018] Thus, the invention relates to a method for manufacturing a silicon watch component 1. Other watch components can be manufactured via the method according to the invention, such as a toothed wheel, an escape wheel, a rocker, a snail, etc. ... Such a method can also be considered for the manufacture of hands, indexes or appliques which require the most careful external surface finish possible.

[0019] According to the invention, the method comprises, as illustrated in the figure 2 , a first step a) which consists of obtaining SOI 10 wafers, that is to say composed of two layers of silicon 11 and 12, linked to each other by a layer of silicon oxide 13. Each of these three layers has one or more very specific roles.

[0020] The top layer of silicon 11, called "device" and formed in a single-crystal silicon wafer (whose main orientations can be varied), has a thickness which will determine the final thickness of the component to be manufactured, typically, in watchmaking, between 100 and 200µm.

[0021] The lower silicon layer 12, called the "handle", essentially serves as a mechanical support, so that the process can be carried out on a sufficiently rigid assembly (which the reduced thickness of the "device" cannot guarantee). It is also formed of a single-crystal silicon wafer, generally with a similar orientation to the "device" layer.

[0022] The oxide layer 13 allows the two silicon layers 11 and 12 to be intimately bonded. In addition, it will also serve as a stop layer during subsequent operations.

[0023] The following step (b) consists of growing a layer of silicon oxide on the surface of the wafer(s) 10 by exposing the wafer(s) to a high-temperature oxidizing atmosphere. The layer thickness varies depending on the device to be structured. It is typically between 1 and 4 µm. Of course, other techniques can be used to form an etching mask; a resin layer, as in the following step (c), may suffice, and if oxide is used, it can be deposited rather than grown.

[0024] Step c) of the process will allow us to define, for example in a positive resin, the patterns that we wish to subsequently create in the silicon wafer 10. This step includes the following operations: The resin is deposited, for example on a spinning wheel, in a very thin layer typically between 1 and 2 µm thick. Once dried, this resin, with its photolithographic properties, is exposed through a photolithographic mask (a transparent plate coated with a layer of chrome, itself representing the desired patterns) using a light source. In the specific case of a positive resin, the exposed areas of the resin are then removed with a solvent, revealing the oxide layer. In this instance, the areas still covered with resin define the zones that are not to be etched in the subsequent deep reactive ion etching (DRE) operation of the silicon.

[0025] In step c), the exposed or resin-coated areas are then exploited. An initial etching process transfers the patterns defined in the resin during the previous steps into the previously raw silicon oxide. To ensure repeatability of the manufacturing process, the silicon oxide is then structured by a dry, directional plasma etching that replicates the quality of the resin's surface, which serves as a mask for this operation.

[0026] Once the silicon oxide has been etched into the open areas of the resin, the silicon surface of the top layer 11 is exposed and ready for DRIE etching. The resin can be retained or removed depending on whether it is to be used as an additional mask during DRIE etching.

[0027] The exposed silicon, unprotected by the silicon oxide, is etched in a direction perpendicular to the wafer surface (Bosch® anisotropic DRIE etching). The patterns formed first in the resin, then in the silicon oxide, are "projected" into the thickness of the "device" layer 11.

[0028] When the etching process reaches the silicon oxide layer 13, which binds the two silicon layers 11 and 12, the etching stops. Indeed, just as the silicon oxide used as a mask in the Bosch® process is resistant to the etching itself, the buried oxide layer 13, of the same nature, also resists it.

[0029] The silicon layer "device" 11 is then structured throughout its thickness by the defined patterns representing the components to be manufactured, now revealed by this DRIE engraving, namely a cam 1 in the illustrated example.

[0030] The components remain attached to the "handle" layer 12 to which they are linked by the buried silicon oxide layer 13. During this step c), part of the upper silicon layer 2 is etched so as to form an internal anchoring element to the watch component, as well as a material bridge 8 linking this anchoring element 7 to the internal wall of the watch component 1, in a non-critical area of ​​the internal wall of the component.

[0031] In the illustrated example, the internal wall of the watch component is the wall of a hole designed to receive a shaft. The hole's shape allows the attachment residue to avoid interfering with the shaft that engages with the silicon wafer. The internal wall could be another internal opening within the component, for example, if it is skeletonized.

[0032] Of course, the process cannot be limited to DRIE etching in step c). As an example, the etching in step c) could just as well be achieved by chemical etching in the same silicon-based material.

[0033] During step c), several cams can be formed in the same wafer.

[0034] In step d), a second photolithography operation similar to the first one performed in step c) is carried out on the back of the wafer 10 (i.e., on the "handle" layer 12 side). To do this, the wafer 10 is inverted, the resin is deposited on it, and then exposed through a mask. During this second photolithography operation, at least one narrow bridge 9 is formed across the back of the watch component, as well as a rear anchor 9' attached to at least one narrow bridge 9. The rear anchor 9' is connected to the anchoring element 7 of the "device" layer by the silicon oxide layer bonding the "device" layer and the "handle" layer.

[0035] According to the invention, an integrated shadow mask is also formed during the DRIE etching of the "handle" layer during step d), so as to create a deliberate and intentional pattern of openings on the back face, said openings then allowing the creation of an elaborate decoration by CVD or PVD deposition during a subsequent step.

[0036] The exposed resin area is then removed using a solvent, revealing the previously formed oxide layer which serves as a mask for deep dry etching, such as DRIE etching, which allows at least one narrow bridge 9, the rear anchorage 9' and the shadow mask to appear.

[0037] In step e), to completely release the components, the various silicon oxide layers are etched by wet etching with a hydrofluoric acid-based solution or by hydrofluoric acid in vapor phase. Advantageously, the formed cams 1 are held to the anchoring element via the material bridge 8, and the whole assembly rests on the rear anchor connected to at least one narrow bridge, itself connected to a frame formed in the "handle" layer.

[0038] According to an optional step in the process, step f) consists of applying various surface finishing operations to the released part, which is still held by the clip. Thus, the front, back, and / or side faces of the watch component can be worked while the component is still held to the wafer. The finishing step may consist of applying layers, textures, or decorations to the different faces of the watch component. These operations are functional (reinforcement, tribological, etc.) or aesthetic (coloring, pattern), using PVD or CVD.

[0039] During this step, the pattern developed in step d) via the shadow mask is also decorated using CVD or PVD coatings.

[0040] This gives us cam 1 as illustrated in the figures 1a and 1b which, advantageously according to the invention, comprises a silicon-based core and a silicon oxide-based coating.

[0041] Advantageously, according to the invention, it is thus possible to manufacture without more complexity a watch component 1 having a functional external profile.

[0042] Finally, the process may also include step h) intended to separate the watch component 1 from the wafer 10 by separating the component 1 from its anchoring element 8.

[0043] Of course, the present invention is not limited to the illustrated example, i.e. the making of a cam, but is susceptible to various variants and modifications which will become apparent to the person skilled in the art.

Claims

1. Method for manufacturing a silicon timepiece component (10), comprising the following steps of: a) procuring an SOI wafer (10) successively comprising a so-called silicon "device" layer (11), a silicon oxide bonding layer (13), and a so-called silicon "handle" layer (12); b) growing a silicon oxide layer on the surface of the wafer (10); c) etching the silicon oxide layer on the front face, then the "device" layer (11) by DRIE, to form the silicon timepiece component (1), as well as an internal anchor element (7) and a bridge of material (8) connecting said anchor element to an inner wall of of a hole arranged to receive a shaft or the wall of an opening internal to the timepiece component; d) etching the silicon oxide layer on the rear face, then the "handle" layer by DRIE, (12) to form at least one narrow bridge (9) and at least one rear anchor (9') integral with the at least one narrow bridge (9), the rear anchor (9') being connected to the anchor element (7) of the "device" layer by the silicon oxide bonding layer (13) bonding the "device" and "handle" layers; e) releasing the timepiece component (1) of a silicon oxide layers by wet etching, the timepiece component (1) being held on the wafer (10) by the anchor element (7) via the bridge of material (8), the oxide bonding layer remaining present only where both the "device" layer and the "handle" layer have not been etched away by the wet etching, and the whole resting on the at least one rear anchor (9') connected to the at least one narrow bridge (9), which is itself connected to the "handle" layer so as to make all faces of the component accessible.

2. Method according to one of claim 1, wherein the silicon oxide bonding layer (13) is partially present between the anchor element (7) and the rear anchor (9') at the end of step e).

3. Method according to claim 1 or 2, wherein the narrow bridge (9) and the bridge of material (8) are not superimposed.

4. Method according to one of claims 1 to 3, characterised in that in step d), an integrated shadow mask is also formed during the DRIE of the "handle" layer so as to produce a deliberate and intentional pattern of openings on the rear face, said openings allowing for an elaborate decoration by deposition.

5. Manufacturing method according to one of claims 1 to 4, characterised in that it comprises a step f) of wafer-finishing the front, rear and / or side faces of the timepiece component (1), the finishing step consisting of depositing layers, patterning and / or decorating for example.

6. Method according to one of claims 1 to 5, wherein the timepiece component is a wheel, a cam, a hand, a lever, a snail, an index or an applique.

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