Timepiece component made of a material without plastic deformation, in particular silicon

Abstract

The present invention relates to a timepiece component made of a material without plastic deformation, in particular silicon, the component having, on at least one portion, at least one structuring element characterised by at least one opening, the vertical walls of which are 20, 50 or 100 times higher than its width, and therefore have an aspect ratio of at least 20, 50, or even 100.

Description

[0001] Watch component made of material without plastic deformation, particularly silicon

[0002] The present invention relates to a watch component in a material without plastic deformation, in particular silicon.

[0003] Silicon is a highly valued material in mechanical watchmaking for its advantageous properties, including its low density, high corrosion resistance, non-magnetic nature, and its ability to be machined using micro-manufacturing techniques. It is used to manufacture balance springs, balance wheels, flexible-guided oscillators, escapement anchors, escapement wheels, and a multitude of other watch components.

[0004] Silicon micromachining consists largely of etching operations. A first manufacturing step is to obtain a wafer-shaped silicon substrate. Of course, the substrate may not be entirely made of silicon or may even be made of doped silicon. The substrate could be formed of silicon on insulator (SOI for its English abbreviation). Such a sandwich-structured substrate consists of two layers of silicon connected by an intermediate layer of silicon oxide. Alternatively, the substrate could consist of a layer of silicon attached to another type of base, such as metal.

[0005] The next step in the process is to deposit and create an openwork etching mask on a horizontal surface of the substrate. The mask is formed from a material capable of withstanding subsequent etching steps. For example, the etching mask is made of silicon oxide. The etching mask is generally obtained by a photolithography process. The substrate is first oxidized, then a layer of photosensitive resin is deposited on a horizontal surface of the substrate and then exposed to radiation (usually UV) through an exposure mask (also called a photomask or reticle). The exposure creates reactions within the resin and causes chemical changes; the irradiated areas will see their solubility change depending on the type of resin (positive or negative). A developer will eliminate the exposed or unexposed resin depending on its solubility and thus form a resin mask on the substrate.The silicon oxide layer is then etched through the resin mask by plasma etching to form the etch mask.

[0006] In the next step, the substrate is etched through this etching mask using the deep reactive ion etching (DRIE) process to form the desired component or set of components in the substrate. The etching mask is then removed. For a silicon-on-insulator or silicon-on-base wafer, the etched layer is released from the support layer along with the various components formed.

[0007] It is also possible to etch the silicon substrate directly through a resin mask, without a silicon oxide layer, for an etching depth of up to 200 pm or even around 300 pm.

[0008] An additional step may be provided consisting of coating the components with a layer of silicon dioxide over their entire outer surface to increase their mechanical strength. This step is carried out by a surface treatment such as a thermal oxidation treatment, chemical vapor deposition (CVD) or physical vapor deposition (PVD), in order to obtain a layer of silicon dioxide of a thickness greater than the layer of so-called "native" silicon dioxide forming naturally in the presence of air. The thickness of the layer of silicon dioxide is preferably at least 0.5 micrometers.

[0009] The above manufacturing process has proven its worth for several years and the manufacturing of parts is becoming increasingly precise. However, the development of new mechanisms and the extensive use of silicon for many components are pushing to achieve even better definitions and manufacturing accuracies.

[0010] The mechanical properties of a silicon watch component are largely determined by the component's overall geometry, thickness, and function within the movement. As noted above, surface treatment steps after component manufacture can also alter certain parameters such as mechanical strength.

[0011] It is noteworthy that the inertia of parts plays an important role in watchmaking. It would therefore be interesting to be able to refine certain mechanical properties of a watch component without affecting its external geometry.

[0012] In this context, the aim of the present invention is to provide a watch component made of a material without plastic deformation, in particular silicon, certain mechanical properties of which are not entirely determined by the external geometry of the component. An aim of the present invention is in particular to provide a watch component with a given geometry and a particular mass, inertia or mechanical resistance.

[0013] The present invention relates to a watch component made of a material without plastic deformation, in particular silicon according to claim 1.

[0014] Other characteristics and advantages of the present invention will appear on reading the description which follows, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

[0015] Figure 1 schematically illustrates the different steps of an example of a method for manufacturing one or more silicon watch components according to one embodiment of the invention.

[0016] Figure 2 illustrates a silicon watch component according to one embodiment of the invention.

[0017] A watch component according to the present invention has at least one structuring on at least one portion of said component. By structuring or structuring, we mean producing at least one but preferably several fine structures in a portion of a surface of the component to lighten it or modify some of its physical or mechanical properties (reduce the mass or the inertia or modify the rigidity for example). By fine structure on a surface of the component, we mean at least one opening passing through the component in a direction essentially perpendicular to the surface and of which one dimension on said surface (or simply width) is below 5 pm or 4.5 pm, typically approximately 4, 3.5, 3, 2.5, 2, 1.5, 1 or even 0.5 pm. These openings are through (i.e. open on each side of the watch component). In unclaimed embodiments, the openings may be blind.Such structures, to be effective, must have a smaller dimension (width) at the surface of the component of a few micrometers wide by a few hundred micrometers high (along an axis essentially perpendicular to the longitudinal axis of the opening or essentially perpendicular to the surface of the component). In particular, the vertical walls of the openings, i.e. the walls essentially perpendicular to the surface of the component or to the transverse axis of the openings, are preferably 20, 30, 40, 50, 60, 70, 80, 90, 100, or even greater than 100 times higher than the width of the opening so that said opening has an aspect ratio greater than or equal to 20, 30, 40, 50, 60, 70, 80, 90, 100, or even 110 or 120.In the following, the aspect ratio of an opening of a structure on the surface of a watch component is therefore defined as the ratio between the height of the opening on its width, that is to say the ratio between its dimension along an axis essentially perpendicular to the opening on the surface of the component and the smallest dimension (width) of the opening on the surface of the component.

[0018] Creating at least one repeated fine structure or structures in a watch component allows it to be made lighter or to modify some of its physical or mechanical properties (inertia, mass, rigidity in different directions, etc.). By structuring a watch component, it is possible to reduce its weight or inertia while ensuring the same rigidity, which can be interesting for high-speed gear trains for which silicon is already used to reduce inertia.

[0019] In another example, the balance spring is sensitive to gravity, especially in vertical steps. Its mass was greatly reduced by switching from a metal balance spring to a silicon balance spring. The mass of the balance spring, m s , is proportional to the ratio p / E 1 / 3 , where p is the density and E, the modulus of elasticity. It is therefore possible to reduce the mass of the hairspring by modifying the above ratio, thanks to a structuring of the hairspring.

[0020] Another advantage of structuring a silicon watch component is to create thin openings in which silicon oxide can be grown for thermal compensation purposes. The thermal compensation effect is determined by the ratio between the width of the part and the thickness of the oxide layer. Thus, with thin structures (openings), the required oxide layer will be thinner. Thermal oxidation is a logarithmic phenomenon since the oxygen that oxidizes the silicon must migrate through the oxide layer that is created. Therefore, the thicker the desired layer, the longer the oxidation time. With a structured watch component, it is possible to reduce this oxidation time.

[0021] The structuring of a watch component can also be useful when it comes to finding a balance between two mechanical characteristics. A chronograph hand, for example, must be light to consume little energy and rigid to mechanically resist when resetting. Appropriate structuring of the hand would allow this ratio between mass and rigidity to be modulated. In another example, flexible guides with separate crossed blades are increasingly used to manufacture pivots or oscillators, particularly of the Wittrick type. These guides must support significant masses. They are therefore required to have high rigidity outside the rotation plane, to avoid bending that can be detrimental to chronometry. On the other hand, they are also required to guarantee a very precise torque in the plane. Appropriate structuring will again allow this ratio between out-of-plane rigidity and in-plane rigidity to be modulated.

[0022] In yet another example, it is possible to produce a leaf spring comprising at least two leaves separated by a fine structure in the form of a slot with a thickness of approximately 1 μm extending over the entire active length of the leaf spring. Depending on the case, the slot may be open at the free end of the leaf spring or closed (leaf spring fixed at its two ends for buckling work). This produces a leaf spring which can provide a significant force thanks to the multiplication of leaves without increasing the stress compared to a single-leaf leaf spring providing the same force. In this example also, the fine structure makes it possible to change the stiffness of the leaf spring in one plane (force plane) without changing the stiffness outside the working plane.

[0023] Finally, structuring can be useful to prevent the propagation of a crack in a high-stress area of ​​the watch component. Silicon, like other materials without plastic deformation, is a very fragile material and it is therefore necessary to avoid stress concentration areas on the component. In general, for spring blades in particular, the maximum stress areas are located on the flanks. However, said flanks machined by DRIE are the site of defects linked to engraving (scalloping, surface defect). It is possible to provide structuring including at least one opening close to the outer surface and the high-stress area of ​​the component. This structuring can be arranged to prevent a crack from propagating to the entire material and breaking it violently. Such breakage could create debris that would pollute the entire movement.After the crack is stopped by the opening(s) in the structure, a loss of rigidity of the component will inevitably be observed, and therefore less efficient operation, but this is less detrimental than debris in a movement. Similarly, it is possible with adequate structuring to avoid the propagation of cracks when tightening components forming a screwed assembly, such as the heel of a spring, screwed into a fixed part of the movement.

[0024] As mentioned, to be effective and useful, these fine structures should have dimensions of a few micrometers wide (smallest dimension of the opening at the surface of the component), preferably below 5 pm, or 4.5 pm, typically about 4, 3.5, 3, 2.5, 2, 1.5, 1 or 0.5 pm wide by a few hundred micrometers high (dimension along an axis essentially perpendicular to the surface of the component or essentially perpendicular to the transverse axis of the opening at the surface of the component). The walls of the opening in its height are typically 20, 30, 40, 50, 60, 70, 80, 90, 100, or even greater than or equal to 100 times higher than the width of the opening at the surface of the component for an aspect ratio greater than or equal to 20, 30, 40, 50, 60, 70, 80, 90, 100, or even greater than or equal to 110 or 120.Preferably, when the structuring is composed of thin and repeated openings, the walls between each opening are also very thin at the surface of the component. In particular, these walls could have the same aspect ratio as the openings. The walls could in another example have a width greater than or equal to 4 pm at the surface of the component.

[0025] Figure 2 illustrates a silicon wafer 1 in which a component 6 comprises on its surface 8 a structure 7 formed of squares 5 pm wide open over the entire height of the wafer 1, i.e. 120 pm.

[0026] Generally, the structuring may have a free form. In particular, the structure(s) may have any suitable shape (square, round, triangular, etc.) and may or may not be uniform in shape, size, and depth. In addition, the structuring may extend over only a portion of the surface of the component, up to covering the entire surface of the component depending on the applications. The watch component made of silicon or of a material without plastic deformation according to the invention may be manufactured using any suitable method, such as, for example, the method mentioned in the introduction. Preferably, the structuring is carried out at the time of manufacture of the watch component.

[0027] A silicon component means a component made of pure or unpure silicon, or doped silicon. Silicon can be monocrystalline or polycrystalline.

[0028] The following is an example of a preferred method for manufacturing a watch component in material without plastic deformation according to the invention. In this example, the aim is to manufacture a watch component in silicon.

[0029] In a first step (Figure 1 a), for example, a silicon wafer 1 is prepared, preferably having a thickness equivalent to that of the components to be manufactured. Alternatively, the wafer could not be entirely made of silicon or could be made of doped silicon. The wafer could also be a silicon-on-insulator (SOI) wafer. Alternatively, the wafer could be made of a layer of silicon applied to another type of base, such as metal. The silicon can be monocrystalline or polycrystalline.

[0030] In a second step (figure 1 b), the plate 1 is oxidized to form a layer of silicon oxide 2. This step can be carried out in particular by a surface treatment such as a thermal oxidation treatment, chemical vapor deposition (CVD) or physical vapor deposition (PVD), in order to obtain a layer of silicon dioxide of greater thickness than the layer of so-called “native” silicon dioxide forming naturally in the presence of air.

[0031] On this silicon oxide layer 2, a metal layer 3 is applied in a third step (figure 1 c). Preferably, the metal layer 3 is a layer of chromium sprayed or evaporated by a physical vapor deposition (PVD) method. Alternatively, the metal layer 3 could be a layer of titanium or tantalum deposited directly on the silicon oxide layer 2 without a prior bonding layer or a layer of nickel, aluminum, indium or tungsten deposited on a bonding layer previously deposited on the silicon oxide layer 2.

[0032] In a fourth step (figure 1 d), a layer of photosensitive resin 4 (hereinafter, simply resin) is deposited on the metal layer 3.

[0033] The fifth step (Figure 1e) is a photolithography step during which the resin layer 4 is exposed to radiation (typically UV) through an exposure mask or reticle. Preferably, this photolithography step is carried out using a stepper. This system, short for step-and-repeat camera or step-and-scan camera for the most recent versions, works like a slide projector. The component to be manufactured - such as a watch component such as an anchor, a balance spring, a balance wheel, a wheel, a spring, an element of the frame or the base plate - is reproduced on a large scale on a reticle (the slide). The reticle is robust and faithfully reproduces all the dimensions and structures forming the component to be manufactured.The reticle is positioned relative to the plate 1 and the resin layer 4, a light source insolates the resin layer 4 through the reticle and a system of lenses which reduce the projected image (generally, between 4 and 10 times). The stepper then moves the reticle relative to the plate 1 and repeats the process. This produces in the resin layer 4 a first etching mask 40 comprising, repeated many times, the precise image of the pattern present on the reticle. The use of a stepper allows very high definition, resolution and precision in the creation of the first etching mask 40.

[0034] The following steps in the process are etching steps.

[0035] In the sixth step (Figure 1 f), the metal layer 3 is etched through the first etching mask 40 formed in the resin layer 4. The technique used is, for example, a reactive ion etching (RIE) technique. This produces a second etching mask 30 in the metal layer 3 in which the precise and repeated images of the first etching mask 40 are transferred with very high precision and resolution. During this step, at least part of the resin layer 4 is consumed.

[0036] In the seventh step (Figure 1 g), the second etching mask 30 in the metal layer 3 is used to etch the silicon oxide layer 2 to form a third etching mask 20 in the silicon oxide layer 2. A plasma etching process can for example be used. During this step, the remainder of the resin layer 4 is generally entirely consumed as well as part or all of the metal layer 3.

[0037] Finally, in an eighth step (Figure 1 h), the third etching mask 20 allows the etching of the silicon wafer 1 by deep reactive ion etching (DRIE). The desired component or a set of desired components is thus formed in the wafer 1. During this step, part or all of the oxide layer 2 is consumed. The openings 50, 51, 52 formed in the silicon wafer 1 can be through (50) or blind with different depths (51, 52).

[0038] An additional step (not shown) may be provided consisting of coating the components with a layer of silicon dioxide over their entire outer surface to increase their mechanical strength. This step is carried out by a surface treatment such as a thermal oxidation treatment, chemical vapor deposition (CVD) or physical vapor deposition (PVD), in order to obtain a layer of silicon dioxide of greater thickness than the layer of so-called "native" silicon dioxide forming naturally in the presence of air. The thickness of this final layer of silicon dioxide is preferably at least equal to 0.5 micrometers.

[0039] Any other additional treatment may be applied to the components obtained after the eighth step or after the additional thermal compensation step above. In particular, a treatment consisting of rounding the edges as described in CH 699 476 may be mentioned. The use of a stepper in the fifth photolithography step makes it possible to obtain a first etching mask 40 in resin with high definition and high precision. It is in particular possible to create a resin etching mask 40 reproducing very fine structures. With the succession of layers and etching steps according to the method described in the example above, these very fine structures of the first etching mask 40 are reproduced with high precision and high resolution on the second and third etching masks 30, 20 and then etched on the plate 1.

[0040] In general, the present invention relates to a watch component made of a material without plastic deformation, in particular silicon (pure or not, doped, poly or monocrystalline) having at least one surface and on at least a portion of this surface at least one opening. Said at least one opening is through in a direction essentially perpendicular to said surface and the vertical walls of the opening in a direction essentially perpendicular to the width of the opening on the surface of the component are at least 20, 50 or 100 times higher than said width, for an aspect ratio at least greater than or equal to 20, 50 or even 100. Said at least one opening is obtained by etching.

[0041] Preferably, the structuring is characterized by the repetition of fine structures formed of openings whose vertical walls are at least 20, 50 or 100 higher than their width (aspect ratio at least greater than or equal to 20, 50 or even 100).

[0042] Preferably, the fine structure(s) have a width of less than 5 pm, typically between 2 and 3 pm or advantageously essentially equal to 4.5, 4, 3.5, 3, 2.5, 2, 1.5, 1 or 0.5 pm wide.

[0043] Preferably, the structuring is chosen so that the inertia of the watch component is reduced compared to a watch component without structuring. Preferably, in the case of a watch component of the leaf spring type, the structuring is chosen to obtain a determined ratio between the rigidity in the working plane of the spring and outside the working plane of the spring.

[0044] In a preferred embodiment, the structuring is arranged in an active portion subjected to stresses when the watch component is in a service position in a timepiece so that a crack propagating in said active portion does not propagate beyond said active portion and does not cause the active portion to break completely with the rest of the component. Watch components are thus produced, in particular wheels, hands, springs, bridges, plates, balance springs, balance wheels which have particular mechanical properties or reduced inertia or a construction making them more resistant to breakage.

Claims

Claims 1. Watch component in a material without plastic deformation, in particular silicon, having a surface and on at least a portion of this surface at least one opening characterized by the fact that said at least one opening is through in a direction essentially perpendicular to said surface and has vertical walls in a direction essentially perpendicular to the width of the opening on the surface of the component which are 20, 50 or 100 higher than said width; and by the fact that said opening is obtained by etching the watch component.

2. Watch component according to claim 1, characterized in that said surface has a repetition of fine structures or structuring formed from openings whose vertical walls in a direction essentially perpendicular to the width of the opening on the surface of the component are 20, 50 or 100 higher than the width of the opening.

3. Watch component according to claim 1 or 2, characterized in that the opening(s) have a width at the surface of the component of less than 5 pm, typically between 2 and 3 pm.

4. Watch component according to one of claims 2 or 3, characterized in that the structuring is chosen so that the inertia of the watch component is reduced compared to a watch component without structuring.

5. Watch component according to one of claims 2 or 3, characterized in that the structuring is arranged in an active portion subjected to stresses when the watch component is in a service position in a timepiece and so that a crack propagating in said active portion does not propagate beyond said active portion and does not cause the total rupture of the active portion with the rest of the component.

6. Watch component according to one of claims 2 to 5, characterized in that said component is a leaf spring and that the structuring is chosen to obtain a determined ratio between the stiffness in the working plane of the leaf spring and the stiffness outside the working plane of the leaf spring.

7. Watch component according to one of the preceding claims, characterized in that it is a toothed wheel, a hairspring, a spring, a hand, a bridge or a balance wheel.

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