EP3839642A1 - Method for manufacturing timepiece springs and etching mask for such a method - Google Patents

Abstract

The method of manufacturing watch springs according to the invention comprises a step consisting in etching the watch springs (10) in a plate (14) made of anisotropic material. The etching is carried out in such a way that the watch springs (10) have different orientations in the plate (14) to reduce their dispersion of stiffness. The invention also relates to an etching mask for implementing this method.

Description

The present invention relates to a method of manufacturing watch springs. By the term “spring” is meant any elastically deformable element to receive energy and / or produce a force or a movement. Examples of watch springs are hairsprings intended to be fitted to balances, rocker, lever or hammer return springs, jumpers or flexible guides.

The present invention relates more particularly to the manufacture of watch springs by etching a plate of material. It is now well known in watchmaking to use engraving techniques such as laser engraving, plasma engraving, deep reactive ionic engraving (known as DRIE) or wet engraving to manufacture components in large numbers and precisely. watchmakers. The most common engraving material is silicon. Several types of silicon have been proposed as an engraving material in watchmaking, in particular monocrystalline silicon oriented along the crystallographic axis <100> (cf. for example the patent EP 1422436 ), monocrystalline silicon oriented along the crystallographic axis <111> (cf. for example the patent EP 2215531 ), monocrystalline silicon oriented along the crystallographic axis <110> (cf. for example the patent EP 3056948 ) and polycrystalline silicon (see for example the patent application US 2018/0142749 ). Both <111> silicon and polysilicon are isotropic. Silicon <100> and silicon <110> are anisotropic. Silicon <100> is the most commonly used.

Currently, several hundred components are generally engraved on a plate of engraving material. Although the geometries of these components are theoretically identical, in practice the etching characteristics are not perfectly homogeneous over the entire plate. The springs manufactured in such a plate therefore cannot all have the same stiffness. This dispersion of stiffness thus obliges, for example, watch manufacturers to classify balance springs according to their stiffness and then to pair them with balances themselves classified according to their inertia in order to obtain the desired oscillation frequency. The number of classes of balance-springs per plate of etching material can be several dozen, which means that large stocks of balance-springs must be managed. In general, the dispersion of stiffness experienced by watch springs from the same batch poses problems in terms of operating precision, whether for mechanical oscillators or for return systems.

The present invention aims to attenuate the dispersion of stiffness of watch springs obtained from the same plate of engraving material.

To this end, a method is proposed for manufacturing watch springs, comprising a step consisting in etching the watch springs in at least part of the thickness of a plate, the plate being made of an anisotropic material in said at least one. part of the thickness, characterized in that the etching is carried out in such a way that the watch springs have different orientations in the plate to reduce their dispersion of stiffness.

The invention further proposes an engraving mask comprising an engraved design representative of watch springs, characterized in that in the engraved design the watch springs have different orientations such that by the engraving of at least a part, in material anisotropic, of the thickness of a plate through the etching mask, it is possible to obtain watch springs whose stiffness dispersion is reduced.

Particular embodiments of the invention are defined in the appended dependent claims.

• la figure 1 est un diagramme représentant le module d'élasticité ou module de Young du silicium dans le plan (100) en fonction de l'orientation par rapport aux axes cristallographiques ;
• la figure 2 est une vue de dessus d'un spiral horloger gravé dans une plaque de matériau de gravure ;
• la figure 3 est un diagramme représentant le nombre de spiraux horlogers obtenus à partir d'une même plaque de matériau de gravure en fonction de leur raideur ;
• la figure 4 montre schématiquement une plaque de matériau dans laquelle sont gravés des spiraux horlogers ;
• la figure 5 montre des étapes successives d'un procédé de fabrication de spiraux horlogers selon l'invention.

Other characteristics and advantages of the present invention will become apparent on reading the following detailed description given with reference to the appended drawings in which:

• the figure 1 is a diagram showing the elastic modulus or Young's modulus of silicon in the (100) plane as a function of the orientation with respect to the crystallographic axes;
• the figure 2 is a top view of a watch balance spring engraved in a plate of engraving material;
• the figure 3 is a diagram representing the number of watch springs obtained from a single plate of engraving material as a function of their stiffness;
• the figure 4 shows schematically a plate of material in which are engraved watch balance springs;
• the figure 5 shows the successive steps of a method for manufacturing watch balance springs according to the invention.

The method according to the invention uses an anisotropic etching material and exploits this anisotropy to reduce the stiffness dispersion of watch springs of the same geometry and of the same size. Silicon <100> commonly used in watchmaking, as well as other materials used or usable such as silicon <110>, sapphire or silicon carbide, is anisotropic in the sense that the modulus of elasticity varies in function of the orientation with respect to the crystallographic axes. By way of illustration, the diagram of the figure 1 represents the modulus of elasticity of monocrystalline silicon in the (100) plane as a function of the crystallographic directions. It can be seen in particular that the modulus of elasticity varies between a minimum value and a maximum value over an angle of 45 °.

If we take as an example of a watch spring a hairspring 10 as illustrated on figure 2 , it can be seen that if its number of turns is not an integer (excluding the possible rigid, non-active intermediate part (s), such as the reinforced portion 11), then the average elastic modulus of the material along the hairspring differs according to the orientation of the hairspring in the plate in which it is engraved. It is therefore possible to adjust the stiffness of the hairspring by choosing a particular orientation, as suggested in paragraph 62 of the patent EP 3056948 B1 .

The teaching of the patent EP 3056948 B1 however does not take into account the inhomogeneity of etching in the same plate. By orienting the spirals of the same plate in a particular direction, one gives them an average stiffness but one does not solve the problem of the dispersion of stiffness. The figure 3 shows, at mark 12, a typical bell-shaped dispersion curve defining a number N of classes of balance springs. The objective of the invention is to reduce the number of classes, in other words to tighten the bell curve 12, as shown by the dotted line 13.

The invention is based on the observation that the etching inhomogeneity, and therefore the dispersion of stiffness, is repeated from one plate of etching material to the next when the parameters of the etching process are kept unchanged. As shown in figure 4 , it is possible to identify on the plate of engraving material, designated by 14, different zones Z1, Z2, Z3, etc. corresponding to different stiffnesses. In the case of plasma etching, in particular deep reactive ion etching, the areas are circular and concentric.

For example, a zone Z2 and a zone Z4 can contain hairsprings having substantially the desired stiffness and therefore corresponding to a class located in the middle of the dispersion curve 12, while zones Z1, Z3, Z5 ... can contain springs. balance springs having different stiffness from the desired stiffness and different from one zone to another. In each zone Zi the stiffnesses are substantially identical and belong to the same class. According to the invention, no measure is taken concerning the balance springs of zones Z2 and Z4 which have the desired stiffness. As regards the other zones, on the other hand, the balance springs are oriented differently in the plate in order to modify their stiffness and to bring it closer to the desired stiffness value.

Thus, in a given zone Zi the hairsprings have the same orientation but the orientation varies according to the zone Zi where they are located, as shown schematically by figure 4 . The orientation is defined for example by the oriented half-line 15 starting from the geometric center of the hairspring and passing through the outer end of the active part of the hairspring, as illustrated in figure 2 . The differences in orientation between the balance springs in the different zones depend on the engraving parameters. The maximum difference in orientation can be 45 °. It is typically at least 10 °, or even at least 20 °, or even at least 30 °.

In practice, the spirals are attached to the plate 14 by one or more bridges of material left during the etching, for example a bridge of material 16 at their outer end. The stiffness of a given hairspring can be measured by coupling the hairspring to a balance of predetermined inertia and by measuring the frequency of the balance-spring assembly, this while the hairspring is still attached to the plate 14 or after its detachment. . The orientation to be given to each hairspring can be calculated as a function of the stiffness measured with respect to the desired stiffness and as a function of the geometry of the hairspring and of the anisotropy of the modulus of elasticity of the etching material.

An example of an implementation of a method for manufacturing watch balance springs according to the invention, with silicon as an etching material, is shown in figure 5 .

At a first stage ( figure 5 (a) ), we use a wafer 1 made of silicon-on-insulator type material known by the acronym SOI (Silicon-On-Insulator). The wafer 1 comprises an upper layer of silicon 2, a lower layer of silicon 3 and, between the two, an intermediate layer of silicon oxide 4. The silicon is anisotropic, that is to say monocrystalline of type <100 > or <110>.

At a second stage ( figure 5 (b) ), a photosensitive lacquer layer 5 is deposited on the upper silicon layer 2 and this layer 5 is structured by photolithography. More precisely, we expose the lacquer layer photosensitive 5 to ultraviolet rays through a mask 6 comprising a plate 6a transparent to ultraviolet rays and a structure to be transferred 6b carried by the plate 6a, the plate 6a being typically made of glass or quartz and the structure to be transferred 6b being typically made of chrome . Then the photosensitive lacquer layer 5 is developed and baked ( figure 5 (c) ). At the end of these operations, the photosensitive lacquer layer 5 has the same shape as the structure 6b and in turn constitutes a mask, said shape corresponding to that of a batch of balance springs to be manufactured.

At a next step ( figure 5 (d) ), the upper layer of silicon 2 is etched through the photosensitive lacquer mask 5 by deep reactive ion etching called DRIE (Deep Reactive Ion Etching) in order to form the balance-springs in this layer 2. The etching is stopped by the intermediate layer d silicon oxide 4, thus making it possible to define a precise thickness for the balance-springs. The number of balance springs is typically at least one hundred, preferably at least five hundred, more preferably at least six hundred. This number depends on the size of the plate 1 (typically 150 or 200 mm in diameter) and on the size of the balance springs.

The photosensitive lacquer mask 5 is then removed by chemical etching or plasma etching ( figure 5 (e) ).

At a next step ( figure 5 (f) ), a plate 8 formed by all or part of the upper silicon layer 2 etched is released from the plate 1. This plate 8 contains a base structure and the balance springs attached to the base structure by bridges of material left during the engraving.

Then the spirals can be subjected to various treatments before being released from the plate 8. One can for example modify their dimensions, by oxidation-deoxidation or other, so that the average stiffness of the spirals in the plate 8 has a predetermined value ( frequency setting), then oxidize them to improve their mechanical resistance and thermocompensate them.

For more details on the implementation of each of the steps described above, reference may be made to the patent application. WO 2019/180596 the contents of which are incorporated into this application by reference.

It will also be noted that a variant may consist in replacing the SOI wafer 1 with a simple wafer of anisotropic silicon and in etching the balance-springs throughout the thickness of the wafer.

In accordance with the present invention, in the engraved pattern that constitutes the structure 6b of the etching mask 6, and therefore in the upper layer of silicon 2 then in the plate 8, the balance springs have different orientations chosen as a function of the anisotropy of the etching material (silicon of layer 2) to reduce their dispersion of stiffness, that is to say tighten the bell curve 12, according to the principle explained above. A batch of balance springs that is more homogeneous in terms of stiffness is thus obtained, which makes it possible to significantly reduce the number of classes and therefore to ease the management of the stock of balance springs. The operation of pairing the balance springs with balances is made easier and may even no longer be necessary.

Instead of being in the form of a plate 6a carrying a structure to be transferred 6b, the etching mask 6 could be a plate opaque to ultraviolet rays and engraved over its entire thickness (perforated mask or “shadow mask”).

The present invention is not limited to balance springs. It can be applied to any type of watch spring, for example to tilt, lever or hammer return springs, to jumpers, to flexible guides, in particular to flexible oscillator guides without pivots. The watch spring in the invention, in particular in the case of flexible guides, can be part of a monolithic component.

Claims (9)

A method of manufacturing watch springs, comprising a step of etching the watch springs in at least part of the thickness of a plate (1, 14), the plate (1, 14) being made of an anisotropic material in said au minus a part of the thickness, characterized in that the etching is carried out in such a way that the watch springs have different orientations in the plate (1, 14) in order to reduce their dispersion of stiffness. Method according to Claim 1, characterized in that at the end of the etching step, areas (Z1-Z5) of the plate (1, 14) each comprise several watch springs, in that the watch springs in each zones (Z1-Z5) have the same orientation and in that the orientation of the watch springs varies according to the zones (Z1-Z5) where they are located. Method according to Claim 2, characterized in that the zones (Z1-Z5) are circular and concentric. Method according to any one of Claims 1 to 3, characterized in that at least one hundred, preferably at least five hundred, preferably at least six hundred, watch springs are engraved in the plate (1). Method according to any one of claims 1 to 4, characterized in that the maximum difference in orientation between the springs watchmakers in the plate (1) is at least 10 °, preferably at least 20 °, preferably at least 30 °, preferably at least 45 °. Method according to any one of claims 1 to 5, characterized in that the watch springs are balance springs, rocker, lever or hammer return springs, jumpers or flexible guides. Process according to any one of Claims 1 to 6, characterized in that the anisotropic material is silicon. Method according to any one of claims 1 to 7, characterized in that the etching step is a plasma etching step, preferably a deep reactive ionic etching step. Engraving mask (6) comprising an engraved design (6b) representative of watch springs, characterized in that in the engraved design (6b) the watch springs have different orientations such that by engraving at least a part, made of anisotropic material, the thickness of a plate (1) through the etching mask (6), it is possible to obtain watch springs whose stiffness dispersion is reduced.

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