EP3781992B1 - Method for manufacturing a timepiece mainspring of silicium based material - Google Patents

Description

The present invention relates to a process for manufacturing a silicon-based watch spring, in particular for a wristwatch or pocket watch.

Silicon is a highly appreciated material in mechanical watchmaking for its advantageous properties, in particular its low density, its high resistance to corrosion, its non-magnetic character and its ability to be machined by micro-fabrication techniques. It is thus used to manufacture spiral springs, balance wheels, oscillators with flexible guidance, escapement anchors and escape wheels.

Silicon nevertheless has the disadvantage of low mechanical resistance, a disadvantage which is aggravated by the etching method generally used for its machining, deep reactive ion etching known as DRIE, which leaves sharp edges and creates flatness defects in the shape ripples (called "scalloping"), as well as defects in the crystal lattice, on the sides of the part. This low mechanical resistance is problematic for the manipulation of the components during their assembly in a movement or in the event of shocks to which the watch is subjected. Components can easily break. To solve this problem, watch components made of silicon are generally reinforced by a coating of silicon oxide with a thickness much greater than that of the native oxide, as described in the patent application WO 2007/000271 . This coating is generally left on the final component but, according to the teaching of the patent application EP 2277822 , it can be removed without significantly affecting the mechanical strength.

In the case of springs, the mechanical strength must also be sufficient for the component to be able to deform elastically without breaking during its operation to perform its function. For a spiral spring intended to equip a balance wheel or for the flexible guidance of an oscillator without pivots, the stresses in operation are relatively low, of the order of a few hundred MPa at most, so that the mechanical resistance provided by the layer of silicon oxide may in theory be sufficient. However, given the oscillation frequencies in operation (4 Hz, 10 Hz or even 50 Hz), the number of cycles is high, which can lead to risks of fatigue failure. For other springs such as mainsprings, in particular barrel springs, or certain hammer or rocker springs, the stresses undergone during their operation are much higher, of the order of a few GPa, and are incompatible with the choice of silicon as the material of manufacture, even coated with silicon oxide. This is why, for this kind of springs, materials with a high elastic limit are used or proposed, such as steels, nickel-phosphorus alloys, Nivaflex ^® (alloy based on Co, Ni, Cr and Fe having an elastic limit around 3.7 GPa), metallic glasses (see patents CH 698962 and CH 704391 ) or metal/diamond or metalloid/diamond composite materials (see patent CH 706020 of the plaintiff).

An alternative to the formation of a layer of silicon oxide on silicon is described in the patent application CH 702431 . It consists in implementing an annealing of the component in a reducing atmosphere in order to round off the edges and attenuate the flatness defects of the sides created by the DRIE etching. This method is not sufficient for springs intended to receive high stresses in operation and does not provide optimum fatigue resistance.

The document EP3181938 describes a spiral spring subjected to an oxidation step and a deoxidation step.

The present invention aims to substantially increase the maximum stress that a silicon-based watch spring is capable of undergoing during its operation and/or the fatigue strength of such a watch spring.

1. a) réaliser à base de silicium une pièce ayant la forme souhaitée du ressort horloger ou comprenant une partie ayant la forme souhaitée du ressort horloger,
2. b) oxyder thermiquement la pièce,
3. c) désoxyder la pièce,
4. d) effectuer un recuit de la pièce dans une atmosphère réductrice,
5. e) former une couche d'oxyde de silicium sur la pièce.

To this end, there is proposed, according to a first embodiment of the invention, a method of manufacturing a watch spring comprising the following steps:

1. a) producing a silicon-based part having the desired shape of the watch spring or comprising a part having the desired shape of the watch spring,
2. b) thermally oxidize the part,
3. c) deoxidize the part,
4. d) annealing the part in a reducing atmosphere,
5. e) forming a layer of silicon oxide on the part.

1. a) réaliser à base de silicium une pièce ayant la forme souhaitée du ressort horloger ou comprenant une partie ayant la forme souhaitée du ressort horloger,
2. b) effectuer un recuit de la pièce dans une atmosphère réductrice,
3. c) oxyder thermiquement la pièce,
4. d) désoxyder la pièce,
5. e) former une couche d'oxyde de silicium sur la pièce.

According to a second embodiment of the invention, there is proposed a method for manufacturing a watch spring comprising the following steps:

1. a) producing a silicon-based part having the desired shape of the watch spring or comprising a part having the desired shape of the watch spring,
2. b) annealing the part in a reducing atmosphere,
3. c) thermally oxidize the part,
4. d) deoxidize the part,
5. e) forming a layer of silicon oxide on the part.

• la figure 1 est un schéma montrant les différentes étapes d'un procédé de fabrication selon un mode de réalisation particulier de l'invention ;
• la figure 2 est un graphique montrant par des points et des boîtes à moustaches des valeurs de contrainte de rupture apparente obtenues dans trois cas différents ;
• la figure 3 est une vue de dessus d'un ressort de barillet réalisé selon le procédé selon l'invention, le ressort de barillet étant représenté à l'état détendu, avant son introduction dans le barillet ;
• la figure 4 est une vue de dessus d'un ressort de marteau réalisé selon le procédé selon l'invention.

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

• the figure 1 is a diagram showing the different steps of a manufacturing method according to a particular embodiment of the invention;
• the picture 2 is a graph showing by dots and boxplots values of apparent breaking stress obtained in three different cases;
• the picture 3 is a top view of a mainspring produced according to the method according to the invention, the mainspring being shown in the relaxed state, before its introduction into the barrel;
• the figure 4 is a top view of a hammer spring produced according to the method according to the invention.

With reference to the figure 1 , a particular embodiment of the method for manufacturing a silicon-based watch spring according to the invention comprises steps E1 to E5.

A first step E1 consists in etching in a silicon wafer, preferably by deep reactive ion etching (DRIE), a part having the desired shape and substantially the desired dimensions of the watch spring, or a part of which a part has the desired shape and substantially the desired dimensions of the watch spring.

Silicon can be monocrystalline, polycrystalline or amorphous. For isotropy of all physical characteristics, polycrystalline silicon may be preferred. The silicon used in the invention can also be doped or not. Instead of the silicon itself, the part can be produced in a composite material comprising thick layers of silicon separated by one or more thin intermediate layers of silicon oxide, by etching in a silicon-on-insulator substrate (SOI substrate).

A second step E2 of the method consists in thermally oxidizing the part, typically at a temperature between 600°C and 1300°C, preferably between 800°C and 1200°C, so as to cover it with a layer of oxide of silicon (SiO ₂ ). The thickness of this layer of silicon oxide is typically between 0.5 μm and a few micrometers, preferably between 0.5 and 5 μm, more preferably between 1 and 5 μm, for example between 1 and 3 μm. This layer of silicon oxide is formed by growth by consuming silicon, which pushes back the interface between the silicon and the silicon oxide and attenuates the surface defects of the silicon.

In a third step E3, the silicon oxide layer is removed, for example by wet etching, vapor phase etching or dry etching.

In a fourth step E4, the annealing treatment described in the patent application is applied to the part. CH 702431 . This thermal annealing treatment is carried out in a reducing atmosphere, preferably at a pressure strictly greater than 50 Torr, or even 100 Torr, and less than or equal to atmospheric pressure (760 Torr), but which may be of the order of atmospheric pressure, and preferably at a temperature between 800°C and 1300°C. The duration of the annealing treatment can be from a few minutes to several hours. The reducing atmosphere can consist mainly or entirely of hydrogen. It can also comprise argon, nitrogen or any other neutral gas. This annealing treatment causes a migration of silicon atoms which leave the convex parts of the surface to accumulate in the concave parts and thus round off the edges and attenuate the ripples and other defects left on the sides by the etching.

In a fifth step E5 of the method, a layer of silicon oxide (SiO ₂ ) is formed on the part, making it possible to increase its mechanical strength. This layer of silicon oxide can be formed by thermal oxidation, in the same way as in the second step E2, or by deposition, in particular chemical or physical vapor deposition (CVD, PVD). It is preferably formed over all or almost all of the surface of the part. Its thickness is typically between 0.5 μm and a few micrometers, preferably between 0.5 and 5 μm, more preferably between 1 and 5 μm, for example between 1 and 3 μm.

Typically, said part is part of a batch of parts produced in the same silicon wafer. At a final stage of the process, the part and the other parts of the batch are detached from the wafer. The final watch spring according to the invention can be the spare part itself or a part of this part.

Surprisingly, it has been found that the oxidation - deoxidation (steps E2 and E3), the annealing (step E4) and the formation of a layer of silicon oxide (step E5) complement each other remarkably well so that the overall effect obtained clearly exceeds what could be expected by combining these steps.

• cas 1 : des éprouvettes fabriquées uniquement par DRIE (étape E1 uniquement),
• cas 2 : des éprouvettes fabriquées par DRIE et revêtues d'une couche d'oxyde de silicium d'environ 3 µm d'épaisseur (étapes E1 et E5 uniquement), ces éprouvettes étant issues de la même plaquette de silicium que celle du cas 1,
• cas 3 : des éprouvettes fabriquées selon le procédé selon l'invention (étapes E1 à E5), la couche d'oxyde de silicium formée à l'étape E5 ayant une épaisseur d'environ 3 µm, ces éprouvettes étant issues de la même plaquette de silicium que celles des cas 1 et 2.

The picture 2 shows the apparent breaking stress in bending measured on several tens of specimens in different cases, namely:

• case 1: specimens manufactured only by DRIE (stage E1 only),
• case 2: specimens manufactured by DRIE and coated with a layer of silicon oxide about 3 µm thick (steps E1 and E5 only), these specimens being taken from the same silicon wafer as that of case 1 ,
• case 3: specimens manufactured according to the process according to the invention (steps E1 to E5), the layer of silicon oxide formed in step E5 having a thickness of about 3 μm, these specimens being taken from the same wafer of silicon than those of cases 1 and 2.

The apparent flexural breaking stress obtained with the process according to the invention is very high. It is on average around 5 GPa, can even reach values close to 6 GPa and the minimum value is greater than 3 GPa. Since silicon is a brittle material, its apparent breaking stress or breaking limit merges with its elastic limit. It is therefore possible to produce silicon springs capable, in current operation, of exerting high-intensity forces, like the springs produced in the most efficient alloys or in metallic glass.

For example, the picture 3 illustrates a mainspring, more precisely a mainspring, intended to store mechanical energy during its winding and to restore it gradually to power the operation of a cog or other watch mechanism. Such a mainspring manufactured according to the method according to the invention will have an excellent energy storage capacity, determined by the ratio of the elastic limit squared to the modulus of elasticity (σ ² /E). This mainspring, shown in picture 3 in its relaxed state when it is out of the barrel, may comprise parts fulfilling additional functions with respect to the storage and the return of energy, for example parts serving as a plug or as a flange as described in the patent CH 705368 .

The figure 4 illustrates a hammer spring whose end is intended to act on a pin carried by a hammer in order to actuate the latter for resetting a chronograph counter. In the case of such a hammer spring or other springs, the very high apparent breaking stress in bending obtained by the method according to the invention can be used to reduce the dimensions of the spring compared to a spring made of a material more classic like steel or nickel-phosphorus, for the same force exerted in normal operation.

It will be noted that the method according to the invention can also be used to increase the fatigue resistance of watch springs exerting forces of moderate intensity but stressed at high frequency, such as spiral springs fitted to balances or flexible guides of oscillators without pivots such as the flexible guide with separate cross blades of the oscillator described in the patent application WO 2017/055983 .

It seems in fact that the excellent complementarity of the treatments implemented by the process according to the invention is due to the diversity of the physical phenomena involved. Oxidation - deoxidation eliminates the thickness of the silicon most affected by the defects of surface. Annealing rearranges the atoms in matter. The formation of the silicon oxide layer brings compressive stress to the silicon surface. The result is that the watch springs obtained are of remarkable quality. Chips and other defects likely to create incipient fractures are greatly reduced or even removed. Surface roughness is smoothed. The ripples and other surface defects created by the DRIE engraving on the sides of the part are reduced or even eliminated. The edges are rounded, which reduces stress concentrations.

The method according to the invention can be applied to watch springs other than those mentioned above, for example to toggle springs, lever springs, pawl springs or jumper springs.

In another embodiment of the invention, step E4 (annealing) is implemented before step E2 (thermal oxidation).

Claims (14)

1. Method for producing a timepiece spring, comprising the following steps:

   a) producing a piece based on silicon, having the desired shape of the timepiece spring or comprising a part having the desired shape of the timepiece spring,

   b) thermally oxidising the piece,

   c) deoxidising the piece,

   d) annealing the piece in a reducing atmosphere,

   e) forming a silicon oxide layer on the piece.
2. Method for producing a timepiece spring, comprising the following steps:

   a) producing a piece based on silicon, having the desired shape of the timepiece spring or comprising a part having the desired shape of the timepiece spring,

   b) annealing the piece in a reducing atmosphere,

   c) thermally oxidising the piece,

   d) deoxidising the piece,

   e) forming a silicon oxide layer on the piece.
3. Method as claimed in claim 1 or 2, wherein step a) comprises an etching operation, preferably a deep reactive ion etching operation.
4. Method as claimed in any one of claims 1 to 3, wherein the thermal oxidation step is carried out at a temperature between 600°C and 1300°C, preferably between 800°C and 1200°C.
5. Method as claimed in any one of claims 1 to 4, wherein the deoxidation step comprises an etching operation, preferably a wet etching operation, a vapour phase etching operation or a dry etching operation.
6. Method as claimed in any one of claims 1 to 5, wherein the annealing step is carried out at a pressure strictly greater than 50 Torr.
7. Method as claimed in any one of claims 1 to 6, wherein the annealing step is carried out at a pressure strictly greater than 100 Torr.
8. Method as claimed in any one of claims 1 to 7, wherein the annealing step is carried out at a pressure lower than or equal to atmospheric pressure.
9. Method as claimed in any one of claims 1 to 8, wherein the annealing step is carried out at a temperature between 800°C and 1300°C.
10. Method as claimed in any one of claims 1 to 9, wherein said reducing atmosphere includes hydrogen.
11. Method as claimed in claim 10, wherein said reducing atmosphere also includes an inert gas, e.g. argon.
12. Method as claimed in any one of claims 1 to 11, wherein step e) is carried out by thermal oxidation.
13. Method as claimed in any one of claims 1 to 12, wherein the silicon is monocrystalline or polycrystalline.
14. Method as claimed in any one of claims 1 to 13, wherein the timepiece spring is a mainspring, preferably a barrel spring, a hammer spring, a lever spring, a rocker spring, a pawl spring, a jumper spring, a hairspring or a flexible guide.

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