EP3159746B1 - Heavily doped silicon hairspring for timepiece - Google Patents

Description

The invention relates to a spiral spring for an oscillator of a timepiece, as well as an oscillator, a timepiece movement and a timepiece as such which comprise such a spiral spring. Finally, it also relates to a method of manufacturing such a spiral spring.

The regulation of mechanical watches relies on at least one mechanical oscillator, which generally comprises a flywheel, called a pendulum, and a spiral wound spring, called spiral spring or more simply spiral. The hairspring can be fixed at one end to the axis of the balance and at the other end to a fixed part of the timepiece, such as a bridge, called a rooster, on which the axis of the balance pivots. The spiral spring equipping the mechanical watch movements of the state of the art is in the form of an elastic metal blade or a silicon blade of rectangular section, the majority of which is wound on itself in a spiral Archimedes. The sprung balance oscillates around its equilibrium position (or dead point). When the pendulum leaves this position, it arms the hairspring. This creates a return torque that acts on the pendulum to tend to bring it back to its equilibrium position. As it has acquired a certain speed, thus a kinetic energy, the pendulum exceeds its dead point until an opposite pair of the spiral stops it and forces it to turn in the other direction. In this way, the hairspring regulates the pendulum swing period.

The accuracy of mechanical watches depends on the stability of the natural frequency of the oscillator formed by the balance and the spiral. When the temperature varies, the thermal expansions of the balance and the balance, as well as the variation of the Young's modulus of the balance spring, modify the natural frequency of this oscillating assembly, thus disturbing the accuracy of the watch.

There are state-of-the-art solutions that attempt to reduce, or even eliminate, frequency variations of an oscillator with temperature. A approach considers that the natural frequency F of an oscillator depends on the ratio between the constant of the return torque C exerted by the balance on the balance and the moment of inertia I of the latter, by the following relation: $$\mathrm{F}=\surd \left(\mathrm{VS}/\mathrm{I}\right)/2\mathrm{\pi }$$

Deriving the previous equation with respect to the temperature, we obtain the relative thermal variation of the oscillator's natural frequency, which is expressed by: $$\left(1/\mathrm{F}\right)\mathrm{dF}/\mathrm{dT}=\left[\left(1/\mathrm{E}\right)\mathrm{of}/\mathrm{dT}+3\mathrm{}{\mathrm{\alpha }}_{\mathrm{s}}-2\mathrm{}{\mathrm{\alpha }}_{\mathrm{b}}\right]/2$$

Where E is the Young's modulus of the oscillator hairspring,
(1 / F) dF / dT is the thermal coefficient of the oscillator, also simply called by the acronym CT,
(1 / E) dE / dT is the thermal coefficient of the Young's modulus of the spiral of the oscillator, also called by the acronym CTE,
α _s and α _b are respectively the coefficients of thermal expansion of the spiral and the pendulum of the oscillator.

Various solutions of the state of the art seek to cancel the value of the thermal coefficient CT of the oscillator by choosing a CTE of the spiral adapted for this purpose. In the case of an anisotropic material, for example silicon, the thermal coefficient varies in the crystalline direction of the stress of the material and therefore varies over the length of the hairspring. Similarly, in the case of a heterogeneous material, such as oxidized silicon, the thermal coefficient varies within the section of the blade. An equivalent or apparent CTE, known to those skilled in the art, is thus considered for the hairspring formed of an anisotropic and / or heterogeneous material. Solutions of the state of the art seek to cancel the value of the thermal coefficient CT of the oscillator by choosing an equivalent or apparent CTE of the spiral adapted for this purpose.

In the description to follow, by "CTE", we mean in particular "CTE equivalent or apparent".

For example, the document EP1258786 proposes using a spiral formed in a particular paramagnetic alloy Nb-Hf comprising an advantageous rate of Hf. The alloy chosen is relatively complex to manufacture.

The document EP1422436 discloses another solution based on a silicon balance spring comprising an oxide layer. This solution requires a thick oxide layer. Its manufacture requires treating the hairspring for a long time at very high temperature, which is a drawback. The document CH 699 780 discloses a silicon spiral or the "CTE" is compensated by a coating comprising a metal or an alloy or by internally doped layers.

The object of the invention is to provide another spiral spring solution that allows the thermo-compensation of the oscillator, to obtain an oscillator whose frequency is independent or quasi-independent of the temperature, which does not have any or some of the disadvantages of the state of the art.

For this purpose, the invention relates to a hairspring for an oscillator of a timepiece, characterized in that it comprises a part, in particular at least one turn or a turn portion, provided with highly doped doping silicon. greater than or equal to 10 ¹⁸ at / cm ³ to allow thermo-compensation of the oscillator.

Said part, in particular said turn or said turn portion, has a locally varying section along its length, in particular along the length of said turn or of said turn portion. This variation is a variation of thickness and / or height.

As a variant or complement, said part may comprise an oxidized outer layer, in particular made of silicon dioxide SiO ₂ .

The invention is more precisely defined by the claims.

• La figure 1 représente schématiquement un spiral d'une pièce d'horlogerie selon un mode de réalisation de l'invention.
• La figure 2 représente l'évolution de l'épaisseur relative du spiral en fonction de son angle défini à partir de son point d'attache selon le mode de réalisation de l'invention.
• La figure 3 représente l'évolution de l'épaisseur relative du spiral en fonction de son angle défini à partir de son point d'attache selon une variante du mode de réalisation de l'invention.
• La figure 4 représente l'épaisseur de la couche d'oxyde d'un spiral dont les variations de section sont conformes à celles du spiral de la figure 3 pour différents rapports de l'épaisseur minimale sur l'épaisseur maximale en fonction de la densité de son dopage afin de former des variantes du mode de réalisation de l'invention.

These objects, features and advantages of the present invention will be set forth in detail in the following description of particular embodiments given as a non-limiting example in relation to the appended figures among which:

• The figure 1 schematically represents a hairspring of a timepiece according to one embodiment of the invention.
• The figure 2 represents the evolution of the relative thickness of the hairspring according to its angle defined from its point of attachment according to the embodiment of the invention.
• The figure 3 represents the evolution of the relative thickness of the hairspring according to its angle defined from its point of attachment according to a variant of the embodiment of the invention.
• The figure 4 represents the thickness of the oxide layer of a spiral whose sectional variations are in conformity with those of the spiral of the figure 3 for different ratios of the minimum thickness to the maximum thickness as a function of the density of its doping to form variants of the embodiment of the invention.

According to one embodiment of the invention, an oscillator for a timepiece comprises a balance-sprung assembly, the spiral being in the form of an elastic blade of rectangular section, wound on itself in an Archimedean spiral . The balance is made of a copper-berrylium alloy, in a known manner. Alternatively, other materials may be used for the balance. Similarly, the spiral could have another basic geometry, such as a non-rectangular section.

The objective of the invention is to propose a solution approaching at most a value of the thermal coefficient (CT) of zero for the balance-spring, whose Oscillations thus become independent or quasi-independent of the temperature. For this, it is necessary to couple the spiral material with that of the balance to obtain a good result. By way of example, with a CuBe2 balance, the hairspring must have a Young's modulus (CTE) thermal coefficient of the order of 26 ppm / ° C to heat compensate the oscillator.

According to an essential element of the invention, the spiral of the embodiments is made of silicon and comprises at least one coil or turn portion of highly doped silicon. By strongly doped, it is understood that the silicon has a doping of an ionic density greater than or equal to 10 ¹⁸ at / cm ³ , or even greater than or equal to 10 ¹⁹ at / cm ³ , or even greater than or equal to 10 ²⁰ at / cm ³ . This doping of the silicon is obtained by means of elements providing an additional electron (doping of p-type or "p-doped silicon") or one less electron (doping of n-type or "n-doped silicon"). It is found that depending on the material used for the balance, for example titanium or a titanium alloy, this single highly doped silicon may be sufficient to obtain a thermo-compensation of the oscillator. For example, the n-type doping is for example obtained by using at least one of: antimony Sb, arsenic As, or phosphorus P. The p-type doping is obtained for example using boron B.

The heavily doped silicon portion advantageously occupies the entire length of the hairspring. In other words, all the silicon turns of a spiral can advantageously be heavily doped. According to one embodiment, the turn or the turn portion is strongly doped over its entire section. In other words, the heavily doped silicon part occupies the entire section of a turn or a portion of turn, that is to say that the doping is massive. According to an alternative embodiment, the heavily doped silicon portion occupies only a surface layer of the section of a turn or of a turn portion, in particular a wall of a turn or a turn portion. In addition, in the embodiments that will be described below, the doping is advantageously uniform on all the turns of the spiral, or on the entire spiral and / or over a section of the spiral. As a variant, it may be non-uniform, variable depending on the turns or the portions of the turns and / or on the section of the turns or portions of the turns of the spiral.

However, it is also noted that heat compensation depends on crystal orientation. In other words, the effect of spiral silicon doping gives an anisotropic thermo-compensation characteristic.

Thus, according to an advantageous embodiment, the geometry of the spiral spring has sectional variations along its length to take into account this anisotropy. In other words, the hairspring exhibits a variation in section as a function of the crystallographic orientation of the highly doped silicon.

A first embodiment is thus based on a modulation of the spiral spiral thickness, that is to say a variation of the dimension of the side of the turns located in a plane parallel to the plane of the spiral, more particularly a variation of the dimension. turns which is locally perpendicular to the neutral fiber of the hairspring in a plane parallel to the plane of the hairspring. This modulation of the thickness is chosen to promote the bending of the first zones of the spiral. These first zones of the spiral have a local CTE higher than the local CTE of second spiral zones. The modulation of the thickness of the turns, more particularly the thinning of the thickness of the turns in these first zones of the spiral, thus makes it possible to optimize the thermo-compensation of the oscillator. As a note, this modulation of the thickness impacts the regularity of the rigidity of the blade, and therefore the mechanical behavior at constant temperature. However, this effect is considered limited compared to the effect of changes in spiral CTE with temperature. Moreover, it is possible to compensate for this effect by related variations in the spiral spiral section.

The figure 1 represents a spiral 1 with a constant pitch at equilibrium or at rest according to one embodiment of the invention, consisting of nine turns, and comprising an evolution of the thickness of the turns presented by the curve of the figure 2 . This figure 2 shows the evolution of the relative thickness (e / e0) of the turns as a function of the angle (α), in a coordinate system in polar coordinates and centered on the center of the spiral. It appears that each turn has thinning 2 on areas extending a given angular range, this angular range varying according to the doping of the spiral silicon and a possible oxidation of the highly doped spiral. This angular range can be between 2 and 80 degrees, especially between 5 and 40 degrees, especially between 5 and 20 degrees. In our particular embodiment, the spiral plane substantially coincides with a {001} plane of the silicon single crystal. In this particular embodiment, the first zones of the spiral, in particular the thinning 2, coincide substantially with the places where the tangent to the neutral fiber is aligned with a <100> direction of the silicon single crystal. In this particular embodiment, the thinning 2 are periodically arranged along the spiral turns in a period of 90 °. In an alternative embodiment in which the spiral plane does not coincide substantially with a {001} plane of the silicon single crystal, the thinning may be periodically disposed along the coils of the spiral at a period of 180 degrees. Apart from thinnings, the thickness may remain substantially constant or not. It should be noted that the thinning, namely the local variations in the size of the turns, may be equal or not. The geometries of the thinning may differ or not. Thus, thinning is periodically arranged according to a given period even if the local variations in the size of the turns or the geometries of the aminicissements differ. It should be noted that with such a geometry, the spiral can have any thickness and not all, while maintaining a good thermal behavior, which allows to determine these parameters according to the criteria set by the search for the best chronometric performance of the oscillator.

The figure 3 alternatively represents a periodic change in the relative thickness (e / e0) of the turns which have a linear profile over 45 degrees. Thus, in this particular variant, each turn has a minimum thickness 2 for the angles 45, 135, 225 and 315 degrees, and maximum thicknesses 3 for the angles 0, 90, 180, and 270 degrees. The 0 degree angle corresponds to the inner end of the hairspring. Between these extreme thicknesses 2, 3, the spiral has a thickness varying linearly with the angle. In this embodiment, the evolution of the thickness is therefore periodic and similar on each turn.

In these two embodiments, the reduction in thickness can range from 5 to 90% relative to the maximum thickness, especially from 10 to 40% relative to the maximum thickness.

According to an alternative embodiment, the variation of the section of the coils spiral can be achieved by a change in the height of turns, that is to say, the dimension perpendicular to the plane of the spiral. This modification can for example be obtained by gray photolithography, for the same purpose of promoting in this way the bending of the first zones of the spiral.

Naturally, other embodiments are possible, based on the variation of the shape and / or dimensions of the section of the turns. For example, it is possible to vary the thickness and the height of the turns, by combining the two embodiments described above. The objective of the geometry modification is to favor the bending of the spiral in the favorable zones, in particular of positive thermal coefficient. Advantageously, this variation of the spiral section as a function of the angle, in a coordinate system in polar coordinates, is periodic. In particular, this period can be 90 or 180 degrees. In addition, this section variation in order to optimize the thermal behavior of the hairspring can be combined with a complementary section variation, generally non-periodic, suitable for optimizing the chronometric behavior of the hairspring.

As a remark, the zones of the spiral to be favored can be determined by a theoretical calculation and / or empirically.

Moreover, it is found that a stronger doping brings a reinforced effect of thermo-compensation. It may also be possible to provide stronger doping in certain areas of the spiral, including the favorable areas mentioned above. It is also possible, alternatively or complement, to provide stronger doping in the areas closer to the surface of the spiral.

This variation of the doping can be made a posteriori by diffusion or ion implantation, to obtain a "fine" adjustment of the CTE of the spiral after its manufacture. Naturally, the different variations described in the previous embodiments can be combined.

It has been found that the variation of the section of a spiral alone makes it possible to obtain good results by using a highly doped silicon. It can be seen that a slight oxidation of silicon, in addition to the characteristics described in the previous embodiments, makes it possible to obtain an equivalent result with a slightly less strongly doped silicon. In other words, oxidation of the highly doped silicon makes it possible to improve the performance in terms of thermo-compensation with equivalent silicon doping, or to reduce the importance of the modulation of the thickness of the turns.

The figure 4 illustrates this effect. The four straight lines 11, 12, 13, 14 respectively represent four spirals each having a different sectional variation, obtained by the periodic modulation of the section of the spiral, whose ratio R between the minimum thickness and the maximum thickness of the turns is equal to respectively 1, 0.55, 0.33, 0.10. These four spirals are associated with the same pendulum CuBe2 to form oscillators. For each of these spirals, the oxide thickness (c) necessary to reach a zero thermal coefficient is represented as a function of the logarithm of the ion density (Log di). It is found in all cases that an ion density doping up to 10 ¹⁸ at.cm ^-3 requires an oxide layer tending towards 3 microns. It is found in all cases that a very strong doping ion density higher than 10 ¹⁸ at.cm ^-3 requires a less oxide layer or zero. In addition, the oxide layer can be advantageously canceled for a rocker formed of a material whose thermal expansion coefficient is substantially lower. As a remark, embodiments with oxide layers of less or even zero thickness remain interesting and are covered by the present invention, even if the thermal coefficient is slightly less good, which is offset by the greater simplicity Manufacturing. In addition, it can be seen that the less pronounced the modulation of the spiral thickness (greater ratio R), the more strongly it is necessary to doping the silicon to obtain a zero thermal coefficient without oxidation. As a remark, it is also noted that these curves remain substantially unchanged if only the type of modulation of thickness is modified, for example according to the figures 2 and 3 , while maintaining an identical R ratio.

It therefore appears that the invention also relates to a spiral comprising a heavily doped silicon part and comprising an outer oxidation layer. In particular, embodiments are obtained by adding an oxide layer to the previously described embodiments. In any case, considering more generally any hairspring of a timepiece according to any embodiment, the oxide layer has a small thickness, its maximum thickness being less than or equal to 5 microns, or even less than or equal to 3 microns, even less than or equal to 2.5 microns, even less than or equal to 2 microns, or even less than or equal to 1.5 microns.

The invention also relates to a method of manufacturing a spiral as described above. This process comprises in particular a spiral cutting step in a wafer of heavily doped silicon, for example by the method of deep reactive ion etching (DRIE), this cutting being such that it allows to form a variable section of spiral turns. More precisely, according to one embodiment, this cutting makes it possible to form turns of variable thickness by the choice of the shape on the mask. Another embodiment consists in forming turns of variable height, for example using a gray photolithography, multiple etching using different masks, or other methods known to those skilled in the art.

As a remark, the wafer can be manufactured from a highly doped silicon ingot itself obtained by a step of high silicon doping during its growth.

Alternatively, the manufacturing method comprises a step of cutting the spiral in a silicon wafer, and then a step of doping the silicon after the cutting, in particular by diffusion or ion implantation, to obtain a spiral comprising highly doped silicon. In this embodiment, an (additional) doping step is therefore added after cutting. The silicon wafer may initially be heavily doped or not. This embodiment makes it possible to more strongly dope the areas close to the surface and more stressed during oscillation deformations. Note that the fact of performing a posterior doping has the advantage of allowing to obtain a higher doping rate and thus avoid the need for oxidation of silicon or reduce the necessary oxide layer.

This manufacturing method also has the advantage of taking advantage of the flexibility of the cutting in a silicon wafer, which makes it possible to achieve very diverse geometries, and in particular to vary with very little limitation the thickness of the blade forming a twist of the spiral.

The wafer may preferably be made of silicon monocrystal oriented in the <100> direction.

According to an alternative embodiment, the manufacturing method comprises a complementary oxidation step. As explained above, the oxidation layer used has a small thickness, in all embodiments, which has the advantage of allowing its realization at a low oxidation temperature, and thus prevent premature wear of the oven used. In addition, this small thickness of the oxidation layer also allows its implementation using oxygen as a precursor, instead of the water vapor used for thicker oxidation layers, thus forming a layer of oxidation of a high quality while minimizing its growth time.

The invention also relates to a timepiece oscillator, a timepiece movement and a timepiece, such as a watch, for example a wristwatch, comprising a hairspring as described above.

Claims (16)

1. A balance spring for an oscillator of a timepiece, characterized in that it comprises a part, in particular at least a coil or a portion of a coil, comprising a cross section varying locally over the length by a reduction in the thickness and/or in the height of said part, said part being provided with heavily doped silicon having an ion density greater than or equal to 10¹⁸ at/cm³, in order to permit the thermo-compensation of the oscillator.
2. The balance spring for an oscillator of a timepiece as claimed in the preceding claim, characterized in that said part, in particular said coil or said portion of a coil, comprises heavily doped silicon having an ion density greater than or equal to 10¹⁹ at/cm³, or greater than or equal to 10²⁰ at/cm³.
3. The balance spring for an oscillator of a timepiece as claimed in claim 1 or 2, characterized in that the variation in cross section is periodic, in particular according to a period of 90 or 180 degrees.
4. The balance spring for an oscillator of a timepiece as claimed in one of previous claims, characterized in that the minimum values of the thickness and/or the height of the coil or the portion of the coil of the balance spring coincide with the places where the tangent to the neutral fiber is substantially in alignment with a direction <100> of the monocrystal constituting the balance spring.
5. The balance spring for an oscillator of a timepiece as claimed in one of the preceding claims, characterized in that said part, in particular said coil or said portion of a coil, comprises the heavily doped silicon for the whole of its thickness and/or for the whole of its height, or only on a layer of its surface.
6. The balance spring for an oscillator of a timepiece as claimed in one of the preceding claims, characterized in that it comprises an external oxidized layer, in particular consisting of silicon dioxide SiO₂.
7. The balance spring for an oscillator of a timepiece as claimed in the preceding claim, characterized in that the external oxidized layer covers said part, in particular said coil or said portion of a coil, and in that said part comprises a cross section varying locally over its length.
8. The balance spring for an oscillator of a timepiece as claimed in Claim 6 or 7, characterized in that it comprises an external oxidized layer having a thickness less than or equal to 5 µm, or less than or equal to 3 pm, or less than or equal to 2.5 µm, or less than or equal to 2 µm, or less than or equal to 1.5 m.
9. The balance spring for an oscillator of a timepiece as claimed in one of Claims 6 to 8, characterized in that it comprises over its entire length a variable cross section, with heavily doped silicon having doping greater than or equal to 10¹⁸ at/cm³ and comprising an external oxidized layer.
10. The balance spring for an oscillator of a timepiece as claimed in one of the preceding claims, characterized in that the part made of heavily doped silicon is of a nature such as to make it possible to nullify or to substantially nullify the expression: $$\mathrm{TCY}+3\mathrm{}{\mathrm{\alpha }}_{\mathrm{s}}-2{\mathrm{\alpha }}_{\mathrm{b}}$$

    

    where

    TCY is the thermal coefficient of the Young's modulus (TCY)

    α_s is the coefficient of thermal expansion of the balance spring

    α_b is the coefficient of thermal expansion of the balance intended to interact with the balance spring.
11. An oscillator for a timepiece, in particular of the balance spring type, characterized in that it comprises a balance spring as claimed in one of the preceding claims.
12. A timepiece, in particular a watch, characterized in that it comprises a balance spring as claimed in one of Claims 1 to 10.
13. A method for producing a balance spring as claimed in one of Claims 1 to 10, characterized in that it comprises a step involving the heavy doping of the silicon and a step involving the production of a wafer made of heavily doped silicon, followed by a step involving cutting said wafer in order to obtain a balance spring comprising heavily doped silicon.
14. The method for producing a balance spring as claimed in one of Claims 1 to 10, characterized in that it comprises a step involving cutting a wafer made of silicon in order to form a balance spring, followed by a step involving the heavy doping of the silicon after cutting, in particular by ion diffusion or ion implantation, in order to obtain a balance spring comprising heavily doped silicon.
15. The method for producing a balance spring as claimed in one of Claims 13 or 14, characterized in that it comprises all or part of the following steps:

    - cutting the balance spring in such a way as to form a modulation of the thickness of the balance spring; and/or

    - second cutting of the balance spring in order to form a variation in the height of at least one coil of the balance spring.
16. The method for producing a balance spring as claimed in one of Claims 13 to 15, characterized in that it comprises a step involving the oxidation of at least one part of the silicon of the balance spring.

Images