EP4212965A1 - Method for limiting the deformation of a silicon timepiece - Google Patents

Abstract

The invention relates to a method for limiting the deformation of a silicon timepiece formed in a wafer, during thermal oxidation, characterized in that the thermal oxidation is carried out on a timepiece highly doped silicon. The invention also relates to a use of a wafer comprising at least one layer of heavily doped silicon, to limit the deformation of a timepiece formed in said layer of heavily doped silicon, during thermal oxidation.

Description

Technical area

The present invention relates to the field of manufacturing parts for watchmaking. The invention relates more particularly to a method for limiting the deformation of a silicon timepiece, in particular a silicon hairspring.

State of the art

The movements of mechanical watches are regulated by means of a mechanical regulator comprising a resonator, that is to say an elastically deformable component whose oscillations determine the rate of the watch. Many watches include, for example, a regulator comprising a hairspring as a resonator, mounted on the axis of a balance wheel and set in oscillation by means of an escapement. The natural frequency of the balance-spring couple makes it possible to regulate the watch and depends in particular on the stiffness of the balance-spring.

Indeed, the frequency f of the regulating organ formed by the balance spring of stiffness R coupled to a balance wheel of inertia I is given by the formula: $$f=\frac{1}{2\pi }\sqrt{\frac{R}{I}}$$

The stiffness of the hairspring also defines its intrinsic vibratory characteristics, such as the natural frequency and the resonant frequencies. In the present application, the natural frequency of an elastic system (a single resonator or a resonator-pendulum couple) is the frequency at which this system oscillates when it is in free evolution, that is to say without exciting force. . Furthermore, a resonance frequency of an elastic system subjected to an exciting force is a frequency at which a local maximum of displacement amplitude can be measured for a given point of the elastic system. In other words, if the elastic system is excited with an excitation source of variable frequency over time, the displacement amplitude slopes upward before this resonant frequency, and slopes downward afterwards, at any point that does not correspond to a node of vibration. Typically, during such a test, the recording of the displacement amplitude as a function of the excitation frequency shows a displacement amplitude peak or resonance peak which is associated with or which characterizes the resonance frequency.

avec :

• ϕ, l'angle de torsion du ressort, et
• M, le couple de rappel du ressort spiral,
• où M, pour un barreau de section constante constitué d'un matériau spécifique, est donné par : $$M=\frac{E\left(\frac{e^3\mathrm{<figure-callout\; id="12"\; label="h"\; filenames="imgf0001.png"\; state="\{\{state\}\}">h</figure-callout>}}{12}\varphi \right)}{L}$$
  
  avec :
• E, le module d'Young du matériau employé pour le barreau,
• L, la longueur du barreau,
• h, la hauteur du barreau, et
• e, l'épaisseur ou la largeur du barreau.

The stiffness of a spiral-type resonator typically depends on the characteristics of the material in which it is made, as well as its dimensions and in particular the thickness (that is to say the width) of its turns along of his bar. The stiffness is given more specifically by: $$R=\frac{M}{\varphi }$$

with :

• ϕ , the torsion angle of the spring, and
• M, the return torque of the spiral spring,
• where M, for a bar of constant section made of a specific material, is given by: $$M=\frac{E\left(\frac{e^3\mathrm{<figure-callout\; id="12"\; label="h"\; filenames="imgf0001.png"\; state="\{\{state\}\}">h</figure-callout>}}{12}\varphi \right)}{I}$$
  
  with :
• E, the Young's modulus of the material used for the bar,
• L, the length of the bar,
• h, the height of the rung, and
• e, the thickness or width of the bar.

The natural frequency of the regulator member formed by the balance spring of stiffness R coupled to a balance wheel of inertia I is in particular proportional to the square root of the stiffness of the balance spring. The main specification of a spiral spring is its stiffness, which must be within a well-defined range in order to be paired with a balance wheel, which forms the inertial element of the oscillator. This operation pairing is essential to precisely adjust the frequency of a mechanical oscillator.

It is very important that the characteristics of the oscillator are as stable as possible, in order to have a rate of the watch which is also stable. The importance of magnetic fields in the modern environment has prompted watchmakers to use silicon hairsprings for several years, which are less sensitive to magnetic disturbances than metal hairsprings.

Very advantageously, several hundred silicon hairsprings can be manufactured on a single wafer using micro-fabrication technologies. It is in particular known to produce a plurality of hairsprings in silicon with very high precision by using photolithography and machining/etching processes in a silicon wafer. The methods for producing these mechanical hairsprings generally use monocrystalline silicon wafers, but wafers made of other materials can also be used, for example polycrystalline or amorphous silicon, other semiconductor materials, glass, ceramic , carbon, carbon nanotubes or a composite comprising these materials. For its part, monocrystalline silicon belongs to the cubic crystalline class m3m whose coefficient of thermal expansion (alpha) is isotropic.

Silicon has a very negative value of the first thermoelastic coefficient, and consequently the stiffness of a silicon resonator, and therefore its natural frequency, varies greatly according to the temperature. In order to at least partially compensate for this drawback, the documents EP1422436 , EP2215531 And WO2016128694 describe a spiral-type mechanical resonator made from a core (or two cores in the case of WO2016128694 ) in monocrystalline silicon and whose variations in temperature of the Young's modulus are compensated by a layer of amorphous silicon oxide (SiO ₂ ) surrounding the core (or cores), the latter being one of the rare materials having a thermoelastic coefficient positive.

It is known to adjust the dimensions of the hairsprings after etching, in particular to reduce the dispersion of stiffness that is observed between the hairsprings, according to their positioning on the wafer in which they are made. This adjustment is made by a succession of oxidation and deoxidation, which makes it possible to precisely modify the dimension of the hairsprings, but also to smooth the sides of these hairsprings. The oxidation operation is carried out thermally, by a heating operation typically carried out between 600 and 1300°C.

It is observed that these operations induce deformations on the hairsprings, particularly the hairsprings having isotropic crystallographic orientations, that is to say particularly the hairsprings made in <100> and <110> silicon. Such a deformation can in particular result in an elongation of the hairspring and therefore a displacement of its outer end. A deformation is also observed under the influence of the own weight of the hairspring, particularly when the oxidation is carried out in a horizontal furnace, in which the wafers are arranged vertically. The turns are then deformed by gravity and are no longer wound regularly.

The document WO2019/180596 proposes to arrange the plates horizontally and on a support, making it possible to limit the deformations related to the own weight of the hairspring and to the heat.

For wafers arranged vertically, it has hitherto been necessary to carry out N oxidation stages, and to perform between each of them a rotation of the wafer in its support of 360/N°, in order to obtain a compensation between the different deformations. This imposes manipulations which slow down the production flow and which create as many opportunities for breakage or pollution of the wafers.

The applicants have found another unexpected solution to this problem. The solution identified can be generalized to other silicon timepieces for which it is essential to control the dimensions and manufacturing tolerances.

Disclosure of Invention

More specifically, the invention relates to a process for limiting the deformation of a silicon timepiece formed in a wafer, during thermal oxidation, characterized in that the thermal oxidation is carried out on a highly doped silicon timepiece.

Another aspect of the invention relates to a use of a wafer comprising at least one layer of highly doped silicon, to limit the deformation of a timepiece formed in said highly doped silicon layer, during thermal oxidation.

According to these two aspects, it is possible to limit or even eliminate the deformations undergone by a silicon timepiece, when it is subjected to thermal oxidation, between 600 and 1300° C., without having to manipulate the wafers.

Brief description of the drawings

• la figure 1 est une représentation schématique d'une installation d'oxydation de plaquettes de silicium dans un four à chargement horizontal, les plaquettes étant disposés verticalement,
• la figure 2 représente un spiral non dopé, déformé après une oxydation thermique dans un four, selon la disposition de la figure 1,
• la figure 3 représente un spiral dopé, après une oxydation thermique dans un four, selon la disposition de la figure 1.

Other details of the invention will appear more clearly on reading the following description, made with reference to the appended drawing in which:

• there figure 1 is a schematic representation of a silicon wafer oxidation installation in a horizontal loading furnace, the wafers being arranged vertically,
• there figure 2 represents an undoped hairspring, deformed after thermal oxidation in an oven, according to the arrangement of the figure 1 ,
• there picture 3 represents a doped hairspring, after thermal oxidation in an oven, according to the arrangement of the figure 1 .

Embodiment of the invention

The document WO2019/180596 describes the steps that make it possible to etch watch components, particularly hairsprings, in a wafer (also called a wafer) of silicon. These lithography steps are well known to those skilled in the art and are incorporated by reference into the present application.

This wafer can be of different types, comprising a single layer of silicon, or a layer of silicon arranged on a layer of silicon oxide (SOI), or several layers of silicon, with a layer of silicon oxide interposed between the silicon layers. Depending on the configurations, the component is etched in the silicon layer or in a group of silicon layers.

Silicon can be of different natures, monocrystalline, polycrystalline or even amorphous.

• ajustement dimensionnel ou lissage des flancs lorsque l'étape d'oxydation est suivie d'une désoxydation,
• ajustement du coefficient thermique d'élasticité du spiral, grâce au fait que l'oxyde de silicium présente un coefficient thermique d'élasticité de signe opposé à celui du silicium,
• renforcement mécanique du composant.

Among the manufacturing steps, one or even several thermal oxidation steps are carried out. These oxidation steps can have different purposes:

• dimensional adjustment or smoothing of the flanks when the oxidation step is followed by deoxidation,
• adjustment of the thermal coefficient of elasticity of the hairspring, thanks to the fact that silicon oxide has a thermal coefficient of elasticity of opposite sign to that of silicon,
• mechanical reinforcement of the component.

As shown in figure 1 , these oxidation steps are carried out in an oxidation furnace 10, at temperatures typically between 600° C. and 1300° C., which makes it possible to form a layer of silicon oxide (SiO ₂ ) which covers the wafer and etched components, consuming silicon from the wafer.

In some processes, the wafers are loaded into the oven by being arranged horizontally. As in the document WO2019/180596 , it is proposed to arrange the wafers on supports provided with spacers, to prevent or at least limit the deformation of the hairsprings under the effect of gravity. However, this solution cannot work for wafers 12 arranged vertically, in particular in furnaces 10 with horizontal loading, as represented on the figure 1 .

The applicants noted with surprise that the deformations undergone by the components during the thermal oxidation operations, under the effect of the heat and the weight of the components themselves, were very limited, even almost zero, by using a wafer whose or the layer(s) of silicon is/are made of doped silicon. Preferably, the doping is high boron or phosphorus doping. By high doping (this expression being equivalent to “heavily doped”) is meant an average concentration of dopant within the same wafer greater than 1x10 ¹⁸ , more particularly greater than 1x10 ¹⁹ , even more particularly greater than 5x10 ¹⁹ atoms per cm ³ . Alternatively, a high doping can be defined as a doping corresponding to a resistivity smaller than 0.01 ohm.cm, or even a resistivity between 0.0045 and 0.0055 ohm.cm, even more particularly a resistivity of 0.005 ohm.cm.

Advantageously, the effects of doping are particularly sensitive with hairsprings made of monocrystalline silicon, in particular having crystalline orientations <100> or <110>. It is in fact the types of silicon which are particularly subject to deformation during a thermal oxidation operation.

It is known to use doped, or even heavily doped, silicon to produce watch components. However, the known effects of such doping concern the mechanical properties of silicon, such as its hardness and Young's modulus. It has also been proposed to dope a portion of a silicon hairspring to obtain thermal compensation for the stiffness of the hairspring, in other words to limit variations in the elastic torque of the hairspring as a function of temperature. The effects of doping on the thermal deformation of silicon have so far not been identified or described.

To illustrate these effects, we have represented on the picture 2 an undoped hairspring produced in a <100> wafer having undergone thermal oxidation while being positioned vertically in an oxidation furnace. The arrow symbolizes the orientation of the gravitational field during the oxidation step. It can be seen that the turns are progressively shifted with respect to their nominal position under the effect of their weight. After the oxidation operation, the induced deformation is final and the balance spring, at rest and flat, remains deformed. To quantify the deformation, a segment was placed radially and parallel to the gravitational field on the figure 2 . It arrives at the level of the ^7th turn on the undoped hairspring, starting from the centre.

We represented on the picture 3 a hairspring produced in a wafer <100> having a high doping, within the meaning of the paragraphs above, having undergone thermal oxidation while being positioned vertically in an oxidation furnace. In its geometry, it is very close to its form before oxidation. A segment of the same length as that shown on the figure 2 and positioned identically. It can be seen that it ends at the level of the ^5th turn, i.e. a shift of 2 turns between an undoped silicon hairspring ( figure 2 ) and a heavily doped hairspring ( picture 3 ).

We also measured the pitch between the penultimate turn and the last turn of hairsprings made in a heavily doped <100> wafer (sample A), and of hairsprings made in an undoped <100> wafer (sample B), the hairsprings having undergone thermal oxidation in a horizontal loading oven, being positioned vertically.

On samples of 20 hairsprings, the average pitch from the center and in the direction of gravity measured for sample B was reduced by more than 15%, while the average pitch from the center and in the direction of gravity for sample A was only reduced by less than 3%, compared to the hairsprings before stage thermal oxidation. The other parameters (oxidation time, oxidation temperature, position of the wafer in the oven, position of the attachment, geometry of the hairspring, etc.) being otherwise constant, this improvement in the behavior of the hairspring to oxidation is due to doping.

Thus, it is found that the use of a highly doped silicon wafer makes it possible to avoid the deformations encountered with undoped silicon wafers, during thermal oxidation steps during which the wafers are arranged vertically, without having to flip or change the orientation of the wafers between successive oxidation steps. The effect is also obtained when the wafers are arranged horizontally, even if this deformation is less noticeable, the fact remains that the hairsprings can deform outside the plane of the wafer, under the effect of their weight, and that such deformation is also limited with heavily doped hairsprings.

While the effects resulting from the invention have been shown on hairsprings, they are found identically with other watchmaking parts in silicon, for which it is essential to control the dimensions and manufacturing tolerances. It is known to those skilled in the art that a thermal oxidation step is present in almost all processes for implementing a silicon part, at least for one of the purposes mentioned above. above (dimensional adjustment, smoothing, thermal coefficient adjustment, mechanical reinforcement). It will be possible in particular to target anchors, wheels, virtual pivot systems such as anchors, pendulums or complete oscillators, that is to say comprising a return member and an inertial mass.

Claims (20)

Process for limiting the deformation of a silicon timepiece formed in a wafer, during thermal oxidation, characterized in that the thermal oxidation is carried out on a highly doped silicon timepiece. Limitation method according to Claim 1, characterized in that the timepiece is made of monocrystalline silicon. Limitation method according to Claim 2, characterized in that the timepiece is made of monocrystalline silicon with orientation <100> or <110>. Limitation method according to one of the preceding claims, characterized in that it comprises a step of supplying a highly doped silicon wafer. Limiting process according to one of the preceding claims, characterized in that the silicon has a dopant concentration greater than 1x10 ¹⁸ , more particularly greater than 1x10 ¹⁹ , even more particularly greater than 5x10 ¹⁹ atoms per cm ³ . Limitation method according to one of the preceding claims, characterized in that the silicon has a resistivity lower than 0.01 ohm.cm, more particularly a resistivity of 0.005 ohm.cm. Limitation process according to one of the preceding claims, characterized in that the dopant is chosen from boron or phosphorus. Limiting method according to one of the preceding claims, characterized in that it is carried out with timepieces formed in a wafer placed vertically in an oxidation furnace. Limiting method according to Claim 8, characterized in that it does not include a step of turning over or rotating the wafer. Limitation method according to one of Claims 1 to 9, characterized in that the timepiece is chosen from among a hairspring, an anchor, a wheel, a virtual pivot system such as an anchor, a balance or an oscillator. Use of a wafer comprising at least one layer of heavily doped silicon, to limit the deformation of a timepiece formed in said layer of heavily doped silicon, during thermal oxidation. Use according to Claim 11, characterized in that the timepiece is made of monocrystalline silicon. Use according to Claim 12, characterized in that the timepiece is made of monocrystalline silicon with orientation <100> or <110>. Use according to one of Claims 11 to 13, characterized in that it comprises a step of supplying a highly doped silicon wafer. Use according to one of Claims 11 to 14, characterized in that the silicon has a dopant concentration greater than 1x10 ¹⁸ , more particularly greater than 1x10 ¹⁹ , even more particularly greater than 5x10 ¹⁹ atoms per cm ³ . Use of embodiment according to one of Claims 11 to 15, characterized in that the silicon has a resistivity lower than 0.01 ohm.cm, more particularly a resistivity of between 0.0045 and 0.0055 ohm.cm, even more particularly a resistivity of 0.005 ohm.cm. Use according to one of Claims 11 to 16, characterized in that the dopant is chosen from boron or phosphorus. Use according to one of Claims 11 to 17, characterized in that it is carried out with timepieces formed in a wafer placed vertically in an oxidation furnace. Use according to claim 18, characterized in that it does not include a step of turning over or rotating the wafer Use according to one of Claims 11 to 19, in which the timepiece is chosen from a hairspring, an anchor, a wheel, a virtual pivot system such as an anchor, a balance wheel or an oscillator.

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