EP3416001B1 - Method for manufacturing an oscillator with flexible pivot - Google Patents

Description

The present invention relates to the manufacture of an oscillator with a flexible pivot, in particular an oscillator intended to serve as a time base in a watch movement.

Flexible pivot oscillators for watchmaking have been described in documents EP 2911012 , EP 2998800 , EP 3037893 , EP 3035127 , EP 2990885 , WO 2012/010408 , WO 2016/096677 and WO 2017/055983 . They include a support, making it possible to fix the oscillator on a fixed or mobile frame, a flexible pivot and a serge suspended from the support by the flexible pivot. The flexible pivot consists of elastic blades arranged to guide the rim in rotation with respect to the support and elastically return the rim to a rest position. The document EP 2990885 explains that an adjustment of the oscillation frequency can be carried out at the level of the elastic blades by heat treatment or ablation of material using a laser beam, and at the level of the inertia of the serge by ablation material also with a laser.

Flexible-pivot oscillators, especially when made in one piece, are typically fabricated by microfabrication techniques such as etching a silicon wafer. There is nevertheless a geometric dispersion between the oscillators of different wafers and even between the oscillators of the same wafer. This dispersion results in a variation of the oscillation frequency from one oscillator to another.

The object of the present invention is to remedy or at least attenuate this drawback by proposing a method of manufacturing an oscillator with a flexible pivot whose dimensions are sufficiently precise not to require reworking.

To this end, a method according to claim 1 is provided.

• la figure 1 est une vue plane de dessus d'un oscillateur à pivot flexible auquel se rapporte l'invention ;
• la figure 2 montre les différentes étapes d'un procédé de fabrication d'un oscillateur à pivot flexible ;
• les figures 3 à 5 montrent une section droite d'une lame élastique de l'oscillateur à pivot flexible à différents moments lors de la mise en œuvre du 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 schematic drawings in which:

• the figure 1 is a top plan view of a flexible pivot oscillator to which the invention relates;
• the figure 2 shows the different steps of a manufacturing process of a flexible pivot oscillator;
• the figures 3 to 5 show a cross section of an elastic blade of the flexible pivot oscillator at different times during the implementation of the method according to the invention.

As shown in figure 1 , the invention relates to an oscillator 1 with a flexible pivot. The oscillator 1 comprises a support or fixing part 2, making it possible to fix the oscillator 1 to a fixed or mobile frame of a watch movement. The support 2 can be in one part, as shown, or in two separate parts as described in the patent application WO 2017/055983 . Oscillator 1 also includes a rim 3 suspended from support 2 by a flexible pivot 4.

The flexible pivot 4 consists of elastic blades 4a, 4b, two in number in the example shown. Each elastic blade 4a, 4b connects the support 2 to the rim 3. In the example shown, the elastic blades 4a, 4b intersect at a point O. They can intersect without contact, the two blades 4a, 4b extending then in two different parallel planes, or intersect with contact, the two blades 4a, 4b then extending in the same plane. The first case, corresponding to a flexible pivot of the “separate crossed blades” type, is preferred to the second (“non-separated crossed blades”) because it allows a greater angular travel of the rim 3 with respect to the support 2. In a another variant, not shown, the flexible pivot 4 could be of the type with a remote center of rotation called “RCC” (Remote Center Compliance).

In all cases, the flexible pivot 4 defines a virtual axis of rotation typically passing through the center of the rim 3 and around which the rim 3 pivots relative to the support 2. The flexible pivot 4 thus guides the oscillations of the rim 3 by report to support 2. It also produces an elastic return torque as soon as rim 3 deviates from a rest position.

Serge 3 is typically in the form of a continuous ring, as shown, but it can alternatively be interrupted.

où K est la raideur du pivot flexible 4 et I est le moment d'inertie de la serge 3.The frequency f of oscillator 1 is given by the following formula: $$f=\frac{1}{2\pi }\sqrt{\frac{K}{I}}$$

where K is the stiffness of the flexible pivot 4 and I is the moment of inertia of the serge 3.

où E est le module d'élasticité du matériau utilisé, h est la hauteur de chaque lame (dimension dans la direction de l'axe de rotation), e est l'épaisseur de chaque lame et L est la longueur de chaque lame. Pour un pivot flexible de type à lames croisées non séparées, la raideur K répond à la formule : $$K=\frac{2}{3}.E.\frac{{\mathit{he}}^3}{L}$$

où les paramètres E, h, e et L sont les mêmes que ci-dessus. Enfin, pour un pivot flexible de type RCC, la raideur K répond à la formule : $$K=\frac{2}{3}.E.\frac{{\mathit{he}}^3}{L}\left(1+\frac{3p}{L}+\frac{3p^2}{L^2}\right)$$

où les paramètres E, h, e et L sont les mêmes que ci-dessus et p est la distance entre le point de croisement fictif des lames et l'extrémité de chaque lame la plus proche de ce point de croisement.The stiffness K depends on the type of flexible pivot 4. For a flexible pivot of the type with separated crossed blades, it corresponds to the formula: $$K=\frac{1}{6}.E.\frac{{\mathit{Hey}}^3}{L}$$

where E is the modulus of elasticity of the material used, h is the height of each blade (dimension in the direction of the axis of rotation), e is the thickness of each blade and L is the length of each blade. For a flexible pivot of the type with non-separated crossed blades, the stiffness K corresponds to the formula: $$K=\frac{2}{3}.E.\frac{{\mathit{Hey}}^3}{L}$$

where the parameters E, h, e and L are the same as above. Finally, for a flexible pivot of the RCC type, the stiffness K corresponds to the formula: $$K=\frac{2}{3}.E.\frac{{\mathit{Hey}}^3}{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}}\left(1+\frac{3p}{L}+\frac{3p^2}{{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}}^2}\right)$$

where the parameters E, h, e and L are the same as above and p is the distance between the fictitious crossing point of the blades and the end of each blade closest to this crossing point.

The above formulas can also be adapted to flexible pivots whose blades have a variable section. Thus, for example, in the case of a flexible pivot with separate crossed blades, the stiffness K can be expressed as follows: $$K=\frac{\mathrm{<figure-callout\; id="6"\; label="E"\; filenames="imgf0002.png"\; state="\{\{state\}\}">E</figure-callout>}}{6}.\frac{1}{{\displaystyle {\int }_0^L\frac{1}{h\left(I\right).{\mathrm{and}}^3\left(I\right)}.\mathit{dl}}}$$

où m est la masse de la serge et r est le rayon de giration de la serge.The moment of inertia I of the serge 3 is given by the formula: $$I=m.{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0001.png"\; state="\{\{state\}\}">r</figure-callout>}}^2$$

where m is the mass of the serge and r is the radius of gyration of the serge.

A method of manufacturing the oscillator 1 with flexible pivot will now be described with reference to the figure 2 .

At a first step E1, the oscillator 1 is formed but in dimensions which are different from the dimensions necessary to obtain a predetermined oscillation frequency of the rim 3 with respect to the support 2.

At step E1, either all the dimensions (in particular height h, thickness e and length L of the strips 4a, 4b, height and thickness of the serge 3) are different from the dimensions making it possible to obtain the predetermined oscillation frequency, or only part of these dimensions are different from the dimensions making it possible to obtain the predetermined oscillation frequency.

Step E1 is preferably carried out by etching a wafer of material. Several oscillators can be made simultaneously on the same wafer. Etching can be deep reactive ion etching (DRIE), chemical etching, focused ion beam (FIB) etching or laser etching, for example. According to the invention, the material is based on silicon.

Preferably, the silicon-based material is monocrystalline silicon whatever its crystalline orientation, doped monocrystalline silicon whatever its crystalline orientation, amorphous silicon, porous silicon, polycrystalline silicon, silicon nitride, silicon, quartz regardless of its crystalline orientation or silicon oxide.

Oscillator 1 formed in step E1 is typically one-piece. It can nevertheless be in several superposed and assembled parts, as described in the patent application EP 2998800 .

Of the techniques mentioned above, the most accurate is deep reactive ion etching. Phenomena that occur during etching or between two successive etchings can nevertheless induce geometric variations.

In a second step E2, the frequency of the oscillator formed in step E1 is measured by measurement means conventionally used in watchmaking. The measurement can be performed while the oscillator is still attached to its etching wafer or on the oscillator previously detached from the wafer, on all or on a sample of the oscillators still attached to the wafer or previously detached from the wafer . Step E2 may consist in determining an average frequency of a representative sample or of all the oscillators formed on the same wafer.

At a third step E3 is calculated, using the aforementioned formulas, a thickness of material to be added to all or part of the oscillator formed in step E1 or to be removed from all or part of the oscillator formed in step E1, to obtain the predetermined oscillation frequency.

It is in fact deduced from the aforementioned formulas that the frequency f can be increased by increasing the stiffness K of the flexible pivot 4 and/or by reducing the moment of inertia I of the serge 3, and conversely that one can reduce the frequency f by decreasing the stiffness K of the flexible pivot 4 and/or by increasing the moment of inertia I of the rim 3. The stiffness K of the flexible pivot 4 can be increased by increasing the section (height h and/or thickness e) slats 4a, 4b and/or by decreasing the length L of the slats 4a, 4b, and can be reduced by reducing the section (height h and/or thickness e) of the slats 4a, 4b and/or by increasing the length L of the blades 4a, 4b. The moment of inertia I of the rim 3 can be increased by increasing the mass m and/or the radius of gyration r of the rim 3, and can be decreased by decreasing said mass m and/or said radius of gyration r.

Consequently, if in step E1 dimensions have been chosen which make the frequency f greater than the predetermined frequency, in step E3 a thickness of material to be added or removed is calculated which makes it possible to reduce the frequency f so that it reaches the predetermined frequency. By analogy, if in step E1 dimensions have been chosen which make the frequency f lower than the predetermined frequency, in step E3 a thickness of material to be added or removed is calculated which makes it possible to increase the frequency f so that it reaches the predetermined frequency.

The calculated material thickness can be a thickness to be added or removed in a homogeneous manner over the entire external surface of the oscillator, a thickness to be added or removed in a non-homogeneous manner over the entire external surface of the oscillator, a thickness to adding or removing homogeneously only on a part of the outer surface of the oscillator or a thickness to add or remove in a non-homogeneous way only on a part of the outer surface of the oscillator.

For example, the addition or removal of material can be provided to vary only the height h of the blades 4a, 4b of the flexible pivot 4, only the thickness e of the blades 4a, 4b or both the height h and the thickness e. The same goes for the serge 3. It is also possible to add or remove material from the support 2 and/or from the serge 3 to vary the length L of the strips 4a, 4b.

It will be noted in particular that adding material to the flexible pivot 4 will increase its stiffness and therefore the frequency, whereas removing material from the flexible pivot 4 will decrease its stiffness and therefore the frequency. A homogeneous addition of material over the entire outer surface of the serge 3 will increase the moment of inertia therefore will decrease the frequency, while a homogeneous removal of material over the entire outer surface of the serge 3 will decrease the moment of inertia therefore will increase frequency. The effects of an addition or removal of material on the flexible pivot 4 and on the rim 3 are therefore opposite but of different magnitudes, so that the predetermined frequency can be obtained even with the addition or removal of a uniform thickness of material over the entire external surface of the oscillator.

At the next step E4, the thickness of material calculated at step E3 is, depending on the case, added to or removed from the oscillator formed at step E1.

Alternatively, in step E3 a thickness of material to be added to the flexible pivot 4 and a thickness of material to be removed from the serge 3 to obtain the predetermined frequency are calculated, or conversely a thickness of material to be removed from the flexible pivot 4 and a thickness of material to be added to the serge 3 are calculated, and these thicknesses are, depending on the case, added or removed in the zones of the oscillator concerned in step E4.

The present invention implements the steps E1 to E4 described above but the material chosen to form the oscillator 1 in step E1 is a silicon-based material and material is removed in step E4. Step E4 in the invention comprises a first step consisting in oxidizing the oscillator 1 in order to transform the thickness of silicon-based material to be removed into silicon dioxide, and a second step consisting in removing the layer of oxide of silicon thus formed. As shown at figures 3 to 5 , which illustrate a cross section of one of the blades 4a, 4b, the oscillator present after oxidation ( figure 4 ) a core 5 of material with silicon base whose dimensions are smaller than the corresponding dimensions h, e of the oscillator before oxidation ( picture 3 ), this core 5 being covered with a layer 6 of silicon oxide. After removal of layer 6 ( figure 5 ), we therefore obtain an oscillator of reduced dimensions.

The oxidation can be carried out thermally, for example between 800 and 1200° C. in an oxidizing atmosphere using steam or oxygen gas. It can be performed locally on the oscillator, for example only on the flexible pivot 4 or on the edge 3, by means of masks such as nitride masks. The oxide formed on the silicon-based material can be removed by a chemical bath comprising, for example, hydrofluoric acid.

Step E4 can end the method according to the invention. However, after step E4, steps E2, E3 and E4 can be repeated one or more times to refine the dimensional quality of the oscillator.

The present invention makes it possible to obtain a very high dimensional precision for the oscillator and therefore to guarantee a more precise oscillation frequency.

After step E4 of the method according to the invention, the oscillator can also be processed to improve some of its properties and characteristics. It can for example be treated to make it less sensitive to thermal variations, that is to say so that the stiffness of its flexible pivot 4 or even its frequency does not vary or varies little depending on the temperature. To do this, a layer of a material having a thermal coefficient of the modulus of elasticity of opposite sign to that of silicon can be formed over the entire silicon oscillator or at least over its flexible pivot 4. This layer is typically made of silicon oxide. It can be formed by thermal oxidation.

Claims (8)

1. Method for manufacturing an oscillator (1) with a flexible pivot of a predetermined frequency, the oscillator (1) with a flexible pivot comprising a support (2), a flexible pivot (4) and a felloe (3) which is suspended on the support (2) by the flexible pivot (4), the flexible pivot (4) being arranged to guide the felloe (3) in rotation with respect to the support (2) and to elastically return the felloe (3) to a rest position, the method comprising the following steps:

   a) forming, in a silicon-based material, an oscillator with a flexible pivot comprising said support (2), said flexible pivot (4) and said felloe (3) but having dimensions different from the dimensions necessary to obtain said oscillator with a flexible pivot of a predetermined frequency,

   b) measuring the frequency of the oscillator formed in step a),

   c) from the measurement effected in step b), calculating at least one thickness of material to be removed from the oscillator formed in step a) in order to obtain said oscillator with a flexible pivot of a predetermined frequency,

   d) from the calculation effected in step c), modifying the oscillator formed in step a) in order to obtain said oscillator with a flexible pivot of a predetermined frequency, this step d) comprising the following steps;

   - oxidising all or part of the outer surface of the oscillator formed in step a) in order to transform said thickness of material to be removed into silicon dioxide, and

   - removing the silicon dioxide.
2. Method as claimed in claim 1, characterised in that in step d), only the flexible pivot (4) is modified.
3. Method as claimed in claim 1, characterised in that in step d), only the felloe (3) is modified.
4. Method as claimed in claim 1, characterised in that in step d), the flexible pivot (4) and the felloe (3) are modified.
5. Method as claimed in any one of the preceding claims, characterised in that step a) comprises an etching step, e.g. deep reactive ion etching, chemical etching, focussed ion beam etching or laser etching.
6. Method as claimed in any one of the preceding claims, characterised in that steps b), c) and d) are repeated one or more times in order to refine the dimensional quality of the oscillator obtained in step d).
7. Method as claimed in any one of the preceding claims, characterised in that the flexible pivot (4) is of the type with separate crossed strips, non-separate crossed strips or remote center compliance.
8. Method as claimed in any one of the preceding claims, characterised in that the oscillator formed in step a) is of one piece.

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