HK1239850B - Method for fabrication of a balance spring of a predetermined stiffness by removal of material - Google Patents

Description

Method for producing a balance spring of predetermined stiffness by removing material

Technical Field

The present invention relates to a method for manufacturing a balance spring of predetermined stiffness, and more particularly to such a balance spring used as a compensating balance spring cooperating with a balance wheel of predetermined inertia to form a resonator of predetermined frequency.

Background Art

European Patent No. 1 422 436, incorporated herein by reference, describes how to form a compensating spring for thermal compensation of the entire resonator: the compensating spring comprises a silicon core coated with silicon dioxide and cooperates with a balance wheel of predetermined inertia.

The manufacture of this type of compensating spring offers numerous advantages, but also drawbacks. Indeed, the process of etching multiple balance springs in a silicon wafer results in significant geometric variations between balance springs from the same wafer, and even greater variations between balance springs from two wafers etched at different times. Furthermore, the stiffness of each balance spring etched with the same etching pattern is variable, resulting in significant manufacturing variations.

Summary of the Invention

One object of the present invention is to overcome all or part of the above-mentioned drawbacks by proposing a method for manufacturing a balance spring of sufficiently precise dimensions not to require further correction operations.

The present invention therefore relates to a method for manufacturing a balance spring of predetermined stiffness, comprising the following steps:

a) forming a balance spring having dimensions greater than those required to obtain a balance spring of a predetermined stiffness;

b) determining the stiffness of the balance spring formed in step a) by measuring the frequency of the balance spring coupled to a balance wheel having a predetermined inertia;

c) calculating the thickness of material to be removed based on the result of the determination of the spring stiffness determined in step b) in order to obtain the dimensions required for obtaining a spring of predetermined stiffness;

d) removing said thickness of material from the balance spring formed in step a) so as to obtain a balance spring having the dimensions required for said predetermined stiffness.

It will therefore be understood that this method makes it possible to guarantee a very high dimensional accuracy of the balance spring and, incidentally, also a more precise stiffness of said balance spring. Consequently, any manufacturing parameter capable of causing the geometrical variations of step a) can be completely corrected for each balance spring manufactured, or averaged over all balance springs formed at the same time, thereby significantly reducing the scrap rate.

According to other advantageous variations of the invention:

- in step a), the dimensions of the balance spring formed in step a) are between 1% and 20% greater than the dimensions required to obtain a balance spring of predetermined stiffness;

- step a) is carried out by means of deep reactive ion etching or chemical etching;

- in step a), forming in the same wafer a plurality of balance springs having dimensions greater than those required to obtain a plurality of balance springs of one predetermined stiffness or a plurality of balance springs of a plurality of predetermined stiffnesses;

- the balance spring formed in step a) is made of silicon, glass, ceramic, metal or a metal alloy;

- step b) comprises a stage b1) of measuring the frequency of an assembly comprising a balance spring formed in step a) coupled to a balance wheel of predetermined inertia, and a stage b2) of deducing the stiffness of the balance spring formed in step a) from the measured frequency;

- according to a first variant, step d) comprises a stage d1) of oxidizing the balance spring formed in step a) so as to transform said thickness of silicon-based material to be removed into silicon dioxide and thus form an oxidized balance spring, and a stage d2) of removing the oxide from the oxidized balance spring in order to obtain a balance spring having the dimensions required for said predetermined rigidity;

According to a second variant, step d) comprises a stage d3) of chemically etching the balance spring formed in step a) so as to obtain a balance spring having the dimensions required for said predetermined stiffness;

- after step d), the method performs steps b), c) and d) at least once more to further improve the dimensional quality;

- after step d), the method further comprises the step e) of forming, on at least a portion of the balance spring having a predetermined stiffness, a portion for correcting the stiffness of the balance spring and for forming a balance spring insensitive to thermal variations;

According to a first variant, step e) comprises a stage e1) of depositing a layer on a portion of the outer surface of said balance spring of predetermined stiffness;

- In a second variant, step e) comprises a stage e2) of modifying the structure of a portion of the outer surface of said balance spring of predetermined stiffness to a predetermined depth;

According to a third variant, step e) comprises a stage e3) of modifying the composition of a portion of the outer surface of said balance spring of predetermined stiffness to a predetermined depth.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages will emerge clearly from the following description, given by way of non-limiting illustration with reference to the accompanying drawings, in which:

- Figure 1 is a perspective view of an assembled resonator according to the invention.

- Figure 2 is an exemplary geometry of a balance spring according to the invention.

- Figures 3 to 6 are cross-sectional views of a balance spring during different steps of the method according to the invention.

- Figure 7 is a perspective view of the steps of the method according to the invention.

- Figure 8 is a diagram of the method according to the invention.

DETAILED DESCRIPTION

As shown in FIG1 , the present invention relates to a resonator 1 of the balance wheel 3 and hairspring 5 type. The balance wheel 3 and hairspring 5 are preferably mounted on the same arbour 7. In this resonator 1 , the moment of inertia I of the balance wheel 3 corresponds to the following formula:

I＝mr ² (1)

Here, m is the mass of the balance wheel and r is the radius of gyration which also depends on the expansion coefficient _αb and the temperature of the balance wheel.

Furthermore, the stiffness C of the balance spring 5 of constant cross section corresponds to the following formula:

Wherein, E is the Young's modulus of the material used, h is the height, e is the thickness, and L is its developed length.

Furthermore, the stiffness C of the balance spring 5 of constant cross section corresponds to the following formula:

Where E is the Young's modulus of the material used, h is the height, e is the thickness, L is the developed length, and l is the abscissa along the curve of the balance spring.

Furthermore, the stiffness C of balance spring 5 , which has a variable thickness but a constant cross section, corresponds to the following formula:

Where E is the Young's modulus of the material used, h is the height, e is the thickness, L is the developed length, and l is the abscissa along the curve of the balance spring.

Finally, the spring constant C of the balance resonator 1 corresponds to the following formula:

According to the invention, it is desirable that the resonator have a frequency variation with temperature that is substantially zero. In the case of a sprung balance resonator, the frequency variation f with temperature T substantially conforms to the following formula:

in:

- is the relative frequency change;

-ΔT is the temperature change;

- is the variation of the relative Young's modulus with temperature, i.e. the thermoelastic coefficient (TEC) of the hairspring;

-α _s is the coefficient of expansion of the hairspring expressed in ppm.℃ ^-1 ;

-α _b is the expansion coefficient of the balance wheel expressed in ppm.°C ^-1 .

The maintenance system may also contribute to thermal dependencies, since the oscillation of any resonator intended for a time or frequency base must be maintained, for example a Swiss lever escapement (not shown) also mounted on arbour 7 and cooperating with impulse stud 9 of disc 11 .

It is therefore clear from equations (1) to (6) that balance spring 5 can be coupled to balance wheel 3 in such a way that frequency f of resonator 1 is almost insensitive to temperature variations.

The present invention relates more specifically to a resonator 1 in which a balance spring 5 is used to temperature-compensate the entire resonator 1—that is, all its components and, in particular, the balance wheel 3. This type of balance spring 5 is often referred to as a compensating balance spring. This is why the present invention relates to a method that ensures very high dimensional accuracy of the balance spring and, incidentally, a more precise stiffness of said balance spring.

According to the invention, the compensation spring 5, 15 is formed of a material that can be coated with a thermal compensation layer and is intended to cooperate with a balance wheel 3 of predetermined inertia. However, it is not possible to avoid the use of a balance wheel with a movable inertia mass that can provide pre-sale or post-sale adjustment parameters for the timepiece.

The use of materials such as silicon, glass or ceramic for the production of balance springs 5, 15 offers the advantages of achieving precision through existing etching methods and having good mechanical and chemical properties while being virtually insensitive to magnetic fields. However, this balance spring must be coated or surface-modified to be able to form a compensating balance spring.

Preferably, the silicon-based material used for the compensation spring may be single-crystal silicon (regardless of its crystal orientation), doped single-crystal silicon (regardless of its crystal orientation), amorphous silicon, porous silicon, polycrystalline silicon, silicon nitride, silicon carbide, quartz (regardless of its crystal orientation), or silicon oxide. Of course, other materials are conceivable, such as glass, ceramic, cermet, metal, or metal alloy. For the sake of simplicity, the following description will refer to silicon-based materials.

Each material type can be surface modified or coated with a layer to thermally compensate the substrate as described above.

Although the step of etching the balance spring in a silicon wafer by means of deep reactive ion etching (DRIE) is carried out with the utmost precision, phenomena occurring during etching or in the interval between two consecutive etchings can cause geometrical variations.

Of course, other types of fabrication can be implemented, such as laser etching, focused ion beam etching (FIB), electroplating growth, growth by chemical vapor deposition or chemical etching, which are less precise and for which this method will make more sense.

The invention thus relates to a method 31 for manufacturing a balance spring 5c. According to the invention, as shown in FIG8 , the method 31 comprises a first step 33 for forming at least one balance spring 5a, for example from silicon, having a dimension D _a greater than the dimension D _b required to obtain a predetermined stiffness C of said balance spring 5c. As shown in FIG3 , the cross section of the balance spring 5a has a height _H1 and a thickness _E1 .

Preferably, the dimension D _a of the balance spring 5 a is approximately 1% to 20% greater than the dimension D _b of the balance spring 5 c required to obtain a predetermined stiffness C of the balance spring 5 c.

Preferably, according to the invention, step 33 is performed by means of deep reactive ion etching in wafer 23 made of silicon-based material, as shown in Figure 7. It should be noted that the opposing faces _F1 , _F2 are undulating because Bosch deep reactive ion etching results in a wavy etching that is constructed by successive etching and passivation steps.

Of course, the method should not be limited to a specific step 33. For example, step 33 can also be implemented by means of chemical etching in wafer 23, for example formed of a silicon-based material. Moreover, step 33 implies the formation of one or more balance springs, i.e., step 33 can form individual discrete balance springs or, alternatively, a balance spring formed in a wafer of material.

Thus, in step 33 , a plurality of balance springs 5a may be formed in the same wafer 23 , the dimensions D _a , H ₁ , E ₁ of which are greater than the dimensions D _b , H ₃ , E ₃ required to obtain a plurality of balance springs 5c having one predetermined stiffness C or a plurality of predetermined stiffnesses C.

Step 33 is also not limited to forming the balance spring 5a with dimensions _Da , _H1 , _E1 greater than the dimensions _Db , _H3 , _E3 required to obtain the predetermined rigidity C of the balance spring 5c using a single material. Therefore, step 33 may also involve forming the balance spring 5a with dimensions _Da , _H1 , _E1 greater than the dimensions _Db , _H3 , _E3 required to obtain the predetermined rigidity C of the balance spring 5c using a composite material, i.e., one containing several different materials.

Method 31 comprises a second step 35 aimed at determining the stiffness of balance spring 5 a. This step 35 may be carried out directly on a balance spring 5 a still attached to wafer 23 or on a balance spring 5 a previously separated from wafer 23, or on all or a sample of balance springs still attached to wafer 23 or on all or a sample of balance springs previously separated from wafer 23.

Preferably, according to the invention, whether or not balance spring 5a is separated from wafer 23, step 35 comprises a first phase aimed at measuring the frequency f of the assembly comprising balance spring 5a coupled to a balance wheel of predetermined inertia I, and then at deriving therefrom, in a second phase, the stiffness C of guide spring 5a using equation (5).

In particular, this measurement phase can be dynamic and carried out according to the teaching of European Patent 2 423 764, incorporated herein by reference. Alternatively, however, a static method according to the teaching of European Patent 2 423 764 can also be implemented to determine the stiffness C of balance spring 5 a.

Of course, as mentioned above, since the method is not limited to etching only one balance spring from each wafer, step 35 may also comprise determining the average stiffness of a representative sample or of all balance springs formed on the same wafer.

Advantageously according to the invention, based on the determination of the stiffness C of balance spring 5a, method 31 comprises a step 37 intended to calculate, using equation (2), the thickness of material to be removed from the entire balance spring in order to obtain the total dimension D _b required for said balance spring 5c to obtain a predetermined stiffness C, that is, the amount/volume of material to be removed, in a uniform or non-uniform manner, from the surface of balance spring 5a.

The method continues with step 39 , which aims to remove excess material from balance spring 5 a in order to achieve the dimensions D _b required of said balance spring 5 c to obtain a predetermined stiffness C. It is therefore understood that it is immaterial whether the thickness and/or height and/or length of balance spring 5 a have been geometrically modified, considering that, according to equation (2), it is the product h·e ³ that determines the stiffness of the coil.

Thus, a uniform thickness may be removed from the entire outer surface, a non-uniform thickness may be removed from the entire outer surface, a uniform thickness may be removed from only a portion of the outer surface, or a non-uniform thickness may be removed from only a portion of the outer surface. For example, step 37 may comprise removing material only from the thickness _E1 or height _H1 of balance spring 5a.

In a first variant involving silicon-based material, step 39 comprises a first stage d1 aimed at oxidizing balance spring 5a in order to convert the thickness of silicon-based material to be removed into silicon dioxide and thereby form oxidized balance spring 5b. This stage d1 can be carried out, for example, by thermal oxidation. This thermal oxidation can be carried out, for example, at temperatures between 800° C. and 1200° C. in an oxidizing atmosphere using water vapor or hydrogen gas, in order to form silicon oxide on balance spring 5a.

As shown in FIG4 , the cross-section of balance spring 5 b has a height H ₂ and a thickness E ₂ . It should be noted that balance spring 5 b is formed from a central silicon-based portion 22, which has the overall dimension Db required for balance spring 5 c of the predetermined rigidity C, and a peripheral silicon dioxide portion 24. Furthermore, it can be seen that the undulating wave-like shape is consistently reproduced over a portion of peripheral portion 24, but is no longer present, or barely present, in central portion 22.

As shown in FIG5 , step 39 ends with a second stage d2, which aims to remove the oxide from balance spring 5 b in order to obtain a balance spring 5 c having only a silicon-based portion 22 having overall dimensions Db required to achieve the predetermined rigidity C, and whose cross-section, in particular, has a height H ₃ and a thickness E ₃ . This stage d2 can be achieved, for example, by chemical etching. The chemical bath can contain, for example, hydrofluoric acid, which is used to remove the silicon oxide from balance spring 5 b.

In a second variant, step 39 comprises only one stage d3, which aims to chemically etch balance spring 5a so as to obtain a silicon-based balance spring 5c having the dimensions Db, H ₃ , E ₃ required for said predetermined stiffness C. Of course, other variants are conceivable, depending on the materials used, such as laser etching or focused ion beam etching, which allow excess material to be removed from balance spring 5a down to the dimensions Db required to obtain said balance spring 5c of predetermined stiffness C.

Method 31 may end with step 39. However, after step 39, method 31 may also perform steps 35, 37, and 39 at least once more in order to further improve the dimensional quality of the balance spring. These repetitions of steps 35, 37, and 39 are particularly advantageous, for example, when a first repetition of steps 35, 37, and 39 is performed on all balance springs or samples thereof still attached to wafer 23, and then a second repetition is performed on all balance springs or samples thereof previously separated from wafer 23 and having undergone the first repetition.

Method 31 may also continue with all or part of process 40 shown in FIG8 , including optional steps 41 , 43 and 45. Advantageously according to the invention, method 31 may thus continue with step 41 for forming, on at least a portion of balance spring 5 c, portion 28 for forming balance spring 5 , 15 insensitive to thermal variations.

In a first variant, step 41 may comprise a phase e1 for depositing a layer of predetermined stiffness C on a portion of the outer surface of said balance spring 5 c.

In the case where portion 22 is made of a silicon-based material, stage e1 may comprise oxidizing balance spring 5 c so as to coat it with silicon dioxide in order to form a temperature-compensated balance spring. This stage e1 may be carried out, for example, by thermal oxidation. For example, this thermal oxidation may be carried out at a temperature between 800° C. and 1200° C. in an oxidizing atmosphere using water vapor or hydrogen gas, in order to form silicon oxide on balance spring 5 c.

Thus, a compensating spring 5, 15 is obtained, as shown in FIG6 , which advantageously comprises, according to the invention, a silicon core 26 and a silicon oxide coating 28. Advantageously, according to the invention, compensating spring 5, 15 thus has very high dimensional accuracy, in particular with regard to height H ₄ and thickness E ₄ , and incidentally achieves very fine temperature compensation of the entire resonator 1.

In the case of a silicon-based balance spring, the overall dimension D _b can be obtained by exploiting the teaching of European patent 1 422 436 and applying it to the resonator 1 to be manufactured, ie compensating all the constituent parts of the resonator 1 as described above.

In a second variant, step 41 may comprise a stage e2 for restructuring, to a predetermined depth, a portion of the outer surface of said balance spring 5 c of predetermined stiffness C. For example, if amorphous silicon is used, said silicon may be crystallized to a predetermined depth.

In a third variant, step 41 may comprise a stage e3 aimed at modifying the composition of a portion of the outer surface of balance spring 5 c to a predetermined depth of predetermined stiffness C. For example, if single-crystal or polycrystalline silicon is used, said silicon may be doped or diffused to fill interstitials or replace atoms to a predetermined depth.

Advantageously according to the invention, it is thus possible to produce, without further complexity, a balance spring 5 c , 5 , 15 as shown in FIG. 2 , which comprises in particular:

One or more coils having a more accurate cross-section than that obtained by means of a single etching;

- Variations in thickness and/or pitch along the coil;

- an integrated inner pile 17;

- an inner coil 19 of the Grossman curve type;

- an integrated balance spring stud attachment 14;

- an integrated external attachment element;

- A portion 13 of the outer coil 12 that is thicker than the rest of the coil.

Finally, method 31 may also comprise a step 45 intended to assemble the compensating spring 5, 15 obtained in step 41 or the balance spring 5c obtained in step 39 on the balance of predetermined inertia obtained in step 43, so as to form a resonator 1 of the balance-with-sprung type, which may or may not be temperature-compensated, that is, whose frequency f may or may not be sensitive to temperature variations.

Of course, the present invention is not limited to the examples described, but is susceptible to various modifications and variations that will be apparent to those skilled in the art. In particular, as described above, even if the balance wheel has a predetermined inertia by design, it may include a movable inertia mass that provides adjustment parameters before or after the sale of the timepiece.

Furthermore, an additional step may be provided between step 39 and step 41 or between step 39 and step 45 for depositing a functional layer or an aesthetic layer, such as a hardening layer or a luminescent layer.

It is also conceivable that, when method 31 carries out one or more repetitions of steps 35 , 37 and 39 after step 39 , step 35 is not systematically performed.

Claims (18)

1. A method (31) for manufacturing a hairspring (5c) with a predetermined stiffness (C), comprising the following steps: a) A hairspring (5a) with dimensions (D _a , H ₁ , E ₁ ) larger than the dimensions (D _b , H ₃ , E ₃ ) required for the hairspring (5c) to obtain a predetermined stiffness (C); b) Determine the stiffness (C) of the hairspring (5a) formed in step a) by measuring the frequency (f) of the hairspring (5a) coupled to the balance wheel with a predetermined inertia; c) Calculate (37) the thickness of the material to be removed based on the determination result of the stiffness (C) of the hairspring (5a) determined in step b) to obtain the dimensions ( _Db , _H3 , _E3 ) required for the hairspring (5c) to obtain the predetermined stiffness (C); d) Remove the material of the thickness (39) from the hairspring (5a) formed in step a) to obtain the hairspring (5c) with the required dimensions ( _Db , _H3 , _E3 ) having the predetermined stiffness (C). 2. The manufacturing method (31) according to claim 1, characterized in that, in step a), the dimensions (D _a , H ₁ , E ₁ ) of the hairspring (5a) formed in step a) are 1% to 20% larger than the dimensions (D _b , H ₃ , E ₃ ) required for the hairspring (5c) to obtain the predetermined stiffness (C). 3. The manufacturing method (31) according to claim 1, wherein step a) is achieved by means of deep reactive ion etching. 4. The manufacturing method (31) according to claim 1, wherein step a) is achieved by means of chemical etching. 5. The manufacturing method (31) according to claim 1, characterized in that, in step a), multiple hairsprings (5a) with dimensions (D _a , H ₁ , E ₁ ) larger than the dimensions (D b, H 3, E ₃ ) required to obtain multiple hairsprings (5c) having a predetermined stiffness (C) or multiple hairsprings (5c) having multiple predetermined stiffnesses ( _C ) are formed in the same wafer ( ₂₃ ). 6. The manufacturing method (31) according to claim 1, characterized in that the hairspring (5a) formed in step a) is made of silicon. 7. The manufacturing method (31) according to claim 1, characterized in that the hairspring (5a) formed in step a) is made of glass. 8. The manufacturing method (31) according to claim 1, characterized in that the hairspring (5a) formed in step a) is made of ceramic. 9. The manufacturing method (31) according to claim 1, characterized in that the hairspring (5a) formed in step a) is made of metal. 10. The manufacturing method (31) according to claim 1, characterized in that the hairspring (5a) formed in step a) is made of a metal alloy. 11. The manufacturing method (31) according to claim 1, characterized in that step b) includes the following stages: b1) Measuring the frequency (f) of the assembly including the hairspring (5a) formed in step a) and coupled to the balance wheel having a predetermined inertia; b2) Derive the stiffness (C) of the hairspring (5a) formed in step a) from the measured frequency (f). 12. The manufacturing method (31) according to claim 6, characterized in that step d) includes the following stages: d1) Oxidize the hairspring (5a) formed in step a) so as to convert the silicon material of the thickness to be removed into silicon dioxide and thereby form an oxidized hairspring (5b); d2) Remove the oxide from the oxidized hairspring (5b) to obtain a hairspring (5c) with the required dimensions ( _Db , _H3 , _E3 ) having the predetermined stiffness (C). 13. The manufacturing method (31) according to claim 1, characterized in that step d) includes the following stages: d3) Chemically etch the hairspring (5a) formed in step a) to obtain a hairspring (5c) with the dimensions ( _Db , _H3 , _E3 ) required for the predetermined stiffness (C). 14. The manufacturing method (31) according to claim 1, characterized in that, after step d), the method is performed again at least once more in steps b), c) and d) to further improve dimensional quality. 15. The manufacturing method (31) according to claim 1, characterized in that, after step d), the method further comprises the following step: e) Forming portions on at least a portion of the hairspring (5c) of a predetermined stiffness (C) for correcting the stiffness of the hairspring (5c) and for forming hairsprings (5, 15) that are insensitive to thermal changes. 16. The manufacturing method (31) according to claim 15, characterized in that step e) comprises the following stages: e1) Deposit a layer on a portion of the outer surface of the hairspring (5c) with a predetermined stiffness (C). 17. The manufacturing method (31) according to claim 15, characterized in that step e) comprises the following stages: e2) Modify the structure of a portion of the outer surface of the hairspring (5c) with a predetermined stiffness (C) to a predetermined depth. 18. The manufacturing method (31) according to claim 15, characterized in that step e) comprises the following stages: e3) Change the composition of a portion of the outer surface of the hairspring (5c) with a predetermined stiffness (C) to a predetermined depth.