Method for manufacturing a return spring with precise stiffness for a timepiece resonator

Abstract

The invention, according to a first embodiment, proposes a method for manufacturing a return spring for a watch resonator, comprising the following steps: a) forming a return spring with dimensions smaller than those required for the return spring to have a predetermined stiffness; b) determining the stiffness of the return spring; c) based on the result of step b) calculating the thickness of oxide to be deposited on the return spring to achieve the predetermined stiffness; d) depositing said thickness of oxide on the return spring at a temperature not exceeding 500°C so that the return spring has the predetermined stiffness. Two other embodiments, also involving oxide deposition at a temperature not exceeding 500°C, are also proposed.

Description

[0001] The present invention relates to the manufacture of a return spring for a watch resonator.

[0002] In a horological resonator such as a balance wheel and hairspring or a resonator with a flexible guide, an inertial element (balance wheel) oscillates under the action of an escapement, which imparts impulses of mechanical energy to it, and under the action of a return spring (hairspring or resonator's flexible guide) which brings it back to an equilibrium position. In the case of a balance wheel and hairspring, the balance wheel is carried by a shaft whose pivots rotate in bearings, and the hairspring is fixed to the shaft at its inner end and to a bridge at its outer end. In the case of a resonator with a flexible guide, the balance wheel is suspended from a base and guided in rotation by an arrangement of elastic parts (the flexible guide), which also serves as a return spring. The resonator constitutes the time base of a horological movement or mechanism. Together with the escapement, it forms an oscillator. However, the term "oscillator" is sometimes used to refer to the resonator alone.

[0003] The present invention relates more particularly to the manufacture of a return spring for a watch resonator by etching a wafer. It is now well known in watchmaking to use etching techniques such as deep reactive ion etching (DRIE), chemical etching, or laser etching to manufacture watch components in large quantities and with precision. The most common etching material is silicon.

[0004] Currently, several hundred components are typically etched onto a wafer of etching material. Etching characteristics are not always stable over time or homogeneous in space, resulting in geometric variation between components on different wafers, or even within the same wafer. In the case of return springs for resonators, this translates into a variation in stiffness. Stiffness is one of the two parameters (the other being the moment of inertia of the pendulum) that determine the resonator's frequency. Therefore, it is crucial to be able to control it as precisely as possible.

[0005] A method for manufacturing a balance spring of precise stiffness is known from patent EP 3181938, according to which: a) a balance spring is formed with dimensions larger than those required to obtain a balance spring of predetermined stiffness; b) the stiffness of the balance spring formed in step a) is determined by measuring the frequency of the balance spring coupled with a balance wheel having a predetermined inertia; c) the thickness of material to be removed to obtain the balance spring of predetermined stiffness is calculated; and d) the calculated thickness of material is removed from the balance spring formed in step a). This method requires material removal, which is uneconomical.

[0006] Another patent, EP 3181939, describes a method for manufacturing a balance spring of precise stiffness, in which a) a balance spring is formed with dimensions smaller than those required to obtain a balance spring of predetermined stiffness, b) the stiffness of the balance spring formed in step a) is determined by measuring the frequency of the balance spring coupled with a balance wheel having a predetermined inertia, c) the thickness of material needed to obtain the balance spring of predetermined stiffness is calculated, and d) material is added to the balance spring formed in step a) to compensate for the missing material thickness. The drawback of this method is that the material added to the balance spring modifies the thermoelastic coefficient (thermal coefficient of Young's modulus, or CTE) and the coefficient of thermal expansion of the balance spring.If we wish to thermocompensate the resonator, that is to say make its frequency almost insensitive to thermal variations by subjecting the spiral to thermal oxidation, as proposed by patents EP 3181938 and EP 3181939 as an additional step in the process, it is necessary to take into account the disturbance caused by step d) on the thermoelastic coefficient and the coefficient of thermal expansion, which complicates the calculations and the theoretical study of the spiral.

[0007] The same disadvantages as those mentioned above are found in the processes described in other patents or patent applications, in particular EP 3416001 (relating to a process for manufacturing a flexible-guided resonator of precise frequency), EP 3982205 (relating to a process for manufacturing a watch spring, in particular flexible guide or part of flexible guide of resonator, of precise stiffness) and EP 4030242 (relating to a process for manufacturing balance springs of precise stiffness, of iterative type).

[0008] The present invention aims to remedy the aforementioned drawbacks, or at least mitigate them, and to this end proposes, according to a first embodiment, a method for manufacturing a return spring for a watch resonator comprising the following steps: a) form a return spring in dimensions smaller than the dimensions required for the return spring to have a predetermined stiffness; b) determine the stiffness of the return spring; c) from the result of step b), calculate a thickness of oxide to be deposited on the return spring so that the latter has the predetermined stiffness; d) deposit said thickness of oxide on the return spring at a temperature not exceeding 500°C so that the return spring has the predetermined stiffness.

[0009] According to a second embodiment, the present invention proposes a method for manufacturing a return spring for a watch resonator comprising the following steps: a) form a return spring in dimensions smaller than the dimensions required for the return spring to have a stiffness within a predetermined range; b) deposit a predetermined thickness of oxide on the return spring at a temperature not exceeding 500°C; c) determine the stiffness of the return spring obtained in step b); d) repeat steps b) and c) if and as long as the stiffness determined in step c) is not within the predetermined range.

[0010] According to a third embodiment, the present invention proposes a method for manufacturing return springs for watch resonators comprising the following steps: a) form return springs in a plate, b) identify the return springs having a stiffness within a predetermined range, c) optionally, detach the return springs identified in step b from the plate, d) deposit, only on the other return springs, a layer of oxide at a temperature not exceeding 500°C so that the stiffness of at least some of them is within the predetermined range, e) detach these other return springs from the plate and, if they have not been detached in step c), the return springs identified in step b).

[0011] The present invention also relates to a clockwork resonator comprising a return spring manufactured by one of these two processes and a timepiece, for example a watch, comprising such a clockwork resonator.

[0012] The present invention is based on the observation that an oxide deposited at low temperature does not, or hardly, alter the thermal characteristics (thermoelastic coefficient and coefficient of thermal expansion) of the material on which it is deposited. It is therefore possible, with the present invention, to adjust the stiffness of the return spring independently of thermal compensation, thus more simply and reliably than in the prior art.

[0013] Depositing a low-temperature oxide onto a return spring for a watch resonator has already been proposed in patent EP 3002638, but for a purpose contrary to that of the present invention, namely, adjusting the thermoelastic coefficient of the spring. To allow the oxide to affect the thermoelastic coefficient, the process described in patent EP 3002638 involves following the oxide deposition with an annealing heat treatment at a temperature of at least 550°C, preferably between 800°C and 1050°C, presumably to densify the oxide and give it a structure similar to that of the thermal oxide conventionally used for the thermal compensation of silicon. Such an annealing treatment is intentionally omitted in the present invention.

[0014] Other features and advantages of the present invention will become apparent from the following detailed description, made with reference to the accompanying drawings, in which: therefigure 1 shows a spiral (with its ferrule) for a balance-spring resonator; the figure 2 shows a flexibly guided clockwork resonator; the figure 3 shows a flexible guide section of a watch resonator; the figure 4 is a block diagram of a first embodiment of the process according to the invention; the figure 5 is a block diagram of a second embodiment of the process according to the invention.

[0015] The return spring for a watch resonator manufactured by the process according to the invention is, for example, (i) a spiral 1 as illustrated in the figure 1 , intended to be mounted on the axis of a balance wheel to form with the latter a balance-spring resonator, or (ii) the flexible guide of a flexible-guided resonator 2 as illustrated in the figure 2 and described in patent EP 3839651, flexible guidance formed for example by a part 3 comprising two crossed elastic blades 4, 5 and intended to be assembled to the resonator's balance wheel 6, or (iii) a flexible guidance part, for example a part 7 (cf. figure 3 ) comprising a blade 8 and intended to be assembled head-to-tail with an identical part to form the flexible guide of a resonator as described in patent application WO 2022 / 009102. Generally, the return spring can be a separate part or part of a monolithic part.

[0016] The oscillation frequency of a clock resonator is given by the following formula: f = 1 2 π K I where K is the stiffness of the spiral or flexible guide and I is the moment of inertia of the balance wheel.

[0017] The stiffness K is given by formulas that are known, for example: in the case of a spiral with a constant blade cross-section: K = E . h . e 3 12 L in the case of a flexible guide with separate crossed blades (crossed blades extending in parallel planes) with a constant cross-section: K = 1 6 . E . he 3 L in the case of a flexible guide with non-separated crossed blades (crossed blades extending in the same plane) with a constant cross-section: K = 2 3 . E . he 3 L in the case of a flexible guide with an offset center of rotation (RCC) and a constant blade cross-section: K = 2 3 . E . he 3 L 1 + 3 p L + 3 p 2 L 2 where E is the modulus of elasticity of the material used, h is the height of the spiral blade or of each flexible guide blade (dimension in the direction of the axis of rotation), e is the thickness of the spiral blade or of each flexible guide blade, L is the length of the spiral blade or of each flexible guide blade and, in the case of RCC, p is the distance between the fictitious crossing point of the blades and the end of each blade closest to this crossing point.

[0018] Formulas (2) to (5) above can be adapted to a spiral or flexible guide whose blade(s) have a variable cross-section. Thus, for example, in the case of a spiral, the stiffness can be expressed as follows: K = E 12 ⋅ 1 ∫ 0 L 1 h l . e 3 l . dl

[0019] The relative frequency variation of a clock resonator as a function of temperature is given by the following formula: df f = 1 2 1 E dE dT + 3 α s − 2 α b . dT where df is the frequency variation, dT is the temperature variation, 1 E dE dT is the relative variation of Young's modulus with temperature, i.e., the thermoelastic coefficient (TEC) of the balance spring or flexible guide; αs is the coefficient of thermal expansion of the balance spring or flexible guide, expressed in ppm.°C⁻¹, and αb is the coefficient of thermal expansion of the balance wheel, expressed in ppm.°C⁻¹. The aim is generally to cancel this frequency variation with temperature by manipulating the thermoelastic coefficient and the coefficient of thermal expansion of the balance spring or flexible guide, typically by thermally oxidizing the balance spring or flexible guide when it is made of silicon, so that the sign of the thermoelastic coefficient, initially negative, becomes positive so that the term TEC + 3αs can compensate for the term 2αb.

[0020] According to a first embodiment, the process according to the invention comprises steps E1 to E6 illustrated in the figure 4 .

[0021] In a first step E1, a return spring is formed having the desired return spring shape but with dimensions smaller than those required for its stiffness to equal a predetermined stiffness (desired stiffness). In step E1, either all the dimensions (height h, thickness e, and length L of the return spring's leaf(s)) are smaller than the dimensions required to obtain the predetermined stiffness, or only some of these dimensions are smaller than the dimensions required to obtain the predetermined stiffness.

[0022] Step E1 is preferably performed by etching a wafer of material. Typically, several return springs, and even a large number of return springs, are made simultaneously on the same wafer. The etching method can be deep reactive ion etching (DRIE), chemical etching, plasma etching, focused ion beam (FIB) etching, or laser etching, for example. The material can be homogeneous or composite. Examples include silicon, quartz, glass (mineral), ceramic (e.g., silicon carbide or silicon nitride), carbon (crystalline or amorphous), metal, or alloy. Silicon-based materials can be monocrystalline silicon (regardless of its crystal orientation, including {001} or {111}), polycrystalline silicon, or amorphous silicon. It can be doped or undoped. However, the chosen material must have a melting point compatible with step E6 described below.Other manufacturing techniques besides engraving can be used to form the return spring, such as galvanic growth, gas-phase chemical deposition growth, or additive manufacturing.

[0023] Of the techniques mentioned above, the most precise is deep reactive ion etching. However, phenomena occurring during etching or between successive etchings can induce geometric variations.

[0024] In a second, optional step E2, which can be implemented in particular when the wafer material is silicon-based or silicon carbide-based, the return spring, and more generally the wafer to which it remains attached with the other return springs, is subjected to a thermal oxidation operation followed by a deoxidation operation to reduce surface defects of the return spring and increase its mechanical strength, as proposed for example in patent EP 3769162. The thermal oxidation is typically carried out between 600°C and 1300°C, preferably between 800°C and 1200°C, under an oxidizing atmosphere (in an oxidation furnace) comprising water vapor or dioxygen gas.The oxide layer, typically silicon dioxide (SiO2), which coats the wafer and in particular the return spring, forms by consuming wafer material, thus pushing back the interface between the wafer material and the oxide. The subsequent deoxidation operation consists of removing the oxide layer, for example by wet etching, vapor deposition, or dry etching.

[0025] A third step, E3, is implemented when the material of the return spring produced in step E1 cannot, by its thermoelastic coefficient (TC) and coefficient of thermal expansion αs alone, compensate for the term 2αb in formula (7) above because, for example, its thermoelastic coefficient is negative. Silicon, some glasses, silicon carbide, and most metals, for example, have a negative thermoelastic coefficient. Conversely, some alloys specifically designed for thermal compensation have a positive thermoelastic coefficient. Therefore, in the case of a material with a negative thermoelastic coefficient, it is subjected in step E3 to a thermal compensation operation aimed at changing the sign of its thermoelastic coefficient.Alternatively, thermal compensation can be implemented to substantially cancel the term CTE+3α s, regardless of the sign of the initial thermoelastic coefficient of the return spring, when the balance wheel is made of a material with a coefficient of expansion α b substantially zero.

[0026] The thermal compensation operation may consist of forming a layer on the material with a thermoelastic coefficient opposite in sign to that of the material itself, for example, by growing a layer of silicon dioxide by thermal oxidation (as described above) on silicon or silicon carbide constituting the etching material of step E1, or by depositing a layer of silicon dioxide on the etching material by chemical or physical vapor deposition (CVD, PVD). Alternatively, the thermal compensation operation may consist of modifying the structure or composition to a predetermined depth of all or part of the surface of the return spring, for example, by crystallization (if an amorphous material is used in step E1), doping, or diffusion of interstitial or substitution atoms.

[0027] At the end of step E3, the dimensions of the return spring are still smaller than the dimensions required to obtain the predetermined stiffness.

[0028] In a fourth step E4, the stiffness of the return spring obtained in step E1, and where applicable in step E2 or E3, is determined. This stiffness determination can be direct, i.e., carried out on the return spring itself, or indirect, i.e., carried out on one or more other return springs manufactured with said return spring in the plate, for example on one or more other return springs manufactured in the same area of ​​the plate as said return spring or on several other return springs distributed over the plate and whose average stiffness can be considered representative of all the return springs of the plate.

[0029] To directly determine the stiffness of a given return spring, it can be coupled to a balance wheel of predetermined inertia, either while the return spring is still attached to the plate or after detaching the return spring from the plate. The frequency of the resulting resonator can then be measured, and the stiffness of the return spring deduced using formula (1) above. If the return spring is a balance spring, the balance wheel of predetermined inertia can be a balance wheel integrated into a counting machine, on whose working axis the balance spring is mounted. Other methods besides coupling the return spring to a balance wheel of predetermined inertia can be used, for example, coupling the return spring to a reference balance spring (see patent EP 2423764) or the methods described in patent application EP 4030243.

[0030] An indirect method for determining the stiffness of the return spring manufactured in step E1 may consist of detaching from the plate a sample of return springs manufactured with said return spring and determining the average of the stiffnesses of these return springs, said return spring and other return springs manufactured with it in step E1 being left on the plate for steps E5 and E6 described below.

[0031] In a fifth step E5, using the result of step E4 and, for example, one of the formulas (2) to (6) above, a thickness of oxide to be deposited on the return spring obtained in step E1, or, if applicable, in step E2 or E3, is calculated so that the stiffness of said return spring is equal to the predetermined stiffness. The thickness or volume of oxide to be deposited may be homogeneous or not on the surface of the return spring. The oxide is preferably silicon dioxide (SiO2), but other oxides may be considered, such as germanium, tantalum, zirconium, or hafnium oxide.

[0032] In a sixth step, E6, a layer of oxide, preferably silicon dioxide, with the thickness calculated in step E5, is deposited on the return spring to give it the predetermined stiffness. To avoid altering the thermal characteristics (thermoelastic coefficient and coefficient of thermal expansion) of the material constituting the return spring, the oxide deposition is carried out at a low temperature, i.e., at a temperature that remains below or equal to 500°C, preferably below or equal to 400°C, preferably below or equal to 300°C, preferably below or equal to 200°C, and preferably below or equal to 150°C. Deposited at such temperatures, the oxides are not sufficiently dense to influence the thermal characteristics. The temperature may vary during deposition as long as it does not exceed the aforementioned values.This oxide layer is typically deposited by the chemical vapor deposition (CVD) technique. However, it can be deposited by other techniques such as physical vapor deposition (PVD), plasma-enhanced chemical vapor deposition (PECVD), high-density plasma chemical vapor deposition (HDPCVD), molecular vapor deposition (MVD), atomic layer deposition (ALD) or the sol-gel process.

[0033] This oxide layer deposited at low temperature not only does not disrupt the thermal characteristics of the return spring but also does not modify the properties of the base material, whether it is single-crystal, polycrystalline or amorphous.

[0034] It is possible to repeat steps E4 to E6 to refine the stiffness value of the return spring.

[0035] At the end of the process, the return spring and the other return springs still located in the plate and which have undergone the same treatment as said return spring are detached from the plate by breaking or eliminating the fasteners (formed during the engraving of step E1) which held them.

[0036] There figure 5This demonstrates a second embodiment of the process according to the invention. This second embodiment differs from the first embodiment in that it is iterative (incremental). After steps F1, F2, and F3, which are identical to steps E1, E2, and E3 of the first embodiment, the second embodiment involves a step F4 of low-temperature deposition of a predetermined thickness of oxide, preferably silicon dioxide, onto the return spring. Step F4 is identical to step E6 of the first embodiment, except that the deposited thickness is predetermined rather than calculated from the result of a stiffness measurement. In a subsequent step F5, the stiffness of the return spring is determined in the same way as in step E4 of the first embodiment. If the stiffness falls within a predetermined range, considered acceptable (step F6), the process ends, for example, by detaching the return spring from the plate.If the stiffness is not within the predetermined range, the process returns to step F4 for an additional low-temperature oxide deposition (according to the predetermined thickness) followed by a new stiffness determination (step F5), the loop repeating until the spring stiffness falls within the predetermined range. The predetermined oxide thickness deposited at each step F4 is chosen to be small enough so that the resulting increase in spring stiffness is less than the width of the predetermined range, preferably less than half the width of the predetermined range, and preferably less than a quarter of the width of the predetermined range.

[0037] A third embodiment of the process according to the invention, not shown, consists of forming several return springs in a wafer, according to any one of the methods and any one of the materials described above in relation to step E1, optionally carrying out steps E2 and E3 described above on the wafer, determining the stiffness of each of the return springs of the wafer, detaching from the wafer the return springs whose stiffness is within a predetermined range, considered acceptable, leaving the other return springs on the wafer and covering them (in the same way as in step E6) with a layer of oxide, preferably silicon dioxide, deposited at low temperature, i.e. at a temperature that remains less than or equal to 500°C, preferably less than or equal to 400°C, preferably less than or equal to 300°C, preferably less than or equal to 200°C,preferably less than or equal to 150°C, so that the stiffness of these other return springs, or part of them, is within the predetermined range, and to detach these other return springs from the plate.

[0038] This third embodiment follows the principle described in patent EP 3769161, except that the oxide is deposited at a low temperature to avoid altering the thermal characteristics of the return springs. The thickness of the oxide layer can be determined by calculation based on the stiffness of the other return springs remaining on the plate. The determination of the stiffness of each return spring can be direct or indirect, as explained in relation to step E4. For example, the stiffness of each return spring can be determined directly by coupling each of them to a pendulum of determined inertia and measuring the frequency of the resonator thus formed, or the stiffness of certain return springs representative of respective areas of the plate can be determined directly, and each of the other return springs can be assigned the stiffness value of the return spring representative of its area.

[0039] The predetermined range is, for example, the second half of the stiffness dispersion range of the return springs manufactured in the wafer during the first step. For return springs whose stiffness is initially within the first half of this dispersion range, the oxide layer they receive increases their stiffness to a value within the second half of this dispersion range. This halves the width of the stiffness dispersion range of the return springs manufactured in the wafer, that is, the number of classes of these return springs, where each class comprises return springs with essentially the same stiffness. A shorter predetermined range can also be chosen, for example, the last third, the last quarter, the last fifth, etc.from the range of stiffness dispersion of the return springs made in the plate during the first step, and perform several iterations to divide by three, by four, by five, etc. the number of classes.

[0040] In a variant of this third embodiment, instead of detaching the return springs from the plate as soon as their stiffness is within the predetermined range, they can be left on the plate and protected, for example by means of masks, from the oxide deposit made on the other return springs, all the return springs then being detached from the plate at the end of the process.

Claims

1. Method for manufacturing a return spring for a watch resonator, comprising the following steps: a) forming a return spring in dimensions smaller than the dimensions necessary for the return spring to have a predetermined stiffness; b) determining the stiffness of the return spring; c) from the result of step b), calculating a thickness of oxide to be deposited on the return spring so that the latter has the predetermined stiffness; d) depositing said thickness of oxide on the return spring at a temperature not exceeding 500°C so that the return spring has the predetermined stiffness.

2. Method for manufacturing a return spring for a watch resonator, comprising the following steps: a) forming a return spring in dimensions smaller than the dimensions necessary for the return spring to have a stiffness within a predetermined range; b) depositing a predetermined thickness of oxide on the return spring at a temperature not exceeding 500°C; c) determining the stiffness of the return spring obtained in step b); d) repeating steps b) and c) if and as long as the stiffness determined in step c) is not within the predetermined range.

3. Method according to claim 1 or 2, characterized in that step a) comprises an etching step, preferably a deep reactive ion etching step.

4. Method according to any one of claims 1 to 3, characterized in that the return spring formed in step a) is based on silicon, quartz, glass, ceramic, carbon, metal or alloy.

5. Method according to any one of claims 1 to 3, characterized in that the return spring formed in step a) is silicon-based.

6. Method according to any one of claims 1 to 5, characterized in that the step of depositing oxide on the return spring at a temperature not exceeding 500°C is carried out by chemical vapor deposition, physical vapor deposition, plasma-enhanced chemical vapor deposition, high-density plasma chemical vapor deposition, molecular vapor deposition, atomic layer deposition or sol-gel deposition.

7. Method according to any one of claims 1 to 5, characterized in that the step of depositing oxide on the return spring at a temperature not exceeding 500°C is carried out by chemical vapor deposition.

8. Method according to any one of claims 1 to 7, characterized in that the oxide is silicon dioxide.

9. Method according to any one of claims 1 to 8, characterized in that it comprises between steps a) and b) a step of thermal oxidation of the return spring followed by a step of deoxidation.

10. Method according to any one of claims 1 to 9, characterized in that it includes between steps a) and b) a thermal compensation step aimed at modifying the thermoelastic coefficient of the return spring, for example to change its sign from negative to positive.

11. Method according to claims 9 and 10, characterized in that the thermal compensation step is carried out after the thermal oxidation and deoxidation steps.

12. Method according to claim 10 or 11, characterized in that the thermal compensation step comprises forming a layer on the return spring, said layer having a thermoelastic coefficient of sign opposite to that of the return spring.

13. Method according to claim 12, characterized in thatsaid layer is made of silicon dioxide.

14. Method according to any one of claims 1 to 13, characterized in that the return spring is a hairspring.

15. Method according to any one of claims 1 to 13, characterized in that The return spring is a flexible guide or a flexible guide part.

16. Method for manufacturing return springs for watch resonators, comprising the following steps: a) forming return springs in a wafer, b) identifying the return springs having a stiffness within a predetermined range, c) optionally, detaching from the wafer the return springs identified in step b), d) depositing, only on the other return springs, an oxide layer at a temperature not exceeding 500°C so that the stiffness of at least some of them is within the predetermined range, e) detaching from the wafer these other return springs and, if they have not been detached in step c), the return springs identified in step b).

17. Method according to claim 16, characterized in that before step e) steps b), c) and d) are repeated one or more times.

18. Clock resonator comprising a return spring manufactured by the method according to any one of claims 1 to 17.

19. Timepiece, for example watch, comprising a watch resonator according to claim 18.

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