Device for determining the stiffness of a spiral

Abstract

The invention relates to a device for determining the stiffness of a silicon-based spiral or spiral blank attached to a silicon-based plate (10), by applying a vibratory excitation and by measuring the vibratory response of the spiral or blank, said device comprising: - a mounting (200) intended to provide support at least on a portion of the plate surrounding the spiral or spiral blank, and comprising an opening to allow free vibration of the spiral or blank, - a source of vibratory excitation (210) of the spiral or spiral blank, arranged to excite the spiral or spiral blank, - means for measuring the vibratory response (220) of said spiral or spiral blank, for measuring the vibratory response of the spiral / blank.

Description

technical field

[0001] The present invention relates to the field of control and manufacture of watch parts. It relates more particularly to a device for determining the stiffness of a silicon-based balance spring or balance blank attached to a silicon-based plate, by applying a vibratory excitation and by measuring the vibratory response of the balance spring or balance blank. State of the art

[0002] Mechanical watch movements are regulated by a mechanical regulator that includes a resonator, a component that can be elastically deformed and whose oscillations determine the watch's accuracy. Many watches, for example, feature a regulator with a balance spring acting as a resonator, mounted on the balance staff and set in motion by an escapement. The natural frequency of the balance wheel and balance spring assembly regulates the watch and depends, in particular, on the stiffness of the balance spring.

[0003] In contemporary watchmaking, balance springs have begun to be manufactured, particularly in silicon, to benefit from the insensitivity of this material to magnetic fields and to take advantage of its machinability.

[0004] Thus, several hundred silicon spirals can be advantageously fabricated on a single wafer using microfabrication technologies. It is well known that multiple silicon resonators can be produced with very high precision using photolithography and machining / etching processes within a silicon wafer. The fabrication processes for these mechanical resonators generally use single-crystal silicon wafers, but wafers made of other materials are also suitable, for example, polycrystalline or amorphous silicon, other semiconductor materials, glass, ceramics, carbon, carbon nanotubes, or a composite of these materials. Single-crystal silicon belongs to the cubic m3m crystal class, which has an isotropic coefficient of thermal expansion (alpha).

[0005] Silicon exhibits a very negative first thermoelastic coefficient, and consequently, the stiffness of a silicon resonator, and therefore its natural frequency, varies significantly with temperature. To at least partially compensate for this drawback, documents EP1422436, EP2215531, and WO2016128694 describe a spiral-type mechanical resonator made from a single-crystal silicon core (or two cores in the case of WO2016128694). Temperature variations in the Young's modulus are compensated by a layer of amorphous silicon dioxide (SiO₂) surrounding the core(s), silicon being one of the few materials with a positive thermoelastic coefficient.

[0006] When spirals are made of silicon or another material by collective manufacturing on a wafer, the final functional efficiency will be given by the number of spirals whose stiffness corresponds to the pairing interval, divided by the total number of spirals on the wafer.

[0007] The patent application bearing the filing number PCT / EP2022 / 050760 explains that there is a dispersion of stiffness between spirals etched on a wafer, and even more so between spirals of wafers etched at different times according to the same process specifications.

[0008] Several solutions have been proposed to reduce this dispersion by applying corrections to the balance springs. Document EP3181938 proposes such a correction, which is based on measuring the stiffness of a balance spring by measuring its frequency when coupled to a balance wheel of known inertia. The aforementioned PCT / EP2022 / 050760 application proposes a method for establishing the stiffness characteristics of the balance springs or balance spring blanks of a dial plate, based on the vibrational response to excitation of at least one balance spring or balance spring blank.

[0009] In microfabrication processes, either SOI (Silicon On Insulator) wafers or simple wafers can be used. Even with SOI, depending on the process used, the silicon layer called the handle layer, which serves as mechanical support, is dissolved to expose the device layer, in which the balance springs are etched. Excitation and vibration measurement can therefore be performed on balance springs embedded in a silicon layer approximately 120 µm thick. At this thickness, the rigidity of the wafer or silicon layer is low, so neighboring balance springs of the one being measured can also be excited, and their own vibration can disrupt the vibrational response of the balance spring in question. Document EP4030243A1 relates to a process for testing and manufacturing watch balance springs.Document EP3915788A1 concerns a process for manufacturing a batch of watch balance springs with thermocompensation. Document CH717357A2 concerns a watch balance spring made of glass or ceramic with a complex geometry.

[0010] The present invention aims to provide a device / apparatus that can at least partially address the aforementioned problem, and improve the responses obtained.

[0011] The invention also relates to a control method implementing a step of using a device according to the invention or similar. Disclosure of the invention

[0012] More specifically, the invention relates to a device for determining the stiffness of a silicon-based spiral or spiral blank attached to a silicon-based wafer, by applying a vibratory excitation and by measuring the vibratory response of the spiral or blank, said device comprising: a mounting intended to provide support at least on a portion of the plate surrounding the spiral or spiral blank, and comprising an opening to allow free vibration of the spiral or blank, a source of vibratory excitation of the spiral or spiral blank, arranged to excite the spiral or spiral blank, means for measuring the vibratory response of said spiral or spiral blank, to measure the vibratory response of the spiral / blank.

[0013] Thus, the mounting allows for at least partial vibrational isolation of the spiral / spiral blank from the rest of the plate, damping parasitic vibrations from neighboring spirals and limiting overall plate vibrations by reducing bending deformability, particularly of the plate and mounting assembly. The vibrational response of the spiral is cleaner; that is, the spectrum characterizing the vibrational response is less noisy due to parasitic vibrations. Consequently, the observed peaks can be directly and solely attributed to the resonances of the measured spiral. The peaks of the observed spectrum are better defined, allowing for higher-quality analysis of the results. Therefore, "encircling" should be understood as enclosing the spiral or spiral blank individually.

[0014] In a preferred embodiment, the vibration excitation source is arranged on one side of the plate, and the means for measuring the vibration response are arranged on a second side of the plate, opposite the first side. In this way, access to the spiral to be measured is much easier for each device, the excitation can be better centered and closer to the spiral. Similarly, the measuring means can be optimally positioned.

[0015] Preferably, the apparatus includes a network of openings, positioned opposite the spirals or spiral blanks to be measured, the openings being sized to allow the vibrations of the spirals or spiral blanks to occur freely. This network of openings makes it possible to isolate a plurality of spirals or spiral blanks, which can then be easily measured one after the other.

[0016] Advantageously, means of support can be provided to press the mounting and the plate against each other, in order to improve the vibration isolation of the spiral to be measured.

[0017] Preferably, the mounting is associated with a clamping plate, the plate being intended to be sandwiched between the mounting and the clamping plate.

[0018] In this case, the fixture and the clamping plate may each include a network of openings, the openings of the fixture and the openings of the clamping plate being arranged opposite each other and intended to be opposite the spirals or spiral blanks to be measured, the openings being dimensioned so as to allow free movement of the spirals or spiral blanks to be measured.

[0019] Other features are also given in the claims and may be combined with each other and / or with the aforementioned features, insofar as they are technically compatible with each other. Brief description of the drawings

[0020] Further details of the invention will become clearer upon reading the following description, made with reference to the attached drawing in which: THE Figures 1A to 1F These are diagrams of certain steps in a process for manufacturing a silicon spiral, the figure 2 is a diagram of a preferred embodiment of a device according to the invention, the figure 3 and the figure 4 illustrate variants or alternatives of the invention. Method of embodying the invention

[0021] We have represented in the following Figures 1A to 1F, certain steps of a process for fabricating a spiral in a wafer 10, also called a SOI (silicon on insulator) wafer. The latter comprises a substrate or handler 20 bearing a sacrificial silicon dioxide (SiO2) layer 30 and a single-crystal silicon layer 40. For example, the substrate 20 may have a thickness of 500 µm, the sacrificial layer 30 may have a thickness of 2 µm, and the silicon layer 40 may have a thickness of 120 µm. The single-crystal silicon layer 40 may have any crystal orientation.

[0022] A lithography stage is shown to Figures 1B and 1C By "lithography," we mean all the operations involved in transferring an image or design onto or above the plate 10. Referring to the figure 1BIn this exemplary embodiment, layer 40 is covered with a protective layer 50, for example, a polymerizable resin. This layer 50 is structured, typically by a photolithography step using an ultraviolet light source and, for example, a photomask (or other type of exposure mask) or a stepper and reticle system. This lithographic structuring forms the patterns for the plurality of resonators in layer 50, as illustrated in the figure 1C .

[0023] Subsequently, in the next step of the figure 1D The patterns are machined, specifically engraved, to form the plurality of 100 resonators in layer 40. The etching can be performed using a deep reactive ion etching technique (also known as DRIE for "Deep Reactive Ion Etching"). After etching, the remaining portion of the protective layer 50 is subsequently removed.

[0024] To the figure 1E , the resonators are released from the substrate 20 by locally removing the sacrificial layer 30 or even by etching all or part of the silicon of the substrate or handler 20. Smoothing (not shown) of the etched surfaces can also take place before the release step, for example by a thermal oxidation step followed by a deoxidation step, consisting for example of wet etching with hydrofluoric acid (HF).

[0025] At the final stage of the manufacturing process at the figure 1FThe 110 turns of the silicon resonator 100 are coated with a 120 layer of silicon dioxide (SiO2), typically through a thermal oxidation step to produce a thermo-compensated resonator. The formation of this 120 layer, which is generally 2–5 µm thick, also affects the final stiffness of the resonator and therefore must be taken into account in the preceding steps to obtain the vibrational characteristics of the balance spring, leading to a specific natural frequency of the balance spring-spring pair in a given watch mechanism.

[0026] As indicated above, at the stage prior to the creation of the thermo-compensation layer, the different resonators formed in the wafer generally exhibit a significant geometric dispersion between them and therefore a significant dispersion between their stiffnesses, notwithstanding that the pattern formation steps and the machining / engraving through these patterns are the same for all resonators.

[0027] Moreover, this dispersion of stiffness is even greater between the spirals of two plates etched at different times even if the same process specifications are used.

[0028] To center the average stiffness of the resonators on different plates relative to a nominal stiffness value as illustrated in the figure 2The resonators obtained in step 1E on the plate 10 in question can be deliberately formed with dimensions d that differ from the dimensions required (for example, larger) to obtain a nominal or target stiffness. Thus, it is possible to implement a control method to estimate the vibrational characteristics of the resonators (natural frequency and / or resonance frequencies) in order to deduce the stiffness and / or the actual dimensions of the resonators 100 and correct their dimensions, thereby obtaining the desired natural frequency of the resonator-balancer pair. The aforementioned PCT / EP2022 / 050760 application discloses such a method. Reference may be made to it for details of the steps involved.

[0029] In summary, for the purposes of understanding the present invention, it should be noted that this method comprises the following steps: a. apply to the spiral or spiral blank a time-varying vibratory excitation to cover a predetermined frequency range, b. identify at least one resonance frequency characteristic, such as a resonance peak, of the spiral or spiral blank during the vibratory excitation over the predetermined frequency range, c. submit to a prediction machine the resonance frequency characteristic identified in step b. to determine a stiffness of the spiral or spiral blank and / or determine if a dimensional correction of the spiral or spiral blank is necessary to obtain the predetermined resonance frequency.

[0030] In practice, it is observed that when excitation is applied to the spiral or spiral blank, neighboring spirals / spiral blanks can also be excited, to a lesser degree, either by indirectly receiving the sound excitation or by vibration diffusion through the wafer. This is particularly noticeable when the wafer consists essentially only of the device layer at this stage, or when the spirals are fabricated in a single wafer, typically around 120 µm thick.

[0031] To limit disruptive influences on the measurement, the invention proposes implementing a fixture 200 designed to provide support on at least a portion of the wafer or silicon layer surrounding the balance spring or balance spring blank to be measured. The fixture 200 includes an opening 202 that allows the vibrations of the balance spring or balance spring blank to pass freely. In other words, the opening is at least slightly larger than the groove formed in the wafer to create the balance spring. The support provided by the fixture is intended to isolate, at least partially, the balance spring / balance spring blank from the rest of the wafer, in terms of vibration.

[0032] In a simple approach, one could have a setup with only one opening to measure a single spiral or spiral blank. However, advantageously, the setup has dimensions close to those of the wafer and offers a network of openings designed to be positioned opposite the spirals. Depending on the desired sampling, one could have an opening for each spiral fabricated on the wafer, or one could have openings opposite only some of the spirals, for example, every other spiral or every third spiral.

[0033] Because the pad is lightweight, its own weight may not be sufficient to generate enough support to adequately isolate the balance spring from disturbing vibrations. Therefore, methods of securing the pad and the mounting plate together can be used. These methods can take various forms, such as screws, weights, or other variations that will be described below. Furthermore, due to the pad's flexibility, there is some modal overlap between the resonances of the balance springs and the pads alone. Thus, by pressing the pad against the mounting plate, its apparent rigidity is increased, which modifies its vibrational response and prevents interference with the responses of the balance springs, which are not in contact with the mounting plate.

[0034] In a preferred embodiment shown on the figure 2The fixture 200 is associated with a clamping plate 300, with the plate 10 being sandwiched between the fixture 200 and the clamping plate 300. Both the fixture and the clamping plate comprise a network of openings; the openings 202 of the fixture 200 and the openings 302 of the clamping plate 300 are arranged opposite each other and are designed to be aligned with the spirals or spiral blanks to be measured. As mentioned previously, the openings 202 and 302 are sized to allow the spirals to vibrate freely.

[0035] Preferably, a system for assembling and / or clamping the clamping plate 300 against the mounting 200 is provided, and, if necessary, indexing means 350 for aligning the openings. For example, pins passing through the mounting 200, the plate 10, and the clamping plate 300 at defined positions can be used. Clamping systems 400, screws, or other means can be provided to stiffen the assembly. The clamping means can be distributed around the mounting 200 and the clamping plate 300, or supplemented by clamping means distributed across the surface of the mounting and the clamping plate. The same indexing means can be provided for indexing the fixture 200, the plate 10 and the clamping plate 300, for example with pins or similar fixed to the fixture or the clamping plate and which pass through the plate to position themselves, respectively, in the clamping plate or in the fixture.We can also provide for separate second indexing means, to position on the one hand, the fixture 200 and the plate 10, and on the other hand, the clamping plate 300 and the plate 10.

[0036] Indexing means can be provided even if there is no clamping plate.

[0037] In all cases, the indexing allows for precise positioning of the fixture 200 relative to the plate 10 and the spirals, which enables the excitation source 210 and the measuring means 220 to be positioned repeatably from one plate to another. The reliability of the measurement is thus improved.

[0038] Preferably, the surfaces of the mounting and, where applicable, the clamping plate are flat, or even ground, to provide even support around the spiral and avoid stress on the plate. The clamping plate and / or the mounting can be metallic or even made of silicon.

[0039] Raised features (not shown) could also be incorporated, forming a ring or portion of a ring around the openings, against which the wafer can rest. These raised features can be elastically deformable to avoid mechanical stress on the wafer and contribute to vibration damping.

[0040] Other methods can also be used to press the mounting plate and the insert against each other. For example, methods can be used to attract the insert onto the mounting plate. In a variant illustrated on the figure 3The attraction means include a suction circuit 420 opening in contact with the wafer 10, to aspirate the wafer. This suction circuit can be connected to a vacuum generator, via a connector 422, to create a vacuum and aspirate the wafer 10 against the pose 200. The suction circuit can be distributed on the pose 200, for example, by providing channels arranged radially around the center of the pose. The pose is intended to be aligned with the center of the wafer, which, in this example, does not have spirals to allow the wafer to be aspirated.

[0041] As an additional, unshown alternative, the attraction means include an electrostatic device, such as an electrostatic attraction plate (also called an electrostatic chuck). The plate or elements located on a clamping plate, of the type described above, can be magnetized to cooperate with the chuck.

[0042] Advantageously, the mounting as proposed in the present invention allows a vibrational excitation source 210 of the spiral or spiral blank to be placed on one side of the plate 10, and the means 220 for measuring the vibrational response of said spiral or spiral blank, typically a laser head, to be placed on the other side of the plate. Thus, it is possible to control the angle of incidence of the laser independently of the position of the excitation source. The latter can be positioned very close to the plate, without having to leave space for the laser head, by passing or emitting the excitation wave inside the opening.

[0043] The 200 fixture can be positioned on either side of the wafer. Depending on the technique chosen for releasing the spirals, if a thickness of the handle layer remains around them, the fixture may preferably be positioned on the device side. In this case, the vibration excitation source can advantageously be positioned on the device layer side, and the measurement equipment on the handle layer side. The reverse arrangement is obviously a possible alternative, particularly when the spirals or blanks are similarly accessible from either side.

[0044] Advantageously, the vibratory excitation source 210 is an acoustic source, and it is coupled to a diverging cone 212 directed towards the spiral or spiral blank to be excited, as shown in the figure 4The dimensions of the cone tip are slightly larger than the engraving dimensions of the spiral, in order to excite the entire spiral.

[0045] The fixture can advantageously be mounted on a movable table, the position of which can be precisely controlled by actuators. Movement can be provided in two orthogonal directions (within the plane of the plate) or even three orthogonal directions (within the plane of the plate and along a direction normal to the plane of the plate). In the frame of reference of the measuring machine, the table is movable, while the excitation source and measuring instruments can be fixed. The fixture can therefore move relative to the excitation source and measuring instruments, allowing for automatic and indexed movement between the different spirals to be measured.

[0046] The present description is given by way of non-limiting example, the scope of the invention being determined by the claims. It should be noted that the mounting serves as a support for a silicon layer of an SOI wafer or what remains of the wafer at the time of measurement, or for a single wafer, if the etching process is performed on a single wafer.

Claims

1. A device for determining the stiffness of a silicon-based hairspring or hairspring blank attached to a silicon-based wafer (10), by application of a vibrational excitation and by measurement of the vibrational response of the hairspring or of the blank, said device comprising: - a fitting (200) intended to provide a bearing at least on a portion of the wafer surrounding the hairspring or the hairspring blank, and comprising an aperture to allow the free vibrations of the hairspring or of the blank, - a source of vibrational excitation (210) of the hairspring or of the hairspring blank, arranged to excite the hairspring or the hairspring blank, - means for measuring the vibrational response (220) of said hairspring or of the hairspring blank, to measure the vibrational response of the hairspring or of the hairspring blank.

2. The determination device according to claim 1, characterized in that said vibrational excitation source (210) is disposed on a first side of the wafer, and in that said vibrational response measuring means (220) are arranged on a second side of the wafer, opposite to the first side.

3. The determination device according to any of claims 1 and 2, characterized in that the fitting (200) comprises an array of apertures (202) intended to be facing the hairsprings or the hairspring blanks to be measured, the apertures being dimensioned so as to allow free vibrations of the hairsprings or of the hairspring blanks.

4. The determination device according to any of claims 1 and 2, characterized in that it comprises holding means arranged to press the fitting and the wafer against each other.

5. The determination device according to any of the preceding claims, characterized in that the fitting (200) is associated with a clamping wafer (300), the wafer being intended to be sandwiched between the fitting and the clamping wafer.

6. The determination device according to claim 5, characterized in that the fitting (200) and the clamping wafer (300) each comprise an array of apertures (202, 302), the apertures in the fitting and the apertures in the clamping wafer being disposed facing each other and intended to be facing the hairsprings or the hairspring blanks to be measured, the apertures being dimensioned so as to allow free vibrations of the hairsprings or of the hairspring blanks to be measured.

7. The determination device according to any of claims 1 to 3, characterized in that it comprises means for attracting the wafer on the fitting.

8. The determination device according to claim 7, characterized in that said attraction means include a suction circuit (420) opening onto the contact point of the wafer.

9. The determination device according to claim 7, characterized in that said attraction means include an electrostatic device.

10. The determination device according to claim 2 or any of the dependent claims of claim 2, characterized in that said excitation source (210) passes into the aperture (202) of the fitting.

11. The determination device according to claim 10, characterized in that the vibrational excitation source is an acoustic source, and in that said acoustic source is coupled to a diverging cone (212) directed towards the hairspring or the hairspring blank to be excited.

12. The determination device according to any of the preceding claims, characterized in that it comprises indexing means (350) to position respectively the apertures of the fitting (200) facing the hairsprings / hairspring blanks of the wafer (10).

13. The determination device according to claim 12 and claim 5, characterized in that said indexing means or second indexing means are arranged to position respectively the apertures of the clamping wafer facing the hairsprings / hairspring blanks of the wafer.

14. The determination device according to any of the preceding claims, comprising a movable table on which the wafer is disposed, to allow it to move with reference to the excitation source and to the measuring means.

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