Method for manufacturing silicon hairspring

Abstract

The invention relates to a method of manufacturing a hairspring having a final stiffness, the method comprising the steps of: manufacturing a hairspring having an excessive thickness; the initial stiffness of the manufactured hairspring is determined in order to remove the volume of material to obtain a hairspring of the desired size with the final stiffness.

Description

[0001] This application is a divisional application of patent application No. 201980017845.2, filed on March 21, 2019, for a "Method of manufacturing a silicon hairspring". TECHNICAL FIELD

[0002] The present invention relates to a method of manufacturing a silicon hairspring, and more particularly to a hairspring used as a compensating spring, which cooperates with a balance wheel of known inertia to form a resonator having a predetermined frequency. BACKGROUND

[0003] It is explained in document EP 1 422 436 how to form a compensating spring comprising a silicon core coated with silicon dioxide and which cooperates with a balance wheel of known inertia to thermally compensate the assembly of the resonator, which document is incorporated by reference in the present application.

[0004] Manufacturing such a compensating spring has many advantages, but also disadvantages. In particular, the step of etching a plurality of hairsprings in a silicon wafer provides a considerable geometrical dispersion between the hairsprings of one wafer and between the same wafer and a wafer etched at a different time. Incidentally, the stiffness of each hairspring etched with the same etching pattern is variable, resulting in a considerable manufacturing dispersion. SUMMARY

[0005] The object of the present invention is to overcome all or some of the above-mentioned disadvantages by providing a method of manufacturing a hairspring whose dimensions are sufficiently precise without having to be changed.

[0006] To this end, the invention relates to a method of manufacturing a silicon hairspring having a known final stiffness, comprising the following steps: a) providing an SOI wafer comprising successively a silicon "handle" layer, a silicon oxide adhesive layer and a silicon "device" layer; b) growing a silicon oxide layer on the surface of the wafer; c) performing a photolithography on the "device" layer to form a resist mask; d) etching the silicon oxide layer through the preformed resist mask; e) performing a deep reactive ion etching to form the silicon hairspring; f) growing a silicon monoxide layer on the surface of the silicon, this oxide layer serving as a protective layer for the assembly; g) etching the "handle" layer to expose the adhesive layer, then releasing the hairspring, which is held on the wafer by at least one appendix; h) determining the initial stiffness of the hairspring and calculating the coil size to be obtained to obtain a hairspring having a final stiffness; i) oxidizing the hairs formed so as to transform the silicon-based material of the thickness to be removed into silicon dioxide, thus forming oxidized hairs; j) removing the oxide from the oxidized hairs, so that silicon-based hairs having the overall dimensions required to obtain the final stiffness can be obtained; k) re-oxidizing the hairs to obtain hairs having the final stiffness and adjusting the thermal behavior of said hairs.

[0007] A compensated hair is thus obtained, which, according to the application, advantageously comprises a silicon-based core and a silicon oxide-based coating. Advantageously, according to the application, the compensated hair thus has a very high dimensional accuracy, and, incidentally, the resonator assembly has a very fine thermal compensation.

[0008] It should therefore be understood that this method makes it possible to guarantee a very high dimensional accuracy of the hairs, and, incidentally, to guarantee a stiffness behavior of the hairs as a function of the temperature, which can compensate for the drift of the assembly formed by the hairs and the balance.

[0009] Other advantageous variants according to the application: - step e) is carried out by chemical etching; - step g) comprises the following stages: gl) carrying out photolithography and dry etching to expose the silicon of the "handle" layer; g2) etching the "handle" layer with a potassium hydroxide solution, a tetramethylammonium hydroxide solution or by DRIE etching; - in step e), a plurality of hairs is formed on the same wafer, the dimensions of which are greater than the dimensions required to obtain a plurality of hairs having one initial stiffness or a plurality of hairs having several initial stiffnesses.

[0010] - step h) comprises the following stages: hl) measuring the frequency of the assembly comprising the hair formed in step e) combined with a balance having a known inertia, and deducing the initial stiffness of the hair formed from the measured frequency; h2) calculating the coil dimensions to be obtained, as a function of the determination of the initial stiffness of the hair, to obtain a hair having a final stiffness, - after step k), the method further comprises the following step: 1) forming a thin layer on at least one portion of said hair having a predetermined stiffness, on a portion of the outer surface of said hair, so that a hair less sensitive to climatic variations and to electrostatic properties can be formed. BRIEF DESCRIPTION OF DRAWINGS

[0011] Other apparent features and advantages will become apparent from the description provided below, by the following purely non-limiting illustration and with reference to the attached drawings.

[0012] - Figure 1 A wafer with a plurality of hairsprings obtained according to the method of the application is shown; - Figure 2 a and 2b respectively show a perspective view and a cross-sectional view of a hairspring obtained according to the method of the application; - Figure 3 The various steps of the method according to the application are shown. DETAILED DESCRIPTION

[0013] The present application relates to a method for manufacturing a hairspring Figure 2 visible in a, and to a method for manufacturing this hairspring 1 which guarantees a very high dimensional accuracy of the hairspring and, incidentally, a more precise stiffness of said hairspring.

[0014] According to the application, the compensating hairspring 1 is formed from a material which is optionally coated with a thermal compensation layer and which is intended to cooperate with a balance of known inertia.

[0015] The use of a material such as silicon-based, glass or ceramic for manufacturing the hairspring has the advantage of being precise by existing etching methods and of having very good mechanical and chemical properties, while being not or less sensitive to magnetic fields. However, it must be coated or surface-modified in order to be able to form a compensating hairspring.

[0016] Preferably, the silicon-based material used as compensating hairspring can be monocrystalline silicon whatever its crystallographic orientation, doped monocrystalline silicon whatever its crystallographic orientation, amorphous silicon, porous silicon, polycrystalline silicon, silicon nitride, silicon carbide, quartz whatever its crystallographic orientation, or silicon oxide. Of course, other materials can be envisaged, such as glass, ceramic, cermet, metal or metal alloy. For simplicity, the following explanations will focus on silicon-based materials.

[0017] Each type of material can be surface-modified or coated with a layer in order to thermally compensate the base material as mentioned above.

[0018] The present application thus relates to a method for manufacturing Figure 3 a silicon hairspring 1 visible in. For the sake of clarity and understanding, the steps of the method represent only the median cross-section along the line A of a single silicon hairspring 1 formed in the wafer 10 of Figure 1 The number of turns 3 of the hairspring 1 is reduced in order to facilitate the understanding of the figures.

[0019] According to the application, as shown in Figure 3 the method comprises a first step a) comprising the provision of an SOI wafer 10, i.e. composed of two layers of silicon 11 and 12 which are bonded to each other by a layer of silicon oxide 13. Each of these three layers has one or more very precise roles.

[0020] The upper silicon layer 11, formed from a single crystal silicon wafer, whose main orientation can vary, is called "device", and its thickness will determine the final thickness of the component to be manufactured, typically between 100 and 200 pm in the watch industry.

[0021] The lower silicon layer 12, called "handle", is mainly used as a mechanical support, so as to be able to carry out the method on a sufficiently rigid assembly (reduced "device" thickness cannot guarantee it). It is also formed from a single crystal silicon wafer, usually having an orientation similar to that of the "device" layer.

[0022] The oxide layer 13 makes it possible to tightly bond the two silicon layers 11 and 12. In addition, it will also serve as a stop layer during the subsequent operations.

[0023] The next step b) consists in growing a silicon oxide layer on the surface of the wafer 10 by exposing the wafer to an oxidizing atmosphere at high temperature. This layer varies according to the thickness of the "device" to be constructed. It is typically between 1-4 pm.

[0024] Step c) of the method will make it possible to define the pattern desired to subsequently produce in the silicon wafer 10, for example in a positive resist. This step comprises the following operations: - deposition of the resist as a very thin layer, with a thickness of 1-2 pm, for example by spin coating, - after drying, exposure of the resist with photolithographic properties using a light source through a photolithographic mask (transparent sheet covered with a chromium layer, which itself represents the desired pattern); - in the case of a positive resist, the exposed areas are then removed by solvent, which then reveals the oxide layer. In this case, the areas still covered with resist define the areas that will not be attacked in the subsequent silicon deep reactive ion etching (DRIE) operation.

[0025] In step d), the exposed areas are then exploited, or conversely, the areas covered with resist. The first etching program makes it possible to transfer the pattern defined in the resist in the previous step to the previously grown silicon oxide. The quality of the sides of the resist, which is used as a mask for this operation, is again reproduced by directional dry plasma etching, still from the point of view of the repeatability of the manufacturing method.

[0026] Once the silicon oxide has been etched in the open areas of the resist, the silicon surface of the upper layer 11 is exposed and ready for the DRIE etching. Depending on whether it is desired to use the resist as a mask during the DRIE etching or not, the resist can be kept or not.

[0027] The exposed silicon not protected by the silicon oxide is etched in the direction perpendicular to the wafer surface (Bosch® DRIE anisotropic etching). The pattern formed first in the resist and then in the silicon oxide is "projected" into the thickness of the "device" layer 11.

[0028] When the etching reaches the silicon oxide layer 13 that joins the two silicon layers 11 and 12, the etching stops. Specifically, just as the silicon oxide acts as a mask in the Bosch® method and resists the etching itself, the buried oxide layer 13, which has the same properties, can also resist the etching.

[0029] The silicon "device" layer 11 is then structured throughout the thickness by a defined pattern that represents the component to be manufactured, now revealed by this DRIE etching, i.e. the spring 1 including the coil 3 and the inner collet chuck 2.

[0030] The components remain firmly attached to the "handle" layer 12, which they are joined to by the buried silicon oxide layer 13.

[0031] Of course, the method is not limited to the DRIE etching during step e). By way of example, step e) can also be obtained by chemical etching in the same silicon-based material.

[0032] During step e), a plurality of springs can be formed in the same wafer, the dimensions of which are greater than those required in order to obtain a plurality of springs with one initial stiffness or a plurality of springs with a plurality of initial stiffnesses.

[0033] After step e), in step el), the passivation resist residue resulting from the Bosch® procedure is removed, then the oxide used as a mask in the DRIE etching is removed in a hydrofluoric acid-based aqueous solution.

[0034] In step f), a silicon oxide layer is again grown on the surface of the silicon (around the "device" layer 11 and the "handle" layer 12), which will serve as a protective layer for the components during the operation for releasing them by separating them from the "handle" layer 12.

[0035] A second photolithography operation similar to the first one performed in step c) is performed on the back of the wafer 10 (and therefore on the "handle" layer 12). To do this, the wafer 10 is flipped over, a resist is deposited on it, then it is exposed through a mask.

[0036] The areas exposed by the resist are then removed by solvent, then the oxide layer formed previously is revealed, then it is structured by dry etching.

[0037] In a next step g), the exposed "handle" layer 12 is completely etched by means of an aqueous solution of potassium hydroxide (KOH), tetramethylammonium hydroxide or by DRIE etching. These solutions are well known to easily etch silicon while leaving the silicon oxide.

[0038] In a step g1) for completely releasing the assembly, the various silicon oxide layers are then etched by means of a wet etching with a solution based on a hydraulic acid. Advantageously, the formed balance spring 1 is held in the frame by at least one attachment, the frame and the attachment being formed during the DRIE etching step e) simultaneously with the balance spring.

[0039] The method comprises a step h) aimed at determining the initial stiffness of the balance spring. Such a step h) can be performed directly on the balance spring still attached on the wafer 10 or on an assembly or on a sample of balance springs still attached on the wafer or on a balance spring separated from the wafer.

[0040] Preferably, according to the application, the step h) comprises a first phase hi) aimed at measuring the frequency of an assembly comprising a balance spring coupled with a balance wheel having a known inertia, from which the initial stiffness of the balance spring is then derived.

[0041] The oscillation frequency of the balance-spring assembly makes it possible to determine the angular stiffness of the measured balance spring and thus the exact dimensions of the coil 3 cross section of the balance spring 1 (mainly its thickness, the height being known since it is the thickness of the underlying substrate "device" layer).

[0042] This measurement phase can in particular be dynamic and performed according to the teachings of document EP 2 423 764, which is incorporated by reference in the present application. However, alternatively, a static method performed according to the teachings of document EP 2 423 764 can also be used to determine the stiffness of the balance spring.

[0043] Of course, as mentioned above, since the method is not limited to etching a single balance spring per wafer, the step h) can also comprise determining the average initial stiffness of a representative sample or of all the balance springs formed on the same wafer.

[0044] During a second phase h2), the coil size to be obtained is calculated by determining the initial stiffness of the balance spring in order to obtain the total size necessary for said balance spring to have the desired stiffness (or final stiffness).

[0045] In order to obtain a balance spring having a final stiffness, the method continues with a step of removing the excess material from the balance spring to the necessary size.

[0046] Step i) comprises oxidizing the balance to transform the thickness of silicon-based material to be removed into silicon dioxide, thus forming an oxidized balance. Such a phase can be obtained for example by thermal oxidation. Such a thermal oxidation can be performed for example under an oxidizing atmosphere between 800 and 1200°C using water vapor or dioxygen gas, thus making it possible to form silicon oxide on the balance. In this step, use is made of the fact that silicon oxide grows uniformly, the oxidation speed and the thickness resulting therefrom being perfectly controlled by the person skilled in the art, which makes it possible to ensure the uniformity of the oxide layer.

[0047] Step i) is followed by step j), which aims to remove the oxide from the balance, thus making it possible to obtain a silicon-based balance having the overall dimensions required to obtain the final stiffness. This step is obtained by chemical etching. Such a chemical etching can be performed for example by means of a solution based on hydrofluoric acid, which makes it possible to remove the silicon oxide from the balance.

[0048] Steps i) and j) can bring the dimensions of the coil 3 to the intermediate values determined in the calculation step h2).

[0049] Finally, step k) comprises oxidizing the balance again to cover it with a layer of silicon dioxide, to form a balance 1 that is thermally compensated. This step can be obtained for example by thermal oxidation. Such a thermal oxidation can be performed for example under an oxidizing atmosphere between 800 and 1200°C using water vapor or dioxygen gas, thus making it possible to form silicon oxide on the balance.

[0050] There is thus obtained a compensated balance 1 as illustrated in figures a and 2b, which advantageously comprises, according to the application, a silicon-based core 30 and a silicon oxide-based coating 31. Figure 2

[0051] This second oxidation makes it possible to adjust the mechanical properties (stiffness) and the thermal properties (temperature compensation) of the future balance 1. At this stage, the dimensions of the coil 3 correspond to the angular stiffness requirements expected, and the silicon oxide grown makes it possible to adjust the stiffness as a function of the dimensional variations of the balance / escapement assembly, depending on the temperature.

[0052] Advantageously, according to the application, it is thus possible to manufacture a balance 1 that comprises in particular: - one or more coils 3 having a cross section that is more precise than that obtained by a single etching; - variations in thickness and / or pitch along the coil; - a one-piece inner collet 2; - an inner coil of the Grossmann curve type; - a one-piece stud-pinning attachment; - a one-piece outer setting element;​ - a part of the outer coil is too thick compared to the other parts of the coil.

[0053] The method can also comprise a metallization step I). In particular, the growth of a considerable layer of silicon oxide on the surface of the hairspring does not only provide advantages. This layer captures and fixes electrical charges which would lead to a phenomenon of electrostatic binding around the hairspring or between the coils.

[0054] This layer also has a hydrophilic nature and it is known that the absorption of moisture causes a drift in the stiffness of the hairspring and therefore in the running of the watch.

[0055] A thin layer of metal such as chromium, titanium, tantalum or alloys thereof thus simultaneously waterproofs and electrically conducts the surface of the hairspring 1, thus eliminating the effects mentioned above. Such a layer can be obtained according to the teachings of document EP 2 920 653, which is incorporated by reference in the present application.

[0056] The thickness of this thin layer is chosen so as to be as thin as possible so as not to impair the properties adjusted above. An appropriate heat treatment can ensure good adhesion of the thin layer.

[0057] Finally, the method can also comprise a step m) which aims to separate the hairspring 1 from the wafer 10 and to assemble them with a known inertia balance so as to form a resonator of the balance-hairspring type, which is optionally thermally compensated, i.e. whose frequency is optionally sensitive to temperature variations.

[0058] Of course, the present application is not limited to the example shown, but has the potential for various alternative forms and modifications which will be obvious to the person skilled in the art. In particular, as mentioned above, the balance, even if it has a predetermined configuration inertia, can comprise a movable inertia mass which makes it possible to provide for setting parameters before or after the sale of the watch.

Claims

1. Method for manufacturing a hairspring, comprising the following steps: a) providing an SOI wafer (10) comprising successively a silicon "device" layer (11), a silicon oxide adhesion layer (13) and a silicon "handle" layer (12); b) growing a silicon oxide layer on the surface of the wafer (10); c) performing a photolithography on the "device" layer (11) to form a resist mask; d) etching the silicon oxide layer through the preformed resist mask; e) performing a deep reactive ion etching to form a silicon hairspring (1); f) growing a silicon oxide layer on the surface of the silicon, said oxide layer serving as a protective layer for the formed hairspring (1); g) etching the "handle" layer (12) to expose the adhesion layer, then releasing the hairspring (1) which is held on the wafer (10) by at least one appendix; h) determining the stiffness of the formed hairspring (1) and calculating the excess material of the coil (3) to obtain a hairspring with a final stiffness; i) oxidizing the formed hairspring so as to transform the thickness of silicon-based material to be removed into silicon dioxide, thereby forming an oxidized hairspring; j) removing the oxide from the oxidized hairspring, thereby making it possible to obtain a silicon-based hairspring having the overall dimensions required to obtain a final stiffness; k) re-oxidizing the hairspring to obtain a hairspring with a final stiffness and adjusting the thermal properties of said hairspring.

2. The production method according to claim 1, characterized by Step e) is performed by chemical etching.

3. The production method according to claim 1 and 2, characterized by, Step g) comprises the following stages: gl) performing a photolithography and etching to expose the silicon of the "handle" layer (12); g2) etching the "handle" layer (12) with a potassium hydroxide solution, a tetramethylammonium hydroxide solution or DRIE etching.

4. The manufacturing method according to any one of the preceding claims, characterized in that, In step e), a plurality of hairsprings is formed in the same wafer, the dimensions of which are greater than the dimensions required to obtain a plurality of hairsprings with one initial stiffness or a plurality of hairsprings with a plurality of initial stiffnesses.

5. The manufacturing method according to any one of the preceding claims, characterized in that, Step h) comprises the following stages: hl) measuring the frequency of the assembly comprising the hairsprings formed in step e), said hairsprings being coupled with a balance wheel having a known inertia, and deducing the initial stiffness of the formed hairsprings from the measured frequency; h2) calculating the coil size to be obtained from the determination of the initial stiffness of the hairsprings to obtain said hairsprings with a final stiffness.

6. The manufacturing method according to any one of the preceding claims, characterized in that, After step k), the method further comprises the following step: 1) forming a thin layer on at least a portion of said hairspring with a final stiffness, on a portion of the outer surface of said hairspring, so that a hairspring less sensitive to disturbances of climatic and electrostatic nature can be formed.

7. The production method according to claim 6, wherein Said thin layer comprises chromium, titanium, tantalum or alloys thereof.

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