CH714815A2 - Process for manufacturing a silicon spiral for watchmaking - Google Patents

Abstract

The invention relates to a method of manufacturing a spiral made of silicon for the watchmaking of a determined final stiffness comprising the steps of manufacturing a spiral according to oversized dimensions, determining the initial stiffness of the spiral formed so as to to remove the volume of material to obtain the spiral to the dimensions necessary for said determined final stiffness.

Description

Description

FIELD OF THE INVENTION The invention relates to a method of manufacturing a silicon balance spring and, more precisely, such a balance spring used as a compensating balance spring cooperating with a known inertia balance to form a resonator comprising a predetermined frequency.

Background of the invention [0002] It is explained in document EP 1 422 436, incorporated by reference into the present application, how to form a compensating hairspring comprising a silicon core coated with silicon dioxide and cooperating with a balance wheel inertia known to thermally compensate all of said resonator.

Manufacturing such a compensating hairspring brings many advantages but also has disadvantages. Indeed, the step of engraving several hairsprings in a silicon wafer offers a non-negligible geometric dispersion between the hairsprings of the same wafer and a greater dispersion between hairsprings of two wafers etched at different times. Incidentally, the stiffness of each hairspring engraved with the same engraving pattern is variable, creating significant manufacturing dispersions.

SUMMARY OF THE INVENTION The object of the present invention is to overcome all or part of the drawbacks mentioned above by proposing a method of manufacturing a hairspring whose dimensions are sufficiently precise to not require retouching.

To this end, the invention relates to a process for manufacturing a silicon hairspring having a known final stiffness comprising the following steps:

a) 1 provide an SOI wafer successively comprising a layer of silicon called "handle", a bonding layer of silicon oxide, and a layer of silicon called "device";

b) i grow a layer of silicon oxide on the surface of the wafer;

c) carry out a photolithography on the “device” layer to form a resin mask;

d) etching the silicon oxide layer through the resin mask previously formed;

e) carry out a deep reactive ion etching to form the silicon balance spring;

f) growing a layer of silicon oxide on the surface of the silicon, the oxide layer serving to protect the components;

g) etching the handle layer to expose the bonding layer and then releasing the hairspring, the hairspring being held in the wafer by at least one clip;

h) determine the initial stiffness of the hairspring and calculate the turn dimensions to obtain in order to obtain the hairspring of a final stiffness;

i) oxidizing the hairspring formed in order to transform said thickness of silicon-based material to be removed into silicon dioxide and thus form an oxidized hairspring;

j) removing the oxide from the oxidized hairspring making it possible to obtain a silicon-based hairspring with the overall dimensions necessary to obtain the final stiffness;

k) oxidize the hairspring again to obtain a hairspring of final stiffness and adjust the thermal performance of said hairspring.

One thus obtains a compensating hairspring which, advantageously according to the invention, comprises a core based on silicon and a coating based on silicon oxide. Advantageously according to the invention, the compensating hairspring therefore has very high dimensional precision and, incidentally, thermal compensation of the whole of the very fine resonator.

It is therefore understood that the method ensures very high dimensional accuracy of the hairspring and, incidentally, a behavior of its stiffness depending on the temperature which will compensate for the drifts of the assembly that it forms with a pendulum.

[0008] In accordance with other advantageous variants of the invention:

CH 714 815 A2 step e) is carried out using chemical etching;

step g) has the following phases:

g1) carry out photolithography and dry etching to expose the silicon of the “handle” layer;

g2) etching the handle layer with a solution of potassium hydroxide, tetramethylammonium hydroxide, or by DRIE etching;

- during step e), several hairsprings are formed in the same wafer according to dimensions greater than the dimensions necessary to obtain several hairsprings of an initial stiffness or several hairsprings of several initial stiffnesses;

step h) has the following phases:

h1) measure the frequency of an assembly comprising the hairspring formed during step e) coupled with a balance wheel with known inertia and deduce from the measured frequency, the initial stiffness of the hairspring formed;

h2) calculate, from the determination of the initial stiffness of the hairspring, the turn dimensions to be obtained in order to obtain said hairspring of a final stiffness;

after step k), the method also comprises the following step:

I) forming, on at least part of said hairspring of a predetermined stiffness, a thin layer on part of the external surface of said hairspring making it possible to form a hairspring less sensitive to climatic variations and to electrostatic interference.

Summary description of the drawings [0009] Other particularities and advantages will emerge clearly from the description which is given below, for information and in no way limitative, with reference to the attached drawings, in which:

fig. 1 illustrates a wafer with a multitude of hairsprings obtained according to a process according to the invention;

fig. 2a and 2b respectively illustrate a perspective view and a sectional view of a hairspring obtained according to a method according to the invention;

fig. 3 illustrates the different stages of a process according to the invention.

Detailed description of the preferred embodiments The invention relates to a compensating hairspring 1 visible in FIG. 2a as well as its manufacturing process making it possible to guarantee a very high dimensional precision of the hairspring and, incidentally, to guarantee a more precise stiffness of said hairspring.

According to the invention, the compensating hairspring 1 is formed from a material, possibly coated with a thermal compensation layer, and intended to cooperate with a known inertia balance.

The use of a material, for example based on silicon, glass or ceramic, for the manufacture of a hairspring offers the advantage of being precise by existing engraving methods and of having good mechanical and chemical properties, in particular being little or not sensitive to magnetic fields. However, it must be coated or modified surface to be able to form a compensating hairspring.

Preferably, the silicon-based material used as the compensating hairspring can be monocrystalline silicon whatever its crystalline orientation, doped monocrystalline silicon whatever its crystalline orientation, amorphous silicon, porous silicon, polycrystalline silicon, silicon nitride, silicon carbide, quartz whatever its crystalline orientation or silicon oxide. Of course, other materials can be envisaged such as glass, ceramic, cermet, metal or a metal alloy. For simplification, the explanation below will be given to a material based on silicon.

Each type of material can be modified superficially or coated with a layer in order to thermally compensate the base material as explained above.

Thus, the invention relates to a process for manufacturing a silicon hairspring 1 visible in FIG. 3. For the sake of readability and understanding, the process steps represent only a middle section along line A of a single silicon hairspring 1 formed in the wafer 10 of FIG. 1, the number of turns 3 of the hairspring 1 being reduced to facilitate the reading of the figures.

CH 714 815 A2 According to the invention, the method comprises, as illustrated in FIG. 3, a first step a) which consists in providing SOI wafers 10, that is to say composed of two layers of silicon 11 and 12, linked to each other by a layer of silicon oxide 13. Each of these three layers has one or more specific roles.

The upper layer of silicon 11, called “device” and formed in a monocrystalline silicon plate (the main orientations of which can be varied), has a thickness which will determine the final thickness of the component to be manufactured, typically in watchmaking, between 100 and 200 μm.

The lower layer of silicon 12, called "handle", essentially serves as a mechanical support, so that the process can be carried out on a sufficiently rigid assembly (which the reduced thickness of the "device" is not able to to guarantee). It is also formed from a monocrystalline silicon wafer, in general with an orientation similar to the "device" layer.

The oxide layer 13 allows the two silicon layers 11 and 12 to be intimately bonded. In addition, it will also serve as a stop layer during subsequent operations.

Step b) which follows consists in growing on the surface of the wafer (s) 10 a layer of silicon oxide, by exposing the wafer (s) to an oxidizing atmosphere at high temperature. The layer varies according to the thickness of the "device" to be structured. It is typically between 1 and 4 μm.

Step c) of the method will make it possible to define, for example in a positive resin, the patterns which it is desired to produce subsequently in the silicon wafer 10. This step includes the following operations:

- the resin is deposited, for example with a spinner, in a very thin layer with a thickness of between 1 and 2 μm,

- once dried, this resin, with photolithographic properties, is exposed through a photolithographic mask (transparent plate covered with a layer of chromium, itself representing the desired patterns) using a light source;

- in the specific case of a positive resin, the exposed areas of the resin are then removed using a solvent, revealing the oxide layer. In this case, the zones always covered with resin define the zones which one does not wish to see attacked in the subsequent operation of deep reactive ion etching (also known by the abbreviation "D.R.I.E.") of silicon.

In step d), the exposed areas or areas covered with resin are then used. A first etching process makes it possible to transfer the patterns defined in the resin in the preceding steps to the previously raw silicon oxide. Still with a view to the repeatability of the manufacturing process, the silicon oxide is structured by dry plasma etching, directional and reproducing the quality of the sides of the resin used as a mask for this operation.

Once the silicon oxide has been etched in the open areas of the resin, the silicon surface of the upper layer 11 is then exposed and ready for DRIE etching. The resin can be preserved or not depending on whether one wishes to use the resin as a mask during DRIE etching.

Exposed silicon not protected by silicon oxide is etched in a direction perpendicular to the surface of the wafer (anisotropic DRIE Bosch® etching). The patterns formed first in the resin, then in the silicon oxide, are "projected" into the thickness of the "device" layer 11.

When the etching leads to the silicon oxide layer 13 bonding the two silicon layers 11 and 12, the etching stops. Indeed, like silicon oxide used as a mask during the Bosch® process and resistant to etching itself, the buried oxide layer 13, of the same kind, also resists it.

The “device” silicon layer 11 is then structured throughout its thickness by the defined patterns representing the components to be manufactured, now revealed by this DRIE etching, namely a hairspring 1 comprising turns 3 and a ferrule 2.

The components remain integral with the "handle" layer 12 to which they are linked by the buried silicon oxide layer 13.

Of course, the method cannot be limited to a DRIE etching during step e). For example, step e) could just as easily be obtained by chemical etching in the same silicon-based material.

In step e), several hairsprings can be formed in the same wafer according to dimensions greater than the dimensions necessary to obtain several hairsprings of an initial stiffness or several hairsprings of several initial stiffnesses.

Following step e), during a sequence e1), the residues of the passivation resin resulting from the Bosch® process are then removed, and the oxide having served as a mask for DRIE etching is eliminated in aqueous solution based on hydrofluoric acid.

During a step f), a layer of silicon oxide is again grown on the surface of the silicon (around the “device” 11 and “handle” layers 12), this oxide layer will serve as protection of the components during the operation used to release them by separating them from the “handle” layer 12.

CH 714 815 A2 A second photolithography operation similar to the first carried out during step c) is carried out on the back of the wafer 10 (therefore on the “handle” layer 12). To do this, the wafer 10 is turned over, the resin is deposited there, then exposed through a mask.

The exposed resin area is then removed using a solvent, revealing the oxide layer previously formed and which is then structured via dry etching.

In step g) below, a complete etching of the “handle” layer 12 exposed is carried out by means of an aqueous solution, based on potassium hydroxide (KOH), tetramethylammonium hydroxide, or well by a DRIE engraving. These solutions are well known for easily etching silicon, while sparing silicon oxide.

In step g1) to completely release the components, the various layers of silicon oxide are then etched by means of wet etching with a solution based on hydrofluoric acid. Advantageously, the hairsprings 1 formed are held to a frame via at least one fastener, the frame and the fasteners having been formed at the same time as the hairsprings during step e) of DRIE etching.

The method comprises a step h) intended to determine the initial stiffness of the hairspring. Such a step h) can be carried out directly on the hairspring still attached to the wafer 10 or on the whole or on a sample of the hairsprings still attached to the wafer or on a hairspring detached from the wafer.

Preferably according to the invention, step h) comprises a first phase h1) intended to measure the frequency of an assembly comprising the hairspring coupled with a balance provided with a known inertia and then, from this deducing the initial stiffness of the spiral.

The oscillation frequency of the balance-spring assembly makes it possible to determine the angular stiffness of the balance spring tested, and thereby the precise dimensions of the coil section 3 of the balance spring 1 (its thickness mainly, the height being known, since it is the thickness of the “device” layer of the base substrate).

Such a measurement phase can in particular be dynamic and carried out according to the teachings of document EP 2 423 764, incorporated by reference into the present application. However, alternatively, a static method, carried out according to the teachings of document EP 2 423 764, can also be used to determine the stiffness of the hairspring.

Of course, as explained above, the method not being limited to the engraving of a single hairspring per wafer, step h) can also consist in determining the initial initial stiffness of a representative sample or of all the hairsprings formed on the same wafer.

During the second phase h2), the turn dimensions to be obtained are calculated, from the determination of the initial stiffness of the hairspring, to obtain the overall dimensions necessary to obtain said hairspring of a desired stiffness (or stiffness final).

The process continues with a sequence intended to remove the excess material from the hairspring to the necessary dimensions in order to obtain the hairspring of a final stiffness.

Step i) consists of oxidizing the hairspring in order to transform said thickness of silicon-based material to be removed into silicon dioxide and thus form an oxidized hairspring. Such a phase can, for example, be obtained by thermal oxidation. Such thermal oxidation can, for example, be carried out between 800 and 1200 ° C. under an oxidizing atmosphere using water vapor or oxygen gas making it possible to form silicon oxide on the hairspring. During this step, use is made of the fact that the silicon oxide increases regularly, the speed of oxidation and the thickness which results therefrom are perfectly controlled by a person skilled in the art, which makes it possible to ensure uniformity. of the oxide layer.

Step i) continues with a step j) intended to remove the oxide from the hairspring making it possible to obtain a hairspring based on silicon with the overall dimensions necessary to obtain the final stiffness. Such a step is obtained by chemical etching. Such chemical etching can be carried out, for example, by means of a solution based on hydrofluoric acid making it possible to remove the silicon oxide from the hairspring.

Steps i) and j) bring the dimensions of the turn 3 to intermediate values determined during the calculation step h2).

Finally, step k) consists in oxidizing the hairspring again to coat it with a layer of silicon dioxide in order to form a hairspring 1 which is thermocompensated. Such a step can, for example, be obtained by thermal oxidation. Such thermal oxidation can, for example, be carried out between 800 and 1200 ° C. under an oxidizing atmosphere using water vapor or oxygen gas making it possible to form silicon oxide on the hairspring.

One thus obtains the balance spring 1 as illustrated in FIGS. 2a and 2b which, advantageously according to the invention, comprises a core 30 based on silicon and a coating 31 based on silicon oxide.

This second oxidation makes it possible to adjust both the mechanical (stiffness) and thermal (temperature compensation) performance of the future hairspring 1. At this stage, the dimensions of the coil 3 meet the requirement of angular stiffness sought and the layer of raw silicon oxide makes it possible to adjust the stiffness as a function of the dimensional change of the balance / hairspring assembly according to the temperature.

CH 714 815 A2 Advantageously according to the invention, it is thus possible to manufacture, without more complexity, a hairspring 1 comprising in particular:

- one or more turns 3 of section (s) more precise (s) than that obtained by a single etching;

- variations in thickness and / or pitch along the turn;

- a one-piece ferrule 2;

- an internal turn of the Grossmann curve type;

- a monoblock pitching attachment;

- a monobloc external mounting element;

- a portion of the outer turn thickened compared to the rest of the turns.

The method may also include a step I) of metallization. Indeed, the growth of a significant layer of silicon oxide on the surface of the balance springs does not only bring advantages. This layer traps and fixes electrical charges, which will lead to electrostatic bonding phenomena either with the environment of the hairspring, or turns between them.

This layer also has hydrophilic properties, 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.

Also, a thin layer of a metal such as chromium, titanium, tantalum or one of their alloys, makes both the surface of hairspring 1 waterproof and conductive, eliminating the effects mentioned above. Such a layer can be obtained according to the teachings of document EP 2 920 653, incorporated by reference into the present application.

The thickness of this thin layer is chosen to be as thin as possible so as not to disturb the performance adjusted above. Adequate heat treatment guarantees good adhesion of the thin layer.

Finally, the method can also include step I) intended to separate the balance springs 1 from the wafer 10 and assemble them with a known inertia balance to form a resonator of the balance spring type which is thermally compensated or not, that is to say, the frequency of which is sensitive or not to temperature variations.

Of course, the present invention is not limited to the illustrated example but is susceptible to various variants and modifications which will appear to those skilled in the art. In particular, as explained above, the balance, even if it has a predefined construction inertia, may include movable weights making it possible to offer an adjustment parameter before or after the sale of the timepiece.

claims

Claims (7)

1. Method of manufacturing a hairspring comprising the following steps: a) provide an SOI wafer (10) successively comprising a layer of silicon called "device" (11), a bonding layer (13) of silicon oxide, and a layer of silicon called "handle" (12) ; b) growing a layer of silicon oxide on the surface of the wafer (10); c) carrying out a photolithography on the “device” layer (11) to form a resin mask; d) etching the silicon oxide layer through the resin mask previously formed; e) carrying out a deep reactive ion etching to form the silicon balance spring (1); f) growing a layer of silicon oxide on the surface of the silicon, the oxide layer serving as protection for the hairspring (1) formed; g) etching the “handle” layer (12) to expose the bonding layer and then releasing the hairspring (1), the hairspring (1) being held in the wafer (10) by at least one fastener; h) determine the initial stiffness of the hairspring (1) and calculate the dimensions of the turn (3) to obtain the hairspring of a final stiffness; i) oxidizing the hairspring formed in order to transform said thickness of silicon-based material to be removed into silicon dioxide and thus form an oxidized hairspring; j) remove the oxide from the oxidized hairspring making it possible to obtain a silicon-based hairspring with the overall dimensions necessary to obtain the final stiffness. k) oxidize the hairspring again to obtain a hairspring of final stiffness and adjust the thermal performance of said hairspring. 2. The manufacturing method according to claim 1, characterized in that step e) is carried out using chemical etching. 3. Manufacturing method according to claims 1 and 2, characterized in that step g) comprises the following phases: g1) carry out a photolithography and an etching to expose the silicon of the "handle" layer (12); g2) etch the handle layer (12) with a potassium hydroxide solution, a tetramethylammonium hydroxide solution, or a DRIE etching. 4. Manufacturing method according to one of the preceding claims, characterized in that, during step e), several hairsprings are formed in the same wafer according to dimensions greater than the dimensions necessary to obtain several hairsprings of an initial stiffness or several hairs of several initial stiffnesses. CH 714 815 A2 5. Manufacturing process according to one of the preceding claims, characterized in that step h) comprises the following phases: h1) measure the frequency of an assembly comprising the hairspring formed during step e) coupled with a balance wheel with known inertia and deduce from the measured frequency, the initial stiffness of the hairspring formed; h2) calculate, from the determination of the initial stiffness of the hairspring, the turn dimensions to be obtained in order to obtain said hairspring of a final stiffness. 6. Manufacturing process according to one of the preceding claims, characterized in that, after step j), the process also comprises the following step: k) forming, on at least part of said hairspring of final stiffness, a thin layer on part of the external surface of said hairspring making it possible to form a hairspring less sensitive to climatic variations and to electrostatic interference. 7. The manufacturing method according to claim 6, characterized in that the thin layer comprises chromium, titanium, tantalum or one of their alloys. CH 714 815 A2 g g g g 9 9 g g g g g g 9 g g g 9 g g g g g g g g g 9 g 9 g g g g g 9 g g 9 g g g g g g ΙβΙ g g g g g g g g g g g g g g g g g g g g g g g g g g g 9 g g g g g g g g g g g g g g g g g g g g g g g 9 9 g g g 9 9 g 9 g 9 g 9 g 9 9 9 g 9 9 g [g g g g g g g g g g g g g g g g 9 9 g g g g 9 g g g 9 g g g g 9 g g g 9 g g g g g g g g g g g g g g g 9 g 9 9 g g 9 g 9 g 9 g 9 g 9 g 9 g 9 g g 9 g 9 9 g 9 g 9 9 9 g 9 g g g g g g g g g g g g g g g g g g 9 g g 9 9 g g 9 9 9 9 9 9 9 g] g g 9 g g g g g g g g g g