EP3769160A1

EP3769160A1 - Method for manufacturing a silicon hairspring

Info

Publication number: EP3769160A1
Application number: EP19712197.3A
Authority: EP
Inventors: Pierre Cusin; Marco Verardo
Original assignee: Nivarox Far SA; Nivarox SA
Current assignee: Nivarox Far SA; Nivarox SA
Priority date: 2018-03-21
Filing date: 2019-03-21
Publication date: 2021-01-27
Also published as: KR102448668B1; JP7100711B2; KR20200120949A; US20210080909A1; JP2021535356A; CN111819501A; EP3543796A1; US11300926B2; WO2019180177A1

Abstract

The invention relates to a method for manufacturing a hairspring with a final stiffness,comprising the steps of manufacturing a hairspring with an excess thickness, determining the initial stiffness of the hairspring produced, in order to remove the volume of material to obtain the hairspring with the dimensions required for said final stiffness.

Description

PROCESS FOR MANUFACTURING A SPI RAL IN S I LICI UM

Field of the invention

The invention relates to a method of manufacturing a silicon spiral and, more specifically, such a spiral used as a compensating spring cooperating with a known balance of inertia to form a resonator having a predetermined frequency.

Background of the invention

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

Making such a compensating hairspring provides many benefits but also has disadvantages. In fact, the step of etching several spirals in a silicon wafer offers a non-negligible geometrical dispersion between the spirals of the same wafer and a greater dispersion between spirals of two wafers etched at different times. Incidentally, the stiffness of each spiral engraved with the same engraving pattern is variable by creating significant manufacturing dispersions.

Summary of the invention

The object of the present invention is to overcome all or part of the disadvantages mentioned above by proposing a method of manufacturing a spiral whose dimensions are sufficiently precise not to require retouching. For this purpose, the invention relates to a method for manufacturing a silicon spiral having a known final stiffness comprising the following steps: a) providing an SOI wafer comprising successively a so-called "handle" silicon layer, a silicon oxide bonding layer, and a so-called "device" silicon layer;

b) growing a layer of silicon oxide on the surface of the wafer; c) photolithography on the "device" layer to form a resin mask;

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

e) performing 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 as protection of the components;

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

h) determining the initial stiffness of the hairspring and calculating the coil dimensions to be obtained to obtain the hairspring of a final stiffness;

i) oxidizing the formed hairspring to transform said silicon-based material material to silicon dioxide and thereby form an oxidized hairspring;

j) removing the oxide of the oxidized spiral to obtain a silicon-based spiral with the overall dimensions necessary to obtain the final stiffness;

k) re-oxidizing the hairspring to obtain a hairspring of final stiffness and adjusting the thermal performance of said hairspring.

A compensating balance spring is thus obtained which, advantageously according to the invention, comprises a silicon-based core and a coating based on silicon oxide. Advantageously according to the invention, the compensating hairspring thus has a very high dimensional accuracy and, incidentally, a thermal compensation of the whole resonator very fine.

It is thus clear that the method makes it possible to guarantee a very high dimensional accuracy of the hairspring and, incidentally, a behavior of its stiffness according to the temperature which will compensate for the drifts of the assembly which it forms with a pendulum.

According to other advantageous variants of the invention:

step e) is carried out using a chemical etching;

step g) comprises the following phases:

g1) 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 a DRIE etching;

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

step h) comprises the following phases:

h1) measuring the frequency of an assembly comprising the hairspring formed during step e) coupled with a balance with a known inertia and deduce from the measured frequency, the initial stiffness of the hairspring formed; h2) calculating, from the determination of the initial stiffness of the hairspring, the turn dimensions to obtain to obtain said hairspring of a final stiffness;

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

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

Brief description of the drawings

Other particularities and advantages will emerge clearly from the description which is given hereinafter, by way of indication and in no way limiting, with reference to the appended drawings, in which:

FIG. 1 illustrates a wafer with a multitude of spirals obtained according to a method according to the invention;

- Figures 2a and 2b respectively show a perspective view and a sectional view of a spiral obtained by a method according to the invention;

- Figure 3 illustrates the different steps of a method according to the invention.

Detailed Description of the Preferred Embodiments

The invention relates to a compensating hairspring 1 visible in Figure 2a and its manufacturing method to ensure a very high dimensional accuracy of the hairspring and, incidentally, to ensure a more precise stiffness of said hairspring.

According to the invention, the compensating spiral 1 is formed based on a material, optionally 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 spiral offers the advantage of being precise by the existing engraving methods and of having good mechanical properties and in particular being little or not sensitive to magnetic fields. It must however be coated or superficially modified to form a compensating hairspring.

Preferably, the silicon-based material used as a compensating spiral may 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 regardless of its crystalline orientation or the oxide of silicon. Of course other materials can be envisioned as a glass, a ceramic, a cermet, a metal or a metal alloy. For simplicity, the explanation below will be focused on a silicon-based material.

Each type of material may be surface-modified or layer-coated to thermally compensate for the base material as explained above.

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

According to the invention, the method comprises, as illustrated in FIG. 3, a first step a) that consists in providing SOI wafers 10, that is to say composed of two layers of silicon 11 and 12, which are bonded together. to each other by a silicon oxide layer 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 (whose main orientations may be varied), has a thickness which will determine the final thickness of the component to be manufactured, typically, in timepieces, between 100 and 200pm.

The lower layer of silicon 12, called "handle", serves essentially as mechanical support, so as to perform the process on a sufficiently rigid assembly (which the reduced thickness of the "device" is not able to guarantee) . It is also formed of a monocrystalline silicon plate, generally of a similar orientation to the "device" layer.

The oxide layer 13 intimately bonds the two silicon layers 11 and 12. In addition, it will also serve as a stop layer for subsequent operations.

The following step b) consists in growing on the surface of the wafer (s) 10 a layer of silicon oxide, exposing the wafer or wafers 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 4pm.

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

the resin is deposited, for example by spinning, in a very thin layer of thickness 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 that one does not wish to be attacked in the subsequent operation of deep reactive ion etching (also known by the abbreviation "D.R.I.E.") of silicon.

During step d), the areas exposed or on the contrary coated with resin are then exploited. A first etching process makes it possible to transfer the patterns defined in the resin in the preceding steps to the previously grown silicon oxide. Always with a view to repeatability of the manufacturing process, silicon oxide is structured by a plasma dry etching, directional and reproducing the quality of the flanks of the resin serving as a mask for this operation.

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

Silicon exposed and unprotected by silicon oxide is etched in a direction perpendicular to the surface of the wafer (Bosch® DRIE anisotropic 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 opens on the silicon oxide layer 13 bonding the two silicon layers 11 and 12, the etching stops. Indeed, like the silicon oxide serving as a mask during the Bosch® process and resistant to etching itself, the buried oxide layer 13, of the same nature, also resists therein.

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 spiral 1 comprising turns 3 and a ferrule 2.

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

Of course, the method can not be limited to a DRIE etching during step e). By way of example, step e) could equally well be obtained by chemical etching in the same silicon-based material.

During step e), several spirals can be formed in the same wafer in dimensions larger than the dimensions necessary to obtain several spirals of an initial stiffness or several spirals 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 the DRIE etching is removed in solution. aqueous 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 a protection for the components during the operation to release them by separating them from the "handle" layer 12.

A second photolithography operation similar to the first carried out in step c) is carried out on the back of the wafer 10 (hence on the "handle" layer 12). To do this the wafer 10 is returned, the resin is deposited therein and then exposed through a mask.

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

In the following step g), complete etching of the exposed "handle" layer 12 is carried out using an aqueous solution, based on potassium hydroxide (KOH), tetramethylammonium hydroxide, or by 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 silicon oxide layers are then etched by wet etching with a hydrofluoric acid solution. Advantageously, the spirals 1 formed are held at a frame via at least one fastener, the frame and the fasteners having been formed at the same time as the spirals during the step e) of etching DRIE. 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 spirals still attached to the wafer or on a spiral 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 having a known inertia and then to deduce the initial stiffness of the hairspring.

The oscillation frequency of the sprung balance assembly makes it possible to determine the angular stiffness of the spiral tested, and thereby the precise dimensions of the turn section 3 of the spiral spring 1 (its thickness mainly, the height being known , since this 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 implemented to determine the stiffness of the hairspring.

Of course, as explained above, the method is not limited to the etching of a single spiral per wafer, step h) may also consist of a determination of the average initial stiffness of a representative sample or the set of spirals formed on the same wafer.

During the second phase h2), the turn dimensions to be obtained, from the determination of the initial stiffness of the hairspring, are calculated to obtain the overall dimensions necessary to obtain said hairspring of a desired stiffness (or final stiffness). The process continues with a sequence for removing the excess material from the hairspring to the necessary dimensions to obtain the hairspring of final stiffness.

Step i) is to oxidize the hairspring in order to convert said thickness of silicon-based material to silicon dioxide and thereby form an oxidized hairspring. Such a phase may, 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 to form silicon oxide on the spiral. During this step, it is exploited that the silicon oxide grows regularly, the oxidation rate and the resulting thickness are perfectly controlled by those skilled in the art which ensures uniformity of the oxide layer.

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

Steps i) and j) make it possible to 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 thermally compensated. Such a step may, 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 to form silicon oxide on the spiral. The compensator spring 1 is thus obtained 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 turn 3 satisfy the requirement of angular stiffness sought and the layer silicon oxide increases the stiffness according to the dimensional change of the balance / hairspring depending on the temperature.

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 section (s) more accurate (s) than that obtained by a single engraving;

variations in thickness and / or pitch along the turn;

a one-piece shell 2;

an internal turn of the Grossmann curve type;

- a monobloc pegging fastener;

- an integral external mounting element;

- A portion of the outer coil thickened relative to the rest of the turns.

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

This layer also has hydrophilic properties, and it is known that the absorption of moisture causes a drift of the stiffness of the hairspring and therefore the running of the watch. Also, a thin layer of a metal such as chromium, titanium, tantalum or one of their alloys renders both the surface of the hairspring 1 waterproof and conductive, eliminating the effects mentioned above. Such a layer can be obtained according to the teachings of EP 2 920 653, incorporated by reference into the present application.

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

Finally, the method may also comprise step I) intended to separate the spirals 1 of the wafer 10 and to assemble them with a known balance of inertia to form a balance-spring resonator which is compensated thermally or otherwise, it is that is, whose frequency is sensitive or not to temperature variations.

Of course, the present invention is not limited to the illustrated example but is susceptible of various variations and modifications that will occur to those skilled in the art. In particular, as explained above, the balance, even if it comprises a predefined construction inertia, may comprise movable weights to provide a setting parameter before or after the sale of the timepiece.

Claims

1. A method of manufacturing a spiral comprising the following steps:

a) providing an SOI wafer (10) successively comprising a so-called "silicon" silicon layer (11), a silicon oxide bonding layer (13), and a "handle" silicon layer (12) ; b) growing a layer of silicon oxide on the surface of the wafer

(10);

c) photolithography on the "device" layer (11) to form a resin mask;

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

e) performing 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 spiral (1) formed;

g) etching the "handle" layer (12) to expose the tie layer and then releasing the hairspring (1), the hairspring (1) being held to the wafer (10) by at least one tie;

h) determining the initial stiffness of the hairspring (1) and calculating the turn dimensions (3) to obtain the hairspring of a final stiffness;

j) removing the oxide of the oxidized spiral to obtain a silicon-based hairsprick with the overall dimensions necessary to obtain the final stiffness.

2. The manufacturing method according to claim 1, characterized in that step e) is carried out using a chemical etching.

3. Manufacturing process according to claims 1 and 2, characterized in that step g) comprises the following phases:

g1) 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 a DRIE etching;

4. Manufacturing method according to one of the preceding claims, characterized in that, in step e), several spirals are formed in the same wafer in dimensions greater than the dimensions necessary to obtain several spirals of an initial stiffness or several spirals of several initial stiffnesses.

5. Manufacturing process according to one of the preceding claims, characterized in that step h) comprises the following phases:

h1) measuring the frequency of an assembly comprising the hairspring formed during step e) coupled with a balance with a 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 spiral, the coil dimensions to obtain to obtain said spiral of a final stiffness.

6. Manufacturing method according to one of the preceding claims, characterized in that, after step j), the method further comprises the following step:

k) forming, on at least a portion of said hairspring of a final stiffness, a thin layer on a portion of the outer surface of said hairspring forming a hairspring less sensitive to weather variations and electrostatic interference.

7. The manufacturing method according to claim 6, characterized in that the thin layer comprises chromium, titanium, tantalum or an alloy thereof.