REINFORCED MICRO-MECHANICAL PART

Abstract

The micro-mechanical part, for example a horological movement part, includes a silicon core (1) all or part of the surface (3) of which is coated with a thick amorphous material (2). This material is preferably silicon dioxide and has a thickness which is five times greater than the thickness of native silicon dioxide.

Description

TECHNICAL FIELD

The present invention concerns a micro-mechanical part made of silicon, said part having been treated in order to give improved mechanical properties. It is for example, but in a non limitative manner, a micro-mechanical part for a horological mechanical movement, i.e. either a part having an active function for example for transmitting and/or transforming an energy to drive hands in order to give a time indication in connection with a dial, or a passive part for example for positioning wheel sets.

BACKGROUND OF THE INVENTION

Silicon is a material which is used more and more often in the manufacture of mechanical parts and in particular of micro-mechanical parts, both “captive” parts i.e. parts which stay connected to a substrate on which they have been etched, or “free” parts such as parts belonging to the kinematic chain of a horological movement.

Compared to metals or metal alloys conventionally used for manufacturing micro-mechanical parts, such as toothed wheels, articulated parts or springs, silicon has the advantage of having a density that is 3 to 4 times lower and therefore of having a very reduced inertia and of being insensitive to magnetic fields. These advantages are particularly interesting in the horological field both for isochronism and the operating duration of the timepiece when the energy source is formed of a spring.

Silicon is however known to be sensitive to shocks, which may be necessary during assembly, inevitable in operation or accidental when for example the user knocks his wristwatch against something or drops it.

EP patent No 1 422 436 discloses a silicon hairspring formed of a spiral shaped bar coated over its entire surface with a layer of amorphous silicon oxide. According to this document, the first thermal coefficient of Young's modulus for silicon oxide is opposite to that of silicon. Thus, the combination of a core made of silicon with an external coating of oxide is said to allow a reduction in said first thermal coefficient.

This prior art document does not mention the problem of shock sensitivity of parts made of silicon.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a solution that aims to improve the mechanical resistance of a silicon micro-mechanical part and in particular its resistance to shocks.

Therefore the invention concerns a silicon micro-mechanical part according to one of independent claim 1 or 6.

The invention also concerns a method for manufacturing a reinforced silicon part according to claim 2. This method enables the formation, in particular by thermal oxidation, the thick amorphous layer which considerably increases the mechanical properties of said part as will be explained in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear more clearly from the following description of an example embodiment, this example being given purely by way of non-limiting illustration with reference to the annexed drawings, in which:

FIG. 1 shows the initial cross-section of a silicon hairspring;

FIG. 2 corresponds to FIG. 1 after the deposition of an amorphous material; and

FIG. 3 shows an additional step of deposition of an anti friction coating.

DETAILED DESCRIPTION OF THE INVENTION

A hairspring mounted in a horological movement the malfunction of which is very easy to detect, simply by observing the movement stop if the hairspring happens to break, as will be explained hereinafter, has been taken here by way of example.

The hairspring is obtained by known etching techniques from a silicon plate of slightly smaller thickness than the desired final height for the hairspring.

One could for example use the reactive ionic etching technique (RIE) and give the hairspring the shape which is considered most appropriate, as disclosed for example in International Patent Application W02004/070476.

Given the very small dimensions of a hairspring, a batch of hairsprings can be manufactured in one time on the same plate.

FIG. 1 shows the cross-section of a hairspring having a core made of silicon, reference 3 designating the initial external surface. When this hairspring is left for a certain amount of time in the surrounding air, it naturally covers itself with silicon dioxide called “native oxide” (not shown) the thickness of which is substantially comprised between 1 and 10 nm.

FIG. 2 shows the same cross-section of the hairspring after it has been treated according to the invention, by a surface thermal oxidation between 900° C. and 1200° C. To this effect the protocol disclosed in the publication “Semiconductors devices: physics and technology (ed. John Wiley & Sons, ISBN 0-471-87424-8, 01.01 1985, p.341-355) is applied. Thus, it takes approximately 10 hours at a temperature of 1100° C. to obtain a thickness of silicon dioxide of about 1.9 micron. As can be seen in FIG. 2 the silicon dioxide is formed using silicon, the surface 3 of which moves backwards in order to create a new interface 5 with the formed SiO₂. Conversely, given that SiO₂ has a lower density, the external surface 7 of SiO₂ extends beyond the initial surface of the hairspring. The positions of these separation lines 3, 5 , and 7 are not shown to scale, but it is obvious that knowledge of the physical properties of Si and SiO₂ and the thermal treatment characteristics allows the initial dimensions to be calculated for etching the hairspring in order to have the desired dimensions at the end of this treatment.

During a first series of tests, the mechanical resistance of non oxidized silicon parts and oxidized silicon parts was tested from the manufacturing stage to the assembly stage.

During the manufacture of a batch of silicon parts, the parts need to be manipulated at different manufacturing stages. For the specific case described in this report, silicon parts originating from two silicon plates which have undergone identical steps are considered.

The parts have subsequently been mounted in a movement. During the tests, the parts are attached to a steel arbour and are pinched with tweezers and measurement setting. During final assembly on the movement, the center of the part is driven on to a solid arbour.

The following table summarizes the results of this test carried out on 19 non oxidized parts and 36 oxidized parts.

	Non oxidized parts	Oxidized parts
Plates	Initial	Remainder	Success	Initial	Remainder	Success
1	10	4	40%	16	16	100%
2	9	6	67%	20	20	100%
Total	19	10	53%	36	36	100%

During this test, comparison of the success rate of a complete chain of operations shows clearly that oxidized silicon parts are less fragile than the same parts without oxidisation.

The mechanical properties of an ordinary silicon hairspring (FIG. 1) and a hairspring modified according to the invention (FIG. 2) have also been compared in a real situation after assembly in the shock test using a shock pendulum of 5000 g.

Two identical movements, in which a non treated hairspring and a hairspring modified according to the invention have been mounted, have been subjected to this mechanical resistance test.

The movements fitted with the non oxidized hairspring or having a very thin deposition of native oxide stopped rapidly because of breakage of the hairsprings due to the shocks.

The movements fitted with the hairspring according to the invention resisted the shocks for a long time and kept the working and isochronism thereof remained satisfactory for more than 30 weeks while being worn.

Thus, surprisingly, replacing one material, silicon, with a material of lower density, silicon dioxide, increases mechanical resistance, while one might logically have expected a decrease in mechanical resistance.

In the example which has just been described, the “thick amorphous layer” was silicon dioxide. In an equivalent manner this layer could be formed with other deposition methods, using other materials such as silicon nitride or carbide or titanium carbide or nitride.

This example shows that all external surfaces of the parts are uniformly coated with a thick amorphous deposition. Of course the use of appropriate masks allows deposition on only selected portions of the part, i. e. on portions which are particularly mechanically stressed. Conversely, for example after a complete coating of SiO₂, it is possible to eliminate certain portions of the coating by chemical etch with BHF, for example for esthetical reasons or for forming another type of coating.

FIG. 3 shows a variant wherein an additional step adds a coating 4 made of a material selected for its tribological properties on the thick amorphous layer.

The foregoing description was made using a hairspring for a horological movement by way of example, but it is obvious that the same advantages would be found for any other parts of a watch movement (toothed wheel, escapement wheel, pallets, pivoted parts, etc. . . . ) and more generally any parts of a micro-mechanism without departing from the scope of the present invention.

Claims

1-10. (canceled)

11. A silicon micromechanical part intended to be integrated in an horological mechanism, said part being selected from the group comprising toothed wheels, escapement wheels, pallets, pivoted parts, and passive parts, wherein said part is coated over its entire surface with silicon dioxide and wherein the thickness of the said coating is five times greater than the thickness of the native silicon dioxide.

12. A method for manufacturing a reinforced silicon micromechanical part, said part being selected from the group comprising toothed wheels, escapement wheels, pallets, pivoted parts, and passive parts, said method including, the successive steps consisting in: etching said part or a batch of said parts in a silicon plate,depositing over the entire surface of said part, in one or several steps, a silicon oxide layer, said deposition being made by thermal oxidation, at a temperature ranging from 900° C. to 1200° C., of the surface of said part(s) for a sufficient period of time to obtain a silicon dioxide layer having a thickness which is at least five times greater than the thickness of native silicon dioxide.

13. The method according to claim 12, wherein, in the first step of the method, the part is etched with slightly smaller dimensions than the desired final dimensions.

14. The method according to claim 12, wherein it further comprises an additional step consisting in coating at least partially the silicon dioxide deposition with a coating of a material selected for its tribological properties, such as diamond like carbon or carbon nanotubes.

15. The method according to claim 12, wherein, after the step of depositing the silicon dioxide layer, it comprises a step of eliminating certain portions of said layer by chemical etching with BHF.

16. Silicon micromechanical part intended to be integrated in an horological mechanism, wherein it is obtained by the method according to claim 12.

17. The part according to claim 16, wherein the silicon dioxide has a thickness greater than 50 nm.

18. The part according to claim 11, wherein the silicon dioxide has a thickness greater than 50 nm.

19. The part according to claim 11, wherein for the portions of silicon dioxide in contact with other parts of a kinematic chain, the silicon dioxide is also at least partially coated with a coating selected for its tribological properties, such as diamond like carbon (DLC) or carbon nanotubes.

20. Silicon micromechanical part intended to be integrated in an horological mechanism, wherein it is obtained by the method according to claim 13.

21. Silicon micromechanical part intended to be integrated in an horological mechanism, wherein it is obtained by the method according to claim 14.

22. Silicon micromechanical part intended to be integrated in an horological mechanism, wherein it is obtained by the method according to claim 15.