Method for making a clock spring from a single crystal material and a clock spring obtained by this method

Abstract

The invention provides a method for making a clock spring (1, 2) from a single crystal material, the method comprising the steps of: delineating the spring (1, 2), identifying one or more zones of weakness (4) in the spring (1, 2), in which, or at least in one of the zones of weakness (4), the spring (1, 2) will break in the event of excessive deformation, and manufacturing the spring (1, 2) from a single crystal material (5, 8, 11) extending in a predetermined plane (P), while orienting the spring (1, 2) so that, when the spring (1, 2) is deformed, the direction of microstresses in the or each zone of weakness (4) is substantially parallel to a cleavage plane of the material transverse to the predetermined plane (P). The invention also provides a clock spring (1, 2) obtained from such a method.

Description

[Technical field]

[0001] The present invention relates to a clock spring made of monocrystalline material, in particular monocrystalline silicon. The invention also relates to a method for manufacturing such a clock spring. The term "spring" means any elastically deformable element for receiving energy and / or generating a force or movement. [Background technology]

[0002] Monocrystalline silicon is a very popular material in mechanical watches due to its advantageous properties, in particular its low density, its high resistance to corrosion, its non-magnetic properties and its ability to be processed by micro-manufacturing techniques; monocrystalline silicon is used to manufacture hairsprings, balance wheels, flexibly guided oscillators, escapement anchors and escapement wheels.

[0003] However, this material has the disadvantage of low mechanical strength: it can easily break under external stress without prior plastic deformation.

[0004] To mitigate this disadvantage, it is customary to reinforce the silicon with silicon oxide, as proposed in patent application WO2007 / 00271.

[0005] In the case of a spring, it is important that its mechanical strength is as high as possible, since in addition to the shocks or accelerations to which it may be subjected during assembly in the movement or as the watch is worn, it must be able to deform without breaking in order to fulfil its function as a spring, and also to resist fatigue. Summary of the Invention

[0006] The object of the present invention is to propose a new approach to increase the strength of watches made of monocrystalline silicon, which may or may not be combined with an approach that consists in coating the silicon with silicon oxide or with any other processing method aimed at improving its mechanical strength.

[0007] More generally, it is an object of the present invention to provide a new approach to increasing the strength of clock springs made of single crystal material.

[0008] For this purpose, a method for making a clock spring from a single crystal material is provided, which method comprises the steps of: a) drawing said spring; b) identifying one or more weakened zones of the spring, at which, or at least one of the one or more weakened zones, the spring will break in the event of excessive deformation; c) manufacturing the spring from a single crystal material extending in a predetermined plane while orienting the spring such that when the spring is deformed the direction of microstress in the or each zone of weakness is substantially parallel to a cleavage plane of the material across the predetermined plane.

[0009] The invention also provides a clock spring made of a single crystal material elastically deformable in a given plane, said single crystal material comprising one or more weakened zones, said spring breaking in at least one of said one or more weakened zones in the event of excessive deformation, characterized in that the direction of the microstresses in the or each of said weakened zones when said clock spring is deformed is substantially parallel to a cleavage plane of said material across said given plane.

[0010] The invention further provides a timepiece and a timepiece including such a spring.

[0011] Other features and advantages of the present invention will become apparent from the following detailed description which proceeds with reference to the accompanying drawings. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a top view of an example of a clock spring that can be made by the method according to the invention, the spring being shown in a deformed state, more precisely in a state of maximum deformation in normal operation, the tensile stress to which the spring is subjected being shown in grey shading. [Diagram 2] FIG. 2 is a perspective view of a portion of the clock spring shown in FIG. 1, with tensile stresses indicated in grey shading. [Diagram 3] 1 is a diagram illustrating the different steps of the method according to the invention; [Figure 4] A single crystal silicon wafer (100) with a wafer flat

[0110] is shown diagrammatically in a top view (FIG. 4(a)) and in a contour view (FIG. 4(b)). [Diagram 5] A single crystal silicon wafer (110) with a wafer flat

[0100] is shown diagrammatically in a top view (FIG. 5(a)) and in a profile view (FIG. 5(b)). [Figure 6] A single crystal silicon wafer (111) with a wafer flat

[0112] is shown diagrammatically in a top view (FIG. 6(a)) and in a contour view (FIG. 6(b)). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Figures 1 and 2 show a clock spring, in this case a rocker spring, comprising a rigid base 1 and an elastic arm 2. The rigid spring 1 is adapted to be attached to a fixed or movable frame of a clock movement, typically to a plate of the movement. The elastic arm 2 extends from the rigid base 1 and performs a spring function. In use, the elastic arm 2 works in bending and acts by its free end on the rocker to return it to a predetermined angular position. To manufacture such a spring in monocrystalline silicon, the invention proposes, according to a particular embodiment, a method as shown in Figure 3 and described below, comprising steps E1-E3.

[0014] The spring is first drawn by computer-aided design (step E1), taking into account the functions it is intended to perform and the place it is intended to occupy in the movement.

[0015] The magnitude and direction of the microstresses that the spring encounters when it is loaded in bending under its normal conditions of use are then calculated by the finite element method (step E2). This calculation takes into account the dimensions of the spring and the elastic properties of the material (elastic modulus and Poisson's ratio). In the case of anisotropic monocrystalline silicon, the average elastic modulus and the average Poisson's ratio can be used at this stage. Since silicon is much less resistant in tension than in compression, it can be restricted to the side of the elastic arm 2 where tension acts during bending, i.e. to the right in FIG. 1. The area 4 of the spring where the tensile stress is the highest is the zone of weakness where the spring becomes susceptible to breakage as from a certain force applied to the free end of the spring. This calculation in step E2 can be carried out for the maximum deformation state of the spring during normal operation of the spring in a movement. However, experience has shown that the location of the zone of weakness 4 and the direction of the microstresses in this zone change very little with the degree of deformation.

[0016] The springs are then produced by etching, for example by deep reactive ion etching (DRIE) or laser etching, such that the springs have a specific orientation in the wafer, i.e. such that the direction of stress D in the weakened zone 4 is parallel to the cleavage plane of the monocrystalline silicon, intersecting the mean plane P along which the wafer stretches and along which the springs will be deformed during use.

[0017] Single crystal silicon has a diamond-like cubic quartz structure with (i) one atom at each of the eight vertices of the cube, (ii) one atom at the center of each of the cube's faces, and (iii) one atom at every four of the cube's eight tetrahedral sites, i.e., one atom at the center of the tetrahedron formed by one corner of the cube and three atoms at the centers of the three adjacent faces of that corner. The most atomically dense crystal planes are cleavage planes, i.e., planes of weakness along which the material breaks when subjected to excessive stress. In the case of single crystal silicon, the cleavage planes are the {111} family of planes.

[0018] FIG. 4 shows a monocrystalline wafer 5 cut in the (100) plane with a wafer flat 6 oriented in the

[0110] direction. The dotted line 7 represents the intersection of the (100) plane with the planes of the {111} family. FIG. 5 shows a monocrystalline wafer 8 cut in the (111) plane with a wafer flat 9 oriented in the (110) direction. The dotted line 7 represents the intersection of the (100) plane with the planes of the {111} family. The dotted line 10 represents the intersection of the (110) plane with the planes of the {111} family. Finally, FIG. 6 shows a monocrystalline wafer 11 cut in the (111) plane with a wafer flat 12 oriented in the

[0112] direction. The dotted line 13 represents the intersection of the (111) plane with the other planes of the {111} family. Said intersections 7, 10 and 13 of the planes along which the wafer is cut with the planes of the {111} family constitute the cleavage directions.

[0019] Thus, for example, in step E3 of the method according to the invention, the spring is oriented in the wafer such that the direction D of stress in the weakened zone 4 is parallel to one of the cleavage directions 7 if the spring is made of silicon (100) with a

[0110] wafer flat, parallel to one of the cleavage directions 10 if the spring is made of silicon (110) with a

[0100] wafer flat, and parallel to one of the cleavage directions 13 if the spring is made of silicon (111) with a

[0112] wafer flat.

[0020] The springs shown in figures 1 and 2 have a single zone of weakness. Other spring shapes can have several zones of weakness where the tensile stress is maximum and where the direction of the stress differs from one another. If such a configuration is found in step E2, the spring is oriented in the silicon wafer so that the direction of the stress in each of the zones of weakness is parallel to one of the cleavage planes. If this is not possible due to the crystallographic orientation of the wafer, the spring is drawn again (step E1) and the stresses are recalculated (step E2) until a shape of the spring is found that includes a single zone of weakness or zones of weakness whose respective stress directions can each be parallel to one of the cleavage planes. To change the position of the zones of weakness and thus the direction of the stress, the thickness of the elastic blade 2 can be changed. Instead of or in addition to changing the shape of the spring, a monocrystalline wafer can be selected that is cut in another crystallographic plane.

[0021] Between steps E2 and E3, the method according to the invention can include intermediate steps consisting in recalculating the stress in the spring based on the exact elastic properties taking into account the anisotropy of the material and the chosen orientation in the wafer, and in modifying the dimensions and / or shape of the spring, respectively, in order to obtain the desired stiffness and the desired breaking stress. These intermediate steps can be carried out iteratively in order to refine the spring properties, if the modification of the spring is such as to change the direction of the maximum stress of the spring in the wafer and therefore the choice of its orientation.

[0022] The mechanical strength of clock springs made according to the invention is significantly increased compared to springs in which the direction of maximum stress of the spring is not parallel to any of the cleavage planes. In particular, tests carried out on two batches of almost 30 samples, each made of (100) silicon coated with silicon oxide and subjected to bending stress, showed that the median fracture stress was about 3.4 GPa when the stress in the weakened zone was at 45° to the cleavage planes, and about 4.7 GPa when the stress in the weakened zone was oriented along the cleavage planes. This difference is much greater than the improvement that could be achieved by the difference in the elastic modulus of the two orientations of the sample. Such a result is surprising, since it could have been expected that the mechanical stress would be reduced by applying stress to the cleavage planes as contemplated by the invention. One possible explanation is that the fracture initiates from a microcrack whose tip is subjected to a microstress that does not have the same direction as the microstress in the weakened zone.

[0023] As mentioned above, the monocrystalline silicon springs made according to the invention can be coated with a reinforcing layer of silicon oxide. The thickness of such a layer is typically at least 0.5 μm, for example 0.5 μm to 5 μm. Other types of reinforcing layers and / or other treatments to further increase the mechanical strength, for example smoothing the surface of the spring, can be considered.

[0024] The improvement in mechanical strength provided by the present invention can help to reduce the spring size for a given force applied under normal conditions, thus reducing the space that the spring takes up in the clock movement.

[0025] The invention can be applied to various types of clock springs, in particular rocker springs, hammer springs, lever springs, jumpers, flexible guiding devices (for example parallel blades with translational guiding or flexible pivots, in particular oscillator flexible pivots), or to elastic parts of clock components (for example gear wheels or collets) used to mount said clock components on a support member. The invention can be applied to the elastic arms of a hairspring collet, shown in figure 10B of patent application EP 2 175 328.

[0026] The single crystal material of the springs made according to the present invention is not necessarily silicon, in alternative embodiments of the present invention the single crystal material can be diamond, aluminum oxide (e.g. sapphire or ruby), or silicon carbide.

[0027] Springs made according to the invention can be used, for example, in wristwatch, pocket watch or miniature clock movements.

Claims

1. A method for making a clock spring from a single crystal material, the method comprising the steps of: a) drawing said clock springs (1, 2); b) identifying one or more weakened zones (4) of said clock spring (1, 2), at which or at least one of said one or more weakened zones (4) said clock spring (1, 2) will break in the event of excessive deformation; c) manufacturing a clock spring (1, 2) from a wafer of single crystal material (5, 8, 11) extending in a predetermined plane (P), while orienting the clock spring (1, 2) such that when said clock spring (1, 2) is deformed the direction (D) of microstress in the or each zone of weakness (4) is substantially parallel to a cleavage plane of said single crystal material transverse to said predetermined plane (P).

2. 2. The method of claim 1, wherein step b) comprises the step of calculating the microstresses experienced by the clock spring when the clock spring is deformed.

3. Between steps b) and c), i) selecting an orientation for said clock spring in said wafer of single crystal material such that, when said clock spring (1, 2) is deformed, the orientation (D) of the microstresses in the or each zone of weakness (4) is substantially parallel to a cleavage plane of the single crystal material transverse to said predetermined plane (P); ii) modifying the clock spring based on the elastic properties of the single crystal material taking into account the anisotropy of the single crystal material and selecting the orientation of the clock spring on the wafer and the clock spring on the wafer.

4. Method according to any one of claims 1 to 3, wherein the one or more weakened zones (4) are located in parts of the clock spring (1, 2) which are subject to tensile stress when the clock spring (1, 2) is deformed.

5. Method according to any one of the preceding claims, wherein the clock spring (1, 2) comprises only one said weakened zone (4).

6. The method according to any one of claims 1 to 5, wherein the single crystal material is silicon, diamond, aluminium oxide or silicon carbide.

7. The method according to any one of claims 1 to 6, wherein the clock spring is a rocker spring, a hammer spring, a lever spring, a jumper, a flexible guiding device or an elastic part of a clock component used for mounting this clock component on a support member.

8. Method according to any one of claims 1 to 7, comprising an additional processing step for increasing the mechanical strength of the clock spring, for example a step for forming a reinforcing layer on the clock spring.

9. A clock spring (1, 2) made of a single crystal material elastically deformable in a given plane, said single crystal material comprising one or more weakened zones (4), said clock spring (1, 2) breaking in at least one of said one or more weakened zones (4) in the event of excessive deformation, characterized in that in said clock spring (1, 2) the direction (D) of the microstresses in the or each of said weakened zones (4) when the clock spring (1, 2) is deformed is substantially parallel to a cleavage plane of said single crystal material transverse to said given plane.

10. 10. A clock spring according to claim 9, wherein said one or more weakened zones (4) are located in parts of said clock spring (1, 2) which are subjected to tensile stress when said clock spring (1, 2) is deformed.

11. 11. The clock spring according to claim 9 or 10, wherein the clock spring (1, 2) comprises only one said weakened zone (4).

12. A clock spring according to any one of claims 9 to 11, wherein the monocrystalline material is silicon, diamond, aluminium oxide or silicon carbide.

13. A clock spring according to any one of claims 9 to 12, wherein the clock spring is a rocker spring, a hammer spring, a lever spring, a jumper, a flexible guiding device or an elastic part of a clock component used to mount this clock component on a support member.

14. A clock spring according to any one of claims 9 to 13, which is coated with a reinforcing layer.

15. A clock movement comprising a clock spring (1, 2) according to any one of claims 9 to 14.

16. A timepiece, in particular a wristwatch or pocket watch, comprising a timepiece spring (1, 2) according to any one of claims 9 to 14 or a timepiece movement according to claim 15.

Images