Manufacturing method of a monoblock device with silicon spring tongues for a clockwork movement

Description

Field of invention

[0001] The invention relates to a method of manufacturing a single-piece flexible silicon blade device, in particular for watchmaking, for example a crossed blade pivot used as a compensator cooperating with a known inertia balance to form a resonator comprising a predetermined frequency. Background of the invention

[0002] Oscillators based on flexible blades are quite commonly formed from silicon and thus benefit from the micromachining performance of this material (DRIE). A particular category of these oscillators is that of so-called "crossed blade" pivots, for which two blades are formed in two separate layers of two substantially parallel planes and cross each other. These pivots allow an element to oscillate in rotation, for example a balance wheel of an oscillator mechanism or an anchor of an escapement mechanism.

[0003] There are several ways to manufacture such pivots. One method is to manufacture the two blades separately and assemble them, leaving a space between them. This method allows for sufficient space between the blades to prevent them from colliding. However, the arrangement of the blades relative to each other is not always precise, which reduces the pivot's performance in terms of chronometry.

[0004] A second method consists of manufacturing a single-piece pivot by machining two layers of an SOI silicon wafer. This method is very effective for arranging the blades relative to each other, but it does not allow for a sufficiently large safety gap between the blades to prevent collision during operation of the pivot. Indeed, the gap depends on the thickness of the oxide binding the two layers of the SOI wafer, the thickness being much less than the required safety gap. An example of application of this second method is described in document EP3326963A1. Summary of the invention

[0005] The aim of the present invention is to overcome all or part of the drawbacks mentioned above by proposing a method for manufacturing a single-piece flexible silicon blade device, the safety space between the planes of the blades of which is sufficient, in particular in width.

[0006] To this end, the invention relates to a method of manufacturing a single-piece flexible silicon blade device, for example a crossed blade pivot, the method being defined in claim 1.

[0007] This results in a flexible blade device, for example a crossed blade pivot, with sufficient safety space between the blades to prevent blade collision during device operation. Indeed, when a silicon oxide layer grows on silicon, a silicon sub-layer is itself oxidized, so that when the silicon oxide layer is removed by etching, the silicon sub-layer has been removed from the initial silicon mass. The final silicon volume is therefore reduced compared to the initial silicon volume.

[0008] Thus, thanks to this effect, the space between the blades can be enlarged by repeating the operations of growing silicon oxide, then removing the silicon oxide layer. Each time a silicon oxide layer is removed, the space widens.

[0009] According to a particular embodiment of the invention, the method comprises the following steps: growing a second layer of silicon oxide on the surface of at least one of the blades bordering the gap, the second layer of silicon oxide being formed from a second silicon sub-layer of the at least one blade, removing the second layer of silicon oxide to further enlarge the gap between the two blades.

[0010] According to a particular embodiment of the invention, the successive steps of growing silicon oxide layers and removing the layer are repeated several times to enlarge the space between the two blades to achieve a desired width.

[0011] According to a particular embodiment of the invention, the device blank comprises crossed blades joined at the crossing by a joint, the joint being formed at least in part of silicon oxide, the method comprising a step of removing the silicon oxide from the joint between the blades to separate them by creating said space between the blades.

[0012] According to a particular embodiment of the invention, each layer of silicon oxide is removed by etching using hydrogen fluoride in the vapor phase.

[0013] According to a particular embodiment of the invention, the growth of silicon oxide is carried out by wet or dry thermal oxidation of silicon.

[0014] According to a particular embodiment of the invention, the device blank is carried out by deep reactive ion etching of the DRIE type, for example carried out using chemical etching.

[0015] According to a particular embodiment of the invention, each growth and elimination of a silicon oxide layer makes it possible to remove a silicon sub-layer of at least 0.10 µm thickness on a blade, preferably at least 0.40 µm.

[0016] According to a particular embodiment of the invention, the method comprises an additional step of growing an additional oxide layer on the device to thermally adjust the stiffness of the device as a function of the temperature, in particular to compensate in temperature an oscillator formed of a balance pivot assembly, and / or to reinforce the device.

[0017] According to a particular embodiment of the invention, the method comprises an additional step of depositing an electrically conductive layer to avoid problems linked to the accumulation of electrostatic charges or absorption of humidity.

[0018] According to a particular embodiment of the invention, the method comprises a step of determining the initial stiffness of the device and calculating its dimensions to obtain a pivot of a desired final stiffness.

[0019] According to a particular embodiment of the invention, the step of forming the device blank comprises the following sub-steps: providing an SOI wafer successively comprising a first silicon layer, a silicon oxide bonding layer, and a second silicon layer; growing a silicon oxide layer on the surface of the wafer; etching the silicon oxide layer on a first side of the wafer through a previously formed mask; performing deep reactive ion etching to form at least a first blade of the single-piece flexible silicon blade device; etching the silicon oxide layer on a second side of the wafer through a second previously formed mask, preferably aligned with respect to the patterns formed on the first side of the wafer, performing deep reactive ion etching to form at least a second blade of the single-piece flexible silicon blade device.

[0020] According to a particular embodiment of the invention, the minimum space obtained between the blades after the steps of the method has a width greater than 10 µm, preferably greater than 15 µm. Summary description of the drawings

[0021] Other features and advantages will become clear from the description given below, for information purposes only and in no way limiting, with reference to the attached drawings, in which: there figure 1 illustrates a top view of a cross-blade pivot according to the invention, the figure 2 illustrates a perspective view of a cross-blade pivot according to the invention, the figure 3 illustrates a side view of a crossed blade pivot obtained using the method according to the invention, the figure 4 is a block diagram illustrating the different stages of a manufacturing process for a flexible pivot with monobloc crossed blades made of silicon according to the invention, the Figure 5illustrates a sectional view of a wafer allowing the manufacture of the pivot, the figure 6 illustrates a side view of a cross-blade pivot obtained after the first step of the method according to the invention, the figure 7 illustrates a side view of a cross-blade pivot obtained after the second step of the method according to the invention, the figure 8 illustrates a side view of a cross-blade pivot obtained after the third step of the method according to the invention, the figure 9 illustrates a side view of a cross-blade pivot obtained after the fourth step of the method according to the invention, the figure 10 illustrates a side view of a cross-blade pivot obtained after the fifth step of the method according to the invention, the figure 11 illustrates a side view of a cross-blade pivot obtained after the sixth step of the method according to the invention. Detailed Description of Preferred Embodiments

[0022] The invention relates to a single-piece flexible silicon blade device, in particular a crossed blade pivot 1 visible in figures 1 to 3 , as well as its manufacturing process allowing to obtain a single-piece pivot whose space between the blades is wide enough to avoid the blades colliding during use.

[0023] Such a pivot 1 comprises a first 5 and a second element 6, as well as two crossed blades 2, 3, the two crossed blades 2, 3 connecting the two elements 5, 6 to each other. The two blades 2, 3 are connected to each element 5, 6, the pivot 1 being in one piece. The first element 5 is for example a support for fixing the pivot to the watch movement, and the second element 6 is a component requiring to be pivoted. Such a component is for example an escapement anchor, a balance or a balance support of a watch movement. The two blades 2, 3 cross between the two elements 5, 6 at a crossing 4. Thanks to the flexible blades 2, 3, the second element 6 is movable relative to the first element 5 around a virtual axis. Thus, the first element 5 is fixed on the clockwork, while the second element 6 performs a periodic back-and-forth movement.The blades 2, 3 bend laterally, to allow the movement of the second element 6 in one direction, then in the other direction.

[0024] The blades 2, 3 are arranged in two parallel planes separated by a minimum space 7 to avoid collision of the blades 2, 3 during operation of the flexible pivot, particularly in the event of parasitic deformation.

[0025] The use of a material, for example silicon-based, for the manufacture of a pivot 1 offers the advantage of being precise using existing engraving methods and of having good mechanical and chemical properties, in particular being little or not sensitive to magnetic fields.

[0026] Preferably, the silicon-based material used may be monocrystalline silicon regardless of its crystalline orientation, doped monocrystalline silicon regardless of its crystalline orientation, amorphous silicon, porous silicon, polycrystalline silicon, silicon nitride, silicon carbide, quartz regardless of its crystalline orientation or silicon oxide.

[0027] Thus, the invention relates to a method of manufacturing 20 a pivot 1 with flexible crossed blades 2, 3 made of silicon visible in the figure 4 .

[0028] The method 20 comprises a first step 21 of manufacturing a silicon pivot blank. This step 21 consists of providing SOI wafers 10, shown in Figure 5, which is composed of two layers of silicon 11 and 12, bonded to each other by a first layer of silicon oxide 13. The layers of silicon 11, 12 are formed in a monocrystalline silicon plate (whose main orientations can be varied). Each layer of silicon 11, 12 will make it possible to manufacture a different blade of the pivot 1 with crossed blades 2, 3. Thus, the blades 2, 3 are arranged on two different levels. The first layer of oxide 13 makes it possible to intimately bond the two layers of silicon 11 and 12. In addition, it will also serve as a stop layer during subsequent operations.

[0029] Multiple pivot blanks can be formed in the same wafer.

[0030] Then, a second layer of silicon oxide 14 is grown on the surface of the silicon layers 11, 12, by exposing the wafer(s) to a high-temperature oxidizing atmosphere. The layer of silicon oxide 14 varies according to the thickness of the silicon layer 11, 12 to be structured. It is typically between 1 and 4 µm.

[0031] Then, we will define, for example in a positive resin, the patterns that we wish to subsequently produce in the silicon wafer 10 using a mask. The pattern includes at least one blade of the pivot. The two blades of the same pivot are each produced on one of the two separate silicon layers 11, 12. Each blade is produced one after the other.

[0032] This step 21 includes the following operations: the resin is deposited, for example using a spin coater, in a very thin layer with a thickness of between 1 and 2µm. 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, thus revealing the first oxide layer 13. In this case, the areas still covered with resin define the areas that are not to be attacked in the subsequent operation of deep reactive ion etching (also known by the abbreviation "DRIE") of the silicon.

[0033] The exposed areas or, on the contrary, areas covered with resin are then exploited. A first etching process allows the patterns defined in the resin in the previous steps to be transferred into the previously grown silicon oxide 14. Still with a view to 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.

[0034] Masks can also be used directly on the wafer, which are called hard masks, such as silicon oxide, silicon nitride or metal masks. These masks are pre-structured with the appropriate pattern and may or may not include resin when used.

[0035] Once the silicon oxide has been etched into the open areas of the resist, the silicon surface of the first top layer 11 is then exposed and ready for DRIE etching. The resist may or may not be retained depending on whether the resist is to be used as a mask during DRIE etching.

[0036] The exposed silicon, not protected by silicon oxide, is etched in a direction perpendicular to the wafer surface (DRIE Bosch ® anisotropic etching). The blade patterns formed first in the resin, then in the silicon oxide, are "projected" into the thickness of layer 11.

[0037] When the etching reaches the first layer of silicon oxide 13 linking the two layers of silicon 11 and 12, the etching stops. Indeed, like the silicon oxide serving as a mask during the Bosch ®< process and resisting the etching itself, the buried oxide layer 13, of the same nature, also resists it.

[0038] The first silicon layer 11 is then structured throughout its thickness by the defined patterns representing at least a first blade of the pivot blank to be manufactured, now revealed by this DRIE etching. Chemical etching could also be used in the same silicon-based material.

[0039] The blades remain attached to the second layer of silicon 12 to which they are bonded by the first buried layer of silicon oxide 13.

[0040] The same photolithography operations are carried out on the second silicon layer 12, in order to form the other blade of the crossed blade pivot. To do this, the wafer 10 is turned over, the resin is deposited there, then exposed through a mask, in order to structure the second silicon layer 12, in a similar manner to the first layer 11.

[0041] There are other embodiment variants for etching the wafer, which can of course be used for this process, whether with resin masks or hard masks, known as hard-masks.

[0042] The first and second elements 5, 6 of the pivot 1 are preferably also formed during the manufacture of the blades 2, 3 on each silicon layer 11, 12 of the wafer 10.

[0043] At this stage the two blades 2, 3 are joined to each other at the intersection 4 of the blades 2, 3, so as to form a joint 8. The joint 8 comprises at least in part silicon oxide from the silicon oxide layer 13 connecting the first and second silicon layers 11, 12. Preferably, the joint 8 is formed entirely of silicon oxide. The pivot blank 1 is shown in the figure 6 The blades 2, 3 are arranged in two different substantially parallel planes, the blades 2, 3 being joined between the two planes by the joint 8.

[0044] A second step 22 of the method 20 consists of removing the silicon oxide at the joint 8 of the blades 2, 3 to separate them by creating a space 7 between the blades 2, 3. The silicon oxide layer 13 is then etched using hydrogen fluoride in the vapor phase.

[0045] Thus, two blades 2, 3 are obtained separated by a space 7 allowing the movement of the blades relative to each other. The space 7 obtained has a width D corresponding to the thickness of the oxide layer of the joint, and therefore of the oxide layer of the wafer.

[0046] However, this thickness is not large enough to avoid the risk of the blades 2, 3 colliding with each other during operation of the pivot 1. In fact, the first layer of silicon oxide 13 is too thin for the blades 2, 3 to be sufficiently spaced apart.

[0047] The method 20 continues with a sequence intended to remove material from the blades up to the dimensions necessary to obtain a space 7 of sufficient width D, as well as adequate blade stiffness. To this end, the thickness of the blades 2, 3 is reduced during the following steps.

[0048] To increase the width D of the space 7 between the blades 2, 3, a third step 23 consists of growing a first layer of silicon oxide 15 on the surface of the blades 2, 3 bordering the space 7. The first layer of silicon oxide 15 is formed in part from a sub-layer of silicon forming the pivot 1. Indeed, during the growth of silicon oxide on silicon, a sub-layer of silicon on which the silicon oxide grows is itself oxidized. Thus, the silicon oxide grows at the expense of the silicon on which it grows. In other words, not only does the silicon oxide grow on the silicon, but it also grows in the silicon.

[0049] 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 dioxygen gas to form silicon oxide on the blades. During this step 23, the fact that the silicon oxide grows regularly is exploited, the oxidation speed and the resulting thickness are perfectly controlled by the person skilled in the art, which makes it possible to ensure the uniformity of the oxide layer.

[0050] In a fourth step 24, the first layer of silicon oxide 15 is removed from each blade 2, 3, to obtain the pivot 1 shown in the figure 9. The silicon sub-layer being oxidized, it is also removed. Thus, the space 7 between the two blades 2, 3 is enlarged. Such removal is achieved by chemical etching. Such chemical etching can be carried out, for example, by means of a solution based on hydrofluoric acid in the vapor phase, which makes it possible to remove the silicon oxide.

[0051] Preferably, the successive pairs of steps of growing silicon oxide layers and removing the oxide layer are repeated several times. Thus, the width D of the space 7 between the two blades 2, 3 can be enlarged to achieve a desired space width.

[0052] For example, as shown in the figure 10, a fifth step 25 consists of growing a second layer of silicon oxide 17 on the surface of the blades 2, 3 on either side of the space, the second layer of silicon oxide 17 being formed in part from a second sub-layer of silicon of the blades 2, 3. Then, in a sixth step 26, the second layer of silicon oxide 17 is eliminated from each blade 2, 3 to further enlarge the space 7 between the two blades 2, 3.

[0053] Each growth and removal of silicon oxide layer allows, for example, to remove a silicon sub-layer of at least 0.10 µm thickness on each blade 2, 3, preferably greater than 0.40 µm. The desired minimum width D of the space 7 between the blades 2, 3 after the steps of the method is, for example, greater than 10 µm, preferably greater than 15 µm. Nevertheless, the width is freely adjustable between a value corresponding to the thickness of the initial oxide layer binding the two silicon layers and a higher value which can go beyond 15 µm, the higher value being limited by the number of operations which one wishes to carry out.

[0054] The silicon oxide layer 13 of the wafer 10 initially has, for example, a thickness of 2 µm, which defines the initial space between the blades 2, 3 after the removal of the joint 8. Thus, to obtain a space of at least 10 µm, a plurality of pairs of silicon oxide growth and removal steps are carried out in series. To reduce the number of operations, the oxidation time of the blades can be extended, so that a thicker silicon sub-layer is removed. Preferably, shorter growth steps are carried out at the beginning and increasingly longer thereafter.

[0055] The method may further comprise an additional step 27 intended to determine the initial stiffness of the flexible blades, in particular to be able to modify the dimensions and obtain the specific physical characteristics sought.

[0056] Finally, once the pivot 1 is the right dimensions, another additional optional step 28 may consist of re-oxidizing the pivot 1 to coat it with a layer of silicon dioxide in order to form a pivot 1 which is thermo-compensated and / or to strengthen the pivot. In the case of an oscillator, this last oxidation makes it possible to adjust both the mechanical (stiffness) and thermal (temperature compensation) performances of the future pivot 1.

[0057] In another additional optional step, not shown in the figures, an electrically conductive layer is deposited on the pivot to avoid problems related to the accumulation of electrostatic charges or moisture absorption. For this purpose, an oxide layer is first deposited on the pivot 1, then the electrically conductive layer is deposited by a PVD type deposition. The electrically conductive layer comprises, for example, chromium, nickel, copper, titanium, zirconium, nickel-phosphorus or titanium-tungsten.

[0058] 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, according to the scope defined by the claims. Thus, it is possible to produce other types of devices with single-piece flexible silicon blades, the device having for example non-crossed blades, and / or which serve for a translational movement instead of a rotational movement.

Claims

1. Method (20) for manufacturing a one-piece silicon watchmaking device (1) with flexible blades (2, 3), in particular for timepieces, for example a pivot with crossed blades, comprising the following steps of: - forming (21) a one-piece silicon watchmaking device (1) blank from a wafer of the SOI type, the watchmaking device (1) comprising two flexible blades (2, 3), each formed in a different layer of the SOI wafer, the blades (2, 3) being arranged in two different substantially parallel planes, the blades (2, 3) being separated by a clearance (7), the method being characterised in that it also comprises the following steps of : - growing (23) a first silicon oxide layer (15) on the surface of at least one of the blades (2, 3) bordering the clearance (7), the first silicon oxide layer (15) being formed from a first sub-layer of silicon of the one or more blades (2, 3), - removing (24) the first silicon oxide layer (15) to increase the clearance (7) between the two blades (2, 3).

2. Manufacturing method according to claim 1, characterised in that it comprises the following steps of: - growing (25) a second silicon oxide layer (17) on the surface of at least one of the blades (2, 3) bordering the clearance (7), the second silicon oxide layer (17) being formed from a second sub-layer of silicon of the one or more blades (2, 3), - removing (26) the second silicon oxide layer (17) to further increase the clearance between the two blades (2, 3).

3. Manufacturing method according to claim 1 or 2, characterised in that the successive steps of growing (23, 25) silicon oxide layers (15, 17) and of removing (24, 26) the layer (15, 17) are repeated several times to increase the clearance (7) between the two blades (2, 3) in order to reach a desired width (D).

4. Manufacturing method according to any of the preceding claims, characterised in that the watchmaking device (1) blank comprises crossed blades (2, 3) joined at the crossover point (4) by a join (8), the join (8) being at least partially made of silicon oxide, the method comprising a step (22) of removing the silicon oxide from the join (8) between the blades (2, 3) to separate same by creating said clearance (7) between the blades (2, 3).

5. Manufacturing method according to any of the preceding claims, characterised in that each silicon oxide layer (13, 14, 15, 17) is removed by etching using hydrogen fluoride in the vapour phase.

6. Manufacturing method according to any of the preceding claims, characterised in that the silicon oxide is grown (23, 25) by wet or dry thermal oxidation of the silicon.

7. Manufacturing method according to any of the preceding claims, characterised in that the watchmaking device (1) blank is produced by deep reactive ion etching (DRIE).

8. Manufacturing method according to any of the preceding claims, characterised in that each growth (23, 25) and removal (22, 24) of a silicon oxide layer (15, 17) enables a sub-layer of silicon that is at least 0.10 µm thick, preferably at least 0.40 µm thick, to be eliminated from a blade (2, 3).

9. Manufacturing method according to any of the preceding claims, characterised in that it comprises an additional step (27) of growing an additional oxide layer on the watchmaking device (1) to thermally adjust the stiffness of the watchmaking device as a function of temperature, in particular to temperature-compensate an oscillator formed by a balance pivot assembly, and / or to reinforce the watchmaking device.

10. Manufacturing method according to any of the preceding claims, characterised in that it comprises a step (28) of determining the initial stiffness of the pivot (1) and of calculating the dimensions thereof to obtain a watchmaking device (1) of a desired final stiffness.

11. Manufacturing method according to any of the preceding claims, characterised in that it comprises an additional step of depositing an electrically conducting layer.

12. Manufacturing method according to any of the preceding claims, characterised in that the step (21) of forming the watchmaking device (1) blank comprises the following sub-steps of: - procuring an SOI wafer (10) successively comprising a first silicon layer (11), a silicon oxide bonding layer (13), and a second silicon layer (12); - growing a silicon oxide layer (14) on the surface of the wafer (10); - etching the silicon oxide layer (14) on a first side of the wafer (10) through a mask; - carrying out a deep reactive ion etching process to form at least a first blade (2) of the one-piece silicon watchmaking device (1) with flexible blades; - etching the silicon oxide layer (14) on a second side of the wafer (10) through a second mask formed beforehand, preferably aligned with the patterns formed on the first side of the wafer; - carrying out a deep reactive ion etching process to form at least a second blade (3) of the one-piece silicon watchmaking device (1) with flexible blades.

13. Manufacturing method according to any of the preceding claims, characterised in that the width (D) of the minimum clearance (7) obtained between the blades (2, 3) after the steps of the method is greater than 10 µm, preferably greater than 15 µm.