Piezoelectric spiral spring and method for manufacturing said spiral spring

Abstract

The piezoelectric spiral spring (3) is intended for a self-regulating circuit (10) for the oscillation frequency of an oscillating mechanical system (2, 3), or for an energy recovery circuit, or for a motor circuit for actuation of the movement or its automatic maintenance. The piezoelectric spiral spring (3) comprises at least one piezoelectric layer (7, 7', 17, 17', 27, 27') disposed on a top (20) or bottom face of a certain number of coils of the spring in planar orientation, and at least two pairs of electrodes (8a, 8b, 8c, 8d) where the electrodes of each pair are disposed on two opposite faces of the same piezoelectric layer or respectively of two piezoelectric layers so as to apply a reverse bias voltage to each pair of electrodes.

Description

Technical field of the invention

[0001] The invention relates to a piezoelectric spiral spring for a circuit for self-regulating the oscillation frequency of an oscillating mechanical system or an energy recovery circuit or a motor circuit for actuating the movement or its automatic maintenance.

[0002] The invention also relates to a method of manufacturing a piezoelectric spiral spring. State of the art

[0003] In the field of watchmaking and from a mechanical point of view, the oscillating mechanical system can be a balance wheel on which a spiral spring is mounted, one end of which is fixed to the axis of rotation of the balance wheel and the other end is fixed to a fixed element of a plate. The mechanical system is kept in oscillation by means of a generally mechanical energy source, which can be a barrel driving a gear train with an escape wheel cooperating with a rotating anchor. The balance wheel with the spiral spring coupled to the escapement can thus form a regulating organ of a watch movement. A significant space in the watch case is used with this totally mechanical regulation which can be a disadvantage in certain cases.

[0004] Patent FR 2 119 482 describes an oscillating mechanical system of a piezoelectric element. This piezoelectric element is preferably arranged on a spiral spring connected to a balance. To do this, it is specified to deposit films of piezoelectric material (PZT) over most of the length of the spring and on an inner face and an outer face of said metal spring. A voltage converter makes it possible to supply an alternating voltage to the piezoelectric element to alternately generate a compression force and an extension force to the spring in order to adjust the oscillation of the balance connected to the spiral spring. In this patent document, it is provided to arrange electrodes along the length of the spring and on each lateral face, which can complicate its production, which constitutes a disadvantage.

[0005] THE Figures 1 and 2of the prior art represent a device 1, which comprises an oscillating mechanical system 2, 3 and a self-regulating circuit 10 of the oscillation frequency fosc of the oscillating mechanical system as described in patent EP 2 590 035 B1. In a mechanical watch, the oscillating mechanical system comprises a balance 2, which is formed of a metal ring connected for example by three arms 5 to an axis of rotation 6, and a spiral spring 3, on which is arranged a piezoelectric element or an electroactive polymer element. A first end 3a of the spiral spring 3 is held fixed by a stud 4 of a balance bridge. This balance bridge is fixed to the plate of the watch movement. A second end 3b of the spiral spring 3 is fixed directly to the axis of rotation 6 of the balance. Piezoelectric or electroactive polymer layers 23, 23' are deposited on two lateral faces of the metal strip 24, which can complicate its production.

[0006] As represented in figure 3of the prior art, patent EP 3 629 103 B1 describes a piezoelectric spiral spring 70 of a timepiece. The spiral spring 70, shown in cross-section, comprises a central body 72 made of silicon, a silicon oxide layer 74 deposited on the surface of the central body to thermally compensate the spiral spring, a conductive layer 76 deposited on the silicon oxide layer, and a piezoelectric material deposited in the form of a piezoelectric layer 78 on the conductive layer 76. Two electrodes 20a and 22a are arranged on the piezoelectric layer 78 respectively on the two lateral sides of the spiral spring. A first portion 80a and a second portion 80b of the piezoelectric layer extend respectively on two lateral sides of the central body 72 and have respective crystallographic structures which are symmetrical relative to a median plane 84 parallel to these two lateral sides.In the two lateral parts 80a and 80b, the piezoelectric layer has two same respective piezoelectric polarization axes 82a, 82b which are perpendicular to the piezoelectric layer and in opposite directions. The production of such a piezoelectric spiral spring structure is complicated and time-consuming to produce, which constitutes a disadvantage.

[0007] In the state of the art, there are several technical difficulties in producing piezoelectric layers and contact electrodes on a spiral spring, for example. One problem may be encountered related to the shadowing effect, i.e., any deposition of one or more layers leading to thickness gradients. Short circuits may occur at the bottom of the turns depending on an insufficient piezoelectric layer thickness. This also leads to restrictions on the dimensions of the spiral spring, since it is necessary to provide a space between the turns and a sufficiently large aspect ratio to mitigate the shadowing effect.

[0008] In the case of jet or spray deposition of material ("sputtering" in English terminology), the piezoelectric layers may have a texture deviated by an angle of several tens of degrees from the normal to the sidewall. The piezoelectric effect is thus reduced, because only the projection on the direction of the electric field contributes to it.

[0009] It should also be noted that the resonance frequency of the balance spring is sensitive to the stiffness of the balance spring, which depends on its high cube thickness. Typical reproducibility of a deposit requires a final state of frequency adjustment after the deposition of the piezoelectric layers. In addition, material jet depositions have a thickness variation of several percent on the surface of the substrates, which complicates the precise correction of the thickness of a deposited layer.

[0010] Non-standard manufacturing processes for a spiral spring are often used, as it is necessary to deposit the layers on structured wafers and structure electrodes on the sides without damaging said layers. Mention may be made in this respect of document CH 714144 A2 and EP 3 457 233 A1, which describe a spiral spring formed from a layer or strip of piezoelectric material for a frequency self-regulation circuit. A first electrode is connected to the self-regulation circuit, and arranged on all or part of one face of the strip and a second electrode is connected to the self-regulation circuit and arranged on all or part of another face of the strip of piezoelectric material. Summary of the invention

[0011] The invention therefore aims to provide an easy-to-produce piezoelectric spiral spring for a self-regulating circuit for the oscillation frequency of an oscillating mechanical system to precisely regulate the oscillation frequency of the oscillating mechanical system, with a limited number of components and to overcome the aforementioned drawbacks of the state of the art. The piezoelectric spiral spring is also intended for an energy recovery circuit or for a motor circuit for actuating the movement or its automatic maintenance.

[0012] To this end, the invention relates to a piezoelectric spiral spring for a circuit for self-regulating the oscillation frequency of an oscillating mechanical system or an energy recovery circuit or a motor circuit for actuating the movement or its automatic maintenance, which comprises the characteristics mentioned in independent claim 1.

[0013] Particular forms of the spiral spring are defined in dependent claims 2 to 13.

[0014] An advantage of such a piezoelectric spiral spring according to the invention lies in the fact that it can be easily produced, because the deposition of the piezoelectric layer is easily controllable if it is deposited on a top or even bottom face. It is also easier to increase the thickness of the piezoelectric layers or to increase their length by depositing them on the top or bottom faces.

[0015] Advantageously, the edges of the spring coil are not affected, which are likely to cause problems with cracks in the piezoelectric layer, as everything is deposited on the top face, or even the bottom, the problems with cracks are reduced.

[0016] Another advantage of depositing a piezoelectric layer on a top face is that we benefit from a better normally perpendicular crystal orientation on the top face than on the side faces where the orientation is inclined. The homogeneity of the deposit on the entire wafer and on each turn is much greater. This does not depend on the space between the different turns of the spiral spring. On the other hand, for the deposition of a piezoelectric layer on the side faces, the smaller the space between the turns, the more difficult it is to deposit such a layer on these side faces. In this case, there is a shadowing effect for the deposition of the layers on the side faces which can also be too thin and where short circuits can appear for an insufficiently thick layer.It is therefore an advantage to deposit a piezoelectric layer on a top face to control manufacturing accuracy, and we limit the difference between designs and reality.

[0017] Another advantage is that it is now easier to deposit piezoelectric layers made of materials that are difficult to structure on the top face compared to the side faces. Generally speaking, a global manufacturing process from the top, including the structuring of the layers and the etching and protection of the structures, is easier than a non-standard structuring of the sides of the turns.

[0018] For this purpose, the invention also relates to a method of manufacturing a piezoelectric spiral spring, which comprises the features of independent claim 14.

[0019] According to the recommended method of the invention, a support in the form of a base plate, for example made of SOI (Silicon-on-Insulator) can be used as the first step. After this first step, the spring can be etched as explained in more detail below, or the electrodes, the piezoelectric layer and the other electrodes above the piezoelectric layer can be deposited and structured before the first silicon layer is etched. Brief description of the figures

[0020] The aims, advantages and characteristics of the piezoelectric spiral spring for a self-regulating circuit of the oscillation frequency of an oscillating mechanical system or an energy recovery circuit or a motor circuit for the actuation of the movement or its automatic maintenance, and the method of manufacturing the spiral spring, will appear better in the following description on the basis of non-limiting embodiments illustrated by the drawings in which: there figure 1 represents in a simplified manner a device, which comprises an oscillating mechanical system and a circuit for self-regulating the oscillation frequency of the oscillating mechanical system according to the prior art, the figure 2 represents a portion of a spiral spring of the oscillating mechanical system, which comprises a piezoelectric element of the device according to the prior art, the figure 3represents a cross-section of a coil of another type of piezoelectric spiral spring according to the prior art, the Figures 4a and 4b represent a three-dimensional partial view of a portion of the coil of the piezoelectric spiral spring and a cross-section of this coil of a first embodiment of the piezoelectric spiral spring according to the invention, the Figure 5 represents a cross-section of a coil of a second embodiment of the piezoelectric spiral spring according to the invention, the figure 6 represents a cross-section of a coil of a third embodiment of the piezoelectric spiral spring according to the invention, the figure 7 represents a cross-section of a coil of a fourth embodiment of the piezoelectric spiral spring according to the invention, and the figure 8represents a formation of turns of the piezoelectric spiral spring on a substrate base and with coverage of a piezoelectric layer on the turns to define the difference between a deposition of the layer on a top face, according to the present invention, compared to a layer on a side face. Detailed description of the invention

[0021] THE Figures 4a and 4b represent a three-dimensional partial view of a portion of the coil of the piezoelectric spiral spring 3 and a cross-section of this coil of a first embodiment of the piezoelectric spiral spring 3. Generally, the spiral spring 3 comprises several coils and is connected to a balance (not shown) to constitute an oscillating mechanical system. A first end of the spiral spring is fixed to a balance bridge, while the second end is fixed to the balance staff. The spiral spring 3 appears as if in a plane between its two ends.

[0022] The piezoelectric spiral spring 3 comprises in this first embodiment, on a top face of the spiral spring, two pairs of electrodes 8a, 8b, 8c, 8d of which the first electrodes 8a and 8b of the two pairs of electrodes side by side are fixed directly on the top face of the spiral spring. The first piezoelectric layer 7 is fixed between the first electrode 8a and the second electrode 8c of the first pair of electrodes, while the second piezoelectric layer 7' distinct from the first layer is fixed between the first electrode 8b and the second electrode 8d of the second pair of electrodes.

[0023] Preferably, silicon is etched on an SOI (or Quartz) wafer to obtain the shape of the spiral spring 3 with the insulator underneath, which comprises on the one hand a layer of SiO2 oxide and on the other hand a base silicon plate. The SOI or quartz wafer may advantageously be covered with an insulating layer of the SiO2 type, with a thickness of the order of 500 nm, to avoid any interference between the activation of the piezoelectric layers and the substrate used for the production of the spiral spring 3. The SOI wafer may have a thickness of the order of 500 µm. According to an alternative embodiment of the method for manufacturing the piezoelectric spiral spring, once the outline of the spiral spring 3 is obtained after etching, the electrodes 8a, 8b, 8c, 8d and the piezoelectric layers 7, 7' can be deposited and structured on a top face of the spiral spring.

[0024] It may also be envisaged to produce the spiral spring 3 on a glass wafer. Under these conditions, a laser-assisted chemical etching step is carried out on the glass wafer to obtain the spiral spring 3. Other types of substrates may be considered, such as ceramics or composites by adapting the manufacturing methods of the spiral spring.

[0025] The first electrode 8a of the first pair of electrodes and the first electrode 8b of the second pair of electrodes are arranged or deposited on the top face 20 of the piezoelectric spiral spring 3 in plan. The first electrodes 8a and 8b are regularly spaced from one another and each follow the shape of turns from a first end of the spiral spring and in the direction of the second end of the spiral spring. The first electrodes 8a and 8b of the two pairs of electrodes are of a substantially equivalent length and over a portion of the length of the spiral spring from the first end of said spiral spring. Preferably, the length of the first electrodes 8a, 8b of the two pairs of electrodes extends from the first end to a second end of the piezoelectric spiral spring 3.

[0026] The first piezoelectric layer 7 is deposited directly on the first electrode 8a of the first pair and preferably of a shape equivalent to said first electrode 8a over at least part of the length of the piezoelectric spiral spring 3. The second piezoelectric layer 7' is deposited directly on the first electrode 8b of the second pair of electrodes and preferably of a shape equivalent to said first electrode 8b over at least part of the length of the piezoelectric spiral spring 3.

[0027] Finally, the second electrode 8c of the first pair of electrodes is arranged or deposited directly on the first piezoelectric layer 7 on a face opposite that of the contact of the first electrode 8a with the first piezoelectric layer 7. The second electrode 8d of the second pair of electrodes is arranged or deposited directly on the second piezoelectric layer 7' on a face opposite that of the contact of the first electrode 8b with the second piezoelectric layer 7'. The shape and length of each second electrode 8c, 8d are equivalent to the shape and length of each first electrode 8a, 8b in this first embodiment.

[0028] Two variants of the manufacturing process of the piezoelectric spiral spring are provided. Since the spiral spring 3 comes from a silicon wafer (SOI) or a quartz wafer, it may be envisaged first of all to etch the silicon or quartz to obtain the base of the spiral spring 3. Subsequently, the electrodes 8a, 8b, 8c, 8d and the piezoelectric layers 7, 7' are deposited on a top or bottom face of the already structured spiral spring 3. In the case of a glass wafer, the base of the spiral spring 3 may first be cut by chemically assisted or non-chemically assisted laser from the top of the wafer.

[0029] According to an alternative embodiment, the electrodes 8a, 8b, 8c, 8d and the piezoelectric layers 7, 7' can already be deposited on the silicon or quartz wafer before structuring, that is to say before etching by a DRIE process to obtain the spiral spring or before laser-assisted chemical etching from above the glass wafer to obtain the spiral spring 3. More details corresponding to the manufacturing process of the piezoelectric spiral spring 3 according to the two variants briefly presented will be given later in the description.

[0030] As shown in the Figure 4a, the self-regulating circuit makes it possible to apply an adaptation voltage to generate a compression stress -T1 on a piezoelectric layer 7 or an inverse adaptation voltage on the other piezoelectric layer 7' generating an extension stress T1 continuously or by determined time periods. This makes it possible to regulate the oscillation frequency of the oscillating mechanical system.

[0031] First of all, a first electrode 8a of a first pair of electrodes and a first electrode 8b of a second pair of electrodes are arranged or structured both on a top face 20 and preferably over a large part of the length of the spiral spring 3 at least half of the length and for example over the entire length of the spiral spring 3 if it is already produced on the basis of the plate. The first electrodes 8a and 8b are arranged next to each other with a predefined spacing for example over the entire length of the spiral spring 3. No electrode is deposited on each side face 22.

[0032] A first piezoelectric layer 7 is subsequently deposited and then structured on the first electrode 8a of the first pair of electrodes. Preferably, the first piezoelectric layer is structured to the lateral dimension and length of the first electrode 8a of the first pair of electrodes. A second piezoelectric layer 7' may be deposited or structured on the first electrode 8b of the second pair of electrodes at the same time as the first piezoelectric layer 7 or after the production of the first piezoelectric layer 7. Preferably, the second piezoelectric layer 7' is structured to the lateral dimension and length of the first electrode 8b of the second pair of electrodes.

[0033] Once the first and second piezoelectric layers 7, 7' are well structured on the first electrodes 8a, 8b of the two pairs of electrodes, a second electrode 8c of the first pair of electrodes is deposited or structured on the first piezoelectric layer 7 opposite the first electrode 8a of the first pair of electrodes. The second electrode 8c is of equivalent shape and size to the first electrode 8a of the first pair of electrodes. A second electrode 8d of the second pair of electrodes is deposited or structured on the second piezoelectric layer 7' opposite the first electrode 8b of the second pair of electrodes. The second electrode 8d is of equivalent shape and size to the first electrode 8b of the second pair of electrodes.

[0034] Once the piezoelectric spiral spring 3 is completed, it can be mounted in an oscillating mechanical system. The two pairs of electrodes in this embodiment are inversely and alternately polarized by a voltage source in particular to maintain a movement of the oscillating system for the oscillation of the piezoelectric spiral spring. To do this, the first electrode 8a of the first pair of electrodes can be connected to the second electrode 8d of the second pair of electrodes. The first electrode 8b of the second pair of electrodes can be connected to the second electrode 8c of the first pair of electrodes. The first electrode 8a and the second electrode 8d can be connected to a first connection terminal arranged at a first end of the piezoelectric spiral spring 3. The first electrode 8b and the second electrode 8c can be connected to a second connection terminal at the first end of the piezoelectric spiral spring 3.

[0035] It should also be noted that it may be provided to deposit only a first piezoelectric layer 7 on the first two electrodes 8a, 8b of the two pairs of electrodes. After this, a separation into two piezoelectric layers 7, 7' may be carried out as shown on each first electrode 8a, 8b of the two pairs of electrodes.

[0036] The electrical connection of the piezoelectric spiral spring 3 can be made on the top, with connection terminals defined in particular at the same time as the deposits of the electrodes 8a, 8b, 8c, 8d and of the piezoelectric layer(s) 7, 7'. Preferably, two connection terminals at a first end of the piezoelectric spiral spring 3 are provided to be connected to the electrodes 8a, 8b, 8c, 8d of at least two pairs of electrodes. The connection terminals are arranged after the balance spring stud, so as not to affect the balance spring mechanically. A resistive layer of the SiO2 type is deposited at least locally on the area where the balance spring is fixed to the stud, so as to avoid any electrical short circuit. An insulated stud can also be used. However, it is also conceivable to use the balance spring stud directly to make the electrical connections.

[0037] As indicated above, to inversely polarize the two pairs of electrodes, the electrodes 8a and 8d are connected to a first terminal, for example represented by Vo-, while the electrodes 8b and 8c are connected to a second terminal, for example represented by Vo+. The voltages Vo+ and Vo- are inverse alternating voltages in time with rectangular or sinusoidal signals or pulse trains to maintain the oscillation of the piezoelectric spiral spring 3.

[0038] It should also be noted that it may be provided to apply voltages of different amplitude for each pair of electrodes to compensate for possible asymmetries. For example, a voltage V0 for the first pair of electrodes 8a and 8c and an inverse voltage V1 of amplitude different from V0 on the second pair of electrodes 8b and 8d. Under these conditions, it is necessary to provide at a first end of the piezoelectric spiral spring 3 four connection terminals each connected to a respective electrode of the two pairs of electrodes.

[0039] The piezoelectric spiral spring 3, which is shown in the Figure 5 , includes features similar to those shown in the Figure 4b. However, in this second embodiment of the piezoelectric spiral spring 3, a single piezoelectric layer 7 is deposited and held on the first two electrodes 8a, 8b. A second electrode 8c, 8d for each pair of electrodes is deposited on the piezoelectric layer 7 opposite each of the respective first electrode 8a, 8b of the pairs of electrodes. However, the piezoelectric layer 7 is deposited both over the width of the first two electrodes 8a and 8b and also over the space separating them. The shape and dimension of the piezoelectric layer 7 is equivalent to the combined shape of the first two electrodes 8a and 8b and including the space separating them.

[0040] The crystalline orientation of the piezoelectric layer 7 deposited on the top face 20 of the spiral spring 3 provides a much better result than layers deposited laterally on lateral faces 22 of the spiral spring. As shown in figure 8, the crystalline orientation of the layer deposited on the lateral faces 22 has an inclined crystalline orientation and therefore not perpendicular like the crystalline orientation of the piezoelectric layer deposited on the top face 20 of the spiral spring.

[0041] The second electrode 8c of the first electrode pair is arranged to receive the voltage Vo+, while the first electrode 8a of the first electrode pair is arranged to receive the voltage Vo- inverse to the voltage Vo+. The second electrode pair is arranged to be polarized inversely to the first electrode pair with the voltage Vo+ supplied to the first electrode 8b of the second electrode pair, while the second electrode 8d of the second electrode pair is arranged to be polarized by the voltage Vo-. However, the bias voltage supplied to the electrodes 8a, 8b, 8c, 8d from the two connection terminals is alternating.The electrodes 8b and 8c are therefore alternately polarized in time by the voltage Vo+, while the electrodes 8a and 8d are alternately polarized in time by the voltage Vo-, which is inverse to the voltage Vo+ to maintain a movement of the oscillating system for the oscillation of the piezoelectric spiral spring 3. Of course, the polarization voltages of the electrodes can vary by rectangular-shaped signals or sinusoidal signals.

[0042] Finite element calculations show that it is possible, with this configuration, to excite the spiral spring 3 in a manner analogous to that of the deposition on the lateral faces 22 of the spiral spring. Even if the piezoelectric effect achieved with this method is lower than a deposition on the lateral faces 22, this can be compensated by the use of materials having larger piezoelectric factors, and which cannot necessarily be successfully deposited on the lateral faces 22 of the spiral spring 3. The method is notably compatible with all piezoelectric materials depositable by vacuum sputtering (AIN, AIScN, PZT, and lead-free piezoelectric materials). Two variants of the manufacturing method are envisaged and described subsequently below.

[0043] There figure 6represents a cross-section of a coil of the piezoelectric spiral spring 3 according to a third embodiment. As for the first embodiment, at least two pairs of electrodes are provided, of which a first electrode 8a of the first pair of electrodes is arranged in contact with the top face 20 of the piezoelectric spiral spring 3 and a second electrode 8c of the first pair of electrodes is arranged on a first set of composite layers 7, 17, 27 of which at least one piezoelectric layer 7, which may be one of the layers from the first layer to the last layer of the first set of composite layers.A first electrode 8b of the second pair of electrodes is arranged on the top face 20 of the turn while the second electrode 8d of the second pair of electrodes is arranged on a second set of composite layers 7', 17', 27' including at least one piezoelectric layer 7', which may be one of the layers from the first layer to the last layer of the second set of composite layers.

[0044] The first set of composite layers and the second set of composite layers may have the layers arranged in series or in parallel between the two electrodes 8a and 8c of the first pair of electrodes or the two electrodes 8b and 8d of the second pair of electrodes. Intermediate electrodes may also be provided between each layer of the set of composite layers to connect the layers in series or in parallel or to short-circuit one or more layers depending on the desired selection. It may also be a functional layer rather than a piezoelectric layer only each layer of the set of composite layers may be made of a different material than the next layer or other layers provided.

[0045] Finally, the fourth form of execution is represented in the figure 7 and compared to the Figure 4b. Two pairs of electrodes are provided, but oriented differently for the first and second electrodes 8a and 8c of the first pair of electrodes and for the first and second electrodes 8b and 8d of the second pair of electrodes. Preferably, in this fourth embodiment, the first piezoelectric layer 7 is first produced on the top face 20 and then or at the same time the second piezoelectric layer 7'. Subsequently, the first and second electrodes 8a, 8c, and 8b, 8d of the two pairs of electrodes are deposited on two opposite side faces of each piezoelectric layer 7, 7'.

[0046] A first piezoelectric layer 7 is arranged and polarized by the first and second electrodes 8a, 8c of the first pair of electrodes vertically relative to the top face 20. And also a second piezoelectric layer 7' is arranged and polarized by the first and second electrodes 8b, 8d of the second pair of electrodes. The crystal orientation may be arranged parallel to the top face 20 unlike the second embodiment or the previous embodiments. However, the realization of the fourth embodiment is more complicated than the previous embodiments.

[0047] According to all the embodiments presented above, the deposition of each piezoelectric layer or electrodes is carried out on the top face 20, even if it can be carried out on the bottom face not shown. In addition, other piezoelectric materials can be used, even if they do not lead to a satisfactory deposition on the side faces 22 as for the present invention the deposition is carried out on the top face of the wafer or directly on the top face 20 of the piezoelectric spiral spring 3. There are also no additional constraints on the dimensions of the spiral spring. The height of the spiral spring contributes to the resonance frequency of the sprung balance, in the same way as its length. On the other hand, a deposition on the top, in particular the top face 20, is more easily controllable than on the side faces.

[0048] Two alternative embodiments of a method for manufacturing a piezoelectric spiral spring 3 are now described using a quartz or SOI wafer by a DRIE etching process or a glass wafer by laser-assisted chemical etching. All four embodiments of the piezoelectric spiral spring 3 can be obtained according to the two alternative embodiments of the method for manufacturing the piezoelectric spiral spring 3.

[0049] As already explained above and according to a first variant embodiment of the manufacturing process of the piezoelectric spiral spring 3, the base of the spiral spring 3 is first produced by a DRIE etching operation from the top of the SOI or Quartz wafer or by chemical etching assisted by laser or pulsed laser of the glass wafer. More conventionally, the spiral spring can be produced first before the deposition of the piezoelectric layers or electrodes necessary to obtain the piezoelectric spiral spring.

[0050] In the case of a jet or spray deposition of material under vacuum, in particular piezoelectric layers 7, 7' ("sputtering" in English terminology), such as AlN or AIScN or PZT, as well as lead-free piezoelectric materials such as KNN (solid solution formed from Potassium Niobate (KNbO3, KN) and Sodium Niobate (NaNbO3, NN), the piezoelectric layers may have a texturing deviated by an angle of several tens of degrees relative to the normal on the lateral faces 22 of the spiral spring 3. The piezoelectric effect is thereby reduced, since only the projection onto the direction of the electric field contributes to it.

[0051] On the other hand, the use of the piezoelectric material KNN can be advantageous if it is deposited on the top face 20 of the spiral spring 3, because it can be deposited over a sufficient thickness, for example 5 µm, without great difficulty.

[0052] In a first series of steps of the manufacturing process of the second variant, it is possible to first produce electrodes or piezoelectric layers taking the programmed shape and length on a top face of the SOI or Quartz wafer to subsequently obtain the piezoelectric spiral spring. Once all the electrodes linked to the piezoelectric layers are obtained on the top face 20 of the SOI or Quartz or glass wafer, an etching or structuring of the spiral spring can be carried out, in particular up to a certain etching depth of the spiral spring. In the final production steps, the base of the SOI wafer is also removed by an etching of the DRIE type (deep reactive ion etching). From this moment the piezoelectric spiral spring is obtained with already the arrangement of the electrodes and the piezoelectric layers on the top face 20 of the spiral spring.

[0053] In the case of using the piezoelectric spiral spring 3 to draw energy or to power a motor circuit, it may be necessary to have a larger and longer spiral spring.

[0054] It should also be noted that with more piezoelectric surface, this implies more amplitude of movement, at equivalent voltages. This is generally favorable for a motor. However, it is possible to work at a higher frequency of a few hundred Hz and smaller amplitude, with the appropriate gear ratios. In this case, a balance spring of similar size can be used. On the other hand, fewer parts can be used in the watch, for example, higher level of the barrel or smaller barrel. The typical outside diameter of a motor balance spring with a large coil would be around 7 mm compared to around 5 mm for a classic chronometric balance spring. It is clear that this remains below the diameter of the balance wheel.

[0055] It is easy to increase the thickness of the piezoelectric layers or to increase the length of the spring itself to adapt, for example, the oscillation frequency. However, the edges of the coil edges that could cause cracking problems are not affected. It is much easier to arrange the piezoelectric layers on one of the top or bottom faces. The length of the spiral spring must be increased if its thickness is increased, to maintain the same resonance or oscillation frequency. Another consequence is that, in this way, layers can be added to a spiral spring with minimal or no modification to its geometry. The effect of a deposit on the side faces is much greater and requires more adjustment of the geometry of the spiral spring.

[0056] It can also be specified that it is easy to use piezoelectric materials that are difficult to structure on the side faces or that may require co-deposition of several different materials, for example AlScN, when a pre-mixed target is not available.

[0057] Another advantage is that we benefit from a better perpendicular crystal orientation on the top face while on the side faces it is inclined. The homogeneity of the deposition on the entire wafer and on each turn is much greater.

[0058] For comparison, the thickness of the deposit on the side faces of the spiral spring also depends on the spacing between the coils of the spring, which relates to the shading effect described above. The closer the coils are, the smaller the thickness deposited on the side faces and the greater the thickness gradient on the side face. In the case of top deposition, the thickness of the piezoelectric layer will be almost identical on each coil. This is an advantage for controlling manufacturing accuracy, and limits the difference between the designs produced and reality.

[0059] The number of electrode pairs used is not limited to two. For example, three or four electrode pairs could be used. With several electrode pairs, different voltage sequences can be applied to the layers, for example, electrical excitation of the central pairs for drift correction on the watch amplitude and energy harvesting or capture on the two outer pairs.

[0060] An odd number of electrode pairs can also be used, for example, three pairs, with the central pair to be used only as a sensor, and the outer layers for action on the hairspring. In this way, different correction circuits can be used, with direct feedback depending on the information collected on the central layer.

[0061] For the first or second variant of the method, the last step of the method may be to remove the base silicon plate forming part of the insulator. Following this, the silicon spiral spring may be connected to a self-regulating oscillation frequency circuit or to an energy recovery circuit or to a motor circuit for the actuation of the movement or its automatic maintenance. In addition, it may be connected to a printed circuit board for connection to the other components of the system. The two connection terminals at one end of the piezoelectric spiral spring are connected to at least two pairs of electrodes 8a, 8b, 8c, 8d arranged on the top face 20 of the piezoelectric spiral spring 3. The electrodes 8a and 8d are connected to a first connection terminal, while the electrodes 8b and 8c are connected to a second connection terminal.

[0062] As already mentioned, thanks to the realization of the electrodes and piezoelectric layers on the top side, it is easy to increase the thickness or even the length of the piezoelectric layers on the spiral spring. It can be easily realized up to a thickness of 3 µm on the top while on the sides only 1 µm is controllable. These values ​​correspond to AIN as piezoelectric layer. Some other materials, like KNN, can be deposited up to 5 µm. Some difficulties may be encountered regarding the structuring, but not regarding the use of such materials with larger piezoelectric factors.

[0063] It should be noted that it can be provided that the electrodes of the two pairs of electrodes and the piezoelectric layer extend only over a first turn of the spiral spring from a first end of the spiral spring where the connection terminals are located. This is advantageous for a low-power self-regulating circuit.

[0064] In addition, the dimension and mainly the width of the electrodes and piezoelectric layers successively deposited on the top face of the spiral spring may vary slightly by a few µm following the manufacturing process. Thus, each first electrode deposited directly on the top face may be slightly wider than the second electrode deposited on the layer

[0065] From the description just given, several other embodiments of the piezoelectric spiral spring can be realized without departing from the scope of the invention defined by the claims. It is possible to use two piezoelectric layers of a material different from each other on the top face of the piezoelectric spiral spring.

Claims

1. Piezoelectric balance spring (3) for a circuit (10) for self-regulating an oscillation frequency of an oscillating mechanical system (2, 3), or an energy recovery circuit or a motor circuit for actuating the movement or for the automatic maintenance thereof, the piezoelectric balance spring (3) comprising at least one piezoelectric layer (7, 7', 17, 17', 27, 27') deposited on a top face (20) or bottom face of a certain number of coils of the spring in a plane, and at least two pairs of electrodes (8a, 8b, 8c, 8d), characterised in that the electrodes of each pair are disposed on the same side as two opposing faces of at least one piezoelectric layer (7) or respectively two separate piezoelectric layers (7, 7') so as to apply a reverse bias voltage on each pair of electrodes.

2. Piezoelectric balance spring (3) according to claim 1, characterised in that the first electrode (8a) of the first pair of electrodes is connected to the second electrode (8d) of the second pair of electrodes, and in that the second electrode (8c) of the first pair of electrodes is connected to the first electrode (8b) of the second pair of electrodes.

3. Piezoelectric balance spring (3) according to one of claims 1 and 2, characterised in that the first electrode (8a) of the first pair of electrodes and the second electrode (8d) of the second pair of electrodes are connected to a first connection terminal at a first end of the piezoelectric balance spring (3), and in that the first electrode (8b) of the second pair of electrodes and the second electrode (8c) of the first pair of electrodes are connected to a second connection terminal at the first end of the piezoelectric balance spring (3).

4. Piezoelectric balance spring (3) according to claim 1, characterised in that all of the electrodes (8a, 8b, 8c, 8d) of the two pairs of electrodes are each connected to a respective connection terminal at a first end of the piezoelectric balance spring (3).

5. Piezoelectric balance spring (3) according to claim 1, characterised in that the first electrode (8a) of the first pair of electrodes is deposited directly on the top face (20) of the piezoelectric balance spring (3), in that the first electrode (8b) of the second pair of electrodes is deposited directly on the top face (20) of the piezoelectric balance spring (3), and in that the first electrodes (8a; 8b) of the two pairs of electrodes are evenly spaced from one another and each take the shape of coils from a first end of the balance spring (3) towards the second end of the balance spring (3).

6. Piezoelectric balance spring (3) according to claim 5, characterised in that the first electrodes (8a; 8b) and the second electrodes (8c; 8d) of the two pairs of electrodes extend from the first end of the balance spring (3) to more than halfway along the full length of the piezoelectric balance spring (3).

7. Piezoelectric balance spring (3) according to claim 5, characterised in that a first piezoelectric layer (7) is deposited on the first electrode (8a) of the first pair of electrodes, in that a second piezoelectric layer (7') is deposited on the first electrode (8b) of the second pair of electrodes, and in that the shape and dimensions of the first piezoelectric layer (7) are identical to the shape and dimensions of the first electrode (8a) of the first pair of electrodes, and in that the second piezoelectric layer (7') is identical to the shape and dimensions of the first electrode (8b) of the second pair of electrodes.

8. Piezoelectric balance spring (3) according to claim 1, characterised in that the first electrodes (8a; 8b) and the second electrodes (8c; 8d) of the two pairs of electrodes and the piezoelectric layer (7) extend from a first end of the balance spring (3) over a first coil of the balance spring (3).

9. Piezoelectric balance spring (3) according to claim 1, characterised in that the first electrode (8a) of the first pair of electrodes is deposited directly on the top face (20) of the piezoelectric balance spring (3), in that the first electrode (8b) of the second pair of electrodes is deposited directly on the top face (20) of the piezoelectric balance spring (3), and in that a piezoelectric layer (7) is deposited on the first electrodes (8a; 8b) both over the width of the two first electrodes (8a; 8b) and over the space therebetween.

10. Piezoelectric balance spring (3) according to claim 1, characterised in that a first electrode (8a) of the first pair of electrodes is disposed in contact with the top face (20) of the piezoelectric balance spring (3) and a second electrode (8c) of the first pair of electrodes is disposed on a first set of composite layers (7, 17, 27) mounted on the first electrode (8a) of the first pair of electrodes, at least one layer whereof is a piezoelectric layer (7), and in that a first electrode (8b) of the second pair of electrodes is disposed on the top face (20) of the piezoelectric balance spring (3), whereas the second electrode (8d) of the second pair of electrodes is disposed on a second set of composite layers (7', 17', 27') mounted on the first electrode (8b) of the second pair of electrodes, at least one layer whereof is a piezoelectric layer (7').

11. Piezoelectric balance spring (3) according to claim 10, characterised in that intermediate electrodes are provided between each layer of each set of composite layers (7, 7', 17, 17', 27, 27') in order to connect the layers in series or parallel or to short-circuit one or more layers depending on the desired selection.

12. Piezoelectric balance spring (3) according to claim 1, characterised in that a first piezoelectric layer (7) is deposited on a top face (20) of the piezoelectric balance spring (3), in that a second piezoelectric layer (7') is deposited on the top face (20) of the piezoelectric balance spring (3), spaced apart from the first piezoelectric layer (7), in that a first electrode (8a) of the first pair of electrodes is disposed on a side face of the first piezoelectric layer (7), whereas a second electrode (8c) of the first pair of electrodes is disposed on an opposing side face of the first piezoelectric layer (7), in that a first electrode (8b) of the second pair of electrodes is disposed on a side face of the second piezoelectric layer (7'), whereas a second electrode (8d) of the second pair of electrodes is disposed on an opposing side face of the second piezoelectric layer (7').

13. Piezoelectric balance spring (3) according to claim 1, characterised in that each piezoelectric layer (7, 7') disposed on the top face (20) of the piezoelectric balance spring (3) is a layer referred to as an AlN, AlScN, PZT or KNN layer.

14. Method for manufacturing a piezoelectric balance spring (3) according to one of the preceding claims, on the base of a SOI wafer (30), a quartz wafer or a glass wafer, characterised in that the base of the balance spring (3) is firstly produced by top-down DRIE of the SOI or Quartz wafer, or by top-down laser-assisted chemical etching of the glass wafer, and in that once the shape of the balance spring (3) has been produced having left the base (30) of the wafer or having completely removed the base (30) of the wafer, by DRIE or by laser-assisted chemical etching, one or more piezoelectric layers (7, 7') are deposited on the top face (20) of the balance spring (3) and the first and second electrodes (8a, 8b, 8c, 8d) of the two pairs of electrodes, combined with the one or more piezoelectric layers (7, 7') are deposited according to the programmed shape and dimensions.

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