Piezoelectric hairspring and method for manufacturing a piezoelectric hairspring

Abstract

The invention provides a piezoelectric hairspring (3) for an electric circuit (10) for automatically adjusting the oscillation frequency of an oscillating mechanical system (2, 3), or an energy recovery circuit, or a motor circuit for an actuating movement or an automatic winding movement. The piezoelectric hairspring (3) comprises at least one piezoelectric layer (7, 7', 17, 17', 27, 27') arranged on the top surface (20) or the bottom surface of a number of turns of the hairspring lying in one plane, and at least two pairs of electrodes (8a, 8b, 8c, 8d), wherein the electrodes of each pair of electrodes are arranged on two opposite surfaces of the same piezoelectric layer or of two piezoelectric layers, in order to apply a counter-biased voltage on each pair of electrodes.

Description

Technical Field

[0001] This invention relates to a piezoelectric hairspring used in the following circuits: a circuit for automatically adjusting the oscillation frequency of an oscillating mechanical system, or an energy recovery circuit, or a motor circuit for actuating a movement or automatically maintaining a movement.

[0002] The present invention also relates to a method for manufacturing a piezoelectric hairspring. Background Technology

[0003] In the field of horology, from a mechanical perspective, an oscillating mechanical system can be a balance wheel with a hairspring mounted on it. One end of the hairspring is attached to the balance wheel's axis of rotation, and the other end is attached to a fixed element on the mainplate. This mechanical system maintains oscillation by means of a typical mechanical power source, which can be a mainspring barrel that uses an escapement wheel engaged with a rotating escapement fork to drive the gear train. Therefore, the balance wheel, where the hairspring is connected to the escapement, can form the regulating component of a watch movement. This fully mechanical regulating mechanism uses a significant amount of space in the watch case, which in some cases constitutes a drawback.

[0004] French Patent No. 2 119 482 describes an oscillating mechanical system using a piezoelectric element. This piezoelectric element is preferably disposed on the hairspring connected to the balance wheel. This is achieved by depositing a piezoelectric material (PZT) film along most of the length of the hairspring and on the inner and outer surfaces of the metal hairspring. An AC voltage is supplied to the piezoelectric element using a voltage converter to alternately generate compressive and tensile forces on the hairspring, thereby regulating the oscillation of the balance wheel connected to the hairspring. In this patent document, electrodes are arranged along the length of the hairspring on each side, which complicates the manufacturing of the hairspring and introduces defects.

[0005] Figures 1 and 2 of the prior art illustrate a device 1 as described in European Patent No. 2 590 035B1, which includes oscillating mechanical systems 2 and 3 and a circuit 10 for automatically adjusting the oscillation frequency fosc of the oscillating mechanical system. In a mechanical watch, the oscillating mechanical system includes a balance wheel 2 and a hairspring 3. The balance wheel 2 is formed by a metal ring, for example, connected to a rotating shaft 6 via three arms 5. A piezoelectric element or an electroactive polymer element is provided on the hairspring 3. The first end 3a of the hairspring 3 is held in place by an outer stud 4 of the balance bridge. The balance bridge is attached to the mainplate of the watch movement. The second end 3b of the hairspring 3 is directly attached to the rotating shaft 6 of the balance wheel. Piezoelectric or electroactive polymer layers 23, 23' are deposited on both sides of the metal bar 24, which complicates the manufacture of the hairspring.

[0006] As shown in Figure 3 of the prior art, European Patent No. 3 629 103B1 describes a piezoelectric hairspring 70 for a watch. The hairspring 70, illustrated in cross-section, comprises a central body 72 made of silicon, a silicon oxide layer 74 deposited on the surface of the central body for temperature compensation of the hairspring, a conductive layer 76 deposited on the silicon oxide layer, and a piezoelectric material deposited on the conductive layer 76 in the form of a piezoelectric layer 78. Two electrodes 20a and 22a are arranged on the piezoelectric layer 78, respectively, located on two sides of the hairspring. A first portion 80a and a second portion 80b of the piezoelectric layer extend on two sides of the central body 72, respectively, and have corresponding crystal structures symmetrical about a central plane 84 parallel to these two sides. In the two lateral portions 80a and 80b, the piezoelectric layer has two identical corresponding piezoelectric polarization axes 82a, 82b, which are perpendicular to the piezoelectric layer and oriented in opposite directions. The manufacturing of this piezoelectric hairspring structure is complex and time-consuming, which constitutes a drawback.

[0007] In existing technologies, several technical challenges arise, such as when fabricating piezoelectric layers and contact electrodes on the hairspring. One potential problem is the shadowing effect, where any deposition of one or more layers results in a thickness gradient. Insufficient piezoelectric layer thickness can lead to short circuits at the bottom of the coil. This also imposes limitations on hairspring dimensions, as sufficient space must be provided between the coils and a sufficiently large aspect ratio must be maintained to mitigate the shadowing effect.

[0008] In the case of sputtering, the piezoelectric layer can be textured at an angle of tens of degrees relative to the normal of the sidewalls. This reduces the piezoelectric effect because only the projection in the direction of the electric field contributes to the piezoelectric effect.

[0009] It should also be noted that the resonant frequency of the balance wheel-hairspring mechanism is highly sensitive to the stiffness of the hairspring, which depends on the cubic thickness of the hairspring. Typical reproducibility of the deposition requires the final frequency setting state after the piezoelectric layer deposition. Furthermore, the thickness of the material deposited by sputtering on the substrate surface varies by several percentage points, making it difficult to accurately correct the thickness of the deposited layer.

[0010] Non-standard manufacturing methods are typically used to produce hairsprings because layers must be deposited on a patterned wafer, and electrodes must be patterned on the sidewalls without damaging the layers. Summary of the Invention

[0011] Therefore, the object of the present invention is to provide an easily manufactured piezoelectric hairspring for a circuit used to automatically adjust the oscillation frequency of an oscillating mechanical system, so as to accurately adjust the oscillation frequency of the oscillating mechanical system using a small number of components and overcome the aforementioned deficiencies of the prior art. This piezoelectric hairspring is also intended for use in energy recovery circuits or motor circuits for actuating movements or automatically sustaining movements.

[0012] Therefore, the present invention relates to a piezoelectric hairspring for use in circuits for automatically adjusting the oscillation frequency of an oscillating mechanical system, or for energy recovery circuits, or for actuating a movement or for automatically maintaining a movement. The piezoelectric hairspring includes at least one piezoelectric layer deposited on the top or bottom surface of a number of coils of the piezoelectric hairspring in a plane, and at least two pairs of electrodes. The characteristic feature is that each electrode in each pair is disposed on the same side as two opposing surfaces of the at least one piezoelectric layer or two separate piezoelectric layers, so as to apply a reverse bias voltage to each pair of electrodes. The first electrode in the first pair of electrodes is connected to the second electrode in the second pair of electrodes, and the second electrode in the first pair of electrodes is connected to the first electrode in the second pair of electrodes.

[0013] One advantage of this piezoelectric hairspring according to the invention is that it can be easily manufactured because the deposition of the piezoelectric layer is easily controlled if it is deposited on the top surface or even the bottom surface. By performing deposition on the top or bottom surface, it is also easy to increase the thickness or length of the piezoelectric layer.

[0014] Advantageously, it avoids touching the edges of the hairspring coils that could cause problems, including cracking of the piezoelectric layer; and because it is deposited entirely on the top or bottom surface, cracking problems are reduced.

[0015] Another advantage of depositing a piezoelectric layer on the top surface is that it produces a better, generally vertical, crystal orientation compared to a tilted deposition on the sides. The uniformity of the deposit across the entire wafer and on each coil is significantly higher. This is independent of the space between the individual coils of the hairspring. On the other hand, when depositing a piezoelectric layer on the sides, the smaller the space between the coils, the more difficult it is to deposit such a layer on these sides. In this case, there is a shading effect for individual layers deposited on the sides, and the individual layers may also be too thin, with insufficiently thick layers potentially causing short circuits. Therefore, depositing the piezoelectric layer on the top surface is advantageous for controlling manufacturing precision and limiting the gap between design and reality.

[0016] Another advantage is that it is easier to deposit piezoelectric layers made of materials that are difficult to pattern onto the top surface compared to depositing them on the sides. Overall, the overall top-down manufacturing approach, including the patterning of each layer and the etching and protection of the patterns, is easier than the non-standard patterning of the coil sidewalls.

[0017] Therefore, the present invention also relates to a method for manufacturing a piezoelectric hairspring.

[0018] According to the method proposed by the present invention, a substrate in the form of a substrate made of SOI (silicon-on-insulator) can be used as the first step. Once the first step is completed, the hairspring can be etched, as described in more detail below, or the electrodes, the piezoelectric layer, and other electrodes above the piezoelectric layer can be deposited and patterned before etching the first silicon layer. Attached Figure Description

[0019] The purpose, advantages, and features of the piezoelectric hairspring, as well as the method of manufacturing the hairspring, will be more clearly seen in the following description based on non-limiting embodiments shown in the accompanying drawings. This piezoelectric hairspring is used in circuits for automatically adjusting the oscillation frequency of an oscillating mechanical system, energy recovery circuits, or motor circuits for actuating or automatically maintaining a movement, as illustrated in the drawings.

[0020] Figure 1 illustrates in a simplified manner an apparatus according to the prior art, comprising an oscillating mechanical system and a circuit for automatically adjusting the oscillation frequency of the oscillating mechanical system.

[0021] Figure 2 shows a portion of the hairspring of an oscillating mechanical system including a piezoelectric element according to a prior art device.

[0022] Figure 3 shows a cross-sectional view of the coil of another type of piezoelectric hairspring according to the prior art.

[0023] Figure 4a and 4b A three-dimensional partial view of the coil portion of a piezoelectric hairspring is shown, along with a cross-sectional view of the coil according to a first embodiment of a piezoelectric hairspring based on the present invention.

[0024] Figure 5 A cross-sectional view of a coil according to a second embodiment of the piezoelectric hairspring of the present invention is shown.

[0025] Figure 6 A cross-sectional view of the coil of a third embodiment of the piezoelectric hairspring according to the present invention is shown.

[0026] Figure 7 A cross-sectional view of the coil of a fourth embodiment of a piezoelectric hairspring according to the present invention is shown, and

[0027] Figure 8 A coil with a piezoelectric hairspring formed on a substrate is shown, and a piezoelectric layer is coated on the coil to define the difference between the layer deposited on the top surface and the layer on the side surface according to the invention. Detailed Implementation

[0028] Figure 4a and 4bA three-dimensional partial view of the coil portion of the piezoelectric hairspring 3 is shown, along with a cross-sectional view of the coil of a first embodiment of the piezoelectric hairspring 3. The hairspring 3 typically comprises multiple coils and is connected to a balance wheel (not shown) to form an oscillating mechanical system. A first end of the hairspring is attached to the balance bridge, while a second end is attached to the balance shaft. The hairspring 3 lies in a plane between its two ends.

[0029] In this first embodiment, the piezoelectric hairspring 3 includes two pairs of electrodes 8a, 8b, 8c, and 8d on the top surface of the hairspring, wherein the first electrodes 8a and 8b of the two pairs of electrodes are directly attached to the top surface of the hairspring. A first piezoelectric layer 7 is attached between the first electrode 8a and the second electrode 8c of the first pair of electrodes, while a second piezoelectric layer 7', separate from the first layer, is attached between the first electrode 8b and the second electrode 8d of the second pair of electrodes.

[0030] Preferably, silicon is etched onto an SOI (or quartz) wafer to obtain the shape of the hairspring 3 with an insulator underneath, comprising a SiO2 oxide layer on one side and a silicon substrate on the other. The SOI or quartz wafer may advantageously be coated with an insulating layer of the SiO2 type with a thickness of approximately 500 nm to avoid any interference between the activation of the piezoelectric layer and the substrate used to produce the hairspring 3. The SOI wafer may have a thickness of approximately 500 μm. According to an alternative embodiment of the method for manufacturing the piezoelectric hairspring, once the outline of the hairspring 3 is obtained after etching, electrodes 8a, 8b, 8c, 8d and piezoelectric layers 7, 7' can be deposited and patterned on the top surface of the hairspring.

[0031] Alternatively, the hairspring 3 can be fabricated on a glass wafer. Under these conditions, the glass wafer is subjected to a laser-assisted chemical etching step to obtain the hairspring 3. By adjusting the fabrication method of the hairspring, other types of substrates, such as ceramics or composite materials, can be considered.

[0032] The first electrode 8a of the first electrode pair and the first electrode 8b of the second electrode pair are disposed or deposited in a plane on the top surface 20 of the piezoelectric hairspring 3. The first electrodes 8a and 8b are evenly spaced apart from each other and each has a coil shape extending from the first end of the hairspring toward the second end of the hairspring. The first electrodes 8a and 8b of the two electrode pairs are approximately equal in length and are located on a portion of the length of the hairspring from the first end. Preferably, the lengths of the first electrodes 8a and 8b of the two electrode pairs extend from the first end to the second end of the piezoelectric hairspring 3.

[0033] The first piezoelectric layer 7 is deposited directly on the first electrode 8a of the first pair of electrodes, and preferably has the same shape as the first electrode 8a for at least a portion of the length of the piezoelectric hairspring 3. The second piezoelectric layer 7' is deposited directly on the first electrode 8b of the second pair of electrodes, and preferably has the same shape as the first electrode 8b for at least a portion of the length of the piezoelectric hairspring 3.

[0034] Finally, the second electrode 8c of the first pair of electrodes is directly disposed or deposited on the surface of the first piezoelectric layer 7 opposite to the contact surface between the first electrode 8a and the first piezoelectric layer 7. The second electrode 8d of the second pair of electrodes is directly disposed or deposited on the surface of the second piezoelectric layer 7' opposite to the contact surface between the first electrode 8b and the second piezoelectric layer 7'. In this first embodiment, the shape and length of each second electrode 8c, 8d are the same as the shape and length of each first electrode 8a, 8b.

[0035] Two alternative embodiments of a method for manufacturing a piezoelectric hairspring are provided. Since the hairspring 3 is made of a silicon (SOI) wafer or a quartz wafer, the silicon or quartz can first be etched to obtain the base of the hairspring 3. Subsequently, electrodes 8a, 8b, 8c, 8d and piezoelectric layers 7, 7' are deposited on the top or bottom surface of the patterned hairspring 3. In the case of a glass wafer, the base of the hairspring 3 can first be cut from the top of the wafer by chemically assisted laser cutting or other laser cutting methods.

[0036] According to one alternative embodiment, electrodes 8a, 8b, 8c, 8d and piezoelectric layers 7, 7' can be deposited on a silicon or quartz wafer before patterning, i.e., before etching using the DRIE method to obtain the hairspring or before performing top-down laser-assisted chemical etching on the glass wafer to obtain the hairspring 3. Further details of the methods for fabricating the piezoelectric hairspring 3 according to the two briefly illustrated alternative embodiments will be given in a later description.

[0037] like Figure 4a As shown, the automatic adjustment circuit allows a matching voltage to be applied continuously or within a specific time period to generate a compressive force -T1 on the piezoelectric layer 7, or an opposite matching voltage to be applied on another piezoelectric layer 7' to generate a tensile force T1. This allows for adjustment of the oscillation frequency of the oscillating mechanical system.

[0038] First, preferably along most of the length of the hairspring 3, at least half of its length, such as the entire length of the hairspring 3 if the hairspring 3 is made of a wafer, both the first electrode 8a of the first pair of electrodes and the first electrode 8b of the second pair of electrodes are disposed or patterned on the top surface 20. The first electrodes 8a and 8b are arranged adjacent to each other at a predetermined spacing, for example, along the entire length of the hairspring 3. No electrode deposition is performed on each side 22.

[0039] The first piezoelectric layer 7 is then deposited and patterned on the first electrode 8a of the first pair of electrodes. Preferably, the first piezoelectric layer is patterned to match the lateral dimensions and length of the first electrode 8a of the first pair of electrodes. Simultaneously with or after the fabrication of the first piezoelectric layer 7, a second piezoelectric layer 7' may be deposited or patterned on the first electrode 8b of the second pair of electrodes. Preferably, the second piezoelectric layer 7' is patterned to match the lateral dimensions and length of the first electrode 8b of the second pair of electrodes.

[0040] Once the first and second piezoelectric layers 7, 7' have been appropriately patterned 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 patterned on the first piezoelectric layer 7, facing the first electrode 8a of the first pair of electrodes. The second electrode 8c has the same shape and size as the first electrode 8a of the first pair of electrodes. A second electrode 8d of the second pair of electrodes is deposited or patterned on the second piezoelectric layer 7', facing the first electrode 8b of the second pair of electrodes. The second electrode 8d has the same shape and size as the first electrode 8b of the second pair of electrodes.

[0041] Once the piezoelectric hairspring 3 is manufactured, it can be installed in an oscillating mechanical system. In this embodiment, the two pairs of electrodes are reverse-biased and alternately biased by a voltage source to maintain the motion of the oscillating system so that the piezoelectric hairspring oscillates. For this purpose, 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 located at the first end of the piezoelectric hairspring 3. The first electrode 8b and the second electrode 8c can be connected to a second connection terminal located at the first end of the piezoelectric hairspring 3.

[0042] It should also be noted that only the first piezoelectric layer 7 can be deposited on the two first electrodes 8a and 8b of the two pairs of electrodes. Thereafter, for each of the two first electrodes 8a and 8b of the two pairs of electrodes, the two piezoelectric layers 7 and 7' can be separated.

[0043] Electrical connection of the piezoelectric hairspring 3 can be achieved from the top, wherein connection terminals are defined, particularly while depositing electrodes 8a, 8b, 8c, 8d and one or more piezoelectric layers 7, 7'. Preferably, two connection terminals are provided at the first end of the piezoelectric hairspring 3 for connection to electrodes 8a, 8b, 8c, 8d in at least two pairs of electrodes. The connection terminals are positioned after the balance wheel stud to avoid mechanically affecting the hairspring-balance wheel mechanism. A SiO2-type resistive layer is deposited at least locally on the area where the hairspring is attached to the stud to prevent any electrical short circuits. An insulated stud may also be used. However, electrical connection can also be achieved directly using the balance wheel stud.

[0044] As described above, in order to reverse bias the two pairs of electrodes, electrodes 8a and 8d are connected to a first terminal, for example, denoted as Vo-, while electrodes 8b and 8c are connected to a second terminal, for example, denoted as Vo+. The voltages Vo+ and Vo- are time-dependent AC voltages with rectangular or sinusoidal signals or pulse trains to maintain the oscillation of the piezoelectric hairspring 3.

[0045] It should also be noted that voltages of different amplitudes can be applied to each pair of electrodes to compensate for possible asymmetry. For example, a voltage V0 can be applied to the first pair of electrodes 8a and 8c, while a reverse voltage V1 with a different amplitude than V0 can be applied to the second pair of electrodes 8b and 8d. Under these conditions, four connecting terminals must be provided at the first end of the piezoelectric hairspring 3, each terminal connecting to a corresponding electrode in one of the two pairs of electrodes.

[0046] Figure 5 The piezoelectric hairspring 3 shown has the same characteristics as... Figure 4b Similar features are shown. However, in this second embodiment of the piezoelectric hairspring 3, only one piezoelectric layer 7 is deposited and maintained on the two first electrodes 8a, 8b. Second electrodes 8c, 8d of each pair of electrodes are deposited on the piezoelectric layer 7, facing each corresponding first electrode 8a, 8b of the pair. However, the piezoelectric layer 7 is deposited both across the width of the two first electrodes 8a and 8b and in the space between them. The shape and size of the piezoelectric layer 7 correspond to the combined shape of the two first electrodes 8a and 8b, including the space between them.

[0047] Compared to the layer deposited laterally on the side surface 22 of the hairspring, the crystal orientation of the piezoelectric layer 7 deposited on the top surface 20 of the hairspring 3 provides better results. For example... Figure 8 As shown, the crystal orientation of the layer deposited on the side surface 22 is tilted, unlike the vertical crystal orientation of the piezoelectric layer deposited on the top surface 20 of the filament.

[0048] The second electrode 8c of the first electrode pair is configured to receive voltage Vo+, while the first electrode 8a of the first electrode pair is configured to receive voltage Vo-, which is opposite to Vo+. The second electrode pair is configured to be reverse-biased relative to the first electrode pair by 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 configured to be biased by voltage Vo-. However, the bias voltages supplied from the two connection terminals to electrodes 8a, 8b, 8c, and 8d are alternating current. Therefore, electrodes 8b and 8c are alternately biased by voltage Vo+, while electrodes 8a and 8d are alternately biased by voltage Vo-, which is opposite to Vo+, to maintain the motion of the oscillating system for the oscillation of the piezoelectric hairspring 3. It goes without saying that the bias voltages of the electrodes can be changed by rectangular or sinusoidal signals.

[0049] Finite element analysis shows that, with this configuration, the hairspring 3 can be excited in a manner similar to deposition on the side 22 of the hairspring. Even if the piezoelectric effect achieved using this method is lower than that deposited on the side 22, it can be compensated for by using a material with a higher piezoelectric coefficient (which may not necessarily be successfully deposited on the side 22 of the hairspring 3). This method is particularly compatible with all piezoelectric materials (AlN, AlScN, PZT, and lead-free piezoelectric materials) that can be deposited by sputtering. Two alternative embodiments of this manufacturing method will be considered and described below.

[0050] Figure 6 A cross-sectional view of the coil of the piezoelectric hairspring 3 according to the third embodiment is shown. Similar to the first embodiment, at least two pairs of electrodes are provided, wherein a first electrode 8a of the first pair of electrodes is positioned in contact with the top surface 20 of the piezoelectric hairspring 3, and a second electrode 8c of the first pair of electrodes is disposed on a first set of composite layers 7, 17, 27, which includes at least one piezoelectric layer 7, which may be one of multiple 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 disposed on the top surface 20 of the coil, and a second electrode 8d of the second pair of electrodes is disposed on a second set of composite layers 7', 17', 27', which includes at least one piezoelectric layer 7', which may be one of multiple layers from the first layer to the last layer of the second set of composite layers.

[0051] Multiple layers of the first and second composite layers can be connected in series or parallel between two electrodes 8a and 8c in the first pair of electrodes or between two electrodes 8b and 8d in the second pair of electrodes. An intermediate electrode can also be provided between each layer of the composite group to allow multiple layers to be connected in series or parallel, or to short-circuit one or more layers, depending on the desired selection. It can also be a functional layer, not just a piezoelectric layer, wherein each layer of the composite group can be made of a different material than the next layer or other layers disposed thereon.

[0052] Finally, Figure 7 The fourth embodiment is shown and is related to Figure 4b A comparison is made. Two pairs of electrodes are provided, but the orientations of the first and second electrodes 8a and 8c in the first pair are different from the orientations of the first and second electrodes 8b and 8d in the second pair. Preferably, in this fourth embodiment, the second piezoelectric layer 7' is formed on the top surface 20 after or simultaneously with the first 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 sides of each piezoelectric layer 7, 7'.

[0053] A first piezoelectric layer 7 is formed and biased by first and second electrodes 8a and 8c of a first pair of electrodes perpendicular to the top surface 20. Furthermore, a second piezoelectric layer 7' is formed and biased by first and second electrodes 8b and 8d of a second pair of electrodes. Unlike the second embodiment or previous embodiments, the crystal orientation may be parallel to the top surface 20. However, the fabrication of the fourth embodiment is more complex than that of the previous embodiments.

[0054] According to all the embodiments described above, each piezoelectric layer or electrode is deposited on the top surface 20, but it can also be deposited on the bottom surface (not shown). Furthermore, other piezoelectric materials can be used, even if they do not produce satisfactory deposition on the side surface 22, because in this invention, deposition is performed on the top surface of the wafer or directly on the top surface 20 of the piezoelectric hairspring 3. Furthermore, there are no additional limitations on the size of the hairspring. The height of the hairspring affects the resonant frequency of the balance wheel-hairspring mechanism in the same manner as its length. On the other hand, deposition on the top, particularly on the top surface 20, is easier to control than deposition on the side surface.

[0055] Two alternative embodiments of the method for manufacturing the piezoelectric hairspring 3 will now be described, using either a quartz or SOI wafer and the DRIE method, or a glass wafer and a laser-assisted chemical etching method. All four embodiments of the piezoelectric hairspring 3 can be obtained using the two alternative embodiments of the method for manufacturing the piezoelectric hairspring 3 described above.

[0056] As explained above and according to a first alternative embodiment of the method for manufacturing the piezoelectric hairspring 3, the base of the hairspring 3 is first formed by top-down DRIE of an SOI or quartz wafer or by pulsed laser or laser-assisted chemical etching of a glass wafer. More conventionally, the hairspring can be formed first before depositing the piezoelectric layer or electrodes required for manufacturing the piezoelectric hairspring.

[0057] In the case of sputtering piezoelectric layers 7, 7', particularly those such as AlN, AIScN, or PZT, and lead-free piezoelectric materials such as KNN (a solid solution formed from potassium niobate (KNbO3, KN) and sodium niobate (NaNbO3, NN)), the piezoelectric layer can be textured at an angle of several tens of degrees to the normal on the side surface 22 of the hairspring 3. This reduces the piezoelectric effect because only the projection in the direction of the electric field contributes to the piezoelectric effect.

[0058] However, it is advantageous to use the piezoelectric material KNN if it is deposited on the top surface 20 of the hairspring 3, because it can be easily deposited to a sufficient thickness, such as 5 μm.

[0059] In the first series of steps of the manufacturing method in the second alternative embodiment, a piezoelectric layer or electrodes with a programmed shape and length are first formed on the top surface of an SOI or quartz wafer, after which a piezoelectric hairspring is obtained. Once a group of electrodes connected to the piezoelectric layer has been obtained on the top surface 20 of the SOI, quartz, or glass wafer, the hairspring can be etched or patterned, specifically to a specific etch depth. In the final manufacturing step, the substrate of the SOI wafer is also removed by DRIE (Deep Reactive Ion Etching). At this point, a piezoelectric hairspring is obtained, which already includes electrodes and a piezoelectric layer disposed on the top surface 20 of the hairspring.

[0060] When using a piezoelectric hairspring 3 to extract electricity or power motor circuits, larger and longer hairsprings may be required.

[0061] It should also be noted that a larger piezoelectric surface area means a greater amplitude of motion at the equivalent voltage. This is generally advantageous for the motor. However, it is possible to operate at higher frequencies of several hundred hertz and smaller amplitudes with appropriate gear ratios. In this case, a hairspring of similar overall size can be used. Conversely, fewer parts can be used in the watch, such as a higher grade of mainspring barrel or a smaller mainspring barrel. The typical outer diameter of a motor hairspring with a wide coil is about 7 mm, compared to about 5 mm for a conventional timepiece hairspring. Obviously, this is still smaller than the diameter of the balance wheel.

[0062] The thickness of the piezoelectric layer can be easily increased, or the length of the hairspring itself can be easily increased, for example, to adjust the oscillation frequency. However, this avoids touching the edges of the coil that could cause cracking problems. It is much easier to place the piezoelectric layer on either the top or bottom surface. If the thickness of the hairspring is increased, its length must also be increased to maintain the same resonant or oscillation frequency. Another consequence is that multiple layers can be added to the hairspring in this way with little or no change in the hairspring geometry. The effect of deposition on the sides is much greater, and requires a much larger adjustment to the hairspring geometry.

[0063] It can also be pointed out that when a premixed target is not available, it is easier to use piezoelectric materials, such as AIScN, which are difficult to pattern on the sides or may require co-deposition of multiple different materials.

[0064] Another advantage is that the top surface has a better vertical crystal orientation, while the sides have a tilted crystal orientation. The uniformity of the deposits across the entire wafer and on each coil is much higher.

[0065] In comparison, the thickness of the deposit on the sides of the hairspring also depends on the spacing between the hairspring coils, which is related to the shading effect described above. The closer the coils are, the smaller the thickness of the deposit on the sides, and the greater the thickness gradient on the sides. In the case of top-down deposition, the thickness of the piezoelectric layer is almost the same on each coil. This is an advantage in controlling manufacturing precision and limits the gap between the resulting design and reality.

[0066] The number of electrode pairs used is not limited to two. For example, three or four pairs of electrodes can be used. With several pairs of electrodes, different voltage sequences can be applied to multiple layers, such as electrically stimulating the central pair of electrodes to correct the drift of the watch amplitude, and performing energy recovery or harvesting on the two outer pairs of electrodes.

[0067] Alternatively, an odd number of electrode pairs can be used, such as three pairs, where the center pair acts solely as a current collector, while the outer pairs act on the hairspring. This allows for the use of different correction circuits, where direct feedback is provided based on information collected on the center layer.

[0068] In a first or second alternative embodiment of the method, the final step may be removing the base silicon liner that forms part of the insulator. The silicon hairspring can then be connected to circuitry for automatically adjusting the oscillation frequency, energy recovery circuitry, or motor circuitry for actuating or automatically maintaining the movement. Furthermore, it can be connected to a printed circuit board for connection to other system components. Two connection terminals at one end of the piezoelectric hairspring are connected to at least two pairs of electrodes 8a, 8b, 8c, 8d disposed on the top surface 20 of the piezoelectric hairspring 3. Electrodes 8a and 8d are connected to the first connection terminal, while electrodes 8b and 8c are connected to the second connection terminal.

[0069] As mentioned above, by forming electrodes and a piezoelectric layer on the top surface, it is easy to increase the thickness and even the length of the piezoelectric layer on the hairspring. It can easily be made 3 μm thick on the top, while only 1 μm can be controlled on the sides. These values ​​correspond to using AlN as the piezoelectric layer. Some other materials, such as KNN, can be deposited up to 5 μm. Patterning may present some difficulties, but not when using materials with such higher piezoelectric coefficients.

[0070] It should be noted that the electrodes and piezoelectric layer in the two pairs of electrodes can extend only on the first coil of the hairspring, starting from the first end of the hairspring where the connection terminal is located. This is advantageous for low-power automatic adjustment circuits.

[0071] Furthermore, due to differences in manufacturing methods, the dimensions, primarily the width, of the electrodes and piezoelectric layers successively deposited on the top surface of the hairspring may vary slightly by a few μm. Therefore, each first electrode directly deposited on the top surface can be slightly wider than the second electrode deposited on that layer.

[0072] Based on the description just given, several other embodiments of the piezoelectric hairspring can be generated without departing from the scope of the invention as defined by the claims. Two piezoelectric layers made of different materials can be used on the top surface of the piezoelectric hairspring.

Claims

1. A piezoelectric hairspring (3) for a circuit (10) for automatically adjusting the oscillation frequency of an oscillating mechanical system, or for an energy recovery circuit, or for a motor circuit for actuating a movement or for an automatic winding movement, Said piezoelectric hairspring (3) comprises at least one piezoelectric layer (7, 7', 17, 17', 27, 27') deposited on the top face (20) or on the bottom face of a number of turns of said piezoelectric hairspring lying in one plane, and at least two pairs of electrodes (8a, 8b, 8c, 8d), characterized in that, each electrode of each pair of electrodes is disposed on the same side of at least one piezoelectric layer (7) or of both opposite faces of two separate piezoelectric layers (7, 7') so as to apply a reverse bias voltage across each pair of electrodes, wherein a first electrode (8a) of the first pair of electrodes is connected to a second electrode (8d) of the second pair of electrodes, and a second electrode (8c) of the first pair of electrodes is connected to a first electrode (8b) of the second pair of electrodes.

2. Piezoelectric hairspring (3) according to claim 1, characterized 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 hairspring (3), and 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 hairspring (3).

3. Piezoelectric hairspring (3) according to claim 1, characterized in that the first electrode (8a) of the first pair of electrodes is deposited directly on a top face (20) of the piezoelectric hairspring (3), the first electrode (8b) of the second pair of electrodes is deposited directly on the top face (20) of the piezoelectric hairspring (3), the first electrodes (8a; 8b) of the two pairs of electrodes are uniformly spaced from each other, and each has a coil shape from the first end of the piezoelectric hairspring (3) towards the second end of the piezoelectric hairspring (3).

4. Piezoelectric hairspring (3) according to claim 3, characterized 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 piezoelectric hairspring (3) along more than half of the total length of the piezoelectric hairspring (3).

5. Piezoelectric hairspring (3) according to claim 3, characterized in that the first piezoelectric layer (7) is deposited on the first electrode (8a) of the first pair of electrodes, the second piezoelectric layer (7') is deposited on the first electrode (8b) of the second pair of electrodes, the first piezoelectric layer (7) has the same shape and size as the first electrode (8a) of the first pair of electrodes, and the second piezoelectric layer (7') has the same shape and size as the first electrode (8b) of the second pair of electrodes.

6. Piezoelectric hairspring (3) according to claim 1, characterized 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 over the first coil of the piezoelectric hairspring (3) from the first end of the piezoelectric hairspring (3).

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

8. Piezoelectric hair spring (3) according to claim 1, characterized in that The first electrode (8a) of the first pair of electrodes is arranged in contact with the top face (20) of the piezoelectrically suspended thread (3), while the second electrode (8c) of the first pair of electrodes is arranged on a first set of composite layers mounted on the first electrode (8a) of the first pair of electrodes, at least one of the layers of the first set of composite layers being a piezoelectric layer (7); and the first electrode (8b) of the second pair of electrodes is arranged on the top face (20) of the piezoelectrically suspended thread (3), while the second electrode (8d) of the second pair of electrodes is arranged on a second set of composite layers mounted on the first electrode (8b) of the second pair of electrodes, at least one of the layers of the second set of composite layers being a piezoelectric layer (7').

9. Piezoelectric hairspring (3) according to claim 8, characterized in that An intermediate electrode is arranged between each of the layers of each set of composite layers in order to connect the layers in series or in parallel or to short-circuit one or more layers according to the desired selection.

10. Piezoelectric hairspring (3) according to claim 1, characterized in that The first piezoelectric layer (7) is deposited on the top face (20) of the piezoelectrically suspended thread (3), the second piezoelectric layer (7') is deposited on the top face (20) of the piezoelectrically suspended thread (3) spaced apart from the first piezoelectric layer (7), the first electrode (8a) of the first pair of electrodes is arranged on one side of the first piezoelectric layer (7), while the second electrode (8c) of the first pair of electrodes is arranged on the opposite side of the first piezoelectric layer (7), the first electrode (8b) of the second pair of electrodes is arranged on one side of the second piezoelectric layer (7'), while the second electrode (8d) of the second pair of electrodes is arranged on the opposite side of the second piezoelectric layer (7').

11. Piezoelectric hair spring (3) according to claim 1, characterized in that Each piezoelectric layer (7, 7') arranged on the top face (20) of the piezoelectrically suspended thread (3) is a layer called AIN, AIScN, PZT or KNN layer.

12. A method for manufacturing a piezoelectric hairspring (3) according to any one of claims 1 to 11 on a substrate (30) of an SOI wafer, a quartz wafer or a glass wafer, characterized in that, The base of the piezoelectrically suspended thread (3) is generated firstly by a top-down DRIE of the SOI wafer or of the quartz wafer or by a top-down laser-assisted chemical etching of the glass wafer and once the shape of the piezoelectrically suspended thread (3) has been generated by DRIE or by laser-assisted chemical etching with the substrate (30) of the wafer left or completely removed, one or more piezoelectric layers (7, 7') are deposited on the top face (20) of the piezoelectrically suspended thread (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, in which the first electrode (8a) of the first pair of electrodes is connected to the second electrode (8d) of the second pair of electrodes and the second electrode (8c) of the first pair of electrodes is connected to the first electrode (8b) of the second pair of electrodes.

13. A method for manufacturing a piezoelectric hairspring (3) according to any one of claims 1 to 11 on a substrate (30) of an SOI wafer, a quartz wafer or a glass wafer, characterized in that, The method is performed directly on the raw top surface of the SOI wafer, quartz wafer or glass wafer, the deposit of one or more piezoelectric layers (7, 7') takes the programmed shape and length of the coil, the deposit of the first and second electrodes (8a, 8b, 8c, 8d) of the two pairs of electrodes takes the programmed shape and length of the coil combined with the one or more piezoelectric layers (7, 7'), and once each electrode (8a, 8b, 8c, 8d) of each pair of electrodes has been placed on one or both piezoelectric layers (7, 7') or on the two facing surfaces of one or two sets of composite layers on the raw top surface (20), a top-down DRIE operation is performed on the SOI wafer or quartz wafer of the piezoelectric balance spring (3) according to the programmed total length and shape, or a top-down laser-assisted chemical etching operation is performed on the glass wafer, and a final DRIE operation or laser-assisted chemical etching operation is performed to completely remove the substrate (30) of the wafer in order to obtain the final piezoelectric balance spring (3), in which the first electrode (8a) of the first pair of electrodes is connected to the second electrode (8d) of the second pair of electrodes and the second electrode (8c) of the first pair of electrodes is connected to the first electrode (8b) of the second pair of electrodes.

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