Flexible piezoelectric device with a ground plane

Abstract

Flexible piezoelectric device having a ground plane. This description relates to a piezoelectric device (1000) comprising a substrate (100) covered by a piezoelectric sensor and electrically conductive tracks (200). The device (1000) comprises a ground plane, a first portion (150) of the ground plane covering the sensor and a second portion (220) of the ground plane covering the electrically conductive tracks (200). The first portion (150) of the ground plane is more flexible than the second portion (220) of the ground plane. The first portion (150) of the ground plane is in contact with the second portion (220) of the ground plane at an intermediate zone (Z3) to form a continuous ground plane. Figure for the abstract: Fig. 1B

Description

Title of the invention: Flexible piezoelectric device having a niasse plane technical field

[0001] This description relates generally to piezoelectric devices, and more particularly to flexible piezoelectric devices, especially those manufactured by a printing technique such as screen printing. Previous technique

[0002] Piezoelectric devices are highly sensitive to external electromagnetic waves (for example, from other nearby electronic devices, which can generate measurement noise generally at the supply frequency (50 Hz in Europe)) or, more generally, to any parasitic signal that can disrupt the device's signal. To protect them from these parasitic signals and improve measurement accuracy, one solution is to cover them with a ground plane.

[0003] The ground plane can also, in the case of a force sensor, eliminate parasitic signals resulting from unwanted electromechanical phenomena such as triboelectricity (electrical charges generated by contact between the sensor and another object).

[0004] A ground plane is generally made of a conductive material (preferably a metal such as copper) surrounding the device and connected to the ground of the measuring electronics. However, since metals are rigid, they are not suitable for flexible electronics.

[0005] Several solutions have been proposed for manufacturing flexible ground planes.

[0006] A first solution is to use a rigid conductor but with a structure non-continuous (as described for example in document US8018410 B2 (titled "Flexible and transparent ground plane for electric paper F"). However, discontinuity reduces protection and only allows flexibility in certain directions.

[0007] Another solution is to use flexible conductive materials such as conductive polymers, or foams, aerogels, or composite materials based on carbon, MXenes, or metallic particles, such as those described in the article by Cheng et al., "Recent Advances in Design Strategies and Multifunctionality of Flexible Electromagnetic Interference Shielding Materials," in Nano-Micro Letters (2022) 14:80. However, these flexible materials are less conductive than rigid materials, and the protection is therefore often less effective.

[0008] A final solution consists of covering a piezoelectric sensor with a ground plane at the time of application to eliminate external disturbances. Such a solution is, for example, presented in the article by Chen et al. 'A method for quantitatively separating the piezoelectric conduct from the as-received “Piezoelectric” signal', Nature Communications (2022) 13:139) where the addition of a ground plane around a piezoelectric sensor at the time of the compression test makes it possible to completely eliminate interference due to triboelectricity.

[0009] However, the integration of a ground plane during the manufacturing of the piezoelectric sensor to ensure its permanent protection implies several additional constraints: - The material of the ground plane must be compatible with the sensor manufacturing method. - The ground plane must protect the sensor as well as the electrical connections between the sensor and the measurement electronics. - The ground plane must be compatible with the polarization step of the piezoelectric material: when the polarization electric field is applied, the presence of the ground plane (which acts as an additional electrode) can cause electrical breakdowns in certain parts of the sensor which can damage it, - the presence of the ground plane must have a minimal mechanical influence on the operation of the sensor.

[0010] Among the various studies attempting to address these constraints, we can cite the article by Kosir et al. ('Manufacturing of single-process 3D-printed piezoelectric sensors with electromagnetic protection using thermoplastic material extrusion', Additive Manufacturing (2023) 73: 103699) which presents a complete piezoelectric sensor with a ground plane manufactured by 3D printing. To prevent breakdown during polarization, the ground plane is also used as a ground electrode for the piezoelectric material at the sensor. A very thick insulator (PLA, thickness between 0.4 and 1.6 mm) is printed between the cable connecting the sensor to the measurement electronics.

[0011] In the article by Fan et al., "Electric polarization-assisted additive manufacturing technique for piezoelectric active poly(vinylidene fluoride) films: Towards fully three-dimensional printed functional material," Additive Manufacturing (2022) 60:103248, polarization is performed directly during the printing of the piezoelectric material and therefore before the printing of the ground plane. To achieve this, the authors integrated a corona polarization system into their 3D printing system to polarize the piezoelectric material immediately after its extrusion.

[0012] However, the main drawback of these two methods is that 3D printing necessarily results in a minimum thickness of 100 pm, which does not allow for the production of highly flexible sensors.

[0013] Finally, in the article by Wang et al. ('Electromagnetic interference shielding effectiveness of carbon-based materials prepared by screen printing' Carbon (2009) 47:1905-1910), or in the article by Wang et al. ('Ultrathin and Flexible Screen-Printed Metasurfaces for EMI Shielding Applications' IEEE Transactions on Electromagnetic Compatibility (2003) 53:3), flexible ground planes, for example based on silver or carbon, are obtained by screen printing. However, these studies are not specific to piezoelectric sensors and therefore do not meet the constraints mentioned above. Summary of the invention

[0014] There is a need to have a piezoelectric device protected from external electromagnetic disturbances or any other parasitic signal that may disturb the signal of the device.

[0015] This objective is achieved by a piezoelectric device comprising a substrate having a first face covered by a piezoelectric sensor and electrically conductive tracks, the piezoelectric sensor comprising an organic piezoelectric layer disposed between a first electrode and a second electrode, the sensor being connected to the electrically conductive tracks intended to be connected to measuring devices, the device comprising a ground plane, a first part of the ground plane covering the sensor, and being separated from the upper electrode by a first layer of dielectric insulation, and a second part of the ground plane covering the electrically conductive tracks and being separated from the electrically conductive tracks by a second layer of dielectric insulation,The first part of the ground plane is in contact with the second part of the ground plane at an intermediate zone to form a continuous ground plane, the first part of the ground plane being more flexible than the second part.

[0016] According to a particular embodiment, the first mass plane is made of a first material having a first Young's modulus and the second part of the mass plane is made of a second material having a second Young's modulus greater than the first Young's modulus.

[0017] According to a particular embodiment, the first part of the ground plane is at a first height relative to the first main face of the substrate and the second part of the ground plane is at a second height relative to the first main face of the substrate, the first height and the second height being different, the second height preferably being greater than the first height.

[0018] According to a particular embodiment, the first part of the ground plane and the second part of the ground plane have different thicknesses.

[0019] According to a particular embodiment, the first part of the ground plane, the first electrode and the second electrode are made of PEDOT:PSS and / or the second part of the ground plane is made of a mixture of PEDOT:PSS and silver.

[0020] According to a particular embodiment, the organic piezoelectric layer comprises a matrix in a copolymer or terpolymer of PVDF, preferably P(VDF-TrFe), the first dielectric insulation layer is in the same material as the matrix of the organic piezoelectric layer and / or the second dielectric insulation layer is in an epoxy or acrylic resin, preferably a cyanoacrylate.

[0021] According to a particular embodiment, the first part of the ground plane is covered by a first protective layer, for example in the same material as the first insulation layer, and / or the second part of the ground plane is covered by a second protective layer, for example in the same material as the material of the second dielectric insulation layer.

[0022] According to a particular embodiment, the device comprises an additional ground plane: - the additional ground plane being positioned on the second face of the substrate, or - the additional ground plane being positioned between, on the one hand, the first face of the substrate and, on the other hand, the sensor and the electrically conductive tracks, or - the additional ground plane covering an additional sensor and additional electrically conductive tracks arranged on the second face of the substrate, a first part of the additional ground plane covering the sensor and a second part of the additional ground plane covering the additional conductive tracks, the first part of the additional ground plane being in contact with the second part of the additional ground plane at an intermediate zone so as to form a continuous additional ground plane, the first part being more flexible than the second part of the additional ground plane.

[0023] This goal is also achieved by a method for manufacturing a piezoelectric device comprising the following steps: - deposit on a first main face of a substrate, a first electrode, a piezoelectric layer, then a second electrode to form a piezoelectric sensor, - to deposit electrically conductive tracks onto the substrate, - Apply a first layer of insulation to the piezoelectric sensor and a second layer of insulation to the electrically conductive tracks, - Cover the piezoelectric sensor and the first layer of insulation with a first conductive material to form the first part of a ground plane, - Cover the electrically conductive tracks and the second layer of insulation with a second conductive material to form the second part of a ground plane. the first layer of insulation and the second layer of insulation being in contact at an intermediate zone, the first part of the ground plane and the second part of the ground plane being in contact at the intermediate zone so as to form a continuous ground plane, the first part of the site plan and the second part of the site plan being screen-printed, the first part of the ground plane being more flexible than the second part of the ground plane.

[0024] According to a particular embodiment, the first part of the ground plane is at a first height relative to the first main face of the substrate and the second part of the ground plane is at a second height relative to the first main face of the substrate, the first height and the second height being different.

[0025] According to a particular embodiment, the first part of the ground plane is in PEDOT:PSS and / or the second part of the ground plane is in a mixture of PEDOT:PSS and silver.

[0026] According to a particular embodiment, an additional ground plane, preferably screen-printed, is formed on the second face of the substrate, or between, on the one hand, the first face of the substrate and, on the other hand, the sensor and the electrically conductive tracks.

[0027] According to a particular embodiment, a first part of the additional ground plane covers an additional sensor formed on the second face of the substrate and a second part of the additional ground plane covers additional conductive tracks formed on the second face of the second substrate, the first part of the additional ground plane being in contact with the second part of the additional ground plane at an intermediate zone so as to form a continuous additional ground plane, the first part of the additional ground plane being more flexible than the second part of the additional ground plane. Brief description of the drawings

[0028] These features and advantages, as well as others, will be described in detail in the following description of particular embodiments, given by way of non-limiting example, in relation to the accompanying figures, among which:

[0029] [Fig.1A], [Fig.1B] and [Fig.1C] schematically represent, respectively in top view, in cross-section at the level of the conductive line connecting the lower electrode and in cross-section at the level of the conductive line connecting the upper electrode, a printed piezoelectric device, according to a particular embodiment of the invention;

[0030] Fig. 2A schematically represents, in cross-section, a piezoelectric bimorph comprising two piezoelectric sensors positioned on two substrates, the two substrates being assembled to each other with an adhesive, according to another particular embodiment of the invention;

[0031] [Fig.2B] schematically and in cross-section represents a piezoelectric bimorph comprising two piezoelectric sensors printed on each side of the same substrate, according to another particular embodiment of the invention;

[0032] [Fig.3] represents, schematically and in cross-section, a piezoelectric device comprising a piezoelectric sensor, positioned on a first face of a substrate, and covered by a ground plane, and having an additional ground plane positioned on a second face of the substrate, according to a particular embodiment of the invention;

[0033] [Fig.4] represents, schematically and in cross-section, a piezoelectric device comprising, on the same face of a substrate, a piezoelectric sensor positioned between two ground planes, according to another particular embodiment of the invention;

[0034] [Fig.5] is a graph representing the charge as a function of the force applied to a sensor with a ground plane, according to a particular embodiment of the invention, and to a sensor without a ground plane given for comparison;

[0035] [Fig. A] is a graph representing the force as a function of time during 3-point bending tests to generate parasitic loads on the track part of a sensor covered by an encapsulation layer, for comparison purposes;

[0036] [Fig.6B] is a graph representing the force as a function of time during 3-point bending tests to generate parasitic loads on the track part of a sensor covered by an encapsulation layer and a ground plane, according to a particular embodiment of the invention;

[0037] [Fig.7] is a graph representing the measurement noise of a sensor without a ground plane given for comparison and of a sensor with a ground plane according to a particular embodiment of the invention, in an environment with water and in an environment with water and noise source;

[0038] [Fig.8] is a photographic image of different sensors (from left to right in the figure): a unit sensor without a ground plane (Cl) given for comparison, a unit sensor with a ground plane (C2) according to a particular embodiment of the invention, a device with a 3x5 sensor matrix without a ground plane (C3) given for illustration and a device with a 3x5 sensor matrix with a ground plane (C4) according to another particular embodiment of the invention.

[0039] The different elements are not necessarily represented at a uniform scale to make the figures more legible. Description of the implementation methods

[0040] The same elements have been designated by the same reference numerals in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same reference numerals and may have identical structural, dimensional and material properties.

[0041] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been represented and are detailed.

[0042] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements coupled together, this means that these two elements can be connected or linked through one or more other elements.

[0043] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "superior", "inferior", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures.

[0044] Unless otherwise specified, the expressions "approximately", "roughly", and "on the order of" mean at 10%, preferably at 5%.

[0045] By between X and Y, we mean that the bounds X and Y are included.

[0046] We will now describe the piezoelectric device in more detail by reference to figures IA, IB, IC, 2A, 2B, 3 and 4.

[0047] The piezoelectric device 1000 comprises a substrate 100 having a first face 100a (also called the first main face) and a second face 100b (also called the second main face). The main faces are parallel or substantially parallel to each other.

[0048] The first face 100a is covered by a sensor and electrically conductive tracks 200. The electrically conductive tracks 200 are connected to the sensor. They are intended to be connected to measuring devices.

[0049] The device includes a ground plane. The ground plane is formed of several parts 150, 220. A first part 150 of the ground plane covers the sensor and is isolated from the sensor by a first layer of dielectric insulation 140.

[0050] A second part 220 of the ground plane covers the electrically conductive tracks 200 and is insulated from the electrically conductive tracks 200 by a second layer of dielectric insulation 220.

[0051] The first part 150 and the second part are in contact with each other to form a continuous ground plane. For example, one of its parts is deposited directly onto the other. In the figures, the first part 150 is on the second part 220, but it could be the other way around.

[0052] The first part 150 of the ground plane is more flexible than the second part 220 of the ground plane. In other words, the first part 150 of the ground plane is less rigid than the second part 220 of the ground plane. To obtain different flexibilities / rigidities, it is possible to vary the material and / or the thickness of the different parts.

[0053] Preferably, the first part 150 of the ground plane is made of a first material having a first Young's modulus. The second part 220 of the ground plane is made of a second material having a second Young's modulus greater than the first Young's modulus.

[0054] First, at the sensor level, the first portion 150 of the ground plane is highly flexible. This first portion 150 is, for example, made of a material having a Young's modulus of less than 10 GPa. The total thickness (thickness of the sensor and the ground plane) is, for example, less than 20 µm. The sensor is thus an ultra-flexible sensor, which allows for good sensitivity.

[0055] The second part 220 of the ground plane, above the electrically conductive tracks 200, can be more rigid but also more efficient.

[0056] Dividing the mass plane into several parts 150, 220 thus makes it possible to retain both a flexible part 150 and a more efficient but more rigid rigid part 220.

[0057] Moreover, as we shall see later, the manufacture of the ground plane, by screen printing, can be integrated into the manufacturing process of the rest of the sensor, which is particularly interesting in terms of speed and cost.

[0058] Such a ground plane structure makes it possible to remedy some or even all of the disadvantages of prior art sensors.

[0059] We will now describe the piezoelectric device 1000 in more detail.

[0060] The device comprises: - a first zone ZI corresponding to the active zone including the sensor, - a second zone Z2 corresponding to the passive zone including the 200 electrical tracks, and - a third zone Z3, arranged between the active zone ZI and the passive zone Z2, corresponding to an intermediate zone and comprising different elements of the active zone and different elements of the passive zone which overlap to ensure the functional continuity of each layer.

[0061] More specifically, in the intermediate part Z3, the following are superimposed and preferably in direct contact: - the first part 150 of the ground plane and the second part 220 of the ground plane, - a part of the electrically conductive tracks and the first electrode 110, - the other part of the electrically conductive tracks 200 and the second electrode 130, - the first layer of 140 insulation and the second layer of 210 insulation, - possibly, the first protective layer 160 and the second protective layer 230.

[0062] These different elements of the different zones are positioned on a substrate.

[0063] The substrate 100 is a flexible substrate but can also serve as a mechanical support for the manufacture and use of the device. It plays an essential role in the conformability of the device.

[0064] The substrate 100 preferably has a very low Young's modulus E (typically 1 MPa < E < 100 MPa, for example E is about 50 MPa). The thickness of the substrate 100 is, for example, between 10 and 500 pm, preferably between 20 and 250 pm, even more preferably between 25 and 200 pm, for example about 100 pm.

[0065] The substrate 100 can be made of polyimide (PI), poly(ethylene naphthalate) (PEN), polyethylene terephthalate (PET), thermoplastic polyurethane (TPU) or polydimethylsiloxane (PDMS).

[0066] The substrate 100 may be made of a thermosetting polymer material. It may be, for example, a substrate marketed under the name BEYOLEX™ by Panasonic.

[0067] The substrate 100 can be formed of two substrates 101, 102 glued to each other by means, for example, of an adhesive layer ([Fig.2A]) or it can be a single-piece substrate ([Fig.2B]).

[0068] The piezoelectric sensor is positioned on the first face 100a of the substrate 100. Preferably, the sensor is a printed sensor (i.e., a sensor in which some or all of the layers are formed by printing).

[0069] The sensor comprises a piezoelectric polymer film 120 disposed between two electrodes 110, 130.

[0070] The piezoelectric film 120 has a thickness, for example, between 1 and 10 µm, preferably between 2 and 5 µm, for example 3 µm. Such thicknesses make it possible to obtain layers 120 with good flexibility.

[0071] Preferably, the piezoelectric film 120 is an organic piezoelectric film. It preferably comprises a polymeric matrix of PVDF, a PVDF copolymer, or a PVDF terpolymer. It may be a copolymer of vinylidene fluoride and at least one other monomer copolymerizable with VDF. Advantageously, the copolymer comprises at least 50% by mole, preferably at least 70% by weight, even more preferably at least 90% by mole of VDF.

[0072] By way of illustration, the copolymerizable monomer(s) are, for example, chosen from chlorotrifluoroethylene (CTFE), chlorofluoroethylene (CFE), hexafluoropropylene (HFP), trifluoroethylene (VF3), methyl methacrylate (MMA), tetrafluoroethylene (TFE), and perfluoro(alkyl vinyl) ethers such as perfluoro(methyl vinyl) ether (PMVE).

[0073] For example, the copolymer is a copolymer of poly(vinylidene fluoride-trifluoroethylene) PVDF / TrFe, also noted P(VDF-TrFe) or PVDF-CTFE.

[0074] It can also be a terpolymer. For example, a PVDF / CTFE / CFE terpolymer will be chosen.

[0075] According to another embodiment, the polymer is not a ferroelectric polymer: it may be PVDF-HFP.

[0076] According to another embodiment, the polymer is polylactic acid (also noted as PLA or PLLA).

[0077] Preferably, the piezoelectric films 120 are piezoelectric polymers selected from PVDF, P(VDF-TrFE) and PLLA.

[0078] The organic piezoelectric film 120 may be made of a composite material. For example, the film may comprise, in addition to a piezoelectric or non-piezoelectric polymer matrix, piezoelectric particles and / or PEDOT:PSS particles. The particles shall be small enough not to alter the roughness.

[0079] For example, ferroelectric particles are in BaTiO3 (BTO), PZT (lead zirconate titanate), AIN, ZnO, or in SBN (Sr-Ba-Nb oxide) or SBT (Sr-Ba-Ti oxide).

[0080] For example, piezoelectric films 120 are formed from composite materials comprising, for example, a polymer (piezoelectric type PVDF, or non-piezoelectric type TPU, PDMS, PVDF HFP) and piezoelectric nanoparticles or nanofibers (PVDF, PZT, BaTiO3, AIN).

[0081] The piezoelectric films 120 are arranged between electrodes 110, 130. The electrodes 110, 130 are flexible electrodes.

[0082] The electrodes 110, 130 have, for example, a thickness between 0.1 and 3 pm, preferably between 0.5 and 1.5 pm, for example 1 pm.

[0083] Preferably, the electrodes 110, 130 are made of an electrically conductive polymer or a metal such as gold. Electrically conductive polymers present a better interface with P(VDF-TrFE). Preferably, this is PEDOT-PSS (poly(3,4-ethylenedioxythiophene).

[0084] The first electrode 110, located between the substrate 100 and the piezoelectric film 120, is also called the lower electrode. The second electrode 130, covering the piezoelectric film 120, is the upper electrode. The second electrode 130 is located between the piezoelectric film 120 and the first part 150 of the sensor's ground plane.

[0085] In the passive zone Z2, electrically conductive tracks 200 are formed on the first face 100a of the substrate 100 to carry an electrical signal between the sensor and electronic measuring devices (not shown in the figures). Voltage measuring devices can also be used.

[0086] The 200 tracks can be printed on the substrate 100.

[0087] Any highly conductive ink that can be printed with a fairly fine resolution (typically the track width is less than 100 pm for high-density matrices) can be used.

[0088] The tracks 200 can be made of a metal (gold or silver, for example), or of an electrically conductive polymer material. For example, the electrically conductive polymer material is PEDOT:PSS. It can also be a polymer in which electrically conductive particles are dispersed, for example, carbon particles.

[0089] The metallic or carbon tracks 200 can be screen-printed using an ink containing metallic or carbon particles and, preferably, a solvent-resistant polymer matrix (for example, with a silicone, TPU, or acrylate base). Preferably, a silver-type ink (in particular, a silicone matrix with silver particles) or a carbon-based ink is used for the electrical connections between the sensor and the measuring electronics.

[0090] The conductive tracks 200 have, for example, a thickness between 5 pm and 20 pm, particularly around 10 pm. Such tracks ensure good conductivity.

[0091] As previously stated, the device 1000 includes a continuous ground plane covering the sensor and the conductive tracks 200 and comprising several parts of different rigidities, which considerably reduces the impact of the ground plane on the mechanical operation of the sensor.

[0092] The first part 150 is at a first height hl relative to the first face 100a of the substrate 100. The second part 220 is at a second height h2 relative to the first face 100a of the substrate 100. The first height hl and the second height h2 are different. The heights hl and h2 depend on the underlying layers.

[0093] The first part 150 of the ground plane preferably has a different thickness from the thickness of the second part 220 of the ground plane. The thickness of the second part of the 220 mass plane may be greater than the thickness of the first 150 part of the mass plane.

[0094] The first part 150 of the ground plane is made of a first conductive material. The second part 220 of the ground plane is made of a second conductive material. Preferably, these are screen-printable materials.

[0095] The first part 150 of the ground plane and the second part 220 of the ground plane are made of different materials.

[0096] On the sensor side, the first portion 150 of the ground plane is as thin as possible to prevent the sensor from losing sensitivity. Preferably, a material with mechanical properties similar to at least one of the materials present in the sensor will be chosen. The material will be selected such that the first portion 150 of the ground plane is no more rigid than the sensor. This prevents the device from losing its very high flexibility and avoids prestressing the piezoelectric material, which could reduce its performance.

[0097] Preferably, one of the sensor materials can be used to manufacture the first part of the ground plane.

[0098] This is for example a PEDOT:PSS type conductive polymer as for electrodes 110, 130. The thickness of the first part of the ground plane is preferably between 0.1 pm and 3 pm, for example 1 pm.

[0099] The second part 220 of the ground plane, having less constraint on mechanical rigidity, is made of a more conductive material than that used for the first part 150 of the ground plane protecting the sensor. It will advantageously be chosen to provide better protection against electromagnetic interference. Preferably, a silver- or carbon-based ink is chosen. The second part 220 of the ground plane has, for example, a thickness between 5 µm and 20 µm, for example around 10 µm.

[0100] The second part 220 of the ground plane can be made of the same material as the electrically conductive tracks 200, the electrodes 110, 130, or a mixture of these materials. The second part 220 of the ground plane is, for example, made of a mixture of PEDOT:PSS and silver. Such a material (PEDOT:PSS / Ag) makes it possible to obtain a uniform and smooth surface, even for a large ground plane. This then allows for easy printing of the second encapsulation layer.

[0101] A first insulating layer 140 is disposed between the sensor and the first part 150 of the ground plane. The first insulating layer 140 is made of a first dielectric material. A flexible material is chosen, preferably having a thickness between 5 µm and 10 µm and / or having a modulus of elasticity between 0.5 GPa and 10 GPa, for example 2 GPa. The first insulating layer 140 may to be printed. The nature and / or thickness of the material will be chosen to withstand the polarization stage of the piezoelectric material (i.e., it will be chosen to have high dielectric strength to prevent breakdown) and to avoid remanent polarization, which would make it an active material. Preferably, it has a dielectric strength greater than approximately 100 V / m. For example, if the piezoelectric active layer is PVDF-based, the same material can be used for the first insulation layer. The thickness of the first insulation layer is, for example, approximately three times that of the active layer.

[0102] The first layer of insulation 140 can be printed in several stages (for example in two stages) to reduce the risk of defects and breakdown phenomena.

[0103] To always guarantee the correct flexibility of the sensor, a total thickness of all printed layers of less than 30 pm is targeted, ideally less than 20 pm.

[0104] On the track side, the thickness and / or nature of the second insulating layer 210 above the conductive tracks 200 is chosen to exhibit good elasticity and dielectric strength properties, similar to the material of the first insulating layer 140 above the sensor. For example, the thickness of the second insulating layer 210 is between 10 µm and 40 µm, typically around 20 µm. However, since the tracks 200 are often thicker than the sensor (electrodes 110, 130 and piezoelectric layer 120) to achieve better conductivity (for example, in the case of silver tracks, their thickness may be around 10 µm), the dielectric material is thicker to ensure good insulation. For gold tracks deposited by photolithography and having a thickness of less than 500 nm, a thinner layer can be used (for example about 10 pm).

[0105] Parts 150 and 220 of the ground plane can be covered, respectively, by a first protective layer 160 and a second insulating layer 230. Thus, layers 160 and 230 protect the different parts of the ground plane. The insulating layers are, for example, layers made of a dielectric material. Although flexibility is less important for this part of the device, a total thickness of the printed layers of less than 60 µm may be chosen, for example.

[0106] The first protective layer 160 is preferably flexible, thin, and has mechanical properties similar to those of the other materials in the active zone ZI so as not to interfere with the measurement. It may be made of the same material as the first insulating layer 140. Advantageously, it is thinner than the first insulating layer 140. The total thickness of the stack in the active zone ZI is advantageously less than 20 or 30 µm.

[0107] Regarding the second protective layer 230, there are fewer constraints on this layer; it can be the same layer as layer 210. On the side of the second zone Z2, a total thickness of less than 60 pm is targeted.

[0108] According to a particular embodiment, for example shown in Figures 2A and 2B, an additional piezoelectric sensor, identical or different from the first piezoelectric sensor, is arranged on the second face 100b of the substrate 100. This double-sided configuration makes it possible to take advantage of the benefits of a bimorphic device (better sensitivity, distinction of types of mechanical stresses) but also to better protect the sensors from electromagnetic disturbances by protecting them with a ground plane on both sides.

[0109] The additional piezoelectric sensor comprises two electrodes 111, 131 and a piezoelectric layer 121. It is preferably arranged so as to be opposite the first piezoelectric sensor disposed on the first face of the substrate 100. The additional piezoelectric sensor is connected to additional electrically conductive tracks 201. A first part 151 of an additional ground plane covers the additional piezoelectric sensor and a second part 221 of the additional ground plane covers the additional electrically conductive tracks 201.

[0110] The first part 151 and the second part 221 have different flexibilities.

[0111] For example, the first part 151 of the additional ground plane has a Young's modulus lower than the Young's modulus of the second part 221 of the additional ground plane and / or the first part 151 and the second part 221 of the additional ground plane have different thicknesses, the thickness of the second part 221 of the additional ground plane preferably being greater than the first part 151 of the additional ground plane.

[0112] The first part 151 of the additional ground plane is at a third height h3 relative to the second main face 100b of the substrate 100, and the second part 221 of the additional ground plane is at a fourth height h4 relative to the second main face 100b of the substrate. The third and fourth heights are different. The first part 151 of the additional ground plane is connected to the second part 221 of the additional ground plane at an intermediate zone, which may coincide with zone Z3.

[0113] The materials previously mentioned for the elements positioned on the first face 100a of the substrate 100 can be used for the elements positioned on the second face 100b of the substrate 100.

[0114] In the various figures, only one sensor is shown on the first face 100a of the substrate 100 and possibly on the second face 100b of the substrate 100. However, several sensors may be arranged on the first face 100a and / or on the second face 100b of the substrate. There can be the same number of sensors on each face of the substrate or a different number of sensors.

[0115] The sensors can be arranged in a line or in matrix form.

[0116] Conductive lines, for example made of silver, can connect the sensors of the matrix or line to each other. Alternatively, the sensors can be connected to the measuring devices independently.

[0117] The conductive lines are preferably covered by a dielectric insulating layer. This may be the same layer as that covering the tracks 200. A ground plane, which may be rigid (for example, made of silver), is positioned above the conductive lines, for example, from the point where all the tracks converge. This may be the second part 220 of the ground plane extending above the lines. The ground plane covering the matrix is ​​advantageously flexible. It is, for example, made of PEDOT:PSS. This may be the first part 150 of the ground plane extending above the other sensors.

[0118] According to another particular embodiment, as shown for example in [Fig. 3], the piezoelectric device 1000 comprises one or more sensors arranged on the first face 100a of the substrate 100 and a ground plane 300 arranged on the second face 100b of the substrate 100. This allows the sensor to be protected by a ground plane on each side for improved efficiency. This ground plane may be an additional electrode formed on the second face 100b of the substrate 100. This ground plane positioned on the second face 100b can be connected to the ground plane printed on the first face 100a by means of a via 310 passing through the substrate 100. This additional electrode can also be screen-printed onto the second face 100b of the substrate 100 using the same materials as for the main ground plane.If the substrate is not compatible with printing on its back side (this is particularly the case with TPU due to the presence of a coating ('liner')), it may be easier to deposit a thin metallic layer by evaporation, such as a layer of gold with a thickness of approximately 100 nm.

[0119] To obtain a dual ground plane, it is also possible to print two ground planes: the first ground plane covers the sensor and the tracks as previously described, and the second ground plane is positioned between, on the one hand, the substrate 100 and, on the other hand, the sensor and the tracks 200 ([Fig. 4]). A first portion 180 of this ground plane is covered by an insulating layer 190 to separate it from the sensor. A second portion 240 of this ground plane is covered by an insulating layer 250 to separate it from the tracks 200. One of the main advantages of this configuration is that, with the lower ground plane positioned between the substrate 100 and the encapsulation, parasitic charges that may arise from the interface between these two materials are reduced. As all the technology is then printed above the The lower ground plane is advantageously uniform. It is, for example, made of PEDOT:PSS. It is covered by inactive dielectric insulation layers 190, 250 (preferably made of P(VDF-TrFE)) to isolate it from the rest of the technology, both the sensor part and the track part.

[0120] The use of a double ground plane offers many possibilities for protecting the device (bimorphic or lower and upper ground planes).

[0121] The manufacturing process for the piezoelectric device comprises the following steps: - deposit on a first main face 100a of a substrate 100, a first electrode 110, a piezoelectric layer 120 and then a second electrode 130 to form the piezoelectric sensor, - deposit electrically conductive tracks 200 on the first face 100a of the substrate 100, - deposit a first layer of insulation 140 on the piezoelectric sensor and a second layer of insulation 210 on the electrically conductive tracks 200, - cover the piezoelectric sensor and the first layer of insulation 140 with a first conductive element so as to form a first part 150 of a ground plane, the first part 150 of the ground plane being at a first height hl relative to the first main face 100a of the substrate 100, - cover the electrically conductive tracks 200 and the second insulation layer 210 with a second conductive element, so as to form a second part 220 of a ground plane, the second part 220 of the ground plane being at a second height h2 relative to the first main face 100a of the substrate 100, the first part 150 being more flexible than the second part 220.

[0122] It would be possible to deposit the second part 220 of the ground plane and then the first part 150 of the ground plane. The same applies to the insulation layers 140, 210, which could be deposited in reverse order.

[0123] The first layer of insulation 140 and the second layer of insulation 210 are superimposed and in contact at the intermediate zone Z3.

[0124] The first part 150 of the ground plane and the second part 220 of the ground plane are superimposed and in contact at the intermediate zone Z3.

[0125] The resulting ground plane is a flexible ground plane.

[0126] Preferably, the first part 150 of the site plan and the second part 220 of the site plan are printed. They can be applied by screen printing.

[0127] Preferably, the other elements of the device 1000 positioned on the substrate 100 are also deposited by screen printing. The piezoelectric device 1000 is ultra-flexible.

[0128] According to a particular embodiment, one or more dielectric layers 160, 230 can be deposited on the ground plane in order to protect it from the external environment.

[0129] Care shall be taken to ensure that the deposition of each layer does not damage the underlying layers. In particular, the solvent used in the printing inks shall be chosen so as not to deteriorate the underlying layers. It is particularly advantageous to use, for the first dielectric layer 140 and for the first part 150 of the ground plane, one of the materials present in the sensor. For example, it is possible to use inks that use water as a solvent. A ground plane made of PEDOT:PSS is, for example, ideal.

[0130] This printing order of the different layers allows them to be superimposed while ensuring good cohesion between each layer. With such a process, it is possible to superimpose the different layers even for large steps without losing continuity.

[0131] The process also includes a step of crystallizing (or aligning the dipoles) the layer 120 of piezoelectric material to improve its piezoelectric performance. This irradiation is carried out, for example, with a UV flash light, with a flash duration, or pulse, of between approximately 500 ps and 2 ms, a fluence (energy delivered per unit area) of between approximately 15 J / cm² and 25 J / cm², and with a light wavelength of between approximately 200 nm and 380 nm. The number of UV flashes, or pulses, produced during this irradiation varies depending on the thickness over which the piezoelectric material is to be crystallized. For example, for a P(VDF-TrFe) thickness of approximately 2 pm, irradiation can be implemented with a fluence of approximately 17 J / cm2, a pulse duration of approximately 2 ms and a number of pulses of 5.

[0132] The piezoelectric material, possibly having undergone previous crystallization, is then subjected to annealing, for example, carried out at about 130°C for about 60 min, to finalize the total crystallization of the piezoelectric material.

[0133] The crystallization of the piezoelectric material can therefore be carried out in two stages: firstly, irradiation by UV light pulse to properly crystallize the second face of the layer in piezoelectric material in order to increase its thermal conductivity, then a thermal annealing completing the crystallization for the rest of the piezoelectric material not crystallized by the previous irradiation.

[0134] When the piezoelectric material is a P(VDF-TrFe)-based copolymer, a biasing step is performed before use. This step can be carried out, for example, by applying a direct current voltage across its terminals via the electrodes to improve the piezoelectric coefficient of the material. This biasing is performed only once for the entire lifetime. of the piezoelectric material. This electric field polarization can be performed at room temperature or under heat (up to approximately 100°C). When polarization is performed at room temperature, a DC voltage of up to approximately 150 V / µm of piezoelectric layer thickness can be applied for a duration ranging from a few seconds to a few minutes. For example, a voltage of 120 V / µm could be applied for 20 seconds. When polarization is performed under heat, for example at a temperature of approximately 90°C, a DC voltage of approximately 50 V to 120 V per micron of piezoelectric layer thickness can be applied for a duration ranging from approximately 1 to 5 minutes. The temperature is then lowered to room temperature, and the electric field applied to the piezoelectric material via the applied DC voltage is then switched off.Such polarizations allow PVDF to achieve piezoelectric coefficients between approximately 10 and 40 pC / N.

[0135] The molecules within the piezoelectric layer remain oriented in this way, even when the material is no longer subjected to this electric field. The material can thus be polarized by applying an initial polarization voltage across the electrodes. Preferably, a piezoelectric material thickness of approximately 3 pm or less is chosen to promote the polarization of the piezoelectric material by this capacitance, and the level of the electrical voltage applied between the electrodes to achieve the initial polarization of the piezoelectric material (when the piezoelectric material is to be initially polarized) is also chosen.

[0136] For example, we will aim for an ideal remanent polarization of 8 pC / cm2.

[0137] Annealing is advantageously carried out at the end of the process, or between the different steps. Annealing is, for example, at a temperature between 100°C and 150°C, preferably around 100°C to remove residual traces of solvent and / or finalize the crystallization of the piezoelectric material.

[0138] It is possible to apply a first DC voltage (low electric field in the piezoelectric material, for example, around 30 V / pm for P(VDF-TrFE)) to remove as much electrical charge as possible from the sensor. Then, a second, high voltage (for example, around 120 V / pm for P(VDF-TrFE)) is applied to eliminate any potential sensor defects. The second voltage can be AC ​​at a first frequency, for example, 10 Hz. It is then possible to apply the same voltage at a lower frequency, for example, 1 Hz, to finalize the alignment of the dipoles.

[0139] Note that during polarization, the P(VDF-TrFE) serving as the insulating dielectric, having a greater thickness, is subjected to an electric field less than 50 V / pm, which is its coercive field, it therefore does not polarize and remains inactive.

[0140] One or more protective layers 160, 230 can then be deposited to protect the ground plane. The protective layer(s) 160, 230 are preferably deposited before the polarization step. This allows all the layers of the device to be printed sequentially. This limits the risk of sensor depolarization during the annealing step after layer deposition.

[0141] The ground plane can be connected to the ground of the measuring electronics.

[0142] According to another embodiment, for example shown in [Fig. IC], it is It is possible to make a through hole in the dielectric insulation layer 140, near the sensor, to short-circuit the first part 150 of the ground plane and the ground electrode. This can be advantageous in case of a possible poor transition between ground planes in the intermediate zone. The ground electrode is preferably the upper electrode 130: it serves as additional protection against electromagnetic interference to complement the performance of the flexible ground plane, and / or to prevent any breakdown at the sensor during polarization.

[0143] According to another particular embodiment, two substrates 101, 102 covered by at least one sensor are assembled face-to-face, for example by means of an adhesive layer, to form a substrate 100 ([Fig. 2A]), or at least two sensors are printed on either side of the substrate 100 ([Fig. 2B]). These embodiment variants make it possible to form piezoelectric bimorphs. The presence of a ground plane above and below the sensors ensures optimal protection.

[0144] In general, the invention can relate to all applications requiring the use of a highly flexible piezoelectric sensor. Thin piezoelectric films are particularly interesting for manufacturing lightweight and flexible force sensors for measuring small forces. The invention finds applications in the medical field: sensor for measuring coaptation forces in a heart valve, tactile sensor on the skin, etc.

[0145] Illustrative and non-limiting example(s)

[0146] In this first example, the sensors have an active diameter (diameter of the smallest electrode) between 2 and 10 mm. The sensors are screen-printed onto a TPU substrate with a thickness of 100 µm and a Young's modulus of 50 MPa. Such a substrate is, for example, commercially available under the name Intexar™ TE-1 IC from DuPont™. A layer of PEDOT:PSS (a solution commercially available, for example, from Heraeus) is first printed to form the lower electrode. Annealing at 135°C for 30 minutes allows the solvent to evaporate. The final thickness is approximately 1 µm thick. Next, the active piezoelectric material, P(VDF-TrFE) 80 / 20 (commercially available from Arkema Piezotech®), is printed, followed by annealing for 3 minutes at 135°C under vacuum. This layer is printed in two stages to eliminate defects. The total layer thickness is 3 µm. The top electrode of PEDOT:PSS is then deposited in the same manner as the bottom electrode.

[0147] The conductive tracks are printed with silver ink (for example, ink marketed by the Taiyo company) followed by annealing for 10 minutes at 135° (thickness approximately 10 pm).

[0148] The first layer of dielectric is then printed on the track part (marketed for example by Loctite under the commercial reference ED AG PF 455B) which is crosslinked by exposure under a mercury (UV) lamp.

[0149] For a speed of 10 m / min and 35% of the power and 3 passes we have:

[0150] [Tables 1] mJ / cm2 mW / cm2 UVA 430 248 UVB 257 155 UVA2 98 61 UVV 340 195

[0151] Depending on the substrate's Young's modulus, this fluence range (the energy delivered per unit area) will be adjusted between 100 and 500 mJ / cm². UVA and UVB radiation account for the majority of the crosslinking energy. If the substrate has a Young's modulus below 0.5 GPa, as is the case with TPU, a fluence of around 150 mJ / cm² will be used to avoid creating excessive stress in the lower layers. The printing process is performed in two stages to eliminate defects (total layer thickness approximately 30 µm).

[0152] The dielectric material is then printed over the sensor. P(VDF-TrFE), which will remain inactive, is used with the same method as before. The printing is done in three stages to eliminate defects (final layer thickness 9 µm). This order of printing is used because, in the intermediate zone where the two dielectric materials overlap, P(VDF-TrFE) adheres well to 455B, but the reverse is not true.

[0153] The ground plane is then printed onto the track area. A mixture of PEDOT:PSS and silver (commercially available under the reference HPS-021) (50 / 50 by volume) is used to obtain a fairly thin (4 / 5 µm), well-crosslinked, homogeneous, and flat layer. The ground plane is then printed onto the sensor area: PEDOT:PSS is used with the same parameters as for the electrodes. The order in which the ground planes are deposited is of little importance here, as their adhesion is quite good in all areas. The difficulty lies rather in the fact that the 1 µm PEDOT:PSS ground plane must cross the 30 µm dielectric step. Therefore, in case the contact is not perfect, it is important that the PEDOT:PSS ground plane also be connected to the ground trace near the sensor.

[0154] Finally, dielectric protective layers are printed to protect the underlying layers (ground planes, insulation layer on the track area (15 µm thick) and insulation layer on the sensor area (1 µm thick). At the sensor level, we used only materials with a relatively low Young's modulus (E < 3 GPa) and the total thickness of the printed layers is well below 20 µm.

[0155] To manufacture a piezoelectric bimorph, once two identical sensors are printed in the same way on two separate substrates, they are laser-cut with the same geometry. They are glued face-to-face substrate to substrate, using the precision of the cutting for alignment, using a double-sided adhesive that we manufacture ourselves, consisting of a 25 µm thick polyimide film onto which a commercial adhesive, for example from 3M under the reference VHB, has been transferred on each side.

[0156] The active piezoelectric layer is then biased using P(VDF-TrFE). A first low DC voltage (electric field in the piezoelectric material ~ 30 V / pm) is applied for 3 minutes to discharge as much electrical charge as possible from the sensor. A first AC voltage ramp-up is then performed at 10 Hz (from approximately 30 V / pm to 120 V / pm) to eliminate certain sensor defects. The same voltage ramp-up is then repeated at 1 Hz to finalize the alignment of the dipoles.

[0157] Note that during polarization, the P(VDF-TrFE) serving as an insulating dielectric having a greater thickness, it is subjected to an electric field of less than 50 V / pm which is its coercive field, it therefore does not polarize and remains inactive.

[0158] The effectiveness of the ground plane is then verified. From a flexibility standpoint, if bending stiffness is considered a good indicator, the ground plane only increases it by 30% at the sensor (1.9 x 10⁵ vs. 2.5 x 10⁵ Nm, considering the presence of the substrate and printed layers). The sensor remains very flexible even with the ground plane.

[0159] The sensitivity of the sensor is then checked. To calibrate the compression device, it is attached to a rigid glass plate with a lightweight double-sided adhesive (commercially available, for example, under the reference Teraoka 9030W). Then, pressure is applied with varying forces using a flat indenter perfectly parallel to the sensor surface at a frequency of approximately 1 Hz. The piezoelectric charges generated are measured with a Kistler 5015 charge amplifier over a period of time A long integration time and a 10 Hz low-pass filter are used. The sensor's sensitivity is then compared with and without a ground plane ([Fig. 5]). Only a slight loss of sensitivity (35%) is observed; the ground plane does not impede the sensor's proper operation. This loss of sensitivity is perfectly explained by the increase in the device's overall thickness due to the ground plane.

[0160] It is also observed that the ground plane helps to eliminate unwanted electromechanical phenomena. Indeed, in application, the entire sensor can be subjected to mechanical stresses, not just the piezoelectric active part. The other materials (notably the substrate and encapsulations) and their interfaces can be the source of weak but unavoidable electromechanical phenomena such as flexoelectricity or triboelectricity. The charges thus generated can then circulate in the conductive tracks of the sensor and disturb the piezoelectric signal. The ground plane helps to reduce this disturbance by partially dissipating it. To verify this, mechanical tests are performed on the track portion of the sensors. The active part is fixed and is not subjected to any stress in order to avoid any piezoelectric effect. The tracks are stretched horizontally and fixed to two clamps spaced 8 cm apart.An indenter applies a 15 mm vertical displacement to the tracks between the two jaws to create a three-point bend. Since the tracks are fixed to the jaws, a mixture of bending and tension is applied to the sensor track portion. The generated charges are then measured with a Kistler 5015. Sensor tracks with only encapsulation and no ground plane generate numerous parasitic electrical charges. With a ground plane, significantly fewer charges are generated (Figures 6A and 6B).

[0161] Finally, the effectiveness of the ground plane for protection against electromagnetic interference is verified. A DDC118 charge amplifier (without a low-pass filter to protect against noise) is used for this purpose, and the noise level between two sensors with and without a ground plane is compared in different environments ([Fig. 7]). To simulate medical applications, the sensors are immersed in water. To add a noise source, the sensor can also be placed in contact with human skin (as with a touch sensor). Since the skin is electrically charged, it releases charges into the water (a conductive environment), thus creating noise. It is observed that the ground plane makes the sensor almost insensitive to changes in the environment and reduces noise by nearly four times when in contact with skin and water.

[0162] Figure 8 shows several piezoelectric devices made with a sensor (Cl and C2) or with a sensor matrix (C3 and C4). Some sensors have a ground plane (C2 and C4) and some sensors do not have a ground plane (Cl and C3).

[0163] Various embodiments and variations have been described. A person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will become apparent to a person skilled in the art.

[0164] Finally, the practical implementation of the embodiments and variants described is within the reach of a person skilled in the art, based on the functional indications given above.

Claims

Demands

1. Piezoelectric device (1000) comprising a substrate (100) having a first face (100a) covered by a piezoelectric sensor and electrically conductive tracks (200), the piezoelectric sensor comprising an organic piezoelectric layer (120) disposed between a first electrode (110) and a second electrode (130), the sensor being connected to the electrically conductive tracks (200) intended to be connected to measuring devices, the device (1000) comprising a ground plane, a first part (150) of the ground plane covering the sensor, and being separated from the upper electrode (130) by a first layer of dielectric insulation (140), and a second part (220) of the ground plane covering the electrically conductive tracks (200) and being separated from the electrically conductive tracks (200) by a second layer of dielectric insulation (210),the first part (150) of the ground plane being in contact with the second part (220) of the ground plane at an intermediate zone (Z3) so as to form a continuous ground plane, the first part (150) of the ground plane being more flexible than the second part of the plane (220).

2. Device according to claim 1, wherein the first ground plane (150) is made of a first material having a first Young's modulus and wherein the second part (220) of the ground plane is made of a second material having a second Young's modulus greater than the first Young's modulus.

3. Device according to any one of the preceding claims, wherein the first part (150) of the ground plane is at a first height relative to the first main face (100a) of the substrate (100) and the second part (220) of the ground plane is at a second height relative to the first main face (100a) of the substrate (100), the first height and the second height being different, the second height preferably being greater than the first height.

4. Device according to any one of the preceding claims, wherein the first part (150) of the ground plane and the second part (220) of the ground plane have different thicknesses.

5. Device according to any one of the preceding claims, wherein the first part (150) of the ground plane, the first electrode (110) and the second electrode (130) are made of PEDOT:PSS and / or wherein the second part (220) of the ground plane is made of a mixture of PEDOT:PSS and silver.

6. Device according to any one of the preceding claims, wherein the organic piezoelectric layer (120) comprises a matrix of a copolymer or terpolymer of PVDF, preferably of P(VDF-TrFe), and wherein the first dielectric insulation layer (140) is of the same material as the matrix of the organic piezoelectric layer (120) and / or wherein the second dielectric insulation layer (210) is of an epoxy or acrylic resin, preferably a cyanoacrylate.

7. Device according to any one of the preceding claims, wherein the first part (150) of the ground plane is covered by a first protective layer (160), for example of the same material as that of the first insulation layer (140), and / or wherein the second part (220) of the ground plane is covered by a second protective layer (230), for example of the same material as the material of the second dielectric insulation layer (210).

8. A device according to any one of claims 1 to 6, wherein the device comprises an additional ground plane: - the additional ground plane being positioned on the second face (100b) of the substrate (100), or - the additional ground plane being positioned between, on the one hand, the first face (100a) of the substrate (100) and, on the other hand, the sensor and the electrically conductive tracks (200), or - the additional ground plane covering an additional sensor and additional electrically conductive tracks (201) disposed on the second face (100b) of the substrate (100), a first part (151) of the additional ground plane covering the sensor and a second part (221) of the additional ground plane covering the additional conductive tracks (201),the first part (151) of the additional ground plane being in contact with the second part (221) of the additional ground plane at an intermediate zone so as to form an additional ground plane, continuous, the first part (151) being more flexible than the second part (221) of the additional ground plane.

9. A method for manufacturing a piezoelectric device comprising the following steps: - depositing on a first principal face (100a) of a substrate (100), a first electrode (110), a piezoelectric layer (120), and then a second electrode (130) to form a piezoelectric sensor, - depositing on the substrate (100), electrically conductive tracks (200), - depositing a first insulating layer (140) on the piezoelectric sensor and a second insulating layer (220) on the electrically conductive tracks (200), - covering the piezoelectric sensor and the first insulating layer (140) with a first conductive material so as to form a first part (150) of a ground plane, - covering the electrically conductive tracks (200) and the second insulating layer (210) with a second conductive material, so as to form a second part (220) of a ground plane mass,the first insulation layer (140) and the second insulation layer (210) being in contact at an intermediate zone (Z3), the first part (150) of the ground plane and the second part (220) of the ground plane being in contact at the intermediate zone (Z3) so as to form a continuous ground plane, the first part (150) of the ground plane and the second part (220) of the ground plane being screen-printed, the first part (150) of the ground plane being more flexible than the second part (220) of the ground plane.

10. A method according to claim 9, wherein the first part (150) of the ground plane is at a first height relative to the first main face (100a) of the substrate (100) and wherein the second part (220) of the ground plane is at a second height relative to the first main face (100a) of the substrate (100), the first height and the second height being different.

11. A method according to any one of claims 9 to 10, wherein the first part (150) of the ground plane is in PEDOT:PSS and / or

12.

13. in which the second part (220) of the ground plane is in a mixture of PEDOT:PSS and silver. A method according to any one of claims 9 to 11, wherein an additional ground plane, preferably screen-printed, is formed on the second face (100b) of the substrate (100), or between, on the one hand, the first face (100a) of the substrate (100) and, on the other hand, the sensor and the electrically conductive tracks (200). A method according to the preceding claim, wherein a first part (151) of the additional ground plane covers an additional sensor formed on the second face (100b) of the substrate (100) and wherein a second part (221) of the additional ground plane covers additional conductive tracks (201) formed on the second face (100b) of the second substrate (100), the first part (151) of the additional ground plane being in contact with the second part (221) of the additional ground plane at an intermediate zone so as to form a continuous additional ground plane, the first part (151) of the additional ground plane being more flexible than the second part (221) of the additional ground plane.

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