PIEZOELECTRIC SENSOR

Abstract

A piezoelectric sensor, comprising at least one first electrode, at least one second electrode, and a piezoelectric material, wherein the piezoelectric material has an anisotropic electromechanical coupling and the at least one first and second electrodes are at least in part embedded in the piezoelectric material, the piezoelectric material having a first surface wherein the electrodes extend vertically within the piezoelectric material from the first surface.

Claims

1. A piezoelectric sensor (1), comprising: at least one first electrode (3); at least one second electrode (5); a piezoelectric material (6); wherein the piezoelectric material (6) has an anisotropic electromechanical coupling, and the at least one first and second electrodes (3,5) are at least in part embedded in the piezoelectric material (6), the piezoelectric material (6) having a first surface (4), wherein the electrodes (3,5) extend substantially at a right angle within the piezoelectric material (6) from the first surface (4).

2. The piezoelectric sensor (1) of claim 1, wherein the first and second electrodes (3,5) are laterally interdigitated first and second electrode fingers forming an intermeshing comb structure within the piezoelectric material and the electrode fingers are individually electrically connectable.

3. The piezoelectric sensor (1) of claim 2, wherein the first and the second electrode fingers (3,5) are arranged in an angle 0<α<90 degrees in relation to one another.

4. The piezoelectric sensor (1) of claim 1, wherein the first and second electrodes (3,5) form a star-shaped arrangement in relation to the first surface (4).

5. The piezoelectric sensor (1) of claim 1, wherein the piezoelectric material (6) is additionally pyroelectric.

6. The piezoelectric sensor (1) of claim 1, wherein the first and second electrodes (3,5) are disc-shaped or elliptical.

7. The piezoelectric sensor (1) of claim 1, wherein the sensor (1) comprises a substrate (2), wherein the piezoelectric material (6) forms a layer on the substrate (2).

8. The piezoelectric sensor (1) of claim 1, wherein the sensor (1) comprises a third electrode (11) spaced apart from the first and second electrodes (3,5).

9. The piezoelectric sensor (1) of claim 1, wherein the sensor (1) comprises a substrate (2), wherein the piezoelectric material (6) forms a layer on the substrate (2) and wherein a third electrode (11) is arranged between the piezoelectric material (6) and the substrate (2).

10. The piezoelectric sensor (1) of claim 8, wherein a primary orientation of the polarization (7) of the piezoelectric material (6) between the third electrode (11) and a top-side (9) of the first and second electrodes (3,5) is substantially parallel to a vertical extension of the first and second electrodes (3,5) and substantially perpendicular to a plane representative of a lateral extension of the third electrode (11).

11. The piezoelectric sensor (1) of claim 1, wherein the sensor (1) comprises low-power circuitry arranged for harvesting electrical energy generated by the piezoelectric material (6) upon a deformation of the piezoelectric material (6) by mechanical stress, and wherein the circuitry is arranged for signal processing using a wireless transmitter.

12. A sensor array (12), comprising a plurality of sensors (1) according to claim 1, wherein the sensors (1) are rotated in respect to one another.

13. The sensor array (12) of claim 12, wherein the sensor array (12) comprises a first and a second sub-array (13,14), each sub-array (13,14) comprising at least two of the plurality of sensors (1), wherein the at least two sensors (1) of each sub-array (13,14) are arranged non-parallel in respect to one another.

14. A method for manufacturing a piezoelectric sensor (1), the method comprising: providing a plurality of first and second electrodes (3,5) in a single layer; disposing an active material (6) over the first and second electrodes (3,5).

15. The method of claim 14, wherein providing the plurality of first and second electrodes (3,5) in the single layer is performed by one of the following or any combination thereof: a printing process; a lithography process; microfluidic structuring.

16. The method of claim 14, further comprising: providing a substrate (2); disposing the active material (6) onto the substrate (2), forming a piezoelectric polymer layer (6); imprinting channels (17) into the piezoelectric polymer layer (6); depositing an electrode material in the channels (17) to provide the plurality of first and second electrodes (3,5).

17. The method of claim 16, wherein the electrode material is a conductive ink (19).

18. The method of claim 17, wherein the method further comprises: thermally/UV curing or photonic sintering the conductive ink (19).

19. The method of claim 16, wherein disposing the active material (6) onto the substrate (2) is performed by one of the following: spin casting; drop coating; bar coating; screen printing; ink-jet printing; gravure printing, physical or chemical vapor deposition, atomic layer deposition.

20. The method of claim 16, wherein the channels (17) have rectangular, trapezoidal or triangular profiles (18), defining a shape of the first and second electrodes (3,5).

21. The method of claim 16, further comprising: treating a surface of the piezoelectric polymer layer after the imprinting.

22. The method of claim 16, wherein the piezoelectric polymer layer (6) has a thickness which is higher than the height of the electrodes (3,5).

23. The method of claim 16, further comprising: poling, to align ferroelectric domains within the piezoelectric polymer layer (6).

24. The method of claim 16, further comprising: forming a third electrode (11), wherein the third electrode is spaced apart from the first and second electrodes (3,5) and wherein forming the third electrode comprises: depositing a layer of electrode material at a surface of the piezoelectric polymer layer (6) substantially perpendicular to the first and second electrodes (3,5).

25. The method of claim 14, wherein the active material is a piezoelectric material, wherein the piezoelectric material (6) has an anisotropic electromechanical coupling, and the plurality of first and second electrodes (3,5) are at least in part embedded in the piezoelectric material (6); and wherein the method further comprises: forming the electrodes (3,5) to extend substantially at a right angle within the piezoelectric material (6) from a first surface (4) of the piezoelectric material.

26. The piezoelectric sensor (1) of claim 7, wherein the substrate (2) is a flexible, elastic substrate (2).

27. The piezoelectric sensor (1) of claim 26, wherein the substrate (2) is a polymer foil.

28. The piezoelectric sensor (1) of claim 27, wherein the polymer foil is polyethylene terephthalate (PET).

29. The piezoelectric sensor (1) of claim 8, wherein the third electrode (11) is arranged at the first surface (4) of the piezoelectric material (6).

30. The piezoelectric sensor (1) of claim 8, wherein the third electrode (11) is arranged at a second surface opposite the first surface (4).

31. The sensor array of claim 13, wherein the at least two sensors (1) of each sub-array (13, 14) are arranged at an angle of 45° degrees in respect to one another.

32. The method of claim 17, wherein the conductive ink (19) is silver (Ag), copper (Cu) or PEDOT:PSS.

33. The method of claim 17, wherein the conductive ink (19) is deposited into the channels (17) by capillary force.

34. The method of claim 16, wherein the imprinting is performed by one of the following: hot embossing, UV imprinting, mold casting.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0038] Exemplary embodiments of the invention are described in relation to the following drawings:

[0039] FIG. 1 illustrates a piezoelectric sensor from the prior art;

[0040] FIG. 2 illustrates a schematic structure of a piezoelectric sensor according to the invention;

[0041] FIG. 3 illustrates a schematic structure of a further embodiment of the piezoelectric sensor according to the invention;

[0042] FIG. 4 illustrates a schematic structure of the piezoelectric sensor according to FIG. 3;

[0043] FIGS. 5 to 7 illustrate a schematic structure of a sensor array according to the invention;

[0044] FIGS. 8a and 8b illustrate the piezoelectric sensor of FIG. 3 in an operating state;

[0045] FIGS. 9a to 9e illustrate process steps in an exemplary manufacturing process for a piezoelectric sensor according to the invention;

[0046] FIG. 10 illustrates a flow diagram of an imprinting process according to an aspect of the invention;

[0047] FIG. 11 illustrates another exemplary sequence of process steps in an exemplary manufacturing process for a piezoelectric sensor according to the invention;

[0048] FIG. 12 illustrates another exemplary sequence of process steps in an exemplary manufacturing process for a piezoelectric sensor according to the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0049] FIG. 1 shows a piezoelectric sensor 1 from the prior art. The sensor 1 comprises a substrate 2. A first electrode, of a first electrical polarity, is disposed at a first (upper) surface 4 of the substrate 2. The sensor 1 further comprises a second electrode 5 of a second electrical polarity. The second electrode is arranged on top of a piezoelectric material 6, forming an active layer between the first electrode 4 and the second electrode 5. The piezoelectric material 6 has a primary orientation of the polarization 7 which is perpendicular in relation to a plane representative of the first electrode 3, the second electrode 5 and the substrate 2. If a force Fn acts upon an upper surface 8 of the sensor 1, the induced mechanical stress will result in a deformation of the piezoelectric material 6 and hence in a displacement of electrical charges in the piezoelectric material 6. A resulting voltage between the first electrode 3 and the second electrode 5 represents quantitative measure for the acting force Fn.

[0050] FIG. 2 shows a schematic structure of a piezoelectric sensor 1 according to the invention. The first and second electrodes are arranged on the substrate 2 and extend substantially at a right angle (vertically) from the upper surface 4 of the substrate 2 into the piezoelectric material 6. The first and second electrodes (3,5) have a substantially rectangular profile and are of alternating electrical polarity. The first and second electrodes form an interdigitating structure within the piezoelectric material 6. The first and second electrodes (3,5) are embedded in the piezoelectric material 6 and arranged substantially parallel in respect to one another. The first and second electrodes (3,5) are spaced apart from the upper surface of the sensor 8. Between a top side 9 of the first and second electrodes (3,5) a layer 10 of piezoelectric material 6 is formed, covering the top sides 9 of the first and second electrodes (3,5). The piezoelectric material 6 has an anisotropic electromechanical coupling. Stress applied upon the piezoelectric material 6 will cause an electrically measurable change in its polarization. Force F comprises normal component Fn and tangential component Ft. The component of F which is substantially parallel to a primary orientation of the polarization 7 of the piezoelectric material 6 between the first and second electrodes (3,5) will cause the major share in the electrically measureable displacement of electrical charges in the piezoelectric material 6. Ft is the component of F acting parallel to the primary orientation of the polarization 7 and hence causing the major share in the measurable shift of polarization in the piezoelectric material 6 between the first and second electrodes (3,5).

[0051] FIG. 3 shows a schematic structure of a further embodiment of the piezoelectric sensor according to FIG. 2. A third electrode 11 is arranged on the piezoelectric material layer 6, forming a top electrode. A footprint of the third electrode 11 covers at least parts of each one of the first and second electrodes (3,5).

[0052] FIG. 4 shows a schematic cross-section of the piezoelectric sensor 1 according to FIG. 3. The third electrode 11 covers the piezoelectric material 6 forming the upper surface 8 of the sensor 1. The third electrode is spaced apart from the top sides 9 of the first and second electrodes (3,5) by the layer 10. In addition to the polarization of the piezoelectric material 6 between the electrodes (3,5) (parallel to the third electrode 11 and substantially perpendicular to the electrodes (3,5)), the piezoelectric material 6 forming the layer 10 above the top sides of the first and second electrodes (3,5) has a primary orientation of the polarization 7 which is substantially parallel to the first and second electrodes (3,5) and substantially perpendicular to the third electrode 11.

[0053] FIGS. 5 to 7 show a sensor array 12 according to the invention. The sensor array 12 comprises a plurality of sensors 1 of the same kind. The first and second electrodes of each of the respective sensor are carried out as intermeshing comb structures. Each sensor 1 of the plurality of sensors 1 has a primary orientation of the polarization 7 between the first and second electrodes (3,5). Each sensor is arranged to detect tangential forces in the same direction as its primary orientation of polarization. In order to generate both a quantitative and a qualitative measure of an acting force, the sensors are rotated in respect to each other. The plurality of sensors is subdivided into a first sub-array 13 and a second sub-array 14. Each sub array comprises at least two sensors 1. Each sensor 1 represents a quadrant of a plane in which a tangential or shear force acts. If a force F1, as in FIG. 6, is applied parallel to an axis of a virtual coordinate system 15, those sensors having first and second electrodes (3,5) which are not parallel to the direction of the force F1, will generate a measurable signal. Thereby, sensors 1 will generate different signal levels according to their rotation relative to the acting force F1. In FIG. 7, the force F2 acts in a 45° degree angel in relation to the coordinate system 15. The sensor 1 whose first and second electrodes are perpendicular to the direction of F2 will generate the highest signal level. The sensor 1 whose first and second electrodes (3,5) are parallel to the direction of F2 will generate no (or the smallest) signal. The sensors 1 whose first and second electrodes (3,5) are rotated by 45° degrees in relation to the direction of F2 will each generate the same signal level but with different signs.

[0054] FIGS. 8a and 8b show the piezoelectric sensor 1 of FIG. 3 in an operating state. An electric connection 16 of the first and second electrodes (3,5) as well as the third electrode 11 as shown in FIG. 3 is employed to distinguish between lateral and normal force (FIG. 8a) or stress (FIG. 8b) components. As shown, the third electrode 11 is connected to a reference potential ϕref and either the potential differences V+, V− or currents I+, I− are measured between reference potential and the first and second electrodes (3,5) of different polarity. The sum signal is proportional to the normal force or stress acting upon the sensor, whereas the difference signal will correspond to the lateral force or stress contributions.

[0055] FIGS. 9a to 9e illustrate process steps in an exemplary manufacturing process of a piezoelectric sensor 1 according to the invention. In FIG. 9a a substrate 2 is provided. In FIG. 9b a polymer piezoelectric material 6 is applied onto the substrate 2 by a printing process to form a layer with a thickness significantly higher than the height of the first and second electrodes (3,5). In FIG. 9c microfluidic channels 17 with rectangular, trapezoidal or triangular profile 18 defining the shape of the first and second electrodes (3,5) are imprinted into the piezoelectric material 6 by means of hot embossing (e.g. T-NIL). The result—the channels—are shown in FIG. 9d. In FIG. 9e, upon optional surface treatment, a conductive ink 19 (e.g. Ag or Cu) is deposited and driven into the microfluidic channels 17 by capillary forces and thermally cured.

[0056] FIG. 10 shows a flow diagram of an exemplary nanoimprint lithography process (NIL) for manufacturing the piezoelectric sensor 1. A UV curable resin is applied onto the substrate 2 and imprinted residual free with a stamp containing the electrode structures and cured. A metal layer is deposited on top of the resin. The resin is removed lifting off the metal layer on top of the resin. Only metal in the imprinted area remains on the substrate 2. The transferred metal layer can be enhanced by electro deposition of the same or another metal, where the prestructured metal layer serves as a cathode during electroforming.

[0057] FIG. 11 shows another exemplary sequence of process steps in another exemplary manufacturing process for a piezoelectric sensor 1 according to the invention. The process involves:

[0058] a) Deposition of conductive feed lines 20 to connect the first and second electrodes (3,5), e.g. by gravure or screen printing (possible on R2R). The substrate 2 is a foil with high thermal stability (up to 180° C.), e.g. polyimide.

[0059] b) P(VDF:TrFE) paste as piezoelectric material 6 is applied with an overlay between the feed lines 20.

[0060] c) Microfluidic channels 17 are hot embossed into the piezoelectric material 6 according to the scheme depicted in FIGS. 9a-9e.

[0061] d) Upon optional surface treatment, e.g. with ozone for a few minutes, the conductive ink 19, e.g. Ag nanoparticles in solution, is deposited into the channels 17 via reservoirs provided on an imprint design. Thereby further channels (not shown) are provided allowing the conductive ink 19 to flow outwards to get in contact with the feed lines 20. Afterwards the conductive ink 19 is sintered at elevated temperature creating conductive first and second electrodes (3,5) embedded in the piezoelectric material 6. Optionally, the first and second electrodes (3,5) can be enhanced by electro deposition of the same or other metal. Applying this process, 7 μm deep and 8.8 μm wide channels are formed in P(VDF:TrFE)=70:30% mol after screen printing on polyimide foil as substrate 2. An Ag nanoparticle conductive ink 19 is dropped into the reservoir and subsequently transported into the channels 17 via micro capillary forces and cured at 150° C. The Ag nanoparticle conductive ink forms a layer that covers both bottom and sidewalls of the channels 17 with a thickness range of 0.2 to 1.3 μm.

[0062] FIG. 12 illustrates another exemplary sequence of process steps in an exemplary process for structuring the first and second electrodes (3,5) of the piezoelectric sensor 1. The first and second electrodes (3,5) are structured by means of photolithography and electroforming. The process is as follows:

[0063] a) The substrate 2 is coated with a conductive, metallic layer 21 serving later as seed layer 21 for electroforming (e.g. using a nickel sulfamate bath on a Ni or Cu seed layer).

[0064] b) A resist 22 is photolithographically structured to serve as a guiding layer 22.

[0065] c) The sample is put into a suitable electrolyte bath and the seed layer 21 is electrically connected such that metal is galvanically deposited in the area uncovered by the resist 22.

[0066] d) The resist 22 is chemically removed.

[0067] e) The seed layer is treated by wet or dry etching where no metal has been electroformed. In a further step (not shown) the P(VDF:TrFE) is applied onto the first and second electrodes (3,5) by means of spin casting or screen printing in order to embed the electrodes (3,5) in the piezoelectric material 6.