Multilayer structure and method for producing a multilayer structure

Abstract

A multilayer structure is provided, comprising a metal substrate and an electrode above the metal substrate. An adhesion layer is positioned between a main surface of the metal substrate and the electrode.

Description

[0001] The present application relates to a multilayer structure and a method for producing such a multilayer structure.

[0002] Multilayer structures in general, and electroceramic thin films in particular, are widely used in various technical fields. These include, in particular, high-energy capacitors, ferroelectric memory devices, piezoelectric actuators such as micromirrors or inkjet printheads, microfluidic pumps, piezoelectric sensors such as pressure sensors or accelerometers, piezoelectric energy harvesters, electrocaloric solid-state cooling devices, thermistors, or applications requiring a material with a high dielectric constant. Such devices often have an electroceramic thin film as their functional unit.

[0003] According to the state of the art, such layers are typically deposited on or over silicon wafers. Such wafers may have an oxide layer. Such structures may feature a classic metal-insulator-metal (MIM) electrode configuration.

[0004] The formation of PZT layers on silicon-based substrates is known from Schwartz et al. (CR Chimie 7, 433-461, 2004). The deposition of PZT layers on stainless steel foils is known from Won et al. (Nano Energy 55, 182-192, 2019). The deposition of an oriented PZT layer on a nickel substrate is known from Yeo et al. (J. Appl. Phys. 116, 014105, 2014). Furthermore, patent application WO 2017 / 045700 A1 discloses that a piezoelectric transducer can be formed using a titanium foil as a substrate.

[0005] However, these or similar approaches to depositing the layers on substrates other than silicon wafers often resulted in unsatisfactory properties of the piezoelectric thin film, such as deterioration of the dielectric or piezoelectric properties, compared to other state-of-the-art thin films, especially those deposited on silicon wafers.

[0006] Accordingly, it is an object of the present application to provide an improved electroceramic thin film in a multilayer structure and devices having such a structure. For example, at least in some cases, electroceramic thin films with significantly improved dielectric and piezoelectric properties can be achieved by means of the invention.

[0007] Some of the disadvantages described above are at least partially overcome by the subject matter of claims 1 and 2, or other advantages are associated with the subject matter of claims 1 and 2. Furthermore, further preferred embodiments or alternative embodiments are described in the further claims.

[0008] According to a first embodiment, a multilayer structure is provided. This multilayer structure comprises a metal substrate and an electrode above the metal substrate.

[0009] An adhesion layer is disposed between a major surface of the metal substrate and the electrode. The inventors have discovered that providing an adhesion layer between the electrode and the metal substrate improves the adhesion of the electrode. Furthermore, the adhesion layer can unexpectedly also improve the properties of an electroceramic layer disposed over the electrode.

[0010] The advantage of using a metal substrate instead of a silicon wafer is that the resulting multilayer structure can be flexible or bendable. These flexible substrates can enable the production of flexible devices.

[0011] "Bendability" or "flexibility" can be understood in the general technical sense in this field. It can be understood as elastic deformation. For example, it can mean that such multilayer structures can withstand a bend of up to 90° over a length of, say, 20 mm. It may be preferable that such bending can be carried out without deterioration of the individual layers or plastic deformation of the overall structure.

[0012] According to one embodiment, the material of the adhesion layer may comprise a transition metal and / or a transition metal oxide. The above formulation implies that more than one transition metal or transition metal oxide may be used. An alloy of the aforementioned metals may also be used to form the adhesion layer, or a transition metal oxide may be formed from the aforementioned alloys.

[0013] The term "transition metal" may be understood here and below in the sense of the general technical understanding in this field. In particular, it may include at least any transition metal from periods 4, 5, and 6 of the Periodic Table. Preferably, the transition metal here and below may be titanium, tantalum, or tungsten. Of these, tantalum in particular has proven advantageous in some cases.

[0014] The inventors have discovered that transition metals or their oxides can be preferred for promoting adhesion between an electrode and a metal substrate and contributing to the formation of high-quality electroceramic layers. The inventors have further discovered that an adhesion layer containing a transition metal or a transition metal oxide helps suppress the migration or diffusion of atoms or substances between the metal substrate and the electrode or layers above the electrode. This can particularly help suppress the diffusion of atoms or substances into electroceramic layers arranged above the electrode, since their composition is less likely to change during the production of the multilayer structures or over time. The properties of many electroceramic layers, such aspiezoelectric layers can be impaired by substances diffusing into these electroceramic layers.

[0015] According to one embodiment, the adhesion layer may comprise a first adhesion sublayer and a second adhesion sublayer. A first adhesion sublayer may, for example, comprise a transition metal, and the second adhesion sublayer may comprise a transition metal oxide. The stacking order of these sublayers is generally not limited. The transition metal of the transition metal oxide of the second adhesion sublayer may be the same transition metal as that contained in the first adhesion sublayer. It may also be a different transition metal.

[0016] As mentioned above, an adhesion layer containing a transition metal or a transition metal oxide can help suppress migration and diffusion. This effect can be enhanced with an adhesion layer comprising a first adhesion sublayer containing a transition metal and a second adhesion sublayer containing a transition metal oxide.

[0017] According to one embodiment, the first adhesion sublayer containing the transition metal can be in direct contact with the metal substrate. The second adhesion sublayer containing the transition metal oxide can be arranged on top of it. According to another embodiment, reverse stacking is also possible, in which the second adhesion sublayer containing the transition metal oxide is arranged directly on the substrate and the first sublayer containing the transition metal is arranged above it.

[0018] According to a further embodiment, the first adhesion sublayer, which contains the transition metal, is arranged directly on the substrate. The second adhesion sublayer, which contains the transition metal oxide, is arranged above it. A third adhesion sublayer, which also contains a transition metal, can be arranged above the second adhesion sublayer. This makes it possible to form a three-layer structure in which the oxide-containing second sublayer is embedded between the first and third sublayers containing the transition metal. In some cases, a structure with these three adhesion sublayers can be particularly advantageous. The inventors believe that the first and third adhesion sublayers can be particularly helpful in achieving improved adhesion, and that the second sublayer, which contains the oxide, contributes to improved diffusion suppression.According to another embodiment, the reverse structure can also be formed, in which a transition metal-based adhesion sublayer is embedded between two transition metal-based adhesion sublayers.

[0019] According to one embodiment, the thickness of the adhesion layer can be between 1 and 100 nm. Preferably, it can be between 10 and 50 nm. For example, the thickness of the adhesion layer can be between 15 nm and 30 nm, which is relevant in many practical scenarios. Even more preferably, it can be between 15 and 100 nm, such as between 15 and 50 nm or between 15 and 30 nm. The inventors of the present invention have found that if the adhesion layer is too thin, detachment can still occur and the diffusion-inhibiting properties are also reduced. In particular, at thicknesses above 15 nm, the adhesion properties and diffusion-inhibiting properties described above are particularly preferred.

[0020] According to one embodiment, the electrode contains or consists of a noble metal, an alloy of noble metals, or a conductive oxide. The aforementioned adhesion layers made of transition metals have proven particularly advantageous for improving the adhesion of a noble metal-containing metal electrode to a metal substrate. Adhesion has also been significantly improved for the aforementioned conductive oxides.

[0021] Here and in the following, "precious metals" are understood in the general technical sense in this field. This includes, at a minimum, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.

[0022] According to one embodiment, the electrode contains or consists of platinum or iridium. For these two precious metals, especially platinum, particularly good adhesion results have been observed with the above-mentioned adhesion layers.

[0023] Iridium oxide, rhodium oxide, rhenium oxide, ruthenium oxide, strontium ruthenium oxide, or lanthanum nickel oxide have proven to be good conductive electrodes, which can be bonded using the adhesion layer described above. Chemical formulas for these oxides can be IrO2, RhO2, ReO2, RuO2, SrRuO3, or LaNiO3.

[0024] According to one embodiment, the thickness of the electrode can be between 50 nm and 500 nm. Especially for more expensive precious metals, a smaller thickness is preferred, such as between 100 nm and 300 nm, for example, between 150 and 250 nm. Furthermore, excessively thick electrode layers tend to cause internal stresses in the device, which can adversely affect the adhesion between the layers.

[0025] In one embodiment, an electroceramic layer can be arranged above the electrode in the multilayer structure. In this case, the electrode can serve as the lower electrode for this electroceramic layer. It is not mandatory for the electrode to be in direct contact with the electroceramic layer. As described further below, intermediate layers can be arranged between the electrode and the electroceramic layer, for example, to enable specific crystal orientations. As already mentioned, the presence of an adhesion layer can surprisingly contribute to improving the quality or properties of the electroceramic layer. This effect is particularly pronounced in the adhesion layers described above containing a transition metal or transition metal oxide. Structures with partial layers are also preferred.Furthermore, the electrodes described above, in particular a platinum-containing electrode, in combination with the adhesion layer can contribute to the formation of an electroceramic layer with improved properties.

[0026] An electroceramic layer can be understood in the general technical sense. It is preferred that the electroceramic imparts functionality to the multilayer structure. Such functionality can be: • High dielectric constant, which can be used in capacitors, for example; • Pyroelectricity, which can be used in infrared sensors, for example; • Ferroelectricity, which can be used in storage devices, for example; • Piezoelectricity, which may be used, for example, in devices described elsewhere in the application; • Temperature dependent resistance; this can be provided by ceramic thermistor materials, such as positive temperature coefficient (PTC) or negative temperature coefficient (NTC) materials.

[0027] The inventors discovered the advantages described above specifically for piezoelectric films. However, the inventors also unexpectedly discovered that these advantages also apply to other types of electroceramic films.

[0028] In the aforementioned embodiment, it may be preferred that an upper electrode be arranged above the electroceramic layer. The upper electrode may be made of the same or a different material as the other electrode, which in this case can be addressed as the lower electrode.

[0029] According to one embodiment, the following electroceramic materials can be used to form an electroceramic layer: Pb[Zrx Ti 1-x ]O3 (abbreviated PZT), (1-x) Pb[Mg 1 / 3 Nb 2 / 3 O3] - x PbTiO3 (abbreviated PMN-PT), [K x N / a 1-x ]NbO3 (abbreviated KNN), (1-x) BiFeO3 - x BaTiO3 (abbreviated BFO-BT), Ba[Zr x Ti 1-x ]O3 (abbreviated BZT), (1-x) [Bi 1 / 2 N / a 1 / 2 ] TiO3 - x [Bi 1 / 2 K 1 / 2 ] TiO3 (abbreviated BNT-BKT), (1-x) BiFeO3 - x Ba [Ti y Zr 1-y ] O3 (abbreviated BFO-BZT), AlMgZrN, or [Al x Sc 1-x ]N. In all these examples, x and y are numbers between zero and one, i.e., 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1 . The electroceramic materials listed above can be modified by doping. Doping is the deliberate addition of impurities to adjust the electrical and / or piezoelectric properties of the electroceramic layer.

[0030] The inventors have discovered that an adhesion layer can improve the quality of the electroceramic layer. In particular, according to one embodiment, the electroceramic layer can comprise crystal grains in the size range of 10 to 1000 nm. Also, according to another embodiment, the electroceramic layer can comprise crystalline columns extending across the entire thickness of the electroceramic layer due to the adhesion layer. These properties can be realized, in particular, in PZT films.

[0031] According to a further embodiment, the electroceramic layer may have a granular structure. For example, grains ranging from 10 nm to 1 µm may be present. Such a granular structure can be realized, for example, by electroceramic layers made of barium titanate.

[0032] According to one embodiment, the electroceramic layer can be applied directly to the electrode. However, in some cases, it is preferable to have a seed layer between the electroceramic layer and the electrode. A seed layer can be a layer that improves the crystal orientation of an electroceramic layer grown thereon. Seed layer materials can be selected, for example, from lead titanate (PbTiO3), lanthanum nickel oxide (LaNiO3), lead oxide (PbO), or titanium oxide (TiO2).

[0033] The thickness of the electroceramic layer is generally not limited. According to one embodiment, the electroceramic layer can have a thickness of 10 nm to 20 µm. Preferably, a lower range for the thickness can be 20 nm, 50 nm, or 100 nm. Preferably, an upper range for the thickness can be 10 µm, 5 µm, 2 µm, or 1 µm. For example, a thickness can be from 100 nm to 5 µm, such as 1 µm to 3 µm, or 1.7 to 2.5 µm. The above-mentioned values ​​for the lower and upper ranges can replace the lower and upper values ​​in all of these examples, respectively.

[0034] In one embodiment, the substrate may be provided in the form of a flexible plate or film.

[0035] According to one embodiment, the substrate may comprise or consist of a base metal or a base metal-containing alloy. The term "base metal" is to be understood in the general technical sense. Base metals or base metal alloys are often comparatively inexpensive and easy to process and may be flexible compared to rigid silicon wafers. Preferred examples of base metals or base metal alloys are titanium, aluminum, nickel, copper, brass, or stainless steel. For example, titanium or certain types of stainless steel can be biocompatible. In particular, substrates made of the aforementioned base metals can also be flexible.

[0036] The inventors have found that the advantages described above are particularly pronounced for non-precious metal substrates in general and for the examples listed in particular.

[0037] Of these substrate materials, titanium is particularly preferred. It has the advantages described above, but also has a coefficient of thermal expansion relatively close to that of many of the electroceramic materials used, especially PZT, which makes multilayer structures easy to manufacture. Furthermore, warpage can be reduced. Furthermore, titanium is chemically compatible with PZT, BZT, BT, PMN-PT, and BFO-BT, as it contains no foreign elements that could diffuse into the PZT and thereby impair the properties of the ceramic material.

[0038] Thanks to inexpensive metal or, in particular, non-precious metal substrates, the invention helps to produce electroceramic thin-film components with excellent performance in a cost-effective manner.

[0039] If the substrates are in the form of wafers or foils, they can be manufactured in a batch process or in a continuous process, such as roll-to-roll production. In contrast, Si wafer-based substrates are generally not only more expensive but can also only be processed in a batch process.

[0040] According to a further embodiment, the thickness of the metal substrate can be in the range of 1 µm to 500 µm. Preferably, and especially for flexibility, a thickness is between 10 µm and 200 µm. Even more preferred for such a use is a thickness between 20 µm and 100 µm.

[0041] According to a further embodiment, the inventors have found that when using an adhesion layer, unpolished substrates can also be used. In particular, the metal substrate can have a surface roughness (R a) of up to 2 µm. In contrast, mirror-polished substrate surfaces have typically been used for silicon substrates. Preferably, the surface roughness can be 1 µm or less. Of course, the invention also works with mirror-polished substrate surfaces, but these are not required. However, a lower surface roughness can also be advantageous for the multilayer structure according to the invention, as it can increase the yield of the functional components. Accordingly, substrates with a surface roughness of over 10 nm can be used without any problems within the scope of the invention. Preferably, they can also have a surface roughness of over 20 nm, over 50 nm, or over 100 nm, as this allows a less thorough polish or even unpolished substrates to be used. In general, polishing is a complicated and expensive process.Furthermore, this process can introduce impurities that can affect component performance and yield. Accordingly, this aspect can help overcome such problems.

[0042] According to a further embodiment, a multilayer structure is described which has the arrangement described above and which has a second electrode arranged above a second main surface of the metal substrate. In this case, a second adhesion layer is arranged between the second main surface and the second electrode. Accordingly, a symmetrical multilayer structure can be formed which has electrodes on two sides of the metal substrate and adhesion layers on both sides that mediate adhesion. The second electrode and the second adhesion layer can have the properties described above. Preferably, the first and second adhesion layers have the same material, which simplifies production. However, the materials can also be different. Similarly, the electrodes on both sides can have the same material, which can be preferred to simplify production.But the material of the electrodes can also be different.

[0043] With two symmetrically arranged adhesion layers on two opposite main surfaces of the substrate, distortion can be reduced. This advantage is particularly pronounced for Ti substrates, as they already have reduced distortion, as described above.

[0044] According to a preferred example of the last embodiment, a second electroceramic layer can be arranged above or on the second lower electrode. The second electroceramic layer can have the above-described properties or advantages of the first electroceramic layer. Here, too, both electroceramic layers can be made of the same material, which may be preferred to simplify manufacturing. However, the material of the electroceramic layers can also be different.

[0045] The present invention is particularly well-suited for the fabrication of such symmetrical structures, as the adhesion layer-based approach described above allows for the use of rough substrates. Accordingly, both surfaces of the substrate can be used without polishing, which would have to be performed on each surface individually. In some cases, polishing can be problematic, as if both surfaces need to be polished, it can be difficult to avoid damaging the polished surface. Therefore, the present application has a synergistic effect for such symmetrical arrangements.

[0046] According to a further embodiment, the multilayer structure can have a further electrode arranged on the electroceramic layer. In addition, a further electroceramic layer can be arranged on or above the further electrode. The properties described above can apply to the further electrode and the further electroceramic layer. The material of the further electroceramic layer can be the same as that of the lower electroceramic layer, but it can also be different. The further electrode can consist of the same material as the other electrode, but it can also be different. Similarly, several layers of electrodes and electroceramic layers can be stacked on top of one another. It is preferred that such stacks be terminated by an upper electrode, which can have the properties described above.

[0047] According to one embodiment, the present application can be used in high-energy capacitors, ferroelectric memory devices, piezoelectric actuators such as micromirrors or inkjet printheads, piezoelectric sensors such as pressure sensors, tire sensors, or accelerometers, microfluidic pumps, piezoelectric energy harvesters, or electrocaloric solid-state refrigerators. For example, the piezoelectric sensor can be integrated into the tire to collect and process data from the wheel or tire, or into a road interface or vehicle bumper to evaluate the location and magnitude of an impact with another vehicle or an obstacle. Accordingly, the multilayer structures described above can be incorporated into these applications.

[0048] According to a further embodiment, a method for producing a multilayer structure is described. The advantages and properties described above apply accordingly to the method, where applicable.

[0049] In one embodiment of the method for forming a multilayer structure, an adhesion layer is first arranged on a major surface of a non-noble metal substrate. The adhesion layer can be deposited or arranged on the metal substrate. Preferably, it is formed by physical vapor deposition, such as magnetron sputtering or vapor deposition. A lower electrode containing a noble metal or a conductive oxide is arranged on the adhesion layer. The lower electrode can be arranged or deposited in any desired manner. Preferably, chemical or physical vapor deposition, such as magnetron sputtering or vapor deposition, can be used. Subsequently, an electroceramic material is deposited on the lower electrode. The electroceramic materials can be, for example, the electroceramic materials defined above.For the deposition of the electroceramic material, for example, chemical solution deposition, magnetron sputtering, atomic layer deposition or pulsed laser deposition can be used.

[0050] The electroceramic layer can be formed by successively depositing thin layers of electroceramic material. For example, layers with a thickness of 50 to 200 nm can be stacked on top of each other per deposition step. In chemical solution deposition, for example, a precursor solution containing an electroceramic starting material can first be deposited. The deposition method can be, for example, spin coating. The solvents can then be removed from the precursor solution by drying. Any organic residues can then be removed by a pyrolysis step. Following this, an annealing step is performed, which leads to the crystallization of the previously amorphous metal oxide. The deposition can be repeated to obtain an electroceramic layer on the order of several micrometers.

[0051] According to one embodiment, a high-quality PZT layer can be formed by deposition and spin coating of a PZT solution on the substrate. The PZT film can be formed from a single solution, creating a sublayer of the PZT film. It can also be produced from two or more solutions with different concentrations of PZT components, from which sublayers of a sublayer are formed. One solution can be Zr-rich, and a second solution can be Ti-rich. The principles of the embodiment explained below can be applied to the cases of two or more solutions.

[0052] According to one embodiment, a method with at least three deposition steps can be used. At least one layer or multiple layers are formed using a first solution, a second sublayer is formed by forming one or more layers using a second solution, and a third sublayer is formed by forming one or more separate layers using a third solution. In this case, the electroceramic material is PZT or comprises PZT. The first solution has a high zirconium to titanium ratio. The first solution is deposited above the bottom electrode, forming the first sublayer. This may also include further deposition and annealing to make the first sublayer thicker than would be achieved by a single deposition.Likewise, a second sublayer is formed from a second solution having an intermediate zirconium-to-titanium ratio, i.e., a lower zirconium-to-titanium ratio than that of the first solution. Further deposition and heating steps may also be used here. A third sublayer is formed on top of this, again by one or more deposition and heating steps. This third sublayer is formed using a third solution having a low zirconium-to-titanium ratio, meaning that the proportion of zirconium to titanium is the lowest compared to the other solutions. This solution, or layer, is deposited on top of the second sublayer. The first solution may be referred to as a zirconium-rich solution. The second solution may be referred to as a morphotropic phase boundary solution. And the final solution may be referred to as a titanium-rich solution.

[0053] This principle can be applied to more than three solutions. For example, four or five or even more solutions with different concentrations of Zr and Ti can be used, gradually changing the concentration from Zr-rich to Ti-rich. As discussed above, a two-solution approach can also be used, also changing from Zr-rich to Ti-rich.

[0054] The invention is described below with reference to exemplary embodiments and figures. The figures also include schematic drawings. Such schematic drawings are not to scale, and dimensions and dimensional relationships may be distorted. Accordingly, no lengths or relationships can be directly derived from the schematic figures unless otherwise stated. Fig. 1 shows a schematic cross section of a first embodiment of a multi-layer structure. Fig. 2 shows a schematic cross section of a second embodiment of a multilayer structure. Fig. 3 shows a schematic cross section of a third embodiment of a multilayer structure. Fig. 4 shows a schematic cross section of a part of a fourth embodiment of a multilayer structure. Fig. 5 shows a schematic cross section of a part of a fifth embodiment of a multilayer structure. Fig. 6 shows a schematic cross section of part of a sixth embodiment of a multilayer structure. Fig. 7 shows a schematic cross section of a part of a seventh embodiment of a multilayer structure. Fig. 8 shows a schematic view of a tire with a tire pressure sensor. Fig. 9 shows a schematic cross section of a part of the tire with the tire pressure sensor. Fig. Figure 10 shows a schematic diagram of a vibration energy collector. Fig. 11 shows an embodiment of a flexible cantilever. Fig. 12 shows a flowchart of a manufacturing method for an embodiment of a multilayer structure. Fig. 13 shows an X-ray diffraction pattern of a PZT thin film of an embodiment of a multilayer structure. Fig. 14 shows a scanning electron micrograph of a PZT film of an embodiment of a multilayer structure in plan view. Fig. 15 shows a scanning electron microscopic cross-sectional image of an embodiment of a multilayer structure. Fig. 16 shows a transmission electron microscopy image of a PZT thin film of an embodiment of a multilayer structure. Fig. 17 shows a polarization-electric field loop of an embodiment of a PZT thin film within a multilayer structure. Fig. 18 shows a relative permittivity electric field loop of an embodiment of a PZT thin film within a multilayer structure. Fig. 19 shows a bipolar displacement curve (English: bipolar displacement curve) of an embodiment of a PZT thin film in a multilayer structure. Fig. 20 shows a unipolar displacement curve of an embodiment of a PZT thin film in a multilayer structure. Fig. 21 shows a photograph of an embodiment of a clamped cantilever device. Fig. Figure 22 shows the first resonant mode of the exemplary embodiment of the clamped cantilever device.

[0055] In Fig. Figure 1 shows a first embodiment of a multilayer structure 1 in schematic cross-section. In this embodiment, an adhesion layer 3 is arranged on a non-noble metal substrate 2.

[0056] The adhesion layer 3 comprises a transition metal or a transition metal oxide. A transition metal can be any transition metal from the fourth, fifth, or sixth period of the periodic table. An electrode layer 4 is located on the adhesion layer 3. The electrode 4 can contain or consist of a noble metal or a conductive oxide. In this stack, the electrode layer 4 can be addressed as the lower electrode. Above the lower electrode 4 is an electroceramic layer 5. The electroceramic layer 5 can be a piezoelectric layer. An upper electrode 6 is arranged above the electroceramic layer 5. The upper electrode 6 can be any conductive electrode. For example, the upper electrode 6 can be a noble metal or a conductive oxide electrode, similar to the lower electrode 4.

[0057] The materials mentioned in the introduction can be used for the various layers. A particularly preferred example is a titanium substrate 2, a tantalum-containing adhesion layer 3, a platinum electrode layer 4, an electroceramic layer comprising a PZT material, and an upper gold electrode 6. In this specific case, the titanium foil can have a thickness of 50 µm, the tantalum-containing adhesion layer 3 a thickness of 20 nm, and the platinum electrode 4 a thickness of 200 nm. The PZT electroceramic layer can have a thickness of 1.7 to 2.5 µm. The upper electrode 6 can have a thickness of 200 nm.

[0058] Not explicitly shown, but preferred in many cases, a seed layer can be arranged between the electroceramic layer and the electrode. The seed layer can further facilitate the crystalline and / or oriented growth of the electroceramic layer.

[0059] The present structure has the advantage that an electroceramic layer 5, such as the aforementioned PZT electroceramic layer 5, can be formed with a high degree of orientation on a low-cost titanium substrate 2 or another low-cost substrate 2. Furthermore, the layers can be fabricated without polishing the substrate 2 or otherwise controlling the surface roughness. As will be shown below, excellent performance can be achieved with such multilayer structures 1. Furthermore, the use of titanium as the substrate 2 has several advantages. Since the coefficient of thermal expansion (CTE) is close to that of PZT, substrate warpage is reduced. Furthermore, the adhesion layer not only facilitates adhesion between the different layers but also acts as a chemical barrier.Furthermore, titanium has been shown to be chemically compatible with PZT because it does not contain any foreign elements that could diffuse into PZT and cause undesirable doping effects on the material properties.

[0060] Fig. 2 shows a schematic cross section of a second embodiment of a multilayer structure 1. The multilayer structure 1 of the Fig. 2 is identical in the upper part to the multi-layer structure of the Fig. 1, i.e., the layers with reference numerals 2, 3, 4, 5, and 6 are identical to those described for the first embodiment. However, a second adhesion layer 3', a second lower electrode 4', a second electroceramic layer 5', and a second upper electrode 6' are also arranged on the other surface of the substrate 2. The materials of these layers can correspond to those of the first adhesion layer 3, the first lower electrode 4, the first electroceramic layer 5, and the first upper electrode 6. Accordingly, a largely symmetrical layer arrangement results with respect to the layers with reference numerals 2 to 6.

[0061] In Fig. 3 shows a schematic cross section of a third exemplary embodiment of a multilayer structure 1. This third exemplary embodiment of a multilayer structure 1 is a modification of the one shown in Fig. 1. It has the first adhesion layer 3, the first lower electrode 4, the first electroceramic layer 5, and the first upper electrode 6 on the substrate 2. A second electroceramic layer 5'' is arranged above the first upper electrode 6. A second upper electrode 6'' is arranged above the second electroceramic layer 5''. In this arrangement, the first upper electrode 6 acts as a lower electrode for the second electroceramic layer 5''. The materials of the second electroceramic layer 5'' can be selected from the same materials from which the material of the first electroceramic layer 5 is selected. The materials of the second upper electrode 6'' can also be selected from the same materials from which the material of the first upper electrode 6 is selected.

[0062] In a similar way as in Fig. As shown in Figure 3, several additional electroceramic layers and upper electrodes can be stacked alternately. Preferably, such a stack is terminated by an upper electrode.

[0063] The third exemplary embodiment can of course be combined with the second exemplary embodiment, ie the two-sided arrangements as in Fig. 2. This allows for the creation of either symmetric or asymmetric stacks, i.e., multilayer arrangements with the same number of layers on both sides of the substrate or with a different number of layers on both sides.

[0064] Fig. 4 shows a schematic cross section through a part of a fourth embodiment of a multi-layer structure 1. Fig. 4 can represent an arrangement of an adhesion layer 3 on a substrate 2 in the first, second and third embodiments of a multilayer structure 1, which in the Fig. 1, Fig. 2 and 3, respectively. In this fourth embodiment, the adhesion layer 3 is arranged on the substrate 2. The adhesion layer 3 comprises or consists of a single type of material and has no internal material layer boundaries. In the present case, the adhesion layer consists only of a transition metal or an alloy of transition metals. Alternatively, it can also consist entirely of an oxide of a transition metal or a mixed oxide of transition metals.

[0065] Fig. 5 shows a schematic cross section through a part of a fifth embodiment of a multilayer structure 1. Fig. 5 can represent an arrangement of an adhesion layer 3 on a substrate 2 in the first, second and third embodiments of a multilayer structure 1, as shown in the Fig. 1, Fig. 2 and 3, respectively. In this case, the adhesion layer 3 comprises a first adhesion sublayer 3a, which consists of a transition metal or an alloy of transition metals. A second adhesion sublayer 3b, which comprises or consists of a transition metal oxide, is arranged thereon. The transition metal of the transition metal oxide can be the same as the transition metal of the first adhesion sublayer 3a, or it can be different from it.

[0066] Fig. 6 shows a very similar structure as in Fig. 5, as a sixth embodiment of a multilayer structure 1, but with an inverted stacking of the first adhesion sublayer 3a and the second adhesion sublayer 3b. Here, the second adhesion sublayer 3b, which comprises or consists of a transition metal oxide, is arranged directly on the substrate 2, and the first adhesion sublayer 3a, which consists of the transition metal, is arranged above it. The stacking in Fig. 6 can be seen an arrangement of an adhesion layer 3 on a substrate 2 in the first, second and third embodiments of a Fig. 1, Fig. 2 and 3 respectively.

[0067] Fig. Fig. 7 shows a schematic cross section of a part of a fifth embodiment of a multi-layer structure 1, which is a modification of the one shown in Fig. 5, ie the fifth embodiment. On the first adhesion sub-layer 3a, the second adhesion sub-layer 3b is arranged in the same way as in Fig. 5. A third adhesion sublayer 3a', which comprises or consists of a transition metal, is arranged on the second adhesion sublayer 3b. The transition metal in the first and third adhesion sublayers 3a and 3a' can be identical or different. Otherwise, the properties described above can apply.

[0068] Not shown here, but also possible, is a reversed arrangement of the layers. For example, a transition metal-containing adhesion sublayer can be arranged between two transition metal oxide-containing adhesion sublayers.

[0069] For the adhesion layers in the Fig. 4 to 7, the inventors discovered that the use of a transition metal can improve the adhesion between a non-noble metal substrate, such as a titanium foil, and an electrode, such as a platinum electrode or another noble metal electrode. Furthermore, the transition metal can prevent the migration of substances between the other layers. In particular, when an oxide layer is present, the diffusion or migration of ions or atoms appears to be suppressed even more effectively.

[0070] In the following Fig. Figures 8 to 10 show some exemplary applications with the multilayer structure described above.

[0071] In Fig. Figure 8 shows a tire 11 provided with a piezoelectric sensor, which is an example of a piezoelectric device 10. In this case, the piezoelectric device 10 may be mounted on the inside of the tire. Fig. 9 shows a part of the tire 11 on a rim 12. As in Fig. As shown in Figure 9, when the tire rolls, the piezoelectric sensor (piezoelectric device 10) can detect repeated contact with the ground 13 because the tire is deformed in the area that comes into contact with the ground.

[0072] This deformation is transferred to the piezoelectric device 10, and a voltage can be recorded that indicates the deformation. The number of such deformation events can be used to determine the rotational speed, from which the vehicle's speed can be derived. The degree of deformation can also contain information about the air pressure in the tire, since the degree of deformation can depend on the pressure. Stresses or unevenness in the tire can also be detected, as these influence the degree of deformation. The inventive concept is particularly suitable for the construction of such piezoelectric sensors, since the invention enables the production of flexible multilayer structures with a sensitive piezoelectric response.

[0073] In Fig. 10 shows a vibration energy harvester. It consists of a piezoelectric device 10 comprising a multilayer structure 1, as shown above. A cantilever 14 is caused to vibrate. As a result, the piezoelectric device 10 generates voltage and current, which can be stored in an energy storage device 15.

[0074] Other examples of applications, especially piezoelectric applications, include micromirrors, microfluidic pumps, inkjet printheads, haptic elements, strain and pressure sensors, tire sensors, shock sensors, energy harvesters, loudspeakers, accelerometers and hearing aids.

[0075] In Fig. Figure 11 illustrates an embodiment of a flexible cantilever. This flexible cantilever-shaped device comprises a multilayer structure according to the present invention, which includes a PZT thin film deposited over a titanium foil. This cantilever device could, for example, be used in an energy harvesting device according to Fig. 10 can be used.

[0076] Fig. Figure 12 shows a flowchart of a process for producing a PZT thin film on a titanium foil as a metal substrate. In the flowchart in Fig. 12 briefly describes the steps explained here.

[0077] First, a titanium foil is provided as a metal substrate. The titanium foil has a thickness of 50 µm.

[0078] The titanium foil is cleaned by rinsing with acetone, isopropanol, and deionized water. It is then treated for 15 minutes in a UV / ozone cleaner. After cleaning, a 20 nm thick Ta adhesion layer is deposited by DC magnetron sputtering at room temperature with 500 W. Polishing is not required during substrate preparation. The adhesion layer can promote adhesion on quite rough surfaces down to 1 µm or even 2 µm.

[0079] A 200 nm thick platinum layer is deposited on this tantalum layer by DC magnetron sputtering at room temperature with 500 W. This creates a layer comprising a Ti foil substrate, an adhesion layer of Ta, and a lower electrode layer of Pt.

[0080] A lead titanate (PbTiO3) seed layer is then deposited on the platinum electrode. For this purpose, a seed layer solution containing PbTiO3 was first prepared. The solution had a concentration of 0.0625 mol / l PbTiO3 and a 10% molar excess of lead to compensate for evaporation during crystallization. Before deposition, the seed layer solution was filtered through a 0.2 µm PTFE syringe filter. The seed layer solution was then applied to the bare platinum surface and spin-coated at a rotation speed of 3000 rpm for 30 seconds. The thus deposited amorphous seed layer was dried on a hot plate at 150 °C and subsequently crystallized in a rapid thermal furnace at 520 °C. This resulted in a seed layer of approximately 10 nm.

[0081] A stack is formed from a titanium foil, an adhesion layer, a platinum electrode layer and a seed layer.

[0082] The PZT electroceramic layer is then formed by depositing several sublayers, each consisting of three sublayers.

[0083] To form a first zirconium-rich sublayer, a zirconium-rich precursor solution is first prepared by dissolving lead(II) acetate in a solution of zirconium(IV) propoxide and titanium(IV) isopropoxide in acetic acid and 2-methoxyethanol. After dissolving the lead precursor substance at 60 °C, the resulting solution was refluxed for 2 hours under a nitrogen atmosphere. After distilling off the by-products, the mixture was diluted to a concentration of 0.5 mol / l. The composition of the solution thus prepared corresponds to a composition of Pb (Zr 0,63 Ti 0,37) O3 for the sublayer to be formed. The solution is prepared to include a 15% molar excess of lead. Before deposition, the zirconium-rich first precursor solution was filtered through a 0.2 µm PTFE syringe filter. The zirconium-rich first precursor solution was then applied to the seed layer and spin-coated for 30 seconds at a rotation speed of 3000 rpm. The thus deposited zirconium-rich sublayer was dried and pyrolyzed at 150 °C or 350 °C.

[0084] Subsequently, to form a medium-concentrated second sublayer, a medium-concentrated second precursor solution was first prepared by dissolving lead(II) acetate in a solution of zirconium(IV) propoxide and titanium(IV) isopropoxide in acetic acid and 2-methoxyethanol. After dissolving the lead precursor substance at 60 °C, the resulting solution was refluxed for 2 hours under a nitrogen atmosphere. After distilling off the by-products, the mixture was diluted to a concentration of 0.5 mol / l. The composition of the solution thus prepared corresponds to a composition of Pb (Zr 0,53 Ti 0,47) O3 for the sublayer to be formed. The solution is prepared so that it also contains a 15% molar excess of lead. Before deposition, the medium-concentrated second precursor solution was filtered through a 0.2 µm PTFE syringe filter. The medium-concentrated second precursor solution was then applied to the dried and pyrolyzed zirconium-rich sublayer and spin-coated for 30 seconds at a rotation speed of 3000 rpm. The thus deposited medium-concentrated second sublayer was dried and pyrolyzed at a temperature of 150 °C and 350 °C, respectively.

[0085] Subsequently, to form a titanium-rich third sublayer, a titanium-rich third precursor solution is first prepared by dissolving lead(II) acetate in a solution of zirconium(IV) propoxide and titanium(IV) isopropoxide in acetic acid and 2-methoxyethanol. After dissolving the lead precursor substance at 60 °C, the resulting solution was refluxed for 2 hours under a nitrogen atmosphere. After distilling off the by-products, the mixture was diluted to a concentration of 0.5 mol / l. The composition of the solution thus prepared corresponds to a composition of Pb (Zr 0,43 Ti 0,57) O3 for the sublayer to be formed. The solution is prepared so that it also contains a 15% molar excess of lead. Before deposition, the titanium-enriched third precursor solution was filtered through a 0.2 µm PTFE syringe filter. The titanium-rich third precursor solution was then applied to the dried and pyrolyzed, medium-concentrated second sublayer and spin-coated for 30 seconds at a rotation speed of 3000 rpm. The thus deposited titanium-rich third sublayer was dried and pyrolyzed at a temperature of 150 °C and 350 °C, respectively.

[0086] The partial layer formed from the three sublayers is amorphous. It has a thickness of 50 to 200 nm. After formation of this amorphous partial layer, it undergoes a crystallization step in a rapid thermal furnace at 650 °C.

[0087] To obtain a PZT electroceramic layer with a thickness between 1.7 and 2.5 µm, the above-described formation of sublayers was repeated 24 to 36 times. In the present example, a thickness of 1.7 and 2.5 µm corresponds to 24 and 36 sublayers, respectively. In this way, a stack consisting of a titanium foil substrate, a tantalum adhesion layer, a platinum electrode layer, a seed layer, and a PZT electroceramic functional layer can be formed. Finally, as described in Fig. 12, an upper electrode may be deposited, for example, by direct current magnetron sputtering. In this case, the upper electrode may be gold or platinum, for example.

[0088] The above procedure can be adapted for other materials or other layer thicknesses.

[0089] Fig. Figure 13 shows an X-ray diffraction pattern of a PZT thin film of an exemplary multilayer structure. The film was produced using the process described above. The diffraction pattern clearly shows that the PZT film is crystallized in the perovskite phase with a {100} preferred orientation. The degree of crystallinity and orientation is high in this case. The high degree of crystallization was achieved in the presence of the adhesion layer and further promoted by the PbTiO3 seed layer.

[0090] In Fig. Figure 14 shows a top-view scanning electron microscopy image. It shows that PZT grains with a size of 100 to 500 nm have formed in the thin film. Furthermore, there is no evidence of secondary phases such as pyrochlore.

[0091] Fig. Figure 15 shows a scanning electron micrograph of a cross-sectional view of a multilayer structure. The multilayer structure has essentially the same arrangement of layers as in Fig. 1. As described below, it has very advantageous properties.

[0092] Fig. Figure 16 shows a transmission electron micrograph of a PZT thin film deposited over a titanium foil. As can be seen in the figure, the electroceramic film 5, consisting of PZT, is located above the lower electrode 4 made of platinum. In this cross-sectional image, crystalline columns 7 can be seen (as indicated in the figure) extending across the entire thickness of the electroceramic layer 5. The width of these columns 7 corresponds to the size of the grains observed in the top view with the scanning electron microscope, as shown in Fig. 14. This very good columnar grain growth is preferred because it leads to a high electromechanical response. TEM examination confirmed the absence of secondary phases such as pyrochlore, which are known to significantly reduce the performance of the electroceramic element.

[0093] In the Fig. Figures 17 to 20 show the results of various electrical and piezoelectric characterizations of the layers. For these measurements, PZT layers were fabricated over a titanium foil substrate using the method described above. For the top electrodes, a 25 nm thick chromium adhesion layer and a 200 nm thick gold electrode were applied by magnetron sputtering.

[0094] The diagrams in the Fig. 17 to 19 were recorded for the same sample. They were taken for a sample with an area of ​​20.65 mm 2recorded. The substrate thickness was 50 µm. The PZT film thickness was 1.7 µm.

[0095] In Fig. 17, a plot of polarization versus electric field is shown, which was recorded at a frequency of 10 Hz. The solid curve, which appears larger with respect to the y-axis, corresponds to the polarization shown on the left side of the diagram, which is also indicated by the corresponding left-pointing arrow. The other, dashed curve corresponds to the right y-axis of the diagram (current density), as also indicated by the corresponding right-pointing arrow. As can be seen, a ferroelectric circuit with a remanent polarization P r of 35 µC cm -2 In addition, a coercive field of E c of 40 kV cm -1 The observed polarization-electric loop is also highly symmetric.

[0096] In Fig. Figure 18 shows the relative permittivity and dissipation factor plotted as a function of the DC bias electric field. The applied frequency here was 1 kHz. The measurement was carried out at a frequency of 1 kHz and with a small signal amplitude of 5 kV / cm. In this case, no leakage currents were observed at high DC fields. The solid curve visible further up in the diagram is the relative permittivity, indicated on the left side of the image by the corresponding left-pointing arrow. The dashed curve below corresponds to the dissipation factor on the right side, also indicated by the corresponding right-pointing arrow. As can be seen, at zero DC bias, the relative permittivity approaches a value of 700, while the loss remains below 10%.

[0097] Fig. Figure 19 shows a bipolar displacement curve. This was recorded at a frequency of 0.5 Hz. The bipolar displacement curve is symmetrical and exhibits the typical butterfly shape of high-quality PZT coatings.

[0098] The stress curve in Fig. 20 was recorded on a similarly shaped, but not identical, electroceramic thin-film element that has the same values ​​as the other substrate. In Fig. 20 a high linearity of the curve can be seen.

[0099] Table 1 shows the results for the piezoelectric coefficient for the graph in Fig. 20 summarized Table 1: -e 31,f + -e 31,f - Average of |e 31,f | Δe 31,f Example of Fig. 20 10,83 C m -2 10,41 C m -2 10,62 C m -2 0,42 C m -2

[0100] The highly effective transverse piezoelectric coefficient |e 31,f | from 10.6 C m -2 and the small difference between -e 31,f + and -e 31,f +, without prior hot-poling of the thin film under a DC electric field, demonstrates the high quality of the formed film. It should be noted that hot-poling or doping by intentionally adding impurities to the piezoelectric thin film can further enhance the performance of the electroceramic thin film.

[0101] In Fig. Figure 21 shows the experimental setup of a clamped cantilever device excited to its resonance frequency. The maximum deflection at the tip of the cantilever is on the order of a few millimeters. Fig. Figure 22 shows the relative amplitude of the cantilever deflection. The deflection is observed in the active part of the cantilever, which corresponds to the area covered by an upper electrode. Reference character list 1 Multi-layer structure 2 Metal substrate 3 Electrode 3' second lower electrode 4 Adhesive layer 4' second adhesion layer 5 electroceramic layer 5', 5'' second electroceramic layer 6 upper electrode 6', 6'' second upper electrode 7th pillar 10 piezoelectric device 11 tires 12 rim 13 Floor 14 cantilevers 15 Energy storage device

Claims

[1] Multi-layer structure a non-precious metal substrate, which may include, for example, titanium, aluminum, nickel, copper, brass or stainless steel, an electrode disposed over the non-noble metal substrate containing a noble metal or a conductive oxide, and a piezoelectric layer disposed over or on the electrode, wherein an adhesion layer comprising a transition metal or a transition metal oxide is arranged between and in direct contact with a main surface of the metal substrate and the electrode, the adhesion layer comprises a first adhesion sub-layer comprising a transition metal and a second adhesion sub-layer comprising a transition metal oxide, and the first adhesion sublayer and the second adhesion sublayer are arranged one on top of the other along a stacking direction, wherein the stacking order of the adhesion sublayers is not limited. [2] Multilayer structure with a metal substrate and an electrode, where an adhesion layer is arranged between a main surface of the metal substrate and the electrode, the adhesion layer comprises a first adhesion sub-layer comprising a transition metal and a second adhesion sub-layer comprising a transition metal oxide, and the first adhesion sublayer and the second adhesion sublayer are arranged one on top of the other along a stacking direction, wherein the stacking order of the adhesion sublayers is not limited. [3] Multilayer structure according to claim 1 or 2, wherein the transition metal is selected from Ti, Ta or W or their alloys. [4] Multilayer structure according to one of claims 1 to 3, wherein the electrode comprises or consists of a noble metal or an alloy of noble metals or a conductive oxide. [5] Multilayer structure according to one of claims 1 to 4, wherein the electrode comprises or consists of Pt or Ir. [6] Multilayer structure according to one of claims 1 to 4, wherein the electrode comprises or consists of an iridium oxide, a rhodium oxide, a rhenium oxide, a ruthenium oxide, a strontium ruthenium oxide or a lanthanum nickel oxide. [7] Multilayer structure according to one of claims 1 to 6, wherein an electroceramic layer is arranged above the electrode or on the electrode. [8] Multilayer structure according to claim 7, wherein the electroceramic layer comprises or consists of a dielectric, pyroelectric, ferroelectric, piezoelectric material or a thermistor material. [9] Multilayer structure according to one of claims 1 to 8, wherein the substrate comprises or consists of a non-noble metal or an alloy comprising a non-noble metal. [10] Multilayer structure according to one of claims 1 to 9, wherein the substrate comprises or consists of titanium, aluminum, nickel, copper, brass or stainless steel. [11] Multilayer structure according to one of claims 1 to 10, wherein the metal substrate has on its main surface a surface roughness (R a ) from 10 nm to 2 µm. [12] A multilayer structure according to any one of claims 1 to 11, wherein a second electrode is disposed over a second major surface of the metal substrate, wherein a second adhesion layer is disposed between the second major surface and the second electrode, and wherein a second electroceramic layer is disposed over or on the second electrode. [13] Multilayer structure according to claim 1, 7 or 8, wherein a further electrode is arranged on the electroceramic layer and a further electroceramic layer is arranged on or above the further electrode. [14] Multilayer structure according to one of claims 1 to 13, wherein the first adhesion sub-layer is arranged directly on the substrate and the second adhesion sub-layer is arranged on the first adhesion sub-layer. [15] Multilayer structure according to one of claims 1 to 13, wherein the second adhesion sub-layer is arranged directly on the substrate and the first adhesion sub-layer is arranged on the second adhesion sub-layer. [16] Multi-layer structure according to claim 14 or 15, wherein the adhesion layer additionally comprises a third adhesion sub-layer, which is arranged on the stack of first adhesion sub-layer and second adhesion sub-layer and comprises the same material as the sub-layer of first adhesion sub-layer and second adhesion sub-layer located closer to the substrate. [17] Multilayer structure with a metal substrate and an electrode, where an adhesion layer is arranged between a main surface of the metal substrate and the electrode, an electroceramic layer is arranged above or on the electrode and has crystal grains in the size range of 10 to 1000 nm. [18] Multilayer structure with a metal substrate and an electrode, where an adhesion layer is arranged between a main surface of the metal substrate and the electrode, an electroceramic layer is arranged above or on the electrode and has crystalline columns extending over the entire thickness of the electroceramic layer. [19] Multi-layer structure a titanium-containing metal substrate, a platinum-containing electrode arranged above the substrate, and a tantalum-containing adhesion layer disposed between and in direct contact with a major surface of the metal substrate and the electrode. [20] High-energy capacitor comprising a multilayer structure according to one of claims 1 to 19. [21] Ferroelectric memory device comprising a multilayer structure according to one of claims 1 to 19. [22] Piezoelectric device comprising a multilayer structure according to one of claims 1 to 19. [23] Electrocaloric solid-state cooling device comprising a multi-layer structure according to one of claims 1 to 19. [24] Energy harvester comprising a multi-layer structure according to one of claims 1 to 19. [25] Pyroelectric device comprising a multilayer structure according to one of claims 1 to 19. [26] Thermistor device comprising a multilayer structure according to one of claims 1 to 19. [27] A method for producing a multilayer structure, wherein an adhesion layer comprising a first adhesion sub-layer and a second adhesion sub-layer is arranged on a main surface of a non-noble metal substrate, wherein the first adhesion sub-layer comprises a transition metal and a second adhesion sub-layer comprises a transition metal oxide, and the first adhesion sub-layer and the second adhesion sub-layer are arranged one on top of the other along a stacking direction, wherein the stacking order of the adhesion sub-layers is not limited, an electrode containing a noble metal or a conductive oxide is arranged above the adhesion layer, an electroceramic material is deposited over the electrode. [28] The method of claim 27, wherein the electroceramic layer is a PZT ceramic layer formed by at least two steps, wherein a first solution having a first Zr / Ti ratio is deposited over the electrode and a first sublayer of the electroceramic layer is formed by drying and / or heating, a second solution having a second Zr / Ti ratio is deposited over the first sublayer of the electroceramic layer, thereby forming a second sublayer, where the second Zr / Ti ratio is smaller than the first Zr / Ti ratio. [29] The method of claim 28, wherein a third solution having a third Zr / Ti ratio is deposited over the second sublayer of the electroceramic layer, thereby forming a third sublayer, the third Zr / Ti ratio being less than the first and second Zr / Ti ratios.