Piezoelectric device and method of forming

Abstract

A piezoelectric device comprising a substrate 360 with a layer of piezoelectric material deposited thereon. The piezoelectric may have a morphological columnar tilting angle greater than 7 degrees, pr

Description

FIELD The present disclosure relates to highly inclined piezoelectric films and a method of manufacturing thereof. BACKGROUND Piezoelectric materials can be used in a wide range of applications, including sensors, actuators, motors, microphones and the like. One particular use of piezoelectric material is in piezoelectric elements for ultrasonic transducers. In this case, the piezoelectric element can be used to generate an ultrasonic signal responsive to an input electrical signal and / or can be used to generate an electrical signal responsive to a received ultrasonic signal. Ultrasound spans the range of sound frequencies that are higher than the range that can be heard by humans, and generally have frequencies of greater than 20kHz. Typical ranges of operation extend from 100kHz up to several GHz. Due to the very high frequencies involved, ultrasonic devices are typically very different from those generally used for audible applications. Analysis using ultrasound waves shows great promise in a range of applications, particularly in imaging such as medical imaging but also in fields such as non-destructive testing (NDT), particularly in industrial NDT. Dental imaging using ultrasonic techniques is one example of a suitable application, wherein the ultrasound imaging can be used to determine properties of the layers of the tooth. However, ultrasound has a wide range of uses and the applications of ultrasound are not limited to these examples. The ultrasound transducer is operable to produce ultrasonic waves that are transmitted into the sample and detect reflections of the ultrasonic waves that are reflected from the interfaces between the layers of the sample. By using techniques such as time of flight and other analyses, it is possible to image the layers of the sample and thereby characterise the sample. The piezoelectric effect or coefficient of the piezoelectric element used in the ultrasonic transducer has a significant effect on the quality and sensitivity of the acquired images. Piezoelectric elements having improved piezoelectric effect, and industrially scalable methods for manufacturing them are therefore desirable. However, as noted above, piezoelectric materials can be applied across a wide range of applications and increases in piezoelectric coefficient could also be beneficial in other applications. At least one aspect of at least one embodiment of the present invention is to provide piezoelectric devices having improved piezoelectric effect and improved methods for manufacturing such devices. SUMMARY Various aspects of the present invention are defined in the independent claims. Some preferred features are defined in the dependent claims. According to a first aspect of the present disclosure is a piezoelectric device comprising: a substrate; and a layer of piezoelectric material deposited on a surface of the substrate; wherein the piezoelectric material has a morphological columnar tilting angle greater than 7 degrees. The piezoelectric material may have a columnar structure. The piezoelectric material may be a sputter coated piezoelectric material. The morphological columnar tilting (or inclined) angle is the angle of columnar structure of atoms or molecules of the piezoelectric material deposited onto the substrate relative to a perpendicular direction to the substrate. This may also be referred to as the columnar tilting angle, the morphological angle, the column angle, the columnar angle or the p angle and these terms can be used interchangeably. The p angle is shown in the example illustrations of Figures 1a and 3b (described in more detail below) and further in the Scanning Electron Microscopy (SEM) image (Figure 7) of an example highly inclined ZnO piezoelectric film. The morphological columnar tilting angle may be greater than 10 degrees, may be greater than 12 degrees; and preferably the morphological columnar tilting angle may be greater than 20 degrees. The morphological tilting angle of a majority, e.g. at least 75% or 90% or more, such as greater than 95%, of the total piezoelectric material that is deposited on the substrate may have the morphological columnar tilting angle greater than 7 degrees. The beta angle may be a beta angle at a front of the piezoelectric material being a part of the piezoelectric material closest to the sputtering target. The beta angle may be a beta angle at the back of the piezoelectric material being a part of the piezoelectric material furthest from the sputtering target. A piezoelectric device having a p angle in this range has been found to provide an increased piezoelectric response (typically known as the piezoelectric coefficient, piezoelectric modulus or c / 33) for piezoelectric material deposition conditions that are otherwise the same. The piezoelectric material may have a piezoelectric response or that is greater than 20pC / N, such as greater than 40pC / N, e.g. greater than 60pC / N. The increased piezoelectric response may mean that a greater piezoelectric displacement can be produced for the same energy expended or the same voltage, or less energy or voltage need be used to produce the same piezoelectric displacement. In this way, the piezoelectric device can be configured to have a very low energy consumption, which may be particularly beneficial for remote sensing and other portable or battery powered applications. The greater ultrasound displacement may result in greater resolution or depth of sensing in applications such as non-destructive testing (NDT) and others. Furthermore, it has been found that the variation of piezoelectric response with p angle is linear, well defined and reproducible (see e.g. Figure 1b) at least over some of its operating range, and so can result in an ultrasound device with very predictable and controllable characteristics. The piezoelectric material may have a porosity value of between 7 and 12 %. The piezoelectric material may be or comprise an inorganic material. The piezoelectric material may be a crystalline, e.g. monocrystalline or polycrystalline, piezoelectric material. The layer of piezoelectric material may be non-polymeric, e.g. it may not contain any polymeric material. The piezoelectric material may comprise Zinc Oxide (ZnO) or Aluminium Nitride (AIN) or other oxide or nitride material having piezoelectric properties. The piezoelectric material may be or comprise a doped or alloyed piezoelectric material comprising at least one dopant or further material (such as an alloying material or a codeposited material), which may be or comprise a transition metal or compound thereof. The dopant or further material may be vanadium, for example. The dopant or further material may be present in the piezoelectric material and / or sputtering target at a level up to 10% with respect to weight, e.g. from 0.01 to 10% w / w. The primary piezoelectric material, e.g. the metal oxide or metal nitride, may be present in the layer of piezoelectric material in levels from 90% w / w up to 99.99% w / w. The substrate may be an electrically conductive rigid or flexible substrate. The substrate may be a metallic substrate. The substrate may be or comprise a metallic foil, such as aluminium foil. The substrate may be or comprise a polymeric material such as polyamide or polyimide. The substrate may be or comprise a material for which the lattice mismatch or strain with the piezoelectric material is less than 33%, preferably 25%, e.g. 15% or less. Aluminium has a moderately low lattice mismatch or strain with ZnO and AIN, which may be beneficial relative to substrate materials with a higher lattice mismatch or strain. The substrate may be an inorganic crystalline material. The substrate may be or comprise sapphire, which beneficially has a very low lattice mismatch or strain with ZnO and AIN. In some examples, the substrate may be or comprise silicon. Even though the lattice mismatch or strain between ZnO and AIN and silicon is higher than for either aluminium or sapphire, with the techniques described herein it may still be possible to produce acceptably low defect and structured files, even on silicon. The substrate may be coated with a layer of a material having a lattice mismatch or strain with the piezoelectric material is less than 33%, preferably 25%, e.g. 15% or less. The substrate may be coated with a layer of the piezoelectric material itself, e.g. prior to sputter coating the piezoelectric material with a morphological columnar tilting angle greater than 7 degrees, e.g. using a restrictor box or collimator. The layer may be a layer of piezoelectric material or doped piezoelectric material. The layer on the substrate may be Aluminium doped Zinc Oxide (AZO). The substrate may be thicker than the layer of piezoelectric material. The substrate and / or the layer of piezoelectric material may be flexible or otherwise conformable. The substrate may be configured or operable as an electrode for operating the piezoelectric device. A surface of the piezoelectric material that is opposite the substrate may comprise at least one further electrode for operating the piezoelectric device. The device may comprise a top electrode, which may be provided on an opposite side of the piezoelectric material form the substrate. The device may be a surface acoustic wave (SAW) device. According to a second aspect of the present disclosure is a piezoelectric device comprising: a substrate; and a layer of piezoelectric material on a surface of the substrate; wherein the deposited piezoelectric material has a porosity value of between 7 and 12 %. A piezoelectric device wherein the porosity of the layer of piezoelectric material is in a range from 7 to 12% has been found to enhance the piezoelectric coefficient and thereby the piezoelectric effect (e.g. as illustrated in the example of Figure 4). The relationship between porosity and piezoelectric coefficient generally resembles a Gaussian function where an optimal porosity may be around 10%. When the porosity is varied significantly from the optimal value, the enhanced effect is no longer present. The piezoelectric material may have a morphological columnar tilting angle greater than 7 degrees, e.g. greater than 10 degrees, may be greater than 12 degrees; and preferably the morphological columnar tilting angle may be greater than 20 degrees. The piezoelectric device may be the piezoelectric device of the first aspect, e.g. may comprise any feature of the piezoelectric device of the first aspect of any combination of features thereof. For example, the piezoelectric material may be or comprise an inorganic material. The piezoelectric material may be a crystalline, e.g. monocrystalline or polycrystalline, piezoelectric material. The layer of piezoelectric material may be non-polymeric, e.g. it may not contain any polymeric material. The piezoelectric material may comprise Zinc Oxide (ZnO) or Aluminium Nitride (AIN) or other oxide or nitride material having piezoelectric properties. The piezoelectric material may be or comprise a doped or alloyed piezoelectric material comprising at least one dopant or further material (such as an alloying material or a co-deposited material), which may be or comprise a transition metal or compound thereof. The dopant or further material may be vanadium, for example. The substrate may be an electrically conductive rigid or flexible substrate. The substrate may be a metallic substrate. The substrate may be or comprise a metallic foil, such as aluminium foil. The substrate may be or comprise a material for which the lattice mismatch or strain with the piezoelectric material is less than 33%, preferably 25%, e.g. 15% or less. The substrate may be an inorganic crystalline material. The substrate may be or comprise sapphire. The substrate may be coated with a layer of a material having a lattice mismatch or strain with the piezoelectric material is less than 33%, preferably 25%, e.g. 15% or less. The substrate may be coated with a layer of the piezoelectric material itself, e.g. prior to sputter coating the piezoelectric material with a morphological columnar tilting angle greater than 7 degrees, e.g. using a restrictor box or collimator. The substrate may be thicker than the layer of piezoelectric material. The substrate and / or the layer of piezoelectric material may be flexible or otherwise conformable. The substrate may be configured or operable as an electrode for operating the piezoelectric device. A surface of the piezoelectric material that is opposite the substrate may comprise at least one further electrode for operating the piezoelectric device. The device may comprise a top electrode, which may be provided on an opposite side of the piezoelectric material form the substrate. The device may be a surface acoustic wave (SAW) device According to a third aspect of the present disclosure is a piezoelectric device comprising: a substrate; and a layer of piezoelectric material deposited on a surface of the substrate at an oblique angle to the surface of the substrate such that columnar structure of the piezoelectric material is obliquely inclined at an angle greater than 7 degrees to a direction perpendicular to the surface of the substrate. The oblique angle at which the layer of piezoelectric material is deposited on the surface of the substrate may be different to the angle at which the columnar structure of the piezoelectric material is obliquely inclined to the direction perpendicular to the surface of the substrate. The columnar structure of the piezoelectric material may be obliquely inclined at an angle of greater than 10 degrees; and preferably greater than 12 degrees to the direction perpendicular to the surface of the substrate. The columnar structure of the piezoelectric material may comprise elongate rod shaped, columnar or other elongate (e.g. high aspect ratio) regions of higher density of the piezoelectric material and / or more regularly structured piezoelectric material than regions surrounding the columns or rods. The angle at which the columnar structure of the piezoelectric material is obliquely inclined relative to the direction perpendicular to the surface of the substrate may be the angle at which the columns, rods or other elongate structure of the piezoelectric material extend, e.g. the angle of a longitudinal direction of the columns, rods or other elongate structure of the piezoelectric material to, the direction perpendicular to the surface of the substrate. The deposition of the layer of piezoelectric material may be by sputter coating, or other suitable physical vapour deposition (PVD) process, using a sputtering target, which may be a metallic sputtering target such as a zinc or aluminium sputtering target. The sputtering target may comprise a doped or alloyed piezoelectric material. The deposition of the layer of piezoelectric material may be by sputter coating using through a plasma, such as an Argon and / or Oxygen and / or Nitrogen plasma. The piezoelectric material may be deposited through an aperture provided between the surface of the substrate and the sputtering target. The aperture may be provided closer to the substrate than to the substrate than to the sputtering target. A distance from the aperture to the substrate may be a half or less, e.g. a third or less, such as a quarter or less, of a distance from the aperture to the sputtering target. By depositing the piezoelectric material through an aperture, any angular components of the incoming flux of atoms outside of a desired range may be blocked, allowing only the atoms approaching in a line-of-sight from the source of piezoelectric material to reach the substrate. This results in piezoelectric devices with a higher morphological angle (typically known as p angle) close to the maximum theoretical achievable p angle. A piezoelectric device having a p angle in this range provides an increased piezoelectric response (typically known as the piezoelectric coefficient, piezoelectric modulus or cfo) for a piezoelectric material deposition that is otherwise the same. The size and / or shape, e.g. square, circle, rectangle, ellipsoid or the like, of the aperture and the position of the aperture relative to the substrate, e.g. distance from the aperture to the substrate, can be customised to control at least one or each of: the thickness uniformity of the deposited piezoelectric material, the p angle distribution across the film, and / or the porosity of the film, which may allow accurate control of the piezoelectric effect of the resulting piezoelectric device. Controlling the restriction of the flux in this way allows more reliable reproducibility of piezoelectric devices with a particular beneficial characteristics to be increased, improving the efficiency and reducing the cost of manufacturing such devices through industrial scalability. The deposition of the layer of piezoelectric material may be by sputter coating or by using another suitable physical vapour deposition (PVD) process such as using a linear array of physical vapour deposition (PVD) sources. The layer of piezoelectric material may be provided or deposited only on one surface, e.g. one planar surface, of the substrate. The layer of piezoelectric material may be provided or deposited on part or all of the surface, e.g. one planar surface, of the substrate. The piezoelectric material may be or comprise a doped or alloyed piezoelectric material. In some examples, the sputter coating may comprise using a sputtering target that is formed from, comprises, or has the same constituents as, the piezoelectric material that forms the layer of piezoelectric material in the final piezoelectric device. In other examples, the plasma comprises a working gas such as Oxygen or Nitrogen that reacts with the material of the sputtering target in order to form the piezoelectric material. For example, the sputtering target may comprise a metal such as zinc or aluminium, and the working gas may comprise Oxygen or Nitrogen that react with the zinc or aluminium to deposit piezoelectric material such as ZnO or AIN. The method may comprise annealing the layer of piezoelectric material, e.g. after deposition. The annealing may comprise annealing the piezoelectric material, which may be done in a controlled environment. The controlled environment may comprise or consist of a gas corresponding to an anion of the piezoelectric material, e.g. the gas of the controlled environment may be or comprise Oxygen if the piezoelectric material is an oxide or the gas of the controlled environment may be or comprise Nitrogen if the piezoelectric material is a nitride. The annealing may improve or increase crystallinity, which may further enhance the piezoelectric effect or cfe in the present case that comprises the piezoelectric material forming columnar structures having defined properties. The piezoelectric material and / or sputtering target may be or comprise a primary piezoelectric material such as a metal oxide or metal nitride, such as zinc oxide or aluminium nitride, or a doped or alloyed metal oxide or metal nitride. The piezoelectric material and / or sputtering target may comprise a dopant or further material (such as an alloying material or a co-deposited material), which may be or comprise a transition metal or compound thereof. The dopant or further material may be vanadium, for example. The dopant or further material may be present in the piezoelectric material and / or sputtering target at a level up to 10% with respect to weight, e.g. from 0.01 to 10% w / w. The primary piezoelectric material, e.g. the metal oxide or metal nitride, may be present in the layer of piezoelectric material in levels from 90% w / w up to 99.99% w / w. The dopant or other material may be integrated, co-deposited or reacted into the primary piezoelectric material, e.g. alloyed with or doped into the primary piezoelectric material, and may not be mixed with or coated onto or in discrete domains with the primary piezoelectric material. The aperture may be configured such that the piezoelectric material is deposited on a predefined portion of the surface of the substrate. The aperture may expose all of the substrate surface. Alternatively, the substrate may only expose a portion of the substrate, e.g. allowing the deposition pattern of the piezoelectric to be tailored to the particular intended use. This allows portions of the substrate to be left exposed. Exposed portions of the substrate may be used to make electrical connections directly to the substrate e.g. such that the substrate is operable as a ground electrode. The sputter coating may comprise depositing the piezoelectric coating using magnetron sputter deposition, e.g. direct current (DC), pulsed DC, radio-frequency, microwave, closed field magnetron (CFM) sputtering and / or high power impulse magnetron sputtering (HIPIMS). Further enhancement may be obtained using substrate biasing (e.g. DC, pulsed DC and / or RF), which may optimise sputter plasma ion energy during film growth. These particular techniques may provide beneficial film growth morphology and / or enhanced piezoelectric properties of the piezoelectric layer. Magnetron sputter deposition is a comparatively faster and cheaper process and is typically performed at room temperature and therefore has more potential for industrial scalability The substrate may be provided on a substrate holder. The substrate holder may be configured to hold the substrate at an oblique angle to the sputtering target. The substrate may be heated, e.g. to elevate the substrate above ambient temperature, during deposition of the layer of piezoelectric material. The substrate holder may be configured to hold more than one substrate. In this way the substrate or substrates is / are held at a repeatable predetermined angle to the target. Additionally, the substrate holder can also be used to transport the substrate(s). The deposition may further comprise providing a box over the substrate, the box comprising the aperture. The aperture may be provided in a planar face of the box. The box may be a cube or cuboid box. The box may be a five or six sided box. The box may be selectively closable, apart from the aperture. The box may have a removable wall or hatch for allowing the substrate or substrates to be inserted into, and removed from, the box. Alternatively, the box may be open at a side that is opposite to the aperture. The open side of the box may be a side facing and / or accommodating the substrate or substrates, in use, e.g. the substrates may be placed on a planar support and the box placed over the substrates so that the open side of the box is closed by the planar support. The box may be configured so that sputtered material passing through the aperture from the sputtering target may be incident on the substrate or substrates. The box may act as a restrictor box, for restricting the sputtered material received by the substrate to sputtered material received from the sputtering target to sputtered material received at a restricted range of angles. In alternative examples, the aperture may be proved in a plate, and the plate may be supported on legs or other supporting structure, for holding the plate and aperture a defined distance from the substrate. With the use of the restrictor box the angular components of the incident atoms are limited and only the atoms reaching in a straight line will interact with the substrate. By using a box, further limitation of atoms approaching from extreme angles is achieved. The box structure ensures repeatable placement of the aperture relative to the substrate surface increasing the reproducibility of the piezoelectric device and therefore further improving its industrial scalability. The layer of piezoelectric material may be, comprise or be comprised in a film of piezoelectric material. The layer of piezoelectric material may be configured and / or operable to produce ultrasound, i.e. the layer of piezoelectric material may be or comprise an ultrasound production layer. The piezoelectric material may be or comprise an inorganic material. The piezoelectric material may be a crystalline, e.g. monocrystalline or polycrystalline piezoelectric material. The layer of piezoelectric material may have a columnar structure. The layer of piezoelectric material may be a layer of non-polymeric piezoelectric material. The piezoelectric material may be or comprise a continuous layer of material having piezoelectric properties, e.g. the piezoelectric material may not comprise discrete domains of piezoelectric material having piezoelectric properties within a matrix of non-piezoelectric material. The piezoelectric material may be uniform and / or may have a low defect rate, which may improve the piezoelectric response. The layer of piezoelectric material may have a thickness in the range of 0.5 to 20pm, such as from 0.5 to 5 pm. The layer of piezoelectric material may be thinner than the substrate. The substrate may be or comprise a metal substrate, e.g. a metal foil, such as aluminium foil. The substrate may be or comprise a flexible and / or conductive substrate. The substrate and the layer of piezoelectric material may together be flexible, e.g. conformable. A flexible substrate may allow the transducer element, e.g. the substrate and / or piezoelectric material to be shaped, non-planar or curved, e.g. to allow for focussing of one or more ultrasound wave(s). The substrate may be or comprise a material for which the lattice mismatch or strain with the piezoelectric material is less than 33%, preferably 25%, e.g. 15% or less. The substrate may be an inorganic crystalline material. The substrate may be or comprise sapphire. The substrate may be coated with a layer of a material having a lattice mismatch or strain with the piezoelectric material is less than 33%, preferably 25%, e.g. 15% or less. The layer of piezoelectric material may be coated onto the layer on the substrate. The substrate may be coated with a layer of the piezoelectric material itself, e.g. prior to sputter coating the piezoelectric material with a morphological columnar tilting angle greater than 7 degrees, e.g. using a restrictor box or collimator. In examples, the substrate may be coated with a layer of aluminium doped ZnO (AZO) prior to deposition of the layer of piezoelectric material, and the layer of piezoelectric material may be deposited onto the layer of aluminium doped ZnO (AZO). The substrate may be or comprise an Aluminium or Copper substrate. The substrate may be or comprise a polymeric substrate such as polyamide or polyimide. In examples, the substrate comprises palladium coated with aluminium doped ZnO (AZO), and the piezoelectric material is coated onto the AZO layer. The layer of piezoelectric material may have a porosity, e.g. a porosity determined by spectroscopic ellipsometry, in a range from 7% to 12%, e.g. greater than 8% and less than 12 %. The porosity may be determined for multiple areas of the same sample, e.g. by using mapping or surface scanning. The deposition of the layer of piezoelectric material may be a deposition, e.g. a sputter coating, in a plasma chamber having a working pressure of greater than 0.3Pa less than 0.8Pa, e.g. less than 0.6Pa, such as 0.5Pa and / or less. The residual pressure prior to deposition may be between 1e-7 and 20e-7 mbar, e.g. between 2e-7 and 12e-7mbar such as around 5.4e-7 mbar. The piezoelectric device may be a piezoelectric device according to any of the first or second aspects or may comprise any feature described in relation to the first or second aspects. According to a fourth aspect of the present disclosure is a method for manufacturing, or for use in manufacturing a piezoelectric device, the method comprising: depositing a layer of piezoelectric material on a surface of a substrate at an oblique angle to the surface of the substrate such that columnar structure of the piezoelectric material is inclined at an oblique angle to the surface of the substrate. The piezoelectric material may be deposited through an aperture. The method may be used to produce the piezoelectric device of any one of the first to third aspects. By depositing the piezoelectric material through an aperture, any angular components of the incoming flux of atoms are blocked, allowing only the atoms approaching in a line-of-sight from the source of piezoelectric material to reach the substrate. As a result, piezoelectric devices with higher morphological angles (typically known as p angle) close to the maximum theoretical achievable p angle may be achievable. A piezoelectric device having a p angle above a certain value may provide an increased piezoelectric response (typically known as the piezoelectric coefficient, piezoelectric modulus or cfe) relative to depositions at other angles with otherwise the same parameters. The size of the aperture and the position of the aperture relative to the substrate can be customised to control the p angle and subsequently control the piezoelectric effect. Controlling the restriction of the flux in this way allows reproducibility of piezoelectric devices with a particular p angle to be increased improving the efficiency and reducing the cost of manufacturing such devices through industrial scalability. The oblique angle at which the columnar structure of the piezoelectric material is inclined (e.g. the morphological angle or beta angle or morphological columnar tilting angle) may be greater than 7 degrees to a direction perpendicular to the surface of the substrate. The oblique angle at which the columnar structure of the piezoelectric material is inclined may be 10 degrees or more or 12 degrees or more to the direction perpendicular to the surface of the substrate. The oblique angle at which the columnar structure of the piezoelectric material is inclined (e.g. the morphological angle or beta angle or morphological columnar tilting angle) may be less than 22°, e.g. in a range from 7° to 22°, or from 10° to 22°, such as from 12° to 22°. The aperture may be configured such that the piezoelectric material is deposited on a predefined portion of the surface of the substrate. The deposition of the layer of piezoelectric material may be by sputter coating using a sputtering target. The sputtering target may be a metallic target such as a zinc or aluminium target. The sputtering target may comprise a doped or alloyed piezoelectric material. The sputter coating may be via an oxygen or nitrogen containing plasma. The sputter coating may comprise using DC, pulsed DC, RF, microwave, closed field magnetron (CFM) sputtering, high power impulse magnetron sputtering (HIPIMS), or other magnetron sputtering method. The substrate may be provided on a substrate holder. The substrate holder may be configured to hold the substrate at an oblique angle (e.g. an alpha (a) angle) to the sputtering target. The aperture may be provided between the substrate and the sputtering target. The aperture may be provided closer to the substrate than to the sputtering target. A distance from the aperture to the substrate may be a half or less, e.g. a third or less, such as a quarter or less, of a distance from the aperture to the sputtering target. The method may further comprise providing a box over the substrate, the box comprising the aperture. The aperture may be provided in a planar face of the box. The box may be a cube or cuboid box. The box may be a five or six sided box. The box may be selectively closable, apart from the aperture. The box may have a removable wall or hatch for allowing the substrate or substrates to be inserted into, and removed from, the box. Alternatively, the box may be open at a side that is opposite to the aperture. The open side of the box may be a side facing and / or accommodating the substrate or substrates, in use, e.g. the substrates may be placed on a planar support and the box placed over the substrates so that the open side of the box is closed by abutting against the planar support. The box may be configured so that sputtered material passing through the aperture from the sputtering target may be incident on the substrate or substrates. The box may act as a restrictor box, for restricting the sputtered material received by the substrate to sputtered material received from the sputtering target to sputtered material received at a restricted range of angles. Alternatively the aperture may be provided in a planar plate. The plate (e.g. an exchange plate) may be selectively installable and removable from a planar face of the box. The plate may be exchangeable for different plates with different apertures, e.g. with different sizes and / or shapes of aperture. The plate may comprise a removable wall or hatch for allowing the substrate or substrates to be inserted into, and removed from, the box. The plate may be supportable on legs or other support structure for holding the plate and aperture a defined distance from the substrate. The plate may comprise a single aperture or a plurality of apertures which may be arranged in uniform pattern or grid. By allowing the aperture size and / or geometry to be changed using a removable plate the restrictive characteristics of the deposition may be quickly changed without removing the box from the deposition system. The layer of piezoelectric material may be a layer of inorganic, crystalline, e.g. monocrystalline, polycrystalline, non-polymeric piezoelectric material. The layer of piezoelectric material may have a columnar structure. The substrate may comprise a metallic foil and the piezoelectric material may be deposited onto the metallic foil. The method may comprise forming the layer of piezoelectric material so as to have a porosity, e.g. a porosity determined by spectroscopic ellipsometry, in a range from 7% to 12%, e.g. greater than 8% and less than 12 %. The method may comprise depositing the layer of piezoelectric material at a plasma chamber working pressure of greater than 0.25Pa and / or less than 1.0 Pa or less than 0.8Pa, e.g. less than 0.6Pa, such as 0.5Pa and / or less. The piezoelectric device may be a device according to any of the first to third aspects. According to a fifth aspect of the present disclosure is an ultrasonic device configured to produce and emit ultrasonic waves comprising the piezoelectric device of any one of the first to third aspects. The ultrasonic device may be configured to receive reflections of the emitted ultrasonic waves. The ultrasonic device may be a medical ultrasound imager. The ultrasonic device may be a non-destructive testing device. The ultrasonic device may be a dental ultrasound imager for imaging the structure or teeth. An ultrasonic device comprising a highly inclined piezoelectric film may be capable of sending and / or receiving ultrasonic waves in an advantageously directional manner when compared to standard non-inclined films. The magnitude of the ultrasonic waves emitted and / or the sensitivity of detection to ultrasonic waves may be anisotropic and may be greater in a preferential direction than in other directions. Inclined piezoelectric films permit scanning angles (closely related to the p angle) relative to the surface of the film. As such, an ultrasound device (or a plurality of ultrasound devices) comprising a plurality of piezoelectric films, each film having a different p angle (and therefore scanning angle) may be arranged such that a volume of space may be scanned without requiring mechanical movement of the device or devices. This may provide an ultrasound device which is simpler to manufacture (less moving parts) and / or operate. This may also permit, for example, inspection of hidden areas or cavities within a structure without requiring a means of mechanically moving or scanning the inspection device. An ultrasound device comprising a highly inclined piezoelectric film, characterised by an increase in piezoelectric coefficient cfa requires less applied voltage to produce the same displacement in the film as an otherwise similar ultrasound device comprising a noninclined or relatively less inclined piezoelectric film. In this way more sustainable and energy efficient ultrasound devices can be produced. Similarly, applying the same electrical stimulus to both a highly inclined and non-inclined piezoelectric film based ultrasound device will result in a more intense ultrasound signal emitted from the ultrasound device comprising the highly inclined film. This may allow a significant improvement in resulting image resolution and / or the ability to carry out nondestructive testing (NDT) with a more in depth and detailed analysis. According to a sixth aspect of the present disclosure is a set of computer readable instructions or process protocols or computer code configured such that, when processed by manufacturing equipment, permit, control or cause the manufacturing equipment, or provide instructions or data for the manufacturing equipment, to produce at least part of the piezoelectric device e.g. the ultrasonic device of the fifth aspect or to perform at least some of the steps of the method of the fourth aspect. The manufacturing equipment may be, comprise or be comprised in a sputtering system. The manufacturing equipment may be computer controlled or controllable. The set of computer readable instructions or computer code may be configured such that, when processed by the manufacturing equipment, permit, control, cause or provide instructions or data for the additive manufacturing equipment to deposit the piezoelectric layer onto the substrate to at least partly form the device of the fifth aspect or that are part of the method of the fourth aspect. According to a seventh aspect of the present disclosure is a method for manufacturing, or for use in manufacturing a piezoelectric device, the method comprising: depositing a layer of piezoelectric material on a surface of a substrate, wherein the method for comprises at least one or more or all of: forming the layer of piezoelectric material so as to have a porosity, e.g. a porosity determined by ellipsometry, in a range from 7% to 12%, e.g. greater than 8% and less than 12 %; and / or the depositing comprises depositing the piezoelectric material at an oblique angle to the surface of the substrate such that columnar structure of the piezoelectric material is inclined at an oblique angle to the surface of the substrate, and the depositing further comprises: depositing the layer of piezoelectric material at a plasma chamber working pressure of greater than 0.25Pa less than 0.8Pa, e.g. less than 0.6Pa, such as 0.5Pa and / or less; and / or depositing the piezoelectric material may be deposited through an aperture, which may be an aperture in a box or plate. The deposition of the layer of piezoelectric material may be by sputter coating using a sputtering target. The sputtering target may be a metallic target such as a zinc or aluminium target. The sputtering target may comprise a doped or alloyed piezoelectric material. The sputter coating may be via an oxygen or nitrogen containing plasma. The sputter coating may comprise using DC, pulsed DC, RF, microwave, closed field magnetron (CFM) sputtering, high power impulse magnetron sputtering (HIPIMS), or other magnetron sputtering method. The substrate may be provided on a substrate holder. The substrate holder may be configured to hold the substrate at an oblique angle (e.g. an alpha (a) angle) to the sputtering target. The aperture may be provided between the substrate and the sputtering target. The aperture may be provided closer to the substrate than to the sputtering target. A distance from the aperture to the substrate may be a half or less, e.g. a third or less, such as a quarter or less, of a distance from the aperture to the sputtering target. The method may further comprise providing a box over the substrate, the box comprising the aperture. The aperture may be provided in a planar face of the box. The box may be a cube or cuboid box. The box may be a five or six sided box. The box may be selectively closable, apart from the aperture. The box may have a removable wall or hatch for allowing the substrate or substrates to be inserted into, and removed from, the box. Alternatively, the box may be open at a side that is opposite to the aperture. The open side of the box may be a side facing and / or accommodating the substrate or substrates, in use, e.g. the substrates may be placed on a planar support and the box placed over the substrates so that the open side of the box is closed by abutting against the planar support. The box may be configured so that sputtered material passing through the aperture from the sputtering target may be incident on the substrate or substrates. The box may act as a restrictor box, for restricting the sputtered material received by the substrate to sputtered material received from the sputtering target to sputtered material received at a restricted range of angles. Alternatively the aperture may be provided in a plate, which may be supportable on legs or other support structure for holding the plate and aperture a defined distance from the substrate. The layer of piezoelectric material may be a layer of inorganic, crystalline, e.g. monocrystalline, polycrystalline, non-polymeric piezoelectric material. The layer of piezoelectric material may have a columnar structure. The substrate may comprise a metallic foil and the piezoelectric material may be deposited onto the metallic foil. According to an eighth aspect of the present disclosure is a piezoelectric device produced by the method of the seventh aspect. The piezoelectric device may be the piezoelectric device of any one of the first to third aspects. The piezoelectric device may be a piezoelectric transducer. The piezoelectric device may be configured as, or comprised in, an ultrasonic transducer. The individual features and / or combinations of features defined above in accordance with any aspect of the present invention or below in relation to any specific embodiment of the invention may be utilised, either separately and individually, alone or in combination with any other defined feature, in any other aspect or embodiment of the invention. Furthermore, the present invention is intended to cover apparatus configured to perform any feature described herein in relation to a method and / or a method of using, producing, repairing or manufacturing any apparatus feature described herein. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying Figures, in which: Figure 1a is an illustration of a perspective view of a simplified representation of an inclined piezoelectric film produced under an OAD process. Figure 1b is a plot of the piezoelectric coefficient of an inclined piezoelectric film as a function of the morphological columnar tilting angle. Figure 2a is a schematic illustration of a conventional OAD piezoelectric device manufacturing system. Figure 2b is a cross sectional view of a piezoelectric device during manufacture using the system shown in Figure 1a. Figure 3a is a schematic illustration of a restricted OAD piezoelectric device manufacturing system according to an embodiment of the present disclosure. Figure 3b is a cross sectional view of a piezoelectric device during manufacture using the system in Figure 2a. Figure 4 is a plot of the piezoelectric coefficient of an inclined piezoelectric film as a function of the porosity of the deposited piezoelectric material. Figure 5 is a plot of the morphological columnar tilting angle as a function of the working pressure of the chamber during deposition. Figure 6 is a plan, side and perspective view of an example restrictor box comprising an aperture according to an embodiment of the present disclosure. Figure 7 is an SEM image of a highly inclined piezoelectric film deposited on a substrate. Figure 8 is a flowchart of a method of depositing a layer of piezoelectric material on a substrate as part of the production of a piezoelectric device. DETAILED DESCRIPTION OF THE DRAWINGS Figure 1a shows an illustration of a perspective view of a simplified schematic representation of a piezoelectric device in the form of an inclined piezoelectric element 100 formed using an Oblique Angle Deposition (OAD) process. The piezoelectric element 100 comprises a substrate 160 and a piezoelectric film 102 on the substrate 160. The piezoelectric film 102 comprises a plurality of columnar structures 105 formed from atoms or molecules of piezoelectric material representing a layer of crystalline piezoelectric material deposited on one planar surface of the substrate 160. The substrate 160 acts to support the layer of piezoelectric material. In this example the piezoelectric material is ZnO but it will be appreciated that other suitable piezoelectric materials such as AIN or other oxide or nitride materials having suitable piezoelectric properties may be used. The columnar structures 105, in at least some examples, are elongate rod shaped, columnar or other elongate regions of higher density of the piezoelectric material and / or more regularly structured piezoelectric material than regions surrounding the columns or rods. The plurality of columnar structures 105 are inclined or tilted relative to a normal vector 115 of the planar surface of the substrate 160. An incident angle a is defined as the angle between the incident atomic flux of atoms during manufacture of the piezoelectric film 100 and the normal vector 115 of the planar surface of the substrate 160. The morphological columnar tilting angle of the film p represents the columnar tilting angle resulting from manufacture using the OAD process. The morphological columnar tilting angle of the film p is the angle at which the columnar structures 105 are tilted with respect to a direction perpendicular to the surface of the substrate upon which they are deposited. The angle p is affected by a shadowing effect in which the front most columnar structures 105a, which the atomic flux of piezoelectric atoms reaches first during manufacture, act as a mask for the substrate behind 125, resulting in the deposited layer of piezoelectric material growing in a tilted nanostructure manner. The morphological columnar tilting angle p is not necessarily the same as the incident angle a. It would be appreciated that the illustration of Figure 1a is a simplified representation of a piezoelectric film comprising a discrete number of uniformly structured and spaced columnar structures 105 for the purposes of describing the inventive concepts of the present disclosure. As described below with reference to Figures 3a and 3b, a manufactured piezoelectric film (e.g. the film shown in Figure 7) may comprise a densely packed array of columnar structures which may not be uniformly deposited on the substrate surface. In practice, the piezoelectric film 102 is a film having a much higher density of columnar structures than is shown, so that the film 102 is essentially a continuous or mostly continuous layer and that the simplified representation with only a few spaced apart columnar structures is for clarity and ease of understanding only. Figure 1b shows a plot of piezoelectric coefficient c / 33 of a piezoelectric film as a function of the morphological columnar tilting angle p. To obtain the data shown, the piezoelectric coefficient c / 33 was measured by means of Laser Doppler Vibrometry (LDV) with the substrate bottom clamped and the angle p was estimated using Scanning Electron Microscopy (SEM). The results are obtained from the inclined piezoelectric film 100 of the type shown in Figure 1a. The plot shows that there is a direct effect on the piezoelectric property c / 33 of the piezoelectric material as a result of tilting the crystalline structure of the piezoelectric film. The value of flat ZnO i.e. with no tilt or inclination present, is represented with p = 0°. The setup used to obtain the values represented in Figure 1b was calibrated using a Lead Zirconate Titanate (PZT) standard with a well-known c / 33 coefficient (370 pmA / ). As the angle p is increased the piezoelectric coefficient also increases providing an improved piezoelectric effect. However, the increase in piezoelectric effect is surprisingly notable for inclined piezoelectric films 100 with a p angle greater than 7°, particularly of greater than 10° and most particularly of 12° and above, e.g. from 12° to 22° or even 18° or 20° and above, e.g. from 18° to 22°. These very uniform and high values of morphological columnar tilting angle p are very difficult to achieve and hence the exceptional benefit to the piezoelectric effect of these values of morphological columnar tilting angle p is surprising. The novel techniques discussed below provide a way of achieving these beneficial morphological columnar tilting angles P using industrially scalable manufacturing processes. Figure 2a shows an illustration of a DC Magnetron Sputtering (MS) deposition system 200 configured for the manufacture of an inclined piezoelectric film using an unrestricted OAD process. In this MS system 200, a substrate 260 is provided on an inclined planar surface of a substrate holder 220. The substrate holder is provided on a surface of a drum 210, for example a rotating drum. The substrate 260 is an electrically conductive metallic foil, in this case an aluminium foil, and the inclined surface of the substrate holder 270 on which the substrate is located is inclined at an oblique angle a to a direction perpendicular to the surface of the drum 210. The system 200 of Figure 1 shows a single substrate 260 located on a single substrate holder 220, however it will be appreciated that a plurality of substrates and / or substrate holders may be provided on the drum 210 to increase throughput and efficiency of the system 200. The drum 210 may be rotated during manufacture to allow more precise control of the duration and nature of the deposition of atoms on the substrate 260. When the drum 210 comprises a plurality of substrates 260 and / or substrate holders 220 and the drum may be rotated during manufacture to increase the longevity of a single deposition session so as to reduce the proportion of time spent loading and unloading the deposition system 200. This results in a further increase in the throughput and control of the deposition system 200. A sputtering target 230 comprising a material that can be used to form a piezoelectric material, in this case Zn, is located on a magnetron 240 at a target-to-substrate distance d. The target-to-substrate distance d is typically 4 to 7 cm in a DC MS system but it will be appreciated that depending on the deposition system the target-to-substrate distance d may vary accordingly. In use, atoms 250 (in this case Zn atoms) are released from the target 230 and accelerated towards the substrate 260 as part of a DC reactive magnetron sputtering process, the Zn atoms 250 reacting with an oxygen plasma (otherwise known as a discharge plasma) in the target-to-substrate space to form ZnO, which is deposited as the piezoelectric material. As the sputtering target 230 provides a large distributed source of atoms 250 travelling in random directions, in part due to scattering processes that occur due to the gas molecules in the discharge plasma, the deposition is not directional enough for the sputtered atoms 250 to reach the substrate 260 with a consistently inclined direction to the surface of the substrate 260, a crucial characteristic for OAD. Figure 2b shows a magnified profile view of the substrate 260 and substrate holder 220 of Figure 2a during the unrestricted OAD process. The angle of incidence of the atoms 250 on the substrate 260 is inconsistent, resulting in a generally unstructured layer of piezoelectric material being deposited on the substrate 260. Figure 3a shows an illustration of a DC MS deposition system 300 configured for the manufacture of an inclined piezoelectric film using a restricted OAD process. The features of the deposition system 300 which are similar to the deposition system 200 of Figure 2a are numbered similarly and their description will not be repeated for conciseness. The deposition system 300 further comprises a restrictor box 380, having an aperture 390 arranged such that a surface of the substrate 360 is exposed to the sputtering target 330 via the aperture. In the system 300, the atoms 350 that are deposited onto the substrate 360 are limited to the atoms 357 that have an angular component sufficient to pass through the aperture 390 of the restrictor box 380. Any atoms 355 which do not pass through the aperture 357 do not reach, and are therefore not deposited on, the substrate 360. This limiting (or restricting) process allows for the synthesis of highly inclined piezoelectric films having an increased morphological columnar tilting angle p and consequentially providing an improved piezoelectric effect. The use of a restrictor box 380 has been found to be particularly beneficial in this specific application. For example, instead of a restrictor box 380, collimators provided near to or adjacent the sputtering target 330 could be used. For example, collimation of the flux could be achieved by the use of a grid of cylinders placed proximate the sputtering target 330. However, this arrangement is not as effective as the restrictor box 380. The solid angle of the material that passes through the collimators can depend on the length of the columns. However, if the deposition is not carried out under ballistic conditions, (i.e., the chamber pressure is low enough to ensure the target-to-substrate distance is shorter than the mean-free path (MFP) of the sputtered particles and / or the deposition is carried at room temperature to ensure no thermal diffusion) there exists a compromise between the total flux reaching the substrate and the length of the collimator. If the collimator length is greater than the MFP of the particles in the incident flux, the atoms travelling on a straight line will not reach the substrate surface as gas scattering processes will occur inside the collimator, causing the atoms to be deposited onto the walls instead of travelling through. Furthermore, this type of collimator requires water cooling to reduce the thermal load on the samples and additionally one must pay attention when designing the hole diameter as it has a direct impact on the lifetime of the collimator. The larger the diameter the longer the lifetime, as the accumulation of materials inside the collimator effect will be observed after longer time of use. The use of the restrictor box 380 eliminates, or at least mitigates against, these problems. In examples, the aperture 390 is provided in a removable exchange plate that is selectively installable and removable from a planar face of the box. The plate may be exchangeable for different plates with different apertures, e.g. with different sizes and / or shapes of aperture. In examples, the plate or box comprises a single aperture or could comprise a plurality of apertures which may be arranged in uniform pattern or grid. Figure 3b shows a magnified profile view of the substrate 360 and substrate holder 320 of Figure 3a during the restricted OAD process. The angle of incidence of the atoms 357 on the substrate 360 which have passed through the aperture 390 of Figure 3a is relatively consistent when compared to the randomly varied angles of incidence of the atoms 250 of Figure 2b. This consistent incident angle produces a uniform deposited piezoelectric layer 102 (see Figure 1) on the substrate 360 in the form of a plurality of columnar structures 305, which may be the columnar structures 105 of the piezoelectric film 100 of Figure 1 a. The plurality of columnar structures 305 are inclined or tilted relative to a normal vector of the planar surface of the substrate 360 by the morphological columnar tilting angle p. The increased consistency of incident angle and subsequent consistent morphological columnar tilting angle p allows for increased reproducibility between runs providing a more industrially scalable manufacturing process. This “obliquely angled” columnar piezoelectric film can have significant benefits over conventionally deposited films. The “obliquely angled” columnar piezoelectric film provides a high surface to area ratio, which enhances the performance and efficiency of sensing platforms. In addition, the directional alteration of some intrinsic physical properties of the materials can be tailored when deposited under oblique conditions. The films can even be configured to create magnetic anisotropy by means of altering the geometry of the film, e.g. by altering the angle of columns of piezoelectric material. In another potential benefit, the ultrasonic properties of the film can potentially be made highly directional, e.g. having an anisotropic properties in which the magnitude of ultrasonic waves produced, and / or detection sensitivity to received ultrasonic waves, is much higher in a certain direction than for other directions. It could be conceived that this can be used for a variety of applications, including directional sensing and / or emission, wave steering, MIMO transmissions and receptions and other applications that make use of directionality. Another possibility is to produce highly focussed beams that can be used to measure properties of a very confined area or volume, which may improve signal to noise ratio by reducing background noise from regions that should not be included in the measurement. The applications are not limited to these and other applications that make use of the directionality could be conceived. Since the technique described above is capable of producing piezoelectric films with a very high uniformity and specificity of morphological columnar tilting angle p, and also very high values of morphological columnar tilting angle p, the piezoelectric effect can be significantly enhanced, as discussed above with respect to Figure 1b. Figure 4 shows a plot of piezoelectric coefficient c / 33 of a piezoelectric film as a function of the porosity of the deposited piezoelectric material produced using the deposition system 300 of Figure 3a. The piezoelectric coefficient c / 33 was measured by means of the quasi-static method (or Berlincourt method) and the porosity was estimated using spectroscopic ellipsometry. Spectroscopic ellipsometry is an indirect method used to calculate the porosity through the application of the effective medium approximation (EMA) model. Polarisation of the reflected light is compared with the incident light and, using the EMA model, from that comparison the number of voids and their distributions is calculated. The fitting between the model and the experimental results, produces a parameter called root mean squared (RMS) roughness. That is, a percentage or other degree of void (equivalent to porosity) in the EMA model can be varied to fit the data and the RMS value can be used as a quality of fit function, with the lower the RMS, the better the fit. The plot shows that there is a direct effect on the piezoelectric property c / 33 of the piezoelectric film when the porosity of the piezoelectric material of the film is changed. The effect generally resembles a Gaussian function where an optimal porosity, in this case around 10%, is capable of enhancing the piezoelectric effect by maximising the piezoelectric coefficient c / 33. When the porosity is varied by a threshold amount from the optimal value the enhanced effect is no longer present. In the example shown, the piezoelectric coefficient c / 33 is optimally enhanced between 7% and 12% porosity. As such, producing the layer 102 of piezoelectric material so that its porosity is in a range from 7% to 12% can significantly enhance the piezoelectric coefficient c / 33. The methods used to characterise the piezoelectric coefficient c / 33 to obtain the results illustrated in Figure 4 (Berlincourt method) are distinctly different from those used to obtain the results illustrated in Figure 1b (LDV). It would be appreciated that a quantitative comparison of values obtained from different characterisation techniques may not produce a representative assessment of piezoelectric coefficient cfe in each. The plots of Figures 1a and 4 show the qualitative effect of p angle (Figure 1a) and inclined film porosity % (Figure 4) on the piezoelectric coefficient of samples characterised using the respective characterisation technique and are intended to be interpreted as such. Figure 5 shows a plot of morphological columnar tilting angle p of an inclined piezoelectric film as a function of the working pressure of the chamber during deposition. The plot shows that there is an observable exponential decay trend of a decrease in p when increasing the working pressure. However, below a certain value of working pressure, the efficacy of the process decreases markedly as there is a deficiency of reactant in the plasma. As such, there is an optimal window of working pressure of plasma in the chamber. SEM analysis indicates that reducing the total gas flux (and thus the pressure) leads to an increase in the resulting columnar tilting angle p for a chosen angle a of 80°. SEM images demonstrate that just by decreasing the pressure from 1.11 Pa to 0.3 Pa an increase from p=8±2° to p=23±2° is observed for the same deposition conditions. Reducing the working pressure limits the interaction between the sputtered atoms and the gases inside the chamber, leading to a more directional deposition. Probability of collision is directly related with the mean free path (MFP) of the sputtered atoms, in this case Zn atoms, which is inversely proportional to the working pressure in the chamber during deposition. In view of the above, the optimal working pressure of the plasma forming material in the deposition for the example deposition system used in this it is in a range from 0.3 Pa to 1.1 Pa and preferably from 0.3Pa to 0.9 Pa, e.g. from 0.3Pa to 0.75 Pa. It would be appreciated that the optimal working pressure for production of highly inclined films in any particular deposition system may be limited by the lowest pressure at which the particular system can achieve a stable discharge plasma. The lower the working pressure the higher the inclination (or beta angle) would be expected in the resulting inclined film. However, the decreased pressure may impact other parameters of the deposition process (e.g. deposition rate) For short MFPs, the atoms reaching the surface of the substrate have a wide distribution of incident angles, caused by scattering processes, leading to a decrease in the structure inclination. When the pressure is reduced, the number of particles reaching the sample are more collimated, with less angular distribution, therefore the deposition results in highly-tilted nanocolumnar films. Figure 6 shows an example five sided cuboid restrictor box 680 used in the deposition system 300 of Figure 3a. The box has a length of 100mm, a width of 60mm and a depth of 40mm. the restrictor box 680 comprises an elongate or rectangular aperture 690 on the upper planar surface of the restrictor box 690. The aperture 690 has a length of 60mm and a width of 20mm. However, these dimensions are only to give a general indication of the order of the sizes and other dimensions could be used depending on the application and deposition equipment used. It would be appreciated that the dimensions of the restrictor box 680 and aperture 690 are particular to both the deposition system 300 of Figure 3a and the desired properties of the resulting piezoelectric film. For example, increasing the height of the restrictor box 680 will increase the distance between the aperture 690 and the substrate onto which the piezoelectric material is to be deposited, changing the morphological structure of the resulting piezoelectric film. Similarly changing the dimensions and / or shape of the aperture 690 (e.g. to a circular or ellipsoid shaped aperture) will change the properties of the resulting piezoelectric film. For example, leaving areas of the substrate exposed to act as electrode connection points. Figure 7 is an SEM image of a cross section through an exemplary inclined ZnO piezoelectric film 705 deposited on a substrate 760 using the deposition system 300 of Figure 3a. Using the disclosed restricted OAM process, a morphological columnar tilting angle p of 47.25°, as indicated in Figure 7, was achieved. The image shows the inclined columnar structure of the deposited piezoelectric film 705 relative to the substrate 760. Figure 8 is a flowchart summarizing a method of depositing a layer of piezoelectric material on a substrate as part of a method of producing a piezoelectric device, such as an ultrasonic transducer. The method comprises, at step 805, locating one or more substrates 360 on a substrate holder 320. The substrate holder 320 is obliquely angled to a sputtering target 330. The substrate holder 320 and one or more substrates 360 are provided in the restrictor box 380 that comprises the aperture 390 (step 810). The chamber of the sputter coater comprises a gas plasma, such as an oxygen or nitrogen plasma, and is controlled to a working pressure in a range from 0.3Pa to 1.0 Pa, e.g. from 0.3Pa to 0.8Pa (step 815). The layer of piezoelectric material 102 is sputter coated onto the substrate 360 or substrates such that the porosity of layer of the piezoelectric material 102 is between 7% and 12% and / or the beta angle is 7° or more (e.g. the beta angle is in a range from 7° to 22°). (step 820). Steps 810 and 815 are optional, i.e. the required beta angle and / or porosity may optionally be achieved in other ways. Although specific examples of improving piezoelectric coefficient are described above in relation to the Figures, it will be appreciated that variations on the above examples are possible. For example, where the thickness of the inclined film is required to be larger, the OAD process must either be run with a higher working chamber pressure or the duration of deposition must be increased. As shown above in reference to Figure 5, a higher working pressure has the effect of decreasing the morphological columnar tilting angle p which has a direct effect on the piezoelectric coefficient cfa as shown in Figure 1b. It has been observed that for a given deposition time, an increase in angle p is observed when the deposition temperature is increased from room temperature to 100°C without a change in film thickness. This effect could be explained by the intrinsic increase of the mean free path (MFP) of the sputtered atoms when the atoms in the discharge plasma have extra energy available, i.e., higher temperature. Increasing the MFP increases the directionality of the deposition, allowing the enhancement of the p angle. However, for longer deposition times (e.g. >5 hours) this effect is reversed indicating thermal diffusion effects are dominating over the influence of the shadowing mechanism, as described above, on the resulting film. For these longer deposition times the increase in temperature also reduces the resulting film thickness for a given deposition time. This reduction can be attributed to atomic diffusion happening naturally when allowing the atoms to slightly diffuse and reorganise forming a more compact layer, filling the naturally occurring void’s structure generated during the OAD process and producing a piezoelectric film with lower porosity, the effects of which are illustrated in Figure 4. Although various mechanisms for producing films with beneficially high piezoelectric coefficient cfa are described, and can be beneficially used in combination, it is not essential that all of these are used in combination and only one, more or all of beta angle in the ranges stated above, e.g. greater than 7°, which may be achieved using the 5 restrictor box, the porosity of the piezoelectric layer in the preferred range of between 7% and 12% and / or the working pressure in the ranges stated above, e.g. from 0.2Pa to 0.9 Pa, and / or other parameters could be used in any combination.

Claims

1. A piezoelectric device comprising:a substrate; anda layer of piezoelectric material deposited on a surface of the substrate; wherein the piezoelectric material has a morphological columnar tilting angle greater than 7 degrees.

2. The piezoelectric device of claim 1 wherein the morphological columnar tilting angle is greater than 10 degrees.

3. A piezoelectric device comprising:a substrate; anda layer of piezoelectric material deposited on a surface of the substrate; wherein the deposited piezoelectric material has a porosity value of between 7 and 12 %.

4. A piezoelectric device comprising:a substrate; anda layer of piezoelectric material deposited on a surface of the substrate at an oblique angle to the surface of the substrate such that a columnar structure of the piezoelectric material is inclined at an oblique angle greater than 7 degrees to a direction perpendicular to the surface of the substrate.

5. The piezoelectric device of claim 4 wherein the oblique angle at which the columnar structure of the piezoelectric material is inclined relative to the direction perpendicular to the surface of the substrate is greater than 10 degrees.

6. The piezoelectric device of claim 4 or claim 5 wherein the deposition of the layer of piezoelectric material is by sputter coating using a sputtering target; and wherein the piezoelectric material is deposited through an aperture provided between the surface of the substrate and the sputtering target.

7. The piezoelectric device of any one of claims 4 to 6, wherein the aperture is configured such that the piezoelectric material is deposited on a predefined portion of the surface of the substrate.

8. The piezoelectric device of claim 6 or any claim dependent thereon, wherein the sputter coating comprises using DC, pulsed DC, RF, closed field magnetron (CFM) sputtering, high power impulse magnetron sputtering (HIPIMS), or other magnetron sputtering method.

9. The piezoelectric device of any one of claims 4 to 8 wherein the substrate is provided on a substrate holder, the substrate holder configured to hold the substrate at an angle to the sputtering target that corresponds to the angle at which the layer of piezoelectric material is deposited on the surface of the substrate.

10. The piezoelectric device of claim 6 or any claim dependent thereon wherein the deposition further comprises providing the substrate in a box, the box comprising the aperture.

11. The piezoelectric device of any one of claims 4 to 10 wherein the layer of piezoelectric material is a layer of inorganic, polycrystalline, columnar piezoelectric material.

12. The piezoelectric device of any one of claims 4 to 11 wherein the substrate comprises a metallic foil and the piezoelectric material is deposited onto the metallic foil.

13. A method for manufacturing, or for use in manufacturing a piezoelectric device, the method comprising:depositing a layer of piezoelectric material on a surface of a substrate at an oblique angle to the surface of the substrate such that a columnar structure of the piezoelectric material is inclined at an oblique angle to the surface of the substrate;whereinthe piezoelectric material is deposited through an aperture.

14. The method of claim 13, wherein the oblique angle of the columnar structure of the piezoelectric material is greater than 7 degrees to a direction perpendicular to the surface of the substrate.

15. The method of claim 13 or claim 14, wherein the aperture is configured such that the piezoelectric material is deposited on a predefined portion of the surface of the substrate.

16. The method of any one of claims 13 to 15, wherein the deposition of the layer of piezoelectric material is by sputter coating using a sputtering target and the aperture is provided between the sputtering target and the substrate, ata location that is closer to the substrate than to the sputtering target.

17. The method of claim 16, wherein the sputter coating comprises using DC, pulsed DC, RF, microwave, closed field magnetron (CFM) sputtering, high power impulse magnetron sputtering (HIPIMS), or other magnetron sputtering method.

18. The method of claim 16 or claim 17 wherein the substrate is provided on a substrate holder, the substrate holder configured to hold the substrate at an angle to the sputtering target that corresponds to the angle at which the layer of piezoelectric material is deposited on the surface of the substrate.

19. The method of any one of claims 16 to 18 comprising:forming the layer of piezoelectric material such that is has a porosity, in a range from 7% to 12%; and / ordepositing the layer of piezoelectric material at a plasma pressure of less than 0.8Pa.

20. The method of any one of claims 13 to 19 further comprising providing the substrate in a box, the box comprising the aperture.

21. The method of any one of claims 13 to 20, wherein the layer of piezoelectric material is a layer of inorganic, crystalline, e.g. polycrystalline, columnar non-polymeric piezoelectric material.

22. The method of any one of claims 13 to 21, wherein the substrate comprises a metallic foil and the piezoelectric material is deposited onto the metallic foil.5 23. An ultrasonic device configured to produce and emit ultrasonic waves comprisingthe piezoelectric device of any one of claims 1 to 12.

24. A set of computer readable instructions or process protocols or computer code configured such that, when processed by manufacturing equipment, permit, 10 control or cause the manufacturing equipment, or provide instructions or data forthe manufacturing equipment, to perform the method of any of claims 13 to 22.

25. The computer readable instructions or computer code of claim 24, wherein the additive manufacturing equipment is, comprises or is comprised in a computer 15 controlled sputtering system.34

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