Devices and method of fabrication thereof

Abstract

A method of forming a device comprising: providing a device substrate; providing a flexible multilayer substrate having an insulating layer and an electrically conductive layer formed on the insulating layer; forming a patterned layer on a surface of the electrically conductive layer, thereby to provide protected and unprotected regions of the electrically conductive layer; etching the unprotected regions of the electrically conductive layer, thereby to form one or more electrically conductive features on the insulating layer of the flexible multilayer substrate; removing the patterned layer; and conformally applying the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate to a target surface of the device substrate.

Description

DEVICES AND METHOD OF FABRICATION THEREOFThe invention relates to devices and a method of fabrication thereof. In particular, but not exclusively, the invention relates to a cleanroom-free method of fabricating surface acoustic wave devices that facilitates improved processing and configurations of devices.Advanced electronic components and devices are conventionally fabricated using photolithography to form intricate structures directly on a substrate. For example, photolithography is traditionally used to form surface acoustic wave (SAW) devices by creating interdigital transducers directly on a piezoelectric substrate. Such fabrication is onerous in respect of the number of processing steps and requirement for complex and expensive cleanroom facilities.Further, not only are such devices costly and time consuming to produce, but in situations where devices are to be used for multiple measurements, such devices are not readily re-usable. For example, in clinical and bio-settings, residual contamination from one set of measurements can make such devices unsuitable for subsequent measurements. Additionally, devices formed by conventional photolithographic processes are provided with predefined dimensions suitable for a particular process and cannot straightforwardly be reconfigured. For example, once formed, conventional SAW devices cannot be reconfigured to change the spacing of electrodes in interdigital transducers to operate at different frequencies. In these cases, a new device needs to be fabricated, resulting in the consumption of further time, cost and resources.These difficulties present a barrier to the effective fabrication of devices.It is an object of the disclosure to at least partly address one or more of the shortcomings in the prior art mentioned above.According to an aspect of the invention, there is provided a method of forming a device comprising: providing a device substrate; providing a flexible multilayer substrate having an insulating layer and an electrically conductive layer formed on the insulating layer; forming a patterned layer on a surface of the electrically conductive layer, thereby to provide protected and unprotected regions of the electrically conductive layer; etching the unprotected regions of the electrically conductive layer, thereby to form one or more electrically conductive features on the insulating layer of the flexible multilayer substrate; removing the patterned layer; and conformally applying the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate to a target surface of the device substrate.The method enables the fabrication of devices in a manner that is easier, quicker and cheaper than standard methods of forming electrically conductive features directly on a substrate using photolithography, thereby reducing the processing steps and removing the need forcomplex equipment and / or cleanroom facilities traditionally associated with such device fabrication. Advantageously, devices formed in accordance with the process can be reused and repurposed in a way that is not possible with devices formed directly on a substrate using photolithography.Optionally, the flexible multilayer substrate comprises a further electrically conductive layer on the opposite side of the insulating layer to the electrically conductive layer. Beneficially, the use of double-sided electrically conductive flexible multilayer substrates enables electrically conductive device functionality not only in combination with an underlying device substrate, but also with additional components on the other side of the flexible multilayer substrate to form more complex devices.Optionally, conformally applying the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate to the device substrate comprises mechanically clamping the flexible multilayer substrate and the device substrate together. Advantageously, devices formed in such a manner are easily disassembled, enabling the effective replacement, cleaning and / or reuse of components.Optionally, mechanically clamping the flexible multilayer substrate and the device substrate comprises positioning the flexible multilayer substrate between the device substrate and at least one further layer. Beneficially, devices are formed in a manner that can distribute pressure points within a device construction whilst enabling further functionality, such as effective inspection of components during operation.Optionally, forming a patterned layer on a surface of the electrically conductive layer of the flexible multilayer substrate comprises: forming a photoresist layer on the electrically conductive layer; selectively masking the photoresist layer; exposing the photoresist layer to ultra-violet light; developing the photoresist layer; and removing developed or undeveloped regions of the photoresist layer, thereby to provide a patterned layer on the electrically conductive layer. Optionally, forming the photoresist layer on the electrically conductive layer comprises laminating a dry photoresist film to the electrically conductive layer. Beneficially, the photofabrication of etched electrically conductive features facilitates controlled, cleanroom-free integration of electrically conductive features, such as photoetched flexible printed circuit boards, into devices. This contrasts with conventional photolithography-based techniques that require the use of a cleanroom to directly pattern a substrate and deposit electrically conductive material directly onto the substrate.Optionally, the target surface of the device substrate is non-planar. Advantageously, the method provides for the effective formation of non-standard, non-planar geometries, such as the formation of 3D geometries for enhanced application and / or integration.Optionally, the device substrate comprises at least one of: a piezoelectric material, a semiconductor material, a solid-state material and a single layer material, optionally wherein the single layer material is a graphene layer. Optionally, the one or more electrically conductive features are electrodes. Beneficially, the method is widely applicable to the formation of different devices with different configurations. Optionally, the electrodes are interdigital transducers. Beneficially, such electrodes are provided in an improved manner to known techniques and, in combination with piezoelectric substrates, provide improved fabrication and device operation of SAW devices.Optionally, the device substrate is a piezoelectric substrate comprising at least one of: lithium niobate, lithium tantalite, lanthanum gallium silicate, quartz, cellulose and piezo-electric polymer-based material. Advantageously, piezoelectric devices, such as SAW devices, are formable in an improved manner, with improved device operation compared to known techniques.Optionally, the method further comprises forming at least part of a cooling device on the opposite side of the flexible multilayer substrate to the electrically conductive layer, optionally wherein the cooling device is a thermoelectric cooling device. Beneficially, the method enables the configuration of devices that are coolable in an improved manner in close proximity to the regions that are to be cooled.Optionally, the method further comprises forming a layer comprising a magnetostrictive material on the flexible multilayer substrate. Advantageously, such integration provides for the effective measurement of magnetic field properties.Optionally, the method further comprises: subsequently separating the device substrate and the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate; cleaning the device substrate; and subsequently conformally re-applying the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate to a target surface of the device substrate, or conformally applying one or more electrically conductive features formed on the insulating layer of a further flexible multilayer substrate to a target surface of the device substrate.Optionally, the method further comprises sterilising the device substrate. Advantageously, the method enable reuse of one or more components of the device, decreases wastage of resources and enables lower cost and / or more easily replaceable components of the device to be replaced, whilst reusing more expensive and / or more difficult to replace components of the device.There is also provided a system configured to perform the method. Beneficially, one or more aspects of the method can be integrated in a system in which one or more steps are automated and / or controlled as part of a computing network.There is also provide a device formed in accordance with the method. Advantageously, devices formed in such a manner are fabricated in a manner that is easier, quicker and cheaper than standard methods of forming electrically conductive features directly on a substrate using photolithography, thereby reducing the processing steps and removing the need for complex equipment and / or cleanroom facilities traditionally associated with such device fabrication. Advantageously, devices formed in accordance with the process can be reused and repurposed in a way that is not possible with devices formed directly on a substrate using photolithography. Further, such devices are formable with configurations and geometries that enable improved devices.There is also provided a method of using the device formed in accordance with the method, wherein the device substrate is a piezoelectric substrate and the one or more electrically conductive features are interdigital transducers, wherein the method comprises: generating surface acoustic waves.Optionally, the method further comprises: detecting and / or characterising biological cells, microbial lifeforms, nanobiomaterials and / or biomolecules. Optionally, the method further comprises: determining a measure of gravity and / or characterising gravity. Optionally, the method further comprises: determining a measure of at least one of: torque, temperature, pressure and / or strain based on a change in the path length of surface acoustic waves travelling along the device substrate. Optionally, the method further comprises: converting an electrical input signal into a surface acoustic wave; and subsequently recombining the surface acoustic wave into a further electrical signal, thereby mechanically filtering the electrical input signal. Optionally, surface acoustic waves are used to control the location of one or more biological and / or non-biological objects in ID and / or 2D on the device substrate. Optionally, the method further comprises: determining a measure of at least one of humidity, viscosity and magnetic field data.Beneficially, SAW devices formed in such a way are used as SAW sensors to detect a wide range of different physical phenomenon thanks to their high degree of sensitivity, ability to operate in both wet and dry environments, and potential to have a small form factor. Beneficially, SAW sensors formed in such a way have a higher power (or lower loss factor) compared with conventionally made SAW sensors, not only meaning that they are more sensitive and have the ability to detect smaller entities, fewer entities, etc., but also where improved sensitivity is not required, the same sensing accuracy can be achieved through the useof a weaker (and less expensive or more flexible, for example) piezoelectric substrate that is not usable in conventional SAW sensors. Additionally, such SAW sensors can be easily and quickly fabricated, as well as being recycled and reused. Accordingly, the fabrication is more efficient and cost effective when compared with conventional techniques.According to an aspect of the invention, there is provided a surface acoustic wave device, comprising: a piezoelectric device substrate; an insulating layer; and one or more electrically conductive features positioned between the piezoelectric device substrate and the insulating layer, the one or more electrically conductive features defining interdigital transducers configured to generate and / or detect surface acoustic waves in the piezoelectric device substrate, wherein the dimensions and / or composition of the insulating layer and / or the geometry of the interdigital transducers is / are configured to substantially avoid coupling of surface acoustic waves between the piezoelectric device substrate and the insulating layer and / or to substantially avoid attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer.Configuring the device specifically to avoid such coupling and / or attenuation provides significant improvements in performance, such as lower power loss and / or improved measurement sensitivity.Optionally, the dimensions and / or composition of the insulating layer and / or the geometry of the interdigital transducers is / are configured to space apart the piezoelectric device substrate from the insulating layer to avoid direct contact or near contact between the piezoelectric device substrate and the insulating layer. Such spacing apart contributes to avoiding coupling of surface acoustic waves between the piezoelectric device substrate and the insulating layer and / or to avoiding attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer.Optionally, the interdigital transducers have a thickness perpendicular to the piezoelectric device substrate of at least 5 microns. Providing such relatively thick interdigital transducers has been found to contribute effectively to avoiding coupling of surface acoustic waves between the piezoelectric device substrate and the insulating layer and / or to avoiding attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer.According to an aspect of the invention, there is provided a method using a device according to the disclosure, wherein the device substrate is a piezoelectric substrate and the one or more electrically conductive features are interdigital transducers, wherein the method comprises: using the interdigital transducers to passively measure surface acoustic waves generated in the piezoelectric substrate by entities in a sample adjacent to the piezoelectric substrate.SAW devices according to embodiments of the present disclosure can have such high performance that they can be applied effectively to a range of passive sensing applications not previously considered available for SAW devices. These include monitoring a response of the entities to a drug, optionally in real time. The inventors have provided specific demonstrations of such a method in the context of monitoring neuronal cells.Further aspects of the invention will be apparent from the description and the appended claims.A detailed description of embodiments is described, by way of example only, with reference to the figures, in which:Figures 1 A to 1H shows processing steps of a method for fabricating a device;Figure 2 shows an exploded perspective view of a surface acoustic wave device;Figures 3 A and 3B show a perspective views of the assembled surface acoustic wave device of Figure 2;Figure 4 shows a perspective view of electrodes formed on a flexible layer and a device formed by the application of the electrodes formed on the flexible layer to a non-planar substrate; andFigure 5 shows an experimental set up including a surface acoustic wave device.Figure 6 is a graph showing measured power propagation of a conventional SAW device without any detected flaws from manufacture.Figure 7 is a graph showing measured power propagation of a SAW device according to the present disclosure.Figure 8 is a graph showing measured power propagation of a SAW device according to the present disclosure in which optical grade quartz is used as a piezoelectric substrate.Figures 9 and 10 depict the results of experiments to demonstrate use of devices of the present disclosure in the context of monitoring cellular response to Amitriptyline.Figures 11-13 depict the result of experiments to demonstrate use of devices of the present disclosure in the context of monitoring cellular response to Paclitaxel.Figures 1 A to 1H show steps of method of forming a device 100 using photoetched electrically conductive features attached to a substrate. Beneficially, the process enables the fabrication of devices in a manner that is easier, quicker and cheaper than standard methods of forming electrically conductive features directly on a substrate using photolithography, thereby reducing the processing steps and removing the need for complex equipment and / or cleanroom facilities traditionally associated with such device fabrication. Advantageously, devices formed in accordance with the process can be reused and repurposed in a way that is not possible with devices formed directly on a substrate using photolithography. Beneficially, the methoddescribed with reference to Figures 1 A to 1H can be used to form surface acoustic wave devices in an improved manner, with improved properties, such as those described in greater detail with reference to Figures 2 to 5.At Figure 1 A there is shown a cross-sectional view of a flexible multilayer substrate having an electrically insulating layer 104 and an electrically conductive layer 106 formed on the electrically insulating layer 104 (hereinafter referred to as an ‘insulating layer’). There is also shown a device substrate 102. The device substrate 102 and the flexible multilayer substrate are provided and processed to form a device 100. In order to form the device, the electrically conductive layer 106 of the flexible multilayer substrate is selectively etched. Figures IB to 1G show steps in a process for selectively etching the electrically conductive layer 106. Whilst there is shown a flexible multilayer substrate having an insulating layer 104 with an electrically conductive layer 106 formed on the insulating layer 106, in further examples there is also a further electrically conductive layer on the opposite side of the insulating layer 104 to the electrically conductive layer 106. Whilst Figure 1 A shows a cross-sectional view of a flexible multilayer substrate with a single insulating layer 104 and a single electrically conductive layer 106, in further examples, the flexible multilayer substrate comprises any number and combination of insulating and / or electrically conductive layers, and / or further layers with different electrical properties in accordance with the functionality described herein.In an example, the flexible multilayer substrate is an adhesive-less Copper-Clad Laminate, such as ESPANEX (RTM) with a polyimide insulating layer having a thickness of 25 microns and an electrically conductive Copper layer formed thereon with a thickness of 18 microns. Advantageously, the use of such a flexible multilayer substrate enables electrically conductive features to be formed with a thickness that is significantly greater than electrically conductive features that are practically formable by conventional metal deposition techniques, such as those used in the conventional formation of electrically conductive features formed directly on a substrate using photolithography. The formation of electrically conductive features 108 with a significantly greater thickness can provide further benefits, for example in the provision of interdigital transducers for SAW devices, where the thickness of the interdigital transducers can be increased compared with conventional devices in order to provide improved operation.Advantageously, the use of an electrically conductive layer 106 formed with a uniform composition (e.g., a Cu-based layer laminate) enables a single etching step, such as the use of a single etch solution. In contrast, multiple electrically conductive layers, for example stacked layers of different types of metals, need to be treated differently, in order to etch throughdifferent materials when forming features in the manner described with reference to Figures 1 A to 1H.In further examples, flexible multilayer substrates with different materials and / or layers with different thickness are used to provide the functionality described herein. For example, flexible multilayer substrates with metal foils and / or flexible insulating layers of different thickness are provided. In an example, the insulating layer 104 is at least partially formed from Kapton (RTM). In further examples, the insulating layer 104 is formed from any suitable material and / or combination of materials.In an example, providing a flexible multilayer substrate comprises forming a magnetostrictive material on at least part of the flexible multilayer substrate. Advantageously, the magnetostrictive material responds to magnetic fields thereby causing a change in the flexible multilayer substrate that can result in a corresponding change at the device substrate 102 when the device 100 has been formed, as described herein. This can be particularly beneficial in combination with a surface acoustic wave device, as described in more detail below.The device substrate 102 used in the formation of the device is any suitable device substrate 102 to provide the desired functionality in combination with one or more electrically conductive features that provide electrical signals to the device substrate 102. In the examples of a surface acoustic wave (SAW) device described herein, the devices substrate is a piezoelectric substrate. In further examples, additionally and / or alternatively, the device substrate comprises semiconductor material, other appropriate solid-state material and / or a single layer material. In an example, the device substrate comprises a single layer material of graphene. Beneficially, as the electrically conductive features 108 described below are formed separately from the device substrate 102, the device substrate undergoes fewer processing steps compared with conventional photolithographical methods to form electrically conductive features directly on a substrate. This facilitates the use of thinner device substrate 102 compared with known systems, such as devices substrates 102 with a thickness of less than 200 microns. Advantageously, the use of thinner device substrates 102 enables inspection of a device 100 through the device substrate 102 itself. For example, in the case of SAW devices, optical inspection of the one or more objects manipulated by surface acoustic waves, such as standing surface acoustic waves (SSAW) through a devices substrate 102 comprising a piezoelectric material is particular advantageous.The formation of a patterned layer is described with reference to Figures IB to IF. At Figure IB, there is shown a cross-sectional view of the flexible multilayer substrate described with reference to Figure 1 A with a photoresist layer 107 formed on the surface of the electrically conductive layer 106. The photoresist layer 107 is formed by laminating a dry photoresist filmto the electrically conductive layer 106 of the flexible multilayer substrate. For example, the photoresist layer 107 is DuPont Riston (RTM) FX900 series photopolymer film. In further examples, the photoresist layer 107 is any suitable dry photoresist film.In further examples, alternatively and / or additionally, a photoresist layer 107 is formed by any appropriate technique, such as deposition by spinning a photoresist material to form a film on the electrically conductive layer 106.Once a photoresist layer 107 has been formed on the electrically conductive layer 106, the photoresist layer 107 is selectively masked with an appropriately formed mask 109, as shown at Figure 1C, which shows a cross-sectional view of the flexible multilayer substrate, photoresist layer 107 and the mask 109. The mask 109 is a photomask that is formed with appropriate apertures that selectively enable transmission of light through the mask 109 thereby to enable photo-patterning of the photoresist layer 107.Once the mask 109 is appropriately aligned and in sufficient proximity to the photoresist layer 107, the mask 109 is illuminated with ultra-violet light, indicated by arrows in Figure ID, in order to expose areas of the photoresist layer 107 that are not selectively masked by the mask 109, as shown in a cross-sectional view at Figure ID.Exposure to ultra-violet light as described with reference to Figure ID causes a change in the properties of the photoresist layer 107 in the regions 111 exposed to the ultra-violet light. The parameter of exposure, such as wavelength, power and duration are determined in order to provide an appropriately patterned photoresist layer 107.Subsequently, as shown in a cross-sectional view at Figure IE, the mask 109 is removed and the photoresist layer 107 is developed and regions exposed to ultra-violet light are removed such that areas of the electrically conductive layer 106 are protected by the remaining photoresist layer 107 that was not exposed to ultra-violet radiation. This leaves unprotected regions 113 associated with the electrically conductive layer 106. The exposed photoresist layer 107 is developed with a developer such as Procirc (RTM) 1120 Aqueous Developer. In further examples, any appropriate alternative and / or additional developer is used. Accordingly, a patterned layer is formed on the surface of the electrically conductive layer 106, thereby to provide protected and unprotected regions of the electrically conductive layer 106.Whilst the exposed and developed regions are removed, in further examples, additionally and / or alternatively, undeveloped regions of the photoresist layer 107 are removed in order to leave a residual patterning of the photoresist layer 107.Once the electrically conductive layer 106 is patterned selectively to protect regions of the electrically conductive layer 106, the electrically conductive layer 106 is etched to provide a photoetched layer 106’, as shown at Figure IF. Figure IF shows a cross-sectional view of aphotoetched layer 106’ where regions 113 that were not protected by the photoresist layer 107 have enabled etching of the electrically conductive layer 106 to remove corresponding regions 115. Etching of the electrically conductive layer 106 in the unprotected regions 113 exposes portions of the underlying insulating layer 104 of the flexible multilayer substrate at the corresponding regions 115. In an example, 40% strength FeCl at a temperature of approximately 50°C is used to etch the Copper of the electrically conductive layer 106. In further examples, any appropriate additional and / or alternative etch solution is used to etch the electrically conductive layer 106.Subsequently, the photoresist layer 107 is removed to provide a flexible multilayer substrate having electrically conductive features 108 on an insulating layer 104, formed by photoetching of the electrically conductive layer 106 of the flexible multilayer substrate. In an example, the residual photoresist layer 107 is removed using a stripping solution such as Procirc (RTM) 1102 Resist Stripper. In further examples, the residual photoresist layer 107 is removed using any appropriate material and technique.Whilst Figures 1 A to 1G illustrate steps for providing electrically conductive features 108 formed on the insulating layer 104 of a flexible multilayer substrate by particular steps of a photoetching process, in further examples additional and / or alternative steps are used to process a flexible multilayer substrate having an insulating layer 104 and an electrically conductive layer 106 formed thereon in order to provide electrically conductive features 108. For example, appropriate masking of the surface of the electrically conductive layer 106 is used to form a patterned layer on the surface of the electrically conductive layer 106 such that selective etching of unprotected regions results in electrically conductive features 108 on the insulating layer 104 of the flexible multilayer substrate.The electrically conductive features 108 are formed with any appropriate geometry for the desired delivery of electrical signals to the device substrate 102. In the examples of a surface acoustic wave (SAW) device described herein, the electrically conductive features 108 are electrodes forming interdigital transducers. In further examples, the electrically conductive features provide electrically connections and / or electrode functionality in accordance with the functionality of the fabricated device 100. Beneficially, the process described with reference to Figures 1 A to 1G enables cleanroom-free photofabrication of photoetched features for the conformal application to an underlying substrate. The cross-sectional view of Figure 1G shows a section of photoetched flexible printed circuit board. It is understood that the flexible multilayer substrate may extend laterally in any appropriate form and dimension and that the distribution of electrically conductive features 108 may provide any appropriate regular and / or irregular distributions on the surface of the insulating layer 104 in accordance with the functionality of thedevice 100 that is to be formed. Whilst the example of Figure 1G shows a cross-sectional view of a photoetched flexible printed circuit board, in further examples the electrically conductive features 108 are formed in appropriate configuration on the insulating layer 104.The electrically conductive features 108 are conformally applied to a target surface of the device substrate 102, as shown in the cross-sectional view of Figure 1H, in order to provide a device 100. Conformal application of the electrically conductive features 108 to the target surface of the device substrate 102 is provided such that electrical signals applied to the one or more electrically conductive features 108 provide functionality in combination with the device substrate 102. The one or more electrically conductive features 108 formed on the insulating layer 104 of the flexible multilayer substrate are held in place with respect to the substrate 102 mechanically, for example by clamping the device substrate 102 to the photoetched flexible multilayer substrate. In further examples, the device substrate 102 and the photoetched flexible multilayer substrate are mechanically clamped together by positioning the photoetched flexible multilayer substrate between the device substrate 102 and at least one further layer.Advantageously, the flexible multilayer substrate with etched electrically conductive features 108 formed on an insulating layer 104 can be seen as a ‘sticker’ that can be applied to a device substrate 102. Such ‘stickers’ are flexible and can be applied to non-planar surfaces, as those described below. Beneficially, such ‘stickers’ can be removed, replaced and / or reused in combination with a device substrate 102. Whilst the ‘stickers’ formed by the flexible multilayer substrate having electrically conductive features 108 formed on an insulating layer 104 are mechanically held in combination with the device substrate 102, in further examples the ‘stickers’ formed by the flexible multilayer substrate having electrically conductive features 108 formed on an insulating layer 104 are attached to a device substrate 102 by any appropriate means. In further examples, the ‘stickers’ formed by the flexible multilayer substrate having electrically conductive features 108 formed on an insulating layer 104 are reversibly bonded to the device substrate 102. In further examples, the ‘stickers’ formed by the flexible multilayer substrate having electrically conductive features 108 formed on an insulating layer 104 are permanently positioned with respect to the device substrate 102. In yet further examples, ‘stickers’ formed by the flexible multilayer substrate having electrically conductive features 108 formed on an insulating layer 104 are held in position with respect to the device substrate 102 using one or more materials, such as adhesive material. Such ‘stickers’ formed by the photoetching of an electrically conductive layer 106 of the flexible multilayer substrate to provide electrically conductive features 108 formed on an insulating layer 104 are configured to provide electrical conductivity in accordance with their intended application. In an example, a‘sticker’ is a photoetched flexible printed circuit board that is conformally applied to a device substrate 102.Whilst the steps of fabricating the device 100 described with reference to Figures 1 A to 1H are shown in a particular order, in further examples the steps are provided in a different order and / or concurrently in order to provide a device 100. In further examples, alternative and / or additional steps are used to form the device 100 through the etching of an electrically conductive layer 106 of a flexible multilayer substrate that is combined with a device substrate 102 to form the device 100.Beneficially, devices formed in the manner described with reference to Figures 1 A to 1H, for example, devices formed by bringing together a device substrate and a flexible photoetched circuit board, are easily disassembled. For example, once the device 100 described with reference to Figure 1H has been formed, it can be used to perform its associated functionality. For example, where the device 100 is a SAW device, such as one of the SAW devices described with reference to Figures 2 to 5, the device can be use to detect and / or characterise biological material, such as biological cells, microbial lifeforms, nanobiomaterials and / or biomolecules. In such cases, the use of the device 100, 200, 400, 512 for such purposes can result in contamination of the electrodes and / or substrate. Conventionally formed devices with electrically conductive features, such as electrodes, formed on a substrate using photolithography cannot be easily or reliably cleaned in order to perform further detections and / or characterisation, due to such residual contamination. However, the method described with reference to Figures 1 A to 1H provides an elegant solution for the disassembly and reassembly, or replacement, of the electrically conductive features 108. Accordingly, once the device 100 has performed its functionality, the device substrate 102 is separated from the one or more electrically conductive features 108 formed on the insulating layer 104 of the flexible multilayer substrate. The device substrate 102 and / or the flexible multilayer substrate are then cleaned. Subsequently, the electrically conductive features 108 formed on the insulating layer 104 of the flexible multilayer substrate are conformally re-applied to a target surface of the device substrate 102, thereby enabling further processes to be performed by the device 100 in the absence of contamination.Alternatively and / or additionally, once separated, the device substrate 102 is cleaned and a different flexible multilayer substrate that has been etched in accordance with the method described with reference to Figures 1 A to 1H is used to conformally apply one or more further electrically conductive features formed on an insulating layer of a flexible multilayer substrate to the device substrate 102. Advantageously, such reuse of one or more components of the device 100 decreases wastage of resources and enables lower cost and / or more easily replaceablecomponents of the device 100 to be replaced, whilst reusing more expensive and / or more difficult to replace components of the device 100.Optionally, the process of cleaning the device 100 comprises sterilising the device substrate 102 and / or the photoetched flexible multilayer substrate. In an example, the device substrate 102 and / or the photoetched flexible multilayer substrate are sterilised with Virkon (RTM). In further examples, additionally or alternatively, different sterilisation agents are used.Advantageously, sterilisation is used to manage potential biohazards. For example, if the device is a device 100, 200, 400, 512 used to detect and / or characterise biological material, such as biological cells, microbial lifeforms, nanobiomaterials and / or biomolecules, and such materials are potentially biohazards, cleaning and sterilisation of the device as part of a disassembly and reassembly / replacement process provides effective recycling of components.Beneficially, devices formed in the manner described with reference to Figures 1 A to 1H enable improved fabrication through the integration of one or more components or layers during the formation of the devices. In an example, at least part of a cooling device is formed on the opposite side of the flexible multilayer substrate to the electrically conductive layer. Beneficially, the electrically insulating layer 104 of the flexible multilayer substrate is thermally conductive. Advantageously, cooling is provided in a region in close proximity to the device substrate 102 without directly cooling the device substrate 102 itself. In an example, the cooling device is a thermoelectric cooling device. In further examples, alternative and / or additional cooling devices are used to provide the functionality. Further, beneficially, cooling is provided in close proximity to electrically conductive features 108 formed by etching the electrically conductive layer 106. Where the thickness of the electrically conductive features 108 corresponding to the thickness of the electrically conductive layer 106 enables thicker features to be formed compared with conventional techniques, such as thicker interdigital transducers, the cooling may enable improved operation at higher powers enabled through the use of thicker interdigital transducers, for example.Figures 1 A to 1H are described with a general process for device fabrication through the combination a photoetched flexible multilayer providing electrically conductive functionality and an underlying device substrate. The process for device fabrication, including the optional cleaning and subsequently reassembly and / or replacement of device components can be carried out by a system comprising one or more processing units that that are at least partially automated and / or coordinated using a computing network of one or more computing devices.Figure 2 shows an exploded perspective view of a surface acoustic wave (SAW) device 200 that can be formed in accordance with the process described with reference to Figures 1 A to 1H.At Figure 2 there is shown a device substrate 202 that is a piezoelectric substrate and a flexible multilayer substrate formed from an insulating layer 204 and a photoetched layer 206’. The photoetched layer 206’ is provided by following the steps described with reference to Figures 1 A to 1H such that an electrically conductive layer 106 formed on an insulating layer 104 is etched to provide electrically conductive features 208. In the case of the SAW device 200 of Figure 2, the electrically conductive features 208 are electrodes forming interdigital transducers. In the example of Figure 2, there are two sets of interdigital transducers 208. In further examples, there may be any number of sets of interdigital transducers 208 arranged to provided different functionality.In between the interdigital transducers 208 there is a region 210 that enables optical access to the surface of the piezoelectric substrate 202 across which surface acoustic waves are propagated and detected. In further examples, region 210 is not provided, for example, where optical and / or other access is not needed.In an example, the piezoelectric substrate 202 is a lithium niobate substrate. In further examples, the piezoelectric substrate 202 alternatively or additionally comprises lithium tantalite, lanthanum gallium silicate, quartz, cellulose and / or piezoelectric polymer-based materials.The interdigital transducers 208 are conformally applied to a target surface of the piezoelectric substrate 202 and are clamped together. The piezoelectric substrate 202 and the flexible multilayer substrate are mechanically clamped together. In order to mechanically clamp the piezoelectric substrate 202 and the flexible multilayer substrate together, the piezoelectric substrate 202 is located in a locating portion 218 of a first portion 216 of a stage. In an example, the locating portion 218 enables access, such as optical access, through the first portion 216 of the stage in order to inspect the piezoelectric device substrate 202. The method described with reference to Figures 1 A to 1H facilitates the use of a thin piezoelectric substrate 202 that enables optical inspection, for example when one or more objects, such as biological objects, are manipulated by surface acoustic waves, such as standing acoustic surface waves, on the opposite side of the piezoelectric substrate 202. In further examples, optical inspection is achieved through apertures on the other side of the piezoelectric substrate 202 to the first portion 218 of the stage. The flexible multilayer substrate is arranged in order conformally to apply the interdigital transducers 208 to the target surface of the piezoelectric substrate 202. A further layer 212, acting as a cushion layer that can be used to redistribute pressure within the device 200, is optionally placed on top of the flexible multilayer substrate. The further layer 212 has an aperture 214 that at least partially corresponds to the region 210 of the flexible multilayer substrate that enables optical access to the surface of the piezoelectric substrate 202 between the interdigital transducers 208.A second portion 222 of a stage is placed on the optional further layer 212. The second portion 222 has an aperture 224 enabling optical access to the surface of the piezoelectric substrate 202. Once the first portion 216 and the second portion 222 of the stage have been suitable arranged with the piezoelectric substrate 202, flexible multilayer substrate with interdigital transducers 208 and the optional further layer 212 between them, the device 200 is clamped together. In the example of Figure 2, the first portion 216 of the stage is arranged with optional members 220 extending outwards, such that the holders 226, 228 can be arranged to surround the first portion 216 and the second portion 222 of the stage in order to mechanically fix the intervening portions together. Whilst the device 200 of Figure 2 is mechanically clamped together, in further examples SAW devices are provided through the appropriate combination and relative positioning of interdigital transducers 208 formed on a flexible multilayer substrate with an underlying piezoelectric substrate.Perspective views of the assembled SAW device are shown at Figures 3A and 3B.Figure 3A shows a perspective view 300A of the device 200 in which the electrodes formed on the flexible multilayer substrate have been conformally applied to the piezoelectric substrate 202. The arrangement of the components of the device 200 is such that optical access to the surface of the piezoelectric substrate between the interdigital transducers 208 is provided. The region 210 of the flexible multilayer substrate is shown at Figure 3 A. The region 210 is a transparent portion of the insulating layer 204 of the flexible multilayer substrate. In further examples, the region 210 of flexible multilayer substrate is an aperture through the flexible multilayer substrate. Figure 3B shows a different perspective view 300B of the assembled SAW device 200.Whilst a particular arrangement of components is provided in relation to the SAW device 200 of Figures 2 to 3B, in further examples, SAW devices are provided in accordance with the process described with reference to Figures 1 A to 1H with any compatible arrangement of components.SAW devices fabricated in accordance with the method described with reference to Figures 1 to 3 can be configured for use in a variety of advantageous configurations that exploit the efficient, simple and versatile fabrication method. For example, the combination of a flexible multilayer substrate with a photoetched layer to provide interdigital transducers in combination with a piezoelectric substrate can be used to generate surface acoustic waves.Advantageously, it has been found that cleanroom-free fabrication of SAW devices by the conformal application of photoetched flexible printed circuit boards to a piezoelectric substrate, in accordance with the method described with reference to Figures 1 to 3, provide more efficient devices with a reduced loss factor compared to conventionally photolithography-based devices. Beneficially, the use of a flexible multilayer substrate having an insulating layer and an electrically conductive layer formed thereon, in mechanical combination with a piezoelectric substrate does not require the high temperature baking steps used with photolithography to provide electrodes on a piezoelectric substrate in a conventional manner. The absence of such baking means that the components of the SAW device, such as the piezoelectric substrate, are not exposed to temperatures that may otherwise have a deleterious effect on the performance of the SAW device. Further, the use of laminated insulating and electrically conductive layers enables the formation of electrodes in an efficient way that provides SAW devices with improved functionality.In an example, such SAW devices are used as sensors based on the combination of a flexible multilayer substrate with a photoetched layer to provide interdigital transducers in combination with a piezoelectric substrate to generate surface acoustic waves. Advantageously, SAW devices using 18 micron thick photo-etched interdigital transducers mechanically clamped in combination with piezoelectric substrates comprising lithium niobate have been shown to already have a loss of factor of as little as approximately 1 dB, thereby showing a considerable improvement compared to conventional techniques.Such SAW sensors are used for a variety of applications. In an example, such a SAW sensor is used to detect chemical vapours based on the use of surface acoustic waves to both identify the quantity of atoms / molecules attached on the piezoelectric substrate by comparing an input signal with an output signal, but also to identify the chemical itself based on how the surface acoustic waveforms are perturbed.In an example, such SAW devices are configured as bio-sensors and used to detect and / or characterise biological cells, microbial lifeforms, nanobiomaterials and / or biomolecules. Beneficially, such devices are usable to determine biohazards.In further examples, such SAW devices are configured as gravity sensors and used to determine a measure of gravity and / or characterise gravity.Gravity sensors are of significant interest, with applications ranging from the geological sciences to defence industry. Accurate and rapid measurements of variations in microgravity unlocks the power to detect the previously unknown (e.g. subterranean structures, which can be used e.g. in underwater navigation, or to traverse more safely in challenging environments). SAW has been previously used to detect changes in gravity (see e.g. Shevchenko et al. Sensors 18:7

[2018] 2301), and the fabrication of SAW-based gravity sensing devices in accordance with the process described herein not only makes such a SAW gravity sensor more sensitive by providing a higher power propagation and lower loss factor, but the intrinsic adaptability of the flexible electrodes enables the conformal application of photoetched electrically conductivefeatures of a flexible multilayer substrate to a target surface of a substrate formed with a shape having various complex (i.e. non-flat) geometries, as described below, with reference to Figure 4. Such fabrication of non-flat gravity sensing SAW devices is not possible with conventional techniques traditional photolithography, nevertheless a route to providing superior results for the purposes of gravity detection is enabled though the conformal application of flexible photoetched interdigital transducers to a non-flat target surface of a piezoelectric substrate.In further examples, such SAW devices are configured as torque, temperature, pressure and / or strain sensors, wherein the devices are configured to determine a measure of at least one of torque, temperature, pressure and strain based on a change in the path length of surface acoustic waves travelling along the piezoelectric device substrate.In further examples, such SAW devices are configured as SAW filters and convert an electrical input signal into a surface acoustic wave and subsequently recombine the surface acoustic wave into a further electrical signal, thereby mechanically filtering the electrical input signal.SAW filters are one of the most common applications of SAW technology, implemented in modern smartphones, and more, for example. One of the key features of SAW filters is the loss factor, which ideally should be as low as possible (or even zero for an ideal filter), and for conventionally made SAW filters this peaks at c. 2.5dB (see e.g. ApiTech White Paper - Introduction to SAW Filter Theory & Design Techniques, 855.294.3800

[2018] ). However, the fabrication of photoetched electrodes from a flexible multilayer substrate enables copper electrodes to be provided on a flexible insulating layer that, when implemented as interdigital transducers in combination with a piezoelectric substrate of lithium niobate have already been shown to produce a loss factor of as little as IdB. Advantageously, processing flexible multilayer substrates to provide photoetched electrodes that are conformally applied to a piezoelectric substrate means that wafers used to form the resultant SAW device (e.g., lithium niobate wafers) do not have to undergo any baking at high temperatures, which is necessary during multiple steps in conventional photolithography and which is a significant source of wafer breakages.An improved loss factor results in superior SAW filters and enables the production of smaller SAW devices to provide equivalent functionality. This in turn means that the SAW devices can be reduced in size and use less material. Further, a reduced loss factor means that alternative piezoelectric substrates may be used that would otherwise be considered unsuitable. For example, quartz may be used instead of lithium niobate. Alternatively, the size of the SAW filter is maintained with dimensions comparable to conventional SAW device, with improved performance.Further, the separate provision of photoetched electrodes and piezoelectric substrates described herein does not required the complex apparatus and processing steps used in the use of conventional photolithography to provide electrodes directly on a piezoelectric substrate and therefore provides significant savings with respect to time, cost and material, facilitating the advance of the production and research of devices formed in such a way.In further examples, such SAW devices are configured to operate as SAW oscillators and / or transformers. These devices share commonality with SAW filters and have analogous benefits.In further examples, such SAW devices are configured to generate standing surface acoustic waves (SSAWs) and use the standing surface acoustic waves to control the location of one or more biological and / or non-biological objects in one dimension and / or two dimensions on the piezoelectric device substrate. In examples, SSAWs are created using the cross-application of pairs of interdigital transducers applied to a piezoelectric substrate to create a control surface on the piezoelectric substrate in between the pairs of interdigital transducers.Biological sciences usually rely upon optical microscopy to investigate their samples (e.g. confocal, TIRFM), which in turn normally require the substrates of samples to have a maximum thickness of c. 200pm. To make a SAW device on a 200pm substrate via conventional photolithography requires complicated processes in order to address a very high attrition rate during manufacture. However, advantageously, the conformal application of a photoetched electrodes of a flexible multilayer substrate to a piezoelectric substrate means that finished electrodes will simply be put onto the substrates in the final step, without requiring the ultra- fragile 200pm wafers to undergo any form of processing before being used during microscopy (and related investigations). SAW (and in particular standing surface acoustic wave (SSAW) patterning) has the potential of revolutionising the kind of microscopy used in biological sciences, as it allows for the regular patterning and separation of cells into grids of one cell by one cell, without harming individual cells, and without utilising any chemicals onto the substrate to achieve this (see e.g. micro-patterned p-Slides™ by Ibidi, or custom made ones using fibronectin), which may affect the results of the experiment from interactions between said chemicals and the cells. SSAW can be activated during seeding, and then deactivated during microscopy, to further minimise the effects on the experimental results.In an example, devices formed in accordance with the process described herein enable the production of SAW devices configured to generate SSAW. Advantageously, the piezoelectric substrate can be formed such that it is thin enough to be used in optical microscopy. In further examples, such devices are adapted for separating and / or sorting biological cells.In further examples, such SAW devices are configured as humidity, viscosity and or magnetic field sensors and determine a measure of at least one of humidity, viscosity and magnetic field data.Advantageously, where the SAW device is configured to determine a measure of humidity at least a part of a thermoelectric cooling device is optionally integrated to the opposite layer of the flexible multilayer substrate to the electrically conductive features. When the SAW device is coupled with a cooling device, such as a thermoelectric cooler, this facilitates the condensation of water vapour to the surface of the piezoelectric substrate of the SAW device, which is subsequently measured. Beneficially, in addition to advantages in respect of cost, ease of production, and higher sensitivity, the characteristics of the flexible photoetched circuit used to provide the SAW device mean that it can be put directly in close thermal contact with the thermoelectric cooler thanks to its electrically insulating layer, to further improve SAW humidity sensors.Beneficially, SAW devices formed in accordance with the process described herein, in an example, are configured to provide magnetic field sensing functionality. For example, it is known that if a film of a magnetostrictive material is deposited in the delay line of a SAW sensor, then dimensional changes of said film from an external magnetic field will stress the underlying substrate, and can subsequently be measured by the SAW device. The SAW device can then be used as a magnetic sensor. SAW based magnetic sensing devices formed in accordance with the process described herein, in addition to benefitting from advantages of cost, ease of production, and higher sensitivity, enable improved integration of magnetic field data detection, for example through the direct incorporation of a magnetostrictive material, such as a magnetostrictive film into a flexible multilayer substrate that is photoetched as described herein.Beneficially, SAW devices formed in accordance with the process described herein, in an example, are configured to provide viscosity sensing functionality. Viscosity is another property that can be measured using SAW, as the resonant frequency of a SAW system submerged in a viscous fluid will ever so slightly change (which can be measured using a spectrum analyser or similar). For the specific application of viscosity sensors, the conformal application of photoetched electrodes to a piezoelectric substrate and temporary clamping enables removal of the electrodes between experiments. Advantageously, the piezoelectric substrate can be cleaned, and a new, clean, set of photoetched electrodes can thereafter be used, for enhanced reusability and reliability. SAW devices formed conventionally through the use of photolithography to form electrodes directly on the substrate are susceptible to contamination from a residue of the previous test(s) affecting the outcome of the current one and / or damage due to difficulty incleaning the deposited electrodes, which will then also affect the results of the current test. The method of forming a SAW device herein addresses these difficulties.Whilst the devices 100, 200 described with reference to Figures 1 to 3 show planar device substrates 102, 202, advantageously, the process for producing such devices is particularly suitable for application to non-planar device substrates.Figure 4 shows a perspective view of a flexible multilayer substrate comprising an insulating layer 404 and one or more electrically conductive features 408 formed on the insulating layer 404 of the flexible multilayer substrate by the selective photoetching of an electrically conductive layer 406 formed on the insulating layer 404, as described with reference to Figures 1 to 3. Figure 4 also shows a perspective view of a device 400 that has a non-planar substrate 402 in combination with the flexible multilayer substrate. The device 400 has a non- planar substrate 402 that is shown with a curved surface that provides a target surface for the conformal application of the one or more electrically conductive features 408 formed on the insulating layer 404 of a flexible multilayer substrate by the selective photoetching of an electrically conductive layer 406 formed on the insulating layer 404, as described with reference to Figures 1 to 3. In the example of Figure 4, the one or more electrically conductive features 408 are shown to have been applied on the upper side of the curved surface of the non-planar substrate 402. In further examples, the electrically conductive features 408 formed on the insulating layer 404 of the flexible multilayer substrate are additionally or alternatively conformally applied to a target surface of the underside of the non-planar substrate 402. In further examples, the electrically conductive features 408 formed on the insulating layer 404 of the flexible multilayer substrate are additionally or alternatively conformally applied to a target surface of the device substrate 402 in any appropriate configuration.In an example, the non-planar substrate 402 is formed through the appropriate processing of a substrate, such as lithium niobate. In an example, a non-planar target surface of a substrate 402 is provided by etching a bulk material. In further examples, the non-planar target surface of a substrate 402 is provided through the use and appropriate configuration of a flexible substrate 402. In further examples, non-planar substrates 402 are formed and / or grown with non-planar target surfaces. Whilst in an example the substrate 402 comprises lithium niobate, in further examples, different piezoelectric materials are formed with non-planar geometries to form the substrate 402. In an example, the non-planar substrate 402 is formed from quartz. In further examples, the non-planar substrate 402 comprises lithium tantalite, lanthanum gallium silicate, cellulose and / or piezoelectric polymer-based material. In further examples, the non-planar substrate 402 is formed from any appropriate material, such as semiconductor material and / orother solid-state material. In further examples, the substrate 402 comprises a single layer material such as graphene.Whilst the device 400 shown at Figure 4 is formed by the application of one or more electrically conductive features 408 formed on the insulating layer 404 of the flexible multilayer substrate to a target surface of the substrate 402 that is curved, in further examples, the device 400 is formed with a target surface that has any appropriate non-planar form. Whilst a target surface of the substrate 402 is provided in the form of a curved surface, in further examples two or more intersecting planes are used to provide a non-planar target surface of a device substrate 402. Beneficially, flexible multilayer substrates forming photoetched electrodes are well adapted for conformal application to such non-single plane, non-flat, geometries.Whilst the method described with reference to Figures 1 A to 1H facilitates the formation of any devices 400 where the target surface of the device substrate 402 is non-planar, the process advantageously also facilitates the formation of particular non-planar devices, such as surface acoustic wave devices, that are not possible to fabricate using conventional photolithographybased techniques.For example, where a non-planar piezoelectric device substrate 402 is provided, photoetched interdigital transducers 408 formed on a flexible multilayer substrate are conformally applied to provide non-planar SAW devices 400. Such non-planar SAW devices 400 can be configured for improved operation compared with known, planar, SAW devices. In an example, a SAW device 400 with a non-planar piezoelectric device substrate 402 to which interdigital transducers 408 formed by photoetching the electrically conductive layer 406 of a flexible multilayer substrate are conformally applied is configured as a gravity sensor. In further examples, the conformal application of photoetched electrodes to a non-flat target surface of a piezoelectric device substrate 402 facilitates improved SAW devices configured to provide sensing functionality, e.g., in relation to torque, rotation and temperature. In further examples, SAW devices formed in accordance with the method described herein are provided with non-flat surfaces in order to provide improved functionality and improved fabrication at least as described in accordance with the examples set out herein.Figure 5 shows an exemplary system in which the SAW devices 200, 400 described with reference to Figures 2 to 4 can be integrated. There is shown a SAW device 512 that receives an input signal from a signal / function generator 502. Optionally, signals are sent to the SAW device 512 by a power amplifier 504 and / or power meter 506 to the SAW device 512. Output signals from the SAW device 512 are measured by an oscilloscope 508, thereby to provide SAW sensing capability and / or are sent to one or more further instruments 510 for characterisation and / or processing of output signals. Optionally, a microscope 514 is used for optical inspectionof the SAW device 512 during operation, for example, through the piezoelectric substrate 202 and / or regions 210, 214, 224 of the components of the SAW device 200 of Figure 2.Whilst the formation of devices from the combination of device substrates with flexible multilayer substrates that have been etched to provide electrically conductive features on an insulating layer have been described in particular arrangements and with particular materials, it will be understood that different arrangements and / or materials can be used to provide devices formed by the combination of device substrates with flexible multilayer substrates that have been etched to provide electrically conductive features on an insulating layer.The method described with reference to Figures 1 to 4 may provide a SAW device having the characteristics described below. The method described with reference to Figures 1 to 4 is particularly well adapted to provide such a SAW device. However, the SAW device described below is not limited to being fabricated using the method described with reference to Figures 1 to 4.As exemplified in Figure 1, a SAW device may be provided that comprises a piezoelectric device substrate 102, an insulating layer 104 and one or more electrically conductive features. The one or more electrically conductive features are positioned between the piezoelectric device substrate 102 and the insulating layer 104. The electrically conductive features may, for example, directly contact either or both of the piezoelectric device substrate 102 and the insulating layer 104. The electrically conductive features may be provided in a layer. The layer may be the only layer between the piezoelectric device substrate 102 and the insulating layer 104 or the layer may be one of a plurality of layers (not shown) between the piezoelectric device substrate 102 and the insulating layer 104. The electrically conductive features may, for example, be formed by patterning an electrically conductive layer 106, as described for example with reference to Figures 1 A-H. The electrically conductive layer 106 may have a substantially uniform thickness. The electrically conductive layer 106 may have a very smooth surface, desirably significantly smoother than can be achieved using typical electroplating processes for example, allowing optimal and maximum contact with the piezoelectric device substrate 102. Desirably, the electrically conductive layer 106 has at least one surface (optionally two surfaces) having a smoothness such that all features on the surface (or surfaces) that cause deviation from a perfectly smooth surface deviate from the perfectly smooth surface by less than about 1 micron, optionally by less than about 500nm, optionally by less than about 50nm, optionally by less than about lOnm, optionally by less than about 5nm, optionally by less than about 2nm.The piezoelectric device substrate 102 may take any of the forms described above with reference to a device substrate, piezoelectric substrate or piezoelectric device substrate, includingcomprising, consisting essentially of, or consisting of, one or more of lithium niobate, lithium tantalite, lanthanum gallium silicate, quartz, cellulose and piezo-electric polymer-based material.The insulating layer 104 and the electrically conductive layer 106 may take any of the forms described above with reference to Figures 1-4.The one or more electrically conductive features define interdigital transducers. The interdigital transducers are configured to generate and / or detect (i.e., to generate, to detect, or to detect and to generate) surface acoustic waves in the piezoelectric device substrate 102. Example configurations of interdigital transducers are shown in Figures 2 and 4 and described above.The dimensions and / or composition of the insulating layer 104 and / or the geometry of the interdigital transducers may be configured to substantially avoid coupling of surface acoustic waves between the piezoelectric device substrate 102 and the insulating layer 104 and / or to substantially avoid attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer. Avoiding such coupling and / or attenuation prevents surface acoustic waves generated in the piezoelectric device substrate 102 from propagating significantly from the piezoelectric device substrate 102 to the insulating layer 104. Avoiding such coupling and / or attenuation reduces undesirable power loss and / or loss of measurement sensitivity.In some examples, the dimensions and / or composition of the insulating layer 104 and / or the geometry of the interdigital transducers is / are configured to space apart the piezoelectric device substrate 102 from the insulating layer 104. The spacing apart may be such as to avoid direct contact between the piezoelectric device substrate 102 and the insulating layer 104 (i.e., such that a minimum distance between material forming the piezoelectric device substrate 102 (e.g., piezoelectric material) and the insulating layer 102 is greater than zero) or such as to avoid near contact. Such spacing apart of the piezoelectric device substrate 102 from the insulating layer 104 reduces or substantially avoids coupling of surface acoustic waves between the piezoelectric device substrate 102 and the insulating layer 104 and / or substantially avoids attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer.In some examples, the interdigital transducers have a thickness perpendicular to the piezoelectric device substrate 102 of at least 5 microns, optionally at least 10 microns, optionally at least 15 microns, optionally at least 20 microns. The electrically conductive features defining the interdigital transducers may, for example, be formed by patterning an electrically conductive layer 106 of a substantially uniform thickness of at least 5 microns, optionally at least 10 microns, optionally at least 15 microns, optionally at least 20 microns. In one example implementation, the interdigital transducers have a thickness of about 18 microns. Such thicknesses are larger than those used in conventional SAW devices. Providing relatively thickinterdigital transducers may contribute to reducing or substantially avoiding coupling of surface acoustic waves between the piezoelectric device substrate 102 and the insulating layer 104. The coupling of surface acoustic waves between the piezoelectric device substrate 102 and the insulating layer 104 (and / or attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer) may alternatively or additionally be reduced or substantially avoided by positioning the individual electrodes of the interdigital transducers relatively close together. The coupling of surface acoustic waves between the piezoelectric device substrate 102 and the insulating layer 104 (and / or attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer) may alternatively or additionally be reduced or substantially avoided by configuring the insulating layer 104 to be relatively or completely rigid, which may be done by suitable selection of the material forming the insulating layer 104 or the thickness of the insulating layer 104. If it is desired to have a relatively flexible insulating layer 104, this can be compensated by arranging for the interdigital transducers to be thicker (in the out-of-plane direction, perpendicular to the piezoelectric device substrate 102) and / or by reducing the gaps between the individual electrodes of the interdigital transducers. In one example class of implementation, the insulating layer 104 is formed from Kapton (or a material having similar mechanical properties) and has a thickness in the range of about 10 microns to about 50 microns, optionally about 20 microns to about 40 microns, optionally about 20 microns to about 30 microns, such as about 25 microns. In such an implementation or other implementations, the interdigital transducers may have a thickness in the range of about 5 microns to about 30 microns, optionally about 10 microns to about 25 microns, optionally about 15 microns to 20 microns, such as about 18 microns. In such an implementation or other implementations, the distance between electrodes (referred to as “A” in the DEMONSTRATIONS OF PERFORMANCE section below) may be in the range of about 20 microns to about 200 microns, optionally about 30 microns to about 100 microns, optionally about 50 microns to about 60 microns, such as around 50 microns or 60 microns. Significantly thicker interdigital transducers and / or smaller electrode separations could be used, for example where the insulating layer 104 is configured to be more flexible than the Kapton example given above. Thinner interdigital transducers (including, optionally, at sub-micron thickness) and / or larger electrode separations could be used where the insulating layer 104 is more rigid. Optionally, in some examples, if the insulating layer 104 is sufficiently rigid to prevent significant coupling and / or attenuation of surface acoustic waves between the piezoelectric device substrate 102 and the insulating layer 104 even at electrode thickness in nanometre, optionally tens of nanometre, optionally hundreds of nanometre, then such a configuration may be assembled using conventional electron beam lithography. For example, the interdigital transducers may be configured to have a thicknessperpendicular to the piezoelectric substrate and / or a width parallel to the piezoelectric substrate and / or a separation parallel to the piezoelectric substrate that is less than a micron, optionally less than about 500nm, optionally less than about 50nm, optionally less than about lOnm, optionally less than about 5nm, optionally less than about 2nm. The interdigital transducers may be formed by electron beam lithography.As described in the DEMONSTRATIONS OF PERFORMANCE section below, SAW devices according to embodiments of the present disclosure can have such high performance that they can be applied effectively to a range of passive sensing applications not previously considered available for SAW devices. Non-limiting examples of such applications are described below.A method of using a SAW device according to the present disclosure is provided, in which the device substrate is a piezoelectric substrate 102 (also referred to as a piezoelectric device substrate) and the one or more electrically conductive features are interdigital transducers, the method comprising using the interdigital transducers to passively measure surface acoustic waves generated in the piezoelectric substrate 102 by entities in a sample adjacent to the piezoelectric substrate 102 (i.e., close enough to the piezoelectric substrate 102 to impart a signal to the piezoelectric substrate 102, optionally in direct contact with the piezoelectric substrate 102). The passive measurement is performed without actively generating surface acoustic waves via the interdigital transducers. The entities may comprise one or more of the following: living cells; living tissue; a living organ; an organism; a pathogen; and a nanoparticle. The method may comprise monitoring a response of the entities to external stimuli, such as a drug, optionally in real time. The approach is demonstrated below in the context of measuring neuronal cells and in particular in monitoring responses of neuronal cells to drugs, which can optionally be done in real time. The method can thus be used more generally to monitor cellular responses to drugs, optionally in real time. Alternatively or additionally, the method may be used to evaluate antimicrobial resistance (AMR) in microbes, such as bacteria.The entities may provide a measurable signal by vibrating or by being otherwise mechanically active. Alternatively or additionally, the entities may emit electrical, electromagnetic, electromechanical and / or electrochemical signals that are such as to cause a reaction in the piezoelectric substrate that leads to a measurable signal in the interdigital transducers.The entities may comprise one or more of the following: living cells, such as eukaryotic neurons or prokaryotic bacteria, living tissue and organs, such as cancerous tissue or brains, whole organisms, such as plants or protozoa, pathogens, such as viruses, bacteria, fungi orprions, nanoparticles, both organic and non-organic. The entities may comprise a wave, such as an acoustic or other mechanical wave, or an electrical or electromagnetic wave.The method may comprise evaluating the viability, general state, or a specific state of living cells, whether eukaryotic or prokaryotic, living tissue, both in vivo and in vitro, as well as whole organisms.DEMONSTRATIONS OF PERFORMANCEPower Comparison TestTests were performed to evaluate the power propagation (or loss factor) of a device (“PEFAG device”) made according to the method of the present disclosure, by comparing it to a conventional SAW device with the same number of IDT electrodes (N = 150), and close to identical electrode configuration (with the width and distance between electrodes, A = 50pm or 60pm). All tests used SAW-grade LiNbCE wafers.The experimental setup was as follow: a signal generator (SG), operating at the theoretically predicted resonant frequency (fp) for the PEFAG device, and at the real resonant frequency (fr, acquired through a frequency sweep) for the conventional device, and always at lOVpp, was connected directly to an oscilloscope (OSC), channel 1. Then, the same SG was connected to one end of the SAW device (whether PEFAG or conventional), without any power amplifiers, and the other end of the SAW device was connected to OSC channel 2 (using conventional grounding for SAW devices). The distance between generating and receiving sets of interdigital transducers on both devices was c. 10mm, without containing any sample or other external sources of attenuation. The ‘input’ of OSC channel 1 (10 Vpp for both devices) was then compared to the ‘output’ of OSC channel 2, and the ratio was deemed as the power propagation of the device in question.Figure 6 is a graph showing the measured power propagation of the conventional SAW device. Curve 601 corresponds to OSC channel 1. Curve 602 corresponds to OSC channel 2. The vertical axis is voltage and the horizontal axis is time. The scales are the same for both OSC channels. No flaws in the electrodes or other issues post-manufacture were detected. The power propagation of this device is in line with published literature.Figure 7 is a graph showing the measured power propagation of the corresponding PEFAG device. Curve 701 corresponds to OSC channel 1. Curve 602 corresponds to OSC channel 2. The vertical axis is voltage and the horizontal axis is time. The scales are the same for both OSC channels. The smaller difference in amplitude between OSC channel 1 and OSC channel 2 in Figure 7 compared to Figure 6 demonstrates that the this PEFAG device (Figure 7) is superior to that of the conventional SAW device (Figure 6).Power Propagation Test (Quartz)The tests described above in the sub-section “Power Comparison Test” were repeated using a variation on the PEFAG device discussed above in which an optical grade quartz wafer is used instead of LiNbCh. No power amplifier was used. The results are shown in Figure 8, where curve 801 represents OSC channel 1 (at lOVpp and 10V OSC scale) and curve 802 represents OSC channel 2 (at IV OSC scale, i.e. increased tenfold compared to 10V OSC scale).Although the quartz performed significantly worse than the LiNbOs (as expected), even without a power amplifier the power propagation / loss factor was demonstrated to be sufficiently good for usage in numerous SAW applications. This demonstrates that devices of the present disclosure generate sufficiently more power than a conventional SAW device that even quartz can be used in SAW applications at low (non-hazardous) voltages, and without using a power amplifier.Passive Listening Amitriptyline - Monitoring Cellular Response to Drugs in Real Time (Neurons Treated with Amitriptyline)Figures 9 and 10 depict the results of experiments to demonstrate use of devices of the present disclosure in the context of monitoring cellular response to Amitriptyline.In one experiment, Fl 1 neuronal cells were provided adjacent to a PEFAG device. SG was connected to the device, generating sinusoidal waves at fp and lOVpp. The other side of the device was connected to OSC. Results were extracted and subjected to a Fourier transform. The results of this “active” case are shown in Figure 9.In addition, measurements were also taken without SG generating any input signal (i.e. no SAW actively generated by electrodes of the device), which constitutes ‘passive listening’, where the only signals detected were generated by the sample itself (i.e. Fl 1 cells). The results of this “passive” case are shown in Figure 10.Measurements (both active and passive) were taken of cells untreated (i.e. control), and then treated with 3pM Amitriptyline, which according to previously published literature should be able to block neuronal Na+ channels (which play a central role in the firing of the action potential) without killing the cells. Less than 5min elapsed between adding the drug and resuming measurements of treated cells, with measurements being taken both actively and passively. Curves 1001 and 1002 in Figure 10 correspond respectively to measurements of the untreated and treated cells (in the passive mode). Curves 901 and 902 in Figure 9 correspond respectively to measurements of the untreated and treated cells (in the active mode).When SAW was being actively generated by the electrodes, then a clear and measurable difference between untreated and Amitriptyline treated Fl 1 could be seen in all measurements (note that the biggest peak is at the frequency of the input signal, and the remaining biggest / sharpest peaks present both before and after are at different harmonics).When SAW was not actively being generated by the electrodes (SG disconnected), and the device merely ‘listened’ to the neurons, then there was also a consistent (albeit less significant) difference between untreated and Amitriptyline treated Fl 1. Several peaks in the frequency spectrum (labelled 1003 in Figure 10) consistently decreased, and the peak at c. 50MHz (labelled 1004), although small, consistently disappeared post treatment in all data sets.Note that the peak at c. 30MHz (labelled 903 in Figure 9) disappeared in post-treatment active SAW (presumed to be suppressed by the combination of drug and active SAW), but is present in both before and after during passive listening. This also indicated that passive listening is less intrusive, and potentially more reliable in measuring certain things, than active (i.e. conventional) SAW. This experiment demonstrated that ‘passive listening’, enabled by the much higher power output of PEFAG devices compared to conventional SAW devices, does not only work, but can achieve experimental feats not currently available to conventional SAW technology.Passive Listening Paclitaxel (Spectrum Analyser) - Monitoring Cellular Response to Drugs in Real Time (Neurons Treated with Paclitaxel)Figures 11-13 depict the result of experiments to demonstrate use of devices of the present disclosure in the context of monitoring cellular response to Paclitaxel (a common drug used to treat cancer, and targeting the microtubules).Fl 1 neuronal cells were provided adjacent to a PEFAG device. No SG or OSC were used. One ‘output’ of the device was instead connected to a spectrum analyser (SA), which was used to see if there were any resonant frequencies intrinsic to the sample investigated. This particular experiment was ‘semi -passive’, as the SA ‘sweeps’ the frequency spectrum investigated at low power, and then the PEFAG device is used to see if any frequency results in a resonance, and subsequent frequency peak in the output. This is a remarkable simple setup, with many uses, whilst conventional SAW devices are largely limited to using SA to find the fir of the device.Figures 11-13 respectively depict frequency spectra at times t = 0, t = 3hr and t = 24 / ir. The peaks observed in the t = 0 “control” (Figure 11) were observed to be identical to peaks observed in the same experiment (with the same cell line and setup) performed two months earlier, as well as another experiment conducted a month after the experiment seen here.This suggest that the peaks are not a mere fluctuation or statistical artefact, but are instead intrinsic to Fl 1 neurons (and possibly all mammalian cells). It was also observed that several peaks (marked by circles / ovals) disappeared completely post application of the drug, indicating that the origin of these peaks were targeted by the drug. Furthermore, several of the peaks that disappeared corresponded to frequencies known to be associated with microtubules, consistent with microtubules being targeted by the Paclitaxel.Thus, the experiment demonstrated not only 1) that the experimental setup can pick up resonant peaks of the sample (i.e. Fl 1 neurons) and 2) that the same peaks were seen in the same places in Fl 1 cells in a period of three months between the first and the most recent experiment, but also 3) that certain peaks, several of which had frequencies known to be related to the microtubules, significantly decreased or completely vanished. To the inventors’ knowledge, no other known method, including conventional SAW, is capable of producing these results.This setup has numerous uses in characterising, and subsequently identifying, a sample, as well as detecting specific changes in a sample over time (e.g. see if a medical treatment works), with measurements being done near instantly, with the pressing of a button.

Claims

CLAIMS1. A method of forming a device comprising: providing a device substrate; providing a flexible multilayer substrate having an insulating layer and an electrically conductive layer formed on the insulating layer; forming a patterned layer on a surface of the electrically conductive layer, thereby to provide protected and unprotected regions of the electrically conductive layer; etching the unprotected regions of the electrically conductive layer, thereby to form one or more electrically conductive features on the insulating layer of the flexible multilayer substrate; removing the patterned layer; and conformally applying the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate to a target surface of the device substrate.

2. The method according to claim 1, wherein the flexible multilayer substrate comprises a further electrically conductive layer on the opposite side of the insulating layer to the electrically conductive layer.

3. The method according to claim 1 or 2, wherein conformally applying the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate to the device substrate comprises mechanically clamping the flexible multilayer substrate and the device substrate together.

4. The method according to any preceding claim, wherein mechanically clamping the flexible multilayer substrate and the device substrate comprises positioning the flexible multilayer substrate between the device substrate and at least one further layer.

5. The method according to any preceding claim, wherein forming a patterned layer on a surface of the electrically conductive layer of the flexible multilayer substrate comprises: forming a photoresist layer on the electrically conductive layer; selectively masking the photoresist layer; exposing the photoresist layer to ultra-violet light; developing the photoresist layer; and removing developed or undeveloped regions of the photoresist layer, thereby to provide a patterned layer on the electrically conductive layer.

6. The method according to claim 5, wherein forming the photoresist layer on the electrically conductive layer comprises laminating a dry photoresist film to the electrically conductive layer.

7. The method according to any preceding claim, wherein the target surface of the device substrate is non-planar.

8. The method according to any preceding claim, wherein the device substrate comprises at least one of: a piezoelectric material, a semiconductor material, a solid-state material and a single layer material, optionally wherein the single layer material is a graphene layer.

9. The method according to any preceding claim, wherein the one or more electrically conductive features are electrodes.

10. The method according to claim 9, wherein the electrodes are interdigital transducers.

11. The method according to any preceding claim, wherein the device substrate is a piezoelectric substrate comprising at least one of: lithium niobate, lithium tantalite, lanthanum gallium silicate, quartz, cellulose and piezo-electric polymer-based material.

12. The method according to any preceding claim, comprising forming at least part of a cooling device on the opposite side of the flexible multilayer substrate to the electrically conductive layer, optionally wherein the cooling device is a thermoelectric cooling device.

13. The method according to any preceding claim, wherein the method further comprises forming a layer comprising a magnetostrictive material on the flexible multilayer substrate.

14. The method according to any of claims 1 to 13, further comprising: subsequently separating the device substrate and the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate; cleaning the device substrate; and subsequently conformally re-applying the one or more electrically conductive features formed on the insulating layer of the flexible multilayer substrate to a target surface of the device substrate, or conformally applying one or more electrically conductive features formed on the insulating layer of a further flexible multilayer substrate to a target surface of the device substrate.

15. The method according to claim 14, further comprising sterilising the device substrate.

16. A device formed in accordance with the method of any of claims 1 to 15.

17. A surface acoustic wave device, comprising: a piezoelectric device substrate; an insulating layer; and one or more electrically conductive features positioned between the piezoelectric device substrate and the insulating layer, the one or more electrically conductive features defininginterdigital transducers configured to generate and / or detect surface acoustic waves in the piezoelectric device substrate, wherein the dimensions and / or composition of the insulating layer and / or the geometry of the interdigital transducers is / are configured to substantially avoid coupling of surface acoustic waves between the piezoelectric device substrate and the insulating layer and / or to substantially avoid attenuation of surface acoustic waves by the interdigital transducers and / or insulating layer.

18. The device according to claim 17, wherein the dimensions and / or composition of the insulating layer and / or the geometry of the interdigital transducers is / are configured to space apart the piezoelectric device substrate from the insulating layer to avoid direct contact or near contact between the piezoelectric device substrate and the insulating layer.

19. The device according to claim 17 or 18, wherein the interdigital transducers have a thickness perpendicular to the piezoelectric device substrate of at least 5 microns.

20. The device according to any of claims 17 to 19, wherein the electrically conductive features are formed by patterning an electrically conductive layer of a substantially uniform thickness of at least 5 microns.

21. The device according to any of claims 17 to 20, wherein the electrically conductive features are formed by patterning an electrically conductive layer and the electrically conductive layer has at least one surface having a smoothness such that all features on the surface that cause deviation from a perfectly smooth surface deviate from the perfectly smooth surface by less than about 1 micron.

22. The device according to claim 17 or 18, wherein the interdigital transducers have a thickness perpendicular to the piezoelectric substrate and / or a width parallel to the piezoelectric substrate and / or a separation parallel to the piezoelectric substrate that is less than a micron.

23. The device according to claim 17, 18 or 22, wherein the interdigital transducers are formed by electron beam lithography.

24. A method of using the device according to any of claims 16 to 23, wherein the device substrate is a piezoelectric substrate and the one or more electrically conductive features are interdigital transducers, wherein the method comprises: generating surface acoustic waves.

25. The method according to claim 24, wherein the method further comprises: detecting and / or characterising biological cells, microbial lifeforms, nanobiomaterials and / or biomolecules.

26. The method according to claim 24, wherein the method further comprises: determining a measure of gravity and / or characterising gravity.

27. The method according to claim 24, wherein the method further comprises: determining a measure of at least one of: torque, temperature, pressure and / or strain based on a change in the path length of surface acoustic waves travelling along the device substrate.

28. The method according to claim 24, wherein the method further comprises: converting an electrical input signal into a surface acoustic wave; and subsequently recombining the surface acoustic wave into a further electrical signal, thereby mechanically filtering the electrical input signal.

29. The method according to any of claims 24 to 28, wherein the method further comprises: generating standing surface acoustic waves; and using the standing surface acoustic waves to control the location of one or more biological and / or non-biological objects in ID and / or 2D on the device substrate.

30. The method according to claim 24, wherein the method further comprises: determining a measure of at least one of humidity, viscosity and magnetic field data.

31. A method of using the device according to any of claims 16 to 23, wherein the device substrate is a piezoelectric substrate and the one or more electrically conductive features are interdigital transducers, wherein the method comprises: using the interdigital transducers to passively measure surface acoustic waves generated in the piezoelectric substrate by entities in a sample adjacent to the piezoelectric substrate.

32. The method of claim 31, wherein the entities comprise one or more of the following: living cells; living tissue; a living organ; an organism; a pathogen; and a nanoparticle.

33. The method of claim 32, wherein the method comprises monitoring a response of the entities to external stimuli, such as a drug, optionally in real time.

34. The method of claim 33, wherein the entities comprise neuronal cells.

35. A system configured to perform the method of claim 14 or 15.

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