Flexible devices including flexible wireless LC sensors containing conductive layers

Abstract

A method for producing a flexible structure, such as a wireless LC sensor, comprising multiple thin film layers of an elastomeric material and at least one layer of a microcorrugated conductive material, and a sensor device incorporating the flexible structure are described. The method includes steps to create a 2D corrugated metallized polydimethylsiloxane (PDMS) layer that exhibits negligible bending failure for angles up to 180 degrees, thereby achieving substantial flexibility.

Description

Technical Field

[0001] The present invention relates to flexible devices comprising flexible conductive layers and methods for fabricating such devices using parylene and PDMS (polydimethylsiloxane, also known as dimethylpolysiloxane or dimethylsiloxane) thin film layers. Devices according to the present invention include flexible wireless LC thin film individual sensors or sensor arrays. Methods for fabricating such devices include methods for fabricating microscale corrugated conductive (typically metallic) layers on PDMS thin film substrates, development of embedded PDMS thin film microstructured layers, and bottom-up wafer-scale sensor assembly. Background Art

[0002] The present invention adopts known principles:

[0003] - Aligned dual-inductor inductive / capacitive (LC) sensors are excellent resonant tanks for inductive wireless communication without the need for power.

[0004] -PDMS elastomers (characterized by low Young's modulus) are the biocompatible "gold standard" material for wearable technology, flexible sensors, and bioelectronics, as well as the limitations of current PDMS metal deposition.

[0005] Elastic microstructured dielectrics exhibit superior deformation properties compared to homogeneous or air-gap dielectrics in the field of flexible sensors, such as increased and tunable low-pressure sensitivity to compression cycles and ultrafast (~ms) highly repeatable spring response without viscoelastic creep.

[0006] - Industry-standard semiconductor micromachining processing methods for large-scale wafer-based batch production, such as photolithography, bottom-up layer-by-layer structuring, photolithographic self-alignment, and thin-film layer deposition by spin coating, sputtering, and evaporation.

[0007] The successful mass production of high-volume flexible thin-film LC pressure sensor arrays relies on achieving highly accurate translational and angular alignment of the dual-inductor structure. This challenge is compounded by the integration difficulties during sandwich processing of the pressure-sensitive microstructures. To date, large-scale LC sensor arrays containing only thin-film elastomers have been difficult to mass produce for the following reasons:

[0008] (1) Cracks in microscale metal structures formed on thin film elastic substrates such as PDMS after deposition, deformation, or bending, resulting in the current use of less flexible / adaptable substrates for inductive structures;

[0009] (2) soft lithographic fabrication methods for forming microstructured layers on flexible substrates, which are not amenable to bottom-up large-scale sensor assembly approaches; and

[0010] (3) Reliance on non-scalable methods used in developing each sensor layer individually and assembling the sensor structure through mechanical alignment, high pressure, and thermal lamination,

[0011] This creates misalignment of the two inductor tracks and structural non-uniformities and thus massively degrades the performance of the sensor array.

[0012] Pressure sensors using passive LC sensor design have been recognized as flexible sensors for competitive applications (e.g., medical sensors, robotics).

[0013] However, most rely on ceramic or polyimide structures, which are either inflexible, not biocompatible, or not amenable to industry-standard semiconductor processing—the present invention uses only PDMS and is therefore fully biocompatible and flexible.

[0014] Furthermore, while soft lithography is commonly used for thin-film PDMS, the fabrication method described here employs specialized sacrificial molding on thin films. This allows for the simultaneous bottom-up layering, microstructuring, and precise alignment of the sensing film layers of the sensor array, enabling the production of high-yield, high-performance sensors on flexible materials at the wafer scale. The novel 2D wrinkled metal PDMS layer provides an unprecedented level of flexibility without metal cracking upon deformation. Summary of the Invention

[0015] Aspects of the present invention that achieve the above-mentioned objects primarily comprise a novel bottom-up method for assembling devices such as LC microsensors by aligning a thin film elastomer to a 2D corrugated conductive (particularly metallized) surface, followed by variable microstructure intra-layer alignment of micro-frustum array geometries produced by a sacrificial mold with adjustable angles, as opposed to standard soft lithography techniques that primarily rely on pre-etched silicon molds and result in set frustum angles.

[0016] Thus, this fabrication method can produce, for example, the wireless, flexible LC sensor array of the present invention, which comprises two thin-film PDMS inductor layers with 2D corrugated surfaces and aligned metal tracks sandwiching the microstructured thin-film layers. Each sensor cell in the array, formed in a single process on a single wafer, can be identical or different from the other sensor cells in the array; for example, the parameters of the individual sensor cells in the array can be varied so that each sensor cell has a unique resonant frequency.

[0017] The fabrication methods disclosed herein facilitate high-volume wafer processing to produce, for example, different sensors simultaneously with high-precision alignment, thereby achieving high yields at reduced manufacturing costs.

[0018] Flexible, densely packed arrays of thin-film sensors—due to their fabrication methods—have high-precision alignment and layer structure consistency, ensuring exceptionally high performance. The passive electromechanical LC design of certain sensors embodying the present invention (which is not novel in itself, but has previously suffered from performance limitations in flexible electronics due to misalignment and the use of less flexible materials) allows for unpowered wireless signal communication.

[0019] The 2D corrugated metallized PDMS layer provided by embodiments of the present invention achieves considerable flexibility with negligible bending failure for angles up to 180°.

[0020] In summary, the methods according to various aspects of the present invention include treating a carrier substrate (typically a silicon wafer) to ensure strong adhesion of a first PDMS film layer, subsequently applied to the substrate, to the periphery of the substrate, and low adhesion of the PDMS layer to a substantial central region of the substrate. One or more additional PDMS film layers of gradually increasing Young's modulus are applied to establish a gradient of decreasing elasticity, followed by a parylene film layer. The substrate is immersed in a solvent for a period of time sufficient to induce low-intensity swelling in the PDMS layer. Another PDMS film layer is applied, followed by another parylene film layer. The parylene layer is applied by vacuum deposition. During the vacuum deposition of the additional parylene layer, diffusion of solvent from the PDMS layer, combined with the strong adhesion of the periphery of the first PDMS layer to the substrate, the elastic gradient, and the initial parylene layer and subsequent PDMS layer, results in permanent, uniform microscale wrinkling of the additional parylene layer. This microscale wrinkling is reflected in a conductive (e.g., metal) layer subsequently applied to the wrinkled parylene, resulting in the wrinkled conductive layer being highly flexible (able to bend without breaking). The same wrinkling mechanism can be used in further layers that are subsequently applied.

[0021] According to one aspect of the present invention, there is provided a method for producing a flexible structure comprising a plurality of film layers of elastic material and at least one layer of micro-corrugated conductive material, the method comprising:

[0022] a) applying a selective anti-adhesion treatment process to the carrier substrate, whereby a peripheral region of the substrate provides a strong adhesion region and a region of the substrate within the peripheral region provides an anti-adhesion core region;

[0023] b) forming a first plurality of continuous PDMS thin film layers on a carrier substrate, each continuous PDMS thin film layer having a smaller ratio of cross-linker to base material and thus having a higher Young's modulus than a previous PDMS thin film layer;

[0024] c) forming a first parylene thin film layer on the last formed PDMS thin film layer;

[0025] d) placing the carrier substrate in an organic solvent for a first period of time to induce swelling in the first plurality of PDMS thin film layers;

[0026] e) forming a first additional PDMS thin film layer on the first parylene thin film layer;

[0027] f) forming a second thin film layer of parylene on the first additional thin film layer of PDMS by vacuum deposition, wherein a permanent microscale corrugated surface morphology is produced due to diffusion of an organic solvent from the first plurality of thin film layers of PDMS during said vacuum deposition, thereby providing a first microscale corrugated parylene layer;

[0028] g) forming and patterning a first layer of conductive material on the first microscale corrugated parylene layer such that the first patterned conductive material has a microscale corrugated surface morphology consistent with that of the first microscale corrugated parylene layer, thereby providing a first microscale corrugated conductive pattern layer.

[0029] In some embodiments, the selective anti-adhesion treatment applied to the carrier substrate includes rendering the peripheral region of the substrate highly hydrophilic and the central region of the substrate within the peripheral region highly hydrophobic. The central region of the substrate can be rendered highly hydrophobic by drying a thin anti-adhesion layer, such as trichloro(1H,1H,2H,2H-perfluorooctyl-silane). The peripheral region of the substrate can be rendered highly hydrophilic by selective O2 plasma etching.

[0030] In some embodiments, the organic solvent and the first time period are selected to induce a degree of swelling in the first plurality of PDMS thin film layers, which results in a permanent micro-scale wrinkled surface morphology in the first additional PDMS thin film layer. The organic solvent and the first time period can be selected to achieve a desired wrinkle fluctuation wavelength for the permanent micro-scale wrinkled surface morphology generated in the first additional PDMS thin film layer.

[0031] In some embodiments, the organic solvent is n-methyl-2-pyrrolidone, dioxane, dimethyl carbonate, pyridine, or dimethylformamide.

[0032] In some embodiments, the Young's modulus of the first additional PDMS thin film layer is equal to the Young's modulus of the last formed layer of the first plurality of PDMS thin film layers.

[0033] In some embodiments, the carrier substrate is a silicon wafer.

[0034] In some embodiments, the second micro-scale corrugated conductive pattern layer is formed by:

[0035] placing the carrier substrate in the organic solvent for a second period of time to again induce swelling in the first plurality of PDMS thin film layers;

[0036] forming one or more additional PDMS thin film layers on the uppermost layer of the aforementioned structure;

[0037] forming a third parylene thin film layer on top of the additional PDMS thin film layers by vacuum deposition, wherein a permanent microscale corrugated surface morphology is generated due to diffusion of an organic solvent from the first plurality of PDMS thin film layers during said vacuum deposition, thereby providing a second microscale corrugated parylene layer; and

[0038] A second layer of conductive material is formed and patterned on the second microscale corrugated parylene layer such that the second patterned conductive material has a microscale corrugated surface morphology consistent with that of the first microscale corrugated parylene layer, thereby providing a second microscale corrugated conductive pattern layer.

[0039] Some embodiments also include forming one or more additional PDMS layers on the first microscale corrugated conductive pattern layer, and patterning one or more of the one or more additional PDMS thin film layers to produce a 3D microstructure. The 3D microstructure can be formed by photolithography. The patterning of the 3D microstructure can provide a single 3D microstructure array corresponding to a single device array. One or more individual 3D microstructures can have one or more physical parameters that are different from one or more other individual 3D microstructures. The second microscale corrugated conductive pattern layer can be formed by the above-mentioned method for forming the second microscale corrugated conductive pattern layer. The one or more additional PDMS thin film layers in the method may include one or more additional PDMS thin film layers for producing a 3D microstructure. Placing the carrier substrate in an organic solvent for a second time period to induce swelling again in the first plurality of PDMS thin film layers can also dissolve the photolithographic photomask used to pattern the 3D microstructure. The 3D microstructure can include an array of individual frustum arrays, and each of the first and second microscale corrugated conductive pattern layers can include an individual array of inductive structures, and each inductive structure of each microscale corrugated conductive pattern layer can be aligned with a corresponding frustum array of the 3D microstructure and a corresponding inductive structure of the other microscale corrugated conductive pattern layer to provide an array of individual devices that can function as wireless LC sensors. The sidewall angle of the frustum array can be determined by UV exposure during photolithographic formation of the 3D microstructure.

[0040] Each layer of conductive material may be patterned using photolithography.

[0041] The patterning of each layer of conductive material may provide a single array of conductive structures corresponding to a single array of devices.One or more individual conductive structures may have one or more physical parameters that differ from one or more other individual conductive structures.

[0042] Each layer of conductive material may be a metal layer. Each metal layer may include one or more of titanium, aluminum, chromium, gold, silver, copper, tungsten, platinum, and lead. Each metal layer may include a first layer of titanium or chromium and a second layer of aluminum.

[0043] According to a second aspect of the present invention, there is provided a flexible structure comprising a plurality of thin film layers of elastic material and at least one layer of micro-corrugated conductive material, the structure comprising:

[0044] a first plurality of consecutive PDMS thin film layers, each consecutive PDMS thin film layer having a smaller ratio of cross-linker to base material and thus having a higher Young's modulus than the previous PDMS thin film layer;

[0045] a first parylene film layer;

[0046] a first additional PDMS thin film layer on the first parylene thin film layer;

[0047] a second parylene thin film layer having a permanent microscale corrugated surface morphology on the first additional PDMS thin film layer, providing a first microscale corrugated parylene layer;

[0048] A first layer of conductive material on the first micro-scale corrugated parylene layer has a micro-scale corrugated surface morphology consistent with the first micro-scale corrugated parylene layer, providing a first micro-scale corrugated conductive pattern layer.

[0049] The first additional PDMS thin film layer may have a Young's modulus equal to the last formed layer of the first plurality of PDMS thin film layers.

[0050] The structure may also include:

[0051] One or more additional PDMS thin film layers on the uppermost layer of the aforementioned structure;

[0052] a third parylene film layer having a permanent microscale corrugated surface morphology, providing a second microscale corrugated parylene layer; and

[0053] A second layer of conductive material is formed on the second microscale corrugated parylene layer such that the second patterned conductive material has a microscale corrugated surface morphology consistent with the first microscale corrugated parylene layer, thereby providing a second microscale corrugated conductive pattern layer.

[0054] The structure may further include one or more additional PDMS layers on the first microscale corrugated conductive pattern layer, the one or more additional PDMS layers being patterned to provide a 3D microstructure. The patterning of the 3D microstructure may provide an array of individual 3D microstructures corresponding to an array of individual devices. The one or more individual 3D microstructures may have one or more physical parameters that differ from one or more other individual 3D microstructures. The structure may include a second microscale corrugated conductive pattern layer. The one or more additional PDMS thin film layers may include one or more additional PDMS thin film layers that provide the 3D microstructure. The 3D microstructure may include an array of individual frustum arrays, and each of the first and second microscale corrugated conductive pattern layers may include an array of individual inductive structures. Each inductive structure of each microscale corrugated conductive pattern layer may be aligned with a corresponding frustum array of the 3D microstructure and a corresponding inductive structure of another microscale corrugated conductive pattern layer to provide an array of individual devices that can function as wireless LC sensors. The sidewall angle of the frustum array may be determined by UV exposure during the photolithographic formation of the 3D microstructure.

[0055] Each layer of conductive material may be patterned using photolithography.

[0056] Patterning of each layer of conductive material may provide an array of individual conductive structures corresponding to a single array of devices.One or more individual conductive structures may have one or more physical parameters that differ from one or more other individual conductive structures.

[0057] Each layer of conductive material may be a metal layer. Each metal layer may include one or more of titanium, aluminum, chromium, gold, silver, copper, tungsten, platinum, and lead. Each metal layer may include a first layer of titanium or chromium and a second layer of aluminum.

[0058] A device embodying the present invention may comprise one of the arrays of individual devices described above. The device may be a wireless LC sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0060] Figure 1A is a circuit diagram illustrating the working principle of a wireless LC pressure sensor that can be implemented using the present invention.

[0061] Figure 1B is a graph showing an example of how the resonant frequency of an LC pressure sensor varies with contact pressure.

[0062] Figure 2 is an exploded view showing the physical structure of a wireless LC pressure sensor 200 that can be implemented using the present invention.

[0063] Figures 3A to 3E A process of manufacturing a wireless LC sensor array according to an embodiment of the present invention is shown.

[0064] Figure 4 is based on Figures 3A to 3E A cross-sectional view of a portion of a sensor manufactured by the method.

[0065] Figure 5A It further shows Figure 3A The selective anti-stiction pretreatment process of the carrier wafer is shown.

[0066] Figure 5B It further shows Figure 3B The permanent frosted wrinkle metallization process shown.

[0067] Figure 5C It further shows Figure 3C Microstructure microfabrication process shown.

[0068] Figure 6 Shown is the permanently frosted, wrinkled surface of the parylene-coated PDMS layer before metallization.

[0069] Figure 7A Examples of tunable microscale metallization and uniformity of fluctuation wavelength on the permanently frosted, corrugated surface of a parylene-coated PDMS layer are shown, where the letters correspond to a = 1 μm, b = 1.5 μm, and c = 2 μm metal depositions and the numerical subscripts (e.g., a1) correspond to different wafer batches.

[0070] Figure 7B The uniformity of the corrugated metallization (here 2 μm thick) is shown on a larger scale.

[0071] Figure 8 It shows that for Figure 8 The three metallization thickness sensors S shown in 11 Amplitude versus thickness graph.

[0072] Figure 9 The embedded thin film microstructured layer of the sensor is shown.

[0073] Figure 10 Showing high alignment yield of a multi-coil dual-inductor structure with track widths as low as 25 μm, where (left) is the top inductor and (right) is the bottom inductor.

[0074] Figure 11 is a graph showing sensors developed in one batch, which have unique resonant frequencies covering the 80 MHz to 1 GHz frequency band.

[0075] Figure 12 The elasticity of the corrugated track to 180° bending is shown.

[0076] Figure 13 The stress relief provided by the wrinkles is shown, while the fracture caused by the blade is stopped and contained to only a few microns near the damage location.

[0077] Figure 14 Shown in Figure 3C and 5C Reservoir formation and preferential dissolution during microfabrication of the microstructure shown: (a) Preferential direction of dissolution of the sacrificial photoresist layer; (b) Photograph of the embedded sacrificial mold on the surface of the first inductor layer before dissolution; (c) Cross-section of the sensor structure looking down into the reservoir region (the unstructured bonding area is located in the lateral direction of the cross-section and is not shown).

[0078] Figure 15 Shown are (a) the optimized heat reduction step after encapsulating the sacrificial die, (b) the same 10x10mm 2 preferential dissolution technique of the sensor array wafer, and (c) microcapillary forces acting during the formation of the microstructured layer.

[0079] Figure 16 The ability to adjust the frustum sidewall angle of the sensor's microstructure layer to 90° and 74° by controlling UV exposure during formation of the sacrificial photoresist layer is shown.

[0080] Figure 17 The photolithographic micro-alignment structure is shown.

[0081] Figure 18 The expansion defect of the sensor array membrane during deposition of the aligned second inductor layer is shown when no vias are formed and the reservoir layer remains encapsulated. DETAILED DESCRIPTION

[0082] Embodiments of the invention will be described herein by way of example with specific reference to a flexible wireless LC pressure sensor. The invention is applicable to other types of sensors and other devices, as discussed in the following description. A preferred embodiment provides for wafer scale production of arrays of sensors / devices, using silicon wafers as carrier substrates for the bottom-up production of flexible multilayer thin film structures which may incorporate micro-corrugated conductive elements and three-dimensional microstructures. The invention enables the simultaneous production of arrays of multiple thin film sensors / devices on a wafer, wherein the physical parameters of the individual sensors / devices in the array may vary between the sensors / devices. References to "wafer" in the following description will be understood to refer to silicon wafers as the preferred carrier substrate for the method. References to "array" will be understood to refer to the array of sensors / devices formed on such a carrier substrate, and general references to "membrane" will be understood to refer to the composite of thin film layers providing the basis for the array of sensors / devices.

[0083] Referring now to the accompanying drawings, Figure 1A is a circuit diagram illustrating the working principle of a wireless LC pressure sensor that can be implemented using the present invention.

[0084] The general LC sensor circuit 100 includes an inductor L s , resistor R s and capacitor C s The physical structure of the sensor makes C s As the pressure applied to the sensor changes, the resonant frequency of the sensor circuit 100 also changes with the pressure.

[0085] This circuit is a general operational circuit for LC sensors. The dual inductor LC structure of the sensor described below can be shown to be equivalent to the circuit of Figure 1. s and C s are the effective (coupled) inductance and capacitance of the two aligned inductors in the structure.

[0086] The wireless readout measurement system 110 includes a readout antenna 120, which includes an inductor L a , inductance L a Can be inductively coupled to the sensor inductance L s The readout system 110 measures the resonant frequency of the sensor circuit in order to determine the pressure applied to the sensor.

[0087] The resonant frequency of the sensor circuit can be measured, for example, by exciting the LC circuit with a swept frequency of radio frequency (RF) energy and then using a phase detector to locate the resonant frequency, or by exciting the LC circuit with pulses of RF energy at a predetermined frequency or set of frequencies and then using a phase-locked loop (PLL) circuit to lock to the resonant frequency of the sensor.

[0088] Figure 1B is a graph showing an example of how the resonant frequency of an LC pressure sensor varies with contact pressure.

[0089] These general types of wireless LC pressure sensors and readout systems are well known to those skilled in the art and will not be described in detail herein except as necessary for understanding the purposes of the present invention, which provides improved methods of making such wireless LC sensors and improved sensors obtained by these methods.

[0090] Figure 2is an exploded schematic diagram illustrating the physical structure of a wireless LC pressure sensor 200 that can be implemented using the present invention. The structure includes a bottom layer 210 containing a first inductive structure 220, a middle layer 230 containing a sensor microstructure 240, and a top layer 250 containing a second inductive structure 260 aligned with the first inductive structure 220. The first and second inductive structures 220 and 260 together provide the inductance and capacitance of the sensor 200, the latter resulting from the overlapping tracks of the two aligned inductive structures 220 and 250, with the middle layer 230 acting as the dielectric of the coupling capacitor. The physical properties of the sensor microstructure 240 are such that the thickness and relative dielectric constant of the middle layer vary with compressive deformation (e.g., applied pressure). Primarily, the reduction in thickness of the microstructured layer in response to compressive deformation is the dominant factor.

[0091] The microstructure 240 can be, for example, a structured elastomer comprising an array of frustums, such as an array of pyramidal structures. The thickness and relative permittivity of the intermediate layer 230, and how they vary with pressure, depend in part on the material used and in part on the physical properties of the microstructure 240. Thus, for a given material, the frequency response of the sensor 200 depends on the physical parameters of the frustum array, such as the size, shape (including wall angle), and spacing / density of the frustums. Aspects of the present invention allow control of these parameters, including the frustum wall angle, to fine-tune the frequency response and sensitivity of the sensor to compressive deformation and allow different sensors in a sensor array to be produced simultaneously with different frequency responses.

[0092] Regarding this latter point, the overall process described here enables:

[0093] (1) Producing a uniform microstructured layer that is adjustable in terms of structural and array configuration parameters, which affects the overall mechanical response of the sensor to compression (i.e., the microstructuring process described below). The direct photolithography method used here forms an embedded microstructured layer directly on a thin film substrate, in principle allowing the microfeatures of the microstructured layer to be selectively designed for individual sensors in a sensor array produced simultaneously (i.e., by incorporating variations between individual sensors into the photolithographic mask design). Variable feature geometries can be applied to the array for each sensor or each sensor array area; for example, the bottom size of the frustum, the spacing / density of the frustum, the frustum geometry (circular, square, triangular, rectangular, etc.). The thickness and frustum angle, although fully adjustable, can be kept uniform across the entire array and are therefore the same for all sensors in each wafer batch. All of the above control the mechanical (structural) response of the sensor to deformation, as well as the physical parameters of the microstructured layer material (e.g., Young's modulus, i.e., elasticity)

[0094] (2) Ultra-dense scalable LC sensor arrays (e.g., 200 μm pitch as shown herein), where each sensor can be designed (inductor size, track dimensions such as loop, width, and pitch) to operate in a unique resonant frequency domain; i.e., to provide a uniquely addressable sensor by virtue of the corrugation / metallization and assembly processes employed. An inherent fundamental relationship between the ambient resonant frequency (i.e., operating frequency range) and sensitivity to pressure of an LC sensor can also be exploited.

[0095] (3) The combination of (1) and (2) above (i.e., the entire process) allows for a sensor array that is fully tunable in terms of performance, operating frequency domain, and sensor size.

[0096] Furthermore, the entire process allows dual-inductor LC sensors to be developed to operate in the advantageous low-frequency band of <80 MHz to 1 GHz when required, e.g. Figure 11 As shown, this is in contrast to prior art dual inductor LC sensors operating at >1 GHz, which are more suitable for medical applications (i.e., less affected by tissue attenuation). However, the present invention can also be used to produce dual inductor LC sensors operating at frequencies >1 GHz.

[0097] Layers 210, 230, and 250, as well as microstructure 240, are formed from an optically transparent, elastic material, making the sensor structure flexible and allowing alignment of features formed by photolithographic techniques, as described below. The present invention employs PDMS (polydimethylsiloxane, also known as dimethylpolysiloxane or dimethylsiloxane) and parylene (preferably parylene C), as discussed in detail below.

[0098] Inductive structures 220 and 260 can be formed from any suitable metal or other conductive material, preferably a combination of nm-thin titanium (Ti) or chromium (Cr), used as a seed layer to provide improved adhesion to the underlying substrate, and micro-scale thick aluminum (Al) forming the body of the conductive layer, as in the embodiments of the present invention described below.

[0099] Other materials that are best suited for these types of inductive structures include, for example, chromium, gold, silver, copper, tungsten, platinum, lead, etc., i.e., materials with a low resistivity coefficient of ρ < ~20*10-8Ωm, which can provide LC sensors with a sufficiently high quality factor for effective wireless readout.

[0100] The inductive structures 220 and 260 typically comprise flat spiral structures, illustrated as right-angled spirals, although other spiral types may be employed, such as other forms of polygonal spirals or circular / elliptical spirals. The shape, size, thickness, and material of the inductive structure may be selected to provide any desired electrical properties, and the material may also be selected to provide desired physical properties, particularly flexibility.

[0101] In addition to LC pressure sensors, other sensor / device types that may utilize the present invention include, but are not limited to:

[0102] - Gas / liquid sensors, such as "electronic noses" or "e-nose." Introducing the sensor into a liquid or gas environment allows liquid / gas access to the sensor's microstructured layer, as the cross-section is exposed after the sensor is separated from the membrane. Monitoring of target quantities can be provided by changing the dielectric constant of the microstructured layer or / and by swelling (increasing thickness), as many solvents can cause significant swelling of PDMS, such as chloroform, tetrahydrofuran, diethyl ether, xylene, pentane, benzene, etc.

[0103] -Proximity sensor. When the sensor is close to a highly conductive material, or vice versa, such as metal, the resonant frequency shifts upwards.

[0104] -Bending and strain sensors. Bending or straining can also cause compressive deformation, so the sensors may be repurposed for this application.

[0105] Flexible antennas / electronics. The microscale wrinkled low-resistivity metallization and thin-film flexible biocompatible PDMS substrates described here could facilitate the development of high-performance flexible antennas (e.g., for wearable or biomedical applications) or serve as a more flexible alternative to polyimide for flexible electronics.

[0106] As described further below, the present invention includes methods for making microscale corrugated ("microcorrugated") conductive layers that can be used to provide the inductive structure of a wireless LC sensor to achieve a high degree of flexibility.

[0107] Referring now to Figures 3 to 5, one aspect of the present invention provides a process for manufacturing a flexible thin film structure that provides devices such as wireless LC sensors, and more specifically, for manufacturing an array of multiple discrete devices / sensors on a single carrier substrate (wafer), where each device / sensor of the array can have different characteristics.

[0108] Figures 3A to 3E An overview of an example embodiment of a process for producing a wireless LC sensor is provided. Figure 4 shows a cross section of the produced sensor structure, and Figures 5A to 5C Shown Figures 3A to 3D Further details of the sub-process shown in .

[0109] The entire process involves depositing successive layers of different materials from bottom to top on a carrier substrate such as a silicon carrier wafer 310. Figure 3AAs shown, the preferred embodiment includes a selective anti-adhesion treatment process applied to the surface of the carrier wafer 310, whereby the periphery 320 of the wafer is made hydrophilic to provide strong adhesion, while the central area 330 is made hydrophobic to provide low adhesion. Figure 5A This preliminary process is described in more detail. It should be understood that for illustration purposes, Figures 3 to 5 show a small portion of a single sensor structure, while the carrier wafer 310 having a hydrophilic periphery 320 and a hydrophobic central region 330 will carry an array of multiple individual sensors.

[0110] Figure 3B The sensor structure is shown after a first set of layers have been successively deposited on a carrier wafer 310; ie Figure 4 Layers L1 to L6 are detailed in Table 1 below. Layers L1 to L5 together correspond to Figure 2 The bottom layer 210, layer L6 corresponds to Figure 2 The first inductor structure 220 is configured as follows.

[0111] Figure 3C The sensor structure is shown after a second set of layers has been deposited on top of the previous layers; i.e. Figure 4 Layers L7 to L9 are as detailed in Table 1 below. Layer L7 encapsulates the first inductor structure (layer L6), and layers L8 and L9 correspond to Figure 2 The intermediate layer 230 and the sensor microstructure 240 are shown.

[0112] Figure 3D shows the sensor structure after a third set of layers has been deposited on top of the previous layers; i.e. Figure 4 Layers L10 to L12 are detailed in Table 1 below. Layers L10 and L11 together correspond to Figure 2 The top layer 250, layer L12 corresponds to Figure 2 The second inductor structure 260 is provided.

[0113] Figure 3E The final sensor structure is shown after drying, with a final layer L13 deposited to encapsulate the second inductive structure (layer L12 ) and the sensor array having been peeled from the carrier wafer 310 .

[0114] As will be further described below, the present invention utilizes PDMS and parylene (preferably parylene C) layers of varying elasticity to provide a parylene surface ("2D corrugated surface") that is uniformly corrugated in two dimensions at the microscale ("microscale corrugations" or "microcorrugations"), onto which metal layers L6 and L12 are deposited, such that the metal layers themselves are microcorrugated and, therefore, highly flexible.

[0115] A specific embodiment of the wireless LC sensor according to the present invention comprises layers L1 to L13 as shown in Table 1 below.

[0116] Table 1

[0117]

[0118]

[0119] **Note that the total thickness of the sensor structure does not include the thickness of metal layers L6 and L12, which are encapsulated in the thickness of layers L7 and L13.

[0120] As can be seen from the table, the sensor structure includes (i) thin film elastomer layers of PDMS, which use two different ratios of cross-linker to base material and therefore have different elasticities: PDMS 1:10 for layers L1 and L7-L9 (with a Young's modulus of 1.7 MPa or greater for L8 and L9 constituting the intermediate layer 230 / microstructure 240 of the sensor 200), PDMS 1:5 (Young's modulus of 3.2 MPa) for layers L2, L4, L10 and L13, and (ii) parylene, with layers L3, L5 and L11 having a much higher Young's modulus (2.7 GPa).

[0121] The present invention uses a thin film PDMS layer with gradually increasing hardness (increasing Young's modulus) to produce a gradient of decreasing elasticity in the direction toward the parylene layer (in this example, L5 and L11), which manifests as micro-scale wrinkles in the final product. The elastic gradient contributes to the formation of the micro-scale wrinkled surface, as further described below. As a minimum, two such PDMS layers are required to produce the required elastic gradient. It is preferred to use this minimum number of two PDMS layers to minimize the complexity of the structure, thereby minimizing the time and cost of production.

[0122] The PDMS elasticity can be varied in two ways, as long as a gradient Young's modulus is maintained towards the upper interface to be metallized, to allow wrinkling to occur (see description of wrinkling method below):

[0123] 1. Elastomer base to crosslinker ratios of native PDMS such as Sylgard 184 from 1:50 to 1:5 result in Young's moduli ranging from 10 kPa to 3.5 MPa, respectively (also depending on cure temperature and cure time)

[0124] 2. PDMS blends such as Sylgard 527 and Sylgard 184 result in a more controllable and stable Young's modulus of 5 kPa to 1.7 MPa (i.e. avoids partial cross-linking and undesirable diffusion that can occur when the ratio is significantly reduced)

[0125] 3. Young's modulus affects the mechanical response of the sensor to compressive deformation, which is particularly important for the L8 (microstructure) layer and further involves the ability to adjust the sensor performance.

[0126] In this example, the PDMS layers have a thickness of 19 μm, except for layer L8 (microstructured layer) which has a thickness of 13 μm, while the parylene layers are much thinner at 2.5 μm.

[0127] Spin coating of PDMS and vacuum deposition of parylene enabled precise control and uniformity of the thickness of each layer across the wafer. As described below, the use of vacuum deposition for the parylene layer also played a role in the wrinkling of parylene layers L5 and L11.

[0128] For the PDMS layer, the thickness can be varied from <10 μm to >200 μm by spin coating.

[0129] Thin film elastomers have higher compliance (bendability), thereby improving flexibility. Increasing the layer thickness t, especially for layers L6-L10, significantly reduces the capacitance of the sensor and the inductive coupling of the sensor's two inductors, resulting in a proportional increase in their resonant frequency (i.e., ~√t). In addition, for layers L6-L10, a significant increase in the total thickness (e.g., t>500μm, i.e., no longer a thin film) will effectively eliminate the inductive and capacitive coupling of the two aligned inductors of the sensor structure, resulting in a decoupled sensor system that is electromagnetically insensitive to compression deformation.

[0130] For the parylene layer, the thickness can be controlled by the deposition rate of the equipment (eg, SCS PDS2010 parylene coater).

[0131] - The tested thicknesses from 0.5 μm to 3 μm had no significant effect on the flexibility and performance of the sensor.

[0132] -Thicknesses < 0.5 μm are expected to result in insufficient coverage of the substrate surface with effective metallization, while thicknesses > 5 μm may result in over-coverage, burying the wrinkled surface and leading to a planar morphology. Planar surfaces can lead to crack formation under bending or compressive deformation (discussed further below with reference to the wrinkling method).

[0133] In this example, the microstructure frustum array provided by layer L8 comprises 50x50 μm 2 components.

[0134] The microstructured layer is formed by photolithography (i.e., by spin coating of a sacrificial micromold photoresist layer and UV exposure). Thus, the size, geometry, and / or spacing of the features can be varied from <1 μm to as large as, for example, >1 mm as desired (i.e., depending on the photomask design and UV exposure system).

[0135] The geometry of the microstructure array can also be varied to include square microfeatures, or circular, triangular, rectangular, etc., or any combination thereof, with or without different / variable feature sizes (i.e., depending on the photomask design). The latter can enable tunable and selective mechanical response of individual sensors or sensor array regions of the film, as desired (discussed further below with reference to the microstructuring process).

[0136] As will be discussed further below, the manufacturing process causes the parylene layers L5 and L11, and therefore the metal layers L6 and L7 deposited thereon, to have a slightly wrinkled surface morphology, while the other layers have a uniform surface morphology.

[0137] Figure 5A It further shows Figure 3A A selective anti-adhesion treatment process is performed, whereby a primary central region 330 of the Si carrier wafer 310 is selectively rendered highly hydrophobic, for example by drying a thin layer of trichloro(1H,1H,2H,2H-perfluorooctyl-silane), thereby providing an active anti-adhesion central region. Any corresponding low surface tension layer can be used to achieve reduced adhesion in the central region to provide similar results. The wafer periphery 320 is made highly hydrophilic, for example by selective O plasma etching, thereby providing a supportive, strong adhesion region.

[0138] The width of the peripheral region 320 is selected to be as small as possible relative to the diameter of the carrier substrate 310 while ensuring that the first layer of the first PDMS layer L1 remains adhered to the substrate during subsequent process steps. It is desirable that the radial width of the strong boundary region be as small as possible because this provides more usable active area for the sensor array to be developed. In the exemplary process described herein, using a 3 inch (7.62 cm) wafer, the width was arbitrarily selected to be approximately 10 mm to ensure a strong bond of the elastomeric film to the carrier wafer and strong planarity of the film during processing, but if desired, this width can be reduced, perhaps as low as 0.5-1 mm.

[0139] A thin nanolayer of trichloro(1H,1H,2H,2H-perfluorooctyl-silane) (3 drops in a vial) can be uniformly deposited on the surface of a Si wafer by adsorption in a desiccator for 3 h (active anti-adhesion zone). Selective O2 plasma etching of the periphery of the wafer surface can be facilitated by attaching a temporary mask to the wafer that shields the central anti-adhesion zone.

[0140] The selective anti-stiction treatment provides strong boundary conditions for the emergence of highly uniform and isotropic fluctuations of the two-dimensional corrugated surface morphology (typically with a wavelength λ on the order of 7.5 μm) over the entire surface area of the wafer for the subsequent corrugation metallization process, while subsequently enabling manual peeling (separation) of the sensor film from the carrier wafer 310 and further enabling high planarity of the film during processing.

[0141] The role of this anti-adhesion process in providing uniform and isotropic corrugations across the surface at a later stage will be described further below.

[0142] Figure 5B The corrugation metallization process of the first inductor layer L6 is shown, which includes:

[0143] 5B-(i)-a Two thin film PDMS layers L1 and L2 are sequentially deposited by spin coating and thermal crosslinking, each with a different Young's modulus (in this example, a PDMS crosslinker to base ratio of 1:10 and 1:5, respectively, using, for example, Sylgard from Dow Chemical Company). TM 184).

[0144] Spin coating PDMS provides controllable thin film (i.e., μm-level) thickness on a wafer scale. Alternatively, typical casting methods used with PDMS result in sub-mm to mm-thick layers with little control over film thickness and surface uniformity / planarity (see also the discussion above on the importance of having a thin film structure for sensor performance). An alternative approach could be spraying (coating), but this requires a dilution (e.g., hexane) to reduce the viscosity of the material used with the airbrush (i.e., through the nozzle) and can reduce the hardness of the polymer.

[0145] 5B-(i)-b is deposited by vacuum evaporation of the first thin film polyparaxylene C layer L3.

[0146] Parylene coatings are typically deposited onto the substrate or material being conformally coated at ambient temperature via a vapor deposition process (i.e., using specialized vacuum deposition equipment, such as the CS PDS 2010, Kisko). Parylene polymer deposition effectively occurs at the molecular level, where the film essentially "grows" one molecule at a time.

[0147] 5B-(ii) The wafer is placed in a bath of n-methyl-2-pyrrolidone for 12 hours. The purpose of this step is to induce a low-intensity swelling in the PDMS layers L1 and L2, which, combined with the elastic gradient mentioned above, causes the parylene layer L5 to spontaneously wrinkle when subsequently applied. This will be discussed further below. Other biocompatible organic solvents with similar properties (e.g., with a low swelling ratio S < 1.1 and a similar solubility for PDMS - δ ~ 11 cal) can be used. 1 / 2 cm -3 / 2 ), such as dioxane, dimethyl carbonate, pyridine or dimethylformamide (for these other solvents, the placement time in the solvent bath can be adjusted according to the S and δ parameters to obtain a wrinkle fluctuation wavelength of λ ~ 7.5μm as described herein).

[0148] 5B-(iii)-a Deposition of a PDMS thin film layer L4 by spin coating and cross-linking at a ratio of 1:5.

[0149] 5B-(iii)-b is deposited by evaporation of a thin film parylene C layer L5, and spontaneously produces a permanently "frosted" and uniform two-dimensional wrinkled surface morphology (with the help of the above-mentioned selective anti-adhesion treatment process), which will be further discussed below.

[0150] NOTE: The terms "frosted" and "permanent frosted" are used in this document to refer to stable, permanent micro-wrinkles similar to those Figure 6 , 7 and 13 show a frosted pattern similar to the micro-folds of the present invention.

[0151] 5B-(iv) O2 plasma etching and activation, photolithographic patterning of the first inductor structure, metallization of the desired Ti / Al thickness by sputtering or e-beam evaporation (or electroplating or thermal evaporation, for example), and lift-off dissolution to form the first inductor layer L6.

[0152] O2 plasma etching increases the adhesion of metal to the parylene surface; an alternative method is O2 reactive ion etching (RIE).

[0153] 5B-(v) Encapsulation with a 1:10 thin film PDMS layer L7 by spin coating.

[0154] The structure thus created, including layers L1 to L6, provides a flexible, elastic substrate comprised of layers L1 to L5, with a flexible, micro-corrugated conductive layer L6 that can be patterned to provide an inductive spiral, as in the embodiments described herein, or any other useful conductive pattern; for example, to provide a flexible antenna for wearable and / or biomedical applications. Thus, layers L1 to L6, independent of the additional layers L8 to L13 of these embodiments, provide a novel and useful flexible structure, with or without a layer such as L7 encapsulating the conductive layer L6.

[0155] The spontaneous generation of surface wrinkles in the polyparaxylene layer L5 on the PDMS substrate is due to the complex mechanical buckling instability at the uppermost interface of the substrate. The gradient mechanical properties of the multilayer substrate film and the low-intensity swelling of the lower layer induced by n-methyl-2-pyrrolidone (for PDMS, S = 1.03 swelling coefficient) synergistically promote this instability, and the strong adhesion of the periphery of the L1 layer to the carrier substrate supports this instability.

[0156] The low-intensity swelling of the heterogeneous multilayer substrate leads to a volume phase transition of the underlying PDMS L1 and L2 layers, which are confined between the conformal coating provided by the L3 parylene layer and the strong adhesion to the periphery of the rigid carrier wafer. Due to the diffusion of solvent from layers L1 and L2, the swollen state is released during the deposition of the L5 parylene layer under vacuum, and undergoes another abrupt volume change. The gradient change in the mechanical properties of the heterogeneous PDMS substrate (L1 and L2) and the abrupt volume change caused by solvent diffusion result in anisotropic osmotic pressure across most of the film thickness, leading to in-plane equibiaxial compressive strain at the interface of the uppermost (L4) PDMS layer, which causes the film to warp and, consequently, a labyrinthine wrinkle pattern ("frosting") on the surface, while the deposition of parylene (L5) simultaneously contributes to the stabilization of the wrinkle formation (frosting). The L3 / L4 interface acts as a "rigid-flexible" interface, promoting the mechanical buckling instability generated at L1 / L2 toward the uppermost "rigid-flexible" interface L4 / L5, and the appearance of surface wrinkling.

[0157] Spontaneous wrinkling effectively acts as a self-organizing mechanism on the membrane surface, minimizing the combined bending energy of the uppermost interface and the elastic strain energy stored in the inner layers. The control of the wrinkle morphology (e.g., the wavelength of the fluctuation) described herein can be achieved by adjusting the mechanical properties of the constituent layers of the substrate (e.g., Young's modulus), the thickness of the layers, and / or the immersion time of the swelling-inducing organic solvent. This method is in contrast to the typical high-intensity swelling methods of homogeneous elastomeric films (i.e., single natural layers) reported in the prior art, which are limited by their inability to obtain ordered wrinkled surfaces on a large scale because the large strains imposed by the solvent are conducive to the formation of wrinkles and folds (e.g., see J. Rodriguez-Hernandez, "Wrinkled Interfaces: Exploiting Surface Instabilities to Pattern Polymer Surfaces," Progress in Polymer Science, 2015, 1-41).

[0158] The uniformity of the undulation wavelength over the substrate surface (i.e., λ ~ 7.5 μm for the parameters reported here) is facilitated by a selective anti-adhesion pre-treatment of the wafer, which effectively creates strong constraints on the periphery of the carrier wafer and, therefore, a strong interface between the elastomeric base and the rigid surface of the wafer, which results in the compressive strain borne by the substrate during the conformal parylene deposition (L5) being directed towards the uppermost interface. The parylene layer L5 also provides a permanent frosting of the wrinkle morphology, which, in combination with the above, Figure 6As shown, and subsequently crack-free metal tracks of adjustable microscale thickness can be formed, the frosted wrinkle morphology is maintained, independent of the deposited metal thickness, as shown in Figure 7 (i.e., in contrast to direct metal deposition on PDMS), because parylene minimizes the thermal expansion of the interfacial surface during metal deposition. The latter is directly derived from the thermal conductivity of parylene (α = 3.5x10 -5 K -1 ), which are respectively greater than the thermal conductivity of PDMS (α=2x10 -4 K -1 ) is an order of magnitude higher and is closer to the thermal conductivity of metals, such as aluminum or titanium used here (α = 20x10 -6 K -1 and α=8.5x10 -6 K -1 ).

[0159] It was observed that conventional uniform anti-adhesion treatment across the entire carrier wafer resulted in non-isotropic and non-uniform wrinkled surface morphology across the entire substrate (i.e., wrinkled and non-wrinkled regions of the substrate had fluctuations of variable wavelength), while significant deviations from the currently described methods or substrate compositions were observed, such as the exclusion of n-methyl-2-pyrrolidone treatment and / or one layer of a multilayer substrate structure, which negated the wrinkled morphology and instead resulted in smooth or cracked surface morphology of the PDMS substrate.

[0160] According to the teachings provided by this disclosure, those skilled in the art will understand that the following combinations:

[0161] (i) a selective anti-adhesion treatment applied to the carrier substrate, whereby a peripheral region of the substrate provides a strong adhesion region and a region of the substrate within the peripheral region provides an anti-adhesion core region,

[0162] (ii) an elastic gradient provided by successive thin film layers of PDMS (L1 and L2) with decreasing ratios of crosslinker to base material,

[0163] (iii) placing the carrier substrate in an organic solvent for a period of time to induce swelling in those PDMS layers, which is constrained by the strong peripheral adhesion forces to the carrier substrate and the overlying parylene layer (L3),

[0164] (iv) applying another PDMS layer (L4) to provide a "hard-soft" interface with the previous parylene layer (L3), and

[0165] (v) diffusion of the solvent during vacuum deposition of the subsequently applied parylene film layer (L5),

[0166] This results in a permanent microscale wrinkled surface morphology in the subsequently applied polyparaxylene film layer (L5). This in turn enables a layer of conductive material to be formed on the microscale wrinkled surface of the polyparaxylene, so that the conductive material has a microscale wrinkled surface morphology consistent with the wrinkled polyparaxylene layer (L5). Therefore, the process provides the basis for a bottom-up wafer-scale manufacturing process for producing flexible structures, which flexible structures include multiple elastic material film layers and at least one layer of micro-wrinkled conductive material, which in turn enables the use of photolithography technology to first pattern and precisely align multiple micro-wrinkled conductive layers and secondly produce adjustable microstructure layers. As will be further described below, the above-mentioned process features (i)-(v) can support the generation of multiple wrinkled polyparaxylene layers within the device structure. Based on this teaching, those skilled in the art will be able to find variations of the specific embodiments and examples provided herein in terms of process parameters, layer thickness and elasticity through routine experimentation, which provides useful results for specific practical applications of the present invention.

[0167] The functionalized surface morphology provides mechanical relief from large compressive and strain stresses due to the increased effective surface area provided by the wrinkles. This allows the membrane to bend up to 180° without failure, as shown in Figure 12 Any induced crack formation, e.g. Figure 13 As exemplified in

[15] , where a film is intentionally damaged by a sharp blade, cracks are blocked and retained only within a few micrometers around the damage location because the distribution of the corrugations effectively prevents any propagation of the metal discontinuity, confined to a single plane, until a perpendicular wavefront is reached. This behavior contrasts with conventional metallization processes on smooth, planar surfaces of PDMS (typically confined to the nanometer scale), where cracks originating from localized stress points propagate across the film.

[0168] The successful development of flexible wireless LC sensors based on PDMS requires the realization of high-quality (i.e., low resistivity) thick (t ≥ 1 μm) metal tracks for the sensor inductive structure in order to provide the sensor with a sufficiently high quality factor (i.e., a 1 / R dependence of the quality factor) to enable wireless readout via inductive coupling, and the tracks to exhibit resilience to failure (i.e., cracking) during deposition, separation from the carrier wafer, bending, and compressive deformation.

[0169] like Figure 8 As shown, increased metal thickness leads to increased return loss magnitude |S 11 | (i.e. increased wireless readout distance), which is due to the higher quality factor provided by the thicker metallization. In addition, the resistivity of the corrugated aluminum metallization was found to be uniform across the entire film, with a very low value of ρ = (10.7 ± 1.6) 10 -8Ω.m, is practically the same as that obtained when aluminum is deposited directly on a silicon wafer (i.e. SiO2 / Si), in which case the value is ρ-=(10.4±2.1)10 -8 Ω.m.

[0170] Figure 5C An interlayer microstructuring process is shown, which, compared to the prior art, enables the formation of an embedded thin-film microstructured layer directly from the bottom up on a thin-film PDMS substrate without the need to separate, move, reposition, and laminate the substrate, for example, from a silicon etching mold as in the prior art.

[0171] As shown, a thin film 1:10 PDMS layer (L8) microstructured with an equiaxed frustum array is formed directly on top of the first inductor layer (L6 / L7) by:

[0172] 5C-(i) A crack-free sacrificial photoresist mold film having a negative image of the microstructure is selectively formed on the central region (ie, within the peripheral region 320 ) of the carrier substrate.

[0173] - To counteract the thermal expansion to which PDMS is subjected due to its elastic properties, which may lead to severe crack formation in the sacrificial photoresist layer due to thermal mismatch (which hinders preferential dissolution and expulsion of the sacrificial mold), thermal activation of the photoresist layer, and crosslinking of the encapsulating PDMS layer in the next step, it can be optimized by lowering the temperature of the wafer to ambient conditions within 105 minutes after each thermal crosslinking and keeping the wafer in an angled position during the cooling period ( Figure 15 a), Successfully produced crack-free sacrificial photoresist micro-mold.

[0174] 5C-(ii) The sacrificial layer is encapsulated with a thin-film spin-coated PDMS layer (L8 / L9), optimizing thermal PDMS cross-linking and forming two exposed reservoir regions and two strongly bonded unstructured regions at the periphery of the wafer (as a result of the aforementioned selective anti-adhesion process).

[0175] 5C-(iii) The sacrificial photoresist layer on the two reservoirs is preferentially dissolved by capillary forces using, for example, n-methyl-2-pyrrolidone (or any other similar biocompatible organic solvent that allows the photoresist to dissolve and has a low swelling strength for PDMS), with the wafer in a vertical position, and an embedded vertical position and embedded microstructured layer are formed. The solvent used can (in this case) be the same as the solvent used in step 5B-(ii). The sacrificial photoresist dissolves very quickly in organic solvents (~1-2 minutes). However, when another wrinkled parylene layer is subsequently to be formed (such as layer L11 in this example), the wafer can be immersed in the solvent for an extended period of time, for example 12 hours, to perform the same function as step 5B-(ii) with respect to wrinkling layer L5. This will be discussed further below.

[0176] 5C-(iv) Thermal drying and dehydration.

[0177] The formation of the embedded microstructured layer is achieved by encapsulating the sacrificial photoresist (PR) containing the microstructured cavities via a thin film spin-coated PDMS layer (L8 / L9). Figure 14 As shown, this is achieved by a sacrificial layer on the substrate (package, Figure 14 ) is selectively photolithographically formed into a PR sacrificial layer in the center (i.e., step 5C-(i) above). Meanwhile, in the subsequent step (i.e., step 5C-(ii), PDMS encapsulation), the two side areas of the substrate (exposed reservoir area, Figure 14 ) in order to serve as reservoir areas (step 5C-(ii), exposed reservoir areas). The latter is facilitated by using a polyimide (PI) adhesive film on these areas, which acts as a temporary mask during the spin coating of the encapsulation layer. In contrast, the top and bottom areas of the substrate (step 5C-(ii), strongly bonded unstructured areas) remain unshielded and therefore unstructured in order to allow strong and uniform bonding of the PDMS substrate (L7) to the encapsulation PDMS layer (L8) ( Figure 14 , unstructured bonding areas). In addition, at this stage, the surface of the PDMS substrate L7 is treated with O2 plasma for 30 seconds to enable the encapsulating PDMS layer (L8) to firmly bond to the underlying PDMS substrate (L7). During this treatment stage, the temperature of the O2 plasma chamber is kept below 53°C, as higher temperatures have been found to cause crack formation in the sacrificial photoresist mold. Thereafter, the PDMS encapsulation layer (L8 / L9) is spin-coated and optimally thermally cross-linked at 22 minutes / 90°C (instead of the 35 minutes / 100°C recommended by Sylgard 184, which has been found to cause undesirable cracking of the sacrificial photoresist mold), and the above process is used to effectively coat all parts except the two shielded reservoir areas (step 5C-(ii), optimal cross-linking).

[0178] Because this PDMS layer L8 is deposited directly on the surface of the obtained now hydrophilic PDMS membrane substrate L7 (due to the O2 plasma), it allows a very strong bond between the two layers since the uncured liquid diffuses in the PDMS substrate L7 at the interface during the thermal cross-linking of the deposited elastomer.

[0179] Dissolution of the sacrificial photoresist mold was achieved by first removing the temporary PI mask at the reservoir and placing the wafer in a bath of n-methyl-2-pyrrolidone for 12 h (see 5C-(iii) above), followed by Figure 14 As shown, through Figure 15 Capillary forces, shown in c, preferentially drain in one direction through the two exposed reservoirs. The latter is achieved by placing the wafer in a vertical position at an angle to the solvent bath, while also reducing the time required for the drainage process from approximately 3 hours to approximately 5 minutes by intermittently depositing small amounts of solvent on the top reservoir using a pipette. Figure 15 As shown in b. Figure 15 As shown in b, the expulsion of the dissolved sacrificial micromold photoresist layer is accelerated (as low as 5 minutes) by holding the wafer in a vertical position and repeatedly depositing small amounts of solvent using a pipette onto one reservoir, now at the top, which is driven by capillary forces to a second reservoir, now at the bottom (see Figure 14 a: dissolution direction). Under the influence of capillary forces, the liquid preferably flows uninterruptedly through the entire microchannel in one direction (see Figure 15 , c1→c3), because the microstructured layer acts as an isotropic and symmetrical microfluidic structure due to the hydrophobic nature of the PDMS. In contrast, simply leaving the wafer in this position for drainage similarly drains the dissolved sacrificial layer to the bottom reservoir over time, but much more slowly (>3 hours) and with lower batch-to-batch yields (due to occasional small amounts of dissolved sacrificial layer being trapped within the structure).

[0180] This is followed by a thorough deionized (DI) water bath and heat and vacuum drying in a desiccator to ensure that any trapped liquid residues are expelled (step 5C-(iv)). Thus, in contrast to current soft lithography processes, the reservoir region allows direct access to the embedded sacrificial layer, enabling the bottom-up formation of a thin-film embedded microstructured layer directly on the substrate, while due to the strong bonding of the encapsulation layer to the unstructured peripheral region of the substrate, during this processing stage, the planarity of the film is ensured by the unstructured outer boundary of the substrate, which firmly holds the film in place.

[0181] In contrast to conventional soft lithographic microstructuring based on potassium hydroxide (KOH) etching of Si molds followed by encapsulation with PDMS, which results in frustums with a fixed sidewall angle of 54.7°, direct photopatterning of the sacrificial layer renders the sidewall angle of the frustum tunable, depending on the UV exposure, e.g. Figure 16 As shown, this can provide additional design freedom for the compressive deformation experienced by the frustum and, therefore, improve the ability to tune the mechanical sensitivity of the sensor to compression. For the developed sensor, a sidewall angle of ~74° was chosen because it results in a reduction of the effective Young's modulus of the frustum by approximately 3 orders of magnitude, thereby improving the sensitivity to deformation.

[0182] like Figure 3D and 3E As shown, the second aligned inductor layer wrinkle metallization process, drying, packaging and stripping process include the following steps.

[0183] 3D-(i) Forming the aligned second inductor layer and the corrugated surface morphology (as described above) by repeating steps (iii)-a, 5B-(iii)-b and 5B-(iv) of the micro-corrugation process Figure 3B and 5B ) to produce layers L10 to L12, including a corrugated parylene layer L11 and a second corrugated metal layer L12, providing that (a) micro-alignment photolithographic design features are utilized included in a photomask for the first inductor layer, and therefore included in the pattern of the first inductor layer itself, and in a photomask for the second inductor layer, so that the first and second inductor structures of the sensor array can be precisely and accurately aligned on the wafer, and preferably, (b) small through-holes are formed at the edges of the encapsulation reservoir areas of the microstructured layers (L8 / L9) prior to metal deposition. Figure 10 Examples of highly aligned multi-coil inductors with track widths as low as 25 μm obtained by this process are shown.

[0184] 3E-(ii) Thermal drying and dehydration.

[0185] 3E-(iii) encapsulating the second inductor layer with a thin film PDMS layer L13 by spin coating and thermal cross-linking; and

[0186] 3E-(iv) The film was cut with a razor blade across the active release area and the film was manually peeled off using the sensor array.

[0187] Referring to 3D-(i) above, the wrinkling mechanism of layer L11 / L12 can be further explained as follows.

[0188] The corrugated second inductor metallization (L12) exploits the same mechanism of low-intensity swelling of layers L1 and L2, as the mechanical compressive instability actually originates there when immersed in an organic solvent for the same set time (in step 5C-(iii)) and during the vacuum deposition of the L11 parylene. Layers L1, L2, L3, and L4, along with their strong peripheral adhesion, play the same role in the corrugation of layer L11 as they do in the corrugation of layer L5.

[0189] In this case, as previously described, immersion in an organic solvent in step 5C-(iii) again leads to low-intensity swelling of the contracted L1-L2 PDMS layer between the L3 parylene layer and the rigid carrier wafer. When the L11 parylene layer is deposited, it similarly undergoes a sudden volume change as the swollen state relaxes again under vacuum due to solvent diffusion. Due to the latter and the complex gradient mechanical properties (elasticity) of the membrane structure, an osmotic pressure is similarly generated across the bulk of the membrane, leading to in-plane equibiaxial compressive strain at the interface of the uppermost (L10) PDMS layer. This in turn leads to membrane warping and, consequently, a labyrinthine wrinkle pattern on the surface. The parylene deposition (L11) simultaneously contributes to the stabilization (frosting) of the wrinkle formation, which is independent of the L12 metal micro-thickness deposition, as previously described.

[0190] If L10 (1:5 PDMS) is omitted from the film structure and the L11 parylene layer is deposited directly onto the surface of L9 (1:10 PDMS), this instead leads to the dissipation of the surface wrinkle morphology (i.e., a planar surface is produced instead) and severe crack formation occurs during metallization despite the introduction of the parylene coating L11. This is because the unconstrained (i.e., due to the exposed reservoir regions) microstructured layer effectively absorbs elastic shear (in-plane) buckling deformation at the interface during parylene deposition (L11) and also experiences enhanced compression in the normal axis at the isolated regions during metal deposition (L12). The introduction of L10 (1:5 PDMS) before depositing the parylene L11 layer restores the complex mechanical properties and structural stability of the film because it reflects the elastic gradient of layers L1 and L2 and also constrains the underlying microstructured layer, thus enabling the generation of the same frosted maze-like wrinkle morphology (i.e., λ ∼ 7.5 μm, for the process parameters described in this paper) at the surface film interface of L10-L11 before depositing the second inductor microscale metallization (L12) of the film as described previously.

[0191] This approach may further provide for the generation of the same corrugation morphology for a subsequent third metal inductor layer, if desired (and potentially for further inductor layers if the approach is similarly adapted).

[0192] It can be noted that the softer 1:10 PDMS (1.7 MPa) composition used for the microstructured layers (L8 and L9) enables the pressure sensor to have the best performance against compressive loads because the lower Young's modulus increases the deformation experienced by the microstructured layers L8 / L9, which in turn significantly improves the sensitivity of the sensor device. The encapsulation of the first inductor structure L6 with the 1:10 PDMS layer L7 was chosen to provide maximum adhesion to the L8 microstructured features and strong resilience (structural stability) to separation of shear forces when the sensor is subjected to dynamic force (tactile) loads (i.e., the features do not separate, and therefore the structure does not separate).

[0193] With further reference to the microscale wrinkling processes and mechanisms discussed herein, it is the inventor's understanding that the complex variability (gradient) in the mechanical elasticity (stiffness) of adjacent layers of the membrane creates a "soft-hard" interface between them, such as L9 / L10, which in turn allows the mechanical compressive instability generated at L1 / L2 to propagate through the membrane, as previously described in detail, and selectively toward the upper interfaces, in this example L4 / L5 and L10 / L11, to permanently wrinkle prior to metallization.

[0194] In general, [see again: J. Rodriguez-Hernandez, "Wrinkled interfaces: exploiting surface instabilities to pattern polymer surfaces," Progress in Polymer Science, 2015, 1-41], a bonded (e.g., constrained to a rigid wafer) elastomeric system consisting of two components with gradient elastic moduli, such as a softer elastomeric substrate, serving as a "base," and a more rigid layer deposited on top, behaving as a solid "skin" at their interface, forms a simple bilayer elastomeric system. When mechanical stress exceeds a critical load value (e.g., introduced by stretching or heating, applying compressive stress, cooling, or by solvent evaporation and osmotic pressure (if the bulk material swells)) and the applied stress is subsequently removed, the membrane "base" is allowed to relax, resulting in a wavy structure called a "wrinkle." In order to drive the interfacial surface to bend out of plane under the influence of mechanical stimuli and promote wrinkle formation, a stiffness difference between the "skin" and the "base" at the polymer interface is required. The "skin" and "base" layers change their dimensions differently due to their different mechanical properties (Young's modulus and Poisson's ratio), thus generating compressive forces at the interface, leading to buckling.

[0195] The LC sensor layer structure described herein can effectively be viewed as a collection of such "soft-hard" and "hard-soft" interfaces created by adjacent layers of the film, namely, L1 / L2, L2 / L3, L3 / L4, L4 / L5, L5 / L7, (L7-L9)* / L10, and L10 / L11 (*L7-L9 are all 1:10 PDMS here, so L7-L9 can be considered as a single effective layer). During vacuum deposition of the L5 or L11 p-xylene layer, respectively, these interfaces continuously promote compressive mechanical buckling instabilities, which are caused by the shrinkage of the swollen L1 and L2 layers bonded to the carrier wafer, across the film and toward the uppermost interface. Regarding the L8 / L9 microstructured layer, as previously mentioned, the L10 layer is also required to constrain the L8 / L9 layer, otherwise the wrinkles will disappear. If the microstructured 1:10 PDMS layers (L7-L9) are omitted entirely and the L10 (1:5 PDMS) / L11 (polyparaxylene) layers are simply deposited in their place, which are effectively exact repeats of layers L4 / L5, the wrinkles reappear as expected, provided the wafer is placed in an organic solvent bath before introducing the required low-intensity swelling.

[0196] Spatial and angular self-alignment of the two inductor layers of the sensor is achieved by exploiting the optical transparency of PDMS and parylene (i.e., of the sensor structure) and by using typical photolithographic micro-alignment design features at typically two locations on the substrate (e.g., four microcrosshairs), which are deposited simultaneously with the first inductor layer (i.e., as part of the L6 layer pattern) and, prior to metallization, are used when a typical UV mask aligner device positions a photomask having the same features for the second inductor layer L12 during the photolithographic formation of the corresponding stripped photoresist of the second inductor layer, respectively. This is achieved by simultaneously optically aligning the micro-alignment features of the first inductor layer L6 with the features of the photomask of the second inductor layer L12 at two locations during processing (in an industrial setting, this is automated) to ensure wafer-scale alignment (i.e., for correct angular and spatial alignment on the wafer).

[0197] Figure 17 The micro-alignment features are depicted in , and a typical photolithographic bottom-up alignment process allows for very low misalignment errors on the order of < 3 μm (depending on the UV mask aligner equipment and the photomask).

[0198] The latter also relates to the advantages of the present invention's bottom-up process over typical prior art techniques involving separate layer development and mechanical alignment, thus allowing the present invention to provide high throughput of wafer-scale sensor arrays.

[0199] like Figure 18As shown, in the absence of the aforementioned through-holes, the embedded microstructured layer may partially swell during the metal deposition of the second inductor layer due to the high vacuum of the sputtering machine. The through-holes allow the trapped air within the embedded microstructured layer to escape, and the second inductor layer can be metallized by simply cutting off a very small film portion (e.g. <1x1 mm) with a blade before metallization of the second inductor layer at the edge (interface) of the encapsulated microstructured area and the now encapsulated reservoir area. 2 ) to achieve it.

[0200] The two floating inductors of the LC sensor structure form an LC system with highly coupled inductance and capacitance because they are aligned and in close proximity (i.e., the film thickness between them results in a very high coupling coefficient k 传感器 ~1).

[0201] As described herein, the present invention enables wafer-scale production of highly flexible thin-film devices incorporating highly flexible conductive layers with very precisely aligned multiple conductive patterns on different layers, as exemplified by the inductive structure of the LC wireless sensor described herein, and also enables the production of sensor microstructures with parameters that can be adjusted and varied across an array of devices.

[0202] Improvements and modifications may be made without departing from the scope of the present invention, which is defined by the appended claims.

Claims

1. A method for producing a flexible structure comprising a plurality of film layers of elastic material and at least one layer of micro-corrugated conductive material, characterized in that: The method includes: a) applying a selective anti-adhesion treatment process to the carrier substrate, whereby a peripheral region of the substrate provides a strong adhesion region and a region of the substrate within the peripheral region provides an anti-adhesion core region; b) forming a first plurality of continuous PDMS thin film layers on a carrier substrate, each of the first plurality of continuous PDMS thin film layers having a smaller ratio of cross-linker to base material and thus having a higher Young's modulus than a previous one of the first plurality of continuous PDMS thin film layers; c) forming a first polyparaxylene thin film layer on the finally formed first plurality of continuous PDMS thin film layers; d) placing the carrier substrate in an organic solvent for a first period of time to induce swelling in the first plurality of continuous PDMS thin film layers; e) forming a first additional PDMS thin film layer on the first parylene thin film layer; f) forming a second parylene thin film layer on the first additional PDMS thin film layer by vacuum deposition, wherein a permanent microscale wrinkled surface morphology is generated due to diffusion of an organic solvent from the first plurality of consecutive PDMS thin film layers during said vacuum deposition, thereby providing a first microscale wrinkled parylene layer; g) forming and patterning a first layer of conductive material on the first microscale corrugated parylene layer such that the first patterned conductive material has a microscale corrugated surface morphology consistent with that of the first microscale corrugated parylene layer, thereby providing a first microscale corrugated conductive pattern layer.

2. The method according to claim 1, wherein The selective anti-stick treatment process applied to the carrier substrate includes a process of making a peripheral region of the substrate hydrophilic and a central region of the substrate within the peripheral region hydrophobic.

3. The method according to claim 2, wherein: The central area of the substrate is made hydrophobic by drying a thin anti-adhesion layer, such as trichloro(1H,1H,2H,2H-perfluorooctyl-silane).

4. The method according to claim 2 or 3, wherein: The peripheral area of the substrate is made hydrophilic by selective O2 plasma etching.

5. The method according to any one of claims 1 to 4, wherein: The organic solvent and the first time period are selected to induce swelling in the first plurality of continuous PDMS thin film layers, the swelling resulting in the permanent microscale wrinkled surface morphology in the first additional PDMS thin film layer.

6. The method according to claim 5, wherein: The organic solvent and the first time period are selected so as to obtain a desired wrinkle fluctuation wavelength of the permanent microscale wrinkled surface morphology produced in the first additional PDMS thin film layer.

7. The method according to any one of claims 1 to 6, wherein: The organic solvent is n-methyl-2-pyrrolidone, dioxane, dimethyl carbonate, pyridine or dimethylformamide.

8. The method according to any one of claims 1 to 7, wherein: The Young's modulus of the first additional PDMS thin film layer is equal to the Young's modulus of the last formed layer of the first plurality of consecutive PDMS thin film layers.

9. The method according to any one of claims 1 to 8, wherein: The carrier substrate is a silicon wafer.

10. The method according to any one of claims 1 to 9, wherein: The second microscale corrugated conductive pattern layer is formed by: placing the carrier substrate in an organic solvent for a second period of time to again induce swelling in the first plurality of continuous PDMS thin film layers; forming one or more additional PDMS thin film layers on the uppermost layer of the aforementioned structure; forming a third parylene thin film layer on top of the additional PDMS thin film layers by vacuum deposition, wherein a permanent microscale wrinkled surface morphology is generated due to diffusion of an organic solvent from the first plurality of consecutive PDMS thin film layers during said vacuum deposition, thereby providing a second microscale wrinkled parylene layer; as well as A second layer of conductive material is formed and patterned on the second microscale corrugated parylene layer such that the second patterned conductive material has a microscale corrugated surface morphology consistent with that of the first microscale corrugated parylene layer, thereby providing a second microscale corrugated conductive pattern layer.

11. The method according to any one of claims 1 to 10, further comprising forming one or more additional PDMS layers on the first microscale corrugated conductive pattern layer, and patterning one or more of the one or more additional PDMS thin film layers to create a 3D microstructure.

12. The method according to claim 11, wherein The 3D microstructure is formed by photolithography.

13. The method according to claim 12, wherein: The patterning of the 3D microstructures provides an array of individual 3D microstructures corresponding to an array of individual devices.

14. The method according to claim 13, wherein One or more of the individual 3D microstructures has one or more physical parameters that are different from one or more of the other individual 3D microstructures.

15. The method according to any one of claims 11 to 14, wherein The second micro-scale corrugated conductive pattern layer is formed by the method according to claim 10.

16. The method according to claim 15, wherein The one or more additional PDMS thin film layers of claim 10 include one or more additional PDMS thin film layers used to create the 3D microstructure.

17. The method according to claim 16, wherein Placing the carrier substrate in the organic solvent for the second period of time to reinitiate swelling in the first plurality of continuous PDMS thin film layers also dissolves the photolithographic photomask used to pattern the 3D microstructure.

18. The method according to any one of claims 11 to 17, wherein The 3D microstructure comprises an array of individual frustum arrays, and each of the first and second microscale corrugated conductive pattern layers comprises an array of individual inductive structures, and wherein each inductive structure of each microscale corrugated conductive pattern layer is aligned with a corresponding frustum array of the 3D microstructure and a corresponding inductive structure of another microscale corrugated conductive pattern layer to provide an array of individual devices that can be used as wireless LC sensors.

19. The method according to claim 18, wherein The sidewall angles of the frustum array are determined by UV exposure during the photolithographic formation of the 3D microstructures.

20. The method according to any one of claims 1 to 19, wherein: Each layer of conductive material is patterned using photolithography.

21. The method according to any one of claims 1 to 20, wherein: Patterning of each layer of conductive material provides a separate array of conductive structures corresponding to a separate array of devices.

22. The method according to claim 21, wherein One or more of the individual conductive structures has one or more physical parameters that are different from one or more of the other individual conductive structures.

23. The method according to any one of claims 1 to 22, wherein: Each layer of conductive material is a metal layer.

24. The method according to claim 23, wherein Each metal layer includes one or more of titanium, aluminum, chromium, gold, silver, copper, tungsten, platinum, and lead.

25. The method according to claim 23, wherein Each metal layer includes a first layer of titanium or chromium and a second layer of aluminum.

26. A flexible structure comprising a plurality of thin film layers of elastic material and at least one layer of micro-corrugated conductive material, characterized in that: The structure includes: a first plurality of continuous PDMS thin film layers, each of the first plurality of continuous PDMS thin film layers having a smaller ratio of cross-linker to base material and thus having a higher Young's modulus than a previous one of the first plurality of continuous PDMS thin film layers; a first parylene film layer; a first additional PDMS thin film layer on the first parylene thin film layer; a second parylene thin film layer having a permanent microscale corrugated surface morphology on the first additional PDMS thin film layer, providing a first microscale corrugated parylene layer; A first layer of conductive material on the first micro-scale corrugated parylene layer has a micro-scale corrugated surface morphology consistent with the first micro-scale corrugated parylene layer, providing a first micro-scale corrugated conductive pattern layer.

27. The structure of claim 26, wherein: The Young's modulus of the first additional PDMS thin film layer is equal to the Young's modulus of the last formed layer of the first plurality of consecutive PDMS thin film layers.

28. The structure according to claim 26 or 27, further comprising: One or more additional PDMS thin film layers on the uppermost layer of the aforementioned structure; a third parylene film layer having a permanent microscale wrinkled surface morphology, providing a second microscale wrinkled parylene layer; as well as A second layer of conductive material is formed on the second microscale corrugated parylene layer so that the second patterned conductive material has a microscale corrugated surface morphology consistent with the first microscale corrugated parylene layer, thereby providing a second microscale corrugated conductive pattern layer.

29. The structure of any one of claims 26 to 28, further comprising one or more additional PDMS layers on the first microscale corrugated conductive pattern layer, one or more of the additional PDMS layers being patterned to provide a 3D microstructure.

30. The structure of claim 29, wherein: The patterning of the 3D microstructures provides an array of individual 3D microstructures corresponding to an array of individual devices.

31. The structure of claim 30, wherein: One or more of the individual 3D microstructures has one or more physical parameters that are different from one or more of the other individual 3D microstructures.

32. The structure of any one of claims 29 to 31 , comprising a second micro-scale corrugated conductive pattern layer according to claim 28.

33. The structure of claim 32, wherein: The one or more additional PDMS thin film layers of claim 28 include one or more additional PDMS thin film layers that provide the 3D microstructure.

34. A structure according to any one of claims 29 to 33, wherein The 3D microstructure comprises an array of individual frustum arrays, and each of the first and second microscale corrugated conductive pattern layers comprises an array of individual inductive structures, and wherein each inductive structure of each microscale corrugated conductive pattern layer is aligned with a corresponding frustum array of the 3D microstructure and a corresponding inductive structure of another microscale corrugated conductive pattern layer to provide an array of individual devices that can be used as wireless LC sensors.

35. The structure of claim 34, wherein: The sidewall angles of the frustum array are determined by UV exposure during the photolithographic formation of the 3D microstructures.

36. A structure according to any one of claims 26 to 35, wherein: Each layer of conductive material is patterned using photolithography.

37. A structure according to any one of claims 26 to 36, wherein Patterning of each layer of conductive material provides a separate array of conductive structures corresponding to a separate array of devices.

38. The structure of claim 37, wherein: One or more of the individual conductive structures has one or more physical parameters that are different from one or more of the other individual conductive structures.

39. A structure according to any one of claims 26 to 38, wherein Each layer of conductive material is a metal layer.

40. The structure of claim 39, wherein: Each metal layer includes one or more of titanium, aluminum, chromium, gold, silver, copper, tungsten, platinum, and lead.

41. The structure of claim 40, wherein Each metal layer includes a first layer of titanium or chromium and a second layer of aluminum.

42. A device comprising one of the array of individual devices in the structure of claim 30 or 31.

43. A wireless LC sensor comprising one of the arrays of individual devices in the structure of claim 34 or 35.

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