A flexible tactile sensor array and its preparation method

Abstract

The present invention discloses a flexible tactile sensor array and a method for fabricating the same. The flexible tactile sensor array comprises a flexible substrate and a tactile sensor array. The tactile sensor array comprises N×M tactile sensing units and an interconnection network. The tactile sensing units are grid-shaped metal foils encapsulated by a conformal polymer, each grid-shaped metal foil being integrated with an in-situ hard micropillar array. The interconnection network comprises N perpendicularly intersecting row lines and M column lines, each insulated from the other by cross-insulating pads at the intersections. Both the row lines and the column lines are encapsulated by a conformal polymer. The flexible substrate and the tactile sensor array are integrally bonded. Based on the mechanism by which the resistance of a metal foil changes under pressure, the present invention constructs a structure in which in-situ hard micropillars squeeze the grid-shaped metal foil. This structure, combined with the in-situ hard micropillars and the interconnection network, forms a novel tactile sensor array with high stability, high sensitivity, low hysteresis, and high integration.

Description

Technical Field

[0001] The present invention relates to the field of tactile sensors, and in particular to a flexible tactile sensor array and a preparation method thereof. Background Art

[0002] With the continuous advancement of materials science and manufacturing technology, society and industry are placing new demands on the functionality, portability, and comfort of various electronic devices. Flexibility has become a key development direction for emerging electronic devices. Compared to traditional rigid electronic devices, bendability and deformability are key features of flexible devices. Their flexibility, elasticity, and ability to withstand arbitrary deformation hold great promise for applications in areas such as medical assistance, wearable health monitoring, and human-computer interaction.

[0003] Flexible tactile sensors can give robotic hands a human-like sense of touch and are key components for enabling intelligent robotic perception and human-machine interaction. These sensors convert tactile signals into electrical signals, enabling rapid and precise transmission of tactile information. By integrating multiple tactile sensor units on a flexible substrate, an NxM flexible tactile sensor array can be formed. This array is connected as a whole via interconnects and, controlled by external circuitry, can simultaneously measure changes in contact pressure at multiple points within a defined area. This array holds enormous potential for application in fields such as medicine, industry, and entertainment. Flexible tactile sensors can be primarily categorized as resistive, capacitive, and piezoelectric based on their sensing mechanism. The operating principle of resistive tactile sensors can be summarized as follows: external contact pressure stimulation causes the sensor's resistance to change, resulting in a change in the output current or voltage, which in turn provides feedback on the magnitude of the contact pressure.

[0004] The core technology of resistive flexible tactile sensors lies in constructing a pressure-sensitive, variable resistance system. Current approaches include: 1) Using flexible pressure-sensitive conductive materials as the sensor's sensitive unit: Flexible pressure-sensitive conductive materials are made by curing powdered or granular conductive materials such as carbon black, carbon nanotubes, or graphene with silicone rubber, hydrogel, or other materials as the conductive phase, to form a flexible, stretchable, pressure-sensitive conductive composite material. Pressure changes the spacing between the conductive particles, which manifests as a change in resistance. 2) Using variable contact resistance as the sensitive unit: Bringing upper and lower electrodes with surface microstructures into contact generates contact resistance. Different pressures produce different contact states and, consequently, different contact resistances, which in turn can alter the output electrical signal.

[0005] Although the existing resistive flexible tactile sensors that use flexible pressure-sensitive conductive materials as the sensor sensitive unit can realize distributed pressure measurement, they have obvious shortcomings in sensitivity, hysteresis and integration. The resistive flexible tactile sensors that use variable contact resistance as the sensitive unit have bottlenecks in range and integration. Summary of the Invention

[0006] The object of the present invention is to provide a novel resistive flexible tactile sensor array with high stability, high sensitivity, low hysteresis and high integration and a preparation method thereof.

[0007] A flexible tactile sensor array comprises a flexible substrate and a tactile sensing array, wherein the tactile sensing array comprises N×M tactile sensing units and an interconnection line network connecting each tactile sensing unit;

[0008] The tactile sensing unit is a grid-shaped metal foil wrapped and encapsulated by a conformal polymer, and a hard micro-pillar array is integrated on the grid-shaped metal foil;

[0009] The interconnection network includes N vertically intersecting row lines and M column lines, with cross insulating pads provided at the intersections of the row lines and column lines. The row lines and column lines are both wrapped and packaged by a conformal polymer. The tactile sensor array is bonded to a flexible substrate.

[0010] In this invention, the flexible substrate is a thin, flexible substrate such as silicone rubber. N row lines and M column lines intersect perpendicularly to form an interconnected network. Intersections are insulated to prevent contact and disconnection between the rows and columns. Tactile sensing units are placed near these intersections, with one end connected to a row line and the other to a column line. These N x M tactile sensing units form an interconnected tactile sensing array via a shared network of row and column lines.

[0011] After the flexible substrate is bonded to the tactile sensor array, thanks to the substrate's flexibility, when contact pressure is applied to the tactile sensor unit, the pressure is amplified by the hard micropillars and squeezes the metal foil underneath. As the flexible substrate concave, the metal foil itself is stretched and bent, causing the metal foil's resistance to change. When the contact pressure is released, the flexible substrate's elasticity causes it to quickly rebound, restoring the shape and resistance of the metal foil. Therefore, during the process of applying and releasing contact pressure, the tactile sensor unit's resistance can respond quickly through the squeezing of the hard micropillars and the concave deformation and rapid rebound of the flexible substrate, thereby changing the output electrical signal and thus achieving the function of sensing pressure.

[0012] By scanning the interconnected network consisting of N row lines and M column lines through an external device, these N×M tactile sensing units can work simultaneously without affecting each other and independently sense pressure signals, thereby enabling the tactile sensor array to achieve the function of multi-point real-time measurement of distributed pressure.

[0013] Based on the mechanism of the change in the resistance of metal foil under pressure, the present invention constructs a structure of hard micro-pillar extruded grid-shaped metal foil, and uses this as the tactile sensing unit combined with an interconnection line network to construct a new tactile sensor array, resulting in a new resistive flexible tactile sensor array that combines the advantages of high stability, high sensitivity, low hysteresis, and high integration.

[0014] The flexible substrate is preferably a thin layer of PDMS (polydimethylsiloxane) with a thickness of preferably 200 microns. Other flexible and elastic materials such as silicone rubber, rubber, hydrogel, dielectric elastomer, etc. can also be selected to set off the tactile sensing array, increase the concave deformation space and the rapid rebound deformation function.

[0015] Preferably, the grid-shaped metal foil adopts a rectangular wave grid shape, and can also be designed as a straight grid shape or a wavy grid shape. The grid width is preferably 20 microns, and the grid spacing is preferably 80 microns.

[0016] The grid-shaped metal foil is composed of a single layer or multiple layers of metal wrapped and encapsulated by a conformal polymer. The thickness of each metal layer is 5 nanometers to 100 microns, and the thickness of the polymer wrapping is 2 microns to 200 microns.

[0017] Preferably, the metal is a chromium-gold double-layer metal, with a chromium thickness of 5 nanometers and a gold thickness of 100 nanometers.

[0018] Preferably, the polymer wrapping is made of PI (polyimide) with a thickness of 5 microns, or may be a polyethylene terephthalate polymer.

[0019] In the hard microcolumn array, the hard microcolumns are cuboids, cylinders or cones, preferably cuboids, with a size of preferably 20*20*35 microns. The material is preferably photocurable resin, and photocurable polymer or photoresist can also be selected.

[0020] Preferably, the cross insulation pad is PI (polyimide) with a thickness of 1-5 microns.

[0021] The present invention also provides a method for preparing a flexible tactile sensor array, comprising the following steps:

[0022] (1) Using PDMS liquid prepolymer, a flexible substrate with a specific thickness is prepared on a silicon wafer by spin coating and thermal curing;

[0023] (2) Using micro-nano manufacturing technology to prepare a tactile sensing array consisting of tactile sensing units and interconnection wire networks;

[0024] (3) The tactile sensor array is transferred and bonded to a flexible substrate using a water-soluble transfer technique, thereby finally obtaining the resistive flexible tactile sensor array.

[0025] In step (1), the spin coating can obtain a flexible substrate of varying thickness by controlling the rotation speed and time, or a flexible substrate of a specific thickness can be prepared using a mold.

[0026] In step (2), the micro-nano manufacturing technology includes photolithography and development technology, metal coating technology, and polymer dry etching technology.

[0027] The specific steps of step (2) are:

[0028] (2-1) Spin-coating a PI liquid prepolymer onto a silicon wafer and thermally curing the prepolymer to prepare a PI bottom layer;

[0029] (2-2) After photolithography, development, metal coating, and metal stripping on the PI substrate, a rectangular wave grid metal foil and an interconnection network are prepared on the PI substrate. During this process, the intersections of the interconnection network must be insulated;

[0030] (2-3) spin-coating a PI liquid prepolymer onto the silicon wafer and thermally curing the prepolymer to form a PI encapsulation layer. The PI is then dry-etched to conform to the rectangular grid metal foil and the interconnection network, thereby forming a sandwich structure.

[0031] (2-4) After hard micropillars were fabricated on the sandwich structure using photosensitive epoxy resin prepolymer via photolithography, the tactile sensing array was fabricated;

[0032] (2-5) The tactile sensing array is released from the silicon wafer by soaking in chemical reagents.

[0033] The specific steps of step (3) are:

[0034] (3-1) Using water-soluble tape, the released tactile sensor array is picked up flatly, with one side of the array completely adhered to the tape and the other side exposed. A thin layer of chromium and a thin layer of silicon dioxide are deposited on the exposed side;

[0035] (3-2) After the flexible substrate is placed in a plasma cleaner and treated with oxygen, the water-soluble tape with the tactile sensor array is quickly hot-pressed to form a whole with the flexible substrate. The flexible substrate is then placed in water to completely dissolve the water-soluble tape, thereby obtaining a flexible tactile sensor array.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. The flexible tactile sensor array of the present invention can integrate N×M tactile sensing units, each of which independently senses pressure signals, thereby enabling the tactile sensor array to achieve the function of multi-point real-time measurement of distributed pressure, with ultra-high sensitivity and the ability to sense extremely slight pressure;

[0038] 2. The hard micro-pillar design of the resistive flexible tactile sensor array of the present invention not only improves sensitivity but also ensures array integration;

[0039] 3. The metal materials and micro-nano manufacturing technology used in the resistive flexible tactile sensor array of the present invention make it have high stability and long life;

[0040] 4. The resistive flexible tactile sensor array of the present invention has a simple preparation method, low preparation cost, high integration, and few wiring and pins. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 1 is a general schematic diagram of a resistive flexible tactile sensor array according to an embodiment of the present invention;

[0042] Figure 2 This is a schematic diagram of the structure of a tactile unit according to an embodiment of the present invention;

[0043] Figure 3 This is a circuit diagram of an interconnection network according to an embodiment of the present invention;

[0044] Figure 4 This is an enlarged schematic diagram of the tactile sensing unit according to an embodiment of the present invention.

[0045] In the figure: 01 - flexible substrate, 02 - tactile sensor array, grid metal foil 21, hard micro-column array 22, row lines 23, column lines 24, cross insulating pads 25. DETAILED DESCRIPTION

[0046] The present invention will be described in further detail below with reference to the accompanying drawings and examples. It should be noted that the following examples are intended to facilitate understanding of the present invention and do not have any limiting effect on the present invention.

[0047] like Figure 1 As shown, a 4×3 flexible tactile sensor array is composed of two stacked layers, including, from bottom to top, a flexible substrate 01 and a tactile sensor array 02. The tactile sensor array 02 includes 4×3 tactile sensor units and a network of interconnects connecting each tactile sensor unit.

[0048] like Figure 2 As shown, the tactile sensing unit is a grid-shaped metal foil 21 wrapped and encapsulated by a conformal polymer, and a hard micro-pillar array 22 is integrated on the grid-shaped metal foil 21.

[0049] To facilitate understanding of the interconnection network, such as Figure 3 As shown, 4×3 rectangular wave-shaped grid-shaped metal foils 21 are connected to 4 row lines 23 and 3 column lines 24 to form an interconnection network. Cross insulation pads 25 are provided at the intersections of the row lines 23 and the column lines 24.

[0050] In this embodiment, the grid-shaped metal foil 21 adopts a rectangular wave grid shape, the material of the flexible substrate 01 is a 200-micron thick PDMS (polydimethylsiloxane) thin layer; the material of the polymer substrate and the polymer package is PI (polyimide), both with a thickness of 2.5 microns; the material of the cross-insulating pad 25 is PI with a thickness of 1 micron; the grid-shaped metal foil 21 and the interconnection line network adopt a double layer of chromium and gold, and 100 nanometers of gold are bonded to the polymer substrate through 5 nanometers of chromium; the material of the hard microcolumn array 22 is a photocurable resin, the shape is a rectangular parallelepiped, and the size is 20*20*35 microns.

[0051] Figure 4 The diagram shows the amplification principle of the tactile sensing unit. Under contact pressure, the hard micropillar array 22 amplifies the pressure and squeezes the grid-shaped metal foil 21 downward. As the flexible substrate 01 concaves, the grid-shaped metal foil 21 itself undergoes stretching and bending deformation, causing the resistance of the grid-shaped metal foil 21 to change. When the contact pressure is released, the elasticity of the flexible substrate 01 causes it to quickly rebound, restoring the shape and resistance of the grid-shaped metal foil 21. Therefore, during the application and release of contact pressure, the resistance of the tactile sensing unit can respond quickly through the squeezing of the hard micropillars and the downward deformation and rapid rebound of the flexible substrate, thereby changing the output electrical signal and thus achieving the function of sensing pressure.

[0052] One method for preparing the flexible tactile sensor array of the present invention is described below.

[0053] (1) Preparation of flexible substrate

[0054] A flexible substrate with a thickness of 200 microns was prepared on a silicon wafer by spin coating and thermal curing using PDMS liquid prepolymer. The spin coating can obtain a flexible substrate of varying thickness by controlling the rotation speed and time, or a flexible substrate of a specific thickness can be prepared using a mold.

[0055] (2) Preparation of tactile sensing array

[0056] Polyimide (PI) is spin-coated onto a clean glass sheet and cured to produce a 2.5-micron-thick PI film as a polymer substrate. Photolithography and development are performed on the PI substrate, and 5 nanometers of chromium are deposited. 100 nanometers of gold are then stripped off using acetone to create a rectangular grid-like metal foil and interconnect network lines on the PI substrate. PI is then spin-coated again and cured to produce a 1-micron-thick polymer package.

[0057] Photoresist is spin-coated on this structure and developed by photolithography. Excess PI is etched away using an ICP (inductively coupled plasma etcher) to create PI cross-insulation pads, and the photoresist is then removed using acetone. Photoresist is spin-coated on this structure and developed by photolithography, and 5 nanometers of chromium are deposited. 100 nanometers of gold are stripped off using acetone to create the column lines of the interconnect network. PI is spin-coated again and cured to create a 2.5-micron thick polymer package. Photoresist is spin-coated on the package and developed by photolithography, followed by 100 nanometers of aluminum as a barrier layer. Excess PI is etched away using ICP. A photocurable resin is spin-coated on this structure and developed by photolithography to create a hard micropillar array measuring 20*20*35 microns.

[0058] Finally, the substrate is placed in an alkaline developer to completely dissolve the aluminum. The substrate is then immersed in hydrofluoric acid, which dissolves the substrate surface, thereby releasing the fabricated tactile sensor array from the substrate.

[0059] (3) Bonding

[0060] The released tactile sensor array is picked up flatly using water-soluble tape, with one side of the array completely adhered to the tape and the other side exposed. A thin layer of chromium and a thin layer of silicon dioxide are deposited on the exposed surface. The flexible substrate is placed in a plasma cleaner and treated with oxygen. The water-soluble tape with the tactile sensor array is quickly hot-pressed to form a whole with the flexible substrate, and then placed in water to completely dissolve the water-soluble tape, thereby obtaining a flexible tactile sensor array.

[0061] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A flexible tactile sensor array, comprising a flexible substrate and a tactile sensor array, characterized in that: The tactile sensing array includes N×M tactile sensing units and an interconnection line network connecting each tactile sensing unit; The tactile sensing unit is a grid-shaped metal foil encapsulated by a conformal polymer, on which a hard micro-pillar array is integrated; the hard micro-pillars in the hard micro-pillar array are in the shape of a cuboid, cylinder or cone, and are made of a photocurable resin, a photocurable polymer or a photoresist; The interconnect network includes N vertically intersecting row lines and M column lines, with cross insulating pads provided at the intersections of the row lines and the column lines, and the row lines and the column lines are both encapsulated and wrapped by a conformal polymer; The tactile sensing array is bonded to a flexible substrate; The flexible tactile sensor array is prepared by the following preparation method: (1) Using PDMS liquid prepolymer, a flexible substrate was prepared on a silicon wafer by spin coating and thermal curing; (2) Spin-coating the PI liquid prepolymer on the silicon wafer and thermally curing it to obtain the PI bottom layer; after photolithography, development, metal plating and metal stripping on the PI bottom layer, a rectangular wave grid metal foil and an interconnection network are obtained on the PI bottom layer, and the intersections of the interconnection network must be insulated during this process; Spin-coating a PI liquid prepolymer onto the silicon wafer and thermally curing it to form a PI encapsulation layer. Dry-etching the PI to conform to the rectangular grid metal foil and the interconnection network, thereby forming a sandwich structure. The tactile sensor array is fabricated by photolithography using a photosensitive epoxy resin prepolymer on a sandwich structure to form hard micropillars. The tactile sensor array is then released from the silicon wafer by immersion in a chemical reagent. (3) Using water-soluble tape, the released tactile sensor array is picked up flatly, with one side of the array completely adhered to the tape and the other side exposed; a thin layer of chromium and a thin layer of silicon dioxide are deposited on the exposed surface; After the flexible substrate is placed in a plasma cleaner and treated with oxygen, the water-soluble tape with the tactile sensor array is quickly hot-pressed and bonded to the flexible substrate as a whole. The flexible substrate is then placed in water to completely dissolve the water-soluble tape, thereby obtaining a flexible tactile sensor array.

2. The flexible tactile sensor array according to claim 1, wherein: The material of the flexible substrate is polydimethylsiloxane.

3. The flexible tactile sensor array according to claim 1, wherein: The grid-shaped metal foil is in the form of a straight grid, a rectangular wave grid or a wavy grid.

4. The flexible tactile sensor array according to claim 1, wherein: The grid metal foil consists of a single or multiple metal layers encapsulated by a conformal polymer wrapper, with the thickness of each metal layer ranging from 5 nanometers to 100 microns.

5. The flexible tactile sensor array according to claim 1, wherein: The polymer material wrapped and packaged on the grid-shaped metal foil is polyimide, and the thickness of the polymer wrapping is 2 microns to 200 microns.

6. The flexible tactile sensor array according to claim 1, wherein: The cross insulation pad is made of polyimide with a thickness of 1-5 microns.

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