An ultra-thin flexible sensor based on edge support strategy and a preparation method thereof

Abstract

An edge support strategy-based ultrathin flexible sensor and a preparation method thereof belong to the field of micro-electro-mechanical systems and micro-nano processing technology, and include a two-layer structure: a lower layer is an edge-thickened ultrathin flexible substrate, and an upper layer is a serpentine sensing unit. First, an ultrathin film is coated on the surface of a substrate, and after patterning, an ultrathin substrate is obtained; second, a layer of PSPI film is spin-coated, and after patterning, an ultrathin substrate with a circular ring-shaped thickened edge is obtained; third, a metal film is deposited on the ultrathin substrate and the exposed surface of the hard substrate, and after spin-coating a positive photoresist and patterning, a serpentine sensing unit is obtained; finally, the sensor is obtained by immersing in water and peeling off from the surface of the substrate. The edge of the sub-micron-thickness ultrathin flexible sensor is thickened to micron thickness, without affecting the conformal fitting and signal acquisition capability of the ultrathin flexible sensor, the defects of easy breaking and difficult unfolding are overcome, and the reliability and operability of the sensor in the preparation and use process are improved.

Description

Technical Field

[0001] This invention belongs to the field of microelectromechanical systems and micro / nano fabrication technology, and relates to an ultrathin flexible sensor based on an edge support strategy and its fabrication method, which is suitable for human wearable vital sign signal monitoring. Background Technology

[0002] Ultrathin flexible electronic devices with micrometer / submicrometer thicknesses are characterized by their thinness, flexibility, and conformability, enabling them to adhere closely to complex surfaces such as human skin for high-quality signal acquisition and a seamless wearable experience. They hold broad application prospects in areas such as wearable vital sign monitoring. However, the mechanical strength of ultrathin flexible electronic devices decreases significantly with decreasing substrate film thickness, making them highly susceptible to breakage and tearing during fabrication and handling. Furthermore, micrometer / submicrometer thick films exhibit uncontrolled curling, folding, and adhesion under van der Waals forces, severely impacting the reliability and controllability of ultrathin flexible electronic devices during operation. Currently, the fabrication and manipulation of ultrathin flexible electronic devices are primarily achieved through transfer printing technology. This involves using a transfer stamp with controllable adhesion to transfer functional structures fabricated on a rigid substrate onto an ultrathin flexible substrate, or using a transfer stamp to transfer ultrathin flexible electronic devices fabricated on a rigid substrate onto a target surface. For example, Chinese invention patent CN109970023A discloses a method for manufacturing flexible microelectrodes. This method uses polydimethylsiloxane as a transfer stamp and a flexible substrate, directly transferring thin-film electrodes prepared on silicon wafers onto polydimethylsiloxane for use as flexible microelectrodes. However, due to the low structural strength of ultrathin flexible electronic devices, peeling using traditional physical transfer methods is prone to damage / tear. To address this issue, researchers have proposed pre-preparing a removable sacrificial layer on the surface of a rigid substrate to achieve non-destructive release of ultrathin flexible electronic devices from the substrate surface. For example, Chinese invention patent CN114397047B discloses a method for fabricating a sacrificial layer-assisted ultrathin flexible pressure sensor. Two functional thin films constituting the ultrathin flexible pressure sensor are prepared on a rigid substrate covered with a sacrificial layer. Then, the two ultrathin functional thin films are aligned and assembled by manipulating the rigid substrate. Finally, the sacrificial layer is dissolved to achieve non-destructive release of the assembled ultrathin flexible pressure sensor.

[0003] The aforementioned methods suffer from several drawbacks. The use of a sacrificial layer significantly increases the complexity of the fabrication process and introduces more complex process compatibility issues. For example, using readily soluble materials as the sacrificial layer conflicts with commonly used wet etching techniques. Furthermore, using materials that are not heat-resistant as the sacrificial layer cannot withstand the thermal processing required for most devices, or stress concentration problems may arise due to the mismatch between the thermal expansion coefficient of the sacrificial layer material and the substrate and functional structure in the device. Therefore, there is a need to develop a simple and universal method for fabricating ultrathin flexible sensors that significantly improves the reliability and controllability of the fabrication and operation processes without compromising the ultrathin characteristics of the device. Summary of the Invention

[0004] To address the problems of existing technologies, this invention proposes a method for fabricating ultrathin flexible sensors based on an edge support strategy. The proposed ultrathin flexible sensor features a thickened edge layer on its periphery. This not only protects the internal ultrathin film from tearing during sensor fabrication and manipulation but also provides support to prevent the ultrathin film from curling and folding. Compared to previously reported methods, the proposed ultrathin flexible sensor structure and its fabrication method overcome the defects of easy breakage / tearing and uncontrolled curling, folding, and adhesion during the fabrication and use of ultrathin flexible sensors without affecting their conformal bonding and signal acquisition capabilities. This effectively improves the reliability and controllability of ultrathin flexible sensors during fabrication and use.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for fabricating an ultrathin flexible sensor based on an edge support strategy includes the following steps: Step 1: A diluted photosensitive polyimide (PSPI) solution is coated on the surface of a rigid substrate 1, and a cured ultrathin film 2 is obtained by initial heating. Then, a circular patterned mask 3 is covered on the surface of the cooled ultrathin film 2 and exposed.

[0006] Furthermore, in step 1, the material of the rigid substrate 1 is selected from glass, silicon wafer or sapphire, with glass being preferred.

[0007] Furthermore, in step 1, the PSPI solution is a negative photoresist; the diluted PSPI solution is a mixture of a diluent and a PSPI solution, wherein the diluent is an N-methylpyrrolidone (NMP) solution, and the volume ratio of the two is 1:1 to 1:3, that is, 100 ml of PSPI solution is mixed with 100 to 300 ml of NMP solution, and the specific mixing ratio is determined according to the required final film thickness.

[0008] Furthermore, the coating methods for the PSPI solution in step 1 include, but are not limited to, spin coating, blade coating, and vapor deposition.

[0009] Furthermore, the heating and curing equipment in this step includes, but is not limited to, hot plates and ovens; the initial heating temperature is 110°C and the heating time is 5 minutes.

[0010] Step 2: The exposed ultrathin film 2 is sequentially immersed in the developer and fixer to be patterned, dried with nitrogen, and then completely heated to obtain a circular ultrathin substrate 4.

[0011] Furthermore, the developing solution mentioned in step 2 is cyclopentanone, and the developing time is 1~3 min; the fixing solution is propylene glycol methyl ether acetate, and the fixing time is 5 s.

[0012] Furthermore, the heating and curing equipment mentioned in step 2 includes, but is not limited to, hot plates and ovens; the complete heating temperature is 250°C and the heating time is 30 minutes.

[0013] Furthermore, the pattern of the ultrathin substrate 4 mentioned in step 2 is circular, and the thickness is less than 1 μm.

[0014] Step 3: Spin-coat a layer of PSPI solution onto the surface of the ultrathin substrate 4 and the exposed rigid substrate 1, and pre-heat to obtain a cured film 5. Then, cover the surface of the cooled film 5 with a circular pattern mask 6 and expose it.

[0015] Furthermore, in step 3, the PSPI solution is the undiluted negative photoresist from step 1.

[0016] Furthermore, the initial heating equipment and parameters in step 3 are the same as in step 1.

[0017] Step 4: The exposed thin film 5 is sequentially immersed in the developer and fixer for patterning, dried with nitrogen, and then fully heated to obtain an ultrathin substrate 7 with a thickened edge in a circular shape.

[0018] Furthermore, the patterning process and parameters in step 4 are the same as in step 2.

[0019] Furthermore, the equipment and parameters for achieving complete heating in step 4 are the same as in step 2.

[0020] Furthermore, in step 4, the edge-thickened ultrathin substrate 7 is composed of an ultrathin substrate 4 and an annular thin film on its outer side; the thickened edge is an annular thin film obtained by patterning the thin film 5, and the thickness of the annular thin film is greater than 5 μm.

[0021] Step 5: Deposit a metal thin film 8 on the surface of the edge-thickened ultrathin substrate 7 and the exposed hard substrate 1, then spin coat a layer of positive photoresist 9 on the surface of the metal thin film 8 and heat to cure it, then cover the cured positive photoresist 9 with a serpentine mask 10 and expose it.

[0022] Furthermore, the metal thin film material mentioned in step 5 is selected from gold, silver, copper, platinum, etc., with gold being preferred.

[0023] Furthermore, the positive photoresist heating and curing equipment mentioned in step 5 includes, but is not limited to, a hot plate and an oven; the heating temperature is 85°C and the heating time is 30 minutes.

[0024] Step 6: Immerse the exposed positive photoresist 9 in the developer solution for patterning, and then use the patterned positive photoresist 9 as a mask to immerse the metal thin film 8 in the etching solution for patterning to obtain the serpentine sensing unit 11. Then, immerse the remaining positive photoresist mask in acetone and ethanol solutions in sequence to dissolve and remove it.

[0025] Furthermore, the positive photoresist mentioned in step 6 is AZ703, and the developer is AZ400K; the metal etching solution is the etching solution corresponding to the metal thin film material selected in step 5.

[0026] Furthermore, the serpentine sensing unit 11 described in step 6 includes a central serpentine electrode, rectangular pads on both sides, and wires for connecting the electrode and the pads. The serpentine electrode is located in the circular non-thickened area of ​​the ultrathin substrate 7, the rectangular pads on both sides are located in the annular thickened area of ​​the ultrathin substrate 7, and the wires for connecting are located in the transition area connecting the circular non-thickened area and the annular thickened area of ​​the ultrathin substrate 7.

[0027] Step 7: Immerse the prepared ultrathin flexible sensor 12 based on the edge support strategy into water until it is released from the surface of the rigid substrate 1.

[0028] The ultrathin flexible sensor based on the edge support strategy prepared by the above method in this invention includes a two-layer structure: the lower layer is an ultrathin flexible substrate 7 with thickened edges, which is composed of an inner submicron thick circular ultrathin substrate 4 and an outer micron thick ring-shaped thickened film; the upper layer is a serpentine sensing unit 11, which is composed of serpentine electrodes, rectangular pads on both sides, and wires for connecting the electrodes and pads.

[0029] Compared with the prior art, the beneficial effects of the present invention are: The ultrathin flexible sensor fabrication method provided by this invention does not require a sacrificial layer. The fabricated ultrathin flexible sensor is simply immersed in water, where the thickened edge supports and pulls the internal submicron-thickness ultrathin substrate away from the rigid substrate surface, avoiding the process complexity and compatibility issues associated with using a sacrificial layer. Simultaneously, the micron-thickness annular thickened region of the ultrathin substrate 7 of the ultrathin flexible sensor fabricated by this invention provides support and protection for the internal submicron-thickness circular ultrathin region without affecting its conformal capability. This overcomes the defects of ultrathin flexible sensors, such as easy breakage and tearing, as well as uncontrolled curling, folding, and adhesion, during fabrication and use, effectively improving the reliability and controllability of the ultrathin flexible sensor during fabrication and use. Furthermore, the sensor pads of the ultrathin flexible sensor fabricated by this invention are located on the thickened edge, which avoids damage and tearing of the ultrathin film caused by wiring on the fragile internal ultrathin film. Attached Figure Description

[0030] Figure 1 A schematic diagram of the fabrication and exposure of an ultrathin PSPI film on a rigid substrate surface; Figure 1 a and 1b are a three-dimensional schematic diagram and a cross-sectional schematic diagram in the thickness direction of an ultrathin PSPI film prepared on a rigid substrate surface and exposed thereon, respectively.

[0031] Figure 2 A schematic diagram illustrating the patterning of an ultrathin PSPI film into a circular ultrathin substrate; Figure 2 a and 2b are respectively three-dimensional schematic diagrams and thickness-direction cross-sectional schematic diagrams of an ultrathin substrate patterned as a circle using an ultrathin PSPI film.

[0032] Figure 3 A schematic diagram illustrating the fabrication of a PSPI thin film on a rigid substrate and exposure by covering it with a photomask; Figure 3 a and 3b are respectively three-dimensional schematic diagrams and thickness direction cross-sectional schematic diagrams of preparing a PSPI thin film on a rigid substrate surface and exposing it by covering it with a mask.

[0033] Figure 4 A schematic diagram illustrating the patterning of a PSPI thin film as a ring-shaped thickened edge connected to an ultrathin substrate; Figure 4 a and 4b are respectively a three-dimensional schematic diagram and a thickness direction cross-sectional schematic diagram of the PSPI thin film patterned as a ring-shaped thickened edge connected to an ultrathin substrate.

[0034] Figure 5 A schematic diagram illustrating the deposition of a gold film, spin-coating of photoresist, and exposure on an ultrathin substrate with thickened edges; Figure 5 5a and 5b are respectively three-dimensional schematic diagrams and thickness direction cross-sectional schematic diagrams of depositing a gold thin film, spin-coating photoresist, and exposing the surface of an ultra-thin substrate with thickened edges.

[0035] Figure 6 A schematic diagram illustrating the patterning of a thin gold film on the surface of an ultrathin substrate with thickened edges into a snake-shaped sensor; Figure 6 a and 6b are respectively three-dimensional schematic diagrams and thickness direction cross-sectional schematic diagrams of patterning a gold thin film on the surface of an ultrathin substrate with thickened edges into a snake-shaped sensor.

[0036] Figure 7 A schematic diagram of an ultrathin flexible sensor based on an edge-support strategy, fabricated for release from a rigid substrate surface in water; Figure 7 a and 7b are respectively a three-dimensional schematic diagram and a thickness-direction cross-sectional schematic diagram of an ultrathin flexible sensor based on an edge-support strategy, which was prepared by releasing it from the surface of a rigid substrate in water.

[0037] Figure 8 This is a flowchart of the method of the present invention.

[0038] In the figure: 1 Rigid substrate; 2 Ultrathin thin film; 3 Circular patterned mask; 4 Ultrathin substrate; 5 Thin film; 6 Circular patterned mask; 7 Ultrathin substrate with thickened edges; 8 Metal thin film; 9 Positive photoresist; 10 Snake-shaped mask; 11 Snake-shaped sensing unit; 12 Ultrathin flexible sensor based on edge support strategy. Detailed Implementation

[0039] The embodiments of the present invention will be described in detail below with reference to the technical solutions and accompanying drawings.

[0040] A method for fabricating an ultrathin flexible sensor based on an edge support strategy, the flowchart of which is shown below. Figure 8 As shown, it includes the following steps: Step (1): 1 ml of negative photoresist PSPI solution was uniformly mixed with 2 ml of NMP diluent solution and then spin-coated onto the surface of a 50 mm × 50 mm × 1 mm glass substrate 1. The spin-coating parameters were low speed 600 r / min for 9 s and high speed 2000 r / min for 30 s. The substrate was then placed in a 110 °C oven for 5 min to solidify and form a 500 nm thick PSPI ultrathin film 2 on the glass substrate 1. After the temperature in the oven dropped to 60 °C, the film was removed. A circular patterned photomask 3 was then applied to the surface of the PSPI ultrathin film 2 and exposed to light at a dose of 100 mJ / cm². 2 ,like Figure 1 As shown.

[0041] Step (2): The exposed PSPI ultrathin film 2 is sequentially immersed in cyclopentanone developer for 1 min and propylene glycol methyl ether acetate fixer for 5 s to complete patterning. Then, it is placed in a 250℃ oven and heated for 30 min to complete imidization, resulting in a fully cured circular polyimide (PI) ultrathin substrate 4, as shown. Figure 2 As shown.

[0042] Step (3): A layer of undiluted PSPI solution was spin-coated onto the surfaces of the PI ultrathin substrate 4 and the exposed glass substrate 1. The spin-coating parameters were low speed 600 r / min for 9 s and high speed 2000 r / min for 30 s. Then, the substrate was placed in a 110°C oven for 5 min to solidify and form a 5 μm thick PSPI film 5 on the surface of the glass substrate 1. After the temperature in the oven dropped to 60°C, the film was removed, and a circular patterned mask 6 was applied to the surface of the PSPI film 5 and exposed to light at a dose of 300 mJ / cm². 2 ,like Figure 3 As shown.

[0043] Step (4): The exposed PSPI film 5 is sequentially immersed in cyclopentanone developer for 3 min and propylene glycol methyl ether acetate fixer for 5 s to complete patterning. Then, it is placed in a 250℃ oven and heated for 30 min to complete imidization, resulting in a fully cured annular PI thickened edge. This is connected to the circular PI ultrathin substrate 4 to form an edge-thickened PI ultrathin substrate 7, as shown. Figure 4 As shown.

[0044] Step (5): A 100 nm thick gold film 8 is deposited on the surface of the edge-thickened PI ultrathin substrate 7 and the exposed hard substrate 1 using magnetron sputtering. Then, an AZ703 positive photoresist 9 is spin-coated onto the surface of the gold film 8. The spin-coating parameters are low speed 600 r / min, 9 s and high speed 2600 r / min, 30 s. Next, it is heated and cured on an 85℃ hot plate. Finally, a serpentine mask 10 is covered on the cured positive photoresist 9 and exposed to light at a concentration of 350 mJ / cm². 2 ,like Figure 5 As shown.

[0045] Step (6): Immerse the exposed AZ703 positive photoresist 9 in AZ400K developer for 30 seconds to complete patterning; then, using the patterned AZ703 positive photoresist 9 as a mask, immerse the gold film 8 in gold etching solution for patterning to obtain the snake-shaped sensor 11; finally, immerse the snake-shaped sensor 11 in acetone and ethanol solutions in sequence to dissolve and remove the residual AZ703 positive photoresist mask on its surface, as shown below. Figure 6 As shown.

[0046] Step (7): Immerse the prepared ultrathin flexible sensor 12 based on the edge support strategy into water until the thickened edge of the ultrathin substrate 7 causes the inner circular ultrathin substrate to detach from the surface of the glass substrate 1, thus obtaining the ultrathin flexible sensor based on the edge support strategy.

[0047] The embodiments described above are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for fabricating an ultrathin flexible sensor based on an edge support strategy, characterized in that, The preparation method includes the following steps: Step 1: Coat a layer of diluted photosensitive polyimide PSPI solution on the surface of a rigid substrate (1); and pre-heat to obtain a cured ultrathin film (2); and expose the cooled ultrathin film (2) by covering it with a circular pattern mask (3); Step 2: The exposed ultrathin film (2) is sequentially immersed in the developer and fixer to be patterned, dried with nitrogen and then completely heated to obtain a circular ultrathin substrate (4). Step 3: Spin-coat a layer of PSPI solution onto the surface of the ultrathin substrate (4) and the exposed hard substrate (1), and pre-heat to obtain a cured film (5). After covering the surface of the cooled film (5) with a circular pattern mask (6), expose it. Step 4: The exposed film (5) is sequentially immersed in the developer and fixer for patterning, dried with nitrogen and then heated completely to obtain an ultrathin substrate (7) with a thickened edge in a circular shape. Step 5: Deposit a metal thin film (8) on the surface of the edge-thickened ultrathin substrate (7) and the exposed hard substrate (1), spin coat a layer of positive photoresist (9) on the surface of the metal thin film (8) and heat to cure, cover the cured positive photoresist (9) with a serpentine mask (10) and then expose. Step 6: Immerse the exposed positive photoresist (9) in the developing solution for patterning. Use the patterned positive photoresist (9) as a mask to immerse the metal thin film (8) in the etching solution for patterning to obtain the snake-shaped sensing unit (11). Remove the residual positive photoresist mask. Step 7: Immerse the prepared ultrathin flexible sensor (12) based on the edge support strategy into water until it is released from the surface of the rigid substrate (1) to obtain the ultrathin flexible sensor based on the edge support strategy.

2. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 1: The rigid substrate (1) material is selected from glass, silicon wafer or sapphire, preferably glass; PSPI solution is a negative photoresist; diluted PSPI solution is made by mixing a diluent and a PSPI solution, wherein the diluent is N-methylpyrrolidone (NMP) solution, and 100 ml of PSPI solution is mixed with 100-300 ml of NMP solution; The coating methods for PSPI solutions include, but are not limited to, spin coating, blade coating, and vapor deposition; The heating and curing equipment is selected from hot plate and oven; the initial heating temperature is 110℃ and the heating time is 5min.

3. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 1: the rigid substrate (1) is glass.

4. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 2: The developer is cyclopentanone, and the development time is 1-3 min; the fixer is propylene glycol methyl ether acetate, and the fixing time is 5 s. The heating and curing equipment is selected from a hot plate or an oven; the complete heating temperature is 250℃, and the heating time is 30 minutes. The ultrathin substrate (4) has a circular pattern and a thickness of less than 1 μm.

5. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 3: The PSPI solution is the undiluted negative photoresist from step 1; The initial heating equipment and parameters are the same as in step 1.

6. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 4: The patterning process and parameters are the same as in step 2; The equipment and parameters for achieving complete heating are the same as in step 2; The edge-thickened ultrathin substrate (7) is composed of an ultrathin substrate (4) and an annular thin film on its outer side; the thickened edge is an annular thin film obtained by patterning the thin film (5), and the thickness of the annular thin film is greater than 5 μm.

7. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 5: The metal thin film material is selected from gold, silver, copper, or platinum; The positive photoresist heating and curing equipment is selected from a hot plate or oven; the heating temperature is 85℃ and the heating time is 30min.

8. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 5, the metal thin film material is selected from gold.

9. The method for fabricating an ultrathin flexible sensor based on an edge support strategy according to claim 1, characterized in that, In step 6: The positive photoresist is AZ703, and the developer is AZ400K; The metal etching solution is the etching solution corresponding to the metal thin film material selected in step 5; The serpentine sensing unit (11) includes a serpentine electrode in the middle, rectangular pads on both sides, and wires for connecting the electrode and the pads. The serpentine electrode is located in the circular non-thickened area of ​​the ultrathin substrate (7), the rectangular pads on both sides are located in the annular thickened area of ​​the ultrathin substrate (7), and the wires for connecting are located in the transition area connecting the circular non-thickened area and the annular thickened area of ​​the ultrathin substrate (7). The remaining positive photoresist mask was dissolved and removed by sequentially immersing it in a mixed solution of acetone and ethanol.

10. An ultrathin flexible sensor based on an edge support strategy, characterized in that, The ultrathin flexible sensor based on the edge support strategy, prepared by any one of claims 1-9, comprises a two-layer structure: the lower layer is an edge-thickened ultrathin substrate (7), which is composed of an inner submicron thick circular ultrathin substrate (4) and an outer micron thick ring-shaped thickened film; the upper layer is a serpentine sensing unit (11), which is composed of a serpentine electrode, rectangular pads on both sides, and wires for connecting the electrode and the pads.

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