A tensile piezoresistive thin film based on laser selective etching and its preparation method

Abstract

This invention provides a tensile piezoresistive thin film based on laser selective etching and its preparation method, belonging to the technical field of tensile piezoresistive thin film preparation. This invention uses a template method to prepare the sensitive material, and then uses an adhesive to bond the electrode layer to the sensitive material. In this preparation process, a porous structure is obtained using sugar or salt. Due to the interference of lateral stretching, the porous structure will exhibit corresponding lateral changes, leading to a change in resistance. Therefore, this application uses laser etching to remove the sensitive material between islands and connects the inter-island electrodes with curved lines. Simultaneously, zero-modulus silicone oil is filled between the islands, allowing tensile strain to transfer to the inter-island area without affecting the remaining sensitive material, thus achieving good tensile resistance.

Description

Technical Field

[0001] This invention belongs to the field of tensile piezoresistive thin film preparation technology, specifically relating to a method for preparing tensile piezoresistive thin films based on laser selective etching. Background Technology

[0002] A piezoresistive thin film is a sensor that converts changes in resistance caused by external stimuli into an electrical signal output. Piezoresistive thin films have advantages such as simple structure, easy integration, and convenient data output, and are currently the most widely studied type of electronic pressure sensor. The fabrication process of a piezoresistive thin film typically involves mixing an active material into a flexible material to form a composite material, connecting electrodes, and then encapsulating it with a flexible material.

[0003] Currently, fabricating tensile-resistant piezoresistive thin films remains a challenging task because piezoresistive films are significantly affected by external strain. During stretching, the resistance change caused by lateral force interferes with the measurement of resistance change under vertical force. Current methods for preparing piezoresistive films involve a template method, where PDMS is mixed with CNTs in a specific ratio, followed by sugar or salt to obtain a porous piezoresistive film. However, when the film is stretched, its unique physical properties cause it to gradually thin, leading to inaccurate resistance measurements. This is because the film's structure changes during stretching, resulting in a change in resistance. Furthermore, cutting the film into specific shapes and fabricating arrays presents complex manufacturing challenges. For example, the size and shape of each film sheet need to be precisely controlled to ensure they form a stable array with specific functions. Additionally, it is necessary to consider how to firmly bond these film materials to the electrode layer to prevent detachment or damage during use.

[0004] Therefore, developing a simple fabrication method for piezoresistive thin films with good tensile strength is of great significance for the practical application of piezoresistive thin films. Summary of the Invention

[0005] To address the limitation of template-based piezoresistive thin film fabrication in achieving effective tensile strength, this invention aims to provide a method for fabricating tensile-resistant piezoresistive thin films based on laser selective etching. This invention designs a tensile-resistant island-bridge structure. Utilizing the stretchable properties of curved wires and filling the spaces between the islands with a zero-modulus material, the film resistance remains unaffected by stretching, thus achieving tensile strength.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A tensile piezoresistive thin film based on laser selective etching, wherein the piezoresistive thin film is an array of island bridge units;

[0008] The island bridge unit includes: an upper electrode layer, a sensitive material layer, and a lower electrode layer. The upper electrode layer and the lower electrode layer are uniformly distributed on the upper and lower surfaces of the sensitive material layer, respectively. The island bridge units are separated by gaps. The upper electrode layers of adjacent island bridge units are connected only in the X direction by curved wires, and the lower electrode layers of adjacent island bridge units are connected only in the Y direction by curved wires. The gaps between the island bridge units are filled with a material with a modulus of 0.

[0009] Furthermore, the upper and lower electrode layers are made of one of copper, iron, aluminum, platinum, or gold, and have a thickness of 4 to 10 micrometers.

[0010] A method for preparing tensile piezoresistive thin films based on laser selective etching, the method comprising the following steps:

[0011] Step 1. Preparation of NaCl powder: Dissolve anhydrous NaCl crystals in water at a ratio of 1:5 to 1:6, evaporate to crystallize, and then grind to obtain NaCl powder;

[0012] Step 2. Preparation of piezoresistive film: Polydimethylsiloxane (PDMS) and CNTs are mixed at a ratio of 100:3 to 100:4 and ultrasonicated for 30 to 40 minutes. Then, a curing agent is added and mixed. CNTs are carbon nanotubes, and the ratio of curing agent to PDMS is 1:10 to 1:12. Next, ground NaCl powder is added at a ratio of PDMS to NaCl of 1:3 to 1:4 and stirred evenly. The mixture is then filled into a glass groove of 6cm×6cm×1mm to 7cm×7cm×1mm and heated at 140℃ to 150℃ for 3 to 4 minutes to cure. After curing, the film is removed and soaked in deionized water for 10 to 12 hours to remove NaCl. Finally, it is placed in an oven at 50℃ to 60℃ for 30 to 40 minutes. All ratios in this method are mass ratios.

[0013] Step 3. Electrode fabrication: Attach the non-adhesive side of a 5cm×5cm to 6cm×6cm water-soluble adhesive tape to an equally sized heat-release adhesive tape and then peel off the coating. Attach an equally sized 4-micron to 10-micron thick copper foil to the water-soluble adhesive tape. Use an ultraviolet laser to cut the copper foil according to the finger pattern, and then peel off the excess portion, leaving the electrode intact.

[0014] Step 4. Prepare the adhesive: Mix the silicone rubber (Ecoflex) and CNT at a ratio of 100:6 to 100:8 until homogeneous;

[0015] Step 5. Use adhesive to attach the electrodes to the piezoresistive film, with the electrodes on the upper and lower sides of the film perpendicular to each other. Heat at 140°C to 150°C for 3 to 4 minutes to cure the adhesive and release the heat release tape. Remove the water-soluble tape with water and place it in an oven at 50°C to 60°C for 30 to 40 minutes before taking it out.

[0016] Step 6. Selective laser etching: Use an infrared laser to etch away the piezoresistive film portion without copper coverage. After etching one side, flip it over to etch the other side. The piezoresistive film on the serpentine electrode portion is etched without etching away the serpentine electrode, thus isolating the piezoresistive film under the island electrode portion.

[0017] Step 7. Encapsulation: Spin-coat Ecoflex at 400 to 500 RPM for 1.5 to 2 minutes to make two films. Place one side of the film on one of the films and heat at 140°C to 150°C for 3 to 4 minutes to cure. Remove the film and place the other side of the film on the other film. Repeat the heating and curing process before removing the film.

[0018] Step 8. Filling the island with low-modulus material: Inject a material with a modulus of 0 into the film using a syringe to obtain a tensile piezoresistive film.

[0019] The mechanism of this invention is as follows: a sensitive material is prepared using a template method, and an electrode layer is bonded to the sensitive material using an adhesive; a porous structure is obtained by using sugar or salt in this preparation process. Due to the interference of lateral stretching, the porous structure will exhibit corresponding lateral changes, resulting in a change in resistance. Therefore, this application uses laser etching to remove the sensitive material between islands and connects the inter-island electrodes with curved lines. At the same time, a zero-modulus material is filled between the islands so that the tensile strain is transferred between the islands without affecting the remaining sensitive material, thereby achieving good tensile resistance.

[0020] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0021] This invention provides a preparation process that enables piezoresistive films prepared by the template method to have good tensile strength. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the process flow for preparing the tensile piezoresistive thin film according to the present invention.

[0023] Figure 2 This is a schematic diagram of a finished tensile piezoresistive thin film.

[0024] Figure 3 The repeatability curves of the resistivity change rate of the tensile piezoresistive thin film prepared in Example 1 under different pressures are shown.

[0025] Figure 4 The sensitivity curves of the tensile piezoresistive thin film prepared in Example 1 under different conditions are shown.

[0026] Figure 5 The resistance variation curves of the tensile piezoresistive thin film prepared in Example 1 filled with different materials are shown. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0028] Since existing preparation methods cannot achieve good tensile strength, this invention adopts a simple template method to prepare a PDMS piezoresistive film with a porous structure. The electrodes are bonded to the film with an adhesive, laser etching is performed, and the islands are filled with zero-modulus silicone oil. Then, an Ecoflex film is encapsulated on the top and bottom to obtain a tensile piezoresistive film.

[0029] A schematic diagram of the fabrication process of a tensile piezoresistive thin film is shown below. Figure 1 As shown, it includes an array of island bridge units.

[0030] Example 1

[0031] The tensile piezoresistive thin film preparation method based on the above process includes the following steps:

[0032] Step 1. Preparation of NaCl powder: Dissolve purchased anhydrous NaCl crystals in water at a ratio of 1:5, evaporate to crystallize, and then grind to obtain NaCl powder;

[0033] Step 2. Preparation of piezoresistive film: PDMS and CNT (carbon nanotubes) are mixed at a ratio of 100:3 and sonicated for 30 minutes. Then, a curing agent (curing agent: PDMS = 1:10) is added and mixed. Next, ground NaCl powder is added at a ratio of PDMS:NaCl = 1:3 and stirred evenly. The mixture is then filled into a 6cm x 6cm x 1mm glass groove and heated at 150℃ for 3 minutes to cure. After curing, the film is removed and then soaked in deionized water for 12 hours (to remove NaCl). Finally, it is placed in an oven at 60℃ for 30 minutes.

[0034] Step 3. Electrode fabrication: Attach the non-adhesive side of a 5cm x 5cm water-soluble adhesive tape to an equally sized heat-release adhesive tape, then peel off the coating. Attach an equally sized 10-micron thick copper foil to the water-soluble adhesive tape. Use an ultraviolet laser (parameters: number of processes 22, speed 300, power 35) to cut the copper foil according to the finger pattern. Under these parameters, the copper foil is unaffected by the laser. Then peel off the excess portion, leaving the electrode intact.

[0035] Step 4. Prepare the adhesive: Mix Ecoflex and CNT at a ratio of 100:6 and stir until homogeneous;

[0036] Step 5. Use adhesive to attach the electrodes to the piezoresistive film, with the electrodes on the upper and lower sides of the film perpendicular to each other. Heat at 150°C for 3 minutes to cure the adhesive and release the heat release tape. Remove the water-soluble tape with water and place it in an oven at 60°C for 30 minutes before taking it out.

[0037] Step 6. Selective laser etching: Use an infrared laser (processing number 7, speed 1000, power 30) to etch away the piezoresistive film without copper coverage (etch one side and then flip to etch the other side; the piezoresistive film on the serpentine electrode part is etched but the serpentine electrode is not etched away) to isolate the piezoresistive film under the island electrode part (square copper).

[0038] Step 7. Encapsulation: Spin-coat the Ecoflex film at 500 rpm for 2 minutes to make two films. Place the film on one film and heat at 150°C for 3 minutes to cure. Remove the film and repeat the process on the other film.

[0039] Step 8. Filling the inter-island low-modulus material: Inject silicone oil with a modulus of 0 into the film using a syringe to obtain a tensile piezoresistive film.

[0040] The tensile piezoresistive thin film prepared in this embodiment exhibits the following sensitivity under different stretching conditions: Figure 4 As shown, the prepared film exhibits the same sensitivity under different stretching conditions.

[0041] Example 2

[0042] The tensile piezoresistive thin film preparation method based on the above process includes the following steps:

[0043] Step 1. Preparation of NaCl powder: Dissolve purchased anhydrous NaCl crystals in water at a ratio of 1:5, evaporate to crystallize, and then grind to obtain NaCl powder;

[0044] Step 2. Preparation of piezoresistive film: PDMS and CNT (carbon nanotubes) are mixed at a ratio of 100:3 and sonicated for 30 minutes. Then, a curing agent (curing agent: PDMS = 1:10) is added and mixed. Next, ground NaCl powder is added at a ratio of PDMS:NaCl = 1:3 and stirred evenly. The mixture is then filled into a 6cm x 6cm x 1mm glass groove and heated at 150℃ for 3 minutes to cure. After curing, the film is removed and then soaked in deionized water for 12 hours (to remove NaCl). Finally, it is placed in an oven at 60℃ for 30 minutes.

[0045] Step 3. Electrode fabrication: Attach the non-adhesive side of a 5cm x 5cm water-soluble adhesive tape to an equally sized heat-release adhesive tape, then peel off the coating. Attach an equally sized 10-micron thick copper foil to the water-soluble adhesive tape. Use an ultraviolet laser (parameters: number of processes 22, speed 300, power 35) to cut the copper foil according to the finger pattern. Under these parameters, the copper foil is unaffected by the laser. Then peel off the excess portion, leaving the electrode intact.

[0046] Step 4. Prepare the adhesive: Mix Ecoflex and CNT at a ratio of 100:6 and stir until homogeneous;

[0047] Step 5. Use adhesive to attach the electrodes to the piezoresistive film, with the electrodes on the upper and lower sides of the film perpendicular to each other. Heat at 150°C for 3 minutes to cure the adhesive and release the heat release tape. Remove the water-soluble tape with water and place it in an oven at 60°C for 30 minutes before taking it out.

[0048] Step 6. Selective laser etching: Use an infrared laser (processing number 7, speed 1000, power 30) to etch away the piezoresistive film without copper coverage (etch one side and then flip to etch the other side; the piezoresistive film on the serpentine electrode part is etched but the serpentine electrode is not etched away) to isolate the piezoresistive film under the island electrode part (square copper).

[0049] Step 7. Encapsulation: Spin-coat the Ecoflex film at 500 rpm for 2 minutes to make two films. Place the film on one film and heat at 150°C for 3 minutes to cure. Remove the film and repeat the process on the other film.

[0050] Step 8. Filling the inter-island low-modulus material: Inject AB glue (epoxy resin) into the film using a syringe, place it on a plate and heat it at 150°C for 3 minutes to cure, then remove it to obtain a tensile piezoresistive film.

[0051] The resistance change of the tensile piezoresistive thin film prepared in this embodiment under different stretching conditions is as follows: Figure 5 As shown by the curve with the square symbol, it can be seen that the prepared film does not change its resistance under different stretching conditions.

[0052] Example 3

[0053] The tensile piezoresistive thin film preparation method based on the above process includes the following steps:

[0054] Step 1. Preparation of NaCl powder: Dissolve purchased anhydrous NaCl crystals in water at a ratio of 1:5, evaporate to crystallize, and then grind to obtain NaCl powder;

[0055] Step 2. Preparation of piezoresistive film: PDMS and CNT (carbon nanotubes) are mixed at a ratio of 100:3 and sonicated for 30 minutes. Then, a curing agent (curing agent: PDMS = 1:10) is added and mixed. Next, ground NaCl powder is added at a ratio of PDMS:NaCl = 1:3 and stirred evenly. The mixture is then filled into a 6cm x 6cm x 1mm glass groove and heated at 150℃ for 3 minutes to cure. After curing, the film is removed and then soaked in deionized water for 10 hours (to remove NaCl). Finally, it is placed in an oven at 60℃ for 30 minutes.

[0056] Step 3. Electrode fabrication: Attach the non-adhesive side of a 5cm x 5cm water-soluble adhesive tape to an equally sized heat-release adhesive tape, then peel off the coating. Attach an equally sized 10-micron thick copper foil to the water-soluble adhesive tape. Use an ultraviolet laser (parameters: number of processes 22, speed 300, power 35) to cut the copper foil according to the finger pattern. Under these parameters, the copper foil is unaffected by the laser. Then peel off the excess portion, leaving the electrode intact.

[0057] Step 4. Prepare the adhesive: Mix Ecoflex and CNT at a ratio of 100:6 and stir until homogeneous;

[0058] Step 5. Use adhesive to attach the electrodes to the piezoresistive film, with the electrodes on the upper and lower sides of the film perpendicular to each other. Heat at 150°C for 3 minutes to cure the adhesive and release the heat release tape. Remove the water-soluble tape with water and place it in an oven at 60°C for 50 minutes before taking it out.

[0059] Step 6. Selective laser etching: Use an infrared laser (processing number 7, speed 1000, power 30) to etch away the piezoresistive film without copper coverage (etch one side and then flip to etch the other side; the piezoresistive film on the serpentine electrode part is etched but the serpentine electrode is not etched away) to isolate the piezoresistive film under the island electrode part (square copper).

[0060] Step 7. Encapsulation: Spin-coat the Ecoflex film at 500 rpm for 1.5 minutes to make two films. Place the film on one film and heat at 150°C for 3 minutes to cure. Remove the film and repeat the process on the other film.

[0061] Step 8. Filling the inter-island low-modulus material: Inject silicone oil with a modulus of 0 into the film using a syringe to obtain a tensile piezoresistive film.

[0062] Comparative Example 1

[0063] The tensile piezoresistive thin film was prepared according to the steps of Example 1, except that the silicone oil with a modulus of 0 filled in step 8 was replaced with other materials, while the other steps remained unchanged.

[0064] The resistance curve of the tensile piezoresistive thin film prepared in this embodiment is as follows: Figure 5 As shown in the figure, the film resistance of the film filled with zero-modulus silicone oil does not change much, while the film resistance of the Ecofelx film and the piezoresistive film, which are made of the same material, both change to some extent.

Claims

1. A method for preparing a tensile piezoresistive thin film based on laser selective etching, the method comprising the following steps: Step 1. Preparation of NaCl powder: Dissolve anhydrous NaCl crystals in water at a ratio of 1:5 to 1:6, evaporate to crystallize, and then grind to obtain NaCl powder; Step 2. Preparation of piezoresistive film: Polydimethylsiloxane and CNTs are mixed at a ratio of 100:3 to 100:4 and ultrasonicated for 30 to 40 minutes. Then, a curing agent is added and mixed. CNTs are carbon nanotubes, and the ratio of curing agent to polydimethylsiloxane is 1:10 to 1:

12. Next, ground NaCl powder is added at a ratio of polydimethylsiloxane to NaCl of 1:3 to 1:4 and stirred evenly. The mixture is then filled into a glass groove of 6cm×6cm×1mm to 7cm×7cm×1mm and heated at 140℃ to 150℃ for 3 to 4 minutes to cure. After curing, the film is removed and soaked in deionized water for 10 to 12 hours to remove NaCl. Finally, it is placed in an oven at 50℃ to 60℃ for 30 to 40 minutes. All ratios in this method are by mass. Step 3. Electrode fabrication: Attach the non-adhesive side of a 5cm×5cm to 6cm×6cm water-soluble adhesive tape to an equally sized heat-release adhesive tape and then peel off the coating. Attach an equally sized 4-micron to 10-micron thick copper foil to the water-soluble adhesive tape. Use an ultraviolet laser to cut the copper foil according to the finger pattern, and then peel off the excess portion, leaving the electrode intact. Step 4. Prepare the adhesive: Mix silicone rubber and CNT at a ratio of 100:6 to 100:8 until homogeneous; Step 5. Use adhesive to attach the electrodes to the piezoresistive film, with the electrodes on the upper and lower sides of the film perpendicular to each other. Heat at 140°C to 150°C for 3 to 4 minutes to cure the adhesive and release the heat release tape. Remove the water-soluble tape with water and place it in an oven at 50°C to 60°C for 30 to 40 minutes before taking it out. Step 6. Selective laser etching: Use an infrared laser to etch away the piezoresistive film portion without copper coverage. After etching one side, flip it over to etch the other side. The piezoresistive film on the serpentine electrode portion is etched without etching away the serpentine electrode, thus isolating the piezoresistive film under the island electrode portion. Step 7. Encapsulation: Spin-coat silicone rubber at 400 to 500 RPM for 1.5 to 2 minutes to make two sheets. Place one side of the film on one sheet and heat at 140°C to 150°C for 3 to 4 minutes to cure. Remove the film and place the other side of the film on the other sheet. Repeat the heating and curing process before removing the film. Step 8. Filling the inter-island low-modulus material: Inject silicone oil with a modulus of 0 into the film using a syringe to obtain a tensile piezoresistive film.

2. The method for preparing a tensile piezoresistive thin film based on laser selective etching according to claim 1, characterized in that, In step 1, polydimethylsiloxane and CNT are mixed at a ratio of 100:3 and ultrasonicated for 30 minutes. Then, a curing agent is added and mixed, wherein the curing agent: PDMS = 1:

10. Next, ground NaCl powder is added according to the ratio of PDMS:NaCl = 1:

3. After stirring evenly, the mixture is filled into a glass groove and heated at 150°C for 3 minutes to cure. After curing, the mixture is removed and then soaked in deionized water for about 12 hours to remove NaCl. Finally, it is placed in an oven at about 60°C and baked for 30 minutes.

3. The method for preparing a tensile piezoresistive thin film based on laser selective etching according to claim 1, characterized in that, In step 4, silicone rubber and CNT are mixed and stirred evenly at a ratio of 100:

6.

4. The method for preparing a tensile piezoresistive thin film based on laser selective etching according to claim 1, characterized in that, In step 3, the laser is an ultraviolet laser with a laser speed of 300 mm / s and a power of 35%.

5. The method for preparing a tensile piezoresistive thin film based on laser selective etching according to claim 1, characterized in that, In step 6, the laser is an infrared laser with a speed of 300 mm / s and a power of 30%.

6. The method for preparing a tensile piezoresistive thin film based on laser selective etching according to claim 1, characterized in that, In step 5, silicone rubber is spin-coated onto the two encapsulation sheets at 500 rpm for 2 minutes. Then, the silicone rubber-coated sides of the two encapsulation sheets are placed on the sides of the two electrode layers away from the sensitive material layers, and heated in an environment of 150°C for 3 minutes.

7. The piezoresistive thin film prepared by the method for preparing tensile piezoresistive thin films based on laser selective etching according to claim 1, characterized in that, This piezoresistive thin film is an island bridge unit arranged in an array; The island bridge unit includes: an upper electrode layer, a sensitive material layer, and a lower electrode layer. The upper electrode layer and the lower electrode layer are uniformly distributed on the upper and lower surfaces of the sensitive material layer, respectively. The island bridge units are separated by gaps. The upper electrode layers of adjacent island bridge units are connected only in the X direction by curved wires, and the lower electrode layers of adjacent island bridge units are connected only in the Y direction by curved wires. The gaps between the island bridge units are filled with a material with a modulus of 0.

8. The piezoresistive thin film prepared by the method for preparing tensile piezoresistive thin films based on laser selective etching according to claim 7, characterized in that, The upper and lower electrode layers are made of one of the following materials: copper, iron, aluminum, platinum, and gold, with a thickness of 4 to 10 micrometers.

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