Process for improving surface adhesive force of polydimethylsiloxane and chemically depositing metal

Abstract

The invention relates to a process for improving surface adhesion of polydimethylsiloxane and chemically depositing metal. The invention discloses a process for improving surface adhesion of polydimethylsiloxane (PDMS) and chemically depositing metal. The process comprises the following main steps: preparing a flexible PDMS substrate with a microstructure on the surface; a surface micro-convex peak structure is formed in a high-temperature annealing mode after strong acid treatment; carrying out plasma process treatment on the flexible substrate; soaking in a PVA (Polyvinyl Alcohol) solution; sputtering an adhesive copper layer; and chemically depositing a copper layer. The prepared polydimethylsiloxane with the metal deposited on the surface shows relatively high stability and continuous hydrophilicity, and has a series of advantages of high adhesive force, good flexibility and the like.

Description

Technical Field

[0001] This invention relates to the field of heterogeneous interface adhesion technology, and more specifically to a process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal. Background Technology

[0002] With the rapid development of flexible electronics technology and smart sensors, flexible sensors have shown broad application prospects in wearable devices, medical and health monitoring, human-computer interaction, and soft robotics. Flexible sensors typically use flexible polymer materials such as polydimethylsiloxane (PDMS), polyimide (PI), and polyurethane (PU) as substrates to achieve good flexibility, stretchability, and mechanical adaptability. However, the interfacial adhesion problem between the flexible substrate and the functional or electrode layers has long been a significant bottleneck restricting device performance and reliability.

[0003] In flexible sensors, metal thin films, electrode materials, or functional inorganic layers need to be deposited or transferred to the surface of a flexible substrate to achieve the acquisition and transmission of electrical signals. However, due to the low surface energy and strong chemical inertness of the flexible polymer substrate, the bonding force between it and the metal layer or other heterogeneous materials is insufficient. Under long-term bending, stretching, or external stress, cracking, peeling, or detachment can easily occur at the interface, causing sensor performance degradation or even failure. This interface instability not only affects the electrical performance and detection accuracy of the device but also severely limits its reliability in dynamic environments and long-term applications.

[0004] To improve the adhesion between flexible substrates and metal layers, existing technologies mainly employ methods such as surface roughening, plasma treatment, the introduction of intermediate buffer layers, or chemical modification. However, these methods still have certain limitations: surface roughening can easily damage the substrate structure; plasma treatment has a time effect, making it difficult to maintain surface activity over a long period; buffer layers may introduce additional resistance or reduce flexibility; and chemical modification suffers from complex processes or poor compatibility with subsequent deposition processes. Therefore, how to significantly improve the interfacial adhesion between flexible substrates and metal layers or other functional layers while ensuring the mechanical properties of the flexible substrate remains a key technological challenge in the current development of flexible sensors. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, the present invention aims to provide a process for improving the surface adhesion of polydimethylsiloxane and chemically depositing metal. Based on flexible PDMS substrates with different microstructured surfaces, the hydrophilicity of the PDMS surface is improved by synergistic treatment of PVA and oxygen plasma. Finally, the process of depositing metal on the surface of polydimethylsiloxane is achieved by magnetron sputtering and chemical plating thickening methods. While maintaining the original performance, this process achieves a long-term stable increase in surface adhesion and simplifies the manufacturing process.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: Step 1: Prepare PDMS with microstructures on the surface by mixing and then thermosetting on sandpaper; Step 2: A surface micro-protrusion peak structure is formed by strong acid treatment followed by high-temperature annealing; Step 3: Use plasma cleaning process to treat PDMS with a certain surface roughness; Step 4: Immerse the PDMS in the prepared PVA solution. Step 5: Sputter the adhered copper layer using magnetron sputtering; Step 6: Based on the above steps, the adhered copper layer is thickened by chemical deposition.

[0007] Furthermore, the specific method for preparing the flexible substrate with microstructure is as follows: 1) Mix the PDMS main agent and curing agent at a mass ratio of 10:1 and stir continuously for 5-10 minutes until they are evenly mixed; 2) Subsequently, a vacuum environment of less than 10 Pa is created by using an air pump to remove air bubbles from the mixed PDMS slurry, making the interior of the cured PDMS more stable and smooth. 3) Apply the PDMS slurry after removing air bubbles to a smooth glass substrate and sandpaper with grits of 220, 400, 600, and 1000 respectively, and place it on a flat spin coater stage to spin coat the PDMS at a speed of 200 rpm to ensure uniform thickness throughout the flexible substrate.

[0008] 4) First cure at 80℃ for 1 hour in a vacuum drying oven, then cure at 60℃ for 2 hours to cure the flexible substrate with surface microstructure.

[0009] Furthermore, the specific method for forming the surface micro-protrusion peak structure by strong acid treatment followed by high-temperature annealing is as follows: 1) Place the cured PDMS in anhydrous ethanol for ultrasonic treatment to remove oil stains from the PDMS surface; 2) Completely immerse the treated PDMS in HNO3 solution, maintain a certain temperature and time, remove it, rinse with plenty of deionized water and dry with N2; 3) In a N2 atmosphere, the temperature is increased to 150℃ at a rate of 2–5℃ / min and held at this temperature for 1–2 hours to allow the surface acid etching layer to undergo further oxidation, cross-linking and thermal shrinkage, generating a strain difference with the untreated substrate that induces a regular surface micro-protrusion peak structure.

[0010] Furthermore, the specific method for using a flexible substrate with a certain surface microstructure for plasma cleaning is as follows: 1) First, flexible substrates with different surface microstructures with varying degrees of roughness are cleaned in anhydrous ethanol and deionized water to remove surface oil. 2) High-voltage ionized oxygen is used to form plasma to impact a flexible substrate with surface microstructure, which increases the surface roughness of the flexible substrate and reduces the surface energy, thereby improving the wettability of the PDMS surface and enhancing the adhesion between the substrate and the metal conductive layer.

[0011] Furthermore, the specific method for preparing the PVA solution and soaking the PDMS is as follows: 1) Weigh out a certain amount of PVA powder at concentrations of 1%, 2%, 3%, 4% and 5%, and slowly add it to 20ml of deionized water in several portions and let it stand for 10 minutes. 2) Heat and stir the solution at 90°C for 1 hour after standing, then heat and stir at 65°C for 3 hours. Finally, add deionized water to 25 ml to make up the volume and compensate for water loss, so as to obtain PVA solutions of different concentrations of 1%, 2%, 3%, 4% and 5%. 3) Immerse the treated PDMS in PVA solutions of different concentrations for 10 minutes, and then dry it on a hot plate at 90°C.

[0012] Furthermore, the method of sputtering the adhered copper layer using magnetron sputtering specifically includes: 1) Place the plasma-cleaned flexible substrate in the sputtering chamber and introduce an inert gas, such as argon, into the chamber. Control the deposition rate by adjusting the gas pressure. 2) When a high-voltage electric field is applied to the target material, gas molecules are excited, forming plasma. In the plasma, negatively charged electrons are accelerated to the target surface and collide with target atoms. The sputtered material escapes from the target surface and forms a high-energy particle beam in the gas. These particle beams move in a direction perpendicular to the target surface and collide with the substrate, forming an adhered copper layer.

[0013] Furthermore, the specific method for thickening the adhered copper layer using chemical deposition is as follows: After the PDMS sample with the sputtered copper layer was removed from the chamber of the magnetron sputtering instrument, it was immediately placed in the electroless copper plating solution and left to stand for 15 minutes before being removed from the solution.

[0014] The purpose of this invention is to solve the technical problems of poor hydrophilicity of polydimethylsiloxane surface, poor adhesion of heterometal interface and thin adhesion metal layer, and to provide a process for improving the adhesion of polydimethylsiloxane surface and chemically depositing metal.

[0015] Compared with existing achievements, the beneficial effects of this invention are as follows: 1) This invention employs a method of curing PDMS on sandpaper surfaces of different grit sizes, which achieves both obtaining different surface microstructures and roughening of the PDMS surface; 2) The micro-peak structure formed on the surface of PDMS after strong acid treatment significantly improves the adhesion and interfacial stability of the PDMS surface by increasing the specific surface area, introducing mechanical interlocking and polar functional groups, and relieving interfacial stress. 3) This invention employs a physical-chemical coupling process, simultaneously performing oxygen plasma treatment on the flexible substrate and chemical grafting of PVA. The synergistic effect of these two methods makes the PDMS surface rougher, which is more conducive to the adhesion of heterogeneous materials. Attached Figure Description

[0016] Figure 1 is a schematic diagram of a process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal according to the present invention; Figure 2 is a schematic diagram of the structure of the PDMS with micro-convex peaks of the present invention. Figure 3 shows a comparison of the water droplet contact angles on the PDMS surface under different process parameters according to the present invention. Figure 4 shows a comparison of the water droplet contact angles on the PDMS surface under different PVA concentrations and different discharge powers according to the present invention. Figure 5 shows a comparison of the water droplet contact angles on the PDMS surface under different PVA concentrations and different sandpaper grits according to the present invention. Figure 6 is a schematic diagram of the principle of chemical deposition of Cu on PDMS according to the present invention. Detailed Implementation

[0017] The present invention will be further explained and described below with reference to the accompanying drawings and embodiments, so as to enable those skilled in the art to better understand the invention.

[0018] As shown in Figure 1, this invention provides a process for improving the adhesion of polydimethylsiloxane to the surface and for chemically depositing metal, comprising the following steps:

[0019] Step 1. Preparation of a PDMS flexible substrate with randomly distributed microstructures on the surface: Mix the PDMS prepolymer and curing agent at a mass ratio of 10:1 and stir continuously for 5-10 minutes until they are uniformly mixed; then create a vacuum environment of less than 10 Pa using a vacuum pump to remove air bubbles from the mixed PDMS slurry; coat the bubble-removed PDMS slurry onto sandpaper with mesh sizes of 0 (smooth surface), 220, 400, 600, and 1000 respectively, and spin coat it on a flat spin coater stage at a speed of 200 rpm to ensure uniform thickness throughout the flexible substrate. After spin coating, let it stand at room temperature for 30 minutes, and then cure it in a vacuum drying oven at 80°C for 2 hours to solidify the flexible substrate with surface microstructures.

[0020] Step 2. The cured PDMS is treated with strong acid followed by high-temperature annealing to form a surface micro-peak structure, as shown in Figure 2. The cured PDMS is ultrasonically treated in anhydrous ethanol at 180W for 15 minutes to remove oil stains from the PDMS surface, ensuring that the subsequent strong acid treatment can act uniformly on the substrate surface. The treated PDMS is then completely immersed in HNO3 solution and kept at a suitable temperature (e.g., 25–40 ℃) in a constant temperature water bath. The immersion time is adjusted according to the required roughness, generally 10–30 min. Too short a time will result in insufficient surface modification, while too long a time may cause excessive corrosion, leading to a decrease in the mechanical properties of the substrate. After removing the sample, it is rinsed with plenty of deionized water and dried with N2. Under an N2 atmosphere, the temperature is increased to 150℃ at a rate of 2–5℃ / min and held at this temperature for 1–2 hours to allow further oxidation, cross-linking, and thermal shrinkage of the surface acid-etched layer, generating a strain difference with the untreated substrate that induces a regular surface micro-peak structure.

[0021] Step 3. Further improve the surface roughness and reduce the surface energy of the flexible substrate by combining oxygen plasma treatment and PVA chemical treatment to enhance its adhesion to the conductive metal layer: High-voltage ionized oxygen is used to form plasma to impact the flexible substrate with surface microstructures, thereby increasing the surface roughness and reducing the surface energy, improving the wettability of the PDMS surface, and enhancing the adhesion between the substrate and the conductive metal layer. To determine the effect of different process parameters on the surface wetting effect of oxygen plasma cleaning, we investigated the experimental effects under different discharge powers (100W and 200W) and different gas flow rates (20sccm and 40sccm). As shown in Figure 3, by measuring the contact angle under different parameters, it was found that the oxygen plasma cleaning effect was the best when the discharge power was 200W and the gas flow rate was 20sccm. As shown in Figure 4, the contact angle of the samples treated at 200W power was smaller than that at 100W power at different PVA concentrations.

[0022] Step four: Weigh out specific amounts of PVA powder according to concentrations of 1%, 2%, 3%, 4%, and 5%, and slowly add them to 20 ml of deionized water in multiple portions. Let the solutions stand for 10 minutes to allow the PVA particles to fully absorb water and partially swell, thus providing favorable conditions for subsequent dissolution. Heat the solutions at 90°C and stir for 1 hour to gradually dissolve the PVA powder, forming a preliminary homogeneous and transparent solution. To further ensure the full unfolding of the molecular chains and the stability of the solution, heat and stir at 65°C for another 3 hours. During this prolonged heating process, some solvent will be lost through evaporation. Therefore, after the dissolution step is completed, cool the system to near room temperature, replenish the evaporation loss with deionized water, and bring the volume to 25 mL to ensure the accuracy of the solution concentration. Finally, PVA solutions with different concentrations of 1%, 2%, 3%, 4%, and 5% are obtained.

[0023] The pretreated PDMS substrate was completely immersed in prepared PVA solutions of different concentrations for 10 minutes to allow PVA molecules to fully adsorb and form a uniform coating on the PDMS surface. Too short an immersion time would result in incomplete surface coverage, while too long an immersion time might cause excessive PVA accumulation in the solution, which would be detrimental to the uniformity of the subsequent drying process. After immersion, the PDMS samples were carefully removed with tweezers and excess solution was gently shaken off to avoid droplet residue forming an uneven coating. The samples were then placed on a uniformly heated 90°C hot plate to dry, allowing the water in the solution to evaporate rapidly, and enabling PVA molecules to fix and crosslink on the PDMS surface, forming a stable hydrophilic modified coating. Treatment with different concentrations of PVA solution resulted in different coating thicknesses and surface energies, thus affecting the hydrophilicity and adhesion properties of the PDMS. The contact angles measured at different sandpaper grits and different PVA concentrations are shown in Figure 5. A smaller contact angle indicates better adhesion to the PDMS surface, which is more conducive to the subsequent growth of copper nanowires.

[0024] Step five involves placing the treated flexible substrate in the sputtering chamber and introducing an inert gas, such as argon, into the chamber. The deposition rate is controlled by adjusting the gas pressure. When a high-voltage electric field is applied to the target, gas molecules are excited, forming a plasma. In the plasma, negatively charged electrons are accelerated to the target surface and collide with target atoms. The sputtered material escapes from the target surface and forms a high-energy particle beam in the gas. These particle beams travel in a direction perpendicular to the target surface and collide with the substrate to form a thin film.

[0025] Step Six: The conductive copper layer is thickened using chemical deposition. After the magnetron sputtered copper film on the PDMS surface, a thicker and denser copper layer needs to be obtained through chemical deposition. The sputtered copper layer is activated by rapid immersion in a 1–2% dilute hydrochloric acid or citric acid solution for several tens of seconds to remove the surface oxide layer and expose the fresh copper surface. It is then thoroughly rinsed with ultrapure water and dried. Next, the sample is transferred to a pre-prepared and temperature-controlled copper deposition solution, which mainly consists of copper sulfate, a complexing agent, a reducing agent (formaldehyde or dimethylamine borane), and a small amount of stabilizer, adjusted to a suitable pH range. During deposition, the solution is gently stirred, and the sample is placed flat to facilitate bubble desorption. The deposition time is set according to the required film thickness, generally lasting several hours. After deposition, the sample is immediately removed, rinsed with ultrapure water, neutralized with a dilute alkaline solution to neutralize residual acidity, and finally dried. To improve the film density and conductivity, a low-temperature tempering treatment can be performed at 80–120°C. Through this process, the thin copper layer formed by sputtering serves as a catalyst surface, guiding the continuous reduction deposition of electroless copper ions, thereby forming a copper film with strong adhesion and excellent conductivity on the flexible PDMS substrate.

[0026] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of protection of the claims.

Claims

1. The technical solution adopted in this invention is a process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal, characterized in that, Includes the following steps: Step 1: Prepare PDMS with microstructures on the surface by mixing and then thermosetting on sandpaper; Step 2: A surface micro-protrusion peak structure is formed by strong acid treatment followed by high-temperature annealing; Step 3: Use plasma cleaning process to treat PDMS with a certain surface roughness; Step 4: Immerse the PDMS in the prepared PVA solution. Step 5: Sputter the adhered copper layer using magnetron sputtering; Step 6: Based on the above steps, the adhered copper layer is thickened by chemical deposition.

2. The process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal according to claim 1, characterized in that, The specific method for preparing the PDMS with microstructures on the surface is as follows: mixing the curing agent and prepolymer of PDMS, spin-coating it onto sandpaper of a certain mesh size, and curing it to obtain PDMS with a certain surface roughness.

3. The process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal according to claim 1, characterized in that, The specific method for forming the surface micro-protrusion structure by high-temperature annealing after strong acid treatment is as follows: the cured PDMS is ultrasonically treated in anhydrous ethanol to remove oil stains on the PDMS surface, the treated PDMS is completely immersed in HNO3 solution, and a certain temperature and time are maintained. After removal, it is rinsed with a large amount of deionized water and dried with N2. Under N2 atmosphere, the temperature is raised to 150℃ at a rate of 2–5℃ / min and maintained at this temperature for 1–2 hours to allow the surface acid etching layer to undergo further oxidation cross-linking and thermal shrinkage, generating a strain difference with the untreated substrate to induce a regular surface micro-protrusion structure.

4. The process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal according to claim 1, characterized in that, The method for preparing PDMS with a certain surface roughness by plasma cleaning is as follows: high-voltage ionized argon / oxygen gas is used to form plasma to impact PDMS with a certain surface roughness for surface treatment.

5. The process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal according to claim 1, characterized in that, The specific method for soaking PDMS in the prepared PVA solution is as follows: calculate the ratio of PVA particles to water according to the required concentration, add a certain mass of PVA particles to the water and heat and stir until the PVA particles are completely dissolved in the water, and then soak the treated PDMS in the prepared PVA solution.

6. The process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal according to claim 1, characterized in that, The method for preparing the adhered copper layer by magnetron sputtering is as follows: argon ions are used to bombard a copper target to excite copper atoms to sputter out and deposit them onto the substrate, forming a uniform and dense copper film.

7. The process for improving the adhesion of polydimethylsiloxane to the surface and chemically depositing metal according to claim 1, characterized in that, The chemical deposition method specifically involves immersing the PDMS with the copper-adhered layer obtained above in a chemical copper deposition solution, where copper ions in the solution are reduced to metallic copper by a reducing agent and deposited on its surface.

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