Method for improving compactness of laser direct writing flexible copper circuit

Abstract

This invention discloses a method for improving the density of flexible copper circuits through laser direct writing, belonging to the field of flexible electronics manufacturing technology. The method includes: preparing a precursor layer on the surface of a cleaned and fixed target substrate using a precursor, wherein the precursor includes a reducing copper ion solution; irradiating the precursor layer with a focused laser to perform laser direct writing, forming a monolayer copper microstructure; repeating the above steps until the copper microstructure is stacked layer by layer to a preset thickness; cleaning the sample surface to remove unreacted residual precursor, obtaining a high-density copper microstructure. This invention, under limited laser energy input, matches the optical penetration depth with the precursor thickness, avoiding thermal damage to the substrate while obtaining a monolayer copper microstructure; employing a layer-by-layer stacking direct writing strategy, the newly added precursor penetrates into the previously formed copper microstructure, forming new nano-copper particles to fill the pores under the action of the laser again, effectively improving the final structure density.

Description

Technical Field

[0001] This invention relates to a method for improving the density of laser-written flexible copper circuits, belonging to the field of flexible electronics manufacturing technology. Background Technology

[0002] Flexible electronic devices, such as wearable health monitoring devices, flexible displays and lighting, electronic skin, and biomimetic sensors, show broad application prospects in fields such as biomedicine, consumer electronics, the Internet of Things, and human-computer interaction. Flexible circuits, referring to bendable / stretchable electronic circuits based on flexible polymers, are the core components for constructing flexible electronic devices. Traditional manufacturing methods rely on subtractive processes such as photolithography, which are costly and environmentally polluting. In contrast, emerging additive manufacturing technologies for printed electronics offer advantages such as low material loss, low cost, and environmental friendliness, and have garnered significant attention in recent years.

[0003] Typical printed electronics technology employs a step-by-step approach, involving nanomaterial synthesis, patterning, and post-processing thermal sintering. Laser direct writing technology offers advantages such as selectable areas, contactless processing, and maskless fabrication, and can efficiently integrate these step-by-step processes based on nano-copper oxides or copper ions. For example, existing technologies prepare a precursor by mixing nano-copper oxides with dispersants and solvents, then utilize the photothermal effect generated by laser irradiation to connect and sinter the nanoparticles, resulting in flexible copper circuits. Other existing technologies directly start from a copper ion precursor containing copper ions and a reducing agent, using laser irradiation to reduce and obtain nano-copper, which is then in-situ connected and sintered to construct flexible copper circuits.

[0004] Obtaining high-density microstructures while ensuring high purity is crucial for fabricating high-performance copper circuits. However, due to the easily oxidized nature of nano-copper and the heat resistance of the substrate, the laser heat input must be maintained at a low level during laser direct writing. Under these conditions, nano-copper only undergoes surface melting, forming sintered necks to complete the connection, resulting in a sintered structure different from macroscopic bulk materials (dense polycrystalline). Simultaneously, the precursors used in laser direct writing must contain ligands such as dispersants, reducing agents, and rheology modifiers to ensure printability / film-forming properties. Their decomposition and volatilization during processing also introduce porosity, significantly limiting the conductivity and stability of the resulting circuits. In recent years, researchers have conducted extensive work on optimizing precursor composition and laser parameters, but the low density problem of copper circuits remains unresolved due to the inherent mechanism of laser direct writing.

[0005] On the other hand, the trend towards multifunctionality and miniaturization in flexible electronic devices places higher demands on the current-carrying capacity of circuits. Most high-performance circuits obtained using existing laser direct writing technology are sub-micron thick, primarily because the optical penetration depth of lasers is limited. Once this thickness is exceeded, the reduction / sintering of the precursor inevitably relies on vertical heat conduction, further exacerbating the difficulty of shape control for flexible copper electrodes. For example, existing technologies indicate that when using laser direct writing of nearly 10 μm nano-copper nanoparticle precursors, due to heat attenuation in the depth direction, even if the nano-copper has completely melted and oxidized, the underlying nano-copper remains a porous, low-density structure.

[0006] In summary, laser direct writing exhibits unique advantages in the fabrication of flexible copper circuits. However, due to limitations such as the heat resistance of flexible thermosensitive substrates, the easy oxidation of nano-copper, the volatilization of ligands in precursors, and the optical penetration depth of lasers, it is difficult to ensure both copper purity and high circuit density. Consequently, the conductivity and reliability of the resulting circuits cannot meet application requirements. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for improving the density of laser-written flexible copper circuits, which uses a layer-by-layer laser-written route to construct high-density flexible copper circuits.

[0008] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0009] In a first aspect, the present invention provides a method for improving the density of laser-written flexible copper circuits, comprising:

[0010] A precursor layer is prepared on the cleaned and fixed target substrate surface using a precursor, wherein the precursor comprises a reducing copper ion solution;

[0011] The precursor layer is irradiated with a focused laser and laser direct writing is performed to form a single-layer copper microstructure.

[0012] Repeat the above steps until the copper microstructure is deposited layer by layer to the preset thickness;

[0013] The sample surface was cleaned to remove unreacted residual precursors, resulting in a high-density copper microstructure.

[0014] Furthermore, the reducing copper ion solution is formed by dissolving copper salts, complexes, and reducing ligands in a solvent.

[0015] Furthermore, the copper salt is selected from at least one of copper nitrate, copper formate, copper sulfate, and copper acetate; the reducing ligand is selected from at least one of monohydric alcohol, polyhydric alcohol, and formic acid; and the complex is selected from at least one of amino alcohol and organic amine.

[0016] Furthermore, the precursor also includes copper nanoparticles mixed in a reducing copper ion solution.

[0017] Furthermore, the mass percentage of the reducing copper ion solution in the precursor is not less than 60%.

[0018] Furthermore, the copper nanoparticles are at least one of copper, cuprous oxide, and copper oxide.

[0019] Furthermore, the target substrate is any one of polyimide, polyethylene terephthalate, polycarbonate flexible thermosensitive substrate, or glass fiber board rigid thermosensitive substrate.

[0020] Furthermore, the precursor layer is prepared by spin coating, micropen direct writing, or bar coating; the single-layer thickness of the precursor layer is no greater than 1 µm.

[0021] Furthermore, the thickness of a single precursor layer is matched with the effective optical penetration depth of the focused laser, so as to achieve a uniform distribution of light-thermal energy within each precursor layer.

[0022] Furthermore, the method for preparing the high-density copper microstructure includes: preparing a precursor layer on the already formed monolayer copper microstructure, and irradiating the newly prepared precursor layer along the same path using a focused laser, so that the newly prepared precursor layer penetrates and fills the pores of the previously formed monolayer copper microstructure to form new nano-copper particles, the new nano-copper particles connecting with the previously formed copper microstructure and filling the pores, and accumulating layer by layer to form the high-density copper microstructure.

[0023] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0024] (1) The present invention decomposes the target thickness into multiple precursor layers, so that the thickness of each precursor layer matches the effective optical penetration depth of the laser, thereby achieving uniform distribution of light-thermal energy and full reaction of reducing copper ion solution within each precursor layer, significantly promoting the reduction and sintering fusion of copper, and ensuring that gaseous byproducts can escape in time.

[0025] (2) The present invention uses a single laser energy lower than the target substrate damage threshold for direct writing, which ensures that the precursor layer reacts fully while avoiding thermal damage and performance degradation of the flexible or rigid thermosensitive substrate due to heat accumulation or local overheating.

[0026] (3) The present invention can accurately and flexibly construct copper microstructures of the required thickness by layer-by-layer stacking and direct writing. In particular, during the repeated preparation of precursor layers and laser direct writing, the newly prepared precursor layers penetrate into the pores of the previously formed copper microstructures, and new nano-copper particles are formed and connected to them under laser irradiation, filling the pores and thus improving the density of the final copper microstructure. Attached Figure Description

[0027] Figure 1 This is a flowchart of the method for improving the density of laser-written flexible copper circuits provided in an embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of the mechanism of the layer-by-layer direct writing method described in this invention;

[0029] Figure 3 This describes the morphology of the circuit obtained by direct writing layer by layer in Embodiment 1 of the present invention;

[0030] Figure 4 This is the microstructure of the circuit obtained by direct writing layer by layer in Embodiment 2 of the present invention;

[0031] Figure 5 This describes the morphology of the circuit obtained by direct writing layer by layer in Embodiment 3 of the present invention;

[0032] Figure 6 This is the microstructure of the circuit obtained by direct writing layer by layer in Embodiment 3 of the present invention;

[0033] Figure 7 This is the morphology of the submicron-thickness circuit obtained by a single direct write in Comparative Example 1 of the present invention;

[0034] Figure 8 This is the microstructure of the circuit obtained by cyclic direct writing in Comparative Example 2 of this invention;

[0035] Figure 9 This is the microstructure of the micron-thickness circuit obtained by a single direct write in Comparative Example 3 of this invention. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0037] like Figure 1 As shown, this invention introduces a method for improving the density of laser-written flexible copper circuits, comprising:

[0038] A precursor layer is prepared on the cleaned and fixed target substrate surface to form a precursor layer, wherein the precursor includes a reducing copper ion solution;

[0039] The precursor layer is irradiated with a focused laser and laser direct writing is performed to form a single-layer copper microstructure.

[0040] Repeat the above steps until the copper microstructure is deposited layer by layer to the preset thickness;

[0041] The sample surface was cleaned to remove unreacted residual precursors, resulting in a high-density copper microstructure.

[0042] like Figure 2 As shown, the method for improving the density of laser-written flexible copper circuits provided by the present invention specifically involves the following steps in its application process:

[0043] S1. Dissolve copper salts, complexes, and reducing ligands in a solvent to form a reduced copper ion solution to obtain a precursor, or mix the reduced copper ion solution with copper nanoparticles to form a precursor.

[0044] In step S1, the precursor is a reducing copper ion solution or a mixture of the reducing copper ion solution and copper nanoparticles. The reducing copper ion solution is prepared by mixing a copper salt, a polyol, a complex, and a polar solvent at room temperature. The complexation effect of the polar solvent can effectively improve the solubility of copper ions and can also act as a weak reducing agent to improve the purity of the structure obtained by direct writing. The copper salt can be one or more of soluble copper salts such as copper nitrate, copper sulfate, and copper acetate; the polyol can be one or more of ethylene glycol and glycerol; the complex can be one or more of amino alcohols and organic amines; and the polar solvent can be one or more of N-methylpyrrolidone and N,N-dimethylformamide. The copper nanoparticles can be one or more of copper, cuprous oxide, or copper oxide (preferably with an average particle size of 20-100 nm); when the precursor is formed by mixing the reducing copper ion solution and copper nanoparticles, the mass percentage of the reducing copper ion solution in the precursor is not less than 60%.

[0045] S2. Clean the target substrate and fix it on the laser processing platform;

[0046] In step S2, the target substrate can be a flexible thermosensitive substrate such as polyimide, polyethylene terephthalate, or polycarbonate (preferably with a thickness of 50-200 μm), or any of the rigid thermosensitive substrates such as glass fiberboard.

[0047] S3. Prepare a precursor layer on the surface of the target substrate by printing / coating or other methods;

[0048] In step S3, the precursor layer can be prepared in the following ways: by spin coating at 500-3000 rpm for 10-60 s to form the precursor layer; or by micropen direct writing, using nozzles of different inner diameters (preferably 10-100 µm) and adjusting the moving speed (preferably 1-20 mm / s) to form precursor traces with controllable width; or by rod coating, using Mayer rods of appropriate specifications to coat and form the precursor layer. Crucially, the thickness of each single-layer precursor layer prepared by these methods is no greater than 1 µm.

[0049] S4. The precursor layer is irradiated with a focused laser to prepare copper microstructures by laser direct writing; the laser wavelength meets the absorbance requirements of the precursor or substrate.

[0050] In step S4, the laser can be a nanosecond, femtosecond, or continuous laser, especially suitable for low-cost long-pulse or continuous lasers. Furthermore, the laser's power, wavelength, spot diameter, and scanning speed are adjusted to match the absorbance of the precursor or substrate.

[0051] S5. Repeat the process described in steps S3-S4 until the copper microstructure is deposited layer by layer to the preset thickness.

[0052] S6. Clean the sample surface with deionized water or a suitable solvent to remove residual unreacted precursors and obtain a high-density copper microstructure.

[0053] The following description, in conjunction with preferred embodiments, explains the contents involved in the above embodiments.

[0054] Example 1

[0055] This embodiment provides a method for fabricating high-density copper circuits by layer-by-layer direct writing, including the following steps:

[0056] (1) Mix 7.3 g of copper nitrate trihydrate, 1 mL of ethylene glycol, and 10 mL of NMP, and stir magnetically until completely dissolved to obtain a reduced copper ion solution with a copper ion concentration of approximately 3 mol / L. The solution is a viscous blue solution, and no significant sedimentation was observed after one year of storage. Compared with existing reduced copper ion solutions (often called particle-free inks), the solution described in this embodiment has advantages such as simple preparation, no need for purification, and good stability.

[0057] (2) Select a glass fiber board with a fire resistance rating of FR4 as the base, clean it with deionized water and ethanol and then dry it before fixing it on the worktable of the laser processing system.

[0058] (3) A reduced copper ion solution was prepared on the substrate surface by spin coating to form a precursor layer. The spin coating parameters were 1000 rpm and 30 s.

[0059] (4) A semiconductor near-infrared laser with a wavelength of 808 nm was used to perform direct writing of the precursor layer by selective irradiation. The parameters used were laser power of 4.2 W, spot diameter of ~600 μm, and scanning speed of 10 mm / s.

[0060] (5) After the first layer is written directly, repeat the spin coating step (3) to prepare the precursor layer again, and repeat the laser writing process (4) using the same scanning path, repeating it three times in total.

[0061] (6) Clean the sample with deionized water and dry it to obtain the final copper circuit.

[0062] like Figure 3 As shown, the resulting electrode surface is continuous and dense, indicating that the circuit obtained by the layer-by-layer manufacturing process is well formed.

[0063] Example 2

[0064] This embodiment provides a method for fabricating high-density copper circuits by layer-by-layer direct writing, including the following steps:

[0065] (1) Mix 2.2 g copper tetrahydrate, 0.9 g isobutanolamine, 1.3 g n-octylamine and 1 mL ethylene glycol, and stir magnetically until completely dissolved to obtain a reduced copper ion solution.

[0066] (2) A 0.1 mm thick polyimide was selected as a flexible substrate, and after being cleaned with deionized water and ethanol and dried, it was fixed on the worktable of the laser processing system.

[0067] (3) Load the reducing copper ion solution into the injection pump and print the precursor traces on the substrate surface using the micropen direct writing method. The nozzle inner diameter is 60 μm (model G34) and the direct writing speed is 5 mm / s.

[0068] (4) The obtained precursor traces were irradiated with a semiconductor near-infrared laser with a wavelength of 808 nm for direct writing. The parameters used were laser power of 1 W, spot diameter of ~50 μm, and scanning speed of 5 mm / s.

[0069] (5) After the first layer is written, repeat the micropen writing step (3) to prepare the precursor layer again on the same trajectory, and repeat the laser writing process (4) using the same scanning path. Repeat five times to obtain a copper microcircuit with a thickness of about 10 μm.

[0070] (7) Clean the sample with ethanol and dry it to obtain the final copper circuit.

[0071] Figure 4The image shown is a scanning electron microscope image of the obtained circuit. It can be seen that there are only a very small number of pores on the surface of the obtained circuit, which is significantly better than the existing sintered structure, indicating that the circuit obtained by the layer-by-layer manufacturing has a high density.

[0072] Example 3

[0073] This embodiment provides a method for fabricating high-density copper circuits by layer-by-layer direct writing, including the following steps:

[0074] (1) Mix 2.2 g copper tetrahydrate, 0.9 g isobutanolamine, 1.3 g n-octylamine and 1 mL ethylene glycol, and stir magnetically until completely dissolved to obtain a reduced copper ion solution.

[0075] (2) Mix 0.5 g of copper nanoparticles (average particle size 50 nm), 0.3 g of reducing copper ion solution and 0.5 g of ethylene glycol, and stir magnetically until completely dispersed to obtain the precursor used for direct writing.

[0076] (3) A 0.1 mm thick polyimide was selected as a flexible substrate, and after being cleaned with deionized water and ethanol and dried, it was fixed on the worktable of the laser processing system.

[0077] (4) A precursor layer was prepared on the substrate surface by bar coating, and the thickness of the single-layer precursor was controlled to be 1 μm by Mayer rods.

[0078] (5) A nanosecond laser with a wavelength of 532 nm was used to perform direct writing of the precursor layer by selective irradiation. The parameters used were laser power of 1 W, spot diameter of ~20 μm, and scanning speed of 5 mm / s.

[0079] (6) After the first layer is written, repeat the rod coating in step (4) to add the precursor layer again, and repeat the laser writing process in step (5) using the same scanning path, repeating it three times in total.

[0080] (7) Clean the sample with ethanol and dry it to obtain the final copper circuit.

[0081] Figure 5 The image shown is a macroscopic photograph of the obtained circuit. It can be seen that the circuit is continuous and has a metallic luster, indicating that it is well formed. Figure 6 The image shown is a scanning electron microscope image of the obtained circuit. It can be seen that the microstructure of the obtained circuit is mainly composed of sintered nanoparticles, which are relatively dense.

[0082] Comparative Example 1

[0083] This comparative example provides a method for fabricating copper circuits by cyclic direct writing after a single coating, i.e., performing multiple laser scans without repeatedly adding precursors. Similar to Example 3, the only difference is that the rod coating in step (4) is not repeated in step (6), and only the number of laser scanning direct writings is increased. Figure 7 The surface morphology of the obtained circuit is shown. It can be seen that the circuit exhibits obvious oxidation color under multiple scans, indicating that the addition of laser irradiation to the precursor layer by layer is the key to avoiding oxidation of the circuit during processing and improving its density.

[0084] Comparative Example 2

[0085] This comparative example provides a method for fabricating submicron-thickness copper circuits in a single coating and direct writing process. Similar to Example 3, the only difference is that step (6) is omitted, while the remaining operations remain the same. Figure 8 The microstructure of the obtained circuit is shown. It can be seen that there are large-scale pores on the circuit surface, mainly caused by the volatilization of ligands in the precursor. This proves that adding the precursor again and performing laser direct writing plays an important role in improving the density of the previously obtained structure.

[0086] Comparative Example 3

[0087] This comparative example provides a method for fabricating micron-thick copper circuits by single-pass direct writing after a single coating. Similar to Example 2, the only difference is that: in step (3), the micropen direct writing speed is reduced to 1 mm / s to increase the thickness of the single-pass precursor trace; in step (4), the laser power is increased to 3 W to achieve complete reduction-sintering of the thick precursor trace; step (5) is omitted, and the remaining operations remain the same. Figure 9 The microstructure of the obtained circuit is shown. It can be seen that the surface density of the circuit has increased slightly, but there is obvious overburning phenomenon (mainly blocky oxide morphology), indicating that it is difficult to guarantee the structural purity of copper circuits formed in one step beyond the laser penetration depth.

[0088] Test Example 1: For the copper circuits prepared in Examples 1, 2, and 3, their thickness was measured using a scanning electron microscope, and their sheet resistance was measured using a Keithley 2450 source meter. The resistivity of each sample was then calculated. The resistivity of the circuit obtained in Example 1 was as low as... The resistivity Ω·m is only about twice that of bulk copper, indicating that the high-density circuit prepared by the method has high conductivity; the resistivity of Example 2 is Ω·m, significantly better than Comparative Example 3. The Ω·m value indicates that, under conditions of similar precursors and comparable target circuit thickness, the layer-by-layer direct writing strategy (Example 2) demonstrates a significant advantage over single-pass thick-layer direct writing (Comparative Example 3). The significant reduction in resistivity directly confirms that the layer-by-layer added precursors can effectively penetrate the existing structure and fill pores, constructing a highly dense copper microstructure under repeated laser irradiation. This proves that the present invention significantly improves the density of copper circuits while effectively maintaining copper purity.

[0089] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for improving the density of a laser direct written flexible copper circuit, characterized by, include: A precursor layer is prepared on the cleaned and fixed target substrate surface using a precursor, wherein the precursor comprises a reducing copper ion solution; The precursor layer is irradiated with a focused laser and laser direct writing is performed to form a single-layer copper microstructure. Repeat the above steps until the copper microstructure is deposited layer by layer to the preset thickness; Clean the sample surface to remove unreacted residual precursors and obtain a high-density copper microstructure. The reducing copper ion solution is formed by dissolving copper salts, complexes, and reducing ligands in a solvent; The method for preparing the high-density copper microstructure includes: preparing a precursor layer on the already formed monolayer copper microstructure, and irradiating the newly prepared precursor layer along the same path using a focused laser, so that the newly prepared precursor layer penetrates and fills the pores of the previously formed monolayer copper microstructure to form new copper nanoparticles. The new copper nanoparticles connect with the previously formed copper microstructure and fill the pores, and the high-density copper microstructure is formed layer by layer.

2. The method of claim 1, wherein the laser direct writing flexible copper circuit is a flexible copper circuit. The copper salt is selected from at least one of copper nitrate, copper formate, copper sulfate, and copper acetate; the reducing ligand is selected from at least one of monohydric alcohol, polyhydric alcohol, and formic acid; and the complex is selected from at least one of amino alcohol and organic amine.

3. The method of claim 1, wherein the laser direct writing flexible copper circuit is a flexible copper circuit. The precursor also includes copper nanoparticles mixed in a reducing copper ion solution.

4. The method for improving the density of laser-written flexible copper circuits according to claim 3, characterized in that, The mass percentage of the reducing copper ion solution in the precursor is not less than 60%.

5. The method for improving the density of laser-written flexible copper circuits according to claim 3, characterized in that, The copper nanoparticles are at least one of copper, cuprous oxide, and copper oxide.

6. The method for improving the density of laser-written flexible copper circuits according to claim 1, characterized in that, The target substrate is any one of polyimide, polyethylene terephthalate, polycarbonate flexible thermosensitive substrate, or glass fiber board rigid thermosensitive substrate.

7. The method for improving the density of laser-written flexible copper circuits according to claim 1, characterized in that, The precursor layer is prepared by spin coating, micropen direct writing, or bar coating; the thickness of a single layer of the precursor layer is no greater than 1µm.

8. The method for improving the density of laser-written flexible copper circuits according to claim 1, characterized in that, The thickness of each precursor layer is matched with the effective optical penetration depth of the focused laser to achieve a uniform distribution of light-thermal energy within each precursor layer.

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