Study On Laser Surface Modification Of Polyimide:Key Technologies And Mechanisms

Polyimide?PI?has a wide application prospect in flexible microelectronics due to its excellent electrical,thermal and mechanical properties.However,its high chemical stability and low surface energy result in weak adhesion to other materials and limit its further applications.Laser surface modification has the advantages of high efficiency,good selectivity and high flexibility,which can change the physical and chemical properties of materials,and has important application value for improving adhesion strength to other materials.Therefore,the effects of laser modification on surface microstructure,chemical composition,wettability and adhesion strength of PI were systematically studied in this dissertation,and the related mechanisms were discussed in depth.The main results were listed as follows:Firstly,in the modes of single pulse and scan,surface microstructure modifications of PI were explored using lasers with different wavelength?355 nm,1064 nm?and pulse width?20 ns,15 ps?.In the single pulse mode,four different microstructures?i.e.,circular concave microstructure,concentric circular microstructure,circular convex microstructure and no obvious structural change?were obtained,respectively.While in the scan mode,depending on the laser fluence and pulse overlap,different microstructures?e.g.,porous flocs,periodic microwells,parallel microgrooves,nanoripples,bubbles and porous sponges,etc.?could be produced.Then,the microtopographies of PI surface ablated with single pulse of ultraviolet?UV?nanosecond laser were simulated by ANSYS software.The simulation accuracy was improved by the subsection calculation of pulse width and the correction of physical model.The simulation accuracies?i.e.,agreement with the experimental results?of both diameter and depth of the microwells were above 90%when the laser fluences were lower than 21.3 J/cm².It proved that the mechanism of UV nanosecond laser ablation of PI was mainly governed by the photothermal effect.Based on the analysis of surface micromorphologies and chemistry,it was proposed that the infrared?IR?pulse laser ablation of PI was governed by photothermal effect,while the UV picosecond laser ablation was dominated by photochemical effect.Secondly,in the scan mode,the effects of laser modification on surface chemistry of PI were systematically explored.Results showed that the O/C and N/C element content ratios of PI surface increased firstly and then decreased with the increase of laser fluence.Under the condition of constant laser fluence,the ratios decreased with the increase of pulse overlap when ablated with the UV nanosecond laser,the IR picosecond laser and the IR nanosecond laser.While for the UV picosecond laser,the O/C and N/C element content ratios increased with the increase of pulse overlap at fluences lower than 5.4 J/cm²;Whereas,the ratios increased firstly and then decreased with increasing pulse overlap at fluences higher than 5.4 J/cm².The changes of O/C and N/C element content ratios were probably due to the removals of oxygen-containing groups and/or nitrogen-containing groups on PI surface.At the same time,the ionized O₂ and N₂ molecules in the air could react with the radicals on PI surface,and such groups as C-O,C=O and C-N-C were produced.Thirdly,in the scan mode,the effects of laser modification on surface wettability of PI were investigated in detail.Results disclosed that after UV nanosecond laser modification,the wettability would transform from intrinsic hydrophilicity to superhydrophobicity with an increase of pulse overlap at low fluences.The superhydrophilic surface could be obtained with increasing pulse overlap at high fluences.However,in the same parameter range,the extrem wettability of PI surface could not be achieved after the other three lasers modification.Therefore,superhydrophobic/superhydrophilic patterns could be obtained by one-step fabrication process using UV nanosecond laser.According to the wetting theory analysis,the wettability of PI was dominated by chemical composition for UV picosecond laser ablation.However,the effect of surface roughness on wettability was not negligible when the PI surface was ablated by the other three lasers.In addition,it could be found that only UV nanosecond laser and IR picosecond laser modification would effectively improve the surface energy of PI.Fourthly,in the scan mode,the effects of laser modification on the adhesion strength between PI and copper layer were studied in detail.Experimental results showed that after UV nanosecond laser and IR picosecond laser modification,PI surface had larger surface energy and higher adhesion strength to copper layer.Therefore,surface energy could be used to qualitatively characterize the adhesion strength between PI and copper layer.Then,the response surface method was used to analyze the effects of UV nanosecond laser and IR picosecond laser processing parameters on the adhesion strength.Optimizing the processing parameters on adhesion strength above 0.6 MPa?the adhesion strength of 3M610 tape?using the established regression equation between the adhesion strength and the parameters,it was found that the scanning speeds of UV nanosecond laser and IR picosecond laser could reach about 1600 mm/s and 4800 mm/s,respectively.The scanning speed improved nearly 3 orders of magnitude on the basis of the existing literatures.Failure analysis of the copper layer showed that the adhesion strength was mainly affected by the depth of microholes after UV nanosecond laser modification,while the number of micropores on PI surface was the key factor to adhesion strength after IR picosecond laser modification.In summary,systematical studies of relevant rules and mechanisms on surface microstructure,chemical composition,wettability,surface energy and adhesion strength of PI after laser modification not only greatly deepe n the understanding of interaction mechanism between laser and PI,but also effectively broaden its application field.At the same time,the dissertation enriches the research direction of advanced laser manufacturing technology and the theoretical system of interaction between laser and materials.