Opto-thermo-fluidic transport phenomena involving thermocapillary flow during laser microfabrication

Abstract

Laser direct synthesis and patterning technology (LDSP) is a promising way to realize flexible electronic circuitry. In LDSP, functional materials microstructure can be fabricated in-situ on flexible substrates using laser induced photo thermal reduction of ions from precursor that is free of nanoparticles. However, previous studies showed that concave curvature was found on the surface of developed LDSP micro-lines. This occurrence of concave curvature is contradictory to the convex structure expected from the temperature distribution of the precursor and on the substrate. Controlled surface morphology is crucially important for the fabrication of electronic circuitry with ultra-fine structure. Therefore, an analysis of the transport phenomena prevailing near the laser processing spot by both numerical simulation and experiment was carried out. The underlying physics behind the surface reconstruction of silver micro-lines fabricated by LDSP was investigated. The analysis included the opto-thermo-fluidic transport phenomena as well as thermo-capillary flow, i.e. the Marangoni effect. Results showed that the Marangoni effect dominated the movement kinetics on the interfacial surface between the ultra-thin silver nanocluster film and the surrounding fluid during the LDSP process. The surface morphology of the synthesized and deposited silver micro-line is therefore affected by the Marangoni effect. An empirical model was developed to determine the values of characteristic parameters for the Marangoni effect in the present case. The model was applied successfully to determine the micro-line surface morphology evolved during multiple laser scans in LDSP process with three different scanning speeds. Theoretical solution on the silver line thickness was compared with that obtained from the experiments, and the discrepancy was less than 12%. The results and analysis techniques developed provide insights to the coupled heat and mass transport phenomena with thermo-capillary effects, and can be used to improve the quality of microstructures fabricated by LDSP and similar technologies.