Abstract
Recent advances of miniaturization and microfluidics technologies bring novel ways of parallely assembling thousands of micron-scale electronic components in fluidic phase. Conventional pick-and-place technology platform in handling micron-scale components assembly processes encounters tremendous difficulties in terms of capacity, efficiency and accuracy. Hence, Fluidic Self-Assembly (FSA) approach provides an alternative means for fast, economic, and precise handling of thousands of micro-scale parts. The present study is intended to delineate the detailed features of fluidic self-assembly process of micro-scale parts and examine the important variables which govern the mechanisms of fluidic self-assembly process by numerical simulations. In order to characterize the strong interactions between the micro-part and lubricant, commercial Computational Fluid Dynamics (CFD) software which can handle Fluid-Structure Interaction (FSI) problems is utilized to investigate the effects of lubricant height and contact angles of interfaces between micropart-substrate and lubricant-substrate on the accuracy of micropart’s self-alignment process. The computational model is based on first principle conservation equations and is constructed by the coupling of two- phase modeling using volume of fraction, solid structure modeling, and fluid-structure coupling. A matching experimental system is set up for the micropart of aspect ratio from 3:1 to 10:1 to validate the 2-D computational simulations. Simulations reveal that high degree of hydrophilicity between lubricant and solid surfaces is required for self-assembly restoring, and lower lubricant height, higher surface tension coefficient and higher viscosity enforce the re-alignment/restoring process. Characterization of the flowfield inside lubricant slug also indicates that the asymmetry of the vortices/stress distribution at both ends of the lubricant meniscus drives the micropart in an oscillation restoring process. The micro parts fabricated from silicon-oxide wafers and ranging in size from 350×350×170 μm3 to 1000×1000×440 μm3, aligned and filled to designated sites in the substrate under water. The effects of micropart sizes and lubricants on the FSA processes are compared. This study provides a fundamental analysis for achieving and optimizing the self-alignment. The polymer or solder adhesion force of the square-patterned micropart immobilized at the larger binding sites were estimated to be 117±15 μN and 510±50 μN, respectively, resulting in higher assembly yield of up to 100% for these samples. Another research is to develop a novel design of two-dimensional modified alignment mark of tear-drop/ elliptical hole with a tip angle of 60 (TDE-1 and TDE-2 pattern shapes) are adopted to improve the recovery angle and reduce the energy barrier to uni-directional micropart alignment. The results of the experimental and surface energy model are compared both qualitatively and quantitatively to examine the feasibility of the new design patterns. Experimental results reveal that the micropart of TDE patterns could be accurately aligned by rotation through 90°and higher capillary force. The acrylate adhesive force of the TDE-2 patterned micropart was estimated to be 41.2±10μN. Fluidic self-assembly (FSA) was performed carried out in an aqueous environment, using a low-temperature solder adhesive for part-substrate lubricant and aligned template-assisted assembly. The standard deviation of aligned angular orientation was 0.9°and that of lateral accuracy was 15 μm ; an assembly yield of 100% was achieved. Micropart self-alignment with a unique in-plane orientation is achieved by combining shape recognition and the adhesive capillary effect. This self-assembly technique could be used to produce a heterogeneous system for packaging, including LSI and MEMS.