In this paper, we report a novel capillary-driven self-assembly technique which proceeds in an air environment and demonstrate it by assembling square piezoelectric transducer (PZT) actuators for 28 diffuser valve micropumps on a 4-inch pyrex/silicon substrate: on the substrate, binding sites are wells of 24 m in depth and the only hydrophilic areas; on the bonding face of the PZT actuator, the central hydrophilic area is a square identical in size to the binding site, and the rim is hydrophobic; acrylate-based adhesive liquid is dispensed across the substrate and wets only the binding sites; the hydrophilic areas on the introduced PZT actuators self-align with the binding sites to minimize interfacial energies by capillary forces from the adhesive droplets; the aligned PZT actuators are pressed to contact the gold coated substrate by their rims and the adhesive is polymerized by heating to 85 C for half an hour, so permanent mechanical and electrical connections are established, respectively, at the center and rim of each PZT actuator. These pumps perform with high uniformity, which is indicated by a small standard deviation of their resonant frequencies to pump ethanol: the average resonant frequency is 6.99 kHz and the standard deviation is 0.1 kHz. Compared with the conventional bonding process with highly viscous silver epoxy, this assembly method has several major advantages: highly accurate placement with self-alignment, controllable adhesive thickness, tilt free bonding, low process temperature and high process repeatability. [1491] Index Terms-Capillary-driven self-assembly, diffuser valve micropump, recessed binding site, interfacial energy minimization. I. INTRODUCTION P ZT actuators can convert electrical energy to mechanical energy with fast response and are widely used as driving elements for several types of microfluidics devices such as micropumps [1], micromixers [2], [3] and microdispensers [4], among others. Typically, piezoelectric transducer (PZT) actuators are manually mounted onto a silicon, glass, or polymer substrate with batch-fabricated microfluidic components using highly viscous silver epoxy, a slow serial process without good control of process parameters such as placement of PZT actuators, adhesive thickness and PZT actuator tilting; therefore this assembly process cannot achieve good repeatability and does not scale well to wafer level packaging. Capillary forces have been exploited to assemble microstructures in two or three dimensions by several research groups [5]-[9]. We previously reported capillary-driven self-assembly Manuscript
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