Self-assembly of PZT actuators for micropumps with high process repeatability

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 mum 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 degC 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