Self-aligned positioning technology to connect ultra-small RFID powder-chip to an antenna

We propose a process of stress-free self-alignment of an ultra-small, ultra-thin (75×75×5 μm) radio frequency identification (RFID) chip (called a “powder chip”) on an antenna film. The process consists of only two simple steps: dispensing a micro-liter liquid droplet with a semispherical diameter of about 1 mm onto a hydrophilic/hydrophobic patterned surface on the antenna film and then releasing a single powder chip onto the droplet. In this method, the dispensed droplet is several tens of times larger than the chip. The hydrophilic surface domain, functioning as an electrode, exists inside the hydrophobic domain. First, the chip released onto the liquid droplet is captured on the liquid/atmosphere interface and is kept floating on the droplet. The droplet gradually shrinks due to evaporation, and the liquid on the hydrophobic domain moves toward the hydrophilic domain. In other words, the liquid that captured the chip is drawn into the hydrophilic electrode domain. When the droplet has completely evaporated, the chip is self-aligned on the hydrophilic electrode domain under stress-free conditions. The key to this technique is determining the balance between the chip size and the liquid droplet size that is necessary to keep the chip afloat. First, the feasibility of the floating condition of the chip on the liquid droplet was determined by calculating isostasy and Laplace pressure force equations. Next, in order to select an appropriate hydrophobic surface, changes in the receding contact angles during the motion of liquid droplet shrinkage on various hydrophobic patterned surface substrates were investigated. On a fluorocarbon-based hydrophobic surface, a greater hydrophobic contact angle of more than 90° was kept stable, even for a droplet size with a substrate contact width of 0.7 mm. Furthermore, a receding contact angle of about 20° was obtained even after the droplet had shrunk to a diameter of 0.2 mm. In these conditions, the powder chip was able to move together with the droplet edge, and was positioned in the predefined 150-um-square hydrophilic domain. The combination of our double-surface-electrode chip and the liquid droplet self-aligned process makes it possible to place a chip on a predetermined area on an antenna film without high-accuracy positioning requiring high-end robotics.