Localization of Microparticles in Solution By Microfluidic Interactions on a Patterned Substrate

Accurate and efficient fabrication is critical to the progress of nanotechnology. Most existing nanofabrication techniques use top-down methods, although some bottom-up methods are used. By comparison, in macroscopic fabrication processes modular prefabrication and subsequent assembly allows for high efficiency and reduced costs. However, the assembly of prefabricated micron-scale particles with high accuracy and throughput remains challenging. One example is the assembly of Micro-LED (𝜇LED) arrays. 𝜇LED displays offer superior brightness, stability, and power consumption¹ but the individual LEDs must be grown on separate wafers then transferred to form a display. Traditional pick and place methods are accurate but limited in throughput. Existing fluidic assembly methods offer a higher throughput, but accurate techniques exist only for the confinement of particles at over 10 𝜇m³ or submicron² size ranges.

This work develops a fluidic assembly technique to localize microparticles onto a patterned wafer. By patterning a silicon wafer with regions of gold and silicon oxide then thiolating the gold to increase hydrophobicity, microfluidic interactions can localize colloidal particles. A microfluidic channel is formed to press a droplet of nanoparticle solution into a thin film, then the cover is removed causing the bulk fluid to recede and leave behind microdroplets that localize the particles as they evaporate. Localization can also be controlled by altering the solvent properties. For example, adding isopropanol to the primarily aqueous solvent increases surface energy but also decreases the cohesion in the bulk solvent to improve droplet formation. On a thiol-treated gold substrate adding 30% isopropanol by volume gave the highest localization density as shown in Figure 1. This study presents preliminary results and proof of concept for fluidic assembly of microparticles with potential applications in microLED technologies and other microelectronics.

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Figure 1: Microscope image of particles localized on a hexagonally patterned substrate.