Shear alignment of particles during spin coating

Abstract

Spin coating is a process for applying uniform coatings to a wide variety of substrates. The final film thickness depends on viscosity, drying rate, percentage of solids, and surface tension and many of these influencing factors have been investigated. Our recent research has been focused on alignment of anisotropic particles being deposited by spin coating. One key aspect of spin coating is the shear forces that are experienced by the fluid before solvent evaporation takes over and freezes in a final particle arrangement. We have calculated the shear experienced by the particles during all stages of the process and tracked the fluid flow to determine the individual particle trajectories. A small fraction of fluid that starts near the center of the wafer contributes to the final coating while the majority of the fluid is flung off. From our calculations and associated measurements, it is evident that the particles away from the center of the wafer are more aligned toward the flow direction due to the higher shearing effect they experienced. Less alignment of particles is found near the center of the wafer as shear is significantly lower at the center. Experimental investigation has been performed to verify the numerical results. Alignment of the particles in the final coating can be advantageous for various optical, electrical and magnetic applications.