Measuring Liquid Drop Properties on Nanoscale 1D Patterned Photoresist Structures.

This communication reports liquid wetting properties of DI-water on one-dimensional nano-patterned photoresist lines atop a silicon substrate as the pattern period is varied from 0.3- to 1.0-µm. Both constant photoresist height and constant width/height ratios are investigated. The line/period ratio was fixed at 0.3 (0.4) for different measurement sequences. The surface of the photoresist was treated with a short CHF3 reactive ion etch to ensure consistent hydrophobic photoresist: water surface energies. Average parallel contact angle (θ||), average perpendicular contact angle (θ⊥), drop width (W), and drop length (L) at constant volume were measured on nano-patterned surfaces fabricated with interferometric lithography. Both θ|| and θ⊥ contact angles increase as the period (0.3- to 1-µm) increases; the θ|| spreading rate is faster than θ⊥ due to pinning on the grooves resulting in an elongated drop shape. The traditional Wenzel and Cassie-Baxter models of drop contact angles were developed for isotropic random 2D roughness and do not account for the anisotropy induced by the 1D line patterns. The observed angular variations with period are not consistent with either model. Understanding liquid wetting properties and hydrophobicity on 1D silicon surfaces has many applications in lab-on-a-chip, micro/nano-fluidic devices, roll-to-roll nano-imprint fabrication, self-cleaning surfaces, and micro-reactors.