Nanopillars by calcium chloride self-assembly and dry etching

Some applications of nanopillars (solar cell, Li ion battery, etc.) require neither long range order nor strict uniformity but more practicality of fabrication methods. Thin film of deliquescent salts may aggregate into nanodots (dry etch mask) by interaction with absorbed water, with enhanced practicalities such as easy mask removing, covering uneven surface, wide tunability and compatibility. However, only cesium chloride succeeded in last decade, based on diffusion. Herein, a new material calcium chloride (CaCl₂) is reported to self-assemble into nanodots for the first time, based on the reaction of crystalline hydrate. Their diameters could vary at least from 100 nm to 1.5 μm with coverage ratio above 35% by the control of film thickness and substrate moisture content. The formed dihydrate dots can keep integrity for more than 1 month in general environments, stable enough for practical use. The whole process flow is possibly controlled within 20 min, due to the fast forming and removing of CaCl₂ dots. Besides, the dots have been made on Si and PMMA substrates and transferred into pillars by dry etch, during which enough etch resistance and good coverage over rough surface are revealed.