Hierarchical structure formation and pattern replication by capillary force lithography

Abstract

The soft lithographic method is gaining popularity in the manufacturing of low-cost micro and sub-microstructures. For several applications, it is desirable to fabricate structures with different lateral size as well as different height, i.e. hierarchical structures, on the same substrate. This is difficult for normal microfabrication methods, particularly for the photolithography method, which is the main method used in the IC industry. Here we demonstrate a simple yet cost-effective process to form hierarchical structures by capillary force lithography (CFL). In our process, the simple principle that the time needed for the fluid to fill up the cavity in the mould is different if the contact angle of the mould or the width of the cavity in the mould is different. Based on this principle, two methods were developed to achieve hierarchical structures by CFL. One is to selectively modify the PDMS mould by UV or oxygen plasma treatment to change its wettability in different areas. The other is to make patterns with variable dimensions in the mould. By using the PDMS mould with selectively modified wettability or patterns with different lateral dimensions, hierarchical structures with a convex/concave microlens array or stripes with convex/concave surfaces have been manufactured successfully.