Fluorosilane-induced softening and collapse of micropillar arrays

Abstract

Replica molding is a widely used technique for the fabrication of polymer microstructures. As structural dimensions decrease, anti-stick surface treatment of the mold becomes increasingly critical to ensure clean demolding and preserve structural integrity. We fabricated arrays of micropillars with 20 µm diameter and 60 µm height using medical-grade polydimethylsiloxane (PDMS), MDX4-4210, and observed a high fraction of collapsed pillars for the first molding after fluorosilanization of the mold to reduce sticking. To address this issue, we systematically investigated the surface treatment protocol for the molds, made from the PDMS Sylgard 184. We provide results from complementary measurement methods, to show that an additional vacuum step partially removes unbound fluorosilane, but does not improve pillar stability. In contrast, a method based on multiple replications, where the first replication effectively removes residual fluorosilane from the mold, significantly enhances structural stability. Mechanical testing further revealed that the presence of fluorosilane lowers the Young’s modulus of both PDMS materials, MDX4-4210 and Sylgard 184, suggesting interference with the curing process. Confocal Brillouin microscopy indicated an elongation of replicated pillars and revealed a softening close to the surfaces, as well as mechanical inhomogeneities in collapsed pillars. We discuss modifications to the molding protocol to improve the reproducibility and mechanical stability of the replicated microstructures, offering insights towards more reliable routes for the fabrication of residue-free, high-aspect ratio features with controlled surface chemistry.