Conformal phase masks made of polyurethane acrylate with optimized elastic modulus for 3D nanopatterning

The pattern resolution of soft lithographic techniques is critically determined by the elastic modulus of the soft mold that can support fine and high-aspect-ratio features with conformal adhesion to target substrates. We present a strategy to fine-tune the elastic modulus of conformal molds made of polyurethane acrylate by optimizing the chemical structures and the composition of prepolymer and modulator. Trimethylolpropane ethoxylated (15) triacrylate plays a key role as a delicate modulator for increasing the elastic modulus of soft aliphatic urethane diacrylate oligomer with its low cross-linking density. The optimized molds have sufficiently high elastic modulus (>23 MPa) for defect-free replication of dense, high-aspect-ratio (>2) nanopillars and nanotrench structures while still preserving their conformality. The conformal mold with good mechanical and optical properties can serve as a semi-permanently usable optical phase mask with a wide range of phase modulations for generating three-dimensional (3D) nanostructures with high precision.