Guiding of LIPSS formation by excimer laser irradiation of pre-patterned polymer films for tailored hierarchical structures

Abstract

LIPSS (Laser Induced Periodic Surface Structures) formation at pre-patterned photoresist films was investigated using a partial polarized excimer laser beam (λ = 248 nm, tp = 20 ns). These line and dot pre-patterns were fabricated by the lithography in a photoresist films and have widths of 1–3 µm and 1–5 µm, respectively. LIPSS with a period of 195 nm and a direction along the laser polarization were formed at very low laser fluences (threshold 7 mJ/cm2) and high pulse numbers. With increasing fluence first LIPSS appear at the top surface, extend than to the sidewalls and finally at fluences higher than (F ∼ 17 mJ/cm2) found only at the sidewalls while the top surface is smooth. The height of the LIPSS increases with the pulse number and saturates at approximately 65 nm. The degree of LIPSS order depends on the alignment of the laser polarization to the pattern direction and shape. For parallel alignment of polarization and pattern edges, a high degree of order was found over several hundreds of micrometers along the stripe structure. This order mechanism can accept alignment mismatches up to 10°. This approach may serve as a general guideline for forming well-ordered LIPSS on large areas.