High fidelity 3D thermal nanoimprint with UV curable polydimethyl siloxane stamps

Abstract

A two-step replication process chain is developed for a microlens array structure with deep three dimensional (3D) reliefs and sharp features enabling the transfer of a photocured acrylic resist patterns into thermoplastic poly-methyl methacrylate (PMMA) with the same structural polarity via an intermediate stamp. By using ultraviolet (UV)-curable polydimethyl siloxane (PDMS), high fidelity negatives were cast from the original microstructures made by two-photon-polymerization and subsequently replicated into PMMA using thermal imprint. The mechanical properties of the new UV-PDMS (X-34-4184, Shin-Etsu Chemical Company, Ltd.), along with its nearly zero process shrinkage, proved to be highly suitable to replicate both 50 lm high concave features and sharp tips with an apex diameter of 500 nm. The results prove that silicone rubber, despite its elasticity, has specific advantages in thermal imprint in structures where both tall microstructures and submicron surface structures have to be replicated. This way, high fidelity PMMA structures with low defects could be prepared by the optimized processing found in this work to have a replication of 3D masters for further upscaling. V

Selective Surface Smoothening of Polymer Microlenses by Depth Confined Softening

“…The material contrast enables to selectively reflow exposed PMMA, while unexposed areas are not affected. To make this possible, a pattern transfer into PMMA is required (Figure b) . The vertical material contrast will be conformally defined along the surface and allow surface smoothening (Figure c) with an amount of modification that decreases exponentially from the surface toward the bulk.…”

“…No treatment: binary resist structure with no treatment prior to reflow adopting a spherical shape after reflow; lateral dose controlled: stepped structure with laterally induced material contrast being transformed into a smooth slope while some areas (dark) remain unaffected; absorption controlled: sloped structure with surface roughness in which absorption controlled, i.e., vertical material contrast is used to smoothen out this roughness at the surface. b) Process of structure fabrication including master structure writing by 2PP, pattern copying into UV‐curing poly(dimethyl siloxane) (UV‐PDMS) by casting and curing as well as final pattern replication into PMMA by thermal nanoimprint lithography . c) Schematic illustration of conformal surface modification of a microlens by 172 nm vacuum ultraviolet (VUV) treatment with corresponding modification, i.e., modification profile one expects.…”

“…To measure the RMS roughness and height as a function of reflow temperature, circular staircase samples were fabricated with 2PP DWL. These samples were replicated into PMMA through intermediate transfer into poly(dimethyl siloxane) (PDMS) . The PMMA used has an M w of 92 kg mol −1 , with a T g of about 122 °C .…”

High-aspect-ratio nanoimprint process chains

“…For process sequences involving more than one replication step, a good balance between hard and soft properties of stamps and the moulding material is needed, along with a good choice of pressure and the ability to achieve high resolution with soft stamps (UV-NIL). Soft stamps may also be used in T-NIL 45 , but the stamp becomes compressed and, owing to lateral expansion, narrow trenches are easily clogged. The use of the PDMS moulds fabricated from the 2PP DWL masters in UV-NIL allowed the formation of nanostructures up to 9.8 μm height with widths down to 350 nm, resulting in an HAR of 28, as shown in Figure 7g.…”

“…Hard stamps, such as silicon or inorganic-organic hybrid polymers, that is, Ormocers, are ideal candidates for T-NIL with squeeze flow in which a pressure is applied for a complete filling of the structures 42 , while soft stamps, such as UV-curable polydimethyl siloxane (UV-PDMS), enable conformation to surface undulations of the moulds by capillary action [43][44][45] . First, the fabrication of the two different masters is presented.…”