Combining nanoimprint lithography and a molecular weight selective thermal reflow for the generation of mixed 3D structures
(Color online) Process sequence based on three different generic processes subsequently applied for the generation of mixed 3D pattern: combining thermal NIL, grey-scale e-beam exposure, and reflow allows a selective transformation of stepped sidewalls into smooth slopes while imprinted surface pattern stays unaffected.
Prepatterning of a 1 μm thick PMMA layer with a molecular weight of 120 kg/mol was accomplished by thermal NIL. The polymer layer on silicon is imprinted on a large area with a binary nanograting (height, width, and distance: 250 nm). During sample cleaving, the resist is distorted and hence slightly detached from the substrate (scale bar: 1 μm).
Normalized dose–depth correlations (contrast curve) are plotted for 1 μm thick PMMA with 120 kg/mol (NIL resist) and 950 kg/mol (standard EBL resist) on silicon. The e-beam exposure was performed at 100 keV/1 nA and the development was accomplished in undiluted MIBK at 20 °C for 30 s. Since the contrast curves exhibit comparable slope characteristics, a grey-scale EBL can also be applied to the NIL resist.
Exemplary mixed 3D pattern was manufactured by combining NIL, grey-scale EBL, and thermal postprocessing (a). The PMMA structures consist of a binary 250 nm line grating on top of a 1 μm thick resist with vertical and sloped sidewalls with an inclination ∼ 30° (b) (scale bar: 2 μm).
SEM micrograph of an exemplary mixed 3D structure in PMMA resist intended for optical application for backlight display devices. The resist pattern is generated by grey-scale EBL onto an NIL prepatterned resist (a). The selective melting of exposed areas eventually integrates prisms with smooth surfaces into the imprinted line grating (b) (scale bar: 2 μm).
SEM micrograph of an OrmoStamp mold (a), replicated from the PMMA master pattern, shows the inverse pattern polarity. Used as a working stamp, it allows a repeated generation of mixed 3D structure by replication techniques (b) (scale bar: 2 μm).
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