(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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