Abstract
Combining microstructures of different dimensions benefits from hybrid manufacturing strategies that use nanoimprint for generating regular large area structures and laser lithography for larger grayscale topography. While the individual processes are straightforward, due to the thermoplastic property of the positive resist used for grayscale lithography, diffraction on surface gratings and degradation of photoactive substances require a careful choice of the order of process steps, and balance of process, temperatures, and dimensions.
Highlights
Direct write lithography (DWL) using a focused laser beam is a powerful maskless patterning process for fabricating 2- and 2.5-dimensional surface structures via binary or grayscale lithography.1 In the DWL 66+ system from Heidelberg Instruments Mikrotechnik GmbH (Heidelberg, Germany), a focused laser beam is scanned within narrow stripes while the substrate stage is moving
The writing speed depends on the choice of the write mode (WM), which correlates to the choice of a focusing lens that results in a specific resolution and scan width
The hybrid structure contains 5 μm long, 400 nm high square pillars imprinted by T-nanoimprint lithography (NIL), and a 150 μm wide, approximately 9 μm deep slope fabricated by DWL
Summary
Direct write lithography (DWL) using a focused laser beam is a powerful maskless patterning process for fabricating 2- and 2.5-dimensional surface structures via binary or grayscale lithography. In the DWL 66+ system from Heidelberg Instruments Mikrotechnik GmbH (Heidelberg, Germany), a focused laser beam is scanned within narrow stripes while the substrate stage is moving. At PSI, we have contributed to hybrid processing by combining mask-based photolithography for the generation of mesa structures with surface patterns or grayscale electron beam lithography (EBL) with NIL For the latter, a subsequent selective surface smoothening process enabled the fabrication of prisms with smooth sloped sidewalls instead of vertical sidewalls in a mesa structure. While the prebake is chosen in a way that most of the residual solvent is removed after spin coating without losing sensitivity, the reflow temperature is above the temperature at which the DNQ starts to degrade It is, a process, after exposure and wet development, where the remaining resist is only used as a thermoplastic material able to flow and assume the shape that is given by surface energy minimization. We explored different thermal imprint temperatures and their effect on the resist contrast
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