How post-processing by selective thermal reflow can reduce the roughness of 3D lithography in micro-optical lenses

Abstract

Most polymeric refractive micro-optics simultaneously demand ultra-smooth 3D topographies and precise geometry for high optical performance and low stray light. We have established a surface selective smoothening for thermoplastic polymers that does not affect the designed optical geometry. For example, high curvature corners required for a 50 µm tall optical diffuser device were maintained while the surface roughness was reduced to about 10 nm RMS. 3D master structures were fabricated using direct write laser-lithography with two-photon absorption. Master structures were then replicated into poly(methyl methacrylate) through a poly(dimethyl siloxane) intermediate copying step and subsequently smoothed-out by high-energy photon exposure and thermal post-processing. The smoothening results in a lower roughness compared to a direct writing strategy using even about 50 nm vertical discretization steps still enables 10 times faster writing times.

Benchmarking surface selective vacuum ultraviolet and thermal postprocessing of thermoplastics for ultrasmooth 3-D-printed micro-optics

“…The NIL process chain with its main elements origination, replication, and pattern transfer (functionalization) is a good example to demonstrate this capability. 25,26…”

“…Instead of improving surface roughness by time-consuming and design-specific direct-writing strategies of 3-D lithographic methods, we chose a postprocessing method based on surface selective thermal reflow. [25][26][27][28] We quickly recapitulate this method for the modification of polymer surfaces being published earlier 28 and especially compare in this work the two strategies of postprocessing and direct-writing for surface smoothening. The principle of smoothening is based on a process known as thermally activated selective topography equilibration (TASTE).…”

Surface smoothening of the inherent roughness of micro-lenses fabricated with 2-photon lithography

“…For the 25x NA 0.8 objective the voxel size is about 300×300×3380 nm 3 for the IpDip and about 380×380×3960 nm 3 for the IpS, including the blurring due to the chemical nature of the resist. In a previous publication [8], on which this paper is based, we have anticipated smaller values (200×200×600 nm 3 for IpDip and 63x, and 300×300×900 nm 3 for IpS and 25x), which have now been improved by measurement [9]. Therefore, while the lateral extension of the voxels is quite small, their vertical extension is large and the height-width ratio ranges from 3.3 from 63x in IpS to 12 for 25x in IpDip.…”

“…The roughness needed for such applications is typically below 5 nm for the visible range, i.e., about 1/100 th of the wavelength used, which results in low stray light, high resolution and high imaging contrast [5,6]. The following publications serve as references for the proposed process and its underlying physical principles [7][8][9].…”

Selective surface smoothening of 3D micro-optical elements

“…We have presented a range of possibilities in Ref. [12] and focus here on the selective smoothening by post processing of surface topographies which were optimized for speed but not resolution and roughness.…”