Multi-material micro-nano-printing advances 3D additive fabrication towards true functional printing. Ultimately, full 3D digital fabrication searches full control over each individual atom. A polymer printing process that comes closest to this is 2-photon direct 3D laser writing. Multi-material printing has three challenges: materials, tools, and scalable processes. The material spectrum is continuously growing. Faster and more scalable processes will come from academic push and industrial pull. We contribute a novel material exchange process for 2-photon laser writers. Our in-situ material replacement exchanges printing material completely with fresh ones.
Multi-photon laser-lithography has become a versatile technology for writing complex 3D structures in high-resolution. Most efforts are focusing on single material printing. To realize multi-material printing, two main steps have to be carried out consecutively: developing the previous material and introducing the next material to the print side. So far, this is done ex-situ, and alignment errors are unavoidable. In this work, a fast in situ material exchange is demonstrated. The designed setup can effectively replace the print material in the optical focus, enabling the writing of detailed 3D structures while conserving efficiency and precision.
State-of-the-art, polymeric, refractive micro-optics simultaneously require an ultrasmooth three-dimensional (3-D) surface and a precise geometry for excellent optical performance with minimal stray light. In earlier work, we have established a surface finishing process for thermoplastic polymer master structures that is only effective on the surface and does not affect the designed optical geometry, thus enabling polishing without touching. Therewith, the high curvature corners of a 50-μm-tall optical diffuser device were maintained while the surface roughness was reduced to about 10-nm root mean square. For this, 3-D master structures were first fabricated by direct write laser-lithography with two-photon polymerization. The master structures were replicated into poly(methyl methacrylate) through a poly(dimethyl siloxane) intermediate replication stamp. Finally, all structures were surface-polished by selective high-energy photon exposure and thermal postprocessing. In this work, we focus on the comparison of the surface smoothening using either postprocessing or dedicated direct writing strategies. For this comparison, strategies for modifying the exposed voxel size and the writing discretization being the primary source of roughness were tested by sweeping the laser exposure dose for two different resist materials and objectives. In conclusion, the postprocessing smoothening resulted in a lower roughness compared to a direct writing strategy—even when 50-nm vertical discretization steps were used—and still enabled 10 times shorter writing times.
Two-photon polymerization by direct laser writing enables to write refractive micro-optical elements with sub-μm
precision. The trajectories and layering during the direct writing process often result in roughness in the range of the
writing increment, which has adverse effects for optical applications. Instead of increasing overlap between adjacent
voxels, roughness in the range of 100 nm can be smoothed out by post-processing. For this a method known as TASTE
was developed, which allows polishing of surfaces without changing the structural details or the overall shape. It works
particularly well with thermoplastic polymers and enables sub-10 nm roughness. The optical quality was confirmed for
an array with several 100 microlenses.
We characterize the impact of high-energy, 172 nm vacuum ultraviolet photons on the molecular weight and the glass transition temperature of poly(methyl methacrylate). We found that the molecular weight is reduced strongly on the surface of the exposed samples with a continuous transition towards the unexposed bulk material being located below the modified region. The glass transition temperature was found to be significantly lowered in the exposed region to well below 50°C compared to that of the 122°C of the bulk region. We could use this material contrast to selectively reflow the top surface of the exposed samples only. This allowed us to create ultra-smooth micro-optical structures by post-processing without influencing the overall geometry that is required for the optical functionality.
We have established a non-contact polishing process for thermoplastic, polymeric microlenses and –prisms with dimensions of up to 50 μm, including sharp convex tips and rims with sub-μm details. The required 3D master structures were fabricated using direct laser-writing lithography with two-photon absorption. Master structures were replicated into poly(methyl methacrylate) through a poly(dimethyl siloxane) intermediate copying step and exposed with 172 nm UV light. Due to the reduction of glass transition temperature in a surface-confined layer, roughness in the range of more than 100 nm could subsequently be smoothed-out to below 10 nm by annealing the surface by heating.
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.
In this contribution, we present the results of a systematic material variation for the development of a resist material for high resolution positive tone electron beam lithography (EBL). Several acrylic copolymer materials with different compositions, that is varying mass fractions of the comonomers and different molecular weights, were synthesized and – as resist solutions – evaluated in terms of EBL performance at acceleration voltages of 30 kV and 100 kV. The resist material exhibiting the best combination of the desired properties, named mr-PosEBR, is two times more sensitive than PMMA 495k and performs comparably to the known high resolution resist ZEP520A at 30 kV. For example, a grating pattern with 29 nm wide lines with a period of 100 nm could be lithographically generated in films of mr-PosEBR with an area dose of 100 μC/cm2. In terms of resolution, single lines of only 35 nm width could be fabricated via metal liftoff. Furthermore, the dry etch stability of mr-PosEBR in a CF4/SF6 process is similar to the one of ZEP520A. Consequently, via dry etching nano patterns in mr-PosEBR could be smoothly transferred into the underlying Si substrate with high fidelity. Moreover, mr-PosEBR was evaluated as electron beam grayscale patterning and reflow resist. It was shown that the resist exhibits a good grayscale and reflow performance very similar to PMMA 120k and ZEP520A. Via these well controllable processes the generation of a wide variety of features and applications is possible.
Due to their small footprint and high sensitivity to biological molecule binding, planar optical microring resonators gained high interest for use as optical biosensors. Typically, microring resonators are made of semiconductor based materials, and are manufactured by time-consuming lithography and etching steps. Semiconductor based waveguides have high refractive indices, and thus, a high refractive index contrast between core and cladding. In this case, due to strong mode confinement, bending loss is a comparably minor issue and becomes relevant only at small bending radii of less than 5 μm. The main loss is determined by surface scattering, and thus, semiconductor based curved waveguides need to be designed and manufactured to have very smooth sidewalls. If polymer materials are used, microring resonators can be cost-efficiently manufactured by nanoimprint lithography. The resulting larger polymer waveguide dimensions facilitate in- and out-coupling, and polymer surfaces allow using established surface biofunctionalization techniques. For polymer waveguides, due to the small refractive index contrast, surface scattering loss is a minor issue, but bending loss becomes dominant for radii of less than 80 μm due to the low mode confinement to the core. In this work, design guidelines for polymer microring resonator waveguides are given and compared to semiconductor based waveguides. Waveguide losses due to bending and surface roughness are determined analytically or numerically by finite element methods. Coupling coefficients are calculated by finite element methods and coupled-mode theory. Resulting conclusions for designing polymer waveguides and semiconductor waveguides are derived.
Combined nanoimprint and photolithography (CNP) is an attractive imprint technology for residual layer free
direct patterning of high aspect ratio polymer structures. Employing the chromium hard mask from the imprint
mold etching process as a light blocking metal layer on top of the imprint mold protrusions is an efficient
manufacturing method for CNP molds. The surface chemistry of the mold and in particular of the chromium
layer is crucial for the realization of antisticking layers (ASLs) on suchlike CNP molds. For the reported ASL
coatings, the stripping process of the electron beam resist was very important, especially for fluorinated resists.
We compared an antireflective chromium photomask surface with the standard imprint mold materials Si (native
oxide), SiO2 (thermal oxide) and quartz. Low surface free energies of ~15.3mN/m and ~10.9mN/m were
achieved by chemical vapor deposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) for the chromium
surface and the natively oxidized Si, respectively. The ASLs were successfully tested on unstructured chromium
imprint molds without sticking problems during imprinting.
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