Arne Schleunitz

Fabrication of 3D nanoimprint stamps with continuous reliefs using dose-modulated electron beam lithography and thermal reflow

3D electron beam lithography and thermal reflow were combined to fabricate structures with multilevel and continuous profiles. New shapes, smooth surfaces and sharp corners were achieved. By using exposure with variable doses, up to 20 steps were fabricated in a 500 nm thick resist with a lateral resolution of 200 nm. Steps were reflowed into continuous slopes by thermal post-processing, and were transferred into silicon substrates by proportional plasma etching. The method can be used for the fabrication of 3D nanoimprint stamps with both sharp features and continuous profiles.

Selective profile transformation of electron-beam exposed multilevel resist structures based on a molecular weight dependent thermal reflow

Novel 3D resist structures are generated with both smooth slopes and stepped resist profiles on the same substrate and in very close vicinity. By performing gray-scale electron-beam exposure and development steps twice with different dose ranges, a selective transformation of multilevel structures into continuous slopes upon thermal treatment was enabled. The molecular weight dependence was analyzed and related to the locally different ability of the resist to flow, which enables one to selectively address resist structures which should be altered while others stay unaffected. The technique has large potential for prototyping elements and devices and can be used to fabricate stamps with complex 3D surface patterns for fabrication using nanoimprint.

Novel 3D micro- and nanofabrication method using thermally activated selective topography equilibration (TASTE) of polymers

Micro- and nanostructures with three-dimensional (3D) shapes are needed for a variety of applications in optics and fluidics where structures with both smooth and sharp features enhance the performance and functionality. We present a novel method for the generation of true 3D surfaces based on thermally activated selective topography equilibration (TASTE). This technique allows generating almost arbitrary sloped, convex and concave profiles in the same polymer film with dimensions in micro- and nanometer scale. We describe its principal mechanism exemplified by pre-patterned poly (methyl methacrylate) resist which is exposed to high energy electrons prior to a thermal annealing step enabling the selective transformation of stepped contours into smooth surfaces. From this we conclude, that TASTE not only offers an enormous degree of freedom for future process variations, but also will advance the patterning capabilities of current standard 3D micro- and nanofabrication methods.

Combining nanoimprint lithography and a molecular weight selective thermal reflow for the generation of mixed 3D structures

Sloped and stepped 3D structures were added to surface-patterned resists using grey-scale electron beam lithography and thermal reflow. A poly(methyl methacrylate) resist with moderate initial molecular weight of 120 kg/mol was chosen, which enabled processing with both nanoimprint and electron beam lithography. Using proper exposure doses, a molecular weight distribution was generated that allowed a selective thermal postprocessing of the exposed steps while the imprinted gratings on top of the resist were preserved. This allows fabricating mixed structures of microprisms surrounded by large-area nanogratings in the same resist layer. Working stamps were casted from the template pattern and subsequently replicated using thermal nanoimprint. As a possible application, backlight devices with arrays of light outcoupling prisms can be seen.Sloped and stepped 3D structures were added to surface-patterned resists using grey-scale electron beam lithography and thermal reflow. A poly(methyl methacrylate) resist with moderate initial molecular weight of 120 kg/mol was chosen, which enabled processing with both nanoimprint and electron beam lithography. Using proper exposure doses, a molecular weight distribution was generated that allowed a selective thermal postprocessing of the exposed steps while the imprinted gratings on top of the resist were preserved. This allows fabricating mixed structures of microprisms surrounded by large-area nanogratings in the same resist layer. Working stamps were casted from the template pattern and subsequently replicated using thermal nanoimprint. As a possible application, backlight devices with arrays of light outcoupling prisms can be seen.

Energy-based thermal reflow simulation for 3D polymer shape prediction using Surface Evolver

An intensive, energy-based analysis of the thermal reflow of thermoplastic polymer structures is presented. Poly (methyl methacrylate) (PMMA) was patterned by grayscale electron beam lithography. The obtained rectangular, micron-scale structures were transformed into lens-like structures by thermal reflow near the glass transition temperature of the original PMMA. Representative parameters obtained from these reflow experiments were used to model the reflow process by using a new, energy-based, finite element, soapfilm method using the free software Surface Evolver. The time-, temperature- and molecular-weight-dependent geometry evolution of the PMMA structures could be described by an apparent contact-angle-evolution time constant and a shape-evolution time constant. The developed model allows the prediction of intermediate geometries during the reflow process occurring between the initial and the final energy optimal geometry. The proposed model is independent from the explicit knowledge of material-specific parameters such as viscosity or glass transition temperature. Extensive experimental data for PMMA reflow is provided. Simulation examples are given for a contact-angle-dominated reflow which demonstrate a good agreement between model and experiment.

Easy mask-mold fabrication for combined nanoimprint and photolithography

Hybrid transparent working stamps with both a surface relief and absorbing mask pattern were fabricated by replicating nanostructures in an Ormostamp film on prepatterned glass substrates. By using a combined nanoimprint and photolithography process, self-aligned mixed patterns of nano- and microstructures can be generated within one single resist layer. In this article the authors present a simple process based on the organic-inorganic hybrid polymer Ormostamp by replication on prepatterned Borofloat substrates. Using these working stamps, grating arrays with 35 nm half pitch were replicated in thermoplastic UV-curable resist. The method is easy to employ for generating mesas with nanostructures on top or the backfilling with microstructures in thermal nanoimprint lithography applications.

Organic-inorganic-hybrid-polymer microlens arrays with tailored optical characteristics and multi-focal properties

Plano-convex microlens arrays of organic-inorganic polymers with tailored optical properties are presented. The fine-tuning of each microlens within an array is achieved by confining inkjet printed drops of the polymeric ink onto pre-patterned substrates. The lens optical properties are thus freely specified, and high numerical apertures from 0.45 to 0.9 and focal lengths between 10 μm and 100 μm are demonstrated, confirming theoretical predictions. Combining nanoimprint lithography approaches and inkjet printing enables using the same material for the microlenses and their substrates, improving the optical performances. Microlens arrays with desired specifications are printed reaching yields up to 100% and high lens reproducibility with standard deviations of the apparent contact angle under 1° and of the numerical apertures and focal lengths under 6%. Microlens arrays involving lenses with different characteristics, e.g. multi focal length, and thus focal planes separated by only few microns are printed with the same reproducibility.

Fabrication of ordered nanospheres using a combination of nanoimprint lithography and controlled dewetting

Arrays with hemispherical dots were produced from nanoimprinted line structures by thermal reflow and coagulation. By adding local nodes into 100 nm wide line cavities of the nanoimprint stamp, the coagulation was controlled and a self-ordered, highly regular dot matrix with almost totally dewetted lines was obtained. The forming of dots of almost equal sizes and distances along the line’s location shows that this coagulation effect is probably only restricted by geometry and surface energy. Simple resist preforms can be transformed into a more complex pattern by thermal-postprocessing in a controlled way.

Fabrication of mesas with micro- and nanopatterned surface relief used as working stamps for step and stamp imprint lithography

A manufacturing concept to fabricate working stamps with defined mesa structures using combined nanoimprint and photolithography is presented. OrmoStamp, an UV-curable organic-inorganic hybrid polymer, was used as mold material. 30 μm high large mesa structures (4×4 mm2) with sharp borders and almost vertical sidewalls were manufactured. On top they featured nanograting patterns with 200 nm height and lateral size as a surface relief. The good thermal decoupling of stamp body and imprinted substrate and the high planarity (divergence <50 nm) make the stamp very suitable for thermal step and repeat nanoimprint lithography of confined patterns with low stitching errors. Up to 210 imprints were performed with a single mesa into a 325 nm thin layer of mr-I 7030E.

Sidewall-angle dependent mold filling of three-dimensional microcavities in thermal nanoimprint lithography

Nanoimprint stamp cavities with vertical and sloped sidewalls show different filling behaviors during thermal nanoimprinting resulting from the varying ability of the viscous polymer to wet the surface during the thermal imprint step. In this paper, the authors describe the formation of prefill states already in the pressure-less contact phase, before squeeze flow begins to fill the cavities. These stable states are created by a constant contact angle formed at the sidewall, and the states themselves generate surface topographies in the resist below the imprint cavities which are dependent upon the surface energy, the geometry of the stamp cavities, and the resist thickness. Therefore, prefilling topographies, formed once a stamp is in contact with a liquid polymer surface and resulting in a transformation of the resist layer with homogeneous thickness into a thickness modulated film, have to be considered as the standard case in molding. Furthermore, due to the local balance of material pinning and wetting mechanisms, cavities preferentially fill from the sloped sidewall and, therefore, this observed phenomenon is highly relevant for the defect-free replication of 3-D surface topographies.Nanoimprint stamp cavities with vertical and sloped sidewalls show different filling behaviors during thermal nanoimprinting resulting from the varying ability of the viscous polymer to wet the surface during the thermal imprint step. In this paper, the authors describe the formation of prefill states already in the pressure-less contact phase, before squeeze flow begins to fill the cavities. These stable states are created by a constant contact angle formed at the sidewall, and the states themselves generate surface topographies in the resist below the imprint cavities which are dependent upon the surface energy, the geometry of the stamp cavities, and the resist thickness. Therefore, prefilling topographies, formed once a stamp is in contact with a liquid polymer surface and resulting in a transformation of the resist layer with homogeneous thickness into a thickness modulated film, have to be considered as the standard case in molding. Furthermore, due to the local balance of material pinning and wetti...