Maika Torge

Laser micromachining of mold inserts for replication techniques: state of the art and applications

The rapid fabrication of microcomponents made from polymers will be presented. The whole fabrication process is divided into three main steps: First, direct patterning of polymers with excimer laser radiation enables the fabrication of first prototypes. Second, laser assisted micromachining using Nd:YAG and KrF-Excimer laser allows a rapid manufacturing of microstructured mold inserts. Third, the application of light induced reaction injection molding (UV-RIM) gives the access to the replication of the previously fabricated mold insert. The fabrication of prototypes made of polymer is carried out highly precisely with excimer laser radiation. With the aid of a motorised aperture mask, CAD data are transmitted directly into the polymeric surface. With an appropriate pretreatment of the polymer surface the debris formation can be drastically reduced. A promising method of micropatterning of mold inserts made of steel is called laser microcaving. This processing technique enables a clean patterning process with only a small amount of debris and melt. The etch rate and surface quality strongly depend on the chemical composition of the steel and the process parameters. Surface qualities with a roughness of about 300 nm can be achieved. Microstructures composed of polymers or ceramic-composites are successfully demolded by using the UV-RIM technique with aspect ratios up to 10. Capillary Electrophoresis-Chips made of PMMA are fabricated, and the functionality of the CE-Chips is demonstrated.

Laser micromachining of polymeric mold inserts for rapid prototyping of PMMA devices via photomolding

New technical approaches in biotechnology call for fast, cost efficient and precise patterning techniques in order to realize first polymeric prototypes in a small-scale production. For this purpose a new promising replication procedure was developed: in the first processing step UV- laser radiation is used for direct precision pattering of chemically stable polymer bulk material such as PSU, PEEK, and PI. In the second processing step thin metallic layers are deposited onto the polymer surface. Finally, in the third processing step these laser generated molds are used for replication of PMMA prototypes via UV-light induced reaction injection molding. The metallic layers on the polymeric surface have to suppress or to avoid an undesired chemical interaction between the polymer surface and the MMA/PMMA resin which is used in photo molding. The deposition of a thin Pt/Au layer system leads to a significant increase of mold inset lifetime form 1-5 up to 20 replication cycles for structures with high aspect ratios. Structures with aspect ratios of larger than 10 can be achieved with a minimal lateral dimension of about 6 micrometers . For this rapid tooling technique the actual impressive technical performances compared to other mold insert fabrication techniques will be presented with respect to the prototyping of microdevices made of PMMA.

Laser adjusted three-dimensional Li-Mn-O cathode architectures for secondary lithium-ion cells

Three-dimensional cathode architectures for rechargeable lithium-ion cells can provide better Li-ion diffusion due to larger electrochemical active surface area and therefore, may stabilize the cycling behaviour of an electrochemical cell. This features show great importance when aiming for long-life batteries, e.g. in stationary or portable power devices. In this study, lithium manganese oxide thin films were used as cathode material with the goal to stabilize their cycling behavior and to counter degradation effects which come up within the lithium manganese oxide system. Firstly, appropriate laser ablation parameters were selected in order to achieve defined three-dimensional structures with features sizes down to micro- and sub-micrometer scale by using mask imaging technique. Laser annealing was also applied onto the laser structured material in a second step in order to form an electrochemically active phase. Process development led to a laser annealing strategy for a flexible adjustment of crystallinity and grain size. Laser annealing was realized using a high power diode laser system operating at a wavelength of 940 nm. Information on the surface composition, chemistry and topography as well as studies on the crystalline phase of the material were obtained by using Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and X-ray diffraction analysis. The electrochemical activity of the laser modified lithium manganese oxide cathodes was explored by cyclic voltammetry measurements and galvanostatic testing by using a lithium anode and standard liquid electrolyte.

Laser modification and characterization of Li-Mn-O thin film cathodes for lithium-ion batteries

The development of future battery systems is mainly focused on powerful rechargeable lithium-ion batteries. To satisfy this demand, current studies are focused on cathodes based on nano-composite materials which lead to an increase in power density of the LIB primarily due to large electrochemically active surface areas. Electrode materials made of lithium manganese oxides (Li-Mn-O) are assumed to replace commonly used cathode materials like LiCoO2 due to less toxicity and lower costs. Thin films in the Li-Mn-O system were synthesized by non-reactive r.f. magnetron sputtering of a LiMn2O4 target on silicon and stainless steel substrates. In order to enhance power density and cycle stability of the cathode material, direct laser structuring methods were investigated using a laser system operating at a wavelength of 248 nm. Therefore, high aspect ratio micro-structures were formed on the thin films. Laser annealing processes were investigated in order to achieve an appropriate crystalline phase for unstructured and structured thin films as well as for an increase in energy density and control of grain size. Laser annealing was realized via a high power diode laser system. The effects of post-thermal treatment on the thin films were studied with Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The formation of electrochemically active and inactive phases was discussed. Surface chemistry was investigated via X-ray photoelectron spectroscopy. Interaction between UV-laser radiation and the thin film material was analyzed through ablation experiments. Finally, to investigate the electrochemical properties, the manufactured thin film cathodes were cycled against a lithium anode. The formation of a solid electrolyte interphase on the cathode side was discussed.

Femtosecond laser patterning of lithium-ion battery separator materials: impact on liquid electrolyte wetting and cell performance

High capacity Li-ion batteries are composed of alternating stacked cathode and anode layers with thin separator membranes in between for preventing internal shorting. Such batteries can suffer from insufficient cell reliability, safety and electrochemical performance due to poor liquid electrolyte wetting properties. Within the electrolyte filling process, homogeneous wetting of cathode, separator and anode layers is strongly requested due to the fact that insufficient electrolyte wetting of battery components can cause limited capacity under challenging operation or even battery failure. The capacity of the battery is known to be limited by the quantity of wetting of the electrode and separator layers. Therefore, laser structuring processes have recently been developed for forming capillary micro-structures into cathode and anode layers leading to improved wetting properties. Additionally, many efforts have been undertaken to enhance the wettability and safety issues of separator layers, e.g. by applying thin coatings to polymeric base materials. In this paper, we present a rather new approach for ultrafast femtosecond laser patterning of surface coated separator layers. Laser patterning allows the formation of micro-vias and micro-channel structures into thin separator membranes. Liquid electrolyte wetting properties were investigated before and after laser treatment. The electrochemical cyclability of batteries with unstructured and laser-structured separators was tested in order to determine an optimal combination with respect to separator material, functional coating and laser-induced surface topography.

Effect of surface topography on hydrophobicity and bacterial adhesion of polystyrene

We report the investigation on effect of surface topography (roughness) and hydrophobicity (contact angle measurement) on bacteria adhesion for polystyrene material. The surfaces of polystyrene substrates were patterned using UV-laser radiation with a wavelength of 193 nm at different conditions. Different surface topographies were fabricated and then measured by an optical surface profiler and contact angle measurements. For bacterial adhesion experiments, an assay of Escherichia coli (E.coli) has been developed and used for bacterial adhesion measurements on both as received and the modified polystyrene surfaces. The method is based upon the staining of attached bacterial cells with the nucleic acid-binding, green fluorescent DAPI stain. The preliminary results show that laser-assisted modification by using laser ablation can make polystyrene substrates either more hydrophilic (with oxygen) or more hydrophobic (with air). The contact angle can be varied from 37° to 108°. The results on bacterial attachment show that the polystyrene substrates as received have no bacteria attached, indicating a good anti-bacterial performance. The treated substrates show some bacterial attachment and, in particular, the surfaces with high contact angle have much higher numbers of bacterial cells attached. This indicates that such laser-assisted process with air can make polystyrene surfaces more attractive to E. coli bacteria.

The SMARTLAM 3D-I Concept: Design of Microsystems from Functional Elements Fabricated by Generative Manufacturing Technologies

Generative manufacturing technologies are gaining more and more of importance as key enabling technologies in future manufacturing, especially when a flexible scalable manufacturing of small medium series of customized parts is required. The paper describes a new approach for design and manufacturing of complex three dimensional components building on a combination of additive manufacturing and e-printing technologies, where the micro component is made up of stacks of functionalized layers of polymer films. Special attention will be paid to the “3-d” modeling approach, requested to support the applicaton developer through provision of design rules for this integrated manufacturing concept . Both, the application concept as well as the related equipment and manufacturing integration currently are currently developed further in the project SMARTLAM, funded by the European Commission.