Horst Exner

Drilling of glass by excimer laser mask projection technique

Presently, there is a growing demand from the industry for microprocessing of materials. In particular, for applications in the field of microsystems technology it is necessary to produce structures with dimensions down to the micrometer scale especially in materials that could not be processed or processed well by conventional microelectronic technologies. We have been investigating the drilling of anodically bondable Pyrex glass by means of laser microprocessing using the excimer laser mask projection technique (248 or 193 nm wavelength, 10 ns pulse duration, 8 mJ pulse energy, 500 Hz repetition rate). We will show the dependence of the processing results on the laser parameters. The diameter of the holes ranges from 30 to 100 μm at the front side and from 1 to 50 μm at the rear side of the 500-μm-thick wafer. We observed the formation of cracks in the laser processed region. Accordingly, we found distinct relationships between the process parameters and the quality of the walls of the drilled holes. Es...

Selective Laser Micro Sintering with a Novel Process

Microparts with a structural resolution of <30μm and aspect ratios of >12 have been generated by selective laser sintering. The technique includes sintering under conditions of vacuum or reduced shield gas pressures. In a novel set-up the material is processed by a Q-switched 1064nm Nd-YAG laser after a special raking procedure. The procedure allows the work pieces to be generated from powders of high melting metals like tungsten as well as lower melting metals like aluminium and copper. Contingent on the parameters, the generated bodies are either firmly attached to the substrate or can be dissevered by a non-destructive method.

Process assembly for μm-scale SLS, reaction sintering, and CVD

A novel device suited for the generation of sintered microparts of metal and ceramics, for reaction sintering and for CVD has been developed and successfully tested. With the production of a functional component it has evidenced professional performance. The set-up is vacuum tight; unstable substances can be processed under various shield gases and pressures; it is equipped with a device suited to rake thin layers of fine powders as well as slurries. Sub micrometer powder can be processed in steps of 1 μm thick sintered layers. In combination with a proprietary sintering regime, micro parts with a structural resolution of <30μm, and aspect ratios of >10 have been achieved.

Micro processing of metals using a high repetition rate femto second laser: from laser process parameter study to machining examples

The paper presents a study of laser micro processing of metals by using a high repetition rate femto second laser. On stainless steel (AISI 304), copper and aluminium the impact of the significant laser processing parameters onto the machining process was investigated, such as laser fluence, repetition rate, lateral pulse distance and polarisation. The machining results were evaluated by the ablation rate, surface roughness, process efficiency, material removal rate and the wall-angle. For complementary discussions the experimental data were compared with results achieved in theoretical analysis. Outgoing from the results appropriate laser processing parameters were derived in order to optimise the machining process. With the application of ultra short laser pulses high-quality machining results with a minimal thermal load and a roughness Ra of the laser processed surface of only some hundreds nano meter were obtained. On other hand high machining throughputs were achieved due to application of high repetition rates. Finally, the possibilities and the limits of the high repetition rate femto second laser technology in laser micro processing are demonstrated by means of three-dimensional micro structured machining examples.The paper presents a study of laser micro processing of metals by using a high repetition rate femto second laser. On stainless steel (AISI 304), copper and aluminium the impact of the significant laser processing parameters onto the machining process was investigated, such as laser fluence, repetition rate, lateral pulse distance and polarisation. The machining results were evaluated by the ablation rate, surface roughness, process efficiency, material removal rate and the wall-angle. For complementary discussions the experimental data were compared with results achieved in theoretical analysis. Outgoing from the results appropriate laser processing parameters were derived in order to optimise the machining process. With the application of ultra short laser pulses high-quality machining results with a minimal thermal load and a roughness Ra of the laser processed surface of only some hundreds nano meter were obtained. On other hand high machining throughputs were achieved due to application of high repet...

Contactless laser bending of silicon microstructures

We are going to present a new technology for laser bending of silicon microstructures based upon a suggestion of and carried out in cooperation with Prof. Dr. J. Frühauf from the Technical University Chemnitz (see acknowledgement). We investigated the influence of various laser process parameters on the bending angle and its reproducibility. Bending of the silicon element as a result of the laser induced thermal stresses in the material occurs toward the incident laser beam. The bending angle depends on a lot of laser process and material parameters. In particular we found that the irra-diation regime is well suited to control the bending angle. First substantial FEM based calculations of laser induced temperature fields using a moving laser heat source show the temperature field propagation in the material and reveal some regions of complicated overheating. As a result of our experiments we show a variety of examples including mul-tiple and also continuous bendings. There are several essential advantages compared to conventional bending technologies with this new method: Laser bending is contactless without using additional tools or external forces. Because of the local laser treatment the heat flux to the neighbouring material is minimized. The laser beam can be applied through windows of glass that means to al-most hermetically sealed micro devices. So laser technology is suitable for machining of already finished microsystems. It opens up a wide field of applications in micro system technologies: clip-chip-mechanism or sliding chips for micro optical benches, the adjustment of optical mirrors or other components or the ability of continuous bending for electro-static drives and so on.

High repetition rate femtosecond laser processing of metals

Previously, in high repetition rate femto second laser processing novel laser matter interacting effects were reported, such as heat accumulation and particle shielding. In this study, high repetition rate laser processing was investigated to discuss and understand the impact of laser repetition rate and accompanied accumulative laser material interacting effects. Therefore, a high repetition rate femto second fibre laser setup joint together with galvo scanner technology was applied in laser micro machining of metals (copper, stainless steel, aluminium). High repetition rate laser processing of aluminium and stainless steel lead to considerably lowered ablation thresholds accompanied with higher ablation rates. Laser ablation behaviour of copper was almost independent of the repetition rate with neither considerable lower ablation thresholds nor higher ablation rates. For explanation, heat accumulation caused by higher repetition rates were assumed as mainly ablation behaviour influencing effect, but thermal material properties have to be considered. Furthermore laser machining examples demonstrate the possibilities and limits of high repetition rate laser processing in 3d micro structuring. Thus, by using innovative scanning systems and machining strategies very short processing times were achieved, which lead to high machining throughputs and attract interest of the innovative laser technology in Rapid Micro Tooling. For discussion, high repetition rate processing results are evaluated by means of comparative machining examples obtained with 1 kHz femto second laser system.

Investigation of cw and ultrashort pulse laser irradiation of powder surfaces: a comparative study

The paper presents results obtained in a comparative study of laser irradiation of tungsten powder surfaces using a continuous wave fiber laser and a high repetition rate femtosecond laser. Depending on the energy input per unit length different melt structures have been produced. In general, if the same average laser power level was applied the structures show the same appearance independent from the laser source. But there was both a little higher degree of initial fusing and cross-linking along the processed path when the powder surface was irradiated with ultrashort pulses. Further, with increasing laser intensity a change in structure formation as well as a broadening of the laser processed path has been occurred, although the energy input per unit length remains constant. However, accumulation of slab-like structures, which was previously observed in high-intense ultrashort pulse laser irradiation, has been become more pronounced in cw laser irradiation above a certain number of consecutive scans. Moreover, characteristic effects, such as formation of ripples and nanomelt structures appearing in ultrashort pulse laser processing have been not detected in cw laser irradiation.

Characterisation of interaction phenomena in high repetition rate femtosecond laser ablation of metals

The paper discusses results obtained in ultrashort pulse laser irradiation of metals in order to characterise interaction phenomena occurring in highly repetitive laser processing, such as heat accumulation and particle shielding. The impact of the temporal pulse-to-pulse distance on the ablation process was investigated using repetition rates ranging between 25.8 kHz and 2.05 MHz. Interacting effects were studied by means of industrial grade metal sheets with various thermo-physical characteristics. The experimental results obtained were evaluated by theoretical calculations of both the ablation rate and surface temperature. Furthermore ultra high speed camera images were taken into discussion.Ablation rates obtained empirically for stainless steel and aluminium indicate increasing material removal at higher repetition rates and, hence, heat accumulation is proven as influencing effect. Thus in case of stainless steel and shorter pulse-to-pulse distances, temperature calculation yields the rise of the surface temperature. Additionally, ultra high speed camera images give evidence of more voluminous ablation plumes at shorter pulse-to-pulse distances, induced by intense laser matter interaction.In contrast, for copper only a marginal impact of the repetition rate on the material removal was found. Thus for highly heat-conductive materials the ablation rate is assumed almost independent from the temporal pulse-to-pulse distance. Even high speed camera images show minor impact of the repetition rate on the ablation process.Finally the application of the laser micro machining technology in micro-mould manufacturing is presented. As a result micro-featured plastic demonstrators were produced by micro injection moulding, offering a wide range of sensor applications, for example in microfluidic systems.The paper discusses results obtained in ultrashort pulse laser irradiation of metals in order to characterise interaction phenomena occurring in highly repetitive laser processing, such as heat accumulation and particle shielding. The impact of the temporal pulse-to-pulse distance on the ablation process was investigated using repetition rates ranging between 25.8 kHz and 2.05 MHz. Interacting effects were studied by means of industrial grade metal sheets with various thermo-physical characteristics. The experimental results obtained were evaluated by theoretical calculations of both the ablation rate and surface temperature. Furthermore ultra high speed camera images were taken into discussion.Ablation rates obtained empirically for stainless steel and aluminium indicate increasing material removal at higher repetition rates and, hence, heat accumulation is proven as influencing effect. Thus in case of stainless steel and shorter pulse-to-pulse distances, temperature calculation yields the rise of the su...

Laser microsintering of tungsten in vacuum

Laser microsintering of tungsten powder is investigated as a function of laser output power, pulse interval and vacuum level. The intensities are calculated for the evaporation thresholds of tungsten powder particles of various sizes. In addition, the powder layer generation and the resulting layer thicknesses are calculated. The powder abrasion occurring during the process was taken into consideration. Polished sections and REM images were prepared in order to analyse the experimental outcomes. The dependence of sinter density on the parameters is discussed.

Laser micro sintering of SiO2 with an NIR-laser

Many materials have already been investigated for laser micro sintering. Nearly all technical metals can be processed with this rapid prototyping technology. A new field of investigation is the sintering of ceramics. For industrial and also for medical, especially dental, application silicon dioxide is a multiply applicable material. One of its interesting features is that the properties of the resulting material can be varied between ceramic on the one and vitreous on the other side, depending on the extent of crystalline or amorphous character of the nano-scale structure. A special problem with laser micro sintering of ceramics is the poor absorption of Nd:YAG laser radiation by most of the materials. Besides that, laser micro sintering of ceramics, in contrary to the process with metals, is not merely a series of aggregational transitions. A solution for the micro part generation of SiO2 is reported. Typical laser sintering results from this material are presented. Material specific behaviors during laser micro sintering are discussed.