Micro Cladding stellt eine Modifikation des klassischen Laserpulverauftragschweißens dar, um großflächig und flexibel Mikrostrukturen auf nahezu beliebigen Oberflächen aufbringen zu können [1][2]. Die Besonderheit des am Laserinstitut Hochschule Mittweida entwickelten Verfahrens sind dabei der Einsatz eines kurzgepulsten Faserlasers mit einer schnellen Strahlablenkung mittels Galvanometer-Scanner sowie die Ausbildung von Mikrostrukturen zur Speicherung des Pulvers [3][4][5]. Hierdurch kann die Laserstrahlenergie auch im Mikrobereich lokal begrenzt eingebracht und die Volumenbaurate gesteigert werden. Die Auflösung des Verfahrens liegt aktuell bei ca. 40 μm. Für mehr industrielle Relevanz wurden die Volumenbauraten durch Anwendung von Hochleistungslasern und schnellen Scannern auf bis zu 3.900 mm³/h deutlich erhöht. Die Ergebnisse der Untersuchungen werden aufgezeigt und die Verfahrensgrenzen diskutiert.

This paper presents a new technology, which allows the reduction of the micro dimensional trench width below the technological limitations of the Deep Reactive Ion Etching (DRIE) process. The high-accuracy and high-throughput femtosecond laser-micro-welding of aluminum was performed for the first time by Micro Electrical Mechanical System (MEMS) fabrication to realize this permanent trench width reduction. Therefore, this technology has been applied for the electrode gap reduction of high precision vibration sensors, based on the capacitive working principle, resulting in a fourfold improvement of the sensitivity without changing the size of the sensor chip itself.

The interaction of laser radiation with glass becomes nonlinear, when the electric eld strength of a femtosecond laser exceeds the dielectric eld strength of the glass. This e ect can lead to a possible void formation along which the material can be precisely shaped. The parameter range of the laser-matter interaction was studied in order to nd the optimal parameters for creating 3D void structures.

We report on selective realignment of the magnetization direction of the exchange biased ferromagnetic layer in two different spintronic layer stacks using laser radiation. The exchange bias effect occurs in an antiferromagnetic/ferromagnetic bilayer system when cooled in an external magnetic field below the Néel temperature and results in a shift of the ferromagnetic hysteresis loop with increased coercivity. The effect is utilized to pin the magnetization direction of the reference ferromagnetic layer in spin valve systems. We investigated the realignment of the pinned magnetization direction in a spin valve system with in plane exchange bias and in a Co/Pt multilayer with perpendicular exchange bias. The layer stacks were heated above the Néel temperature in a defined lateral area by using rapidly deflected laser radiation. Two different laser assisted annealing techniques were investigated applying either continuous or pulsed laser radiation. During laser annealing, the sample was subjected to an external magnetic field in order to selectively realign the magnetization direction of the pinned ferromagnetic layer. Magnetic structuring was performed by heating narrow single tracks as well as irradiating single pulses. By using a magneto optical sensor in combination with a polarization microscope, the magnetic structures have been visualized. After laser annealing of larger-scaled areas, the exchange bias field strength and the coercive field strength were analyzed using a magneto optical Kerr effect set up (MOKE). The impact of the processing parameters laser peak intensity, laser pulse duration, scan speed (continuous wave) and magnetic field strength on the resulting reversed exchange bias field was evaluated.

We report on a fast machining process for cutting silicon wafers using laser radiation without melting or ablating and without additional pretreatment.For the laser induced cutting of silicon materials a defocused Gaussian laser beam has been guided over the wafer surface. In the course of this, the laser radiation caused a thermal induced area of tension without affecting the material in any other way. With the beginning of the tension cracking process in the laser induced area of tension emerged a crack, which could be guided by the laser radiation along any direction over the wafer surface. The achieved cutting speed was greater than 1 m/s. We present results for different material modifications and wafer thicknesses. The qualitative assessment is based on SEM images of the cutting edges.With this method it is possible to cut mono- and polycrystalline silicon wafers in a very fast and clean way, without having any waste products. Because the generated cracking edge is also very planar and has only a small roughness, with laser induced tension cracking high quality processing results are easily accessible.

We report on selective realignment of the exchange biased magnetization direction in spintronic layer stacks using rapidly deflected focused laser radiation in a direct-write technique. Laser-based magnetic field cooling by applying either pulsed or continuous wave laser radiation was investigated. The magnetic properties of laser-based field cooled layer stacks were investigated by using magneto optical Kerr effect (MOKE) measurements. The dependencies of the processing parameters peak intensity and external magnetic field strength on the resulting exchange bias field strength were evaluated. In addition, temperature field simulations gain deeper insights into the mechanisms of laser-based field cooling. Our results show significant influence of the laser processing regime. Field cooling induced by continuous laser radiation caused higher exchange bias field strengths, compared to pulsed laser radiation. Moreover, the external magnetic field strength affected the resulting exchange bias field strength only by irradiating low-intensity laser beams. (C) 2014 Published by Elsevier B.V.

Die Erfindung betrifft Verwendungen von Laserstrahlung hoher Leistung zum Spannungsrisstrennen von Korpern aus Halbleitermaterialien und Einrichtungen zum Spannungsrisstrennen von Korpern aus Halbleitermaterialien. Die Verwendungen von Laserstrahlung hoher Leistung und die Einrichtungen zeichnen sich durch ein schnelles Trennen von Korper aus Halbleitermaterialien aus. Zum Spannungsrisstrennen mit hoher Geschwindigkeit der aus Halbleitermaterialien bestehenden Korper wird dazu entweder kontinuierliche Laserstrahlung eines Lasers mit einer Wellenlange zwischen 1.000 nm und 1.100 nm, einer mindestens eindimensionalen nahezu gaus- oder besselformigen Strahlverteilung, einem kein Aufschmelzen des Halbleitermaterials verursachenden Wirkdurchmesser und einer zu applizierenden Leistung von 200 bis 20.000 W oder gepulste Laserstrahlung eines Lasers mit einer Wellenlange zwischen 800 nm und 1100 nm, einer eindimensionalen nahezu gaus- oder besselformigen Strahlverteilung, einer kein Aufschmelzen des Halbleitermaterials verursachenden Wirkbreite der Laserstrahlung und einer zu applizierenden Pulsleistung von 1.000 bis 1.000.000 W verwendet.

The joining of micron devices by bonding of thin films is required for the electrical or mechanical connection of thin film components. Current developments of flexible electronics call for advanced micron interconnection technologies. For the joining of thin film devices on flexible polymer substrates a novel technique called “laser microriveting” (LMR) is introduced and demonstrated which enables the bonding of two thin metal layers of different materials for mechanical and electrical connections. In this approach the mechanical and electrical connections are achieved by geometrical interlocking of the thin film materials as a consequence of pulsed laser irradiation. Imaging by SEM and EDX shows clearly that a hollow rivet of copper is formed interlocking the copper layer with the overlaying molybdenum film. The copper hollow rivet formation is the result of highly-dynamic laser-driven processes of melting, evaporation, recoil pressure formation, material transport processes, and resolidification of the molten copper within a single laser pulse. A contact resistance of less than 100mΩ for one rivet of approximately 20μm size was measured. This new approach provides a solution for current problems of joining very dissimilar materials.

In laser ablation cutting, irradiation of high-intense laser beams causes ejection of molten and evaporated material out of the cutting zone as a result of high pressure gradients, induced by expanding plasma plumes. This paper investigates highspeed laser ablation cutting of industrial grade metal sheets using high-brilliant continuous wave fiber lasers with output powers up to 5 kW. The laser beam was deflected with scan speeds up to 2700 m/min utilizing both a fast galvanometer scan system and a polygon scan system. By sharp laser beam focusing using different objectives with focal lengths ranging between 160 mm and 500 mm, small laser spot diameters between 16.5 μm and 60 μm were obtained, respectively. As a result high peak intensities between 3*108 W/cm² and 2.5*109 W/cm² were irradiated on the sample surface, and cutting kerfs with a maximum depth of 1.4 mm have been produced. In this study the impact of the processing parameters laser power, laser spot diameter, cutting speed, and number of scans on both the achievable cutting depth and the cutting edge quality was investigated. The ablation depths, the heights of the cutting burr, as well as the removed material volumes were evaluated by means of optical microscope images and cross section photographs. Finally highspeed laser ablation cutting was studied using an intensified ultra highspeed camera in order to get useful insights into the cutting process.

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...

In this paper processing of transparent materials by laser radiation from various sources with short (nanoseconds) and ultrashort (femtoseconds) pulse lengths at different wavelengths is discussed. The investigations were carried out with a short pulse Nd:YVO(4) laser (1064 nm, 532 nm) and a high repetition rate femtosecond fiber laser (1030 nm). In our experiments the laser beam was guided across the probe either through the motion of a coordinate table or through a laser scanner with an f-theta-objective.In our study we investigated in detail the influence of important process parameters like wavelength, pulse width, and irradiation regime upon micro defect generation inside bulk glass (BK glass, fused silica) and polymers (polymethylmethacrylate, polycarbonate, cyclo-olefin-copolymers). By applying an irradiation regime with optimal process parameters these locally confined material defects can be aligned as to yield cut surfaces for the excision of 3d parts that consist of transparent material with bulk properties. Especially for the production of irregularly shaped 3d parts a CAD-CAM software tool was developed that automatically converts geometry data into a processing program.

The paper presents an overview about high repetition rate femto second laser machining and its applications. Novel phenomena in laser matter interaction with the repetition rate as one of the mainly influencing parameter are discussed. Depending on temporal distances between consecutive femto second laser pulses, either heat accumulation or particle shielding effects were detected. Heat accumulation enhanced the ablation behavior and higher ablation rates were found. On other hand energy losses due to particle shielding caused lowered ablation rates. Furthermore laser induced periodical surface structures, such as ripple formations and conical micro structures, were observed due to the high average laser energy input and accumulative effects. Formation und shape properties of these structures depend on laser processing parameter.Joining high repetition rate laser technology together with high speed beam deflection systems, significantly higher ablation rates and short processing times were achieved. Exemplarily demonstrated in 3D micro structuring, processing times were reduced more than 40 times compared to fs laser processing using repetition rates of some kHz. In consequence, prospectively high machining throughputs were reached, which attract increasing interest of the novel technology in industrial applications.

This paper discusses the machining of transparent materials using two short pulse (nanoseconds) and one ultrashort pulse (femtoseconds) laser sources. The investigations were carried out with short pulse Nd:YAG lasers (1064 nm, 532 nm, and 355 nm) as well as a high repetition rate fslaser (1030 nm). In our experiments the laser beam was focused onto the sample with both a stationary objective and a laser scanner with an f-theta-objective. In our study we investigated in detail the dependencies of controlled defect generation inside bulk glass material on important process parameters like wavelength, pulse width, pulse repetition rate, and irradiation regime. Due to a smart arrangement of these defects in lines, planes, and shells cut surfaces can be generated. Finally, cutting of 3d parts, consisting of bulk transparent material, becomes possible.

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.

As the result of research started in 2001 with a project on the possibility of generating parts with an improved resolution by Selective Laser Sintering (SLS), the research group at University of Applied Sciences Mittweida and the attached Laserinstitut Mittelsachsen e.V developed the process of Laser Micro Sintering (LMS).The technology uses as essentials sub-micrometer powders, a ring rake and a q-switched solid state laser for the successful generation of solid bodies from various metal powders. The resolution and the surface roughness are by more than one order of magnitude better than achieved by previous selective laser sinter technologies. Presently the technology shows advancements in selective laser sintering of highly resolved specimens of densely sintered Al2O3 and SiC ceramics too.This paper reports the process mechanism of LMS and its principal differences compared to SLS methods. A variety of laser micro sintered parts from different metals and the newest results in laser micro sintering of ceramic parts are presented. Material specific behaviour in laser micro sintering is discussed.It also will be shown the ability of the method to generate parts of layer wise different materials (laminate sintering) with one sintering machine.

In this paper first results on 3d laser cutting of glasses as a technology for rapid tooling, obtained at the laser institute of the University of Applied Sciences Mitt-weida, are presented. The investigations were carried out with a short pulse Nd:YVO4 slab laser from Edge-wave (Aachen) and a high repetition rate femtosecond laser of Clark-MXR Inc. Michigan. In the experiments the laser beam was focussed onto the sample with both a stationary aspheric lens and a laser scanner with an f-theta-objective combined with a high precision xyz-axis stage.The technique utilizes nonlinear absorption effects to induce local heating, followed by stress generation and finally micro defect formation inside the transparent bulk material. Several side effects have to be considered in machining of transparent materials, for example, nonlinear processes like self focussing and the focal shift.In the beginning for various glasses the dependence of the micro defect formation on the process parameters were investigated in detail - possibilities and limits are discussed. The development of strategies of applicable arrangements of micro defects to produce fracture lines, planes and shells enclosing the required component are object of the investigations in order to produce 3d parts. These components created with the presented technology are consisting of bulk material - the key benefit in comparison to laser sintered parts.

PurposeThe purpose of the paper is the elucidation of certain mechanisms of laser material processing in general and laser micro sintering in particular. One major intention is to emphasize the synergism of the various effects of q‐switched laser pulses upon metal and ceramic powder material and to point out the non‐equilibrium character of reaction steps.Design/methodology/approachRecent results and observations, obtained in development of “laser micro sintering,” are surveyed and analyzed. By breaking down the overall process into relevant steps and considering their possible kinetics, an approach is made towards interpreting specific phenomena of laser micro sintering. Thermodynamics upon heating of the material as well as its photo‐electronic response to the incident radiation are considered.FindingsThe findings corroborate a model whereby short pulses of high intensity provide non‐equilibrium pressure conditions at the location of incidence, that allow for the melting of metal powder with an almost immediate expansion of a plasma and/or vapor bulb. Thereby the molten material is condensed and propelled towards the substrate. A final boiling eruption after each pulse is the reason for the morphology of the laser micro‐sintered surfaces and can prevent oxidation when the process is conducted under normal atmosphere. In sintering of ceramics, the short pulsed and intensive radiation increases the chance to excite the material even with photon energies below the bandgap value and it lowers the risk of running into a destructive avalanche.Research limitations/implicationsOwing to the stochastic character of the respective sintering event, that is initiated by each individual pulse, the gathered data are not suitable yet for the formulation of an exact quantitative function between sintering behavior and laser parameters.Practical implicationsThe qualitative findings yield a good rule of thumb for the choice of parameters in laser sintering on a micrometer scale and the model is conducive for advanced interpretation of other phenomena in laser material processing besides sintering.Originality/valueThe kinetics and thermodynamics of laser sintering with q‐switched pulses are approached by a qualitative explanation. The heterogeneous and non‐equilibrium character of the processes is taken into account; this character is often neglected by researchers in the area.

PurposeExamples are given for the technical applicability of a novel development of selective laser sintering called “laser micro sintering”.Design/methodology/approachTogether with a specific method to produce powder layers, the controlled application of pulsed radiation for the processing of sub‐μm grained metal powders was exploited to produce micro‐tools with a heretofore unattained structural resolution.FindingsHigh resolution micro bodies are displayed. Instruments could be generated which proved to fulfil their designation as grip bits for micro manipulators. The micro‐bodies can be generated detachably from or firmly fixed to the construction substrate. The material of the generated bodies withstands the traction forces when used as an injection mold for polymer casts.Research limitations/implicationsDensities and structural resolutions can still be improved especially with a newly updated version of the equipment. Laser micro sintering of materials, other than metal (e.g. ceramics), has still to be developed. The introduction of the equipment and the technique into the market is on its way.Practical implicationsMicro‐tools can be generated with an overall structural resolution of 30 μm and with all the advantages of a freedom technique.Originality/valueThe paper informs the technical community on a new novel modification of the freeform technique selective laser sintering, demonstrating the solution of some problems that have hampered the progress of metal laser into resolution ranges below 100 μm. It also supplies evidence for the technical applicability.