Beneath lithography, etching and coating, laser technology is necessary to support the production of innovative devices in MEMS, electronic and optical industry. Laser beam sources emitting pulses with durations in the regime between nano- and femtoseconds are winning more and more importance for processing metal, semiconductor and optical materials. Finest structures, cuts and drilings can be manufactured in a high variety of materials like silicon, glasses but also in composite materials like polymers. In this paper a survey on a number of laser based processes for precision manufacturing is presented. With regard to this context, effects and material interactions are discussed and attached to different laser beam sources. The quality of processes as well as their economical meaning from perspective of laser technology is evaluated. An overview about machining and actual trends for ns-, upcoming ps- and fs-laser technology is presented.
The development of the recent years led to an increased importance of frequency-converted diode-pumped solid-state lasers (DPSSL) for industrial drilling, cutting and structuring applications. The UV laser systems show favorable beam absorption in a broad range of MEMS and MOEMS relevant materials like ceramics, metals and polymers. Their short pulses in the range of tH = 20 nanoseconds and the excellent beam quality offer the possibility of manufacturing with a minimum heat affected zone (HAZ) in the surrounding material and thereby a decreasing of initial fusing and debris. To obtain the reachable ablation quality, especially on metals, in this paper copper, tungsten, tantalum, molybdenum, nickel, iron, aluminum and titanium were machined under identical conditions. Material properties like heat conductivity, optical and thermal penetration depth, are decisive for the magnitude of the mentioned side-effects. The correlation of these physical values of different metals to those effects is the subject of this paper. Results of systematically accomplished experiences using a frequency tripled DPSSL with a wavelength of λ = 355 nm in order to investigate this correlation are presented. Due to thermal effects, the ablation quality of metals differ from each other extremely. These information enable a prediction of the reachable quality of the desired structure.
Laser beam sources emitting pulses with durations in the regime between nano- and femtoseconds are winning more and more importance for processing optical and semiconductor materials. Beneath lithography, etching and coating, laser technology is necessary to support the production of innovative electronic and optical devices. Finest structures, cuts and drillings can be manufactured in a high variety of materials like silicon, glasses but also in composite materials like polymers.
In this paper a survey on a number of laser based processes for microelectronic and optoelectronic manufacturing is presented. With regard to this context effects and material interactions are discussed and attached to different laser beam sources. The quality of processes as well as their economical meaning from perspective of laser technology is evaluated. An overview about machining and actual trends for ns-, upcoming ps- and fs-laser technology are presented.
This paper deals with basic investigations in order to control the laser spot micro welding process when packaging electronic components onto three dimensional molded interconnect devices (3-D MID) or flexible printed circuit boards. A wide range of experiments has been carried out for both successful and fail welds. Typical failures appearing during welding are either damage of the circuit board due to overpower or loss of connection between the welded components due to gap formation between the leads of the component and the circuit board. The optical radiation emitted from the process was firstly measured off-axially and co-axially with a spectrometer. To aid the spectrometric analysis, an optical sensor based on a silicon photo diode and an appropriate optical filter was applied for detecting the emitted radiation. The signal was acquired, analyzed, and saved using a dedicated software program. Changes in the detected radiation due to different weld conditions were evaluated. Moreover, the weld quality was investigated by Scanning Electron Microscope (SEM) measurements and cross-sectional analysis. A correlation has been found between the signal course and the weld quality. Primarily, there are three relevant signal phases (high peak, flat stage, and small peak) appearing during the weld. Any changes in the characteristic signal during these process phases can be used to predict the quality of the welds.
Femtosecond laser systems offer a good solution for the creation of straight microcuts and grooves on macroscopic workpieces, as they are becoming more established in industrial applications. Although such linear ablation processes have been investigated and improved before, the main obstacle is still the long processing time. Increasing the processing speed by applying high pulse energies usually leads to a significant quality loss. Using high pulse repetition rates at low pulse energies would lead to the best results, but the repetition rate of commercially available laser sources is mostly restricted to one to several kilohertz. However, a systematic investigation of further relevant parameters enables the processing quality and speed to be optimized. To demonstrate these relations, cuts and grooves using different motion parameters and focusing strategies are presented at the example of metal and silicon samples. With regard to the focusing strategy, it is shown that by using linear focus shapes in the direction of the cut, cutting speeds can be increased while maintaining high edge qualities of the cuts and grooves. The presented results prove the potential of femtosecond lasers for high quality cuts in different industrially relevant materials.
The precision machining of glass by laser ablation has been expanded with the short wavelength of the 157 nm of the F2 excimer laser. The high absorption of this wavelength in any optical glass, especially in UV-grade fused silica, offers a new approach to generate high quality surfaces, addressing also micro-optical components. In this paper, the machining of basic diffractive and refractive optical components and the required machining and process technology is presented. Applications that are addressed are cylindrical and rotational symmetrical micro lenses and diffractive optics like phase transmission grating and diffractive optical elements (DOEs). These optical surfaces have been machined into bulk material as well as on fiber end surfaces, to achieve compact (electro)-optical elements with high functionality and packaging density. The short wavelength of 157 nm used in the investigations require either vacuum or high purity inert gas environments. The influence of different ambient conditions is presented.
Modern optical systems must satisfy high demands in terms of functionality and performance. With complex optical elements the problems in manufacturing surfaces with sufficiently high quality require new approaches in manufacturing technology. Although the geometrical shape of such structures can be generated by different means, achieving a high surface quality is not always possible for complex surfaces.
A concept for the manufacturing of high quality complex glass surfaces is presented within this publication. The idea is to develop a microwave assisted thermal polishing process supported by CO2-laser radiation. This combination aims to the reduction of thermal gradients in the glass, by heating a definite glass volume during surface treatment using laser radiation. To realise the high surface quality it is absolutely necessary to control the process temperatures. With temperature measurement devices i.e. pyrometers, the average volume temperature is monitored as well as the temperature in the laser spot on the glass surface. By controlling the temperatures of the volume and the temperature on the optical glass surface, it is possible to implement a stress reduced thermal polishing process.
The results of the software based process controlling will be shown by means of CO2-laser polished samples. Additionally the method of process optimisation by analysing the control parameters will be explained and demonstrated. The use of such a system allows the processing of many temperature based laser process applications of amorphous materials. The request for quick polishing processes, independent from the 3D-shape of the surface will be reached by this innovative technique.
From the general trend towards higher functional integration and miniaturization results an increasing demand for metallic parts of smallest dimensions. Metal forming processes are best suited for these applications in terms of productivity and accuracy. But problems arise from the so called "size effects" related to these small dimensions, e.g. the influence of the microstructure becomes an important aspect to consider. An approach to these problems is the laser-assistance of the microforming process. Laser light is used to increase the temperature of the material during forming, reducing the flow stress and increasing the ductility. By controlling the temperature in the workpiece via laser radiation the microstructure can be modified by inducing recrystallization and thus increasing the formability in the required area of the part. This can also be verified by FEM simulations of the forming process. To enable the transmittance of laser light into the workpiece, sapphire tools are used. The machining of these tools has been carried out by laser ablation with wavelengths in the UV range, e.g. with excimer lasers. Experimental investigations have shown that the manufacturing of sapphire tools and their use in laser-assisted microforming processes is a suitable method for the mass production of microparts.
The precision machining of glass by laser ablation has been expanded with the short wavelength of the 157 nm of the F2 excimer laser. The high absorption of this wavelength in any optical glass, especially in UV-grade fused silica, offers a new approach to generate high quality surfaces, addressing also micro-optical components. In this paper, the machining of basic diffractive and refractive optical components and the required machining and process technology is presented. Applications that are addressed are cylindrical and rotational symmetrical micro lenses and diffractive optics like phase transmission grating and diffractive optical elements (DOEs). These optical surfaces have been machined into bulk material as well as on fiber end surfaces, to achieve compact (electro) -- optical elements with high functionality and packaging density. The short wavelength of 157 nm used in the investigations require either vacuum or high purity inert gas environments. The influence of different ambient conditions is presented.
μFor drilling fused silica, mechanical techniques like with diamond drills, ultrasonic machining, sand blasting or water jet machining are used. Also chemical techniques like laser assisted wet etching or thermal drilling with CO2-lasers are established. As an extension of these technologies, the drilling of micro-holes in fused silica with VUV laser radiation is presented here. The high absorption of the 157 nm radiation emitted by the F2 excimer laser and the short pulse duration lead to a material ablation with minimised impact on the surrounding material. Contrary to CO2-laser drilling, a molten and solidified phase around the bore can thus be avoided. The high photon energy of 7.9 eV requires either high purity nitrogen flushing or operation in vacuum, which also effects the processing results. Depending on the required precision, the laser can be used for percussion drilling as well as for excimer laser trepanning, by applying rotating masks. Rotating masks are especially used for high aspect ratio drilling with well defined edges and minimised debris. The technology is suitable particularly for holes with a diameter below 200 μm down to some microns in substrates with less than 200 μm thickness, that can not be achieved with mechanical methods. Drilling times in 200 μm fused silica substrates are in the range of ten seconds, which is sufficient to compete with conventional methods while providing similar or even better accuracy.
Dielectric sensors with smallest electrode structures are used to monitor several technical applications. Thus, durable substrate materials are essential for the exploitation of the sensors in hostile process environment. Here, ceramics and different glasses show ideal material properties, but are difficult-to-machine in the micrometer range. UV-laser beam sources are well suited for an economic manufacturing of micro-structures in these brittle materials. Especially, advanced laser tools like excimer- and frequency converted solid-state lasers show excellent machining results. This paper presents the development of new laser based production techniques and innovative process chains for the fabrication of distinctive electrode structures in high stable materials. Two different machining concepts are shown. A conductive Indium Tin-Oxide (ITO) layer with a thickness of about 600nm has been structured with a KrF excimer laser (λ = 248 nm, H = 5 J/cm2), without damaging the underlying borosilicate glass substrate. The dimension of the electrodes and insulation channels of the sensor are as small as 50μm. For the second approach of manufacturing a dielectric sensor, aluminum oxide as a bulk substrate materials has been machined with the same laser type. No thermal damage was observed by an operating fluence H = 20 J/cm2. The obtained extreme durable embedded-electrode-type sensor can completed by filling the cavity with conductive material by standard electroplating techniques. For the manufacturing process, a high flexible NC-controlled machining concept is presented, which allows a time and pulse minimized fabrication as well as an optimization of the surface quality of the micro-sensor, including a process optimization via simulation.
In this paper investigation of using frequency-converted diode-pumped solid-state laser for substituting conventional industrial machining processes like in the MST is presented. For industrial application a decreasing of the processing time is demanded. This study shows that the pulse energy of current available DPSS lasers is nearly sufficient to meet the demands for industrial laser cutting and selective ablation processes.
Excimer lasers are proven tools to machine 2½-D microstructures with variable lateral dimensions. Therefore developed techniques are limited in the vertical dimension since material is removed along the optical axis perpendicular to the target plane. This paper presents 3D structures produced with such UV-lasers. In contrast to optical set-ups for machining 2½-D structures, this approach tilts the target plane and ablates material underneath the target superficies. The tilting angle adds two major difficulties to laser machining: the distortion of the image on the target and the alteration of the ablation cross section. These two difficulties were studied in experiments with different tilting angles β L between target plane and optical axis of the laser. The impact of β L was identified on the achieved geometry of 3D structures. A first theoretical approximation integrates the material reflectance and the target cross-section in order to give an estimation of the influence of further effects within the ablation process. This theoretical analysis is a starting point for producing undercutting structures and can additionally be applied to changeable shaped surfaces. Such compel 3D structures have the potential to be sued in micro- tribology as well as in micro guidance systems and are estimated being an important step in micro-mechanics.
This paper presents results on machine and process development for laser welding of surface mounted devices on thermal resistant polymer based molded interconnect devices with copper layers of 35 micrometers and 70 micrometers thickness. Characteristics, advantages and problems of this technology are shown and possibilities to achieve reproducible results are discussed. The investigations are carried out with pulsed Nd:YAG-lasers wit a maximum average power of 300 W. Additionally, a process control concept evaluating the reflected process radiation is discussed.
F2 lasers emitting radiation with a wavelength (lambda) equals 157 nm allow the direct laser based micro-structuring of materials like PTFE (e.g. Teflon) or quartz glass for MST-applications. This paper reports about the fundamental basis of controlling and modelling the structuring process and about existing tools for the 3-D micron structuring with F2 excimer lasers. The concept of three-dimensional micro-structuring is based on gradual ablation, using single laser pulses. A CNC-controlled precision machining centre with a vacuum encapsulated beam guiding system has been developed. For the material processing with F2 lasers a model of the single ablation size was qualified, which provides an exact description of the laser pulse taking the material properties, the energy density and the lateral dimensions of the single ablation into account. The non- linear characteristics between pulse geometry and process parameters were experimentally determined and implemented into software for automated NC-data generation. A special software tool makes the simulation of manufacturing micro-components possible. It is based on the complex NC-code and on the exact model of the single ablation. Using this simulation, the process parameters can be optimised before processing with a laser micro-machining centre. The described technology was verified by manufacturing 3-D micro-structures in several materials, such as quartz glass and PTFE.
Using CAD-CAM software tools in a production chain from the design to the NC code generation and manufacturing of workpieces is state of the art in common technologies such as milling or turning. Today these software tools are equipped with intelligent algorithms consider the characteristics of the respective production technology like e.g. the consideration of different tools to manufacture different volume sizes or the quality of special surface finishes.
Currently there is a strong demand for refractive optical elements made from glass in 21/2D and 3D-structures. Due to the characteristics of brittle materials like glass, only a limited number of manufacturing methods can be used to machine these materials with sub-micron resolution. Thus, current microstructures made out of glass are mainly manufactured by photolithography and etching process. Lithography techniques are only for economic purposes for a series production, but is not suitable for manufacturing prototypes or a small series. Micromachining done with Excimer Lasers in combination with high precision CNC- controlled handling systems offers flexible design possibilities for optical components. Due to the limitations of conventional machining techniques for brittle materials, a new laser machining system for material processing at a wavelength of 193 nm has been designed and built. The better absorption of 193 nm compared to 248 nm or larger wavelengths leads to damage free microstructuring of most glasses. Data generation for the volume to be ablated starts with the mathematical description of the surface shape of the optical component. The contour can be derived from a mathematical function or individual xyz-data point information from any CAD-program. A pre-processor calculates the CNC-data for laser triggering, xyz-table and the CNC- mask control. Each laser pulse leads to a material removal, defined by the illuminated surface on the workpiece as well as the energy density. Superposition or overlapping of pulses allows the creation of the desired surface. The surface roughness is determined by the wavelength as well as the chosen ablation strategy. To achieve best results, the process has to be carefully adjusted for a specific material. This technique is a sufficient method for structuring grooves in ceramics or glass as well as producing aspherical transparent optical surfaces or micro lens arrays. This paper shall describe the potential of 193 nm treatment of 3D micro surfaces with an optimized process machine and data handling system in comparison with results originating from 248 nm laser processing.
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