Christian Kulik

Ablation of metals and semiconductors with ultrashort pulsed lasers: improving surface qualities of microcuts and grooves

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.

Short and ultrashort laser pulses: an upcoming tool for processing optical and semiconductor materials

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.

Processing thin silicon with ultrashort-pulsed lasers: Creating an alternative to conventional sawing techniques

When processing thin silicon, an important workpiece category in modern IT industry, femtosecond laser pulses enable to create cuts and also holes of utmost precision. This could contribute to the ongoing miniaturization by reducing back-end processing resolutions.However, it bears special challenges: On the one hand, decreasing material thickness involves chipping effects and raises general handling questions. On the other hand, laser processing of silicon generally involves debris effects which are hard to handle in comparison to other common materials. In addition, processing speeds are usually still significantly lower than those of cutting with conventional mechanical methods.Therefore an overview of influences will be given that are of special interest for the processing of thin silicon with ultrashort-pulsed lasers. To demonstrate the competitiveness of the process in the near future, general advantages of this technique will be explained. Methods that have improved quality in case of chipping and debris and at the same time increased the processing speed will be presented.When processing thin silicon, an important workpiece category in modern IT industry, femtosecond laser pulses enable to create cuts and also holes of utmost precision. This could contribute to the ongoing miniaturization by reducing back-end processing resolutions.However, it bears special challenges: On the one hand, decreasing material thickness involves chipping effects and raises general handling questions. On the other hand, laser processing of silicon generally involves debris effects which are hard to handle in comparison to other common materials. In addition, processing speeds are usually still significantly lower than those of cutting with conventional mechanical methods.Therefore an overview of influences will be given that are of special interest for the processing of thin silicon with ultrashort-pulsed lasers. To demonstrate the competitiveness of the process in the near future, general advantages of this technique will be explained. Methods that have improved quality in case of chipping and ...

Machining of optical microstructures with 157 nm laser radiation

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.

Basic investigations for controlling the laser spot-welding process when packaging 3-dimensional molded interconnect devices

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.

Production of innovative geometries with UV lasers

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.

Automatic control of a thermal-based polishing process for aspherical optics

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.

Short and ultrashort laser pulses: application-driven comparison of source types

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.

Approach to fast fabrication of micro-sensors with high repetition 4ω-diode-pumped solid-state lasers

A modification of standard photolithographic processes for thin film technology by Laser Direct Writing (LDW) is introduced. Employing a high frequency UV-Diode-Pumped Solid-State Lasers (DPSSL) combined with a scanner beam guiding system, the demonstrated approach optimizes development cycles of new sensor devices. The presented image-mask-free technique allows a structuring of photo-sensitive resist in micrometer range on a base of CAD templates, which insures a high flexibility concerning modifications of the micro-machined geometries. Especially during the development of sensor systems (prototyping) and small batch series, a distinct cost advantage can be obtained, since time-and cost-consuming image mask technology is avoided.The potential of LDW has been proved by machining components of a micro-eddy current sensor with a minimum lateral dimension of some 10 µm in photo resists of 2 µm, 24 µm and 52 µm thickness with excellent geometrical features (smooth edges, flank angle αFl ∼ 90°). Whereas the machining duration in sub-second range (for 2 µm thick resists) emphasizes the potential for high flexible prototyping in thin film technology.A modification of standard photolithographic processes for thin film technology by Laser Direct Writing (LDW) is introduced. Employing a high frequency UV-Diode-Pumped Solid-State Lasers (DPSSL) combined with a scanner beam guiding system, the demonstrated approach optimizes development cycles of new sensor devices. The presented image-mask-free technique allows a structuring of photo-sensitive resist in micrometer range on a base of CAD templates, which insures a high flexibility concerning modifications of the micro-machined geometries. Especially during the development of sensor systems (prototyping) and small batch series, a distinct cost advantage can be obtained, since time-and cost-consuming image mask technology is avoided.The potential of LDW has been proved by machining components of a micro-eddy current sensor with a minimum lateral dimension of some 10 µm in photo resists of 2 µm, 24 µm and 52 µm thickness with excellent geometrical features (smooth edges, flank angle αFl ∼ 90°). Whereas the m...

Precision shaping of transparent materials for optical devices with VUV laser radiation

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.