Andreas Ostendorf

Sub-diffraction limited structuring of solid targets with femtosecond laser pulses

Possibilities to produce sub-diffraction limited structures in thin metal films and bulk dielectric materials using femtosecond laser pulses are investigated. The physics of ultrashort pulse laser ablation of solids is outlined. Results on the fabrication of sub-micrometer structures in 100-200 nm chrome-coated surfaces by direct ablative writing are reported. Polarization maintaining optical waveguides produced by femtosecond laser pulses inside crystalline quartz are demonstrated.

Microdrilling of metals with ultrashort laser pulses

The pulse duration requirements for melting-free and burr-free drilling of metals have been observed in detail. It will be shown that the highest process efficiency and hole quality can be achieved by using pulse durations in the subpicosecond time scale. Furthermore, it will be demonstrated that nonlinear distortions due to high intensities can be successfully minimized by applying diffractive optical elements for beam forming. The influence of the polarization of the laser radiation on the process quality will be discussed. Experimental results will show that the rotation of linear polarized radiation during percussion drilling significantly improves the quality of the hole geometry.

Microstructuring with femtosecond lasers

Using ultrashort laser pluses can cause the ablation of nearly all kinds of materials, leading to very precise machining results with minimal damage. This includes even the machining of delicate materials with high heat conductivities or comparatively low melting temperatures, such as metals, as well as optical transparent and organic materials. The best results are obtained using pulses with sub-picosecond duration. This report summarizes recent progress in ultrashort pulse laser machining. Several promising applications in microstructuring, nanotechnology and medicine are highlighted.

Compact electron-based extreme ultraviolet source at 13.5 nm

Generation of extreme ultraviolet (EUV) radiation from solid targets is studied and a compact EUV source for small-scale lithographic applications and EUV metrology is developed. This source is based on a transfer of conventional x-ray tube technology into the EUV spectral range. As in an ordinary x-ray tube, electrons are generated by a tung- sten filament and accelerated in a high-voltage electric field toward a solid target. In the demonstrated EUV tube beryllium and silicon tar- gets are used to generate radiation at 11.4 and 13.5 nm, respectively. The absolute conversion efficiencies into EUV photons at 13.5 nm are measured. Prospects for a further power scaling of the EUV source are discussed.

Micromachining using femtosecond lasers

Femtosecond laser systems have been proved to be effective tools for high precision micro-machining. Almost all solid materials can be processed with high precision. The dependence on material properties like thermal conductivity, transparency, heat- or shock sensitivity is strongly reduced and no significant influence on the remaining bulk material is observed after ablation using femtosecond laser pulses. In contrast to conventional laser processing, where the achievable precision is reduced due to a formed liquid phase causing burr formation, the achievable precision using femtosecond pulses is only limited by the diffraction of the used optics. Potential applications of this technique, aincluding the structuring of biodegradable polymers for cardiovascular implants, so-called stents, as well as high precision machining of transparent materials are presented.

Structuring silicon with femtosecond lasers

In this paper, basic examinations on the laser cutting of silicon using ultrashort ((tau) H equals 150 fs) laser pulses are presented. The influence of the polarization on the cutting process is investigated. It was found that significant deviations from the ideal cut geometry occur if the polarization is parallel to the cutting motion. An innovative automated method using image processing to assess the quality of cuts is discussed. On the basis of this method, it is shown that the deviations increase with the depth of the cut. Hence, it is suggested that deviations are caused by reflection. Two models for simulating the influence of different polarizations on the intensity distribution on the ablation ground for cutting and drilling are discussed.

Femtosecond laser ablation and nanostructuring

At the Laser Zentrum Hannover investigations of possibilities to use femtosecond laser pulses for direct ablative writing and microstructuring of solid materials have been started in 1995. Since then considerable progress in understanding and in the application of different femtosecond technologies has been obtained. At present, we are able to produce high quality microstructuring and large area patterning of solids with structure sizes between one and ten micrometers. By using tightly focused femtosecond pulses it is possible to produce even sub-micrometer structures. In this paper we pursue the goal to find and characterize the limits of femtosecond laser micromachining. Detailed investigations of possibilities to use femtosecond lasers for the sub-wavelength microstructuring of metals and for fabrication of periodic structures in transparent materials with the scale length of the order of several hundreds nanometers are reported.

Laser spot welding of electronic micro parts

This paper deals with parameter optimization and online monitoring of laser spot welding (LSW). Using Nd:YAG laser, a wide range of experiments regarding the welding process have been carried out for both successful and failed welds. The typical failures appearing during packaging of surface mounted devices (SMDs) on flexible printed circuits (FPC) include gaps, a loss of connection between the welded components, and damage of the printed circuit boards. A flip-flop device called SO16 and lead frames as two components of widely used SMDs were packaged on FPCs in the experiments. The reproducibility of the weld quality for SO16 (FeNi) is greater than for lead frames (CuFe2P); this points out the difficulties appearing during copper or copper alloy welding. However, a correlation between the weld quality and the detected emission signals recorded during the weld process has been found for both components. The detected signals of the optical process emission for successful welds depict identical characterisics which are divided into three relevant signal phases. Changes in the signal characteristics, especially in these phases, imply information about the weld quality. While monitoring the welding processes for both components are possible, the detected signals for SO16 are less sensitive to process variations compared to those for lead frames. Based on spectral analysis, the intensity of the detected emission due to SO16 welding is slightly higher than the intensity due to lead frames welding.

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.