Alexander Horn

Fundamental investigations of micromachining by nano- and picosecond laser radiation

The removal processes of ceramics and metals were investigated using pico- and nanosecond laser radiation produced by diode-pumped Nd:YAG lasers. The laser radiation was focused to spot diameters smaller than 10 mm, yielding power densities up to 5 = 10 12 Wrcm 2 . The threshold fluence for removal and the removal depth per pulse were determined for 40 pico- and 10 nanosecond laser pulses using the fundamental wavelength, the second harmonic and the third harmonic laser radiation of the laser system. For 40 ps laser pulses pump and probe investigations were used to study the interaction of intense ultrashort laser beams with matter. By this technique ultrashort processes can be photographed with a time resolution determined by the pulse length of pump and probe pulses. The measurements allow a detailed characterization of the material removal including melting, vaporization and fast resolidification as well as the feedback of the surrounding atmosphere to the processed microstructures. The threshold fluences for material removal and the removal rates per pulse were determined for Si N , SiC and WC as a function of laser pulse length and laser wavelength. Using picosecond laser 34 radiation microstructures were produced in different ceramics and metals demonstrating the suitability of short laser pulses for the production of microstructures with dimensions smaller than 10 mm and for ultra-precise removal of thin layers. q 1998 Elsevier Science B.V.

Dynamical detection of optical phase changes during micro-welding of glass with ultra-short laser radiation

The accelerating developments in micro- and nanotechnology require faster and more precise tools for application and diagnostics. A new ultra-fast diagnostics is presented as an advancement of a conventional phase microscopy method (Iatia QPm™). In contrast to the conventional method using one CCD to detect three object planes, three CCDs detect these planes separately and simultaneously. The measurement technique named TQPm has been analyzed and validated by measuring the optical phase of a commercial fiber. This novel visualization technique affords reliable quantitative time-resolved measurements of the optical phase, the transient refractive index or the dynamical geometry changes. The structural and optical modifications of welded glass have been observed coaxially in situ during melting and welding by TQPm. Technical glass plates (Schott D263) have been welded using an ultra-short pulsed laser. By the use of femtosecond laser radiation (tp = 350 fs, frep = 1 MHz) focused by a microscope objective (2ω0 ≈ 4 µm) at the interface, multi-photon absorption is the dominant phenomenon. This causes heat accumulation and thereby glass melting and welding.

In situ measurement of plasma and shock wave properties inside laser-drilled metal holes

High-speed imaging of shock wave and plasma dynamics is a commonly used diagnostic method for monitoring processes during laser material treatment. It is used for processes such as laser ablation, cutting, keyhole welding and drilling. Diagnosis of laser drilling is typically adopted above the material surface because lateral process monitoring with optical diagnostic methods inside the laser-drilled hole is not possible due to the hole walls. A novel method is presented to investigate plasma and shock wave properties during the laser drilling inside a confined environment such as a laser-drilled hole. With a novel sample preparation and the use of high-speed imaging combined with spectroscopy, a time and spatial resolved monitoring of plasma and shock wave dynamics is realized. Optical emission of plasma and shock waves during drilling of stainless steel with ns-pulsed laser radiation is monitored and analysed. Spatial distributions and velocities of shock waves and of plasma are determined inside the holes. Spectroscopy is accomplished during the expansion of the plasma inside the drilled hole allowing for the determination of electron densities.

High-throughput process parallelization for laser surface modification on Si-solar cells: determination of the process window

The laser is an extremely suitable non-contact tool for fast and automated in-line processes for example used to improve the efficiency of solar cells. With ultra-short pulsed laser radiation it is possible to decrease the reflectivity by modifying the surface topology of silicon. For the proposed modification, the optimum process window for altering the silicon surface topology on a micrometer scale is found at small laser fluencies at finite repetition rates. A promising up scaling method is process parallelization using in parallel a multiple set of interaction zones with the optimized process characteristics for single process interaction. Based on the single process, required laser process parameters and optical parameters for parallel processing are derived theoretically in order to enable a wafer processing in standard cycle times. Exemplarily 5-inch mc-silicon solar wafers are machined using a linear 7-times diffractive optical element (DOE), and in a second step solar cells are built up to determine the efficiency gain by the laser surface modification. A preliminary absolute efficiency gain of Δη > 0.2 % is achieved.

Temporal femtosecond pulse tailoring for nanoscale laser processing of wide-bandgap materials

Nanoscale laser processing of wide-bandgap materials with temporally shaped femtosecond laser pulses is investigated experimentally. Femtosecond pulse shaping in frequency domain is introduced and applied to two classes of shaped pulses relevant to laser nano structuring. The first class, characterized by a symmetric temporal pulse envelope but asymmetric instantaneous frequency allows us to examine the influence of the sweep of the photon energy. In contrast, asymmetric temporal pulse envelopes with a constant instantaneous frequency serve as a prototype for pulses with time-dependent energy flow but constant photon energy. In our experiment, we use a modified microscope set up to irradiate the surface of a fused silica sample with a single shaped pulse resulting in ablation structures. The topology of the laser generated structures is measured by Atomic Force Microscopy (AFM). Structure parameters are investigated as a function of the pulse energy and the modulation parameters. We find different thresholds for surface material modification with respect to an asymmetric pulse and its time reversed counterpart. However, we do not observe pronounced differences between up- and down-chirped radiation in the measured structure diameters and thresholds.

Microholes in zirconia-coated Ni-superalloys for transpiration cooling of turbine blades

Drillings in zirconia coated Ni-superalloys is done by melt extraction with pulsed laser radiation provided by a Nd:YAG slab laser with microsecond pulse duration. This laser system distinguishes itself by a high beam quality and offers the possibility to investigate drilling of holes with a diameter of 200 micrometer by percussion drilling and trepanning. The quality of drilled holes, e.g. the heat affected zone (HAZ), the recast layer and the conicality, are presented. During drilling different process gases are used. The results in drilling velocities, melt thickness and chemical composition of the melting zone are shown for oxygen, argon and nitrogen by SEM and EDX. A numerical simulation of the trepanning process will be presented. The different time scales of the contributing physical processes related, for example, to the small melt film layer during trepanning are described. A coating is distributed on the multilayer system to protect the blade from recast. Aim of the investigation is the production of holes in a multilayer system, consisting of CMSX-4, VPS-MCrAlY and EB-PVD-zirconia. With this used laser system inclined holes up to 60 degrees in this layer system can be drilled. No recast layer and no spalling of the zirconia-layer are observed.

Ultrafast pump and probe investigations on the interaction of femtosecond laser pulses with glass

The change of the matrix structure of glass is investigated during and after irradiation with ultrashort pulsed laser radiation (100 fs < tp; < 3 ps) at the wavelength λ = 810 nm. The dynamics of the plasma expansion and the stress formations are visualized by time-resolved Normarski-photography. Optical microscopy visualizes the structural changes in glass. The spatial stress distribution and the refractive index change are shown in the time range 100 fs < t < 120 ns. The ionization state of atoms and/or the formation of color centers has been investigated by transient absorption spectroscopy (TAS) in the time regime 100 fs < t < 120 ns. The temporal change of the spectra shows different regimes, which can be explained by the electron and phonon relaxation.

Electron beam-physical vapor deposition - thermal barrier coatings on laser drilled surfaces for transpiration cooling

In this paper the deposition behavior of zirconia on laser drilled surfaces is reported. The samples were prepared with laser drilled holes (diameter 200 μm) with tilting angles of 0°, 15°, 30°, 45° and 56°. As substrate material, VPS-MCrAlY-coated nickel-based superalloy was used. The zirconia thermal barrier coatings were deposited by the electron beam-physical vapor deposition (EB-PVD) technique. The extent by which the tilted holes were closed by the deposition process were examined. With increasing tilting angle the holes were closed during the deposition of the thermal barrier coating with decreasing of coating thickness. In additional experiments, a gas (argon) flowed through the holes during the deposition process. Due to the lower temperature of the surrounding area to the holes caused by the cold gas stream, the holes were closed faster by the ceramic vapor compared with the deposition without gas flow. Another effect of the gas flow was the formation of non-stoichiometric zirconia near the holes which was evident by the darker color.

Micromachining of metals and ceramics by nano- and picosecond laser radiation

The removal processes of Si3N4- and SiC-ceramics and tungstencarbide were investigated using 40 ps and 10 ns laser pulses. The threshold fluence for removal and the removal rate per pulse were determined. Changes in the chemical composition of the processed surfaces are described and the influence of the removal strategy on the processing results are discussed. Micro-structures were produced in combination with high- resolution optics and precision motion control systems. In SiC-ceramics grooves were produced with geometries smaller than 30 micrometer. In Si3N4-ceramics holes were drilled with diameters smaller than 6 micrometer. The influence of scanning velocity, overlap of the laser pulses and pulse energy on material removal and surface finish were discussed.

Formation of subwavelength-laser-induced periodic surface structures by tightly focused femtosecond laser radiation

Sub-wavelength (1/4*λ-3/4*λ) laser induced periodic surface structures are generated by irradiation of either bulk fused silica and silicon or Er:BaTiO3 thin films by scanning a tightly focused beam (Θ = 1 μm) of femtosecond laser radiation (λ = 800 nm, tp = 100 fs) on the surface. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarization of the laser radiation. The dependence of the ripple spacing on the spacing of successive pulses, the direction of polarization and the properties of the material is investigated. The evolution of the ripples is investigated by applying pulse bursts with 1 - 20 pulses. The development conditions of the stuctures are specified and possible mechanisms of ripple growth are discussed.