We combine laser processing and the technique of a scanning near-field optical microscope (SNOM) for realization of
laser-patterning on a nanoscale, laser ablation near the fiber tip, and micro-analysis of solid surface samples by laserinduced
breakdown spectroscopy (LIBS). We describe an universal SNOM-like setup allowing to produce near-field
laser patterns by laser heating and laser ablation, laser-induced breakdown spectroscopy, and atomic force microscope
(AFM) topography investigation with the same optical fiber tip, which is used as near-field emitter or as probe. With
solid Si and Al samples, three laser processing regimes were observed with increasing laser pulse energy: (1) cone
formation (only for Si, smallest features with 500 nm width and 100 nm height), (2) formation of small craters (smallest
features with 450 nm width and 250 nm depth), and (3) crater formation with a width of more than 2 μm with emission
of evaluable plasma emission line spectra.
Pulsed-laser deposition (PLD) is an excellent technique to grow thin films and multi-layers of complex oxide materials.
We present our recent results on deposition, characterization and nano-patterning of novel oxide high-temperature
superconductors (HTS) and piezoelectric materials. HTS thin films of improved superconducting properties are
fabricated from ceramic targets consisting of Y2Ba4CuMOy (M = metal) nano-particles embedded in YBa2Cu3O7
(YBCO) matrix. The nano-composite ceramics are UV-laser ablated and the optical plasma emission is monitored by
laser - induced breakdown spectroscopy. The HTS films reveal superconducting critical current densities that depend on
target composition and are strongly enhanced as compared to phase pure YBCO films. Application of HTS layers in
future nano-electronic devices requires novel techniques for nano-patterning. Masked ion-beam structuring enables the
nano-patterning of YBCO thin films in a direct and single-step process. Gallium orthophosphate (GaPO4) thin films and
piezoelectric ZnO multi-layers are reported also. GaPO4 is an outstanding piezoelectric material with very high
transition temperature (~ 970 °C). Epitaxial GaPO4 films are fabricated on quartz substrates by PLD and thermal post-annealing.
Long-term annealing at high temperature does not degrade the GaPO4 films. Multi-layers of Al and Li doped
ZnO are pulsed-laser deposited on various substrates and investigated for applications in thin film sensors.
The article describes the modification of fluoropolymers by UV-irradiation with wavelengths below 200 nm in a reactive or inert atmosphere. Light sources employed are excimer lamps, excimer-lasers or F2-lasers. Special emphasis is put on adhesion phenomena of fluids, biological cells and metal coatings at the modified surfaces.
Advanced thin film materials with giant dielectric permittivities up to ≈ 10000 were produced by pulsed-laser deposition. Composite targets of barium titanate (BaTiO3) and polytetrafluorethylene (PTFE) were ablated with 248 nm KrF-laser radiation in Ar atmosphere. The synthesized films have a complex microstructure and contain product species which are formed during the pulsed-laser ablation / deposition process. The dielectric permittivities of films exhibit pronounced dispersion for frequencies higher than 10 kHz. Strong dependencies of the film permittivity on target composition, layer thickness, ambient gas pressure and relative humidity are revealed. The large dielectric permittivity of these film materials may be attributed to space-charge polarization phenomena. Films deposited from the same targets in oxygen atmosphere have much lower dielectric permittivity (ετ'≈ 30).
Pulsed-laser deposition is a unique technique that has been employed for thin film growth of a broad variety of materials. We report on the deposition and characterization of advanced piezoelectric thin films and ceramic/polymer composite layers. Pulsed-laser ablation and micro-patterning of piezoelectric GaPO4 is presented.
Two-dimesional (2D) lattices of microspheres formed by self-assembly from colloidal solutions have been used for laser-induced surface patterning by ablation, etching, deposition, and surface modification. The imaging properties of microspheres and the related intensity- and temperature-distributions on nearby substrates are studied and compared with experimental results on the deposition of Pd from aqueous solutions of PdCl2 in NH3.
Recent achievements in laser-induced surface patterning obtained in our group are summarized. Here, we have employed both a SNOM-type setup and two-dimensional lattices of SiO2 microspheres formed by self-assembly processes. With the SNOM-type setup we have
demonstrated nanoscale photochemical and photothermal etching, mainly of Si in Cl2 atmosphere. With 2D lattices of microspheres a large number of single features can be generated by a single or a few laser shots. Among the examples presented is the surface patterning by ablation, etching, deposition, and surface modification.
Pulsed-laser deposition is a unique technique that has been employed for thin film growth of a broad variety of materials. Tuning of deposition parameters allows to produce advanced thin films with optimized materials properties. We report on the deposition and characterization of various kinds of high-temperature superconducting thin films and of ceramic/polymer composite layers.
We report on rapid in-situ analysis of liquid and solid steel samples under reduced ambient pressure by laser-induced breakdown spectroscopy (LIBS) using a transportable system. LIBS denotes a technique where a pulsed laser beam is used to ablate small amounts of the target material. The characteristic optical emission line intensities of the excited species in the laser-generated plasma plume allow a quantitative chemical analysis of the target material. Over a wide range of parameters the expansion of the plume can be described by a generalized shock wave model. LIBS is a fast, non-contact method, which can be carried out under various atmospheric conditions allowing large working distances between the sample under investigation and the detection system. These properties make LIBS applicable to process control especially for vacuum devices used in metallurgy.
Laser-induced material processing is reviewed with special emphasis on recent achievements mainly obtained by the Linz group. Among those are investigations using optical fiber tips for nanoscale photophysical etching, laser-induced ablation using self-assembled microspheres, the pulse-laser deposition of thin films of high-temperature superconductors (HTS) and polytetrafluoroethylene (PTFE), and the modification and cleaning of surfaces.
We report on experiments on the underlying physical mechanisms in the Dry-(DLC) and Steam Laser Cleaning (SLC) process. Using a frequency doubled, Q-switched Nd:YAG laser (FWHMequals8 ns), we removed polystyrene (PS) particles with diameters from 110-2000 nm from industrial silicon wafers by the DLC process. The experiments have been carried out both in ambient conditions as well as in high vacuum (10-6mbar) and the cleaned areas have been characterized by atomic force microscopy for damage inspection. Besides the determining the cleaning thresholds in laser fluence for a large interval of particle sizes we could show that particle removal in DLC is due to a combination of at least three effects: thermal substrate expansion, local substrate ablation due to field enhancement at the particle and explosive evaporation of absorbed humidity from the air. Which effect dominates the process is subject to the boundary conditions. For our laser parameters no damage free DLC was possible, i.e. whenever a particle was removed by DLC we damaged the substrate by local field enhancement. In our SLC experiments we determined the amount of superheating of a liquid layer adjacent to surfaces with controlled roughness that is necessary, in good agreement with theoretical predictions. Rough surfaces exhibited only a much smaller superheating.
A model for ns dry laser cleaning that treats the substrate and particle expansion on a unified basis is suggested. Formulas for the time-dependent thermal expansion of the substrate, valid for temperature-dependent parameters are derived. Van der Waals adhesion, the elasticity of the substrate and particle, as well as particle inertia is taken into account for an arbitrary temporal profile of the laser pulse. Time scale related to the size of the particles and the adhesion/elastic constants is revealed. Cleaning proceeds in different regimes if the duration of the laser pulse is much shorter/longer than this characteristic time. Expressions for cleaning thresholds are provided and compared with experiments on the cleaning of Si surfaces from spherical SiO2 particles with radii between 200 and 2585 nm in vacuum with 248 nm KrF excimer laser and 532 nm frequency doubled Nd-YAG laser. Large discrepancies between the experimental data and theoretical results for KrF laser suggest that ns dry laser cleaning cannot be explained on the basis of thermal expansion mechanism alone.
We present a method for directly imaging the undisturbed near field of a particle resting on a surface. A comparison with numerical computations shows good agreement with the results of our experiments. These results have important consequences for laser-assisted particle removal where field enhancement may cause local surface damage and is one of the physical key processes in this cleaning method. On the other hand, the application of near fields at particles allows structuring of surfaces with structure dimensions in the order of 100 nm and even below.
Pulsed-laser deposition (PLD) is a unique technique that has been employed for thin film growth of a broad variety of materials. In this contribution, PLD of high-temperature superconducting films of YBA2Cu3O7-(delta ), Bi2Sr2CaCu2O8+(delta ) and (Hg,Re)Ba2CaCu2O6+(delta ) is reported. Emphasis is put on the optimization of deposition parameters and the growth of so-called tilted films on vicinal cut substrates. Such films offer the unique possibility to measure in-plane and out-of- plane transport properties which is especially important for materials not available as single crystals. Experiments on photodoping and on vortex string channeling are presented. The electrical properties and the microstructure of the vicinal films are investigated with respect to the tilt angle and the film thickness.
The cleaning of silicon surfaces from submicron dust particles has been studied by means of the 'Steam Laser Cleaning' (SLC) process and compared to 'Dry Laser Cleaning' (DLC) which is used nowadays in many applications. For SLC a thin liquid layer (e.g. a water- alcohol mixture) is condensed onto the substrate, and is subsequently evaporated by irradiating the surface with a short laser pulse. The DLC process, on the other hand, only relies on the laser pulse, without application of a vapor jet. We have systematically investigated the efficiency of these two processes for the removal of well-characterized polymer, silica and alumina particles of various sizes down to 60 nm in diameter, and have also studied the influence of light wavelength and laser pulse duration for nanosecond and picosecond pulses. The results demonstrate that for the gentle cleaning of silicon wafers SLC is a very efficient method and is superior to DLC. An effect which so far has only rarely been taken into account for laser cleaning is the field enhancement under the particles, which can give rise to serious surface damage, in particular when cleaning pulses in the picosecond and femtosecond range in the DLC are applied.
Polytetrafluoroethylene (Teflon PTFE) films were grown by pulsed-laser deposition (PLD). Films prepared by ablation from press-sintered targets are found to be highly crystalline, with spherulite sizes adjustable over more than one order in magnitude by suitable thermal annealing. As revealed by dielectric dilatometry, PLD-PTFE films show characteristics remarkably similar to those of conventional PTFE, i.e. the same structural first-order phase transitions. Dielectric losses are low and indicate no tendency to film oxidation. PLD-PTFE films additionally show an excellent charge-stability, comparable and even superior to commercially available Teflon-PTFE foils. PLD-PTFE enlarges the family of Teflon materials and may thus become interesting for potential miniaturized electret devices. Furthermore, dielectric dilatometry provides an elegant means for the determination of the coefficient of thermal expansion in thin nonpolar films.
Single-shot laser ablation of polyimide has been investigated with UV Ar+-laser radiation (lambda approximately equals 302 nm) for pulse lengths between 140 ns and 5000 ns. The dependences of the ablation rate on laser pulse length and intensity were measured by means of an atomic force microscope (AFM). These data are compared with mass-loss measurements using a microbalance. The experimental data are analyzed by taking into account both mass losses related to volatile product species and real material ablation.
UV-laser irradiation of polymers leads to different types of surface modifications, including the formation of structures with sub-micrometer dimensions. These structures may influence the adhesion of surface coatings. In this paper we investigate the temperature dependence of the growth of branched dendritic structures. Growth rates in excess of 1 nm/s were measured near the glass transition temperature of poly(ethylene terephthalate). Periodic surface structures are investigated with respect to physical formation processes.
In this paper we present recent results on laser-induce surface modifications and surface patterning by ablation. Different types of structure formation are discussed. The modeling of UV-laser ablation in nonstationary regimes is studied. Numerical calculations on the ablation rate are compared with experimental data.