Dieter Baeuerle

Optical near field effects in surface nanostructuring and laser cleaning

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.

Laser cleaning of silicon wafers: mechanisms and efficiencies

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.

Dynamic particle removal by nanosecond dry laser cleaning: theory

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.

Laser-induced single step micro/nanopatterning

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.

Laser-enhanced adhesion and thin film formation

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.

UV-laser-induced polymer ablation: the role of volatile species

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.

Photophysical mechanism of UV laser action: the role of stress transients

The joint influence of electronic excitations and mechanical stresses originated from UV laser action on solids (primarily polymers) decomposition kinetics is discussed.

Laser-induced surface modifications, structure formation, and ablation of organic polymers

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.

Modeling of nanosecond-laser ablation: calculations based on a nonstationary averaging technique (spatial moments)

Semi-analytical approach to a quantitative analysis of thermal ns laser ablation is presented. It permits one to take into account: (1) Arbitrary temperature dependences of material parameters, such as the specific heat, thermal conductivity, absorptivity, absorption coefficient, etc. (2) Arbitrary temporal profiles of the laser pulse. (3) Strong (Arrhenius- type) dependence of the ablation velocity on the temperature of the ablation front, which leads to a non-steady movement of the ablation boundary during the (single) pulse. (4) Screening of the incoming radiation by the ablated products. (5) Influence of the ablation (vaporization) enthalpy on the heating process. (6) Influence of melting and/or other phase transformations. The nonlinear heat conduction equation is reduced to three ordinary differential equations which describe the evolution of the surface temperature, spatial width of the enthalpy distribution, and the ablated depth. Due to its speed and flexibility, the method provides powerful tool for the fast analysis of the experimental data. The influence of different factors onto ablation curves (ablated depth h vs. fluence (phi) ) is studied. Analytical formulas for (phi) th and h((phi) ) dependences are derived and discussed. The ablation curves reveal three regions of fluence: Arrhenius region, linear region, and screening region. Threshold fluence (phi) th and Arrhenius tails at (phi) less than (phi) th, are affected heavily by the temperature dependences in material parameters, surface evaporation rate, and pulse duration and shape. In contrast, the slope of the ablation curves at (phi) greater than (phi) th, is determined almost exclusively by the latent heat of vaporization, high temperature dependence of absorptivity, and, in the case of screening, by the absorption coefficient of the plume (alpha) g. In the screening region ablated depth increases logarithmically with fluence and its qualitative behavior is weakly affected by the temperature dependence in (alpha) g (T). Small vaporization enthalpy results in a sub-linear h((phi) ) dependence, which, nevertheless, remains faster than logarithmic. With weakly absorbing materials ablation may proceed in two significantly different regimes -- without or with ablation of the heated subsurface layer. The latter occurs at higher fluences and reveals significantly higher ablation temperatures, but is weakly reflected on the ablation curves. Calculations are performed in order to study the: (1) Influence of the duration and temporal profile of the laser pulse on the threshold fluence, (phi) th. This is particularly important for strong absorbers were the heat conduction determines the temperature distribution. (2) Influence of the temperature dependences in material parameters on the ablation curves (ablated depth versus laser fluence) for regimes (phi) approximately equals (phi) th and (phi) very much greater than (phi) th. (3) Consequences of shielding of the incoming radiation at high fluences. (4) Differences in ablation curves for materials with big and small ablation enthalpy (e.g., metals and polymers which ablate differences in ablation curves for materials with big and small ablation enthalpy (e.g., metals and polymers which ablate thermally). Nanosecond laser ablation has been studied for a large variety of different materials and laser wavelengths. As an illustrative example, the method is applied to the quantitative anlaysis of the single pulse ablation of polyimide Kapton TM H.

Perspectives of laser processing and chemistry

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.