Ablation of Materials with Low Absorption

Research project P 14476 Ablation of Materials with Low Absorption Johannes HEITZ 26.6.2000 Recently it has been demonstrated that nanosecond pulsed-laser deposition (PLD) permits one to fabricate high- quality thin films of dielectric materials, even if they are weak absorbers at the laser wavelength under consideration. Here, the type of target material employed is of fundamental importance. With (homogeneous) crystalline or fused glassy target materials, the ablation rates are very low and the deposits consist mainly of large particulates and fragments. With pressed and sintered powder targets with optimized grain size, however, smooth and stoichiometric oriented films can be fabricated. Within the frame of this research-project we want to elucidate the fundamental interactive mechanisms. between laser light and pressed powder and ceramic targets. In particular we want to investigate systematically the dependence of ablation and deposition rates on the variation of the energy density deposited near the surface. For each material the energy density can mainly depend on the grain size, the number density of grains, the laser wavelength, the laser intensity, and the number of laser pulses per area. The films deposited with various values of these experimental parameters shall be characterized especially with respect to their thickness, morphology, stoichiometry, and orientation by employing various techniques such as profilometry, AFM, XRD, IR- Spectroscopy, and resistivity and dielectric measurements. An analysis of the experimental data will clarify which parameters are really relevant and will allow us to describe the ablation process by a model. The results of this proposal may lead to the possibility for pulsed-laser deposition of optimized thin dielectric films from non-absorbing materials. We know from our industrial partners that such films would have a number of promising technological applications, for instance as electret layers. From a fundamental point of view the results could help for a better understanding of the basic interaction mechanisms of intense laser-light at surfaces. The results may also be applicable to other fields as the laser-cleaning of surfaces from inhomogeneous but non- absorbing contaminants.

The basis of the current project was the fact, that we were able to prepare high-quality thin films by laser deposition also from materials, which were weak absorbers at the wavelength of the laser-light, by employing pressed and sintered powder targets. One example is the production of thin Teflon films. The material transport here is mainly due to a strongly directed particle jet. The appearance of the particle jet depends thereby strongly on the microstructure of the target material. Extensive experimental series at various laser-wavelengths were performed. The results show an unexpected strong and complex influence of the wavelength onto the deposition process, which has never been reported before in literature. The ablation rates were very high in comparison to rates achieved with other materials. The optical constants were numerically calculated on the basis of a model for light-propagation in a turbid medium. In a numerical simulation we determined effective optical penetration depths, which was much larger than the measured ablation-rates. The ablation mechanism therefore cannot be explained on the basis of "simple" models for photothermal or photochemical ablation. We are also interested in technical applications of this method for production of thin functional coatings. At the moment we are engaged in the development of Teflon protection coatings for medical implants and Teflon layers on electret microphones.