Depending on the laser parameters, the underlying mechanisms of laser ablation are of different nature. The influence of the specific laser properties on the laser-material interaction and the resulting ablation is discussed in chapter 2. This work mainly discusses the structuring of semiconductors and insulators with focus on technologically important materials like silicon or silicon carbide where laser ablation is shown to be a promising process solution compared to conventional structuring methods.
The properties of these materials and the particular samples used in this work are discussed in chapter 3. By looking deeper into the ablation process it can be split up into an absorption process to generate free electrons and the consecutive heating of the electron gas by free carrier excitation. As mentioned before the wavelength has to be short enough to obtain efficient linear absorption. On the other hand, the heating of the electron gas scales with the square of the wavelength. Because of this λ2-dependence of the free carrier heating rate, IR-irradiation is much more efficient for the further energy deposition into the electron system. Based on these considerations a "two-color scheme" is discussed in this work, where for both processes the suitable wavelength is provided to enhance the overall ablation process. The possibility to generate harmonic wavelengths by simply inserting a nonlinear crystal into the beam allows two-color ablation with the fundamental wavelength and a fraction of the second and third harmonic, respectively. The ablation enhancement by "harmonics seeding" is discussed in chapter 7 with focus on semiconductor materials. Besides the wavelength, the pulse duration of the laser source is the main parameter influencing the nature of the laser-material coupling.
Pulse durations of down to a few femtoseconds are available from state of the art laser systems. The high peak intensities provided by ultrashort pulses initiate multiphoton absorption mechanisms, bridging a band gap much larger than the photon energy. Thus, by applying ultrashort pulses the ablation of nearly any material is possible by multiphoton absorption. In addition the ablation process happens on time scales so short that no significant thermal diffusion can take place.
Moreover, compared to pulse durations in the nanosecond time scale or longer, no radiation is scattered by ablated products. The non-thermal nature of ultrashort pulsed ablation together with the ultrafast mechanism offers well-defined ablation threshold fluences and enables laser structuring with high precision. This well-defined ablation threshold was demonstrated in literature to allow even sub-wavelength structuring if the laser fluence is controlled to be close to the ablation threshold. Chapter 6 of this work capitalizes on these ablation characteristics of ultrashort pulses to be able for selective material removal from a substrate. The use of ultrashort pulsed ablation for structuring thin film solar cells is demonstrated where the removal of minimized line widths at high process speed and without thermal damage is required. As prototypical systems, the ablation of Molybdenum and two different transparent conductive oxides (TCOs) on glass substrates were studied. Next to thin film structuring, the selective ablation scheme is a tool for 2-dimensional patterning. One possible application is the ablation of thick photoresist from substrates. In this case the substrate acts as an etch-stop layer. The structuring of thick photoresist is discussed, showing the potential of this mask- and developmentless method to build prototypes faster than with conventional lithography methods where a mask is needed for the pattern generation.