In this work, we applied spatially shaped ultra-short pulse laser micro-machining for a new processing approach of $\mu$-TLM test structures. These structures are used for resistivity measurements of multilayer systems with highly resistive interface layers, e.g. in TCO top contacts or oxide passivation layers for solar cells.For precise measurements of sheet resistances of the individual layers as well as contact resistivities of the respective interfaces, isolating trenches and homogenous ablation areas are required that can be fabricated by matching of pulse overlapping based on rectangular laser spots in $\mu$m-dimensions.
Ultrashort pulses with 10 ps and 200 fs pulse duration of different laser wavelength (532 nm, 1030 nm) as well as optical beam shaping elements for a redistribution of the intensity from gauss to top-hat profiles enables a selective removal of top metallic Ag and oxide layers on the multilayer stack. The method is applied to metal/oxide/silicon stacks as model system, in which the metal/oxide junction has negligible contact resistivity and the electrical behavior is determined by the oxide/Si interface. The thermal damage (HAZ) is minimized in underlying material and in adjacent region of the laser trenches. Small effective optical penetration and sub $\mu$m-adjustable ablation depth were achieved by an ultrafast “non-thermal” ablation mechanism via absorption at the several interfaces of multilayer systems. Morphology and microstructure of heat affected zones (HAZ) at the laser structures are characterized by scanning electron microscopy (SEM). Furthermore, electrical properties of the multilayers obtained from systematic processing variations in the $\mu$-TLM patterns are presented. Laser induced local modification of the material structure (e.g. crystallinity, residuals) are discussed with regard to measured resistivity values in order to optimize laser processing recipes for enhanced ablation selectivity and lateral confinement.