Strukturierung von Zinkoxid-Dünnschichten mittels Zweistrahlanordnung unter Ausnutzung einer Excited-State-Absorption

The present work demonstrates the ns-laser pulse assisted excited state absoption (ESA)-based processing of Zinc(II)-oxide thin films with a layer thickness of 50 300 nm on crystallographically differently oriented quartz-, as well as crystalline sapphire-, and amophous fused silica substrates using a dual laser beam setup. The goal was to achieve an ablation efficiency improvement compared to single-beam ablation by decreasing the required pulse fluences. For the ESA-based ablation, the optical excitation with radiation of a laser with photon energies Ep above the optical band gap energy EBL was used to excite electrons into the conduction band (LB), which then partially relax into defect-levels. This allows the transient single-photon absorption of radiation of a second spatially and temporally aligned laser pulse with Ep < EBL, exciting electrons to the LB from these defect-levels. By utilizing different ESA-wavelengths in the visible spectrum with ESA = 410 600 nm, ESA = 450 nm was determined as the most effective wavelength for the ESA-based ablation due to the increased Ep, allowing the absorption from an increased number of suitable defect-levels. To quantify the efficiency improvements, a model was developed to allow for a comparison with single-beam ablation by determining the pulse fluence dependent size of the ablated structures. For a more detailed analysis of the ESA-based dual-beam process and to determine optimal processing parameters, a second model was developed to describe the temporal process and the impact of the temperature increase by the pump-pulse. In addition to ablation of variable ablation depths, the ablation quality was improved by decreasing resolidified molten structures and an ablation was achieved even with a temporal pulse delay in the s-range.