Fs-laser micro machining for μ-TLM resistivity test structures in photovoltaic TCO multilayers

In this work we developed a new approach for resistivity measurements based on fs laser micro machining of μ-TLM test structures. This method is applied to highly resistive interfacial and conductive bulk multilayer systems in photovoltaic TCO thin film devices. Resistivity data has been acquired by a new TLM based method at μm-dimensions (μ-TLM, patent pending, DE 102014211352.0). For this approach, isolating trenches are prepared in the µm range with reasonable effort using fs laser processing. The application of ultrashort pulses with a laser wavelength in the IR range (λ = 1.03 μm) allows selective removal of the top SnO 2 layer of the TCO multilayer stack by a reduced thermal influence on the layers beneath and in the adjacent region of the laser trenches. Small effective optical penetration and ablation depth was achieved by an ultrafast thermal ablation mechanism via free carrier absorption at the interface of the SnO 2 /ITO layers. Therefore the risk of laser induced modification of the electric layer properties is negligible. The μ-TLM test structure results in highly accurate and reproducible resistivity data. Applied to SnO 2 /ITO/glass double layer stacks, the obtained resistivity values for the SnO 2 interfacial layer (ρTO = 40.5 kΩμm) and for the indium tin oxide thin film (ρITO = 1.3 Ωμm) agree with reference data from four-point-probing and from literature.