Pulsed laser material interaction with thin indium tin oxide films
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The interaction of short and ultrashort laser pulses with very thin films is currently an important area of research for laser material interactions. The interaction between the laser pulse and the thin transparent conductive film is key to the fabrication of touch panel sensors, light emitting devices, and photovoltaics. This thesis presents results obtained from an investigation of short and ultra-short pulses with very thin transparent conducting oxide films (TCOs). The experiments were all performed on indium tin oxide (ITO), which remains of critical importance to the many applications of TCOs in the industrial sector. The selective patterning processes was characterised for nanosecond, picosecond and femtosecond laser pulses, using off-line techniques such as atomic force microscopy and real time plasma imaging. The results were analysed by the development of multi-physics simulations based on finite element modelling techniques. The film removal process was found to be dependent on laser wavelength, laser pulse duration, substrate material, and other beam delivery properties such as numerical aperture, laser spot radius, and laser shot number. In nanosecond laser patterning, selective removal of the ITO film on glass substrates, for pulses with photon energies less than the ITO direct transition bandgap, is found to be non-ablative, principally driven by thermal melt flow. Pulses with photon energies greater than the bandgap result in ablation by vaporisation. The laser spot overlap is an important parameter for selective patterning; high shots per area result in film removal with smoother profiles and less glass damage. Also for nanosecond processing, the size of the laser spot was found to affect the threshold fluence; this is interpreted using an electronic localisation and diffusion model in the thesis. In ultrashort laser patterning of ITO on glass substrates, the film removal process was found to be highly dependent on the applied laser fluence for picosecond and femtosecond laser pulses. The development and application of a two temperature simulation model, predicts that film ablation is principally driven by ultrafast lattice deformation. This is induced by the hot electron blast force, which leads to fracture of the ITO film at the film grain boundaries. Film removal of ITO on flexible Polyethylene terephthalate (PET) polymer substrates, was found to be through thermo-elastic thin film delamination. The delamination process is found to be principally generated by the thermal expansion of the PET substrate during laser heating. This release mechanism was found to be independent of the wavelength and pulse duration studied. The selective patterning process is characterised by the ability to create a clean removal process using low numbers of shots per unit area. Finally, the study examined the dependence of laser processing with a spatial beam profile using custom fabricated diffractive optics. A novel two-step fabrication method was used to generate high efficiency reflective diffractive optics. The spatially shaped, top-hat, laser beam profile was used to selectively remove the ITO thin film, which resulted in optimal processing, consisting of minimised re-solidified crater edges for nanosecond pulses. In summary, the thesis provides a concise in-depth study highlighting the opportunities for selectively patterning highly degenerate semiconductors used as TCOs in current and future devices.
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