Processing Oxide Thin Films with Light: a Path to High-Throughput Solution-Processed Oxide Electronics
Daunis, Trey Benjamin
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High-throughput roll-to-roll manufacturing of solution-deposited metal oxide electronics has the potential to enable the widespread and low-cost availability of thin film electronic devices including solar panels, large-area displays and lighting, and flexible medical devices and sensors. However, several challenges have prevented the realization of this potential: (1) The processing speed of solution-deposited oxides is severely limited by the length of the annealing process, usually greater than 20 minutes, which is typically necessary to convert solution-deposited precursor films into metal oxides, (2) the quality of the resultant metal oxide films is limited by the low upper operating temperatures of low-cost, roll-to-roll compatible, plastic substrates, typically much less than 250 °C, and (3) equilibrium heating of devices to these temperatures often results in mechanical failure of the thin films due to the different thermal responses of the films and substrates. In this dissertation, we address these challenges by using light, rather than heat, as a source of energy for curing solution-deposited metal oxide thin films. We use this method to develop solution-deposited oxide thin film transistors (TFTs) on shape memory polymer (SMP) substrates. We demonstrate the direct patterning of Al₂O₃ and In₂O₃ precursor films on SMP by exposure to UV light through a shadow mask as a method to overcome the mechanical failure of blanket-coverage films during thermal annealing. The patterned precursors are then converted to oxides for the gate dielectric and the channel semiconductor of the TFTs by thermal annealing without causing damage to the films. The unexpectedly high mobility displayed by these TFTs is studied and the absorption of water from the atmosphere by the gate dielectric is identified as the cause. Finally, we demonstrate the high-speed photonic curing of ZrO₂ dielectric films on polyethylene naphthalate substrates. Using intense pulsed light to both heat and cure the oxide film in as little as 100 milliseconds without significantly heating the substrate, we achieve a process for fabricating metal oxide electronic devices that is compatible with roll-to-roll processing speeds exceeding 30 m/min.