The feasibility of using Cu(Mg) alloy film as a gate electrode for thin-film transistor (TFT) liquid crystal displays has been investigated. When pure Cu was used as a gate electrode, severe interdiffusion occurred between Cu and the gases SiH4, NH3, and CF4 during plasma-enhanced chemical vapor deposition of a gate dielectric, SiNx, and dry etching of the SiNx. On the other hand, the deposition of a Cu(Mg) alloy film gives rise to the formation of a MgO/Cu bilayer structure with low Cu resistivity, good adhesion to SiO2, higher leakage current density, and excellent passivation capability. A hydrogenated amorphous silicon TFT with a MgO encapsulated Cu gate exhibited a gate voltage swing of 0.91 V/dec. and a threshold voltage of 6.8 V, resulting in a reduction of process steps and better performance.
Diffusion barrier characteristics and electrical properties of self-aligned MgO layers obtained from a Cu(Mg) alloy film have been investigated. Self-aligned surface and interfacial MgO layers were formed upon annealing a Cu(Mg) film in an oxygen ambient and prevented interdiffusion of Cu in SiO2 up to 700 °C. The thermal stability of a pure Cu/TiN/Si multilayer system has been significantly enhanced up to 800 °C by the MgO layers by forming a MgO/Cu/MgO/TiN/Si multilayer system. A combined structure of Si3N4(500 Å)/MgO(100 Å) increased the breakdown voltage up to 20 V from 15 V and reduced the leakage current density down to 3×10−9 A/cm2 from 1×10−8 A/cm2 compared to a pure copper system. Consequently, the deposition of Cu(Mg) alloy followed by annealing in an oxygen ambient gives rise to the formation of a self-aligned MgO layer with excellent diffusion barrier and electrical characteristics and the film can be applied as a gate electrode in thin-film transistor/liquid-crystal displays, resulting in a reduction of process steps.
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