Optical EngineeringTransparent indium tin oxide films prepared by ion-assisted deposition with a single-layer overcoat
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Transparent indium tin oxide (ITO) films have many applications as electrodes in Liquid Crystal displays and EMI/RFI shielding. We have compared two processes for obtaining ITO thin films with low resistivity (below 15B/sq) and high transmittance (above 90%) in the visible region. The first process was deposition using electron beam gun evaporation. The second process was using Ion Assisted Deposition (lAD) of low energy oxygen ions from an end-Hall ion source. In both cases the conductive layer was overcoated with 50-250 nm thickness of a MgF2 layer to enhance the transmittance. When ITO produced by IAD was overcoated with a MgF2 layer, the surface was found to be conductive. The surface was analyzed using the Auger Electron and Second Ion Mass Spectroscopy methods. Traces of indium were found on the MgF2 layer. We speculate that indium that diffused through the overcoat layer gives assistance to the conductivity and enables the measurement of the resistivity from the top surface of the coating. This effect occurred only when the ITO was deposited using IAD. This is an advantage in manufacturing of optical conductive windows. Otherwise there is a need to leave exposed areas of ITO for conductive contacts. The diffusion process was not observed with oxide layers. Atomic Force Microscopy scans show that the IAD process decreased the average roughness of the surface.