Transparent conducting zinc oxide and indium–tin oxide films prepared by modified reactive planar magnetron sputtering

Abstract

Transparent conducting layers for liquid crystal displays and plastic substrates must be deposited without significant heating. A planar magnetron sputtering process which provides a suitable combination of resistivity and transparency on unheated substrates has been developed. Transparent conducting films of ZnO and indium tin oxide (ITO) have been deposited by sputtering Zn and In0.9Sn0.1 targets in a planar magnetron system modified to introduce oxygen near the substrate and to reduce the conductance for oxygen to the target. As the oxygen flow rate f2 increased, the dc voltage Va, required to maintain constant sputtering current, first increased and then decreased rapidly as the critical flow rate f2c, at which the target oxidizes, is approached. In the ZnO case, Va had a maximum value of 525 V and was 355 V after the target oxidized; transparent conducting films were obtained at 435 V but the best films were obtained by activating an rf discharge close to the substrate to increase the oxidation rate. These polycrystalline films had a transmittance of 80% and a resistivity of 0.03 Ω cm but their chemical stability was too poor for useful application. In the ITO case, the rf discharge was necessary to obtain transparent conducting films. Even at f2/f2c=0.94, films were opaque in the absence of this extra oxidation mechanism. Films with a transmittance of 79% and a resistivity of 1×10−4 Ω cm were obtained by coupling an rf power of 105 W to the substrate holder, with a substrate self-bias voltage less than 5 V, at f2/f2c=0.77. These films are amorphous and chemically stable. This sputtering process provides a maximum deposition rate of 48 nm/min on unheated substrates and is suitable for fabricating MIM-addressed liquid crystal displays.