Reactive magnetron sputtering of tin-doped indium oxide (ITO): influence of argon pressure and plasma excitation mode

Tin-doped indium oxide (ITO) films have been prepared by reactive magnetron sputtering from it metallic alloy target onto unheated substrates. In a systematic study, the influence of the working pressure and the discharge mode (DC, Pulsed DC or RF) on the electrical and structural properties of ITO films was investigated. It was found that the RF excitation (13.56 MHz) of a magnetron discharge is the most effective one with respect to the usage of the oxygen for oxidising the sputtered metal atoms on the substrate surface. When varying the working pressure (0.3-3.4 Pa), the oxygen partial pressure for the resistivity minimum of the ITO films is independent of the working pressure for RF excitation. Exciting the plasma by DC, an increasing oxygen amount is necessary in order to prepare low resistant ITO films when increasing the working pressures. This behaviour can be explained by different sources for the input of energetic particles into the growing film for RF and DC excitation. In a DC discharge, most of the energetic particles stem from the target surface (e.g. reflected neutral argon atoms or negative oxygen ions), while in a RF magnetron discharge, the largest part of the energetic particles has its origin in the plasma in front of the substrate. By Rutherford backscattering analysis the chemical composition of the ITO films as a function of the oxygen partial pressure was investigated. Fully oxidised ITO films exhibit an oxygen excess of approximately 15%. Low resistant ITO films show an oxygen-to-metal ratio of approximately 1.5, suggesting a film structure where the tin atoms occupy indium lattice sites. From the XRD analysis, a compressive strain in the ITO films of up to 2% was measured, which seems to be caused by the excess oxygen in the films. Furthermore, the ITO films exhibit a density which is up to Mlo lower than that of bulk indium oxide. (C) 2001 Elsevier Science BN. All rights reserved.