Influence of the Gas Phase on the Water Vapor Barrier Properties of SiO x Films Deposited from RF and Dual-Mode Plasmas

Abstract

The influence of different gas phase species and the positive ion density on the water vapor barrier performance of SiO x thin films is investigated in this study. A reactor equipped with a remote 2.45 GHz slot antenna plasma source and a 13.56 MHz-biased substrate holder was used to deposit films from gas mixtures of oxygen and HMDSO. Besides determining the WVTR of films, different excited gas phase species were monitored by means of OES and the positive ion density in pure oxygen was measured by LP. The dependencies of the positive ion density and of the actinometric trends of species on the oxygen flow rate as well as on the power input by RF and MW source were investigated. Both gas-phase diagnosis techniques reveal a higher excitation of the gas-phase in dual mode than in pure RF plasma. Production of atomic species seems to be enhanced under such conditions, which however leads to enhanced oxidation of HMDSO and its fragments by atomic oxygen. Hence, at oxygen flow rates above 200 sccm, enough atomic oxygen seems to be obtained so that the film growth mechanism is dominated by homogeneous combustion reactions, overriding the beneficial effect of the substrate self bias. Thus, a complete loss of the water vapor diffusion barrier is observed. Apart from these defective deposits, typical films revealed low WTVR for the entire variation range in RF mode and below 200 sccm of oxygen flow also in dual mode. Here, a decrease of the WVTR with RF power (i.e. DC self bias) and the oxygen flow rate was obtained, while WVTR could be reduced to 0.1 g · m -2 · d -1 corresponding to a barrier improvement by a factor of 150 if compared to uncoated substrate polymers.