In situ ellipsometry and infrared analysis of PECVD SiO 2 films deposited in an O 2/TEOS helicon reactor

Abstract

Silicon dioxide thin films have been deposited at room temperature on silicon substrates in oxygen/tetraethoxysilane (O 2/TEOS) helicon diffusion plasmas at low pressure (5 mTorr) and 300 W rf power. The properties of the films have been measured by in and ex situ ellipsometry, ex situ infrared spectroscopy, and chemical etching (p-etch) as a function of the TEOS flow rate ( Q TEOS). The growth rate ( V d) is determined in situ using an ultra violet-visible phase modulated spectroscopic ellipsometer (1.5 to 5 eV). Two different kinetic regimes appear: at low TEOS flow rate ( Q TEOS < 5 sccm) V d increases linearly and no carbon species are detected while the OH content rises strongly. For higher values of Q TEOS, V d saturates at ≈ 11 nm. The change in the kinetics corresponds to the appearance of carbon impurities. The increase in the deposition rate is accompanied by a decrease in the refractive index and an increase in the p-etch rate. The Bruggeman effective medium approximation (BEMA) is used to determine the fraction of voids incorporated in the layer. It is shown that porous films incorporate water when exposed to the atmosphere. Based on this result, an explanation is proposed for the insensitivity of the stretching peak of SiOSi to the deposition conditions. Good quality SiO 2 films with optical properties close to that of a thermal oxide can be obtained at low deposition rates ( V d < 5 nm/min).