Optical spectroscopic analyses of OH incorporation into SiO2 films deposited from O2/tetraethoxysilane plasmas

Abstract

Silicon dioxide thin films are deposited on (100) silicon substrates at low pressure (5 mTorr), from O2/tetraethoxysilane (TEOS) helicon plasmas. The reactor is operated at 300 W radio frequency power without any intentional heating or biasing of the substrate. The samples are characterized using infrared spectroscopy, ultraviolet-visible ellipsometry, and complementary density measurements. Changes in film properties are observed varying the TEOS fraction in the gas mixture. Good quality SiO2 films, insensitive to postdeposition exposure to atmospheric water, are deposited for low TEOS fractions (<5%) in the mixture. As the O2 flow rate decreases, porous SiO2 and polymeric SiOxCyHz samples are successively obtained. Aging over 5 months of intentionally produced porous films has been investigated using Fourier-transform infrared spectroscopy. The 2900–3800 cm−1 OH absorption stretching band is quantitatively analyzed with three deconvolution bands. These films are hygroscopic and they show changes in the infrared spectra indicating an incorporation of additional highly associated hydroxyl groups. In addition, the development of the 935 cm−1 Si–OH stretching band and the evolution of the Si–O–Si stretching peak are due to interactions between the airborne absorbed water and silica network. On the other hand, isolated silanol species are rather insensitive to the postdeposition exposure to the atmospheric water. The respective contribution of growth induced and after growth ex situ incorporated Si–OH groups is established. Using the Bruggeman effective medium approximation, we found that water molecules account well for the dielectric properties of these highly associated SiOH groups mainly originating from postdeposition silica hydrolysis.