Investigation on optical and physico-chemical properties of LPCVD SiO_xN_y thin films

¹ LEMEAMED Laboratory, Electronics Department, Constantine 1 University, 25000 Constantine, Algeria
² CNRS, LAAS, 7 Avenue du Colonel Roche, 31400 Toulouse, France
³ Université de Toulouse, UPS, LAAS, 31400 Toulouse, France

^a e-mail: b.kaghouche@gmail.com

Received: 3 December 2013
Revised: 23 April 2014
Accepted: 30 April 2014
Published online: 5 June 2014

Abstract

In this work, the correlation between BEMA model and FTIR analysis was employed to investigate the chemical composition of silicon oxynitride (SiO_xN_y) films deposited by low pressure chemical vapour deposition (LPCVD) technique at temperature of 850 °C from nitrous oxide N₂O, ammonia NH₃ and dichlorosilane SiH₂Cl₂. Different stoichiometries were obtained for different values of relative gas flow ratio NH₃/N₂O while keeping the SiH₂Cl₂ flow constant. The optical properties were studied using spectroscopic ellipsometry. Apart from films thickness, their refractive index as well as their SiO₂ and Si₃N₄ volume fractions were deduced using the Bruggeman effective medium approximation (BEMA) model. It was found that the refractive index increases from 1.45 to 1.60 with increasing nitrogen incorporation. The Fourier Transform Infrared spectroscopy was carried out to study the evolution of chemical bonding of LPCVD SiO_xN_y films. In order to improve the qualitative analysis of their Si-N and Si-O vibrational modes, a correlation between Fourier transform infrared and spectroscopic ellipsometry measurements was established. A shift of infrared peaks position with the increase of relative gas flow ratio is observed. Furthermore, the calculated areas of absorption bands were used to estimate the SiO_xN_y stoichiometry. This quantitative analysis was proved with an adequate method in the literature. We found a decrease of x values from 1.94 to 1.26 and an increase of y from 0.04 to 0.49, when the NH₃/N₂O gas flow ratio increases. This behavior was explained by the increase of nitrogen content as well as the decrease of oxygen content in the SiO_xN_y film.