Diagnostics in O-2 helicon plasmas for SiO2 deposition

O-2 and O-2/TEOS helicon plasmas used for plasma enhanced chemical vapour deposition of SiO2 films are investigated in the 1-10 mTorr pressure and 0-800 W rf power ranges. The positive oxygen ions are analysed by energy selective mass spectrometry and Langmuir probes. The oxygen atom concentration is monitored by actinometry and ionization threshold mass spectrometry. In oxygen plasmas it is shown that O-2(+) is the major positive ion, and that the oxygen molecules are far from being completely dissociated, due to a very high oxygen atom recombination frequency on the reactor wails. The dissociation degree increases with the rf power reaching 10% at 500 W. In O-2/TEOS plasmas, the plasma density and electron temperature decrease as the TEOS fraction increases. In contrast, the degree of oxygen dissociation increases sharply with the addition of a few per cent TEOS, is maximum for about 5% TEOS and decreases as TEOS fraction is further increased. In a 95:5 O-2/TEOS plasma (5 mTorr, 300 W) the fluxes of oxygen positive ions and atoms impinging onto a floating substrate are estimated to be 4 x 10(15) cm(-2) and 10(18) cm(-2) s(-1) respectively. Under these plasma conditions, near-stoichiometric SiO2 films, with low OH content, are deposited at ambient temperature. The corresponding atom to ion flux ratio is about 250, which suggests the dominant role of oxygen atoms in the deposition kinetics. The comparison of the compositions of layers grown in a 5 mTorr 95:5 O-2/TEOS plasma at two rf powers confirms the major role of oxygen atoms.