Phenomenology of a dual-mode microwave/RF discharge used for the deposition of silicon oxide thin layers

A remote plasma enhanced chemical vapour deposition (RPECVD) reactor has been developed to deposit silicon oxide films. It consists of a microwave discharge (created by surface waves) along a quartz tube and a capacitively coupled radiofrequency (RF) discharge on a planar electrode (substrate holder) perpendicular to the tube and facing the gas flow. Plasma diagnostics have been performed in Ar, O-2, Ar-O-2 and Ar-He-O-2 discharges, at two different positions of the reactor. The densities of electrons (a few 10(11) cm(-3)), argon metastables (a few 10(10) cm(-3)) and oxygen atoms (10(13)-10(14) cm(-3)) have been determined as a function of different plasma parameters, using microwave interferometry and optical emission spectroscopy (self-absorption and actinometry) respectively. In the case of Ar-M, the gas flow has little effect on the local equilibrium along the discharge tube due to fast quenching on the walls and quenching in the plasma bulk by slow electrons and oxygen molecules, In contrast, the O atom density profile is governed by the gas flow velocity due to a slow recombination probability on the walls. However it clearly appears that the O atom recombination probability is much larger in the microwave discharge region (due to ion bombardment and plasma heating of the walls) than in the afterglow region. We have also shown that the fraction of O atoms with respect to O-2 molecules is enhanced by using helium and/or argon dilution due to the production of O atoms by the quenching reactions of Ar-M or He-M metastables with O-2. Comparing the densities of electrons, argon metastables and oxygen atoms and their respective rate constants for the reaction with silane (SiH4), we have deduced that the plasma chemical kinetics leading to silicon oxide deposition can be summarized in a simple scheme: electrons dissociate O-2 into O atoms and produce Ar-M which subsequently react with O-2 to enhance the O atom density. In the flowing afterglow, SiH4 is almost entirely decomposed by O atoms, the direct electron impact dissociation being negligible except when applying an RF discharge in the substrate region.