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 h as been developed to deposit silicon oxide films. It consists of a mic rowave discharge (created by surface waves) along a quartz tube and a capacitively coupled radiofrequency (RF) discharge on a planar electro de (substrate holder) perpendicular to the tube and facing the gas flo w. 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 densit ies 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 interfe rometry and optical emission spectroscopy (self-absorption and actinom etry) respectively. In the case of Ar-M, the gas flow has little effec t on the local equilibrium along the discharge tube due to fast quench ing on the walls and quenching in the plasma bulk by slow electrons an d oxygen molecules, In contrast, the O atom density profile is governe d by the gas flow velocity due to a slow recombination probability on the walls. However it clearly appears that the O atom recombination pr obability is much larger in the microwave discharge region (due to ion bombardment and plasma heating of the walls) than in the afterglow re gion. 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 meta stables and oxygen atoms and their respective rate constants for the r eaction with silane (SiH4), we have deduced that the plasma chemical k inetics leading to silicon oxide deposition can be summarized in a sim ple scheme: electrons dissociate O-2 into O atoms and produce Ar-M whi ch subsequently react with O-2 to enhance the O atom density. In the f lowing afterglow, SiH4 is almost entirely decomposed by O atoms, the d irect electron impact dissociation being negligible except when applyi ng an RF discharge in the substrate region.