Analysis of surface-reaction layers formed by etching Si3N4 with pulsed-microwave plasma

Abstract

The mechanism of highly selective etching of Si3N4 by a pulsed-microwave electron-cyclotron-resonance plasma was investigated by analyzing surface-reaction layers formed by etching using a CH3F/O2/Ar gas chemistry. A hydrofluorocarbon (HFC) layer formed not only on nonpatterned materials (Si3N4 and others) but also at the bottoms of line-and-space patterns were analyzed by X-ray photoelectron spectroscopy. Thermal reactivity between the HFC layer and the Si3N4 layer was also investigated by thermal-desorption spectroscopy. The investigation results show that nitrogen contained in the Si3N4 layer thermally reacted with the HFC layer to form NH3 or HCN, and silicon contained in Si3N4 had high reactivity with fluorine contained in the HFC layer. Owing to the high reactivity between the fluorine-rich HFC layer and the Si3N4 layer in the pulsed-microwave plasma, the HFC layer became thin, even at a low wafer bias, and thus promoted ion-assisted etching. A wide process window was provided by the formation of the fluorine-rich thin HFC layer using the pulsed-microwave plasma.