A reduction–nitridation process of molybdenum films in expanding microwave plasma: Crystal structure of molybdenum nitrides

Abstract

This work reports the structure of molybdenum nitride phases crystallizing in molybdenum films coated on Si (100) substrates, and exposed to (Ar–N2–H2) gas mixtures with various gas contents in an expanding microwave plasma reactor. Compared with the numerous data found in literature, the formation of βMo2N and γMo2N phases strongly oriented in the [204] and [220] crystalline directions, respectively is unexpected. The βMo2N phase is synthesized in molybdenum films exposed to pure N2 plasma for 1h whereas the molybdenum films exposed to (Ar–33%N2–1%H2) plasma for only 30min consists almost entirely of γMo2N phase. The Mo2N crystallites about 30nm wide display a columnar morphology in the growth direction after processing in (Ar–33%N2–1%H2) plasma. In contrast with most processes where energetic ions improve the nitrogen transfer into the surface layers of the metal film, the expanding plasma process promotes the chemical reactions on the surface of the material. The remaining oxides are reduced by hydrogen species such as NHx radicals produced in (Ar–N2–H2) plasma. The reactions are exothermic and could involve the formation of intermediates such as hydrogen molybdenum bronze HxMoO3 that would promote the crystallization of the high temperature γMo2N phase to the detriment of the low temperature βMo2N phase during the reduction–nitridation process.