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

International audienceThis 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 expandingmicrowaveplasma reactor. Compared with the numerous data found in literature, the formation of βMo2N and γMo2Nphases strongly oriented in the [204] and [220] crystalline directions, respectively is unexpected. The βMo2Nphase is synthesized in molybdenum films exposed to pure N2 plasma for 1 h whereas the molybdenumfilms exposed to (Ar–33%N2–1%H2) plasma for only 30 min consists almost entirely of γMo2N phase. TheMo2N crystallites about 30 nm wide display a columnar morphology in the growth direction after processingin (Ar–33%N2–1%H2) plasma. In contrastwithmost processeswhere energetic ions improve the nitrogen transferinto the surface layers of the metal film, the expanding plasma process promotes the chemical reactions on thesurface of the material. The remaining oxides are reduced by hydrogen species such as NHx radicals producedin (Ar–N2–H2) plasma. The reactions are exothermic and could involve the formation of intermediates such ashydrogen molybdenum bronze HxMoO3 that would promote the crystallization of the high temperatureγMo2N phase to the detriment of the low temperature βMo2N phase during the reduction–nitridation process