A nitriding process of very thin molybdenum films in an expanding microwave plasma at low temperature

Abstract

A nitriding process of very thin molybdenum films in an expanding microwave plasma at low temperature Abstract. A transfer of nitrogen of about 20 at % is detected in very thin molybdenum films of about 200 nm thick coated on Si (100) wafers heated at 673 K and exposed to ternary (Ar-N 2 -H 2 ) plasma. The nitrogen diffusion goes with a noticeable decrease of the remaining oxide layers in the whole film thickness. On the contrary, pure N 2 gas exposure leads to a slight diffusion of nitrogen into the first molybdenum layers up to a depth of about 40 nm, only and an enhancement of oxygen amount also. Hydrogen species contained in the plasma reduce the oxide layers which act as nitrogen diffusion barrier. The nitrogen diffusion decrease with increasing distance of the workpiece surface from the centre of the discharge as well as the occurrence of nitrogen diffusion in molybdenum layers at room temperature highlight the role of NH x<3 active species of the plasma on the reactivity of the surface. The morphology of the nitrided surface consists of slightly smaller grains compared with those corresponding to untreated molybdenum films ranging in size from 30 to 50 nm wide. In contrast to substrates heated at 873 K, the tetragonal Mo 2 N structure has not been detected in molybdenum films heated at 673 K. IntroductionIn addition to attractive physical and chemical properties of transition metal nitrides such as high hardness, melting point and electrical conductivity, good chemical stability, a growing scientific and technological interest is focused on their catalytic properties. Mo 2 N and W 2 N compounds are catalytically active for NO reduction in presence of hydrogen. The NO conversion on Mo 2 N is equal to 89 % at 723 K and remains constant for 10 h. Besides, the catalytic properties of Mo 2 N can be compared to those of noble metals for hydroprocessing, H 2 -D 2 exchange reactions and so on… [1][2]. Moreover, since metal nitrides exhibit a good corrosion resistance, a very thin layer forms a good diffusion barrier preventing the metal film from oxidizing [3]. A thermochemical process including expanding plasma activated by microwave discharge has been successfully developed for metal nitriding [4,5]. The method consists of producing gaseous species and carrying them out along the discharge up to the workpiece surface. Since the density of electrons in the discharge is strongly reduced by inelastic collisions occurring between electrons and molecules, high dilutions of molecular gas in Ar gas are realized. So, this method allows selecting active gaseous species such as NH x<3 simple radicals since, they are mainly detected in (Ar-N 2 ) gas mixtures with low N 2 as H 2 is injected within the discharge [6]. The purpose of this work is to carry out measurements on the transfer of nitrogen into very thin molybdenum layers heated at low temperature and exposed to various (Ar-N 2 -