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This paper reports on mass spectrometry analysis performed downstream a microwave discharge in Ar-N 2-H 2 gas mixture in nitriding conditions. Investigations are focused on the main simple radicals NH 2 , NH and N, and on the molecular species NH 3 and N 2 H 2 produced. Because of wall desorptions due to catalytic effects, we must develop a specific method taking into account both wall desorption and dissociative ionization effects, in order to correct the mass spectrometer signal intensity. The relative concentrations of the previous species are studied in various gas mixtures. Correlations are made between the plasma chemistry and plasma parameters (electron density and energy electron distribution function), measured by means of Langmuir probes spatially resolved within the plasma expansion. These results show the efficiency of ternary gas mixtures (Ar-N 2-H 2) to produce electrons and N x H y species used in plasma nitriding process. 1. introduction. Microwave discharge sustained in Ar-N 2-H 2 gas mixtures are used in a wide range of industrial applications. Like ammonia synthesis /1/, nitriding processes for surface treatment of metallic or polymer pieces /2,3,4/ and for the elaboration of optical or electronic devices /5/. The efficiency of such plasma processes depends on the knowledge of the various elementary mechanisms involved in the gas phase and at the surface of the reactor wall or of the work pieces. In previous papers /6, 7/, we have shown that the surface oxide reduction of thin molybdenum films, in the nitriding treatment, depends on the gas composition in the plasma expansion and on the distance between the work piece surface and the discharge centre. We confirm correlations between nitrided layers characteristics, discharge expansion

The mechanical and chemical properties of transition metal nitrides are very attractive for numerous industrial applications. Thin nitride films are expected to be good diffusion barrier in microelectronic devices. Nitrogen diffuses into the whole thickness of the molybdenum film heated at low temperature and exposed to expanding plasma of (Ar-N 2 -H 2 ) compared with pure N 2 plasma. NH x species in the plasma are produced by different homogeneous or heterogeneous reactive mechanisms that results in an expansion of the plasma compared with pure N 2 plasma. NH x species and probably H atoms improve the transfer of nitrogen into the metal by preventing the formation of MoO 2 oxides which act as a barrier of diffusion for nitrogen.