Plasma material surface investigation in a nitriding process of thin molybdenum films using an expanding (Ar-NH2-H2) plasma
Résumé
In a thermochemical treatment of metal films exposed to expanding plasma of various (Ar-N2-H2) gas mixtures activated by microwave discharge of 2.45 GHz, reactive plasma and surface processes occur. Plasma species as H2, N, NH, NH2, NH3, N2, N2H2 and Ar are detected downstream the discharge with a mass spectrometer. (Ar-30%N2-12%H2) and (Ar-25%N2-30%H2) produce a larger amount of NH3 radicals than (Ar-8%N2-10%H2) plasma. However, since reactive species are largely diluted in (Ar- 8%N2-10%H2) plasma, the effect of dilution must be considered. The nitriding treatment is carried out on very thin molybdenum films about 200 nm thick coated on Si(001) substrates by e-beam evaporation and heated at a temperature of 400°C. A large transfer of nitrogen from the film surface up to the Mo-Si interface is seen by secondary neutral mass spectrometry in molybdenum films exposed to (Ar-25%N2- 30%H2) whereas pure N2 and (Ar-1%N2) plasma exposures lead to slight nitrogen diffusion. The role of hydrogen gas in the plasma as a reduced agent is confirmed by the strong decrease of oxygen amount in the film. In contrast to nitriding treatments carried out at 600°C, the tetragonal Mo2N phase is not detected by X-ray diffraction. The morphology of the surface investigated by atomic force microscopy consists of small grains ranging in size from 30 to 50 nm wide.