Investigations on a nitriding process of molybdenum thin films exposed to (Ar–N 2–H 2) expanding microwave plasma

Abstract

Nitriding treatments using various (Ar–N 2–H 2) gas mixtures are performed on thin molybdenum films coated on Si (100) substrates, in an expanding microwave plasma reactor. The electron density measured in (Ar–25%N 2–30%H 2) and (Ar–8%N 2–10%H 2) plasma as well as the electron energy distribution function (EEDF) vary with the distance from the centre of the discharge and are large compared to the corresponding parameters found in (Ar–30%N 2–12%H 2) gas mixture and pure N 2 gas. In such ternary gas mixtures, the mean electron energy ranges between 0.5 and 0.7 eV. A nitride compound of tetragonal-like structure is formed in the whole film thickness of molybdenum heated at 873 K and exposed to (Ar–25%N 2–30%H 2) and (Ar–8%N 2–10%H 2) gas mixtures for exposure times of 20 min duration. It consists of small columnar grains ranging in size from 20 to 30 nm in the growth direction. The nitrogen diffusion profiles are fitted to the Fick's second law by introducing a diffusion coefficient ranging between 5 × 10 − 10 and 5 × 10 − 9 cm 2s − 1 within the accuracy of the method. The diffusion coefficient found from previous measured pressure transients is equal to about 10 − 10 cm 2s − 1 . The plasma composition as well as the distance from the centre of the discharge seems to play a role on the reduction of oxides, only. An (Ar–25%N 2–30%H 2) nitriding treatment carried out at 8 cm from the centre of the discharge leads to oxide amount twice lower than the one measured after an (Ar–8%N 2–10%H 2) nitriding treatment carried out at 12 cm from the centre of the discharge. Because of the low ion energies, the reduction of oxides is probably mainly confined to hydrogen species.