Study of optical properties of MoxC1−x films

Metallic optical mirrors are foreseen to be exploited in plasma diagnostic systems in ITER. They will suffer from deposition of impurities like carbon causing marked changes of the reflectivity. In this paper, we have focused on the co-deposition of MoxC1−x films (0 ⩽x ⩽ 0.61) and have studied the influence of the deposited thin film composition on the optical properties. Deposits were performed by magnetron co-sputtering with argon as process gas in a high vacuum system. In-situ XPS measurements, real time reflectometry, ex-situ spectrophotometry and spectroscopic ellipsometry were used to characterize the resulting layers. Carbide formation was evidenced. As expected, optical constants of the films and reflectivity were found to depend strongly on the film composition. Simulations of the impact of thin film composition on the reflectivity of a molybdenum mirror show how a thin deposited layer can drastically degrade the reflectivity.