Mechanisms for compositional variations of coatings sputtered from a WS2 target

Abstract

Magnetron sputtering from compound targets is widely used for the deposition of compound films since it is easy to scale up and it exhibits a high reproducibility. Controlling film stoichiometry is crucial for obtaining films with desired properties. However, the process is rather complex and sputtering from a compound target frequently results in film compositions that deviate significantly from that of the target. This is due to a number of effects related to the nature of the sputtering process which include preferential re-sputtering due to energetic particle bombardment at the substrate, different take-off angles, scattering in the gas phase, and different sticking coefficients.In this work, we have investigated how sputtering from a WS2 target results in different film compositions as a function of the position in the chamber, for different processing conditions. Hence, the films have not been characterized with respect to structural or morphological properties. A Monte-Carlo based software, accounting for different take-off angles and scattering in the gas phase, was developed to simulate the compositional variations at various positions in the chamber. Further, a number of experiments were performed by varying the target voltage, process pressure, as well as the location of the substrate (on and off axis). Simulations and experiments reveal significant compositional variations for different processing conditions. Experiments show that these variations are only slightly affected by the target voltage, while the most significant variations result from the processing pressure and position on the chamber. From the qualitatively good agreement between experiments and simulations it is clear that gas phase scattering must be taken into account to explain the observed compositional trends, while the other effects are less important and sticking coefficients effects may even be negligible. It is therefore concluded that the major effect responsible for the compositional variation of the film is the different scattering behaviour of S and W in the gas phase.