Variations in composition and structure occurring in multicomponent alloy targets during their service term

Abstract. Main part of the magnetron sputtering (MS) technology is a target that serves as a source for film/coating material. The target properties directly influence the properties of a film/coating deposited by MS. The target composition and microstructure may change during its exploitation, especially if the target has complex composition, e.g., multicomponent alloy. However, only a small part of the target in the MS serves as a source of film-forming particles. This is an erosion zone, an annular groove, which forms over time under the ion bombardment (IB) of the target. Usually, the groove area is about 25% of the target square while the rest square remains unchanged. As the groove develops, the composition and angle of expansion of the film-forming particles change, that affects the composition and thickness uniformity of the film. Therefore, the information of the composition and microstructure of the groove at the end of the target's service term allows predicting the possible changes in the properties of the film. In view of the told above, we used SEM and microanalysis to study the structure and composition of the grooves of CoCrCuFeNi, CoCrCuFeMnNi, AlCoCrCuFeNiV high entropy alloy targets at the beginning and at the end of their service. Initial microstructure of the surface of all targets was smooth and flat and composed of matrix and inclusions homogenously distributed in the matrix. The microrelief of the bottom and side walls of the grooves was different and depended on the intensity of the IB. In the area of strong IB the microrelief acquired features of a melted surface. In the area of moderate IB the microrelief was crossed in various directions by micro-trenches of various length and width. It was revealed that the shapes and distribution of these micro-trenches coincided with those of the inclusions observed on the original targets surfaces. It was also revealed that at the end of the service term the composition of the grooves became depleted with elements with maximal enthalpy of formation. Analysis of the obtained results led us to the following conclusions: (i) the signs of melting on the targets surfaces indicate that its temperature is close to the melting temperatures of the alloys; (ii) a new type of the effect of preferential sputtering has been established. The main condition for its existence is the presence in the target composition of the second phase in the form of microinclusions, which sputtering yield is noticeably higher than of the components of the first phase; (iii) the formation of specific microrelief on the targets surfaces upon IB with ions with energy typical for DC MS is a result of the combined action of two factors: high temperature and sputtering. Key words: DC MS, target, high entropy alloys, composition, structure