Stress-field in Sputtered Mo Thin Films and Mo/Ni Superlattices: Origin and Evolution after Ion-Irradiation
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Stress-field in Sputtered Mo Thin Films and Mo/Ni Superlattices: Origin and Evolution after Ion-Irradiation A. Debelle, G. Abadias, A. Michel, C. Jaouen, Ph. Guérin and M. Drouet Laboratoire de Métallurgie Physique, UMR 6630, Université de Poitiers, SP2MI, Téléport 2, 86962 Chasseneuil-Futuroscope, FRANCE ABSTRACT In the present study the ion irradiation technique is used to investigate the origin of the stressfield in Mo layers grown by ion beam sputtering. Strain measurements were performed by X- ray Diffraction (XRD) using the sin2ψ method. The evolution of the sin2ψ plots with ion irradiation shows that the usual assumption of a biaxial stress state is not adequate to determine the true stress-free lattice parameter a0 of the film. A new stress model based on a triaxial state of stress, which includes a hydrostatic component linked to point defects induced volume distortions, is derived to interpret the XRD results. For pure Mo films, the obtained a0 parameter is close to the bulk value, while for Mo sublayers in Mo/Ni superlattices, the a0 value is lower due to intermixing between Ni and Mo. These results demonstrate that ion irradiation is a powerful tool for stress relaxation, which allows to obtain additional information on the respective contribution of chemical effects and growth defects to the a0 value. INTRODUCTION Stress generation and evolution during thin film growth has attracted a widespread interest over the two last decades because the stress can significantly affect the optical, electronic and magnetic properties of the film. Consequently, the need to understand the origin of film stress and control it is vital to the integrity and performance of films. X-ray Diffraction (XRD) as well as wafer curvature methods are generally used to measure the residual stress in a large number of systems. Large compressive stresses are usually reported in sputtered thin films [1], which can lead to undesirable effects such as buckling or interfacial delamination. Although extensive experimental studies have been dedicated to investigate the influence of deposition parameters on the residual stresses in a wide variety of sputtered thin films[2-5], a complete description and a comprehensive picture relating the stress-field to the microscopic origin of compressive stresses is still lacking. Indeed, a usual and standard assumption of a biaxial stress field is made for describing the true stress state in thin films, which is in contradiction with the general agreement that intrinsic compressive stress is caused by point defect-induced volume distortion, known as the atomic peening process [6]. The energetic species (sputtered atoms or backscattered neutrals) cause the atoms of the growing film to be incorporated into spaces smaller than the usual atomic volume. Hereby the film expands outwards from the substrate and, in the plane of film, since the film adheres to the substrate, compressive stresses are generated. Recently, Kamminga et al. [7] have proposed a stress model which includes a hydrostatic stress
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