Electrical Resistivity of Multilayers During Ion Beam Mixing
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ELECTRICAL RESISTIVITY OF MULTILAYERS
DURING ION BEAM MIXING
J. GRILHE, J.P. RIVIERE, J. DELAFOND, C. JAOUEN, C. TEMPLIER Laboratoire de Mdtallurgie Physique L.A. 131 du C.N.R.S. 40, avenue du Recteur Pineau 86022 Poitiers (France) ABSTRACT A new approach is developed,-employing "in situ" electrical resistivity measurements, as a tool to study ion beam mixing of evaporated metal-metal multi or bilayers. The electrical resistivity variations measured continuously during the ion bombardment exhibit a monotonical increase and a tendency toward a saturation process allowing to detect precisely the total mixing of the film. The volume fraction of intermixed atoms can be determined within the framework of a simple conduction model. Experimental results are given in the case of Fe-Al and Al-Ag multilayers. INTRODUCTION The application of energetic ion beams to surface processing of materials has been recently developed in order to improve the mechanical properties of the surface on engineering components (1, 2). The possibility to produce surface alloys by a process which do not involve heating the bulk of the material represents an important advantage of ion implantation. In addition, the high defect concentrationproduced in the collision cascades modifies considerably the diffusion coefficients and the classical compositional limitations connected with the solid solubility are surpassed and new metastable alloys can be produced either single phase solid solution or amorphous alloys. The practical importance of these thin adherent alloys layers has been demonstrated in many cases (3) for wear and corrosion properties. Unfortunately, high dose implantations are necessary to produce concentrated alloys, generally above 1017 ions/cm2 and a physical limitation to the maximum concentration achievable is imposed by the sputtering phenomenon. Consequently, direct implantation is restricted to low concentrations and small alloyed depth. Tsaur et al.(4, 5) have tried successfully to overcome the precedent limitation by using the ion beam mixing technique combining thin layer evaporation and ion bombardment. Successive layers are deposited on the material and bombarded with Xe ions in order to homogenize the different layers to an atomic scale. The process necessitates Xe ion doses several orders of magnitude lower than direct implantation reducing the influence of sputtering. Moreover, the average film composition can be varied easily in a wide concentration range by only adjusting the relative thickness of the individual layers. Different configurations of layered materials have been studied bilayers, multilayers or also thin markers. However, for potential applications, the multilayer structure appears more convenient in producing alloys of desirable composition than the bilayer one because the composition is forced to a predetermined value. From the experimental stand point most of the studies have used the RBS technique (6, 7, 8) which appears powerful in very specific cases for bilayers or thin markers. But in the case of
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