Glancing angle x-ray study of the effect of oxygen on interface reactions in Al/Ni bilayers
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Glancing angle x-ray reflectivity and EXAFS measurements have been made on a series of UHV prepared Al/Ni bilayers with varying amounts of oxygen impurities. These samples show an intrinsic reacted region prior to annealing, and for clean samples further reaction occurs at 250 °C. Oxygen is found to influence strongly the course of the reaction with an effect which depends on its location. A few percent O impurity within the Al film strongly suppresses the grain boundary diffusion path, which allows the growth of a smooth NiAl3 layer. Interfacial O exposures of 60 and 600 Langmuir both inhibit the initial reaction and raise the temperature at which further reaction occurs to as much as 300 °C with an effect which depends on exposure. The thickness of the intrinsic reaction zone is about 60 A for clean samples, and is nearly eliminated for contaminated interfaces. The results indicate that surface/interface, grain boundary, and bulk diffusion all play important roles in the formation of these interfaces, and that each of these is influenced by O impurities.
I. INTRODUCTION
Interfaces between transition metals and Al are important in applications such as advanced composite materials and semiconductor processing. Accordingly, there is a large body of work concerning their characterization. In this paper we use glancing angle x-ray methods to take a close look at the influence of O impurities on the interfacial reaction in Ni-Al couples. Reactions at Al/Ni interfaces have been the subject of a number of papers x~4 which have reported somewhat contradictory results. There is agreement that NiAl3 is the first phase to form, but in some work a smooth reaction front is observed, while others observe an irregular reaction front indicative of substantial reaction along grain boundaries (GB). Reaction temperatures are generally reported in the range of 250-320 °C. It has been suggested that impurities, especially oxygen, could be a primary reason for the differing results.4 This seems to be the case for the reaction temperature, where deliberately contaminated samples showed no reaction at temperatures as high as 400 °C.4 Our previous work56 on Cu/Al bilayers also demonstrated the important role that O plays in interface reactions with Al. The glancing angle x-ray techniques used are reflectivity and x-ray absorption measurements.7'8 The reflectivity measurements provide a depth dependent density profile of the bilayer and are useful in following the interdiffusion and reactions occurring at the inter-
Current address: Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77251.
face. The x-ray absorption measurements can be used to identify the phases being formed, and perhaps more importantly can detect very small amounts of interface reactions by detecting Ni-Al bonds. These techniques are generally more sensitive than conventional studies using Rutherford Backscattering (RBS) and avoid the difficult and destructive sample preparation needed for electron microscopy. They allow us t
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