Electromigration Resistance and Mechanical Strength: New Perspectives for Interconnect Materials?
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ELECTROMIGRATION RESISTANCE AND MECHANICAL STRENGTH: NEW PERSPECTIVES FOR INTERCONNECT MATERIALS? E. ARZT*, 0. KRAFT*, J. SANCHEZ*, S. BADER** and W.D. NIX** *Max-Planck-Institut fuir Metallforschung, Inst. fdr Werkstoffwissenschaft, Stuttgart, Germany "**Stanford University, Department of Materials Science and Engineering, Stanford, CA, USA ABSTRACT A brief review is given of models which propose a correlation between electromigration resistance and the mechanical strength of thin film interconnects. In an attempt to achieve metallurgical strengthening and improved electromigration resistance, aluminum films were implanted with oxygen ions. Preliminary electromigration tests on line arrays patterned from these films resulted in lifetimes comparable to the standard Al films. The lack of improvement is attributed to enhanced hillock/whisker growth during electromigration in the implanted interconnects. This behavior is coincident with a lower compressive strength in similarly treated continuous films at elevated temperatures as measured by the substrate curvature technique. INTRODUCTION Electromigration is the major failure mechanism for conductor lines in integrated circuits and, with further miniaturization, its importance is likely to increase in the future. Strategies to inhibit electromigration behavior of pure aluminum lines have involved the addition of alloying elements, such as copper, silicon, magnesium, and nickel, among others. However, due to the lack of a generally accepted explanation for alloying effects, progress is mainly made by trial and error. The second strategy, i.e. grain coarsening, is somewhat more clear-cut because grain boundaries are known to provide the dominant diffusion paths at the low homologous temperatures that are relevant for chip operation. It is curious that the measures of alloying and of grain structure manipulation seem to parallel the steps taken to achieve high mechanical strength in advanced aluminum alloys for structural applications. Therefore it might be speculated that strength is somehow related to electromigration resistance. In order to provide a more rational basis for the development of conductor line alloys, a better understanding of this possible connection is hence desirable. In this paper we attempt to carry this hypothesis a step further. We first review the classical Blech model and more recent theories, all of which seem to propose a connection between electromigration resistance and yield or creep strength. Then a first attempt to test these predictions is described: aluminum thin films, which were ion implanted in an attempt to raise the mechanical strength, were tested for strength (using continuous films) and for electromigration resistance (using patterned interconnects). The results, while still preliminary, help to shed new light on the electromigration process. MODELS FOR THE EFFECT OF MECHANICAL STRENGIH ON ELECIROMIGRATION Mechanical considerations have influenced the understanding of E-M ever since the discovery that the directed diffusion of m
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