Electromigration Failure and Mechanical Stress in thin Film Conductors
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ELECTRONMIGRATION FAILURE AND MECHANICAL STRESS IN THIN FILM CONDUCTORS J.R. Lloyd, Digital Equipment Corporation, 77 Reed Road, Hudson MA, U.S.A. and MaxPlanck-Institut fur Metallforschung, Institut fur Werkstoffwissenschaft, Seestrasse 92, D-7000 Stuttgart, Germany. ABSTRACT The role of mechanical stress in electromigration of thin films is reviewed and discussed. The role of stress in the driving force and the diffusivity are seen both to be important. The concept of the activation volume is discussed with regard to information available from its measurement. It is seen that void nucleation by electromigration or stress voiding is an unsolved theoretical problem. INTRODUCTION Barring serious design errors and defect induced problems, failure of metal conductors in thin film microcircuits can occur via corrosion, stress voiding and electromigration. Corrosion is beyond the scope of this paper and will not be discussed further. Stress voiding is the electronics industry's name for what metallurgists refer to as creep, in this case resulting from thermal stresses induced during processing. The third mechanism, and the main subject of this paper, is electromigration. Electromigration is defined as diffusion biased due to the momentum exchange between conducting electrons and diffusing metal atoms. There has been an unfortunate trend in the literature recently to refer to any motion due to an electric field as electromigration. It is the considered opinion of many of us in the field that these phenomena have their own vocabulary and that the definition presented here be used exclusively for the term. The reader is asked to note the distinction so as not to add to the present confusion. Although it has been shown repeatedly that electromigration lifetime can be negatively affected by the presence of defects as might arise from either stress voiding or corrosion, [1,2] the various failure modes have traditionally been treated independently. Recent experimental and theoretical developments, however, show that electromigration and stress voiding may be intimately connected [3-7] and that the role of mechanical stress in electromigration failure needs to be carefully explored. It will be seen that mechanical stress plays a very important role in the reliability of integrated circuit conductors, with one of its most important effects being that on electromigration lifetime. ORIGIN OF ELECTROMIGRATION INDUCED STRESS Electromigration is a diffusion process. Under conditions of conunercial importance, such as in integrated circuit use, diffusion is almost entirely via the grain boundaries. In bulk materials, current densities high enough to promote electromigration cannot be attained before catastrophic thermal runaway due to Joule heating, but thin films, because they are in intimate contact with a relatively large and massive heat sink, can withstand very high current density with no immediate damage. In addition, thin films are extremely fine grained compared to bulk material and the increased number of grain boundary diff
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