Effect of Modified Metal/Passivation Interfaces on Stress Voiding in Interconnects +
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ABSTRACT Stress and electromigration voids are believed to nucleate at the metal/passivation interface. In this study, modified interfaces were synthesized to experimentally elucidate the role of the interface on stress voiding for comparison with theoretical predictions. Interfaces were modified by depositing photoresist in a controlled manner on Ti/Algfi lines prior to passivation deposition. Stress voiding susceptibility was assessed in as-received lines as well as those subjected to storage anneals for 1, 5, and 25 days at 190'C to accelerate void formation. The global textures of all the lines were fiber and were not significantly affected by the interface modification. Differences in stress voiding were attributed to the different interfaces. Higher ratios of voids in modified regions to voids in unmodified regions were observed both in as-received lines and those given a 1 day storage anneal due to the heterogeneous nucleation sites provided by the photoresist contamination in the modified regions. However, the void ratio appears to decrease exponentially with longer time anneals. Lowered driving force for void formation in the modified regions leads to less voiding with time, while higher driving force and a limited number of nucleation sites in the unmodified regions leads to more voiding with time. Local textures at a limited number of voided sites in modified and unmodified regions were identical to the global texture of the line. In our limited sampling, voids did not form in local regions of weaker texture. INTRODUCTION The critical role of the metal/passivation interface on stress voiding was initially shown by the suppression of stress voiding as a result of sputter etching the metal surface prior to passivation deposition [I]. Presumably, the sputter etch provided an additional cleaning step. Stress-induced or electromigration-induced void nucleation at contaminants along the metallpassivation interface was initially examined by Flinn [2] and recently analyzed in further detail [3-6]. Classical nucleation theory calculations indicated that the stress required for void nucleation at a flaw on the interface was not only considerably lower than for vacancy condensation [3], but was consistent with experimentally measured, volume-averaged values. An alternative model of void nucleation proposes that the contamination results in a debond along the interface, which alters the stress profile enabling void formation by crystallographic slip [5,6]. Growth of the voids is expected to be kinetically restricted to sites that are intersected by a grain boundary. An additional effect of contaminated interfaces could be to significantly decrease the triaxial stress in the line by weakening metal/passivation adhesion which reduces the mechanical constraint. Thus the driving force for stress voiding would be greatly decreased.
219 Mat. Res. Soc. Symp. Proc. Vol. 563 © 1999 Materials Research Society
Stress voiding severity is strongly influenced by the line texture. Increased stress voiding has been associated wi
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