Electromigration Voiding in Argon-Implanted Interconnects

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aluminum interconnect lines, we introduce an initial defect population (argon bubbles) of controlled size and location. Tests were performed in a high-voltage SEM (120 keV), which enables in-situ observation of the voiding process through the passivation layer.3 Images taken throughout the in-situ tests were analyzed to determine void nucleation times and locations. In the argon-implanted interconnects, ten of the 15 voids that nucleated were within implanted regions. Voids nucleated in the interior of the line within the implanted regions, as well as at the passivation/sidewall interface where voids are typically seen in conventional electromigration tests. In addition, voids in implanted regions nucleated much more quickly than those in unimplanted regions. These observations support the idea of argon bubbles reducing the nucleation barrier. TEM was used to analyze the microstructure of both control and implanted interconnect lines. INTRODUCTION The energy barrier for void nucleation is becoming an important consideration as studies have shown that the incubation time to void nucleation can constitute a large portion of interconnect lifetime.4' 5 Electromigration voids in passivated interconnects nucleate at a grain boundary/line sidewall intersection at the cathode end of a polygranular segment.6 8 With improved metallization quality, however, very few voids form in electromigration testing compared to the large number of such potential nucleation9 sites. In previous in-situ tests, some sites were able to nucleate multiple voids throughout a test. The observations of fewer voids than potential nucleation sites and sometimes multiple voids at the same site suggest that there is something unique about these sites that allows voids to form there. A possible explanation is that a defect or free surface must be present for voids to nucleate at a reasonable rate. Experiments have shown that the cleaning step that follows dry etching of aluminum has a large effect on interconnect reliability. 10°'11 The etch residue may act as the defect required to lower or eliminate the nucleation barrier. Previous electromigration experiments have tested the theories that void formation is energetically not possible unless nucleation occurs at a pre-existing defect.' 2"13 These experiments involved electromigration testing of argon-implanted aluminum interconnects with no Ti underlayer. Argon ions were selectively implanted in aluminum interconnect lines to introduce defects of controlled size and location. The argon precipitates to form small bubbles following implantation.""'5 Since aluminum will not bond to the noble gas atoms, the bubbles act as free surfaces with high surface energy, reducing the barrier for potential void nucleation. There was evidence suggesting the argon bubbles reduced the nucleation barrier, but some questions were left unanswered. 97 Mat. Res. Soc. Symp. Proc. Vol. 563 ©1999 Materials Research Society

The experiments that will be discussed here were designed to investigate further the theories regarding