Abstract
Electromigration in an Al-27% Cu metallization thin film deposited on a silicon transistor structure has been studied in situ using the electron beam induced current mode (EBIC) of the scanning electron microscope. In this mode electrons which are transmitted through the metallization generate a signal which depends on the thin film thickness and its mass density. During the initial stages of the electromigration experiment the copper rich precipitates on the surface coarsened. Also, there was a preferential coarsening towards the positive end. Once the negative end became depleted of these surface precipitates grain thinning occurred. Voids nucleated and grew in from the sides of the metallization in these depleted regions finally causing film failure. However almost no voids were observed at grain boundaries or triple points. The copper rich precipitates in the interior of the metal film were stationary and showed no signs of coarsening or depleting any region. These observations suggest that surface diffusion is the predominant mode of atom migration in these alloys; grain boundary diffusion has been reduced compared to pure aluminum.
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