Semiconductor interlevel shorts caused by hillock formation in Al-Cu metallization

A new failure mode in AlCu and AlCuSi metallization is described in which interlevel metal short circuiting occurs between two or more levels of metal. Shorts are caused by theta-phase (Al₂Cu) hillocks which nucleate and grow during high-temperature vacuum heat treatment and processing, Hillock growth occurs at high-energy sites, such as silicon precipitates and grain boundary nodal points. The growth of Al₂Cu hillocks depends on the heat-treatment/processing temperature and aluminum film purity. The growth kinetics indicates that grain boundary diffusion is the dominant mass transport mechanism. Methods used to limit theta-phase hillock formation and growth concentrate on the diffusion and nucleation mechanisms involved. Decreasing the heat-treatment/processing temperature slows the atomic diffusion required for hillock growth, and it delays, but does not prevent, theta-phase hillock formation. A 1-h heat treatment (213 Pa, N ₂ ambient) at 350°C produces a high density of large hillocks. Hillock density and height are generally reduced at 300°C. Altering the layered structure of a metallization alters Al₂Cu hillock growth. Deposition of a hard coating as a cap on the layered structure of an aluminum-based metallization mechanically suppresses hillock formation. A layer of pure aluminum deposited beneath the aluminum-copper layer acts as a sink for copper and delays hillock formation. Increasing film copper content reduces hillock formation: theta-phase hillocks, up to 1.3 μm in height, are observed in films with 1 wt.% copper, whereas negligible (<0.2 μm in height) hillock formation is observed in 11 wt.% Cu films