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

A failure mode in AlCu and AlCuSi metallization in which interlevel metal short-circuiting occurs between two or more levels of metal is described. Shorts are caused by theta-phase (Al₂Cu) hillocks that 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 heat treatment/processing temperature and aluminum film purity. Methods used to limit theta-phase hillock formation and growth concentrate on the grain-boundary diffusion and nucleation mechanisms involved. Decreasing heat treatment/processing temperature slows the atomic diffusion required for hillock growth and reduces, but does not prevent, theta-phase formation. Deposition of a hard coating as a cap on the layered structure of an aluminium-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