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The effects of Si addition and of deposition temperature on the precipitation processes of Al2Cu (θ) and Si particles in Al-Cu-Si alloy films were studied with in-situ hot stage transmission electron microscopy (TEM). Deposition of an Al-1.5Cu-1.5Si (wt%) film at 305°C, in the three-phase, Al(α)-Al2Cu-Si region resulted in formation of fine, uniformly distributed spherical θ -phase particles due to the coprecipitation of the θ and Si phase particles during deposition. For deposition in the two-phase, Al(α)-Si region (435°C), fine θ -phase particles precipitated during wafer cooldown, while coarse Si nodules formed at the sublayer interface during deposition. In-situ heat treatment of the film revealed that excess Si existed in a supersaturated Al matrix. Si addition decreased film susceptibility to corrosion induced by the θ -phase precipitates, since extensive Cu segregation can be reduced by coprecipitation at 305°C and the Al matrix supersaturated with Si reduced galvanic action with respect to the θ -phase precipitate.