Mechanism of stress-induced migration in VLSI aluminum metallization

Void formation in aluminum lines caused by high-temperature heat treatment is investigated in connection with passivation stress. Passivation layers bulge due to their own compressive stress, forcing the aluminum they encapsulate to expand. When the temperature is high enough, the aluminum moves, forming voids. This process is comparable to that caused by thermal expansion mismatch. This mechanism is considered to explain the dependence of the observed phenomenon on treatment temperatures, cooling rate, line width, passivation type, and passivation thickness.<>