Mechanisms for success or failure of diffusion barriers between aluminum and silicon

Diffusion barrier layers with improved stability are needed in very large scale integrated (VLSI) devices to prevent aluminum from interacting with silicon, under the demanding constraints of shallow junctions and processing heat cycles. We present a comparison of the mechanisms controlling the integrity or failure of diffusion barriers at temperatures above 500 °C. Three classes of high-temperature barrier materials are discussed: amorphous metal alloys, conducting nitrides, and conducting oxides. The advantage of an amorphous barrier is shown to be minimal. Amorphous refractory metal alloys (W–Re) fail by intermetallic reaction with Al at ∼525 °C, well below their crystallization temperature, to form Al12(W,Re). This performance is only slightly better than a polycrystalline film of the same composition, and offers little improvement over pure W. Conducting nitrides (TiN, W–N) have been shown to be effective barriers up to 600 °C or higher, but only if exposed to air prior to Al metallization. In this case, we show evidence for the formation of interfacial aluminum oxide, which prevents further Al reaction. When not exposed to air, these nitrides fail as Al barriers at much lower temperature (450–500 °C) through the formation of Al3Ti and Al12W. Conducting oxides (RuO2, Mo–O) also prevent Al penetration up to 600 °C. Here, the growth of interfacial aluminum oxide is even more pronounced, and reduction of the barrier layers occurs. We conclude that the presence of an interfacial aluminum oxide layer is responsible for the high-temperature (600 °C) stability of diffusion barriers between Al and Si.