The interaction of hafnium and aluminium thin films on silicon substrates

A thin hafnium film (100–200 Å) is frequently used as a current-carrying bottom layer when anodization is used in aluminium interconnect patterning for integrated circuits. Unfortunately severe pitting in contact holes occurs on annealing (400–500°C), precluding the use of the process for shallow junctions. A detailed investigation on this failure mechanism has been undertaken. Thin film stacks of hafnium and aluminium sputtered onto silicon substrates were annealed for various times at temperatures ranging from 400 to 500°C. The failure mechanism of this metallization structure is found to be compound formation. HfAl 3 formation is followed by a rapid reaction to HfSi 2 and Hf(Si,Al). Hafnium acts as a sacrificial barrier between aluminium and silicon. The ternary compound is formed not only in contact holes but also along large parts of the aluminium line as a result of the rapid diffusion in the ternary compound. Hence severe pitting and silicon dissolution occur.