Etch pit formation on metal films by wet chemicals

It is well known that the voids formed on metal interconnects shorten their lifetimes substantially in VLSI by inducing electromigration. Voids are often found on metal films after stripping photoresist films on metal films. These voids turn out to be etch pits formed by wet chemicals attacking the necks of the hillocks of CuA1₂ and Si precipitates. Etch pits are formed most easily by phenol-based strippers, while very few etch pits are formed by the ashing treatment (photoresist stripping employing a barrel type-plasma etching system). Also, it was found that etch pits tend to form more often on the metal films of Al-l%Si-0.5%Cu than on these of Ai-I%Si. Therefore, the remedy for void formation after photoresist stripping may be as follows: 1) Minimization of hillock formation by lowering residual gas contamination during the metal deposition process, 2) using phenol-free organic strippers or a plasma etching technique for photoresist stripping, and 3) using AI-I%Si alloy instead of AI-I%Si-0.5Cu alloy for the metal interconnect.