Interconnects for future generations of integrated microcircuits need to be both highly conductive and electromigration resistant. Copper doping of aluminum interconnects significantly improves electromigration resistance due to the precipitation of CuAl.sub.2, or a sub-stoichiometric alloy along the grain boundaries. These grain boundary precipitates retard grain boundary diffusion in the thin films, thereby reducing total mass transport and improving electromigration resistance. The CuAl.sub.2 precipitates at grain boundaries and triple points are formed after annealing Al--Cu films at 425.degree. C. Similarly Al--Cu thin films annealed at 200.degree. to 300.degree. C. are enriched with copper at the aluminum grain boundaries.
Copper doping can be achieved by sputtering an Al--Cu film or by annealing a sandwiched stack of Al and a pure Cu or Cu containing film. In situ sputtered copper-containing films have been used as copper doping sources for chemical vapor deposited (CVD) aluminum at temperatures as low as 230.degree. C. Diffusion of copper from the copper containing film can result in a copper concentration in the CVD Al approaching solid solubility limits. From a sputtered pure copper source a maximum of 0.37 wt. % of copper is incorporated into the aluminum film. The copper containing film may also be deposited using metal-organic CVD (MOCVD) precursors. A small amount of copper diffuses into the aluminum layer during deposition, and complete diffusion of the copper into the aluminum is achieved by a rapid thermal anneal at a higher temperature.
Typically, electromigration characteristics improve with increasing copper doping in the range of zero to two weight percent. However Al--Cu alloys with copper levels above one weight percent are difficult to etch. This is due to the fact that copper byproducts formed during the etch have extremely low vapor pressure and form residues during the etch. These residues can make the etched surface rough as well as result in micromasking and uneven etching.
It is, therefore, an object of the instant invention to provide a method for introducing copper into an aluminum film and still being able to effectively and efficiently etch the Al--Cu film.