Pulsed Laser Planarization of Metals for IC Interconnect

We consider the feasibility of using an excimer laser to planarize copper deposited over polyimide. Even though the polyimide is thermally sensitive, a 2.8 μm thick copper film, deposited over polyimide, was planarized with 248 nm, 25 ns, laser fluences from 1.7 to 2.6 J/cm2. Although planar surfaces were readily produced, a variety of defects were observed in the underlying structures. The defects observed include stress induced fractures of the interconnect structure, polyimide surface roughening, and changes in the electrical characteristics of the polyimide. We also describe cross sectional TEM images of cavities formed in interlayer contacts during laser planarization that support recent descriptions of the surface tension driven flow.