Electromigration in sputtered copper interconnection with polyimide as interlevel dielectric or passivation

Abstract

Electromigration damage (EMD) is one of the major causes for the failures of interconnection. The use of copper and low dielectric constant dielectric has been proposed to reduce the RC time delay and to improve EMD. In this study, the Electromigration of Cu with polyimide is investigated with an empirical formula dR/dt.1/R-0 = AJ(n) exp(-Q/RT). Secondary ion mass spectrometry (SIMS) reveals the interdiffusion between Cu and polyimide during curing of the polyimide and/or annealing of Cu metallization. Thin layers of TiW is deposited between polyimide and Cu as a barrier to reduce the interdiffusion. The activation energy Q for the electromigration of Cu on polyimide is 0.77eV from 120 degrees C to 230 degrees C, while activation energies for samples with titanium tungsten as an interlayer are 0.79eV (140 degrees C to 190 degrees C) and 1.73eV (190 degrees C to 230"C). The presence of TiW barrier enhances the high temperature electromigration resistance and promotes the adhesion between Cu and polyimide interface. However, films with TiW are found to be more sensitive to current stressing than those without. Polyimide is also employed as a passivated layer on top of Cu metallization. Resistance of the passivated samples decrease in the initial stage of the electromigration experiment. Possible causes are discussed on the decrease in R. The geometry of the metallization also affects the electromigration, the current exponent (n), calculated from EMD data, are different for interconnection with different geometries.