The impact of substrate properties on the electromigration resistance of sputter-deposited Cu thin films

Abstract

Copper (Cu) is commonly used as metallization for a wide range of microelectronic devices. Typically, organic circuit boards as well as ceramic and glass-ceramic substrates use galvanic deposited Cu films for this purpose. However, due to a thickness of several microns the lateral resolution in the μm-region being required e.g. for novel high frequency applications can not be guaranteed when applying this technology. Hence, sputter deposition is envisaged for the realization of Cu thin films on glass, LTCC (low temperature co-fired ceramics) and alumina substrates. The reliability of 300 nm thick Cu thin films is investigated under accelerated aging conditions, utilizing a test structure which consists of 20 parallel lines stressed with current densities up to 1•10⁺⁶ A•cm^-2 at temperatures between T= 100°C and 200°C. To detect the degradation via the temporal characteristics of the current signal a constant voltage is applied according to the overall resistance of the test structure. Knowing the mean time to failure (MTF) and the activation energy at elevated temperatures conclusions on the migration mechanism can be drawn. Whereas on LTCC substrates the activation energy of E_a~ 0.75 eV is similar to other face centered cubic metals such as silver, the higher activation energies of about E_a~ 1 eV on glass and alumina indicate a suppression of back diffusion especially at enhanced temperature levels. Therefore, the overall electromigration resistance is lower compared to Ag. This effect is predominantly caused by a stable oxide layer being formed at high temperatures acting as passivation layer.