The origin of internal stress in low−voltage sputtered tungsten films

Abstract

The internal stress can be of importance in the tungsten metallization process used for the fabrication of large−scale integrated circuits. The present work is an extension of an earlier study on the dependence of the stress in low−voltage triode sputtered tungsten films upon deposition conditions and substrate materials. As a function of film thickness, the stress was found to decrease with increasing thickness at various substrate temperatures. The effect of higher substrate temperatures is just to change from large compressive stress to smaller compressive stress and finally into tension. For example, the stress in a 5000−Å film decreases from 1.6×10¹⁰ dyn/cm² in compression to 5×10⁹ dyn/cm² in tension as substrate temperature increases from 370 to 850 °C. Generally, no gross difference was found for films deposited on SiO₂, Al₂O₃, or Si₃N₄ at higher substrate temperatures. As a function of deposition rate, the stress can be described in three regions. The stress was found to be small and relatively constant in the low−deposition−rate region. However, as the deposition rate increased to the transition region, a sharp stress increase was found. Finally, above a critical deposition rate, the large internal stress remains constant again. By measuring the stress and characterizing the microstructure of the film, it is concluded that the grain size is the dominant factor determining the stress. These experimental results are discussed in light of the grain boundary relaxation model of Finegan and Hoffman and annealing of disorder proposed by Klokholm and Berry.