A study on the plating and wetting ability of ruthenium-tungsten multi-layers for advanced Cu metallization

Abstract

In this study, the plating and wetting ability of Cu/Ruthenium-Tungsten (RuW)/silicon oxide (SiO2) multi-layer stacks were investigated. RuW alloy films were prepared on a SiO2 layer by sputtering, followed by a deposition of a Cu thin film by electro-plating. Scanning electron microscopy (SEM) top view images shows that Cu films can be electroplated on RuW, with smaller Cu nuclei size but in a more highly concentrated and uniform distribution than Cu films electroplated on Ta or TaN. The rate of Cu nucleation decreases and larger Cu clusters were formed on the RuW alloy films with increasing W content. Cu/RuW/SiO2 samples were annealed at 400°C for 30min and then characterized using SEM. There were fewer pin holes on the surface of a pure Cu/Ru stack compared to Cu/RuW multilayers. Cu/RuW/SiO2 multi-layers had fewer pin holes than Cu/Ta/SiO2 structures processed under similar conditions. The wetting angle, measured by SEM, of Cu on a RuW substrate (43°) was still lower than that of Cu on a Ta substrate (123°), which suggests that the adhesion strength of Cu onto RuW alloy is better than that onto Ta. Preliminary studies of Cu diffusion through 25nm RuW layers at 650°C showed no Cu penetration into the underlying Si in a Cu/RuW/Si multi-layer test structure.