Investigation of copper agglomeration at elevated temperatures

In this work, the agglomeration behavior of copper thin films after high temperature annealing was investigated. Cu (200 or 50 nm)/Ta (10 nm, or no Ta)/TaN (50 nm)/Ta (10 nm) layers were deposited onto SiO₂ (270 nm)/Si substrates by magnetron sputtering. All samples were annealed in vacuum at temperatures ranging from 400 to 800°C. The sheet resistance, phases, surface morphology, elemental depth profiles, and chemical binding states were investigated by four-point probe, θ-2θ X-ray diffraction, scanning electron microscopy (SEM), Auger electron spectroscopy, and X-ray photoelectron spectroscopy (XPS). Experimental results revealed that 50 nm thick copper films deposited directly onto TaN agglomerated after annealing at 600°C. No copper agglomeration was observed for 200 nm thick copper films even after annealing at 800°C. It is also observed that copper agglomeration was prevented while a Ta layer was interposed between Cu and TaN. SEM and XPS results showed that, with a Ta interposed interlayer, copper grain growth was slowed down and Ta out-diffused to the copper surface to form a TaO_x layer. The slow grain growth rate of copper and forming of TaO_x cap layer are believed to be the main reasons for preventing copper agglomeration.