We describe the formation of RuO 2 thin films grown using atomic layer deposition (ALD) on (100) Si substrates from Ru(EtCp) 2 and O 2 , and the subsequent influence of annealing temperature and atmosphere on the surface morphology and structure of the deposited layers. The films are characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction (ED). The as-deposited films consist of RuO 2 islands. No significant changes in composition or morphology are observed following annealing in N 2 for 4 h at either 500 or 7008C. Higher temperature annealing in N 2 (8208C, 4 h) results in some modifications to the morphology and structure where ED data indicate the formation of some Ru metal. However, complete transformation from as-deposited RuO 2 to Ru metal is, obtained after annealing in forming gas (95% N 2 /5% H 2) at 4208C for 5 min.