The microstructure and the chemical binding state of the interfacial layer in W/WNx / poly Si was investigated by using high-resolution transmission electron microscopy (HR- TEM) and X- ray photoelectron spectroscopy (XPS). After a short period of annealing, the Si- N layer and island- type precipitation formed between WNx and poly Si as a result of the thermodynamic instability of WNx films at 900degreesC. HR- TEM imaging and XPS analysis indicated that the island- type precipitation is composed of W, Si, and N. With increasing annealing time, the Si- N layer became thinner and the island- type precipitation became a continuous layer having an amorphous phase about 3 nm thick. However, above a critical annealing time, the interfacial layer thickness remained constant due to the formation of a stable W- Si- N layer. (C) 2004 The Electrochemical Society.