Titanium thin films are used as diffusion barriers between a copper interconnect layer and a silicon substrate. Plasma treatment, by modifying the film microstructure, can improve the film properties and its electrical conductivity. To clarify the correlation, the microstructure of a series of Ti thin films deposited using a metal-chemical vapor deposition (CVD) technique have been analyzed by sheet resistance, X-ray diffraction, and transmission electron microscopy (TEM). TEM analysis results show that films are made of quasi-amorphous thin films, and no texture is observed as no plasma is applied. The titanium films with plasma treatment are actually a multilayer structure. The chlorine content in the film is reduced with plasma treatments. Copper barrier performances of ultrathin 10 nm film of plasma enhanced CVD-Ti, Ti/TiNx, Ti/Ti(H), Ti/Ti(N), and physical vapor deposited-Ti have been investigated. Thermal stability tests, including Rs measurement and XR TEM observation, showed comparable performance between the 10 nm Ti/Ti(H) and Ti/Ti(N), whereas the thermal stability of 10 nm Ti/TiNx barrier was superior to any other barrier films. It also reveals that hydrogen radicals can reduce the chlorine concentration, and nitrogen radicals and ions are attributed to nitridation formation. It is believed that hydrogen plays an important role in the decrease of chlorine concentration and the nitridation effect promotes thermal stability against copper diffusion. (C) 2004 The Electrochemical Society.