Secondary ion mass spectrometry (SIMS) was successfully used as an analytical method to characterize chemical vapor deposited (CVD) tungsten (W) processes. Blanket CVD tungsten film on titanium nitride (TiN) barrier layers generally begins with the deposition of a tungsten nucleation layer by silane (SiH 4) reduction of tungsten hexafluoride (WF 6), followed by hydrogen (H 2) reduction of WF 6 alone to form the bulk tungsten layer. In the present work, the tungsten nucleation layer was formed by simultaneous SiH 4 and H 2 reduction of WF 6. A two-step SiH 4 gas flow scheme was used to determine the effects of magnitude and duration of SiH 4 flow on the Si concentration at the W/TiN interface. SIMS was used to characterize the Si distribution at the CVD W/TiN interface. SIMS depth profiles indicate, with a constant SiH 4 flow time of 3 s, the Si concentration at W/TiN interface does not vary significantly with the increase of SiH 4 flow from 20 to 30 sccm. However, it increases dramatically with the increase of SiH 4 flow from 30 to 48 sccm. With a constant SiH 4 flow of 40 sccm, the Si concentration at the W/TiN interface increases linearly with the increase of the SiH 4 flow time. These results amply demonstrate that SIMS analysis can be used to evaluate the deposition process so as to meet the fill and barrier protection requirements for narrow trench or small via.