XPS study of tungsten and barrier film transition at various stages of chemical mechanical polishing endpoint and of surface compositions post-CMP cleaning

The present study aims to determine wafer surface film compositions at various stages of optical end point during W CMP process. Wafers taken at reflectance peak time, peak plus time, slope time, end point time and overpolish time were analyzed by XPS at various locations of the wafer, with special focus on the compositional identification of the visual “residues”. Results showed that W and barrier film removal proceed in a non-uniform manner at XPS probe depth scale of ∼6 nm. At the optical reflectance peak time the wafer is substantially covered with W, and Ti barrier only appeared on the edge of the wafer due to center slow - edge fast removal profile of the polishing. Using peak to end point time to calculate Ti barrier removal rate may under-estimate the true barrier polishing time and led to artificially lower barrier rate. Ammonia-based post CMP cleaning process and its impact on W film surface composition is also discussed.