STUDY OF ULTRATHIN TA DIFFUSION BARRIER FOR CU INTERCONNECTS

摘要

This study focuses on 5 nm Ta thin film of the 150 nm Cu/5 nm Ta/200 nm SiO_2/Si multi-layered structure to study the impact of ultrathin diffusion barriers on Cu interconnects. In order to investigate the barrier's thermal stability, samples undergo a thermal treatment between 150 and 850℃, which accelerates Cu diffusion. There is a drastic increase in the sheet resistance of the copper layer after 550℃ annealing. The surface roughness and grain size of the copper layer are analysed with respect to the annealing temperature. ToF-SIMS data show an early stage of Cu diffusion through the barrier at 550℃ as a result of grain boundary diffusion. At temperatures of 750℃ and above, it becomes more pronounced. XRD data, being different from 30 nm films, show that metallic oxides are formed during annealing and lend themselves to more diffusion paths for Cu to diffuse into SiO_2. Cross-sectioned TEM micrographs show agglomeration in the copper film and voids in the barrier layer at high temperatures. HRTEM analysis reveals Cu_xSi_y particulates near the Ta/SiO_2 interface in the 750℃ sample. More Cu_xSi_y particulates are observed in the 850℃ sample. In conclusion, the ultrathin Ta barrier is still reliable and functioas effectivelyup to around 500℃.