Scaling interconnects to nanodimensions: interfacial chemistry considerations

The continued requirements of ULSI indicate that Cu diffusion barriers must soon shrink to widths of < 4 nm. This scaling requirement, combined with the possible replacement of Cu seed layers by Ru for plating applications, places a renewed emphasis on control of the surface and interfacial chemistry in the plating stack. Additionally, the current 3-component barrier stack $TaN_x/Ta/Cu seed (or Ru) is extremely complex to process at nm length scales and continued scaling would be greatly simplified if one could go to a 2-component stack. This paper will review challenges raised specifically by the shrinkage of the barrier width to nm length scales, recent work done to address these problems, and suggest avenues for future research and development. Specific issues include (1) inhibition of oxide growth during plating on Ru; (2) Ru alloying with Ta, and (3) Ta interfacial reaction with Si:O:C low-k dielectrics. Recent work suggests that iodine monolayers may enhance the smoothness and nucleation of Cu ad-layers while inhibiting Ru oxide formation. This suggests that Ta, rather than TaN_x be removed from the barrier stack. This raises issues of Ru/TaN_x adhesion and alloying. The application of surface science techniques to investigating and solving these challenges is also discussed