Abstract
We present an ab initio evaluation of electron scattering mechanisms in Al interconnects from a back-end-of-line (BEOL) perspective. We consider the ballistic conductance as a function of nanowire size, as well as the impact of surface oxidation on electron transport. We also consider several representative twin grain boundaries and calculate the specific resistivity and reflection coefficients for each case. Lastly, we calculate the vertical resistance across the Al/Ta(N)/Al and Cu/Ta(N)/Cu interfaces, which are representative of typical vertical interconnect structures with diffusion barriers. Despite a high ballistic conductance, the calculated specific resistivities at grain boundaries are 70-100% higher in Al than in Cu, and the vertical resistance across Ta(N) diffusion barriers are 60-100% larger for Al than for Cu. These results suggest that in addition to the well-known electromigration limitations in Al interconnects, electron scattering represents a major problem in achieving low interconnect line resistance at fine dimensions.
Highlights
Copper (Cu) has been the conductor-of-choice for back-end-of-line (BEOL) interconnect technology since the late 1990s (220nm half-node) when it replaced aluminum-based alloys Al(Cu).[1]
The Fermi surface of Cu, on the other hand, is largely preserved when interfaced with TaN, allowing moderate electron transmission across the barrier. These results indicate that if a refractory metal nitride like TaN is to be used as a diffusion barrier in next-generation interconnect technology, vertical resistance can be expected to be greater for Al interconnects than for Cu interconnects
We have systematically evaluated several electron transport properties of Al interconnects using first-principles calculations
Summary
Copper (Cu) has been the conductor-of-choice for back-end-of-line (BEOL) interconnect technology since the late 1990s (220nm half-node) when it replaced aluminum-based alloys Al(Cu).[1]. The existence of such a cross-over point confirms our previous assertion that metals will have a smaller size effect than in Cu if their product of bulk resistivity ρ0 and electron mean free path λ is smaller than the corresponding product for Cu.[15] this appears to be the case for Al. In this study, we apply a first-principles approach to quantitatively determine the resistivity scaling of Al, without relying on the empirical classical models
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