Probing ultrathin film continuity and interface abruptness with x-ray photoelectron spectroscopy and low-energy ion scattering

The authors have examined ultrathin (≤10 Å) tantalum nitride (TaNx) thin films deposited by atomic layer deposition (ALD) on three surfaces relevant to interconnect layers in microelectronic devices: thermally grown SiO2; a Cu thin film grown by physical vapor deposition, and a carbon-doped SiO2 porous low-κ thin film. The authors have employed ex situ angle-resolved x-ray photoelectron spectroscopy (ARXPS), low-energy ion scattering spectroscopy (LEISS), and atomic force microscopy (AFM) to determine the continuity of these thin films, and by implication, the abruptness of the thin film/substrate interface. On SiO2 and low-κ, the authors find similar results: both ARXPS and AFM indicate that smooth, uniform thin films are deposited, consistent with nearly layer-by-layer growth of TaNx on these surfaces. Examination of these films using LEISS reveals that while the 10 Å TaNx thin films are continuous, the 5 Å TaNx thin films are not continuous and may possess on the order of ∼10% exposed substrate in the form of small subnanometer inclusions. On Cu, the situation is quite different. The TaNx thin films on these surfaces are not continuous, and our results point to a mixed layer of TaNx and Cu forming during ALD. In all cases, if one were to rely solely on results from ARXPS, the picture would be incomplete as the results from LEISS are ultimately decisive concerning thin film continuity.