Via resistance reduction using "cool" PVD-Ta processing

Different processes, including "cool" physical vapor deposition (PVD), of Ta barrier and Cu seed deposition were compared in Cu interconnect development. In the cool Ta process, the substrate temperature was <50°C, compared to a temperature of about >100°C in the standard process. With the cool process, via resistance (0.19 μm in via size) was reduced by about 25%, although 40% thicker Ta was measured at the via bottom. This was not in agreement with the common understanding that the thicker the Ta film is, the higher the via resistance. Blank film studies suggested that a mixed texture of α- and β-Ta was formed at via bottom in the new Ta/Cu process. X-ray diffraction spectra clearly exhibited the existence of α-Ta in addition to the β-Ta, where the latter is usually observed in the standard process. Electron diffraction spectra further supported the claim of mixed α-/β-Ta formation at via bottom. Moreover, Rutherford backscattering data suggested that the mixed α-/β-Ta had even higher thermal stability.