Understanding reversal effects of metallic aluminum introduced in HfSiON/TiN PMOSFETs

We evaluate the insertion of metallic aluminum in TiN metal gate over HfSiON/SiON for a gate first CMOS integration with an equivalent oxide thickness of 15. From capacitance versus voltage measurements, we report for the first time a non-linear Vfb shift associated to aluminum thickness variation (a). To understand this observation the metal gates have been reproduced on various dielectric stacks having either (b) beveled SiON or (c) 150 HfSiO. From beveled samples we extract the effective work function, which presents the same variations with aluminum thickness as in nominal devices (a). Aluminum diffusion at the bottom high-k interface is prevented in samples (c) thanks to thick HfSiO and leads to a negative Vfb shift. We conclude that the observed reversal shifts with metallic aluminum thickness in TiN are due to a +50 mV aluminum induced dipole at the HfSiON/SiON interface associated to an opposite metal work function decrease.