Process-induced positive charges in Hf-based gate stacks

Hf-based gate stacks will replace SiON as a gate dielectric even though our understanding of them is incomplete. For an unoptimized SiO₂ layer, an exposure to H₂ at a temperature over 450 °C can lead to positive charging. In this work, we will show that a thermal exposure of Hf-based gate stacks to H₂ can also induce a large amount of positive charge (∼10¹³ cm^-2). There is little information available on this process-induced positive charge (PIPC) and the objective of this work is to fill this knowledge gap. The work is divided into two parts: an investigation of the features and properties of PIPC, followed by an exploration of its dependence on process conditions. It will be shown that PIPC does not originate from the generation of interface states, is stable both thermally and electrically, and has a large sample-to-sample variation. It consists of two components: fixed and mobile. Regarding its dependence on process conditions, PIPC occurs in both HfO₂ and Hf-silicate stacks, in devices with either TaN or poly-Si gates, and in both p metal-oxide-semiconductor field-effect transistors (pMOSFETs) and nMOSFETs. PIPC is generally enhanced by nitridation, although it can also be observed in some Hf-based gate stacks without nitridation.