Effective-Work-Function Control by Varying the TiN Thickness in Poly-Si/TiN Gate Electrodes for Scaled High- $k$ CMOSFETs

We have investigated the controllability of the effective work function (phi_m,eff) of TiN as a work-function-determining metal (WFM) for various gate-electrode structures in HfSiON MOSFETs. phi_m,eff was controllable from 4.7 to 4.44 eV by changing the TiN thickness from 30 to 2 nm in poly-Si/TiN gate electrodes, without any distinct increase in EOT. Therefore, thin-TiN and thick-TiN WFMs are preferred for the reduction in threshold voltage in nMOSFETs and pMOSFETs with poly-Si/TiN gate electrodes, respectively. A similar controllability was not observed with W/TiN gate electrodes but was evident with W/TaSiN/TiN gate electrodes. This means that controllability is a characteristic of metal gate electrodes with a structure including a Si-rich layer (such as poly-Si and TaSiN)/TiN. It is considered that Ti suboxides, which increase phi_m,eff as a thin insulator with negative fixed charges, or interface dipoles in the TiN/HfSiON interface, are reduced by oxidation of the Si-rich layer, producing the required result of phi_m,eff decrease when the TiN thickness becomes as thin as 2 nm.

I. Introduction

Metal gate electrodes and high- gate dielectrics are required instead of conventional poly-Si gate electrodes and SiON dielectrics to suppress gate depletion and gate leakage current increase, which are associated with the continued scaling of CMOS. A dual-metal-gate concept, in which two different conductive materials are used as gate electrodes for nFETs and pFETs, seems to be effective for obtaining two different effective work functions ('s) for nFETs and pFETs. However, as long as the gate-first process and a single high- dielectric such as HfSiON are employed, without other material incorporation such as La and Al [1]–[4], the difference between dual metal gates for nMOS and pMOS are actually much narrower than that between and poly-Si on (1.1 eV) [5], [6]. As a result, the threshold voltages of both nFETs and pFETs become unacceptably high. Moreover, it is very difficult to simultaneously etch dual-metal-gate electrodes that are composed of different conductive materials [7].