Dependence of Effective Work Function Modulation with Phosphorous Segregation on Ni to Si Ratio in Ni Silicide/SiO₂ Systems

Abstract

The effect of Ni to Si ratio on effective work function (È eff ) modulation with phosphorus (P) segregation at the Ni silicide/ SiO 2 interface was systematically investigated. To discriminate the P segregation effect from other parasitic possibilities of È eff modulation, which are changes in Ni-Si ratio and crystallinity including crystal grain orientation near the interface, we applied a P post-doping process for Ni 2 Si (Ni 3 Si 2 )/SiO 2 , NiSi/SiO 2 , and NiSi 2 /SiO 2 systems. In the post-doping process, P atoms were segregated to the Ni silicide/SiO 2 interface after Ni silicide gate formation; hence, independent control of a desired interface P density can be realized. In addition, we newly developed a thin Ti-inserted NiSi 2 formation process, by which NiSi 2 can be formed at a temperature as low as 450 C, thus revealing the P-segregated È eff modulation effect at the NiSi 2 /SiO 2 interface for the first time. As a result, the largest È eff modulation (ÁÈ eff ¼ À0:45 eV) occurs at the P-segregated NiSi 2 /SiO 2 interface, and a È eff of near the Si conduction band edge (4:13 AE 0:04 eV) was realized, although the obtained È eff values with P segregation at Ni 2 Si (Ni 3 Si 2 )/SiO 2 and NiSi/SiO 2 were 4:38 AE 0:01 eV (ÁÈ eff ¼ À0:32 eV) and 4:37 AE 0:01 eV (ÁÈ eff ¼ À0:29 eV) for the same P dose, respectively. Moreover, we found that ÁÈ eff at the NiSi 2 /SiO 2 interface is larger than those at the NiSi/SiO 2 and the Ni 2 Si (Ni 3 Si 2 )/SiO 2 interfaces, even when the segregated P densities are the same. These results can be explained by an increase in the density of effective interface dipole for È eff modulation at a high-Si-content interface.