Dependence of Effective Work Function Modulation with Phosphorous Segregation on Ni to Si Ratio in Ni Silicide/SiO2Systems

The effect of Ni to Si ratio on effective work function (Φeff) modulation with phosphorus (P) segregation at the Ni silicide/SiO2 interface was systematically investigated. To discriminate the P segregation effect from other parasitic possibilities of Φeff 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 Ni2Si (Ni3Si2)/SiO2, NiSi/SiO2, and NiSi2/SiO2 systems. In the post-doping process, P atoms were segregated to the Ni silicide/SiO2 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 NiSi2 formation process, by which NiSi2 can be formed at a temperature as low as 450 °C, thus revealing the P-segregated Φeff modulation effect at the NiSi2/SiO2 interface for the first time. As a result, the largest Φeff modulation (ΔΦeff = -0.45 eV) occurs at the P-segregated NiSi2/SiO2 interface, and a Φeff of near the Si conduction band edge (4.13 ±0.04 eV) was realized, although the obtained Φeff values with P segregation at Ni2Si (Ni3Si2)/SiO2 and NiSi/SiO2 were 4.38 ±0.01 eV (ΔΦeff = -0.32 eV) and 4.37 ±0.01 eV (ΔΦeff = -0.29 eV) for the same P dose, respectively. Moreover, we found that ΔΦeff at the NiSi2/SiO2 interface is larger than those at the NiSi/SiO2 and the Ni2Si (Ni3Si2)/SiO2 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 Φeff modulation at a high-Si-content interface.