Analysis of Origin of Threshold Voltage Change Induced by Impurity in Fully Silicided NiSi/SiO2 Gate Stacks

To investigate the origin of the threshold voltage (Vth) change by impurity segregation in a fully silicided (FUSI) NiSi/SiO2 gate stack, for the first time we directly examined the vacuum work function (vac) of the electrodes, the electrical dipole moment (Dinter), and the chemical state of the impurity at the NiSi/SiO2 interface by backside X-ray photoelectron spectroscopy (XPS). We found that the impurity causes neither a change in the vac nor the formation of a fixed charge in SiO2, and that the interface dipole is a dominant factor to cause the Vth change. We propose that the origin of the interface dipole is the impurity atoms with large electronegativity bonded to both NiSi and SiO2 at the NiSi/SiO2 interface.