Physical mechanism of effective work function modulation caused by impurity segregation at Ni silicide/SiO2 interfaces

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An effective work function (Φeff) modulation caused by impurity doping in Ni-FUSI/SiO2 systems has been systematically investigated. To clarify the physical origin, we reveal the relationship between changes of Φeff modulation factors at gate electrode interfaces and Φeff modulation ranges (ΔΦeff) in two different doping processes (predoping and post-doping). In the predoping process, in which impurities (As or B) are implanted into polycrystalline Si gate before Ni fully silicided gate formation, both the crystal structure of the Ni silicide layer and the impurity concentration at the Ni silicide/SiO2 interface change depending on the impurity species. On the other hand, the impurity post-doping process, in which impurities are implanted and introduced to the Ni silicide/SiO2 interface after Ni silicide formation, makes it possible to introduce the desired amount of impurities into the interface without any structural change of the Ni silicide. In both cases, dependence of ΔΦeff on interface impurity concentration is almost the same, indicating that impurity segregation is the main cause of Φeff modulation. From the results of hard x-ray photoelectron spectroscopy and back-side x-ray photoelectron spectroscopy, the segregated As atoms, which change Φeff to lower values, are mainly located at the Ni silicide side of the Ni silicide/SiO2 interface. On the contrary, B atoms, which change Φeff to higher values, diffuse into SiO2 and are located at the SiO2 side of the interface. Based on these experimental results, we propose a physical model for Φeff modulation with impurity doping such that the position and concentration of impurity at the interface determine the direction and the extent of Φeff modulation with impurity segregation.