Experimental study of electron mobility characterization in direct contact La-silicate/Si structure based nMOSFETs

Abstract

This study focuses on studying the effective electron mobility in direct contact La-silicate/Si structure based nMOSFETs and searching for the difference of the mobility characteristics compared with the SiO2 MOSFETs. In this study, three types of gate electrode structure were prepared to investigate the mobility characteristics over a wide EOT range; W for EOT of 1.63nm, TiN/W for EOT of 1.02nm and metal-inserted poly-Si (MIPS) for EOT of 0.71nm. Since the silicate formation is basically caused by the presence of oxygen, Si layer in MIPS can suppress the oxygen in-diffusion from atmosphere, resulting in scaled EOT. It is found that the Eeff dependence of mobility with La-silicate is observed to differ from the mobility of SiO2 MOSFETs. The electron mobility with La-silicate shows the weaker Eeff dependence than the mobility of SiO2 nMOSFETs in middle and high Eeff region. This suggests an existence of additional mobility component related to the direct contact La-silicate/Si structure. The effective electron mobility is degraded with decreasing EOT in entire Eeff region. This means that the scattering sources including Coulomb scattering, phonon scattering and surface roughness scattering are located not at La-silicate/Si interface but the inside of gate stacks and approach the Si inversion channel. Coulomb scattering and phonon scattering are thought to be strengthened by increasing k-value because of the enhancement of Coulomb scattering potential and higher ionicity in La-silicate gate dielectrics. The influence of metal/high-k interface is also considered to affect on the mobility with decreasing the EOT.