Physical and optoelectronic characterization of reactively sputtered molybdenum-silicon-nitride alloy metal gate electrodes

Abstract

With continued transistor scaling, work function tuning of metal gates has become important for advanced complementary-metal-oxide-silicon applications. The work function tuning of reactively sputtered MoxSiyNz (also referred to as MoSiN) gates has been studied through the incorporation of nitrogen. The nitrogen concentration in the MoSiN films was altered by controlling the gas flow ratio, RN=N2/(N2+Ar), during gate deposition. The sheet resistance (Rs) of blanket MoSiN films, measured using four-point resistance method, was found to increase as the gas flow ratio was varied from 10% to 40%. Current-voltage measurements confirmed excellent electrical stability of MoSiN/SiO2/p-Si gate stack for applied electric fields ranging up to 6 MV/cm. High frequency capacitance-voltage measurements were used to extract the MoSiN work function (Φm) using the relationship between the flatband voltage (VFB) and the oxide thickness (tox). The extracted MoSiN/SiO2 interfacial barrier heights, obtained through the internal photoemission of electrons, were used to corroborate the extracted values of MoSiN work function. The MoSiN work functions (Φm), extracted independently using both techniques, were consistent and were observed to decrease with increasing gas flow ratio [N2/(N2+Ar)]. Secondary ion mass spectrometry depth analysis revealed uniform distribution of nitrogen throughout the bulk MoSiN films, with no piling up at gate-dielectric interface. X-ray photoelectron spectroscopy surface analysis suggested a steady increase in the Mo–N bonds, and therefore the total nitrogen concentration (from ∼20% to 32%), as the gas flow ratio is increased from 10% to 40%. A similar trend was observed in the nitrogen concentration (in percent), measured using Rutherford backscattering spectroscopy, for these gate deposition conditions. These material characterization results demonstrate that the increase in nitrogen concentration in MoSiN films is consistent with the lowering of MoSiN work function. Moreover, the work function of reactively sputtered MoxSiyNz films was found to be variable over ∼0.3 eV by adjusting the nitrogen concentration.