Investigation of the interlayer characteristics of Ta2O5 thin films deposited on bare, N2O, and NH3 plasma nitridated Si substrates

Abstract

Low-temperature N2O and NH3 plasma nitridations on Si surfaces are conducted to prevent the thermodynamic instability at the Ta2O5/Si interface. The surface and interface of the Ta2O5/Si systems, without or with nitridation, were examined by x-ray photoelectron spectroscopy (XPS), atomic force microscopy, transmission electron microscopy, and secondary ion mass spectrometry. Capacitance–voltage (C–V) measurements were carried out to investigate the electric/defect characteristics of the Ta2O5/Si systems. For the non-nitrided Si substrate XPS detects no surface oxide formed prior to Ta2O5 deposition. In contrast, NH3 plasma nitridation forms a nitrogen-rich SiOxNy layer, while N2O plasma nitridation produces an oxygen-rich SiOxNy layer on Si. C–V measurement reveals high densities of fixed charges, trapping sites, and interface states in the Ta2O5/non-nitrided Si sample, indicative that a defective interlayer was formed during Ta2O5 deposition. Ta2O5 on both nitrided systems exhibits a reduced amount of fixed charges and trapping sites. Nevertheless, NH3 plasma nitridation is not as effective as N2O plasma nitridation in diminishing the density of interface states. The different electric/defect characters between the NH3 and N2O nitrided systems are attributed to the further oxidation of the nitrogen-rich SiOxNy layer produced by NH3 plasma nitridation, which induces defects at the dielectric/Si interface. Effectiveness of the nitridation process, therefore, depends on the stability of the nitrided layer during deposition of Ta2O5.