Impact of Si substrate nitridation on electrical characteristics of Ta2O5 stack capacitors

Abstract

A comparison of the effect of rapid thermal nitridation (RTN) of the Si surface in N2O and NH3 ambient at different temperatures (700–850 °C) on the dielectric and electrical characteristics of thin (∼20 nm) Ta2O5 stacks has been made. The electrical parameters of capacitors (film permittivity, oxide charge, densities of bulk traps, interface and slow states, leakage current) are discussed in terms of the impact of N incorporation in the interface region. The films on both types of RTN-treated Si exhibit ∼100 times lower leakage current than Ta2O5 on bare Si, but among the two RTN processes NH3 nitridation is more beneficial since only it simultaneously increases also the stack permittivity. This improvement in parameters is suggested to be due to a real nitridation of Si surface which occurs under the NH3 rapid thermal process. RTN in N2O does not produce resistance to the oxidation substrate and it could explain the observed lack of stack dielectric constant improvement. The composition of the interfacial layer under NH3 RTN appears to be TaSi-oxinitride-like, while the interface region at N2O-nitrided Si seems to be SiO2-like. Each RTN process, however, modifies the Si surface and constitutes a specific interface layer different from that at the bare Si substrate. The composition of this layer defines parameters of the traps close to the substrate, the barrier height at the Ta2O5/interface layer and influences the conduction mechanisms in the stacks.