Stability of TaRh_x as a potential diffusion barrier for Cu metallization: capacitance–voltage tests after bias temperature stress

Amorphous TaRh_x was integrated in metal oxide semiconductor (MOS) capacitors with Cu gate metallization (Al/Si/25 nm SiO₂/10 nm TaRh_x/Cu). The stability of TaRh_x diffusion barriers was investigated under bias temperature stress testing using capacitance–voltage (C–V) measurements. The stability of these capacitors was compared with similar capacitors with TaN_x as the diffusion barrier layer or with capacitors with no diffusion barrier. The electrical measurements were compared with compositional information obtained by backside secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM). Comparison of C–V measurements on capacitors with TaRh_x/Cu and TaRh_x/Al gate metallizations revealed that both mobile alkali ions and Cu⁺ contribute to the flatband voltage shift of the MOS capacitors. C–V measurements of capacitors with various barriers showed that the mobile ion concentration (Cu⁺) in the capacitors with TaRh_x diffusion barriers is reduced to 32 % of the value for capacitors with no diffusion barrier. In contrast, capacitors with TaN_x barriers showed the lowest mobile ion concentration among all the barrier types. Based on flatband voltage shift values, TaN_x barriers outperform TaRh_x barriers. However, if the percentage of deformed C–V curves is used as the reliability criterion, both barriers perform quite similarly. Compositional analysis data suggests that the concentrations of the diffused species are below the detection limits of SIMS and TEM. This work demonstrated the importance of employing electrical reliability tests for evaluation of potential diffusion barrier materials.