Preparation of NiO Monolayer by Langmuir–Blodgett Technique and Its Characterization as Diffusion Barrier for Copper Metallization

Abstract

Langmuir–Blodgett (LB) technique was used to prepare monolayers of NiO over SiO₂/Si substrate. Diffusion barrier capability of NiO layer against the diffusion of copper into the dielectric was evaluated. Deposition and structure of the NiO layer were analyzed using X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and atomic force microscopy (AFM) techniques. Thermal stability of Cu/SiO₂/Si and Cu/NiO/SiO₂/Si test structures was compared using X-ray diffraction (XRD), scanning electron microscope (SEM), and four probe techniques. The samples were annealed at different temperatures starting from 473 K up to 873 K (200 °C up to 600 °C) in vacuum for 30 minutes each. XRD and SEM results indicated that combination of NiO/SiO₂ worked as diffusion barrier up to 773 K (500 °C), whereas SiO₂ alone could work as barrier only up to 573 K (300 °C). Sheet resistance of these samples was measured as a function of annealing temperature which also supports XRD results. Capacitance–voltage (C–V) curves of these structures under the influence of biased thermal stress (BTS) were analyzed. BTS was applied at 2.5 MV cm⁻¹ at 423 K (150 °C). Results showed that in the presence of NiO barrier layer, there was no shift in the C–V curve even after 45 minutes of BTS. Little shift was observed after 60 minutes of BTS while in the absence of barrier there was a significant shift in the C–V curve even after 30 minutes of BTS. For the structure with NiO barrier, the threshold voltage (V _t) was almost unchanged up to 60 minutes under BTS while V_t of structure without barrier changed significantly even after 15 minutes of stress. Further, these test structures were examined for leakage current density (j _L) at same BTS conditions. It was found that the Cu/NiO/SiO₂/Si test structure could survive about one and half time more than the Cu/SiO₂/Si test structure.