TUNING THE EFFECTIVE WORK FUNCTION OF TITANIUM NITRIDE DEPOSITED BY PLASMA ENHANCED ATOMIC LAYER DEPOSITION

The implementation of nitride-based metallic compounds as metal gate materials is attractive for complementary metal-oxide-semiconductor (CMOS) applications. The metal`s effective work-function (EWF) determines the transistor`s threshold voltage, and its tuning is important and challenging. The plasma enhanced atomic layer deposition (PEALD) technique enables the deposition of various metals and their nitrides or carbides and provides additional degrees of freedom for EWF engineering.

The current work presents the EWF tuning of TiN deposited by PEALD using various reactive gases: N₂, H₂, NH₃ and N₂/H₂ mixtures. The thickness, microstructure, chemical composition and electrical resistivity of the films were determined by X-ray reflectivity (XRR), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and four-point probe measurements, respectively. The EWF of TiN on SiO₂ was determined using capacitance-voltage measurements and plotting the flat-band voltage versus the oxide thickness.

TiN films, with a thickness of 25±1[nm], were found to be polycrystalline and possessed low resistivity values of 242±15, 134±3, 207±8 [μΩ∙cm] for N_2, NH₃, H₂ reactive gases, respectively. The preferred orientation of the films depended on the reactive gas used: they were (111) oriented when H₂ plasma was used, (100) oriented for N₂ plasma, and NH₃-produced TiN was in an intermediate state, as presented in Fig. 1. These differences affected the TiN/SiO₂ interface electrical and structural properties. The obtained EWF values were 4.5±0.1, 4.85±0.05, 5.2±0.1 [eV] for N_2, NH₃, H₂ reactive gases, respectively. Thus, the position of TiN Fermi level at the metal/oxide interface can be tuned from Si midgap level towards Si valence band edge.