Evaluation of lanthanum based gate dielectrics deposited by atomic layer deposition

Abstract

In this article, we evaluated physical and electrical characteristics of La-based gate dielectrics (La2O3 and LaAlxOy) deposited by atomic layer deposition (ALD). The precursors used for La2O3 and LaAlxOy are lanthanum tris[bis(trimethylsilyl)amide] La[N(SiMe3)2]3, trimethyl aluminum [Al(CH3)3], and water. Physical properties of these dielectric films were studied using ellipsometry, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). To investigate electrical properties of these La-based dielectrics, metal oxide semiconductor capacitors (MOSCAPs) were fabricated using metal gates (Ta–Si–N, TiN and Pt). Linear growth rate characteristics were observed for ALD (La2O3 and LaAlxOy films deposited at temperatures of 225to275°C. XPS and XTEM analysis of La-based films grown on a chemical oxide starting surface revealed a rough La-based dielectric/Si interface and chemical interaction with the Si substrate. In general, adding Al into La2O3 improved electrical properties of the films. Devices with La based dielectric deposited on a ∼10Å Al2O3 underlayer had better capacitance-voltage characteristics compared to those deposited directly on a chemical oxide surface. Adding Al to the dielectric also resulted in lower leakage current and smaller hysteresis. For devices with Ta–Si–N gates, a significant decrease in maximum capacitances was observed after forming gas annealing, probably due to interaction between the gate electrode and the dielectric. XTEM images for these devices indicated an indistinct interface between the Ta–Si–N gate and the La-based dielectrics. The XTEM images also showed microcrystals in Ta–Si–N that may be formed in Si deficient regions of the metal gate. No interaction between TiN or Pt with La gate dielectrics was observed by XTEM up to 800°C annealing temperature. After 900°C annealing, some interaction between LaAlxOy and Pt gate was observed. Our results indicated that silicon substrate interactions may limit the utilization of ALD La based dielectrics in future complementary metal-oxide semiconductor processing.