Effect of process condition for crystallinity and energy band structure at atomic layer deposited ZrO2 films using Cp-Zr precursor

Abstract

With scaling down of dynamic random access memory (DRAM) technology, several alternative materials were applied to dielectric material of metal-insulator-metal (MIM) capacitor instead of SiO2 or SiON used in the past. Capacitor dielectric materials are required to thin equivalent oxide thickness (EOT) and wide band gap. Half pith 2x nm of DRAM is required to lower than 0.5 nm of EOT. However, thin EOT leads to disadvantage such as high leakage current density by direct tunneling. Therefore, DRAM capacitor dielectrics have been investigated for high value of dielectric constant and wide energy band gap. Although several binary oxides such as TiO2, HfO2, and ZrO2 are investigated due to their high value of dielectric constant, HfO2 exhibits insufficient thermal stability and causes chemical reaction with poly-Si. In addition, TiO2 shows high leakage current because of its low band gap and a particular noble metal as a bottom electrode is required to representing its high dielectric constant. On the other hand, ZrO2 is attracted as replacement for capacitor dielectric due to compatibility with silicon and high-k material and wide band gap. In this study, ZrO2 films were deposited using thermal ALD process by CpZr(N(CH3)2)3 as a precursor and ozone as a reactant, respectively. Si (100) substrate was used for the process window test and diluted HF (200:1) cleaning was carried out before ZrO2 deposition. In process window test, to measure the growth rate and chemical composition, it was used to aspectroscopic ellipsomerty (SE) and Auger electron spectroscopy (AES), respectively. Moreover, chemical binding energy state and the crystallinity of films were investigated by X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), respectively. The optical band gap was measured by UV-SE. Fermi level was extracted from ultra-violet photoelectron spectroscopy (UPS). The MIM capacitor was fabricated for electrical characteristic measurements using TiN and Au as a bottom electrode and top electrode, respectively. ZrO2 films exhibit strong phase transition and energy band gap change by tuning the process temperature and film thickness. The ZrO2 films appeared nearly stoichiometric in the process window region. The phase transition from monoclinic to tetragonal was observed as the temperature increased. Furthermore, the crystallinity of the ZrO2 is significantly affected by the physical thickness. In addition, the energy band gap was differed with crystallinity of ZrO2. We discuss that relation between energy band structure and crystalline phase transition in terms of the crystal field splitting. Either, it was demonstrated that valence band maximum was decrease under decreasing ozone concentration and which is related to oxygen vacancy reduction. Also, leakage current density of negative bias decreased with a reduction of oxygen vacancy defects.