Comprehensive Study and Atomic Layer Deposition of HfO2 Process Development Using Novel Hf Alkoxide Precursors

Abstract

Due to the scale down of the semiconductor device, the SiO2 based dielectric has been replaced with a high dielectric constant dielectric due to the decrease in the reliability of the gate oxide against electric breakdown and the increased leakage current due to the direct tunneling of electrons. Among the various high-k dielectric materials, HfO2 is considered one of the most suitable materials due to its high dielectric constant, good thermodynamic stability, and high band gap energy. Atomic layer deposition of high quality HfO2 has emerged as a key technology in ultra-thin and high-k dielectrics. The thin film deposited by ALD is formed through surface chemical reactions that occur only on the substrate surface because precursors and reactants are sequentially injected and do not contact in the gas phase during the ALD process. The surface reaction depends on the chemical properties, which can affect the growth properties and film properties. Therefore, proper selection of precursors is very important for the potential application of ALD HfO2. Numerous studies have been reported investigating the effect of Hf precursors on the growth and film properties of ALD HfO2. In the case of halide precursors, it was used to avoid electrical degradation due to carbon impurities. However, it is difficult to apply the halide precursor to the ALD HfO2 process due to the high thermal budget. On the other hands, alkylamide precursors are the most widely used metalorganic precursors owing to their low melting points and high vapor pressure, but thermal decomposition of the alkylamide precursors easily occurs at temperature of ~300 °C due to their poor thermal stability. Among the various type of Hf precursors, alkoxide precursors are considered as attractive precursor to deposit C-free films owing to its strong inherent metal-oxygen bonding. However, ALD of HfO2 using alkoxide precursors exhibits non-saturated growth characteristics caused by undesired chemical reaction. Chain reaction of alkoxide precursor hydrolysis or ligand exchange occurs continuously because vapor phase H2O, alcohol and surface hydroxyl (O-H) can be generated from ligand degradation by β-hydride elimination. In addition, the alkoxide precursor requires a relatively high deposition temperature due to the ligand exchange reaction between the alkoxide ligand and the surface OH group due to strong Hf-O bonds. It is difficult to apply alkoxide precursors for ALD process due to thermal decomposition due to high process temperature. To solve the problem, it is necessary to modify the chemical properties of the precursors, such as thermal stability, reactivity. Suitable properties of the precursor for ALD application can be obtained by changing the precursor structure. Despite numerous efforts to develop new precursors, only limited success has been achieved by previous studies. Thus, the development of new types of ligand and precursors is needed to solve non-saturation behavior and improve thermal stability. In addition, a systematic study is needed to reveal the effect of modifying the precursor molecular structure on improving growth properties and thermal stability. In this study, we report a comprehensive investigation into the synthesis and evaluation of new precursors to overcome the weaknesses of existing alkoxide precursors. Hf alkoxide precursors were newly synthesized by Korea Research Institute of Chemical Technology designed for deposition of HfO2 with low impurities. New type of β-diketonates ligand was employed to improving thermal stability and inhibition of undesired reactions in ALD process. In addition, Cyclopentadienyl ligand was employed to develop heteroleptic precursor. We also investigated the impact of ligand substitution on growth characteristics, chemical compositions, film density and crystallinity of ALD HfO2 using new precursors. The electrical properties including the dielectric constant and leakage current density were evaluated for applications. Furthermore, we theoretically investigated the effects of ligand substitution on mechanism of precursor intermolecular reactions which can cause non-saturated growth by density functional theory (DFT) calculations used for calculating the reaction energy.