A study on the material and device characteristics of hafnium oxynitride MOSFETs with TaN gate electrodes

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Kang, Changseok

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HfO2 have been under intense investigation for gate dielectric application into the 70 nm technology nodes and beyond to replace conventional SiO2 or oxynitrides since it possesses a dielectric constant of 22 – 25, a large band gap of 5.6 eV with sufficient band offsets of larger than 1.5 eV, and is thermally stable in contact with silicon and metal gates. However, HfO2 is vulnerable to the diffusion of oxygen that causes formation of a low-k interfacial (silicon oxide or silicate) layer at the Si interface and boron penetration when combined with the p+ poly-Si gate technology. In addition, HfO2 crystallizes at relatively low temperature (<600 oC) unlike SiO2 that remains in amorphous phase through the semiconductor process involving high temperature annealing higher than 1000 oC. In this research, the focus was placed on the effects of nitrogen in HfO2 dielectrics to improve thermal stability (i.e. equivalent oxide thickness (EOT) increase by anneal and crystallization) of the dielectrics by surface treatment and nitrogen incorporation into the HfO2. In addition to the Hf-based dielectrics themselves, effects of Hf into conventional SiO2 dielectrics and the characteristics of TaN for gate electrode application were studied. The effects of Hf implanted into p-type Si substrates on the properties of n+ polycrystalline-Si/SiO2/Si capacitors and MOSFETs have been investigated. Flat-band voltages (Vfb ) and substrate doping concentrations ( NA ) calculated from high frequency C-V curves of the capacitors were not dependent on the doses of Hf. Also, electron channel mobility was not degraded by Hf contamination. The amount of Hf diffused into the Si substrate during the high- k dielectric imposed negligible effects on silicon based MOS device characteristics in terms of C-V, J-V characteristics and electron mobility. In this work, sputtered-TaN was mainly used as a gate electrode. Work functions of tantalum nitride (TaN) film before and after post-metal-annealing were ~ 4.15eV, and ~ 4.7 eV, respectively. A surface nitridation technique using NH3 anneal has been investigated to reduce interfacial reactions and consequently the equivalent oxide thickness (EOT) of TaN/HfO2/ Si MOS capacitors. For the same EOT, the nitrided samples showed 1 ~ 2 order of magnitude lower leakage current density compared to the non-nitrided ones. However, nitridation induced higher interface state density and larger hysteresis. The degraded interface quality due to the nitridation was improved by post-metal annealing (PMA). Electrical and material characteristics of hafnium oxynitride (HfON) gate dielectrics have been studied in comparison with HfO2. HfON was prepared by a deposition of HfN followed by post-deposition-anneal (PDA). By secondary ion mass spectroscopy (SIMS), incorporated nitrogen in the HfON was found to pile up at the dielectric/Si interface layer. Based on the SIMS profile, the interfacial layer (IL) composition of the HfON films appeared to be similar to hafnium- silicon-oxynitride (HfSiON) while the IL of the HfO2 films seemed to be hafnium-silicate (HfSiO). HfON with nitrogen of ~ 5 atomic % at the interface resulted in an increase in crystallization temperature, higher dielectric constant, lower leakage current at the same equivalent oxide thickness (EOT) and high dielectric strength compared to HfO2. The improved electrical properties of HfON over HfO2 can be explained by the thicker physical thickness of HfON for the same EOT due to its higher dielectric constant as well as a more stable interface layer. High temperature forming gas anneal at 600oC for 30 min was effective in improving carrier mobility of nMOSFETs with HfON gate dielectrics. The effects of silicon and nitrogen profiles in gate dielectrics on the electrical and material properties of the Hf-based dielectric were investigated. To vary nitrogen profiles in the HfON films, 6-Å-thin Si layers were inserted in the different position of HfON films. By the insertion of Si layer, nitrogen profiles were modulated. The inserted Si leading to the nitrogen incorporation increased thermal stability. Highest mobility was observed at the dielectrics with Si layer inserted in the middle of HfON films.