Evaluation of nitrogen incorporation effects in HfO₂ gate dielectric for improved MOSFET performance
Cho, Hag-ju, 1969-
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The aggressive scaling of Si integration technology requires the thinning of SiO2 gate oxide. However, as the oxide thickness continues to decrease, the leakage current density becomes significantly excessive due to direct tunneling. Thus, high-k dielectric, which has a higher dielectric constant than SiO2, has been intensively studied for future I.C. application. Among various high-k dielectrics, HfO2 is considered to be the most promising candidate because it has a high dielectric constant ~25, large heat of formation, relatively large band gap ~6 eV, and is thermally stable with silicon, and appears to be compatible with Si processes. In this study, nitrogen incorporation in HfO2 has been studied for improving HfO2 device performance. Thermal nitridation of Si prior to HfO2 deposition is one of the methods for incorporating N. However, it resulted in degraded interface. Thus, top nitridation was explored to prevent oxygen and boron penetration into Si substrate while maintaining HfO2/Si interface. As a result of using HfON on HfO2, thermal stability and immunity to boron diffusion were improved. In addition, MOSFET device using the top HfON layer showed about 2 times higher drive current compared to HfO2. The improvement was enhanced by applying high temperature forming gas annealing at 600o C prior to Al metal deposition. However, it turned out that such advantages from the top HfON layer were very limited due to a very small amount of nitrogen (< 1%) To achieve higher nitrogen concentration at the top, HfSiON (k~12-16) was used. Nitrogen was incorporated in HfSixOy in a range of 12-28 at.%. None of nitrogen in the upper part of the dielectric diffused to the interface of HfO2 and Si substrate for anneals up to 800o C. Dielectric constant and crystallization temperature were found to increase as N increased. In addition to improved thermal stability and reduced boron diffusion, HfSiON/HfO2 (TSN) devices showed higher channel mobility and higher drive current compared to HfO2 devices. SiON interfacial layer between TSN and Si further reduced EOT without sacrificing hysteresis and Dit. Even higher nitrogen concentration at the top was achieved by NH3 annealing of TSN gate dielectric. The application of NH3 annealing to in-situ processed TSN resulted in EOT < 10 Å. The experimental results of this study suggest that nitrogen profile engineering for high-k materials is a promising technique to improve MOSFET performance.