Evaluation of nitrogen incorporation effects in HfO₂ gate dielectric for improved MOSFET performance
Abstract
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.