Paper EM-FrM8
Thermal Stability of Rare Earth Oxides as High-k Gate Dielectrics

Friday, October 19, 2007, 10:20 am, Room 612

Chip performance has drastically increased during the past decades, pushed forward by the ITRS roadmap that projects a doubling of the amount of transistors on a chip every two years. This task has been mainly accomplished by down scaling the dimensions of the transistor, including the gate dielectric thickness down to a few atomic layers. As a result, gate leakage current densities have reached unacceptable levels. As a solution alternative gate dielectrics, replacing the standard SiON, were researched, leading to the identification of Hf-based dielectrics, recently announced to be integrated in the 45-nm technology node. Another class of alternative dielectrics that receives a lot of attention is the rare earth oxides, both as binary or ternary compounds. Rare earth oxides are being explored for their property to shift the work function of a metal gate towards n-type, which is of interest to engineer a proper transistor threshold voltage. In addition, ternary rare earth oxides have been studied because of their thermal stability that allows obtaining a material that remains amorphous during the entire CMOS process. We have studied the thermal behavior and stability of rare earth based oxides, such as Dy₂O₃, La₂O₃, LaAlO₃, HfLaO_x, and DyScO₃, as function of anneal temperature, time, and ambient, as well as function of their composition. Observations were made by Spectroscopic Ellipsometry, Time-Of-Flight-Secondary Ion Mass Spectroscopy, Transmission Electron Microscopy, X-Ray Reflectometry, and X-Ray Diffraction. The rare earth oxide strongly intermixes with the SiO₂ layer underneath, in agreement with the tendency of rare earth elements to form silicates.¹ More interestingly, for ternary rare earth oxides, this behavior is heavily dependent on the composition of the deposited layer. The system evolves to a stable composition that is controlled by the thermal budget and the rare earth content of the layer. Understanding this behavior is important, since it provides a better insight in the behavior of these dielectrics during the thermal treatments inherent to a CMOS process flow. Finally, as an increased Si-content, resulting from silicate formation, can increase the thermal stability of a material, proper care needs to be taken interpreting thermal stability measurements, most often based on X-Ray Diffraction.

¹ H. Ono et al., Appl. Phys. Lett., 78, 1832 (2001).