Scaling of hafnium-based high-k dielectrics

In this paper, various approaches to extend scalability of Hafnium-based dielectrics are reported. Among the three crystal phases of HfO₂ (monoclinic, cubic and tetragonal), the tetragonal phase has been reported to have the highest dielectric constant. Tetragonal phase stabilization by crystallizing the thin HfO₂ using a metal capping layer and by adding zirconium is demonstrated. The microstructure, morphology, optical properties and impurities of Hf_xZr₁-_xO₂ dielectrics (for 0>x>1) are discussed. Subtle but important modification to high-k / Si interface characteristics resulting from addition of Zr into HfO₂ is reported. To further boost the dielectric constant of hafnium-based dielectrics, incorporation of TiO₂, which has been reported to have high dielectric constant, is explored. Hf_xZr ₁-_xO₂/TiO₂ bilayer films were fabricated. 30 & TiO₂ films were deposited on a 5, 8, 12 or 15 & Hf_xZr₁-_xO₂ underlayer to determine the minimum thickness needed to maintain good thermal stability with Si substrate. CV and IV results indicated that 12-15 & is the optimal thickness range for the Hf_xZr₁-._xO₂ underlayer. A dielectric constant as high as 150 for TiO₂ layer is extracted from TiO₂ thickness series deposited on 12 & Hf _xZr₁-_x. O₂ underlayer. In addition to increasing the k-value of Hafnium-based dielectrics, it is important that the threshold voltage of these high-k devices is low. Here we report the use of thin A1₂O₃ capping layers to modulate PMOS threshold voltages. About 100 mV reduction in threshold voltage is achieved by capping HfO₂ with a 5D Al₂O₃ film. Finally, dielectric scaling by modifying the Si / high-k interfacial layer is attempted. Nitrogen incorporation into Hf_xZr₁-_xO₂ is shown to be a simple and effective method to lower the capacitance equivalent thickness (CET) of Hafnium-based dielectrics.