16 Hafnium - based High - k Gate Dielectrics

Scaling of silicon dioxide dielectrics has once been viewed as an effective approach to enhance transistor performance in complementary metal-oxide semiconductor (C-MOS) technologies as predicted by Moore’s law [1]. Thus, in the past few decades, reduction in the thickness of silicon dioxide gate dielectrics has enabled increased numbers of transistors per chip with enhanced circuit functionality and performance at low costs (Fig. 1). However, as devices approach the sub-45 nm scale, the effective oxide thickness (EOT) of the traditional silicon dioxide dielectrics are required to be smaller than 1 nm, which is approximately 3 monolayers and close to the physical limit (Fig. 2), thus resulting in high gate leakage currents due to the obvious quantum tunneling effect at this scale (Fig. 3). To continue the downward scaling, dielectrics with a higher dielectric constant (high-k) are being suggested as a solution to achieve the same transistor performance while maintaining a relatively thick physical thickness [2]. Many candidates of possible high-k gate dielectrics have been suggested to replace SiO2 and they include nitrided SiO2, Hf-based oxides, and Zr-based oxides. Hf-based oxides have been recently highlighted as the most suitable dielectric materials because of their comprehensive performance. One of the key issues concerning new gate dielectrics is the low crystallization temperature. Owing to this shortcoming, it is difficult to integrate them into traditional CMOS processes. To solve these problems, additional elements such as N, Si, Al, Ti, Ta and La have been incorporated into the high-k gate dielectrics, especially Hf-based oxides. In the following sections, the requirements of high-k oxides, brief history of high-k development, various candidates of high-k, and the latest hafnium-based high-k materials are discussed.