(A) study on the $HfO_2/Al_2O_3$ nanolaminated thin film by plasma-enhanced atomic layer deposition

Abstract

The downscaling of the DRAM devices is necessary to achieve higher speed with less power consumption. It is getting difficult to meet the new requirements with the existing $SiO_2$ or $Si_3N_4$ due to their low dielectric constants and tunneling leakage currents through the thin layers. For this reason, high-k materials enabling high-k and low leakage currents with physically thicker film have received considerable attention. $HfO_2$ and $Al_2O_3$ nanolaminated films attract particular attention as a promising alternative to $SiO_2$ or $Si_3N_4$ from high-k and low leakage current considerations. In this work, $HfO_2$ and $Al_2O_3$ nanolaminated films deposited by plasma enhanced atomic layer deposition (PEALD) using tetrakis(ethylmethylamide) hafnium $(TEMAHf, Hf[N(CH_3)(C_2H_5)]_4)$ and tri-methyl-aluminum $(TMA, Al(CH_3)_3)$ as metal precursors, respectively, and $O_2$ plasma as an oxidant were studied. The film characteristics were analyzed by structural, electrical, and compositional points. To control the sub-layer thickness in nanolamination, the sub-layer thickness depending on the number of unit cycles was examined by TEM image. When the number of unit cycle exceeded 10, the deposited film thickness linearly increased with the number of cycle, showing the typical ALD characteristic. Therefore, precise thickness control was possible by varying the number of cycle. Crystallization of $HfO_2$ film was suppressed by nanolamination with $Al_2O_3$. As the $HfO_2$ sub-layer thickness increased, more crystallization of the nanolaminated film was observed after annealing. However, the nanolaminated films with $HfO_2$ sub-layer thickness below 50Å had micro-crystals embedded in amorphous matrix, and 2 orders lower leakage current density than that of fully crystallized films, such as pure $HfO_2$ film. The leakage current density had relationship with crystallization which was affected from the $HfO_2$ sub-layer thickness. Therefore, the $HfO_2$ sub-layer thi...