(A) study on the fabrication and characteristics of ruthenium nanocrystal formed by atomic layer deposition

Abstract

Nanocrystal used as discrete floating gate has been particularly attractive due to its potential for next-generation nonvolatile memory devices. Major advantage of nanocrystal memory compared with conventional continuous floating gate memory is its immunity from local oxide defects by suppressing lateral migration of electrons between discrete floating gates. Moreover, metal nanocrystals have several advantages such as higher density of states around Fermi level and a wide range of available work functions compared to their semiconductor nanocrystal counterparts. Ruthenium is an attractive candidate for the nanocrystal memory because of its large work function(4.71eV) and chemical stability. In this work, two main schemes were applied to improve nanocrystal memory performance. The one is to obtain maximum nanocrystal density, by optimizing deposition condition. For better reliability, high nanocrystal density and uniform size are necessary. Ruthenium (Ru) nanocrystals were grown by plasma-enhanced atomic layer deposition (PEALD) using $Ru(C_5H_4C_2H_5)_2$ as precursor and $NH_3$ as reactant gas at a temperature of 270℃. By adopting $Al_2O_3$ film as substrate and $NH_3$ plasma as reactant gas, high density of Ru nanocrystal can be accomplished. As a result, maximum nanocrystal densities of $2.5*10^12/cm^2$, mean diameter of 3.8nm with narrow size distribution can be obtained and observed by transmission electron microscopy (TEM). To measure the electrical characteristics of the nanocrystal memory, MOS structure $(Si/Al_2O_3/Ru NC/Al_2O_3/Pt)$ was fabricated. $Al_2O_3$ is used as tunnel oxide and blocking oxide, because it has low leakage current, high bandgap, and high crystallization temperature. As nanocrystals and dielectric film are deposited by ALD process, devices were fabricated in one chamber without vacuum break. Charging characteristics of Ru nanocrystal structure has been investigated by capacitance-voltage (C-V) curve analysis. Large C-V hysteresi...