Charge storage characteristics of self-assembled NiSix nanocrystals embedded in a metal-oxide-semiconductor structure

Abstract

Recently, memory-cell structure using discrete traps as the charge storage media has received much attention as the promising candidate for future high speed, ultrahigh density flash type memory devices. The metal nanocrystal memory possesses several advantages, such as a contorlable work functions and higher density of states around the Fermi Level. Nickel silicide is considered to be one of the most suitable materials for deep submicron, self aligned silicide (salicide) applications due to their lower morphological and thermal stability >550°C which allows them to form self-assembled nanocrystals by controlling annealing temperature and time. Here, we investigated the charge storage characteristics of self-assembled NiSix nanocrystals embedded in a metal-oxide-semiconductor (MOS) structure. A p-type Si with ~3 nm surface oxide was used as the substrate. Rf sputtering was used to deposit 3~5 nm thick NiSix on the oxide surface. Then 20 nm of SiO2 was sputtered as the control oxide. Rapid thermal annealing (RTA) was performed to breakup the thin film into self-assembled nanocrystals. Cr/Au electrodes were defined and capacitance-voltage (C-V) measurements were performed to evaluate the charge storage characteristics depending on sputtered thickness and RTA temperature and duration. Varying the RTA temperature between 700~900 °C and the duration between 30 ~ 120 s resulted in nanocrystals with 3~10 nm diameters. Threshold voltage shifts between 2 to 4 V were observed for NiSix with initial thickness between 3 to 5 nm. It was found that the C-V characteristics of NiSix nanocrystals formed without the control oxide deposed before RTA, showed variation in threshold voltages compared to nanocrystals formed within the control oxide, which may be due to higher surface mobility resulting in a variation of nanocrystal dimensions giving rise to variations in nanocrystal capacitance. We will also demonstrate non-volatile memory operation using self-assembled NiSix nanocrystals as the nano-floating gate.