Atomic Layer Deposition of Ru Nanocrystals with a Tunable Density and Size for Charge Storage Memory Device Application Sung-Soo Yim, a Do-Joong Lee, a Ki-Su Kim, a,c Moon-Sang Lee, a,d Soo-Hyun Kim, b, * and Ki-Bum Kim a, * ,z a Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea b School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Gyeoungsangbuk-do 712-749, Korea We propose a deposition method capable of independently controlling the spatial density and average size of Ru nanocrystals by using both plasma-enhanced and thermal atomic layer deposition ALD. Plasma-enhanced ALD is used to promote the nucleation of Ru nanocrystals, while thermal ALD is used to assist their growth. By the rigorous selection of each stage, we can demonstrate the formation of Ru nanocrystals with a density variation from 3.5 10 11 to 8.4 10 11 cm -2 and sizes from 2.2 to 5.1 nm, which is in the optimum density and size range of nanocrystal floating-gate memory application. © 2008 The Electrochemical Society. DOI: 10.1149/1.2952432All rights reserved. Manuscript submitted April 14, 2008; revised manuscript received May 12, 2008. Published July 11, 2008. Nanocrystal NCfloating-gate memory NFGMdevices have been extensively investigated for the replacement of current flash memory devices, because a discrete NC layer provides charge- storage sites which are immune to stress-induced leakage through the tunnel oxide; thus, a relatively thin oxide can be used for the low-voltage operation. 1 In this type of device, it has been reported that the density and size of the NCs, as well as material types of the NC and the surrounding dielectrics, strongly affect device perfor- mance such as the threshold voltage shift V th , charging effi- ciency, and charge retention time. 2-4 Theoretically, it has been re- ported that lower density and larger size of the NCs are favorable in the aspects of charging efficiency and retention characteristics. 2 At the same time, however, a reasonably high density of the NCs up to 1 10 12 cm -2 is required in order to guarantee a sufficient memory window conventionally 1–2 V difference of the V th be- fore and after the NCs are charged. In addition, high density of the NCs is advantageous when the device size is as low as a few tens of nanometers because the deviation in the number of NCs per device can be statistically reduced. Therefore, the density and size of the NCs should be rigorously controlled in order to obtain the optimum NFGM performance. However, independent control of the density and size is a very dif- ficult task by employing any type of deposition processes such as physical vapor deposition PVDfollowed by thermal treatment, 5,6 chemical vapor deposition CVD, 7-9 and atomic layer deposition ALD. 10 For instance, in the case of a PVD-based process, in which many of the processes commonly have utilized thin-film agglomera- tion, density and size of the NCs are determined simultaneously by the nominal thickness of a starting film and the subsequent anneal- ing temperature. In the case of CVD and ALD processes, the origin of these difficulties comes from the fact that the nucleation and growth occur simultaneously during the formation of the NCs. Thus, the density and size of NCs are determined only by deposition time at the given deposition conditions, including precursor injection time, partial pressure, and the type of substrate. Certainly, one of the most promising ways to control density and size of NCs is to independently control the nucleation and growth stage during deposition. Ideally, it is hoped that the density of NCs is determined only by the nucleation stage, and the growth of these nuclei to the desirable size is controlled by the separated growth stage. A similar approach has been reported in the CVD-Si system using two different precursors of SiCl 2 H 2 and SiH 4 . 11 In their ap- proach, Si NCs were nucleated by exposure of SiH 4 to the SiO 2 substrate and followed by selective growth of NCs using SiH 2 Cl 2 . They could deposit Si NCs with the narrower size distribution by the two-step method. However, they did not report on the independent control of the density and size. In our previous publication, we pro- posed an ALD process for the formation of NCs, because ALD provides more exact control of the supply of adatoms through rep- etition of the ALD cycles, which is based on the self-limiting surface-saturated reaction mechanism. In particular, nucleation in the ALD Ru was found to strongly depend on both the method of reducing the precursors and the surface state of the substrate. 10,12-14 In this article, we propose a method capable of producing Ru NCs with a tunable density and size using a two-step deposition process that combines plasma-enhanced ALD PEALDand thermal ALD in order to find out the optimum process window for density and size of NCs. Ru NCs were deposited using a showerhead-type PEALD system at a temperature of 300°C and a pressure of 400 Pa. Ar was used as both a carrier and a purging gas. Diethylcyclopentadienyl ruthenium RuEtCp 2 vapor was generated in a bubbler at 80°C and carried by Ar at a flow rate of 100 sccm. For the PEALD, radio frequency plasma of NH 3 was used as a reactant at a plasma power of 100 W. The flow rates of NH 3 during pulsing and Ar during purging were both 150 sccm. For the thermal ALD, the reactant was O 2 at a flow rate of 20 sccm. The pulsing sequences were RuEtCp 2 pulsing, purging, reactant pulsing, and purging with durations of 5, 5, 15, and 3 s for the PEALD, and 5, 5, 10, and 5 s for the thermal ALD, respectively. Between PEALD and thermal ALD, we introduced an intermediate O 2 pulsing of 20 sccm for 500 s in order to suppress further nucleation. Thermally grown SiO 2 was used as a substrate for all of the processes. Detailed information about Ru ALD can be found elsewhere. 10,13 The spatial density, average size, and size dis- tribution of the Ru NCs were measured using plan-view and cross- sectional transmission electron microscopy TEM, JEOL JEM- 3000F with a field emission gun operated at 300 kV. We investigated the nucleation behavior of Ru deposited by PEALD and thermal ALD. As shown in Fig. 1, the PEALD process is quite effective in forming a high density of NCs, although the overall deposition rate is much lower. The data shows that the maxi- mum density of the NCs obtained from PEALD is almost 50 times higher than that obtained from thermal ALD. More importantly, there is quite a difference in the size distribution of the NC array. There is a small variation in the size of NCs in PEALD, while thermal ALD induces a large variation. This result clearly demon- strates that the overall deposition process of PEALD occurs by the formation of a high density of NCs and the slow growth of them, * Electrochemical Society Active Member. c Present address: Semiconductor Business, Samsung Electronics Co., Ltd., Yongin, Gyeonggi-do 446-711, Korea. d Present address: Samsung Advanced Institute of Technology, Suwon, Gyeonggi-do 440-600, Korea. z E-mail: kibum@snu.ac.kr Electrochemical and Solid-State Letters, 11 9K89-K92 2008 1099-0062/2008/119/K89/4/$23.00 © The Electrochemical Society K89 Downloaded 14 Jul 2008 to 147.46.136.237. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp