Atomic layer deposition of high-density Pt nanodots on Al2O3 film using (MeCp)Pt(Me)3 and O2 precursors for nonvolatile memory applications
© Ding et al.; licensee Springer. 2013
Received: 30 November 2012
Accepted: 8 January 2013
Published: 15 February 2013
Pt nanodots have been grown on Al2O3 film via atomic layer deposition (ALD) using (MeCp)Pt(Me)3 and O2 precursors. Influence of the substrate temperature, pulse time of (MeCp)Pt(Me)3, and deposition cycles on ALD Pt has been studied comprehensively by scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Therefore, Pt nanodots with a high density of approximately 2 × 1012 cm-2 have been achieved under optimized conditions: 300°C substrate temperature, 1 s pulse time of (MeCp)Pt(Me)3, and 70 deposition cycles. Further, metal-oxide-semiconductor capacitors with Pt nanodots embedded in ALD Al2O3 dielectric have been fabricated and characterized electrically, indicating noticeable electron trapping capacity, efficient programmable and erasable characteristics, and good charge retention.
KeywordsAtomic layer deposition Pt nanodots Nonvolatile memory
Platinum (Pt) nanodots or nanoparticles have been attracting more and more attention due to their various potential applications. As a catalyst, Pt nanodots have been extensively used in the petroleum reforming and petrochemical industries as well as in fuel cells because of their excellent catalytic activity [1–4]. On the other hand, Pt nanodots have also been investigated for memory devices that utilize discrete metal nanodots as charge storage medium [5, 6]. This is attributed to the potential that the nanodot-based memories can lessen the impact of localized oxide defects, lateral coupling of charge storage layers between adjacent devices, and stress-induced leakage current . Moreover, Pt has a high work function of 5.1 eV, low diffusivity, and excellent thermal stability [6–8]. Therefore, the employment of Pt nanodots can obtain a deep potential well in memory devices to ensure good data retention, together with good compatibility with CMOS processing. However, most researchers used high-temperature rapid thermal annealing (RTA) of ultrathin Pt films to achieve high-density Pt nanodots [5, 8, 9], which might cause the formation of an additional interfacial layer between the high-permittivity (high-k) tunnel layer and silicon substrate as well as crystallization of the tunnel layer.
In recent years, atomic layer deposition (ALD) of Pt nanoparticles have been investigated on various surfaces such as micron-sized porous silica gel particles , SrTiO3 nanocubes , WC , and SiO2 film . However, most of them are used for catalyst. Although Novak et al. reported ALD Pt nanoparticles for memory applications, their study relates only to deposition cycles rather than the effect of substrate temperature and pulse time of the precursor on the growth behavior of Pt nanoparticles . Moreover, the ALD technique is also attempted to grow other metallic nanodots for memory applications, such as Ru, WN, and RuO x nanodots [13–15]. It is worthwhile to mention that by means of the ALD technique, high-density metal nanodots can be obtained at much lower temperatures compared to high-temperature RTA of ultrathin metal films [16, 17]. On the other hand, to further improve retention time and ensure low-voltage operation, recent efforts have been focused on high-k dielectrics to replace SiO2 as a gate oxide in nanodot floating gate memories . Among high-k dielectrics, Al2O3 has been widely studied due to its dielectric constant of approximately 9, a large bandgap of 8.9 eV, a large band offset of 2.8 eV with respect to silicon, good chemical and thermal stabilities with the silicon substrate, and amorphous matrix at high temperature . Therefore, in this article, the ALD growth of Pt nanodots on Al2O3 films has been investigated comprehensively, and the experimental parameters are optimized for high-density Pt nanodots. Further, metal-oxide-semiconductor (MOS) capacitors with Pt nanodots have been fabricated, and the corresponding memory characteristics are measured.
Firstly, around 8-nm Al2O3 films were deposited on cleaned P-type silicon substrates by ALD using the precursors Al(CH3)3 and water. Subsequently, the ALD growth of Pt nanodots were carried out on the surface of Al2O3 film using (MeCp)Pt(Me)3 and O2 precursors in a commercial tool (TFS 200, Beneq, Vantaa, Finland). Herein, the precursor (MeCp)Pt(Me)3 was kept at 70°C, the vapor of which was pulsed into the reaction chamber by the carrier gas argon (99.999%). High-purity O2 (99.999%) was pulsed into the reaction chamber through a separate gas line with a flow rate of 100 sccm. During the ALD process, the working pressure in the deposition chamber was maintained at 5 mbar, and the O2 pulse time was fixed at 0.1 s. To obtain the optimal process conditions, the influences of substrate temperature, pulse time of (MeCp)Pt(Me)3, and reaction cycles on Pt nanodot growth were investigated respectively. Further, to investigate the characteristics of Pt nanodots as charge storage nodes, the Al gate MOS capacitors with 8-nm Al2O3/Pt nanodots/24-nm Al2O3 were fabricated; herein, Pt nanodots were deposited under optimized conditions (shown later). As a comparison, a MOS capacitor without Pt nanodots was also fabricated.
The thicknesses of Al2O3 film was measured by an ellipsometer (SOPRA GES 5E, Courbevoie, France). ALD of Pt was characterized by field emission scanning electron microscope (FE-SEM; JSM-6700 F, JEOL, Tokyo, Japan), high-resolution transmission electron microscope (HR-TEM), and X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra DLD). Capacitance-voltage (C-V) measurements were performed on a LCR meter (Keithley 590, Cleveland, OH, USA), and voltage pulses were generated by a pulse/pattern generator (Keithley Model 3402).
Results and discussion
Impact of substrate temperature on ALD Pt nanodots
Influence of (MeCp)Pt(Me)3 pulse time on ALD Pt nanodots
Influence of deposition cycles on ALD Pt
Memory characteristics of MOS capacitors with Pt nanodots
Growth of Pt nanodots on the surface of Al2O3 has been investigated by ALD using (MeCp)Pt(Me)3 and O2 precursors. By optimizing substrate temperature, pulse time of (MeCp)Pt(Me)3, and deposition cycles, Pt nanodots with a high density of approximately 2 × 1012 cm-2 have been achieved, i.e., the process parameters are as follows: substrate temperature 300°C, (MeCp)Pt(Me)3 pulse time 1 s, and 70 deposition cycles. Further, the fabricated MOS capacitor with Pt nanodots exhibits noticeable programmable and erasable characteristics even under low voltages of ±8 V, a large memory window, and good charge retention at room temperature.
The authors thank the financial support of the National Key Technologies R&D Program (2009ZX02302-002), National Natural Science Foundation of China (no. 61076076, 61274088), the Program for New Century Excellent Talents in University (NCET-08-0127), and the Key Project of the Chinese Ministry of Education (108052).
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