Atomic Layer Deposition of SrO: Substrate and Temperature Effects
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Atomic Layer Deposition of SrO: Substrate and Temperature Effects Han Wang, Xiaoqiang Jiang, and Brian G. Willis Department of Chemical, Materials & Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269. ABSTRACT The atomic layer deposition (ALD) of SrO was conducted on various oxide surfaces by using strontium bis(tri-isopropylcyclopentadienyl) and water at deposition temperatures of 200 and 250°C. The initial and steady growth behaviors were studied by in-situ spectroscopic ellipsometry and ex-situ X-ray photoelectron spectroscopy. For initial growth, the growth per cycle (GPC) of SrO not only depends on the concentration of hydroxyl groups but also the formation of interfacial Sr-O-Si bonds. For the steady growth, in-situ annealing was used to enhance the growth rate and multiple growth regions were identified. INTRODUCTION Atomic layer deposition (ALD) has attracted much attention due to its capability for accurate thickness control and superior conformal growth [1]. Challenges for ALD include nonideal nucleation and substrate effects often encountered at the interfaces between dissimilar materials [2-4]. These substrate effects are particularly problematic for the growth of more complicated materials including ternary systems because of the difficulty encountered in achieving steady, predictable growth [5]. The growth of metal oxides is one of the most extensively studied and promising areas of ALD. SrO ALD is critical for the ALD of the ternary strontium titanate (STO), which is of major interest for use with high-density metal-insulatormetal (MIM) capacitors [6-8]. SrO is also of interest for the growth of epitaxial perovskite oxides on semiconductors where it acts as a buffer layer between the reactive semiconductor and the metal oxide layers [9,10]. In this work, we investigate strontium oxide (SrO) as a model system to better understand the fundamental origins of substrate and temperature effects on ALD. Especially, we study both initial and steady growth of SrO on two insulating oxide substrates at 200 and 250°C using insitu real-time spectroscopic ellipsometry (RTSE) [11]. We observe complex ALD growth characteristics with several different ALD operating regimes.
EXPERIMENT SrO thin films were deposited in a warm wall stainless steel reactor on approximately 1 cm2 1.6 nm native SiO2, or 28 nm ALD-grown Al2O3/Si(100) substrates [11]. The Sr-metal precursor was the THF adduct of Sr(C5iPr3H2)2 (Air Liquide). H2O vapor (Millipore 18 Mȍ) was used as the oxidizer. The solid Sr source compound was heated to 140–145°C in a glass
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container where it becomes a light yellow liquid. The temperature of the H2O container was 25°C. Both inert gas purging and vacuum pumping were used between reactant exposures. Argon was regulated to maintain 1 Torr reactor pressure using leak valves; vacuum pumping was to below 10 mTorr. The whole ALD system was heated to a temperature higher than 150°C to prevent precursor condensation on the reactor and tubing walls. Typical purging and pumping times w
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