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Role of Initial Precursor Chemisorption on Incubation Delay for Molybdenum Oxide Atomic Layer Deposition Charith E Nanayakkara, Abraham Vega, Guo Liu, Charles L. Dezelah, Ravindra K. Kanjolia, and Yves J. Chabal Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03423 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016
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Chemistry of Materials
Role of Initial Precursor Chemisorption on Incubation Delay for Molybdenum Oxide Atomic Layer Deposition Charith E. Nanayakkara†, Abraham Vega†, Guo Liu‡, Charles L. Dezelah‡, Ravindra K. Kanjolia‡, Yves J. Chabal†* † Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States ‡
EMD Performance Materials, 1429 Hilldale Avenue, Haverhill, MA, 01832, United States
Abstract In an effort to grow metal oxide films (i.e. MoO3) at low temperatures, a novel molybdenum precursor, Si(CH3)3CpMo(CO)2(η3-2-methylallyl) or MOTSMA, is used with ozone as the coreactant. As is often observed in atomic layer deposition (ALD) processes, the deposition of molybdenum trioxide displays an incubation period (~ 15 cycles at 250 °C). In-situ FTIR spectroscopy reveals that ligand exchange reactions can be activated at 300 °C, leading to a shorter incubation periods (e.g. ~9 cycles). Specifically, the reaction of MOTSMA with OHterminated silicon oxide surfaces appears to be the rate limiting step, requiring a higher temperature activation (350 °C) than the subsequent ALD process itself, for which 250 °C is adequate. Therefore, in order to overcome the nucleation delay, the MOTSMA precursor is initially grafted at 350 °C, with spectroscopic evidence of surface reaction, and the substrate temperature then lowered to 250 °C or 300 °C for the rest of the ALD process. After this initial activation, a standard ligand exchange is observed with formation of surface Si(CH3)3CpMo(η32-methylallyl) after precursor and its removal after ozone exposures, resulting in Mo(=O)2 formation. Under these conditions, the ALD process proceeds with no nucleation delay at both temperatures. Post-deposition X-ray photoelectron spectroscopy spectra confirm that the film
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composition is MoO3. This work highlights the critical role of precursor grafting to the substrate as essential to eliminate the nucleation delay for ultra-thin ALD grown film deposition. 1.1 Introduction Atomic layer deposition (ALD) is an attractive technique to use for thin film deposition due to its sequential and self-limiting surface reactions leading to conformal and controlled film growth.1, 2 The nucleation of the precursor molecule is extremely important to get an ideal ALD process.2 Any nucleation delay may initially lead to non-uniform island growth, requiring several cycles to obtain continuous and more homogeneous films. Such inhomogeneity is an issue even for thick films.2 Therefore, effective nucleation by chemical reaction of the precursor molecule with the substrate is critical to obtain uniform ALD grown films, particularly ultra-thin films. Molybdenum oxide thin films are important for applications in microelectronics, catalysis, optics and even for hybrid heterostructures.3-5 Various techniques have been used to grow molybdenum oxide films including more recently atomic layer deposition (ALD),6-14 as new precursors have become available.7 For instance, deposition using molybdenum hexacarbonyl (Mo(CO)6) and ozone as the co-reactants was reported to yield, within a very narrow temperature window (152 - 172 °C), amorphous films that crystallize into a α-MoO3 phase upon annealing in air at 600 °C.7 Later, bis(tert-butylimido)bis(dimethylamido) molybdenum was used as an alternative to the less thermally stable metal carbonyls. Thermal ALD of bis(tert-butylimido)bis(dimethylamido) molybdenum and ozone was achieved over a temperature window of 250 – 300 °C with the highest growth per cycle (GPC) at 300 °C;8 films deposited at 300 °C also had the lowest amount of C and H (
