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Title:Coating and filling of nanometer-scale structures using chemical vapor deposition
Author(s):Talukdar, Tushar Kanti
Director of Research:Abelson, John R.
Doctoral Committee Chair(s):Ertekin, Elif
Doctoral Committee Member(s):Girolami, Gregory S.; Tawfick, Sameh H
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Bottom-up fill
Superconformal coating
Chemical vapor deposition
Trench fill
Dielectric fill
Seam-free fill
Oxide growth
Thin film
Abstract:Void-free filling of high aspect ratio (AR = 3 to 10) structures, such as trenches or vias, is necessary in nanoscale device fabrication. Examples include shallow trench isolation, metallization, and reverse tone patterning in integrated circuits and optical waveguides in photonic devices. Gas-phase coating methods such as chemical vapor deposition (CVD) and atomic layer deposition (ALD) can be operated in a regime that is kinetically limited by the surface reaction rate rather than by the precursor transport rate. In that regime, the precursor diffuses everywhere within the structure, which affords a nearly uniform coating thickness. However, complete filling of a deep structure is ultimately sensitive to the geometry. In a trench with parallel sidewalls, the width of the opening along the centerline decreases as film builds up on the sidewalls, and the rate of precursor transport by molecular diffusion declines until it becomes reaction rate limiting. As the supply of precursor deep within the structure dwindles, a residual void (seam) is left along the centerline. This seam is unacceptable in many applications because it leads to a degradation of performance, including thermal, mechanical, or electrical properties, or to a higher etch rate during surface planarization processes. In this dissertation, I report a newly discovered superconformal CVD method that affords bottom-up filling of trenches with oxide: the film growth rate increases with depth such that the profile of material develops a “V” shape that fills in along the centerline without a seam of low density material. The method utilizes low pressures of a metal precursor plus a forward-directed, lower-pressure flux of co-reactant (water). Under these conditions, many of the co-reactant molecules travel ballistically to the trench bottom where a fraction of them reflect. This scattering, which creates a virtual source of co-reactant from the trench bottom, leads to a superconformal growth process whose rate is highest at the bottom and declines towards the opening. Simultaneous with this superconformal component is the typical subconformal growth process due to the portion of the co-reactant flux that enters the trench opening isotropically; with a sufficiently large forward-directed flux, however, the overall profile is superconformal. I demonstrate this approach for filling trenches with HfO2 using 0.09 mTorr tetrakis-(dimethylamido)hafnium (TDMA-Hf) precursor and 0.009 mTorr H2O co-reactant. Precursor-rich growth conditions at a substrate temperature ≤ 270 °C are used to assure that the growth rate is kinetically limited (determined) by the H2O flux and is nearly independent of the TDMA-Hf flux. Under these conditions, the growth rate in a trench with an AR of 3.5 increases from 0.6 nm/min at the top to 1.0 nm/min at the bottom sidewalls (step coverage = 1.6). I simulate the precursor transport-reaction problem within the trench using a Markov chain model to account for both the forward-directed and isotropic reactant fluxes, including the multiple re-emission events within the trench, as a function of the surface sticking probability. The model predicts the fraction of the total incident flux that must be forward-directed in order to afford seam-free filling as a function of the sticking probability and the starting AR. Experimentally, I find that the opening of the trench accumulates a slightly greater thickness (a ‘bread-loaf’ profile) that tends to pinch off the trench just before complete filling. To eliminate this effect, I use a molecular growth inhibitor, H(hfac) or H(acac), to reduce the growth rate near to the opening. The result is seam-free filling of trenches with HfO2 up to an AR of 10. I also report CVD of highly conformal MgO film at a high growth rate (up to 300 nm/min) and a low substrate temperature (≤ 350 °C) using Mg(DMADB)2 precursor plus H2O co-reactant. The film stress is low enough, and the adhesion to the Si substrate is strong enough, to grow 2 µm thick MgO film at 350 °C. Conformal growth in microtrenches is demonstrated by controlling the precursor sticking probability: using a precursor-rich condition at 270 °C, a step coverage of 98 % is obtained in a trench of AR 9 at a growth rate of 7.5 nm/min. Films grown at a very high rate (> 90 nm/min) incorporate boron as B2O3, but those grown at a moderately high rate (7-25 nm/min) have a very low (~ 1 at. %) boron incorporation. The refractive indices are lower than bulk MgO due to a reduced physical density (~ 85 % of that of bulk). The measured electrical dielectric constant (9.5) and breakdown strength (6 MV/cm) agree well with literature values for MgO thin films. This dissertation also reports extremely conformal HfB2 coating in carbon nanotube (CNT) forests with height of ≤ 1.7 mm by employing static (unpumped) chemical vapor deposition (SCVD), utilizing a high pressure of Hf(BH4)4 precursor up to its full vapor pressure. When the substrate temperature is kept low (≤ 200 °C), the growth saturation occurs at a pressure that is orders of magnitude lower than the maximum precursor pressure limit, thereby affording a high degree of conformality in the forests. A step coverage of 92 % is achieved when the forest height is 80 µm. In addition to high conformality, HfB2 coating creates strong joints wherever the neighboring CNTs touch each other; this converts the CNT forest with van der Waals interactions into a mesoscale composite foam with greatly enhanced mechanical performance. Flat punch nanoindentation measurements of HfB2-CNT composites with different HfB2 coating thicknesses show that the modulus and compressive strength follow a power law relationship with density (~ ρ^n ) where the exponent n = 3 and 3.4, respectively.
Issue Date:2018-04-18
Rights Information:Copyright 2018 Tushar K. Talukdar
Date Available in IDEALS:2019-08-23
Date Deposited:2018-05

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