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Title:Nucleation inhibition and enhancement in chemical vapor deposition
Author(s):Zhang, Zhejun
Director of Research:Abelson, John R
Doctoral Committee Chair(s):Abelson, John R
Doctoral Committee Member(s):Girolami, Gregory S; Cao, Qing; Krogstad, Jessica A
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Thin film
Abstract:Area selective deposition (ASD) is becoming increasingly attractive as a bottom-up approach to nanomanufacturing. Most ASD processes developed so far concern selective deposition of metal on metal (i.e., not on non-metallic surfaces) or oxide on oxide (i.e., not on metallic surfaces), but nanomanufacturing also demands other film-substrate combinations such as deposition of metal on one oxide vs. another oxide or a metal surface. In this dissertation, I report one example of ASD that affords these new capabilities: chemical vapor deposition (CVD) of cobalt from Co2(CO)8 is fast on some oxides (Al2O3) but slow on others (SiO2). We also show that the addition of ammonia as an inhibitor improves the selectivity between oxides: a coflow of ammonia strongly inhibits nucleation on SiO2 (an acidic oxide) but has negligible effect on the nucleation and growth on Al2O3 (a basic oxide). We also show that the cobalt deposition process can, in some cases, be tuned to enable nucleation of Co on metal but not on oxide, or Co on oxide but not on metal. If, however, film growth is required on a relatively unreactive surface (e.g., one otherwise used for nongrowth), then the nucleation step will be kinetically difficult: a relatively small areal density of islands will form over an extended period of time. As a consequence, islands of different sizes populate the surface, and full coalescence (coverage of the substrate) occurs only when a relatively large thickness has been deposited, and the morphology is rough due to the distribution of island heights. To enhance film smoothness on unreactive substrates, I demonstrate that the sequential use of (i) a self-limiting substrate pretreatment by tetrakis-(dimethylamino)¬metal (TDMA-M) molecules (M = V, Hf, or Ti), followed by (ii) growth inhibition using a co-flow of ammonia during film growth of cobalt. The film grown by the combination of steps has a much smaller roughness than either step alone. I also investigate the nucleation of HfB2 from Hf(BH4)4 on Al2O3 vs. on SiO2 substrates. In both cases nucleation begins rapidly. However, on Al2O3 a high density of nuclei forms, and these nuclei rapidly coalesce into a smooth continuous film; by contrast, on SiO2 the island density remains smaller and coalescence occurs at larger film thickness. The method of pretreatment from self-limiting adsorption of tetrakis¬(dimethylamino)¬hafnium can increase nuclei density, and thus, speed up nucleation and reduce film roughness. Chemical design of a CVD precursor can be used to afford rapid nucleation but slow film growth, thus, smooth ultra-thin films. In a collaborative effort with S. Liu in the group of G. S. Girolami, I demonstrate the CVD of smooth platinum films using the newly-synthesized Pt[CH2CMe2CH2CH=CH2]2 precursor. This molecule has a rapid nucleation, which is the consequence of the availability of low barrier C-H activation pathways, and slow growth rate due to the formation of carbon-containing species that passivate the Pt surface. I used reflection IR absorption in real time to analyze the steady-state population of adsorbates during CVD. This information is needed to fully understand the surface kinetics that govern conformal and superconformal growth, as well as nucleation inhibition and enhancement. We show that use of a metal substrate used at high angle of incidence provides enough signal enhancement in p-polarization to observe sub-monolayer coverages. The intrinsic cancellation of the absorption signal in s-polarization means that the unwanted absorption due to gas phase molecules in the beam path, and molecules adsorbed on the IR chamber windows, can be cancelled out by subtraction of the p- and s- signals. In the last section, I demonstrate a superconformal process for cobalt deposition by adding a consumable inhibitor. The films are, however, contaminated by incorporation of the inhibitor molecules. Therefore, I propose an innovative approach that can potentially achieve superconformal growth of contamination-free films. This method requires two precursors that deposit the same film, but with quite different rates of adsorption and reaction.
Issue Date:2020-07-16
Rights Information:Copyright 2020 Zhejun Zhang
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08

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