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Title:CVD of magnetic iron-cobalt alloy thin films: surface poisoning from carbonyl precursors and use of ammonia as a chemical inhibitor to restore growth kinetics
Author(s):Zhang, Pengyi
Director of Research:Abelson, John R.
Doctoral Committee Chair(s):Abelson, John R.
Doctoral Committee Member(s):Girolami, Gregory S.; Dillon, Shen J.; Shoemaker, Daniel P.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Soft magnetic thin films
Chemical vapor deposition (CVD)
Surface poisoning effect
Abstract:Iron-cobalt alloys are excellent soft ferromagnetic materials that are widely used in data storage applications such as writer heads for hard disc drives and magnetic tunnel junctions in spin-transfer torque magnetic random-access memories. Although the FexCo(1-x) alloys in such devices are currently sputter deposited, there is considerable interest in developing low temperature chemical vapor deposition (CVD) processes to this material, so as to extend scalability to future technology nodes. In this dissertation I report the following developments in the area of low temperature CVD of transition metals. First, we show that it is possible to grow dense, smooth, and high-purity FexCo(1-x) thin films by chemical vapor deposition using a co-flow of Fe(CO)5 and Co2(CO)8 precursors. Films with the composition Fe0.55Co0.45 show excellent soft magnetic properties: the saturation magnetization of 2.45 ± 0.05 Tesla approaches the theoretical maximum, and the coercivity is less than 20 Oe. Conformal films have also been grown successfully in a trench structure with an aspect ratio of 4:1. We found, however, that the morphology and composition are extremely sensitive to small variations in the deposition temperature or precursor partial pressures. To explore the mechanism of the instability, we then explored the deposition of iron by CVD from Fe(CO)5. We show that at a constant temperature (e.g., 300 °C) the growth rate decreases monotonically with time. Growth eventually ceases altogether at a certain film thickness and cannot restart, even under conditions that are favorable for nucleation. We propose that the reduction in Fe deposition rate observed here and in previous literature reports results from surface poisoning: the dissociative chemisorption of CO molecules on the Fe surface at elevated temperature forms inactive surface species, termed graphitic carbon, which accumulate on the surface and eventually stop Fe growth. Remarkably, we found that the surface poisoning effect can be inhibited, so that Fe deposition occurs at a constant rate with no self-limiting growth behavior, by co-flowing NH3 along with the Fe(CO)5 precursor during growth. The adsorbed NH3 inhibits CO chemisorption by displacing CO from the growth surface and inhibiting CO chemisorption. The resulting Fe films are high purity, with carbon and nitrogen contents each below 1 at. %. Based on the study of surface poisoning during Fe growth, we then demonstrate that the instabilities in FexCo(1-x) alloy growth are also a result of a surface poisoning effect involving chemisorption of carbon monoxide, and that adding a co-flow of NH3 during CVD eliminates the poisoning effect without introducing measurable quantities of nitrogen into the deposit. The use of ammonia thus enables growth of FexCo(1-x) films over a wide temperature window with highly reproducible morphology and stoichiometry. The implications of this thesis extend beyond the scope of FexCo(1-x) films. We discuss, based on the surface science literature, the possible use of NH3 in combination with other transition metal carbonyl precursors that are of interest for low temperature CVD.
Issue Date:2016-07-11
Rights Information:Copyright 2016 Pengyi Zhang
Date Available in IDEALS:2016-11-10
Date Deposited:2016-08

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