Synthesis and characterization of precursors for chemical vapor deposition of metal oxide thin films

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Virginia Tech


Optimal precursor behavior during Chemical Vapor Deposition (CVD) is crucial for reproducible synthesis of high quality thin films. Desirable precursor properties include: 1) volatility and thermal stability at <180 °C (10 - 100 millitorr vapor pressure at atmospheric pressure) 2) low decomposition temperature (350-550 °C) to metal oxide with minimal organic ligand contamination and 3) ambient stability and minimal toxicity. Optimal selection and usage of CVD precursors is implemented by synthesis and characterization studies. Additionally, precursor synthesis and characterization studies render the development of novel precursors, which are specifically engineered for the CVD process.

Lead bis-tetramethylheptadione [Pb(thd)₂], lead bis-heptafluorodimethyloctadione [Pb(fod)₂], and zirconium tetrakis-tetramethylheptadione [Zr(thd)₄], which are lead and zirconium precursors for CVD of Pb(ZrxTi1-x)O₃ thin films, and T8- hydridospherosiloxane, which is a silica precursor, were synthesized and purified. Free ligand was the predominant impurity from the lead and zirconium precursor syntheses, and the T8 synthesis produced several byproducts including T10-hydridospherosiloxane and a polymer. The lead and zirconium precursors were purified by recrystallization from toluene, and the T8 was purified by extracting the byproducts with pentane. Purity of Pb(thd)₂, Pb(fod)₂ and Zr(thd)₄ was confirmed by melting point determination, carbon and hydrogen elemental analysis and proton nuclear magnetic resonance spectrometry (NMR). Purity of T8 was confirmed by proton NMR.

Isothermal gravimetric analysis (TGA) was used to study volatility and thermal stability of the precursors. The Zr(thd)₄ isotherms ranged from 180-260 °C, the Pb(thd)₂ and Pb(fod)₂ isotherms were 80-200 °C, and the T8 isotherms were 80-140 °C. Vapor pressure was calculated from TGA data, with use of diffusion equations. Precursors exhibited vapor pressure ranging from 0.2 - 600 millitorr, over their respective vaporization temperature ranges. Enthalpy of vaporization was calculated from Arrhenius plots of vapor pressure as a function of temperature. The Zr(thd)₄ and T8 were observed to be thermally stable over the temperature ranges and durations of experiments. The Pb(thd)₂ and Pb(fod)₂ are not thermally stable over the vaporization temperatures, and undergo oligomerization when heated at high vaporization temperatures. Addition of a polyether adduct to the lead precursor was proposed to inhibit oligomer formation at high vaporization temperatures.

Precursor decomposition studies were executed in sealed quartz tubes. Products of decomposition were examined by infrared spectrometry, mass spectrometry, and solid state NMR spectrometry. Metal oxide formation from decomposition of precursors was observed at; 300-350 °C for T8, 350-550 °C for Zr(thd)₄, 250-350 °C for Pb(thd)₂, and 300 °C for Pb(fod)₂. Lead fluoride became the dominant phase of Pb(fod)₂ decomposition products above 300 °C. Intermediate decomposition products of all the precursors were documented and discussed. Synthesis and/or isolation of intermediate decomposition products of Zr(thd)₄ , Zr2O(thd)₄, was proposed as a novel precursor for ZrO₂ deposition.