Chongying Xu

Principles of molecular precursor selection for aerosol synthesis of materials

The influence of precursor design on gas-phase and liquid/solid-phase (spray pyrolysis) aerosol materials synthesis is described. The necessity for the incorporation of reaction mechanisms that lead to complete removal of organic by-products is emphasized by the examples of aerosol-assisted CVD of Cu and Ag films and the formation of ZnS films and powders. The role of “single-source” precursors in the formation of complex materials including ZnS, perovskite phase PbTiO3 and yttrium aluminum garnet, Y3Al5O12 is described.

Germanium ALD/CVD Precursors for Deposition of Ge/GeTe Films

In order to deposit conformal films in the high aspect ratio trench and via structures in future high-density phase-change memory devices, suitable ALD/CVD precursors are needed. We report on the development of novel germanium(II) metal-organic ALD/CVD precursors containing amide, cyclopentadienyl, and amidinate ligands. The physical properties, volatility, and thermal behavior of the precursors were evaluated by simultaneous thermal analysis (STA) and vapor pressure measurements. Stability studies were conducted to investigate the suitability of the precursors for use as ALD/CVD precursors for device manufacturing.

Conformal MOCVD Deposition of GeSbTe in High Aspect Ratio Via Structure for Phase Change Memory Applications

We have demonstrated conformal deposition of amorphous GeSbTe films in high aspect ratio structures by MOCVD. SEM analysis showed the as-deposited GeSbTe films had smooth morphologies and were well controlled for void free amorphous conformal deposition. GeSbTe films adhere well to SiO 2 , TiN, and TiAlN. The morphology and adhesion are stable in 420°C post process. By annealing at 365°C, amorphous GeSbTe films converted into crystalline GeSbTe with polycrystalline grain sizes of 5nm. Film resistivity in the crystalline phase ranged from 0.001 to 0.1 Ω-cm, suitable for device applications. Phase change devices fabricated with confined via structures filled with MOCVD GeSbTe showed cycle endurances up to 1×10 10 with a dynamic set/rest resistance of two orders of magnitude.

Thermal Stability Studies on 1, 3, 5, 7 - Tetramethylcyclotetra-Siloxane(TMCTS), a Low к CVD Precursor

Chemical studies on 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) were conducted to elucidate its thermal behaviors with water and under various reaction conditions. TMCTS was heated in the presence of 316L stainless steel and in the presence of water. The heated TMCTS then was evaluated using 1 H NMR (proton nuclear magnetic resonance) spectroscopy, GC-MS (gas chromatography-mass spectrometry) as a function of time, temperature and residual water concentration. The thermal degradation kinetics of gas-phase TMCTS were investigated using FTIR (Fourier transform infrared) spectroscopy at elevated temperatures. These initial results indicated that TMCTS degradation rates increased with both temperature and water concentration. This work spawned the development of a “dry” TMCTS that is expected to exhibit enhanced thermal stability relative towards uncontrolled decomposition.

Metalorganic Chemical Vapor Deposition of Thin Film ZrO2 and Pb(Zr,Ti)O3: Precursor Chemistry and Process Characteristics.

Metalorganic chemical vapor deposition (MOCVD) process characteristics of several zirconium source reagents were investigated. These source reagents included metal β-diketonates [e.g., Zr(thd)4 where thd=(2,2,6,6-tetramethyl-3,5-heptanedionate)] and metal alkoxide/β-diketonates [e.g., Zr(OiPr)2(thd)2 and Zr(OtBu)2(thd)2]. Thermal properties and transport behaviors of these precursors were examined by thermogravimetric analysis. Zirconium oxide films were deposited on silicon substrates at reduced pressure. Under the process conditions examined, the deposition behavior was mass-transport controlled, and Zr(OiPr)2(thd)2 and Zr(OtBu)2(thd)2 behaved similarly. The films exhibited low carbon content. Pb(Zr, Ti)O3 (PZT) films were deposited on iridium-coated silicon substrates under reduced pressure. Zirconium incorporation efficiency was significantly improved for Zr(OiPr)2(thd)2 when compared to Zr(thd)4. Use of M(OtBu)2(thd)2 (where M=Zr or Ti) as source reagents for MOCVD of PZT was examined and compared to M(OiPr)2(thd)2 analogues. In this case, higher process pressures were needed to improve the incorporation efficiencies of M(OtBu)2(thd)2 precursors.

Synthesis and characterization of Lewis-base adducts of 1,1,1,5,5,5-hexafluoroacetylacetonatosilver(I)

The compounds [{Ag(hfacac)}mLn][hfacac = 1,1,1,5,5,5-hexafluoroacetylacetonate, L = norbornadiene (nbd), m= 2, n= 1; L = SMe2, SEt2, SPrn2 or SBUn2, m= 1, n= 1; L = 1,4-oxathiane. m= 1, n= 1 or 2] were prepared by the reaction between the Lewis bases, L, Hhfacac and Ag2O in the appropriate ratios. In addition, the intermediate [{Ag(hfacac)}2(H2O)], formed by the reaction of Ag2O with 2 equivalents of Hhfacac in the absence of L was isolated. These species were characterized by 1H and 13C NMR and Fourier-transform IR spectroscopy and by combustion elemental analysis. Three examples were structurally characterized by single-crystal X-ray diffraction. The compound [{Ag(hfacac)}2(H2O)] is oligomeric by virtue of intermolecular hydrogen bonding between the co-ordinated water molecule and the oxygen atoms in the hfacac ligand in an adjacent molecule. In addition there is a bonding interaction between the methine carbon in the hfacac ring of one molecule and the silver centre in an adjacent molecule. The compound [{Ag(hfacac)}2(nbd)] is dimeric leading to a tetranuclear molecular unit in which the hfacac ligands both chelate and bridge, with unidentate nbd ligands. The compound [Ag(hfacac)(C4H8OS)2] is monomeric in the solid state with the 1,4-oxathiane ligands co-ordinated to the silver(I) centre exclusively viathe S atoms. The silver has a severely distorted tetrahedral geometry with an enlarged S–Ag–S angle [149.6(1)°] and a reduced O–Ag–O angle [74.4(2)°] which is characteristic of (β-diketonato)bis(ligand)metal compounds where M = Ag or Cu.