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Title: Hafnium oxide-based dielectrics by atomic layer deposition
Author: King, Peter
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2013
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In 2007 there was an important change in the architecture of nanotransistors - the building blocks of modern logic and memory devices. This change was from utilising thermally grown silicon dioxide as a dielectric to so-called high-κ hafnium oxide dielectrics grown by atomic layer deposition. The first production logic devices of this era used a hafnium oxide dielectric layer deposited by thermal atomic layer deposition; using HfCl₄ and H₂O as the precursors. Present day fabrication makes use of hafnium oxide-based atomic-layer-deposited dielectric films. The latest nanotransistor devices utilise a third generation hafnium oxide-based dielectric material. This thesis examines hafnium oxide-based thin film dielectric materials prepared by thermal atomic layer deposition on silicon substrates. Specifically the enhancement of the dielectric response of hafnium oxide by the addition of other elements is examined. Two ternary materials systems were deposited by thermal atomic layer deposition and analysed: titanium-hafnium oxide and cerium-hafnium oxide. Hafnium oxide films were deposited to be used as measurement benchmarks. Cerium oxide films were also deposited and analysed in their own right as potential dielectric layers. The hafnium oxide and both ternary deposition experiments used (MeCp)₂Hf(OMe)(Me) as the hafnium precursor. The titanium-hafnium oxide growth used Ti(iOPr)₄ as a titanium source and the cerium oxide and cerium-hafnium oxide work utilised Ce(mmp)₄ as a cerium source. Post-deposition specimen sets consisted of an as-deposited sample, a sample spike-annealed in N₂ at 850°C and a sample annealed for 30 minutes at 500°C. These annealing regimes were performed to mimic typical gate-first and gate-last transistor processing steps. The compositions and thicknesses of the films were measured using medium energy ion scattering. The structure of the films was analysed by X-ray diffraction and Raman spectroscopy. Capacitance-voltage and current density-field measurements were taken from fabricated MOS capacitor specimens to assess the dielectric response of the films. X-ray diffraction and Raman measurements showed that un-doped HfO₂ had monoclinic crystallinity as-deposited and after the two annealing regimes. The dielectric constant and leakage current density, 17 and 1.7x10⁻⁷ A/cm² at -1 MV/cm respectively, are consistent with values reported in the literature for HfO₂ films. The addition of titanium suppressed the crystallinity of the material resulting in amorphous films in compositions with Ti₀.₃Hf₀.₇O₂ titanium and above. The optimum electrical results were recorded for the titanium-hafnium oxide material in the composition Ti₀.₅Hf₀.₅O₂ which had a dielectric constant of 35 as-deposited and a leakage current density of 1.0x10⁻⁷ A/cm² at -1 MV/cm. This composition of film demonstrated similar values after the 500°C/30 min anneal but both dielectric constant and leakage current density suffered after the 850°C/spike anneal; 22 and 1.8x10⁻⁶ A/cm² at -1 MV/cm respectively. Films with compositions of Ti₀.₁Hf₀.₉O₂ demonstrated much lower dielectric constant and higher leakage current density, especially after heat treatment. The addition of cerium in a Ce₀.₁₁Hf⁰.₈₉O₂ composition was found to suppress crystallinity as-deposited and then provoke a lattice-substitutional phase change to the metastable tetragonal/cubic phase after both types of heat treatment. This ceriumactivated phase change resulted in a molar volume modulation compared to un-doped HfO₂. An increased dielectric constant compared to un-doped HfO₂ of 31 was recorded for the 500 °C/30 min anneal with the 850°C/spike anneal resulting in a lower value of 21. Leakage current density was 1.3x10⁻⁷ A/cm² and 3.2x10⁻⁷ A/cm² at -1 MV/cm respectively for the same anneals. Deposition with Ce(mmp)₄ and water was found to result in cubic crystalline films across a growth temperature range 150-350 °C. The frequency dependency of the dielectric properties was found to be influenced by the crystallite size which was governed by the deposition temperature. The highest dielectric constant, 42, was measured for the 150 °C growth temperature with C-V measurements performed at 1 MHz. The two doped HfO₂-based materials systems studied have demonstrated potential as dielectric materials for use in future nanoelectronic devices.
Supervisor: Chalker, P. R.; Potter, R. J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)