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spectroscopic detection of intrinsic defects in nano-crystalline transition metal elemental oxides scales of order (0.5 to 5 nm) for nano- and non-crystalline thins Gerry Lucovsky, NC State University - PowerPoint PPT Presentation

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  • spectroscopic detection of intrinsic defects in nano-crystalline transition metal elemental oxides scales of order (0.5 to 5 nm) for nano- and non-crystalline thinsGerry Lucovsky, NC State Universitygraduate students and post docs S. Lee, H. Seo, J.P. Long, C.L. Hinkle and L.B. Flemingcollaborators J.L. Whitten (ab-initio theory), J. Lning (NEXAS, SSRL) , D.E. Aspnes (SE), M.D. Ulrich. J.E. Rowe (SXPS, NSLS-BNL)outlinerecent technology advancesintroduction to spectroscopic techniquesconduction and valence band electronic statesintrinsic bonding defects in Ti, Zr and Hf elemental oxidesengineering solutions - HfO2, Hf silicates, tphy < 2 nm

  • recent technology advancestwo different dielectrics have emerged as candidates for introduction at the 32 nm process nodenano-crystalline HfO2 (< 2 nm) and non-crystalline HfSiONmost significant published technology advances from SEMATECH group:Gennadi Bersuker, Pat Lysaght, Paul Kirsch, Manuel Quevedo, Chad Young, et. althis report: science base for quantifying differences in electronic structure between these two classes of materials intrinsic defects assigned to O-atom vacancies clustered on nano-crystalline grain boundaries defect densities significantly reduced when film thickness is ~2 nm

  • spectroscopic approachesnear edge x-ray absorption - NEXAS - SSRL 200 to 1200 eV x-rays, S/N ~ 5000:1, resolution, 0.1 eV soft x-ray photoelectron spectroscopy - SXPS - BNL40 to 200 eV, S/N ~1000:1, resolution, ~0.15 eVvis-vacuum UV spectroscopic ellipsometry - vis-VUV SE 1.8 to 6 eV, and 4 eV to ~ 8.5 eV

  • molecular orbital model valence band (SXPS, UPS) and conduction band (NEXAS) reveal d-state featurestwo contributions to lifting of d-state degeneracy crystal field symmetry/coordination nano- and non-crystalline filmsD[Eg(2) - T2g(3)] ~ 2.5 - 4 eVJahn-Teller bonding distortions rutile and distorted CaF2degeneracies removed Eg 2 states & T2g 3 statesd[Eg(2)]~d[T2g(3)] ~ 0.5-1 eV

    octahedral bonding of Ti with 6 Ozeroth order MO approximation Ti molecular orbitals Ostates labeled wrt to molecular symmetry considerable mixing: 3d, 4s and 4p states however, atomic labeling provides useful description of band edge electronic structure, intrinsic defects

  • x-ray absorption spectroscopy - a novel way to study conduction band d*-states in transition metal oxidesinter-atomic spectraZr5s*,p* + O 2p*Zr4d* s(5s*,p*) + O 2p* sZr4d* p(5s*,p*) + O 2p* pZr5s*,p* + O 2p*Zr4d* s+ O 2p* s Zr 4d* p + O 2p*, p O 1sO 2p nbcore level and and band edge transitions terminate in similar Zr-O molecular orbital statesvis and VUV spectroscopic ellipsometrysimilar transitions to Ti 3d*, etc.

  • conduction and valence band edge electronic statesTiO2; roadmap for other dielectrics: HfO2 and "Hf SiON"D(Eg-T2g)av ~3 eVcrystal field splitting 6-fold coordination octahedral symmetryD(Eg-T2g)av ~2.1 eVband gaps, BG, scale with atomic d-state energiesoxide BG at. d-stateTi: 3.2 eV, -11.0 eVZr: 5.5 eV, -8.1 eVHf: 5.7 eV, -8.4 eV

  • Ti d-state degeneracy removalO K1 edge inter-atomic O 2p + 3d, 4s and 4pTi L3 edge intra-atomic Ti 3d2nd derivative of absorption respective OK1 - L3 spectraTi 3d, 4s, and 4p

  • Ti d-state features in O K1 (x-axis) and Ti L3 (y-axis) edgesslope of ~ 1 indicatescrystal field - average Eg-T2g splittings, andJahn-Teller term-splittingsessentially the same in O K1 and Ti L3 NEXAS spectraL3 is intra-atomic transition O K1 are projections of same d-states, but different transition matrix elements

  • comparison of average (C-F) d-state splittings valence and conduction band band edge statesO K1, empty conduction band states, filled valence band statesD(Eg,Tg): 3.0 eV, 2.1 eVD(4s, 4p): 3.5 eV, 2.6 eVanti-bonding states split more than bonding states

  • linear scaling (slope 1) between Ti L3, and both O K1 and e2gives linear scaling (slope 1) between e2 and O K1O K1 edge - wider spectral range, than lab VUV SE

  • limitations on NEXAS approach p-state core hole life-times scale inversely as Zeff2.5 (Slater rules) for M3,N3 absorptions - Zr 3p to 4d, Hf 4p to 5d, too much broadening to resolve d-state splittingshowever,J-T separations are easily obtained from O K1 edgegaussian fits and 2nd derivatives term splittings for Eg and T2g compared with epsilon 2 (e2) from VUV-SE, SXPS valence band spectra, and studied for different scales of order film thickness

  • intrinsic conduction band edge electronic structure O K1 edge in XAS d-states - same splittings as conduction band d-states in SE VUV e2 and anano-crystalline - 800C anneal Eg - 532.5, 533.5 (0.15 eV) T2g - 535.2, 536.3, 537.4 (0.15 eV) DEg=10.2 eV - D(T2g - Eg)=2.70.2 eV epsilon 2 (e2) spectrum DEg = 0.80.2 eV D(T2g - Eg) = 2.30.2 eV

  • summary - part I experimental determination of electronic structure of conduction and valence band edge statesNEXAS - OK1 for conduction band states SXPS - for valence band statesbonding and localized nature of d-states molecular orbital description basis for correlating spectral features with atomic states of transition metal atoms of high-k dielectrics

  • defect states - electron and hole injection into HfO2 through a thin SiO2/SiON interfacial layer electron and hole trapping/transport asymmetriesfirst studied by IMEC group confirmed at NC Statemodel calculations for defects Robertson/Schlugerspectroscopic studies TiO2 VB and e2 spectra -- defect state electronic structure

  • Massoun et al., APL 81, 3392 (2002)Si-SiO2-HfO2 gate stackstraps are in high-k material of stack 2x1013 cm -2 -- s ~ 1.5x10-17 cm-2 coulombic center - lower x-section than Pb centers in Si substrate screened by high dielectric constant of HfO2Z. Xu et al., APL 80, 1975 (2002)substrate injection electrons gate injection electrons electron trap ~0.5 eV below conduction band edge HfO2substrate injection holes

  • J-V asymmetry - IMEC modelcontinuity of eE - e(SiO2) ~ 3.9 < e(HfO2)~20 asymmetry in potential distribution across stacktraps accessible for injection from n-type substrate using mid-gap gate metal - TiNtraps not accessible for injection from mid-gap metal -- TiN

  • from M. Houssa, IOP, Chapter 3.4 Lucovsky group NCSUsubstrate electron injectionelectron transportsubstrate hole injection hole trapping>500xC-V -- surface potentials of Si substrate are negative hole injection

  • spectroscopic (SXPS, SE) identification of defect states in TiO2energy of defect state with respect to valence band edge analysis of SXPS spectrum defect state (peak) 2.40.2 eV above VB edgeanalysis of e2 - band gap of 3.2 eV defect state at 2.50.2 eV above VB edgeTiO2 valence band & defect states TiO2 conduction band & defect states

  • SXPS valence band spectra qualitative similarities between TiO2 and HfO2 symmetry driven reversal of Eg and T2g statesHfO2 - mid 1019 cm-3 -- TiO2 - high 1019 to low 1020 cm-3) greater de

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