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recent technology advances

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

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recent technology advances

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  1. 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 graduate students and post docs S. Lee, H. Seo, J.P. Long, C.L. Hinkle and L.B. Fleming collaborators J.L. Whitten (ab-initio theory), J. Lüning (NEXAS, SSRL) , D.E. Aspnes (SE), M.D. Ulrich. J.E. Rowe (SXPS, NSLS-BNL) outline recent technology advances introduction to spectroscopic techniques conduction and valence band electronic states intrinsic bonding defects in Ti, Zr and Hf elemental oxides engineering solutions - HfO2, Hf silicates, tphy < 2 nm

  2. recent technology advances two different dielectrics have emerged as candidates for introduction at the 32 nm process node nano-crystalline HfO2 (< 2 nm) and non-crystalline HfSiON most significant published technology advances from SEMATECH group:Gennadi Bersuker, Pat Lysaght, Paul Kirsch, Manuel Quevedo, Chad Young, et. al this 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

  3. spectroscopic approaches near 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 - BNL 40 to 200 eV, S/N ~1000:1, resolution, ~0.15 eV vis-vacuum UV spectroscopic ellipsometry - vis-VUV SE 1.8 to 6 eV, and 4 eV to ~ 8.5 eV

  4. NEXAS O K1 edge conduct. band UPS, SXPS valence band octahedral bonding of Ti with 6 O zeroth order MO approximation molecular orbital model valence band (SXPS, UPS) and conduction band (NEXAS) reveal d-state features two contributions to lifting of d-state degeneracy “crystal field”symmetry/coordination nano- and non-crystalline films D[Eg(2) - T2g(3)] ~ 2.5 - 4 eV “Jahn-Teller” bonding distortions rutile and distorted CaF2 degeneracies removed Eg 2 states & T2g 3 states d[Eg(2)]~d[T2g(3)] ~ 0.5-1 eV Ti molecular orbitals O states 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

  5. intra-atom core level spectra Zr Zr 5s*,p*, (Ti 4s*,p*) 5s*,p* + O 2p* s s s Zr Zr 4d* 4d* (Ti 3d* s) (5s*,p*) + O 2p* p p p Zr Zr 4d* 4d* (Ti 3d* p) (5s*,p*) + O 2p* deep core level transitions terminate in empty localized 4d* and 5s*(p*) states Zr 3p (Ti 2p) final states display local bonding symmetry of Zr (Ti) atoms Zr 1s x-ray absorption spectroscopy - a novel way to study conduction band d*-states in transition metal oxides inter-atomic spectra Zr Zr 5s*,p* + O 2p* 5s*,p* + O 2p* s s s + O 2p* s Zr Zr 4d* 4d* (5s*,p*) + O 2p* p p Zr 4d* (5s*,p*) + O 2p* Zr 4d* p + O 2p*, p core level and and band edge transitions terminate in similarZr-O molecular orbital states O 2p nb vis and VUV spectroscopic ellipsometry O 1s similar transitions to Ti 3d*, etc.

  6. conduction and valence band edge electronic states TiO2; roadmap for other dielectrics: HfO2 and "Hf SiON" band gaps, BG, scale with atomic d-state energies D(Eg-T2g)av ~3 eV oxide BG at. d-state Ti: 3.2 eV, -11.0 eV Zr: 5.5 eV, -8.1 eV Hf: 5.7 eV, -8.4 eV D(Eg-T2g)av ~2.1 eV crystal field splitting 6-fold coordination octahedral symmetry

  7. Ti d-state degeneracy removal O K1 edge inter-atomicO 2p + 3d, 4s and 4p 2nd derivative of absorption respective OK1 - L3 spectra Ti L3 edge intra-atomicTi 3d Ti 3d, 4s, and 4p

  8. Ti d-state features in O K1 (x-axis) and Ti L3 (y-axis) edges slope of ~ 1 indicates crystal field - average Eg-T2g splittings, and Jahn-Teller term-splittings essentially the same in O K1 and Ti L3 NEXAS spectra L3 is intra-atomic transition O K1 are projections of same d-states, butdifferent transition matrix elements

  9. comparison of average (C-F) d-state splittings valence and conduction bandband edge states O K1, empty conduction band states, filled valence band states D(Eg,Tg): 3.0 eV, 2.1 eV D(4s, 4p): 3.5 eV, 2.6 eV anti-bonding states split more than bonding states

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

  11. 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 splittings however, J-T separations are easily obtained from O K1 edge gaussian 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

  12. 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 a nano-crystalline - 800°C anneal Eg - 532.5, 533.5 (0.15 eV) T2g - 535.2, 536.3, 537.4 (0.15 eV) DEg=10.2 eV - D(T2g - Eg)=2.70.2 eV epsilon 2 (e2) spectrum DEg = 0.80.2 eV D(T2g - Eg) = 2.30.2 eV

  13. summary - part I experimental determination of electronic structure of conduction and valence band edge states NEXAS - OK1 for conduction band states SXPS - for valence band states bonding and localized nature of d-statesmolecular orbital description basis for correlating spectral features with atomic states of transition metal atoms of high-k dielectrics

  14. defect states - electron and hole injection into HfO2 through a thin SiO2/SiON interfacial layer electron and hole trapping/transport asymmetries first studied by IMEC group confirmed at NC State model calculations for defects Robertson/Schluger spectroscopic studies TiO2 VB and e2 spectra -- defect state electronic structure

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

  16. J-V asymmetry - IMEC model continuity of eE - e(SiO2) ~ 3.9 < e(HfO2)~20 asymmetry in potential distribution across stack traps accessible for injection from n-type substrate using mid-gap gate metal - TiN traps not accessible for injection from mid-gap metal -- TiN

  17. EOT~7 nm EOT~1.7 nm from M. Houssa, IOP, Chapter 3.4 Lucovsky group NCSU substrate electron injection electron transport >500x substrate hole injection hole trapping C-V -- surface potentials of Si substrate are negative hole injection

  18. spectroscopic (SXPS, SE) identification of defect states in TiO2 Ti3+ T2g TiO2 valence band & defect states TiO2 conduction band & defect states energy of defect state with respect to valence band edge analysis of SXPS spectrum defect state (peak) 2.4±0.2 eV above VB edge analysis of e2 - band gap of 3.2 eV defect state at 2.5±0.2 eV above VB edge

  19. SXPS valence band spectraqualitative similarities between TiO2 and HfO2symmetry driven reversal of Eg and T2g states HfO2 - mid 1019 cm-3 -- TiO2 - high 1019 to low 1020 cm-3) greater departurses (d) from stoichiometry in TiO(2-d)

  20. Robertson et al., IEEE Trans DMR, 5, 84 (2005) K. Xiong, J. Robertson, S.J. Clark, APL 87 (2005) vacancy (VO) and interstitial (IO) O-atom defects in ZrO2similar results for HfO2 adds two more charge states for O-atom vacancies VO- and VO2+ O-atom mono-vacancy defects do not describe exp. results states too close to Si conduction band edge ~4.2-4.5 above valence band edge of Zr(Hf)O2 ~ 2 eV below lowest d-state feature in Hf(Zr)O2

  21. Schluger group no mono-vacancy states at valence band edge Robertson, et al. Schluger group

  22. model for intrinsic O-vacancy defects in TiO2 comparison with hydrated Ti3+ ion spectrumclassic example in Molecular Orbital Theory texts - Ballhausen and Gray Ti3+ in Ti(H2O)63+ proposal: Ti3+ in TiO2O-divacancies clustered along grain-boundaries Ep(wrt VB) ~ 2.4 eV, D ~ 1 eV

  23. clustered vacancy model for TiO2 TiO(2 - d) = b(TiO2) + a(Ti2O3) b + 2a = 1 2 - d = 2b + 3a a = d b = 1 -2d concentration of Ti3+ = 2d if defects are divacancies, then 2d ~ 10-3 or 2-3x1019 cm-3 similar calculations apply to ZrO2 and HfO2 and defect states are labeled accordingly divacancies - clustered on grain boundaries two Ti, Zr or Hf atoms of each divacancy defect nearest neighbors to 2 missing O-atoms

  24. model for Ti3+ defect states in TiO2 band gap model calculation TiO2 valence & conduction bands degeneracy in T2g defect state removed by J-T distortion (as in Ti2O3) HB Gray, and HB Gray and CJ Ballhausen

  25. SXPS and UPS valence band spectra of TiO2 SXPS valence band spectra for HfO2 and UPS valence band spectra of ZrO2 4 10 ZrO 2 1000 100 0 -2 -4 -6 -8 -10 -12 -14 -16 HfO 2 SXPS 60 eV 6d 5/2 6d 3/2 6s p O2p 6p nb UPS He I Ti3+ T2g photoelectron counts Hf3+ Eg symmetry Zr3+ range of UPS reliable data binding energy (eV) Ti 3d-state contributions to VB qualitative similar spectra 4d and 5d states

  26. band edge defect spectral features ZrO2 XAS, VUV SE, PC defect state defect state O K1 VUV SE PC

  27. comparison between Zollner and NCSU VUV SE measurements and analysis

  28. summary - part II spectroscopic detection of band edge defects valence band edge - SXPS, conduction band edge - O K1 NEXAS, VUV SE and PC not described by mono-vacancy calculations of Robertson (and Schluger) energy of formation (>5-8 eV) much too high for concentrations > 1019 cm-3 defect states in TiO2 identified by analogy with hydrated Ti3+ion in solution, and Ti2O3 band gap Ti3+in divacancies clustered along grain boundaries similar assignments for HfO2 and TiO2 intrinsic defects grain boundary defect model supported by measurements of TJ King for HfO2 as function of annealing as crystal sizegrows, grain boundary density decreases and defect signature in VUV SE is reduced

  29. defect states as annealing temperature is increased band edge, and discrete defect concentrations are each reduced

  30. scales of order for electronic structure/defect formation grain boundaries are well-defined when crystallite size extends to several (~3-4) “primitive” cell units cell dimensions are typically ~ 0.5 nm, so that critical dimension for defect formation is ~1.5 nm to 2 nm experimental observations by SEMATECH and STM defect densities are significantly reduced when physical film thickness is reduced below 2 nm is there a spectroscopic signature for this change in crystallite size? yes, relative strengths of p and s-anti-bonding states in OK1 spectra

  31. s-bonding is intra-primitive cell in character it therefore occurs on a 0.5 nm scale [O-s1-s1'-Ti-s2'-s2-O]-s3-s3'-Ti-, etc s1, s2, s3, etc., are different O-atom s-bonds s1', s2', s3'. etc., are different Ti-atom s-bonds [....] indicates the “primitive” cell bonding unit s-bonds within the cell - localized on intra-cell atoms p-bonding is inter-primitive cell in character occurs on a scale > 1.5-2.0 nm p1-[O-p1-p1'-Ti-p1'-p2-O]-p2-p3'-Ti-, etc p1-O-p1-p1'-{Ti-p1'-p2-O-p2-p3'-Ti}-, etc p1, p2, p3, etc., are different O-atom p-bonds p1', p2', p3'. etc., are different Ti-atom p-bonds [....] indicates the “primitive” cell bonding unit {.....} indicates the coupling of primitive cells via O-p-bonds O p2 bond couples 2 different primitive cells

  32. chemical phase separation: Zr silicates - thickness > 20 nm 0.6 0.6 ~30% ZrO ~30% ZrO 2 2 0.55 0.55 900 C C o o 0.5 0.5 anneal anneal IRO Zr 4d* Zr 4d* 0.45 0.45 4d 4d 4d 4d 3/2 3/2 5/2 5/2 0.4 0.4 absorption (arb. units) absorption (arb. units) D D E ~ 0.5 eV E ~ 0.5 eV 0.35 0.35 as-deposited as-deposited 0.3 0.3 300 300 C C SRO o o 0.25 0.25 0.2 0.2 530 530 532 532 534 534 536 536 538 538 540 540 photon energy (eV) photon energy (eV) 0.5 eV spectral shift 1/2 of D(4d3/2/Eg) of nano-crystalline-ZrO2 degeneracy removal ~3 nm nano-crystallite grains 3 nm Zr silicate x~0.25 after 900°C anneal

  33. change in energy of first peak in nano-crystalline ZrO2as a function of film thickness

  34. thickness dependence of lowest p-stateas function of thickness ZrO2, high ZrO2 content silicate alloy films of ~ 2 nm each show low defect densities for EOT < 1 nm

  35. scales of order for SiO2 ab initio calculation for ~ 1 nm cluster gives excited states that correlate with absorption spectrum for thick 10 nm SiO2 fix energy gap of 8.90.1 eV with derivative feature at 529.6 eV and e2 peaks are in good agreement with O K1

  36. cluster for electronic transition calculationswhen relaxed by CI gives the experimental bond angle, bond angle distribution, and IR effective charges physical size ~ 1 nm Si* are effective potentials that ensure there is no dipole moment, and that core levels of Si and O within cluster are correct

  37. O K1 edges of SiO2spectra for films as thin as 1.5 nm are essentially the same as those of films10 nm thick molecular Si-O-Si units are coupled by s-bonds of four-fold coordinated Si atoms via different orbitalsvery from p-coupling of Ti, Zr and Hf elemental oxides

  38. application to SiO2correlation between OK1 and reflectivity spectrum of Herb Phillip for non-crystalline fused silica and crystalline alpha quartz e2 peaks est. from plot (±0.2 eV) 10.13 eV 12.35 eV 13.92 eV 16.55 eV is there a linear correlation between features in reflectivity and O K1 edge? spectral peaks at same energies in non-crystalline and crystalline SiO2

  39. radiation effects in nano-crystalline HfO2 and non-crystalline Hf silicate MOSCAPs midgap voltage shift vs. does for 67.5 nm HfO2 and Al2O3 ALD dielectrics on 1.1 nm Si oxynitride for different annealing - effects different than SiO2 due to grain-boundary defects (a) midgap voltage and (b) flatband voltage shifts for total dose irradiations for Hf silicate capacitors with 4.5 nm EOT gate dielectrics - SiO2 like

  40. summary band edge electronic structure NEXAS O K1 edge “replicates: conduction band states SXPS gives valence band states defect states are vacancies clustered on grain boundaries of nano-crystalline oxides densities >1019 cm-3 and easily detected by O K1, SXPS and vis-VUV SE scale of order for suppression of band edge p-state degeneracy removal is ~2 nm two engineering solutions for 32 nm nodeultra thin HfO2, Hf silicate alloys (well done SEMATECH!!) scales of order for p- and s-bonding differentiated by extent to which “primative” unit cells are coupled through O-atoms SiO2 is qualitatively different scale of order for conduction band “resonance excitons” is 1 nmdemonstrated by ab initio theory and verified by experiment - O K1 edge

  41. Ti, Zr and Hf trapped in Si3N4-SiO2 matrixno chemical phase separation to 1100°C EOTs to 32 nm node Zr SiON 40% Si3N4 2.2 eV for 4-fold, ~4 eV for 8(7) fold high-lighted d-state splittings in Zr and Ti SiON consistent with 4-fold coordination of Zr and Ti Hf SiON - decreased hole trapping in rad testing (later in review)

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