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 departurses (d) from stoichiometry in TiO(2-d)

Robertson et al., IEEE Trans DMR, 5, 84 (2005)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)O2vacancy (VO) and interstitial (IO) O-atom defects in ZrO2 similar results for HfO2K. Xiong, J. Robertson, S.J. Clark, APL 87 (2005)adds two more charge states for O-atom vacancies VO- and VO2+

Schluger groupno mono-vacancy states at valence band edgeRobertson, et al.Schluger group

Ti3+ in Ti(H2O)63+model for intrinsic O-vacancy defects in TiO2 comparison with hydrated Ti3+ ion spectrum classic example in Molecular Orbital Theory texts - Ballhausen and GrayEp(wrt VB) ~ 2.4 eV, D ~ 1 eVproposal: Ti3+ in TiO2 O-divacancies clustered along grain-boundaries

clustered vacancy model for TiO2TiO(2 - d) = b(TiO2) + a(Ti2O3)b + 2a = 12 - d = 2b + 3aa = db = 1 - 2dconcentration of Ti3+ = 2dif defects are divacancies, then 2d ~ 10-3 or 2-3x1019 cm-3similar calculations apply to ZrO2 and HfO2 and defect states are labeled accordinglydivacancies - clustered on grain boundaries two Ti, Zr or Hf atoms of each divacancy defect nearest neighbors to 2 missing O-atoms

HB Gray, and HB Gray and CJ Ballhausenmodel for Ti3+ defect states in TiO2 band gapmodel calculation TiO2 valence & conduction bandsdegeneracy in T2g defect state removed by J-T distortion (as in Ti2O3)

SXPS and UPS valence band spectra of TiO2Ti 3d-state contributions to VBSXPS valence band spectra for HfO2 and UPS valence band spectra of ZrO2qualitative similar spectra 4d and 5d statesUPS He IHfO2SXPS 60 eVrange of UPSreliable data6p6s6d3/26d5/2O2ppnbHf3+photoelectron countsbinding energy (eV)Eg symmetryTi3+ T2g Zr3+

O K1VUV SEPC

comparison between Zollner and NCSU VUV SE measurements and analysis

summary - part II spectroscopic detection of band edge defects valence band edge - SXPS, conduction band edge - O K1 NEXAS, VUV SE and PCnot described by mono-vacancy calculations of Robertson (and Schluger) energy of formation (>5-8 eV) much too high for concentrations > 1019 cm-3defect states in TiO2 identified by analogy with hydrated Ti3+ ion in solution, and Ti2O3 band gap Ti3+ in divacancies clustered along grain boundariessimilar assignments for HfO2 and TiO2 intrinsic defectsgrain boundary defect model supported by measurements of TJ King for HfO2 as function of annealing as crystal size grows, grain boundary density decreases and defect signature in VUV SE is reduced

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

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

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 units-bonds within the cell - localized on intra-cell atomsp-bonding is inter-primitive cell in character occurs on a scale > 1.5-2.0 nmp1-[O-p1-p1'-Ti-p1'-p2-O]-p2-p3'-Ti-, etcp1-O-p1-p1'-{Ti-p1'-p2-O-p2-p3'-Ti}-, etcp1, 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

chemical phase separation: Zr silicates - thickness > 20 nm3 nm0.5 eV spectral shift 1/2 of D(4d3/2/Eg) of nano-crystalline-ZrO2degeneracy removal ~3 nm nano-crystallite grainsZr silicate x~0.25 after 900C anneal

change in energy of first peak in nano-crystalline ZrO2 as a function of film thickness

thickness dependence of lowest p-state as function of thickness ZrO2, high ZrO2 content silicate alloyfilms of ~ 2 nm each show low defect densities for EOT < 1 nm

scales of order for SiO2ab initio calculation for ~ 1 nm cluster gives excited states that correlate with absorption spectrum for thick 10 nm SiO2 fix energy gap of 8.90.1 eV with derivative feature at 529.6 eV and e2 peaks are in good agreement with O K1

Si* are effective potentials that ensure there is no dipole moment, and that core levels of Si and O within cluster are correctcluster for electronic transition calculations when relaxed by CI gives the experimental bond angle, bond angle distribution, and IR effective charges

O K1 edges of SiO2 spectra for films as thin as 1.5 nm are essentially the same as those of films10 nm thickmolecular Si-O-Si units are coupled by s-bonds of four-fold coordinated Si atoms via different orbitals very from p-coupling of Ti, Zr and Hf elemental oxides

application to SiO2 correlation between OK1 and reflectivity spectrum of Herb Phillip for non-crystalline fused silica and crystalline alpha quartzspectral peaks at same energies in non-crystalline and crystalline SiO2is there a linear correlation between features in reflectivity and O K1 edge?e2 peaksest. from plot(0.2 eV) 10.13 eV12.35 eV13.92 eV16.55 eV

(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 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 defectsradiation effects in nano-crystalline HfO2 and non-crystalline Hf silicate MOSCAPs

summary band edge electronic structure NEXAS O K1 edge replicates: conduction band states SXPS gives valence band statesdefect 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 SEscale of order for suppression of band edge p-state degeneracy removal is ~2 nm two engineering solutions for 32 nm node ultra 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-atomsSiO2 is qualitatively different scale of order for conduction band resonance excitons is 1 nm demonstrated by ab initio theory and verified by experiment - O K1 edge

high-lighted d-state splittings in Zr and Ti SiON consistent with 4-fold coordination of Zr and TiHf SiON - decreased hole trapping in rad testing (later in review)Ti, Zr and Hf trapped in Si3N4-SiO2 matrix no chemical phase separation to 1100C EOTs to 32 nm nodeZr SiON 40% Si3N42.2 eV for 4-fold, ~4 eV for 8(7) fold