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 - HfO 2, Hf silicates, tphy < 2 nm

recent technology advances two different dielectrics have emerged as candidates for introduction at the 32 nm process node nano-crystalline HfO 2 (< 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

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

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  [E g (2) - T 2g (3)] ~ eV “Jahn-Teller ” bonding distortions rutile and distorted CaF 2 degeneracies removed E g  2 states & T 2g  3 states  [E g (2)]~  [T 2g (3)] ~ eV octahedral bonding of Ti with 6 O zeroth order MO approximation 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 NEXAS O K 1 edge conduct. band UPS, SXPS valence band

x-ray absorption spectroscopy - a novel way to study conduction band d*-states in transition metal oxides intra-atom core level spectra Zr 1s Zr5s*,p* + O 2p* Zr4d*  (5s*,p*) + O 2p*  Zr4d*  (5s*,p*) + O 2p*  Zr5s*,p*, (Ti 4s*,p*) Zr4d*  (Ti 3d*  Zr4d*  (Ti 3d*  Zr 3p deep core level transitions terminate in empty localized 4d* and 5s*(p*) states final states display local bonding symmetry of Zr (Ti) atoms (Ti 2p) inter-atomic spectra Zr5s*,p* + O 2p* Zr4d*  (5s*,p*) + O 2p*  Zr4d*  (5s*,p*) + O 2p*  Zr5s*,p* + O 2p* Zr4d*  + O 2p*  Zr 4d*  + O 2p*,  O 1s O 2p nb core level and and band edge transitions terminate in similar Zr-O molecular orbital states vis and VUV spectroscopic ellipsometry similar transitions to Ti 3d*, etc.

conduction and valence band edge electronic states TiO 2 ; roadmap for other dielectrics: HfO 2 and "Hf SiON"  (E g -T 2g ) av ~3 eV crystal field splitting 6-fold coordination octahedral symmetry  (E g -T 2g ) av ~2.1 eV band gaps, BG, scale with atomic d-state energies oxide BG at. d-state Ti: 3.2 eV, eV Zr: 5.5 eV, -8.1 eV Hf: 5.7 eV, -8.4 eV

Ti d-state degeneracy removal O K 1 edge inter-atomic O 2p + 3d, 4s and 4p Ti L 3 edge intra-atomic Ti 3d 2nd derivative of absorption respective OK 1 - L 3 spectra Ti 3d, 4s, and 4p

Ti d-state features in O K 1 (x-axis) and Ti L 3 (y-axis) edges slope of ~ 1 indicates crystal field - average E g -T 2g splittings, and Jahn-Teller term-splittings essentially the same in O K 1 and Ti L 3 NEXAS spectra L 3 is intra-atomic transition O K 1 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 states O K 1, empty conduction band states, filled valence band states  (E g,T g ): 3.0 eV, 2.1 eV  (4s, 4p): 3.5 eV, 2.6 eV anti-bonding states split more than bonding states

linear scaling (slope 1) between Ti L 3, and both O K 1 and  2 gives linear scaling (slope 1) between  2 and O K 1 O K 1 edge - wider spectral range, than lab VUV SE

limitations on NEXAS approach p-state core hole life-times scale inversely as Z eff 2.5 (Slater rules) for M 3,N 3 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 K 1 edge gaussian fits and 2nd derivatives term splittings for E g and T 2g compared with epsilon 2 (  2 ) from VUV-SE, SXPS valence band spectra, and studied for different scales of order film thickness

intrinsic conduction band edge electronic structure O K 1 edge in XAS d-states - same splittings as conduction band d-states in SE VUV  2 and  nano-crystalline - 800°C anneal E g , (  0.15 eV) T 2g , 536.3, (  0.15 eV)  E g =1  0.2 eV -  T 2g - E g )=2.7  0.2 eV epsilon 2 (  2 ) spectrum  E g = 0.8  0.2 eV  T 2g - E g ) = 2.3  0.2 eV

summary - part I experimental determination of electronic structure of conduction and valence band edge states NEXAS - OK 1 for conduction band states SXPS - for valence band states bonding 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 HfO 2 through a thin SiO 2 /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 TiO 2 VB and  2 spectra -- defect state electronic structure

Massoun et al., APL 81, 3392 (2002) Si-SiO 2 -HfO 2 gate stacks traps are in high-k material of stack 2x10 13 cm  ~ 1.5x cm -2 coulombic center - lower x-section than P b centers in Si substrate screened by high dielectric constant of HfO 2 Z. Xu et al., APL 80, 1975 (2002) substrate injection electrons gate injection electrons electron trap ~0.5 eV below conduction band edge HfO 2 substrate injection holes

J-V asymmetry - IMEC model continuity of  E -  (SiO 2 ) ~ 3.9 <  (HfO 2 )~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

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

spectroscopic (SXPS, SE) identification of defect states in TiO 2 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  2 - band gap of 3.2 eV defect state at 2.5±0.2 eV above VB edge TiO 2 valence band & defect states TiO 2 conduction band & defect states Ti 3+ T 2g

SXPS valence band spectra qualitative similarities between TiO 2 and HfO 2 symmetry driven reversal of E g and T 2g states HfO 2 - mid cm TiO2 - high to low cm -3 ) greater departurses (  ) from stoichiometry in TiO(2-  )

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 ~ above valence band edge of Zr(Hf)O 2 ~ 2 eV below lowest d-state feature in Hf(Zr)O 2 vacancy (V O ) and interstitial ( I O ) O-atom defects in ZrO 2 similar results for HfO 2 K. Xiong, J. Robertson, S.J. Clark, APL 87 (2005) adds two more charge states for O-atom vacancies V O - and V O 2+

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

Ti 3+ in Ti(H 2 O) 6 3+ model for intrinsic O-vacancy defects in TiO 2 comparison with hydrated Ti 3+ ion spectrum classic example in Molecular Orbital Theory texts - Ballhausen and Gray E p (wrt VB) ~ 2.4 eV,  ~ 1 eV proposal: Ti 3+ in TiO 2 O-divacancies clustered along grain-boundaries

clustered vacancy model for TiO 2 TiO (2 -  ) =  (TiO 2 ) +  (Ti 2 O 3 )  + 2  =  = 2  + 3   =   = 1 -  2  concentration of Ti 3+ = 2  if defects are divacancies, then 2  ~ or 2-3x10 19 cm -3 similar calculations apply to ZrO 2 and HfO 2 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

HB Gray, and HB Gray and CJ Ballhausen model for Ti 3+ defect states in TiO 2 band gap model calculation TiO 2 valence & conduction bands degeneracy in T 2g defect state removed by J-T distortion (as in Ti 2 O 3 )

SXPS and UPS valence band spectra of TiO 2 Ti 3d-state contributions to VB SXPS valence band spectra for HfO 2 and UPS valence band spectra of ZrO 2 qualitative similar spectra 4d and 5d states UPS He I HfO 2 SXPS 60 eV range of UPS reliable data 6p 6s 6d 3/2 6d 5/2 O2p  nb Hf 3+ photoelectron counts binding energy (eV) E g symmetry Ti 3+ T 2g Zr 3+

band edge defect spectral features ZrO 2 XAS, VUV SE, PC defect state O K 1 VUV SE PC

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 K 1 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 > cm -3 defect states in TiO 2 identified by analogy with hydrated Ti 3+ ion in solution, and Ti 2 O 3 band gap Ti 3+ in divacancies clustered along grain boundaries similar assignments for HfO 2 and TiO 2 intrinsic defects grain boundary defect model supported by measurements of TJ King for HfO 2 as function of annealing as crystal size grows, grain boundary density decreases and defect signature in VUV SE is reduced

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

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  and  -anti-bonding states in OK 1 spectra

 -bonding is intra-primitive cell in character it therefore occurs on a 0.5 nm scale [O-   -   '-Ti-   '-   -O]-   -   '-Ti-, etc    ,  , etc., are different O-atom  -bonds   '    ',   '. etc., are different Ti-atom  -bonds [....] indicates the “primitive” cell bonding unit  -bonds within the cell - localized on intra-cell atoms  -bonding is inter-primitive cell in character occurs on a scale > nm   -[O-   -   '-Ti-   '-   -O]-   -   '-Ti-, etc   -O-   -   '-{Ti-   '-   -O-   -   '-Ti}-, etc    ,  , etc., are different O-atom  -bonds   '    ',   '. etc., are different Ti-atom  -bonds [....] indicates the “primitive” cell bonding unit {.....} indicates the coupling of primitive cells via O-  -bonds O  2 bond couples 2 different primitive cells

chemical phase separation: Zr silicates - thickness > 20 nm 3 nm 0.5 eV spectral shift 1/2 of  (4d 3/2 /E g ) of nano-crystalline-ZrO 2 degeneracy removal ~3 nm nano-crystallite grains Zr silicate x~0.25 after 900°C anneal absorption (arb. units) photon energy (eV) as-deposited 300 o C 900 o C anneal Zr 4d* 4d 3/2 4d 5/2  E ~ 0.5 eV absorption (arb. units) photon energy (eV) as-deposited 300 o C o C anneal Zr 4d* 4d 3/2 4d 5/2  E ~ 0.5 eV ~30% ZrO 2 2 SRO IRO

change in energy of first peak in nano-crystalline ZrO 2 as a function of film thickness

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

scales of order for SiO 2 ab initio calculation for ~ 1 nm cluster gives excited states that correlate with absorption spectrum for thick 10 nm SiO 2 fix energy gap of 8.9  0.1 eV with derivative feature at eV and  2 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 correct cluster for electronic transition calculations when relaxed by CI gives the experimental bond angle, bond angle distribution, and IR effective charges physical size ~ 1 nm

O K 1 edges of SiO 2 spectra 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 orbitals very from  -coupling of Ti, Zr and Hf elemental oxides

application to SiO 2 correlation between OK 1 and reflectivity spectrum of Herb Phillip for non-crystalline fused silica and crystalline alpha quartz spectral peaks at same energies in non-crystalline and crystalline SiO 2 is there a linear correlation between features in reflectivity and O K1 edge?  2 peaks est. from plot (±0.2 eV) eV eV eV eV

(a) midgap voltage and (b) flatband voltage shifts for total dose irradiations for Hf silicate capacitors with 4.5 nm EOT gate dielectrics - SiO 2 like midgap voltage shift vs. does for 67.5 nm HfO 2 and Al 2 O 3 ALD dielectrics on 1.1 nm Si oxynitride for different annealing - effects different than SiO 2 due to grain- boundary defects radiation effects in nano- crystalline HfO 2 and non- crystalline Hf silicate MOSCAPs

summary band edge electronic structure NEXAS O K 1 edge “replicates: conduction band states SXPS gives valence band states defect states are vacancies clustered on grain boundaries of nano-crystalline oxides densities >10 19 cm -3 and easily detected by O K 1, SXPS and vis-VUV SE scale of order for suppression of band edge  -state degeneracy removal is ~2 nm two engineering solutions for 32 nm node ultra thin HfO 2, 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 SiO 2 is qualitatively different scale of order for conduction band “resonance excitons” is 1 nm demonstrated by ab initio theory and verified by experiment - O K 1 edge

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) Ti, Zr and Hf trapped in Si 3 N 4 -SiO 2 matrix no chemical phase separation to 1100°C EOTs to 32 nm node Zr SiON 40% Si 3 N eV for 4-fold, ~4 eV for 8(7) fold