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Dopant and Self-Diffusion in Silicon and Silicon Germanium

Dopant and Self-Diffusion in Silicon and Silicon Germanium. Eugene Haller, Hughes Silvestri, and Chris Liao MS&E, UCB and LBNL FLCC Tutorial 4/18/05. Outline. Motivation Background Fick’s Laws Diffusion Mechanisms Experimental Techniques for Solid State Diffusion

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Dopant and Self-Diffusion in Silicon and Silicon Germanium

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  1. Dopant and Self-Diffusion in Silicon and Silicon Germanium Eugene Haller, Hughes Silvestri, and Chris Liao MS&E, UCB and LBNL FLCC Tutorial 4/18/05

  2. Outline • Motivation • Background • Fick’s Laws • Diffusion Mechanisms • Experimental Techniques for Solid State Diffusion • Diffusion of Si in Stable Isotope Structures • Future Work: Diffusion of SiGe in Stable Isotope Structures • Conclusions FLCC Tutorial

  3. Motivation • Why diffusion is important for feature level control of device processing • Nanometer size feature control: - any extraneous diffusion of dopant atoms may result in device performance degradation • Drain extension Xj < 10 nm by 2008* • Extension lateral abruptness < 3 nm/decade by 2008* • Accurate models of diffusion are required for dimensional control on the nanometer scale *International Technology Roadmap for Semiconductors, 2004 Update FLCC Tutorial

  4. Semiconductor Technology Roadmap (International Technology Roadmap for Semiconductors, 2004 Update) FLCC Tutorial

  5. Planar Bulk-Si Structure Thin-Body Structure scaling to Lg < 20nm MOSFET Scaling Si1-xGex in the S/D regions will be needed for thin-body PMOSFETs in order to • enhance mobility via strain • lower parasitic resistance • S/D series resistance • contact resistance  Si and Ge interdiffusion, as well as B diffusion in Si1-xGex and Si must be well understood and characterized Courtesy of Pankaj Kalra and Prof. Tsu-Jae King FLCC Tutorial

  6. Jin Jout dx Example: Vacancy Mechanism Fick’s Laws (1855) Fick’s 1st Law: Flux of atoms 2nd Law Diffusion equation does not take into account interactions with defects! Jout Jin -RS +GS FLCC Tutorial

  7. Analytical Solutions to Fick’s Equations D = constant - Finite source of diffusing species: Solution: Gaussian - Infinite source of diffusing species: Solution: Complementary error function FLCC Tutorial

  8. Solutions to Fick’s Equations (cont.) D = f (C) Diffusion coefficient as a function of concentration Concentration dependence can generate various profile shapes and penetration depths FLCC Tutorial

  9. Solid-State Diffusion Profiles Experimentally determined profiles can be much more complicated - no analytical solution Kennel, H.W.; Cea, S.M.; Lilak, A.D.; Keys, P.H.; Giles, M.D.; Hwang, J.; Sandford, J.S.; Corcoran, S.; Electron Devices Meeting, 2002. IEDM '02, 8-11 Dec. 2002 B implant and anneal in Si with and without Ge implant FLCC Tutorial

  10. Direct Diffusion Mechanisms in Crystalline Solids (no native defects required) Pure interstitial Elements in Si: Li, H, 3d transition metals Direct exchange No experimental evidence High activation energy → unlikely FLCC Tutorial

  11. Vacancy-assisted Diffusion Mechanisms (native defects required) Vacancy mechanism (Sb in Si) Dissociative mechanism (Cu in Ge) FLCC Tutorial

  12. Interstitial-assisted Diffusion Mechanisms (native defects required) Interstitialcy mechanism (P in Si) Kick-out mechanism (B in Si) FLCC Tutorial

  13. Why are Diffusion Mechanisms Important? • Device processing can create non-equilibrium native defect concentrations • Implantation: excess interstitials • Oxidation: excess interstitials • Nitridation: excess vacancies • High doping: Fermi level shift FLCC Tutorial

  14. Oxidation Effects on Diffusion • Oxidation of Si surface causes injection of interstitials into Si bulk • Increase in interstitial concentration causes enhanced diffusion of B, As, but retarded Sb diffusion • Nitridation (vacancy injection) causes retarded B, P diffusion, enhanced Sb diffusion Oxidation during device processing can lead to non-equilibrium diffusion (Fahey, et al., Rev. Mod. Phys. 61 289 (1989).) FLCC Tutorial

  15. Implantation Effects on Diffusion Transient Enhanced Diffusion (TED) -Eaglesham, et al., Appl. Phys. Lett.65(18) 2305 (1994). • Implantation damage generates excess interstitials • Enhance the diffusion of dopants diffusing via interstitially-assisted mechanisms • Transient effect - defect concentrations return to equilibrium values • TED can be reduced by implantation into an amorphous layer or by carbon incorporation into Si surface layer • Substitutional carbon acts as an interstitial sink • Stolk, et al., Appl. Phys. Lett.66 1371 (1995) FLCC Tutorial

  16. Ec 0.11 eV V--/- 0.57 eV V-/o 0.35 eV Io/+ 0.13 eV V+/++ 0.05 eV Vo/+ Ev Doping Effects on Diffusion Heavily doped semiconductors - extrinsic at diffusion temperatures • Fermi level moves from mid-gap to near conduction (n-type) or valence (p-type) band. • Fermi level shift changes the formation enthalpy, HF, of the charged native defect • Increase of CI,V affects Si self-diffusion and dopant diffusion V states (review by Watkins, 1986) FLCC Tutorial

  17. Doping Effects on Diffusion The change in native defect concentration with Fermi level position causes an increase in the self- and dopant diffusion coefficients FLCC Tutorial

  18. Experimental Techniques for Diffusion Creation of the Source • Diffusion from surface • Ion implantation • Sputter deposition • Buried layer (grown by MBE) Annealing Analysis of the Profile • Radioactivity (sectioning) • SIMS • Neutron Activation Analysis • Spreading resistance • Electro-Chemical C/Voltage Modeling of the Profile • Analytical fit • Coupled differential eq. FLCC Tutorial

  19. Primary Experimental Approaches • Radiotracer Diffusion • Implantation or diffusion from surface • Mechanical sectioning • Radioactivity analysis • Stable Isotope Multilayers – new approach • Diffusion from buried enriched isotope layer • Secondary Ion Mass Spectrometry (SIMS) • Dopant and self-diffusion FLCC Tutorial

  20. Radiotracer Diffusion • Diffusion using radiotracers was first technique available to measure self-diffusion • Limited by existence of radioactive isotope • Limited by isotope half-life (e.g. - 31Si: t1/2 = 2.6 h) • Limited by sensitivity • Radioactivity measurement • Width of sections Mechanical/Chemical sectioning Generate depth profile Application of radio-isotopes to surface Concentration (cm-3) Measure radioactivity of each section annealing Depth (m) FLCC Tutorial

  21. Diffusion Prior to Stable Isotope Multilayer Stuctures What was known about Si, B, P, and As diffusion in Si Si: self-diffusion: interstitials + vacancies known: interstitialcy + vacancy mechanism, QSD ~ 4.7 eV unknown: contributions of native defect charge states B: interstitial mediated: from oxidation experiments known: diffusion coefficient unknown: interstitialcy or kick-out mechanism P: interstitial mediated: from oxidation experiments known: diffusion coefficient unknown: mechanism for vacancy contribution As: interstitial + vacancy mediated: from oxidation + nitridation experiments known: diffusion coefficient unknown: native defect charge states and mechanisms FLCC Tutorial

  22. a-Si cap nat. Si 28Si enriched FZ Si substrate Stable Isotope Multilayers • Diffusion using stable isotope structures allows for simultaneous measurements of self- and dopant diffusion • No half-life issues • Ion beam sputtering rather than mechanical sectioning • Mass spectrometry rather than radioactivity measurement FLCC Tutorial

  23. Ion gun Mass spectrometer Ion detector Secondary Ion Mass Spectrometry • Incident ion beam sputters sample surface - Cs+, O+ • Beam energy: ~1 kV • Secondary ions ejected from surface (~10 eV) are mass analyzed using mass spectrometer • Detection limit: ~1012 - 1016 cm-3 • Depth profile - ion detector counts vs. time • Depth resolution: 2 - 30 nm FLCC Tutorial

  24. Diffusion Parameters found via Stable Isotope Heterostructures • Charge states of dopant and native defects in diffusion • Contributions of native defects to self-diffusion • Enhancement of extrinsic dopant and self-diffusion • Mechanisms which mediate self- and dopant diffusion FLCC Tutorial

  25. Si Self-Diffusion • Enriched layer of 28Si epitaxially grown on natural Si • Diffusion of 30Si monitored via SIMS from the natural substrate into the enriched cap (depleted of 30Si) • 855 ºC < T < 1388 ºC • Previous work limited to short times and high T due to radiotracers • Accurate value of self-diffusion coefficient over wide temperature range: 1153 ºC, 19.5 hrs 1095 ºC, 54.5 hrs (Bracht, et al., PRL 81 1998) FLCC Tutorial

  26. ni Interstitialcy mechanism Vacancy mechanism Si and Dopant Diffusion Arsenic doped sample annealed 950 ˚C for 122 hrs extrinsic intrinsic Io I- I-- FLCC Tutorial

  27. ni Interstitialcy mechanism Vacancy mechanism Si and Dopant Diffusion Arsenic doped sample annealed 950 ˚C for 122 hrs IoI-I-- FLCC Tutorial

  28. ni Interstitialcy mechanism Vacancy mechanism Si and Dopant Diffusion Arsenic doped sample annealed 950 ˚C for 122 hrs IoI-I-- FLCC Tutorial

  29. Si and Dopant Diffusion Supersaturation of Io, I+ due to B diffusion Io andI+ mediate Si and B diffusion Enhancement due to Fermi level effect Diffusion mechanism: Kick-out • Bi0 Bs- + I 0 + h • Bi0 Bs- + I + FLCC Tutorial

  30. Si and Dopant Diffusion Phosphorus Diffusion Model: Interstitialcy or Kick-out mechanism – Io, I- Pair assisted recombination or dissociative mechanism – V0 Annealed 1100 ˚C for 30 min FLCC Tutorial

  31. f C D = + + = - - - I I I D ( n ) f C D f C D D ( n ) o o o + + + Si i tot Si i I I I I I I Native Defect Contributions to Si Diffusion (Bracht, et al., 1998) Diffusion coefficients of individual components add up accurately: (B diffusion) (B, P diffusion) (As, P diffusion) FLCC Tutorial

  32. Diffusion in Ge Stable Isotope Structure Annealed 586 °C for 55.55 hours Ge self-diffusion coefficient determined from 74Ge/70Ge isotope structure Fuchs, et al., Phys. Rev B51 1687 (1995) FLCC Tutorial

  33. Intel’s 90nm CMOS Technology Si1-xGex in PMOS S/D regions to enhance on-state drive current without increasing off-state leakage compressive strain  30% Idsat increase Diffusion in Si1-xGex • SiGe is used as new material to enhance electronic devices • Will face same device diffusion issues as Si • Currently, limited knowledge of diffusion properties Si1-xGex in the S/D regions will be needed for thin-body PMOSFETs in order to • enhance mobility via strain • lower parasitic resistance • S/D series resistance • contact resistance  Si and Ge interdiffusion, as well as B diffusion in Si1-xGex and Si must be well understood and characterized T. Ghani et al., 2003 IEDM Technical Digest Courtesy of Pankaj Kalra and Prof. Tsu-Jae King FLCC Tutorial

  34. Diffusion in SiGe Isotope Structures • Diffusion of Si in pure Ge • Si and Ge self-diffusion in relaxed Si1-xGex structures • Si and Ge self-diffusion in strained Si1-xGex structures • Simultaneous Si and Ge dopant and self-diffusion FLCC Tutorial

  35. ~ 1020 cm-3 Si Ge epilayer Ge substrate [Si] [C] - - - Si Diffusion in Pure Ge • Before determination of Si and Ge self-diffusion in SiGe can be made must determine Si diffusion in Ge and Ge diffusion in Si • Large amounts of data on Ge diffusion in Si - used as a tracer for Si self-diffusion due to longer half-life • Much less data on Si diffusion in Ge • MBE grown Ge layer • 100 nm spike of Si (1020 cm-3) FLCC Tutorial

  36. Si Diffusion in Pure Ge Annealed at 550 °C for 30 days FLCC Tutorial

  37. 200 nm nat. Si1-xGex 400 nm 28Si1-x70Gex 200 nm nat. Si1-xGex SiGe graded buffer layer Si substrate Si and Ge Self-DiffusionRelaxed Si1-xGex Structures • Use isotope heterostructure technique to study Si and Ge self-diffusion in relaxed Si1-xGex alloys.(0.05 ≤ x ≤ 0.85) • No reported measurements of simultaneous Si and Ge diffusion in Si1-xGex alloys • Proposed isotope heterostructure: • MBE grown - Group of Prof. Arne Nylandsted Larsen, Univ. of Aarhus, Denmark FLCC Tutorial

  38. 100 nm nat. Si1-yGey 200 nm 28Si1-x70Gex 100 nm nat. Si1-yGey SiGe graded buffer layer Si substrate Si and Ge Self-DiffusionStrained Si1-xGex Structures • Study Si and Ge self-diffusion in strained Si1-xGex alloys. • 0.15 ≤ x ≤ 0.75 • Vary composition between layers to generate: • Compressive strain (x - y < 0) • Tensile strain (x - y > 0) • x - y≈ 0.05 Proposed isotope heterostructure: MBE grown - Group of Prof. Arne Nylandsted Larsen FLCC Tutorial

  39. amorphous Si cap 100 nm 28Si1-x70Gex 100 nm nat. Si1-xGex SiGe graded buffer layer Si substrate Simultaneous Dopant and Self-DiffusionSi1-xGex Multilayer Structures • Five alternating 28Si1-x70Gex (0.05 ≤ x ≤ 1) and natural Si1-xGex layers with amorphous cap • Implant dopants (B, P, As) into amorphous cap • Simultaneous Si and Ge self-diffusion and dopant diffusion Proposed isotope heterostructure: MBE grown - Group of Prof. Arne Nylandsted Larsen FLCC Tutorial

  40. Conclusions • Diffusion in semiconductors is increasingly important to device design as feature level size decreases. • Device processing can lead to non-equilibrium conditions which affect diffusion. • Diffusion using stable isotopes yields important diffusion parameters which previously could not be determined experimentally. • Technique will be extended to SiGe alloys with variation of composition, strain and doping level. FLCC Tutorial

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