1. FLCC 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. FLCC 4/18/2005 FLCC Tutorial 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

3. FLCC 4/18/2005 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 X j < 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

4. FLCC 4/18/2005 FLCC Tutorial 4 Semiconductor Technology Roadmap (International Technology Roadmap for Semiconductors, 2004 Update) Difficult Challenges ≥ 45nm Through 2010Summary of Issues Front-end Process modeling for nanometer structures Diffusion/activation/damage models and parameters including SPE and low thermal budget processes in Si-based substrate, i.e., Si, SiGe:C, Ge (incl. strain), SOI, and ultra-thin body devices Characterization tools/methodologies for these ultra shallow geometries/junctions and low dopant levels Modeling hierarchy from atomistic to continuum for dopants and defects in bulk and at interfaces Front-end processing impact on reliability

5. FLCC 4/18/2005 FLCC Tutorial 5 MOSFET Scaling Planar Bulk-Si StructureThin-Body Structure scaling to L g < 20nm Si 1-x Ge x 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 Si 1-x Ge x and Si must be well understood and characterized Courtesy of Pankaj Kalra and Prof. Tsu-Jae King

6. FLCC 4/18/2005 FLCC Tutorial 6 Fick’s Laws (1855) Diffusion equation does not take into account interactions with defects! J in J out +G S -R S Example: Vacancy Mechanism 2nd Law J in J out dx Fick’s 1st Law: Flux of atoms

7. FLCC 4/18/2005 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

8. FLCC 4/18/2005 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

9. FLCC 4/18/2005 FLCC Tutorial 9 Solid-State Diffusion Profiles Experimentally determined profiles can be much more complicated - no analytical solution B implant and anneal in Si with and without Ge implant 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

10. FLCC 4/18/2005 FLCC Tutorial 10 Direct Diffusion Mechanisms in Crystalline Solids Pure interstitial Direct exchange Elements in Si: Li, H, 3d transition metals No experimental evidence High activation energy → unlikely (no native defects required)

11. FLCC 4/18/2005 FLCC Tutorial 11 Vacancy-assisted Diffusion Mechanisms Dissociative mechanism Vacancy mechanism (Sb in Si) (Cu in Ge) (native defects required)

12. FLCC 4/18/2005 FLCC Tutorial 12 Interstitial-assisted Diffusion Mechanisms Interstitialcy mechanism Kick-out mechanism (P in Si) (B in Si) (native defects required)

13. FLCC 4/18/2005 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

14. FLCC 4/18/2005 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 (Fahey, et al., Rev. Mod. Phys. 61 289 (1989).) Oxidation during device processing can lead to non-equilibrium diffusion

15. FLCC 4/18/2005 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)

16. FLCC 4/18/2005 FLCC Tutorial 16 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, H F, of the charged native defect –Increase of C I,V affects Si self-diffusion and dopant diffusion EcEc EvEv V --/- V -/o V +/++ V o/+ I o/+ 0.11 eV 0.57 eV 0.35 eV 0.13 eV 0.05 eV V states (review by Watkins, 1986)

17. FLCC 4/18/2005 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

18. FLCC 4/18/2005 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.

19. FLCC 4/18/2005 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

20. FLCC 4/18/2005 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. - 31 Si: t 1/2 = 2.6 h) –Limited by sensitivity –Radioactivity measurement –Width of sections Application of radio- isotopes to surface annealing Mechanical/Chemical sectioning Measure radioactivity of each section Depth (  m) Concentration (cm -3 ) Generate depth profile

21. FLCC 4/18/2005 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, Q SD ~ 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

22. FLCC 4/18/2005 FLCC Tutorial 22 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 28 Si enriched FZ Si substrate nat. Si a-Si cap

23. FLCC 4/18/2005 FLCC Tutorial 23 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: ~10 12 - 10 16 cm -3 Depth profile - ion detector counts vs. time –Depth resolution: 2 - 30 nm Ion gun Mass spectrometer Ion detector

24. FLCC 4/18/2005 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

25. FLCC 4/18/2005 FLCC Tutorial 25 Si Self-Diffusion Enriched layer of 28 Si epitaxially grown on natural Si Diffusion of 30 Si monitored via SIMS from the natural substrate into the enriched cap (depleted of 30 Si) –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: (Bracht, et al., PRL 81 1998) 1095 ºC, 54.5 hrs 1153 ºC, 19.5 hrs

26. FLCC 4/18/2005 FLCC Tutorial 26 Si and Dopant Diffusion Arsenic doped sample annealed 950 ˚C for 122 hrs Vacancy mechanism Interstitialcy mechanism nini extrinsic intrinsic I o I - I --

27. FLCC 4/18/2005 FLCC Tutorial 27 Si and Dopant Diffusion Arsenic doped sample annealed 950 ˚C for 122 hrs nini IoI-I--IoI-I-- Vacancy mechanism Interstitialcy mechanism

28. FLCC 4/18/2005 FLCC Tutorial 28 Si and Dopant Diffusion Arsenic doped sample annealed 950 ˚C for 122 hrs nini IoI-I--IoI-I-- Vacancy mechanism Interstitialcy mechanism

29. FLCC 4/18/2005 FLCC Tutorial 29 Si and Dopant Diffusion Supersaturation of I o, I + due to B diffusion I o and I + mediate Si and B diffusion Enhancement due to Fermi level effect Diffusion mechanism: Kick-out –B i 0  B s - + I 0 + h –B i 0  B s - + I +

30. FLCC 4/18/2005 FLCC Tutorial 30 Si and Dopant Diffusion Phosphorus Diffusion Model: Interstitialcy or Kick-out mechanism – I o, I - Pair assisted recombination or dissociative mechanism – V 0 Annealed 1100 ˚C for 30 min

31. FLCC 4/18/2005 FLCC Tutorial 31 Native Defect Contributions to Si Diffusion Diffusion coefficients of individual components add up accurately: D Si (n i ) tot  f I o C I o D I o  f I  C I  D I   f I  C I  D I   D Si (n i ) (Bracht, et al., 1998) (B diffusion) (As, P diffusion)(B, P diffusion)

32. FLCC 4/18/2005 FLCC Tutorial 32 Diffusion in Ge Stable Isotope Structure Annealed 586 °C for 55.55 hours Fuchs, et al., Phys. Rev B 51 1687 (1995) Ge self-diffusion coefficient determined from 74 Ge/ 70 Ge isotope structure

33. FLCC 4/18/2005 FLCC Tutorial 33 Diffusion in Si 1-x Ge x SiGe is used as new material to enhance electronic devices –Will face same device diffusion issues as Si –Currently, limited knowledge of diffusion properties compressive strain  30% I dsat increase Intel’s 90nm CMOS Technology Si 1-x Ge x in PMOS S/D regions to enhance on-state drive current without increasing off-state leakage Si 1-x Ge x 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 Si 1-x Ge x 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

34. FLCC 4/18/2005 FLCC Tutorial 34 Diffusion in SiGe Isotope Structures Diffusion of Si in pure Ge Si and Ge self-diffusion in relaxed Si 1-x Ge x structures Si and Ge self-diffusion in strained Si 1-x Ge x structures Simultaneous Si and Ge dopant and self-diffusion

35. FLCC 4/18/2005 FLCC Tutorial 35 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 (10 20 cm -3 ) Ge substrate Ge epilayer ~ 10 20 cm -3 Si [Si] [C] - - -

36. FLCC 4/18/2005 FLCC Tutorial 36 Si Diffusion in Pure Ge Annealed at 550 °C for 30 days

37. FLCC 4/18/2005 FLCC Tutorial 37 Si and Ge Self-Diffusion Relaxed Si 1-x Ge x Structures Use isotope heterostructure technique to study Si and Ge self- diffusion in relaxed Si 1-x Ge x alloys. (0.05 ≤ x ≤ 0.85) –No reported measurements of simultaneous Si and Ge diffusion in Si 1-x Ge x alloys Proposed isotope heterostructure: –MBE grown - Group of Prof. Arne Nylandsted Larsen, Univ. of Aarhus, Denmark Si substrate SiGe graded buffer layer 200 nm nat. Si 1-x Ge x 400 nm 28 Si 1-x 70 Ge x

38. FLCC 4/18/2005 FLCC Tutorial 38 Si and Ge Self-Diffusion Strained Si 1-x Ge x Structures Study Si and Ge self-diffusion in strained Si 1-x Ge x 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 Si substrate SiGe graded buffer layer 100 nm nat. Si 1-y Ge y 200 nm 28 Si 1-x 70 Ge x Proposed isotope heterostructure: MBE grown - Group of Prof. Arne Nylandsted Larsen

39. FLCC 4/18/2005 FLCC Tutorial 39 Simultaneous Dopant and Self-Diffusion Si 1-x Ge x Multilayer Structures Five alternating 28 Si 1-x 70 Ge x ( 0.05 ≤ x ≤ 1 ) and natural Si 1-x Ge x layers with amorphous cap Implant dopants (B, P, As) into amorphous cap Simultaneous Si and Ge self-diffusion and dopant diffusion amorphous Si cap Si substrate SiGe graded buffer layer 100 nm nat. Si 1-x Ge x 100 nm 28 Si 1-x 70 Ge x Proposed isotope heterostructure: MBE grown - Group of Prof. Arne Nylandsted Larsen

40. FLCC 4/18/2005 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.