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Mobility Enhancement in Strained Silicon-Germanium Channels: Collaborative Workshop Insights

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  1. SiNANO Workshop David LeadleyUniversity of Warwick Carrier mobility enhancement in strained silicon germanium channels

  2. Collaborators • Warwick Tim Grasby, Andy Dobbie, Chris Beer, Jon Parsons, Evan Parker, Terry Whall • IMEC Gareth Nicholas, Marc Meuris, M Heyns, P Zimmerman, Matty Caymax, ++ • SINANO partners KTH,Udine, Chalmers, AMO

  3. Why is mobility still critical ? High mobility - light mass and minimal scattering • ITRS – long term years • By 2020, Lg=5nm, Vdd=0.7V • Mobility enhancement factor of 1.04 each node • Ballistic enhancement factor x2 • Double gate structures – light doping

  4. Why add Ge? • ADVANTAGES • Smaller bandgap • Lighter hole mass • Strain – splits bands and reduces scattering • BUT… • Native oxide no good • Band-to-band tunnelling

  5. Pseudomorphic Si0.64Ge0.36 on Si Mobility doubled

  6. Impact Ionisation Reduced impact ionisation for SiGe, despite higher mobility Nicholas et al, Electronics Letters41, 20052074

  7. Metal HfO2 SiO2 SiGe Si High-k gated SiGe • Deposited HfO2 • Si cap – oxidises to thin SiO2 interlayer Lowest EOT 12Å < 1nm

  8. Metal gate/HfO2 gate stack development for Si(cap)/Si0.8Ge0.2 channel Interface state density (W gate) Gate leakage Warwick, AMO, Chalmers

  9. High-k/metal gated Si0.8Ge0.2 surface p-channel devices ........enhancements beating Intel (IEDM 2004) KTH, Warwick, Chalmers, AMO

  10. Effective Mobility Best SiGe (25%) devices show mobility enhancement over silicon control Mobility degradation compared to universal – interface roughness and Coulomb scattering important KTH, Warwick, Chalmers, AMO

  11. [001] [011] [110] [011] (110) substrate (100) substrate HOT SiGe • Hybrid Orientation Technology • n on (100), p on (110) in Si, Yang et al. IEDM 2003 • in Si, no variation with orientation for long channels, but <110> best for short channels, Saito et al VLSI 2006 • 50% enhanced Idsat for SiGe (110) over Si (100),3.3x for mobility, Liu et al VLSI 2006

  12. = 3.2 nm Novel p-channel/substrate orientations … enhancement beating Liu et al(VLSI Symp 2006) KTH, Warwick, Chalmers, AMO

  13. Unstrained Ge pMOS • High performance Ge pMOS devices using a Si-compatible process flow • Zimmerman et al. IEDM 2006 4nm HfO2 2μm Ge on Si substrate 12Å EOT, TDD 107-108 cm-2 PMOS fabricated with Lg ~125 nm IMEC

  14. Unstrained Ge Ge + anneal Si IMEC

  15. Nit spread 0.2-8x1012cm-2 • affects Ids, gm and Vt Uniformity restored after anneal IMEC

  16. Thinner EOT? Si Starts to leak when too thin ! • Mobility increase with thinner HfO2 • less charge trapping • Ragnarsson et al. IEEE TED 53, 1657 ( 2006) IMEC

  17. Mobility in unstrained Ge μ > 300 cm2/Vs Correction required for Rsd in short channel devices Lg<0.25μm IMEC

  18. Investigating trapped charge Dit varies across wafer 1011 - 2x1012cm-2 Warwick, IMEC

  19. Extract Dit in 3 ways at 300K: • from Vt – average all energies • subthreshold slope – specific energy • charge pumping Interface charge density Warwick, IMEC

  20. Mobility modelling at 4K • At low T, with confinement only have one HH subband. Fit using ni and D as parameters ... Warwick, IMEC

  21. As grown Annealed Modelled 4K mobility ni agrees with values from Dit Δdecreases after anneal from 0.6nm to 0.5nm,hence μSR increases by 35% μ300Kmust also depend on surface roughness Warwick, IMEC

  22. Strain tuning buffers • Strained-Si by local strain now in production • Si1-yGey virtual substrates for global strain • y~0.2 for s-Si • y~0.5 for s-Si1-xGex • y~0.8 for s-Ge • Need low TDD and zero pile-up

  23. Terrace Graded VS – better than industrial quality 20% Ge TDD ≤ 105cm-2, PUD=0 XTEM 10x10um2 AFM image indicating RMS roughness 1.5nm Warwick, LETI, Jeulich

  24. State-of-the-art sSi electron mobilityfrom TG-VSs y = 15 -27% strain 0.6 – 1.1% State of the art Warwick, Udine, KTH F. Driussi et al., ULIS 2007

  25. sGe global platform- 80% terrace grade TDD=3x105/cm-2, PUD = 0, RMS ≈ 8nm Warwick, FZ-Jeulich, LETI

  26. Strained Si Strained Ge 300nm Si0.2Ge0.8 Grade Seed layer Si Sub Novel thin VS for sGe 20x20 um2 AFM image. 1.3nm RMS, fully relaxed 80% platform TDD ≈ 106/cm2 range Warwick

  27. 150nm TiN 4nm ALD HfO2 Positive Vt due to gate workfunction Strained Ge pMOS ASM, Warwick, IMEC

  28. Record Ge mobility via CMOS process Peak μ = 650 cm2/Vs Warwick, IMEC G. Nicholas et al., IEEE EDL 28, 825 (2007)

  29. Further comments on s-Ge • Full band Monte Carlo calculations, incl. BTBT, SCE etc. show best prospect for pMOS … • … strained-Ge directly on Si ! Krishnamohan et al (Stanford) VLSI (2006) • For s-SiGe OI, Idsat enhancement in short channels exceeds μ increase in long channel.Tsutomu Tezuka et al. VLSI (2006) • nMOS still a problem for Ge …… but tensile strain Si0.1Ge0.9 on relaxed Ge promising.

  30. Summary • Addition of Ge improves mobility • High-k makes Ge viable • Mobility enhancements relevant for nanometre scale devices

  31. 30% TDD ≈ 4 x 104 cm-2 PUD = 0 Terrace grading – new generation of virtual substrates Warwick

  32. LG 30% ≈105 cm-2 TDD Pile-up ≈1 cm-1

  33. High stability sSi layers Linear Grading Relaxation (%) Terrace Grading Strained Si thickness (nm) Warwick, FZ-Jeulich J. Parsons et al., ULIS 2007

  34. Extended Stacking Faults PVTEM image of 180nm strained silicon layer Defect etch image of 20nm strained silicon layer Warwick, FZ-Jeulich J. Parsons et al., ULIS 2007

  35. Extended Stacking Faults “Perfect” dislocation Stacking fault Trailing (30°) “partial” dislocation Leading (90°) “Partial” dislocation S-Si SiGe

  36. Stacking faults– effective misfit dislocation blockers? Defect etch images of 30nm strained silicon layer Warwick, FZ-Jeulich J. Parsons et al., ULIS 2007

  37. 50% terrace graded (for sSi) TDD = 2x105 cm-2 PUD = 0 Warwick

  38. ≈ 20% TG - slower growth(+sSi) TDD = 4 x 104/cm2 PUD = 0

  39. Schottky Barrier MOSFET- world best? Tunnelling f(m*) Thermionic emmision Measurement Modelling UCL, Warwick, Glasgow D.J. Pearman et al., IEEE Trans ED (accepted 2006)

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