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Advancing Strained Silicon

Advancing Strained Silicon. A O’Neill , S Olsen, University of Newcastle P-E Hellstrom, M Ostling, KTH K Lyutovich, E Kasper, University of Stuttgart. Scope. First investigation of thin VS MOSFETs 10x reduction in virtual substrate thickness Reduced self-heating Reduced growth time

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Advancing Strained Silicon

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  1. Advancing Strained Silicon A O’Neill, S Olsen, University of Newcastle P-E Hellstrom, M Ostling, KTH K Lyutovich, E Kasper, University of Stuttgart

  2. Scope • First investigation of thin VS MOSFETs • 10x reduction in virtual substrate thickness • Reduced self-heating • Reduced growth time • Compare thin with thick VS MOSFET

  3. SiNANO collaboration • WP1 • Stuttgart • KTH • Newcastle

  4. Strained Si Material Growth ID IL growth T (°C) Processed wafers A1669 550 A1670 575 T1 A1671 600 A1672 625 A1673 650 T2 A1674 675 Si control

  5. Processing • Device isolation: deposited oxide • Gate oxide: 2.8 nm • Poly-Si: 150 nm • Spacer formation: TEOS/Si3N4 • Source/Drain implants: As + 950 °C RTA • Silicide: 20 nm NiSi • Isolation: 200 nm low temperature oxide • Metalisation: TiW (120 nm) and Al (500 nm)

  6. nMOSFET • no cross-hatching or dislocation pile-ups

  7. Wafer yield T2 T1 Si control • 53.4% 89.7% 65.5% • yield not determined by material quality (yield on Si control is only 65%) Lg = 10 mm

  8. T2 T1 Si control Short-channel performance Lg = 0.35 mm Vg-Vt = 1,2,3 V • High performance strained Si MOSFETs • Only small self-heating, despite high Id • High knee voltage for T2 devices

  9. Series resistance: silicide Rsh (ohm/sq) T2 T1 Si control Minimum 49 10 7 Maximum 99 11 7 Median 69 10 8 • RshSi < RshT1 < RshT2 • Same trend for NiSi on gate => primarily a process issue

  10. Series resistance: contacts Rc (ohms) T2 T1 Si control Minimum 42 14 3.8 Maximum 7700 979 15.6 Median 3090 126 8.6 • RcSi << RcT1 << RcT2 • Problems with Al-TiW-NiSi source/drain contacts for strained Si wafers

  11. FIB investigation of contacts T1 smooth contact, no overetch T2 rough contact, much overetch (~160 nm) • Overetch of vias resulting in contact to SiGe virtual substrate (caused by thin silicide layer reducing etch selectivity?)

  12. Impact of Rs on short channel performance Lg = 0.35 mm 400 400 T2 T2 T1 350 350 T1 Si control 300 300 Si control = 1.0 V (mS/mm) = 1.0 V (mS/mm) 250 250 200 maximum transconductance 200 maximum transconductance 150 150 d d at V at V 100 100 max 50 max 50 m g m g 0 0 0 20 40 60 80 100 1 10 100 1000 10000 source/drain sheet resistance R (ohms/sq) contact resistance R (ohms) sh C High Rsh and Rc degradegmmax

  13. 30 25 20 15 maximum transconductance d 10 at V 5 max m g 0 1 10 100 1000 10000 contact resistance R (ohms) c Impact of Rs on long channel performance Lg = 10 mm 30 25 = 1.0 V (mS/mm) 20 = 1.0 V (mS/mm) 15 maximum transconductance d 10 at V T2 T2 T1 5 T1 max Si-control Si-control m g 0 0 20 40 60 80 100 source/drain sheet resistance R (ohm/sq) sh Little impact of high Rsh and Rc on long channel performance => concentrate analysis on large geometry devices

  14. Device performance: Ioff dependence on material Lg = 10 mm T2 Vd = 0.1 V Vd = 1.0 V Vd = 0.1 V T2 T1 T1 • Ioff(T2) > Ioff(T1) > Ioff(Si) • Only T2 wafer exhibits large cross-wafer variation in Ioff

  15. Cross-wafer variation: Ioff T2 T1 Si control Vd = 0.1 V, Lg = 10 mm

  16. Sub-threshold summary Lg = 10 mm, Vd = 0.1 V

  17. Device performance: Ioff dependence on Vd T2 T2 T1 T1 Lg = 10 mm • Both SSi wafers exhibit large dependence of Ioff on Vd • Electrical measurements confirm Ioff dominated by substrate current

  18. Origin of leakage: n+/p junction n=1.45  recombination sites xj ~120 nm intermediate SiGe layer

  19. Origin of leakage: defects T2 T1 100 mm 100 mm Etch pit density: 2.2x106 cm-2 Etch pit density: 9x105 cm-2 • Reverse processing on best-performing devices (gate regions) • Schimmel etch consisting of CrO3/HF • Increased defect density on material grown at T2

  20. C-V characteristics Low T High T Si control • No difference in EOT between wafers (~ 3 nm) • C-V measurements carried out on 50 mm x 100 mm MOS capacitors

  21. Gate oxide quality best 10 mm die best 0.35 mm die 1.E+13 1.E+13 Median Median Best Best ) -1 ) eV -1 eV -2 1.E+12 1.E+12 -2 (cm (cm it D it D 1.E+11 1.E+11 T1 T2 Si Control T1 T2 Si Control • Regardless of gate length: • No impact of SiGe virtual substrate on Dit • No correlation between gmmax and Dit!

  22. Surface roughness • AFM measurements carried on 20 mm x 20 mm scan areas • No clear impact of growth T on surface roughness T2 T1

  23. Analysis by Raman Spectroscopy l = 514.5 nm Si-Si bond in Si substrate Si-Si bond in Si channel Si-Si bond in SiGe VS IL growth T = 675 degC • Raman spectra provide information on Ge composition, channel thickness, virtual substrate thickness and channel strain. • Shift in peak for Si-Si in SiGe indicates fluctuation in VS Ge composition • Spectra may also be influenced by defectivity

  24. Ge-strain correlation T1 T2 • As-grown channel stress follows VS Ge composition • Processed channel strain measurements in progress

  25. Drain current enhancements T2 T1 Si T1 T2 L=W=10um ~ Uniform enhancements in Ion with Vd suggest little self-heating

  26. Mobility enhancement ~ 50%

  27. Device performance: gm Lg = 10 mm T2 T1 Si Vd = 0.1 V

  28. Cross-wafer variation: gm 30 T2 T1 25 Si 20 15 Count 10 5 0 7 9 11 13 15 17 19 21 23 25 Maximum transconductance gm (mS/mm) Vd = 1.0 V, Lg = 10 mm

  29. Impact of gate length on gmmax T1 T2 Si T1 T2 Si Vd = 0.1 V Vd = 1.0 V • Expected increases in gmmax at smaller Lg observed

  30. T1 Impact of gate length on gmmax Reduced enhancements for high Vd – self heating?

  31. T1 Impact of gate length on gmmax Olsen et al, J Appl Phys (2005) Reduced enhancements for high Vd – self heating? Greater impact of self heating for thick virtual substrate

  32. T1 Impact of gate length on gmmax Olsen et al, IEEE Trans ED (2003) Reduced enhancements for high Vd – self heating? Greater impact of self heating for thick virtual substrate Same increase in enhancement for low Vd for thick virtual substrate (x=0.15)

  33. Cut-off frequency vs. gate length W = 5 μm Vd = 1.2 V ~100 % enhancement in cut-off frequency for strained devices

  34. Cut-off frequency vs. gate length W = 5 μm Vd = 1.2 V Lg = 1 μm Vd = 1.2 V ~100 % enhancement in cut-off frequency for strained devices gate width increases cut-off frequency de-embedded pads (not circuit model)

  35. Summary • First thin virtual substrate MOSFETs • Enhanced performance • reduced self heating (cf thick VS) • RF performance demonstrated

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