+ V gate M O. Trend: As k, E g. Part 6: High Dielectric Constant (k), Gate Electrode, & Channel Materials. Bandgap versus Dielectric Constant (k) k k

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1 Part 6: High Dielectric Constant (k), Gate Electrode, & Channel Materials O 2 gate oxide is approaching physical limits Thickness & Current M O S poly-crystalline V Source W Source Contact Insulator n + source + V gate n ++ Poly Gate Contact or Electrode O 2 - Gate oxide channel p- Wafer Drain Contact Insulator n + drain V Drain + Crystalline L t ox C ox okoxa t ox t EOT k k t ox highk highk Frank, Dennard, Nowak, Soloman, Wong & Taur, Proc. IEEE Circuit & Devices, 89 (2001) 259 M. Houssa et al., Materials Science and Engineering R, 51 (2006) Bandgap versus Dielectric Constant (k) Trend: As k, E g e E e Hik c e e E Hik c Robertson, MRS Bulletin (March 2002) p. 217 Robertson, J. Vac. Sci. Technol. B, 18(3), May/Jun

2 Band Offsets: High k on Aspects to Consider: 1. E g 2. E c & E v 3. m eff Recall in Barrier Region: e So: kx J e * 2kx k k 2 m ( V E ) eff m eff e m, J eff e E e Hik c e e E Hik c Robertson, MRS Bulletin (March 2002) p. 217 J g ( kt ) k fleftde Ef frightde E f dk pˆ 1 E where p mg & g & g m k 3 Band diagrams of MOS compare O 2 to high k materials O 2 ; κ~3.9 HfO 2 ; κ~25 PbZr (0.53) Ti (0.47) O 3 κ~200 Low S/C doping Southwick &, IEEE TDMR, 6(2), (2006)

3 Need to consider the Thermodynamics of the materials system Ellingham diagram G vs- Temperature The more negative G is, the more stable the materials system is. Example: Grow Y 2 O 3 on, will steal oxygen from Y 2 O 3 to form interfacial layer (IL) of O 2. Why? Y 2 O 3 O 2 Al 2 O 3 G G O2 Y2O3 O more stable than Y O R. DeHoff, Thermodynamics of Materials, (Prentice Hall, 1996) Ch. 11, fig Interfacial layer (IL) of O 2 Present for HfO 2 TiN 3.5 EOT WRT t O2 & t HfO2 3 O 2 IL HfO 2 E OT n m t IL Crystalline Channel t HfO2 nm k t t t IL ox, eff IL highk khighk 6 3

4 EOT: 7 nm HfO 2 & 1nm O 2 : EOT ~ 2 nm 8 nm HfO 2 : EOT ~ 1.25 nm Southwick &, IEEE TDMR, 6(2), (2006) NVM Floating Gate NVM Versus SONOS (O 2-3 N 4 -O 2 -) NVM SONOS Advantages over Floating Gate: Replace poly- floating gate with 3 N 4 Stored charge lies in defect (bound) states below 3 N 4 conduction band Improved endurance - single defect will not cause the discharge/leakage of carriers Can reduce Thickness of TO 3 N 4 thinner than floating gate Poly Carriers not Floating around Minimizes interaction with neighboring memory cells Thus, can scale down memory cell size BL BL = Blocking Layer CTL = Charge Trapping Layer TL = Tunnel Layer CTL TL Todd Wallinger, SONOS Eases Non-Volatile Memory Integration in SoC, Semiconductor International (2007) 8 4

5 - NVM BL CTL TL BL CTL TL BL TL Flat Band Condition Poly Floating Gate Poly Energy Band Diagram Energy Band Diagram 3 N 4 = CTL Gate stack scaled down in thickness & cell area Todd Wallinger, SONOS Eases Non-Volatile Memory Integration in SoC, Semiconductor International (2007) 9 - NVM Flat Band Condition BL TL Poly 3 N 4 Energy Band Diagram Gate stack scaled down in thickness & cell area Todd Wallinger, SONOS Eases Non-Volatile Memory Integration in SoC, Semiconductor International (2007) 10 5

6 - NVM Multilayer high k dielectric films for memory applications SONOS (poly O N O ) MANOS (metal Al 2 O 3 N O ) TANOS (/O 2 /N/A 2 O 3 /TaN) Sanghun et al., IEEE TED 52 (2005) 2654.pdf Lee et al., Symposium on VLSI Technology Digest of Technical Papers ( Low Dielectric Constant (k) Materials What About Low-k Dielectric Materials? What would they be used for? C ox okoxa t ox 12 6

7 Compare and Contrast following Memory Gate Stacks: Stack #: Metal BL CTL TL -S/C Stack 1: TiN O2 3 N4 O 2 p Stack 2: TiN Al 2 O 3 Ta 2 O 5 HfO 2 p Stack 3: TiN La 2 O 3 ZnO ZrO 2 p

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10 19 MOSFETs Bandgap Engineering of Channel Consider: Bandgap, mobility, effective mass,lattice matching, quantum confinement of carriers Lattice Constants: a = Å a Ge = Å Cullity, Elements of X-ray Diffraction, 2 nd Ed (1978) Appendix 5 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap ( McGraw-Hill, 2005) 20 10

11 MOSFETs Bandgap Engineering of Channel Consider: Bandgap, mobility, effective mass, lattice matching, quantum confinement of carriers Lattice Constants: a = Å a Ge = Å Cullity, Elements of X-ray Diffraction, 2 nd Ed (1978) Appendix 5 IBM RJ Antoniadis et al., Continuous MOFET Performance Inc with Scaling - Strain & Channel Matl (2006) 21 11