1 / 38

Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach

Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach. PhD Final Examination Anisur Rahman PhD Co-Advisors: Professor Mark Lundstrom and Professor Gerhard Klimeck School of Electrical and Computer Engineering Purdue University, West Lafayette, IN 47907.

zaide
Download Presentation

Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Exploring New Channel Materials for Nanoscale CMOS Devices: A Simulation Approach PhD Final Examination Anisur Rahman PhD Co-Advisors: Professor Mark Lundstrom and Professor Gerhard Klimeck School of Electrical and Computer Engineering Purdue University, West Lafayette, IN 47907 PhD Defense, Anisur Rahman

  2. Acknowledgements • Doctoral Advisory Committee: • Professor Supriyo Datta (ECE) • Professor Ron Reifenberger (Physics) • Dr. Avik Ghosh (ECE) • Present Colleagues: • Dr. Diego Kienle • Dr. Jing Wang • Sayed Hasan • Neophytos Neophytou • Siyu Koswatta • Former Colleagues: • Dr. Zhibin Ren (IBM) • Dr. Ramesh Venugopal (TI) • Dr. Jung-Hoon Rhew (Intel) • Asst. Prof. Jing Guo (Univ. of Florida) Funding: SRC and MARCO/FCRP-MSD PhD Defense, Anisur Rahman

  3. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 PhD Defense, Anisur Rahman

  4. Background: CMOS Scaling • Historical Development: • Four decades of steady scaling • Present day MOSFETs are nanoscale (LG~30nm) • Scaling is motivated by performance and • integration density issues • Moore’s Law describes this steady growth • ITRS guides the future scaling trends Moore’s Law • Scaling Challenges: • Implementation of Moore’s law becoming challenging • Off-state leakage limits scaling of planar CMOS • Additional gates needed to curb SCE (dual/tri-gate) • Gate oxide scaling reached the limit (direct tunneling) • High-κ dielectric + metal gate in near future • Interface properties of Si-SiO2 becoming less critical 65nm Node Deviecs LG=35nm PhD Defense, Anisur Rahman

  5. Background: New Materials Electron Transport Property • Motivation for Novel Materirals: • Ge and III-V display very high mobility, • saturation velocity, and scattering • mean-free path • New process technology allows high-quality • channel-insulator interface • Ultra-high-speed,very-low-power logic • application • Experimental research is underway Ashley et al., ICSICT 2004 Current Calculation Qtop vinj • Key Device Physics Issues: • Treatment of quantum mechanical effects • Trade-off between high velocity and low DOS • Atomic level fluctuation become significant • Full-band treatment become necessary Source Drain Current=QtopX vinj Qtop=CG(VG-VT) where, CG<COX PhD Defense, Anisur Rahman

  6. Objective: • For New Channel Material Nanoscale CMOS Devices: • Develop simulation tools and theoretical approaches • Perform design studies, assess performance limits, and explore • scaling characteristics • Investigate relevance of carrier mobility • Identify key bandstructure related issues • Provide an improved understanding of their operation. • Theoretical Approaches: • Effective-mass-equation based quantum transport using NEGF formalism • for novel-channel material n-MOSFETs • Atomistic tight-binding approach for incorporating bandstructure effects in • deeply scaled ballistic n- and p-MOSFETs PhD Defense, Anisur Rahman

  7. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary and Conclusion • Future Work Chapter 2 Chapter 3 PhD Defense, Anisur Rahman

  8. Theory: Effective Mass Approach Ellipse Crystal in Effective Mass Equation (EME) Channel Bottom Gate Drain Top Gate Source • Key Features of EMA: • Most widely used approach for n-MOSFETs • Very successful to treat quantum transport • in (100) Si n-MOSFETs • NEGF and mode-space decomposition of the • transport problem is highly efficient Device Outstanding Issue: When DCS and ECS are not aligned, EMT is a full 3X3 matrix and solution of EME is very difficult PhD Defense, Anisur Rahman

  9. Theory: Generalized Effective Mass Unitary transformations conserves density of states and group velocity PhD Defense, Anisur Rahman

  10. Results: Ge n-MOSFETs LT=LG+2Lul Ch S D VD = 0.4 V Lul Lul LG NSD NSD IDS [μA/μm] Doping Density Profile Ballistic NEGF ID-VG Ballistic NEGF ID-VD VG = 0.4 V (4) (2) IDS [μA/μm] (1) Valley Deg. Si (001)/[100] Ge (111)/[110] Ge (001)/[100] • Simulation Setup: • End of ITRS 2001 UTB DG Ge n-MOSFETs • NEGF ballistic and scattering simulation • Design study performed • nanoMOS 2.5 modified to treat Ge n-MOSFETs • Ge devices on (100) and (111) wafers • Process variation and mobility effects examined EOT=0.6nm PhD Defense, Anisur Rahman Rahman et al., IEDM 2003

  11. Results: Ge n-MOSFETs Ch S D NSD NSD Energy μCh μSD μSD tbody Sensitivity to process variation NEGF Scattering Ballistic limit W/C device Y ch=1000cm2/V-sec Thick body ITRS 2016 IOFF [μA/μm] Nominal IDS [μA/μm] tbody = 2.5 nm ch=300cm2/V-sec Thin body VG = VD = 0.4V VD = 0.4 V; VG = 0 V Gate Length, LG [nm] S/D mobility, SD [cm2/V-sec] PhD Defense, Anisur Rahman

  12. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary and Conclusion • Future Work Chapter 4 PhD Defense, Anisur Rahman

  13. Top-of-the-Barrier Ballistic Model CQ Circuit Model for 2-D Electrostatics VG COX CS CD VS VD Gate Electrostatics • Key Features: • Semiclassical ballistic transport • Poisson equation solved only at top-of-barrier • Quantum capacitance, CQ, treated • Treats arbitrary bandstructure • Treats floating source potential • Two carrier fluxes, FS and FD present • Sum of carrier density in FS and FD is total charge • Difference of current from FS and FD is net current FS FD Source Drain UTB DG Model Device Top Gate Ch S D Rahman et al., TED, 2003 Bottom Gate PhD Defense, Anisur Rahman

  14. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 5 PhD Defense, Anisur Rahman

  15. Theory: Tight-binding (001) (111) (010) (110) a (100) Channel Drain Source Device — Ultra-thin Body Symmetric Dual-gate MOSFET Two FCC Lattices — Red andBlue • Challenges: • Appropriate TB model • Finite dimensional systems • Remove surface states • Treat electrostatics • Sparse matrix techniques • Computing I-V • Key Features: • Localized atomic orbital-like basis set • Suitable for modeling nanostructures • Correct full bandstructure • Strain, hetero-channel, novel materials PhD Defense, Anisur Rahman

  16. Approach: A Good TB Model Silicon CB ΔEc = +100meV • “Well-behaved” • TB Parameter Set • Manageable size — two • centerintegrals • Correctbandgap and • effective masses • Scalablefor strained • materials Vogl NEMO NN-sp3s* 5 orbitals NN-sp3d5s* 10 orbitals Boykin et al., PRB 69(11), 2004 PhD Defense, Anisur Rahman

  17. Approach: Band Bending TB Domain Calculate Bandstructure Transport Potential Charge EC VG Bulk/HOI MOSFETs EV nZ atoms Poisson Domain Poisson Charge Potential UTB MOSFETs TB Domain Self-consistence nZ atoms Band bending along thickness is less important in UTB. EC VG VG EV Band bending along thickness is important in bulk/HOI devices. PhD Defense, Anisur Rahman

  18. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Validity of single band parabolic E-k in Ge n-MOSFETs • Compare Si, Ge, GaAs, InAs as channel materials • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 6 PhD Defense, Anisur Rahman

  19. Study: Validity of Parabolic E-k in Ge • Simulation Setup: • Validity of parabolic E-k in UTB • unstrained Ge n-MOSFET examined • 20 band sp3d5s*-SO TB used • Three thicknesses: 16nm, 4nm, 2nm • Band bending not treated • I-V calculated from top-of-barrier model NN-sp3d5s*-SO ΔEc=300meV ΔEc=170meV • Bulk Ge Bandstructure: • The CB Δ, Λ,Г within 250meV. • Parabolic E-k valid at low energy • Quantum confinement can • alter the order of the bands. Red-Eff. mass Black-TB PhD Defense, Anisur Rahman Rahman et al., IEDM 2004

  20. Results: Thick Body Limit (16nm) L Г X 1 3 3 1 2D DOS 2 2 2D E-k • ~ 16nm (113 atomic layers) Ge body • Size of H ~ 2200 X 2200 • Electrostatic potential not treated • (100) wafer  quantization along [100] • Conduction band  subbands • Non-parabolicity important at high energy • I-V shown for (001)/[100] device [110] [100] Ballistic I-V VDS = 0.4V VT not adjusted PhD Defense, Anisur Rahman

  21. Results: Going Thinner (4nm) L Г X 1 3 1 3 2 2 2D E-k • ~ 4nm (30 atomic layers) Ge body • Size of H ~ 600 X 600 • Conduction band  Subbands • L and Г- Valleys came closer • Non-parabolicity affects ground state • Ge (001)/[100] device • VT shifted by 55 meV (not adjusted) [110] [100] Ballistic I-V 2D DOS 25% lower ION for eff. mass VDS = 0.4V VT not adjusted ΔVT=55 meV ΔVT PhD Defense, Anisur Rahman

  22. Results: Extreme Scaling (2nm) L Г X 1 3 1 3 2 2 2D E-k • ~ 2nm (12 atomic layers) Ge body • Size of H ~ 240 X 240 • Conduction band  Subbands • At Г, X Valleys form ground state • Strong non-parabolicity affects L-valleys • VT shifted by 570 meV (adjusted) [110] [100] Ballistic I-V 2D DOS 15% higher ION for eff. mass VT adjusted ΔVT=570 meV ΔVT PhD Defense, Anisur Rahman

  23. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Validity of single band parabolic E-k in Ge n-MOSFETs • Compare Si, Ge, GaAs, InAs as channel materials • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 6 PhD Defense, Anisur Rahman

  24. Study: New Channel Materials Group IV • Objective: • To explore and compare scaling characteristics of CMOS with Si, Ge, GaAs, InAs as channel materials. • Simulation Setup: • 2016 device specification from ITRS 2004 (22nm HP) • UTB DG with, body: 19 AL (~2.5nm), EOT=0.5nm • Unstrained material • Semiclassical, top-of-barrier, ballistic model Eeff = 1.3eV Si Energy, [eV] Eeff=1.04eV Ge Group III-V • Bandstructure: • CB and VB split into subbands • Quantum confinement increase effective band gap • Very high vinj expected for III-V • Lowest CB are X2(Si), L4(Ge), Г1(GaAs, InAs) • Si and Ge display higher CB DOS compared to III-V Eeff=1.72eV GaAs Energy, [eV] InAs Eeff=0.78eV Rahman et al., to appear in IEDM 2005. PhD Defense, Anisur Rahman

  25. Results: New Channel Materials ID-VD Ge Ge Si GaAs InAs vinj-VG • Injection velocity, vinj vs. VG: • Very high vinj for III-V materials • Electron vinj in InAs is highest, as expected • Beyond 0.4V, GaAs vinj drops (Г→L transfer) InAs GaAs Si Ge • Carrier density, Qtop vs. VG: • Ge: High CB DOS is key to its success • InAs: Electron Qtop and CG is strongly degraded • Beyond 0.4V, CG in GaAs improves (Г→L transfer) Qtop-VG Ge Si GaAs Low CB DOS strongly degrades deeply scaled III-V device performances InAs • Device: Deeply scaled body (19AL) • and oxide (0.5nm) • Ballistic ID-VD: • Ge performs best (n or p-FET) • GaAs or InAs cannot compete with Si or Ge • InAs n-FETs performs worst PhD Defense, Anisur Rahman

  26. Results: New Channel Materials Thin Oxide Ge tox=1.0nm InAs InAs & GaAs Ge Si GaAs Si tox=0.5nm InAs tox=0.5nm InAs GaAs GaAs Ge Ge Si Si Si tox=1.0nm • Thin EOT + Thick Body (Small CQ/COX): • Ge n-MOSFETs perform best • Qtops in III-V suffer, CG strongly degraded tox=0.5nm Ge Si Ge GaAs GaAs InAs InAs ID-VD Thick Oxide • Effects of ratioCQ/COXexplored: • 100 AL (~15nm) body thickness • Only n-MOSFETs treated • Ballistic transport using top-of-barrier model • Less QC effect, subbands closely separated • Thick body increase 2D DOS, consequently, CQ • EOT =1.0nm, and 0.5nm (thin) considered. vinj-VG • Thick EOT + Thick Body (Large CQ/COX): • III-V n-MOSFETs perform best • Silicon performs worst (slow X4 valleys) tox=1.0nm Qtop-VG PhD Defense, Anisur Rahman

  27. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Mobility behavior in strained bulk p- and n- MOSFETs • Hole density profile in HOI structure • Summary • Future Work Chapter 7 PhD Defense, Anisur Rahman

  28. Study: Strained Bulk p-MOSFETs Source Drain SiGe SiGe Si Substrate Induced Biaxial Tension Process Induced Uniaxial Compression Unstrained Intel 90 nm (Uniaxial) Rim et al. (Biaxial) 1995 Drain Source Source Drain Mobility [cm2/V-sec] Si Si Si Universal Hole Mobility (Unstrained) Si Si SiGe Si Rim et al. (Biaxial) 2002 Eeff [MV/cm] • Overview: • Devices on (100) wafers • Strain can be substrate-induced (biaxial) or • process-induced (uniaxial) • Strain deforms crystal by changing bond lengths • and bond orientations • Biaxial tensile strain: Hole mobility improves • at low gate bias but disappears at high bias • Uniaxial compressive strain: Hole mobility • improvement at low VG retained at high VG Experimental Hole Mobility Thompson et al., TED 04 PhD Defense, Anisur Rahman

  29. Consequence of Strain: sp3d5s*-SO Strained Bulk Si VB Strained Top VB HH Second VB LH Unstrained Bulk Si VB Unstrained PhD Defense, Anisur Rahman

  30. Results: Strained Bulk p-MOSFETs Intel 90 nm (Uniaxial) Rim et al. (Biaxial) 1995 2D E-k, Low VG and High VG Mobility [cm2/V-sec] Universal Hole Mobility (Unstrained) Rim et al. (Biaxial) 2002 Eeff [MV/cm] Experimental Hole Mobility Ballistic ION Ratio vs. Eeff Eeff=q(Ndep+p/3)/εSi • Bulk p-FETs: Self-consistent sp3d5s*-SO TB approach • Low VG: Top VB LH for both uniaxial and biaxial • High VG: Top VB HH for biaxial, LH for uniaxial • QC nullify strain splitting of LH,HH in biaxial case • Ballistic simulation explains mobility behavior in • strained p-MOSFETs PhD Defense, Anisur Rahman

  31. Study: Strained Bulk n-MOSFETs Process Induced Uniaxial Tension Source Drain SiGe SiGe Si • Mobility Behavior in n-MOSFETs: • Experimentally, both substrate-induced biaxial- • tension and process-induced uniaxial-tension • improves electron mobility • Electronic mobility enhancement is observed over the • entire range of gate bias • Such mobility enhancement is often explained in terms • of degeneracy removal of X2 and X4 valleys, which is • recently questioned (Fischetti et al., JAP, 2002.) • Simulation Setup and Observation: • Self-consistent sp3d5s*-SO TB model and the top-of- • barrier ballistic model does not show any enhancement • of strained device performance • Bandstructure alone cannot explain the electronic • mobility enhancement in strained planar n-MOSFETs Ballistic n-MOSFETs Eeff=q(Ndep+p/2)/εSi PhD Defense, Anisur Rahman

  32. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Mobility behavior in strained bulk p- and n- MOSFETs • Hole density profile in HOI structure • Summary • Future Work Chapter 7 PhD Defense, Anisur Rahman

  33. Study: Hole Profile in HOI Poly-Si ε-Si ε-SiGe Buried Oxide Schematic Representation • Heterostructure on Insulator (HOI) • Utilizes the high mobility central SiGe • channel to improve hole mobility • Band discontinuity moves holes to center • Band bending due to VG rearranges hole • profile TOX E 30 AL ~ 4 nm Biaxial tensile ε-Si (y=0.24) Biaxial compressive 21 AL ~ 3 nm ε-Si0.5Ge0.5 30 AL ~ 4 nm ε-Si (y=0.24) Biaxial tensile EV EC BOX • Simulation Setup: • The 20 band sp3d5s*-SO TB model with self- • consistent electrostatics. • Compressively strained Si0.5Ge0.5 sandwiched • between two Si layers under tensile strain. • Top and bottom oxides are 2 nm and 10 nm • thick, respectively. PhD Defense, Anisur Rahman Hoyt Group (MIT)

  34. Results: HOI Q-VG VB Profile Hole Profile EF = 0.45 eV Si/SiGe/Si BOX EF = 0.45 eV • Observations: • Tri-layer (Si-SiGe-Si) structure • tOX= 2 nm(top), tOX=10 nm(bot) • Low VG: band discontinuity • moves holes to central channel. • High VG: hole profile moves • near the surface. BOX Si/SiGe/Si Si/SiGe/Si EF = 0.45 eV BOX PhD Defense, Anisur Rahman

  35. Outline: • Background • Objective • Generalized Effective-mass Approach • Assessment of Ge n-MOSFETs • A Top-of-the-barrier Ballistic Model • Semi-empirical Tight-binding Approach • Tight-binding Application: UTB DG CMOS • Tight-binding Application: Self-consistent Electrostatics • Summary • Future Work Chapter 8 PhD Defense, Anisur Rahman

  36. Summary: • Effective mass approach extended to treat n-MOSFETs on arbitrary wafer • orientations. • An NEGF study of LG=10nm end of ITRS Ge n-MOSFET reveals that: • Ge (001)/[100] device performs best and can meet target ION • Gate under-lap improves short-channel-effects • High mobility in the S/D region is crucial to limit RS degradation • A strict process tolerance in body thickness necessary to limit VT fluctuation across the chip. • A physics based top-of-the-barrier semiclassical ballistic transport model • developed and its application demonstrated. Continued to next slide PhD Defense, Anisur Rahman

  37. Summary (cont.): • A semi-empirical 20 band sp3d5s*-SO TB model used to assess strained and • unstrained UTB novel channel material CMOS devices, and was reveled that: • Ge n-MOSFETs: Below 4nm thickness, use of single band parabolic E-k is limited by non-parabolicity. Below 2nm thickness, its use is further limited by band reordering and multi-valley conduction. • A trade-off exists between high vinj and low DOS. III-V devices outperform Si or Ge n-MOSFETs only for thick body and thick EOT. For deeply scaled devices Ge displays the best ballistic performances. • Self-consistent gate electrostatics was treated in sp3d5s*-SO TB model • and was revealed that: • Experimental hole mobility behavior in strained planar p-MOSFETs can beexplained by bandstructure modulation. Similar behavior for electron mobility in n-MOSFETs cannot be explained by bandstructure alone. • HOI simulation shows that at high gate field, the hole-profile moves near the surface and not at the central high-mobility SiGe layer. PhD Defense, Anisur Rahman

  38. Future Work • Separate the effects of bandstructure and scattering in published experimental mobility data • Discretize the TB Hamiltonian for arbitrarily oriented wafers—Only (100) wafers treated here • Mode-space representation of TB Hamiltonian for UTB MOSFETs—A full 2D representation is not feasible, computationally • Employ zone-unfolding technique to treat SRS and random alloy effects • Employ self-consistent TB approaches for III-V HEMT and QWFET devices— Ballistic simulation is more relevant here due to their very high mobility PhD Defense, Anisur Rahman

More Related