6_Mobility_Boosters_in_Silicon_MOS_part1

6_Mobility_Boosters_in_Silicon_MOS_part1 - Mobility...

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Unformatted text preview: Mobility Boosters in Silicon MOS 
 1. Strain 2. Crystal Orientation Prof. Krishna Saraswat / Aneesh Nainani Department of Electrical Engineering Stanford University Stanford, CA 94305 saraswat@stanford.edu / nainani@stanford.edu araswat tanford University 1 EE311/Strained Si High Mobility Channels Historic CMOS Performance vs. Scaling: the 1/LG “law” Carrier velocity increase is paramount for performance scaling HMOSFET delay has continued tothere (LG=10nm) and igh mobility channel: Getting decrease because of Saturationto boost velocity … str-Si in carrier velocity ! proceeding beyond … araswat tanford University Courtesy: D. Antoniadis (MIT) 2 EE311/Strained Si 1 Impact of Channel Mobility on MOS Performance Injected (vinj) Back scattered (r) Nsource Source Drive Current: ballistic transport model IDsat = qN source" inj Drive Current: Near ballistic transport model Channel $1 # r ' IDsat = qN source" inj & ) %1 + r ( ! Gate Delay: Drain Lgate " VDD CLOADVDD = ID (VDD # VT ) " v inj ! K. Natori, J. Appl. Phys. 15 October 1994, p. 4879 M. Lundstrom, IEEE EDL, June 2001 p.293 ! r is back scattering coefficient νinj =µES is the injection velocity at the source ! Note that at the source µ is at low E-field ⇒ Need high mobility High νinj Low r Increasing µ at source (low field) brings us closer to ballistic transport limit araswat tanford University 3 EE311/Strained Si Transistor scaling: Mobility mb 1000 ulo Surface Roughness Co 500 Eeff -0.3 tox=35 Å tox=70 Å Eeff µeffeff (2/V.sec)" µ (cm cm2 /V-s) µ ic Phon on Electron 300 200 -0.3 Eeff -2 Eeff Hole 100 -1 50 30 0.1 1 µm CMOS 0.2 0.3 0.1 µm CMOS 0.5 1 Eeff (MV/cm) Eeff 2 3 Mobility degrades with scaling   Eeff increases with scaling   Increase in substrate doping increases ionized impurity scattering   Reduced gate oxide thickness increases remote charge scattering   High k dielectrics have higher Coulombic scattering due to surface araswat states and soft phonon scattering tanford University 4 EE311/Strained Si 2 History : Strain in Semiconductors (50-60’s) araswat Smith 1954 tanford University 5 EE311/Strained Si History : Strain in Semiconductors (80-90’s) Wafer Bending Strain gauge Silicon/Steel <100> 45° <110> Current Flow araswat tanford University Y Kanda TED 1982 6 EE311/Strained Si 3 Mobility Enhancements in Strained-Si MOSFETs (90-till date) NMOS Gate + en poly lectrode gate oxide PMOS LTO spacer Strained Si n+ Relaxed Si1-xGex n+ Si1-xGex Graded layer Mobility Enhancement Factor Si Substrate NMOS PMOS S. Thompson, et al. IEEE EDL, April 2004 araswat tanford University 7 EE311/Strained Si What is strain? - Types of loading Tensile Compressive Shear Torsion For tensile and compressive strain Stress and strain: Positive for tensile loads Negative for compressive loads araswat tanford University 8 EE311/Strained Si 4 What is strain? – Hooke’s Law E Elastic strain is reversible Plastic strain is irreversible and is accompanied by damage to crystal araswat tanford University 9 EE311/Strained Si What is strain? – Poisson’s Ratio Elastic constants Material 0.26 1000 0.1 748 0.18 GaAs 86 0.31 InAs 51.4 0.35 SiO2 94 Al 10 103 SiC tanford University 0.28 C araswat 130 Ge[100] Low Poisson’s ratio Poisson’s Ratio Si [100] Typically, High Young’s modulus Young’s Modulus (GPa) 64 Cu 124 W 406 EE311/Strained Si 5 Types of strain - definition Hydrostatic Biaxial Uniaxial Hydrostatic – 3 directions Biaxial – 2 directions Uniaxial – 1 direction araswat tanford University 11 EE311/Strained Si Formation of Energy Bands (1)  If atoms are close to each other, potential barrier is strong, energy bands are narrowed and spaced far apart. (Corresponds to crystals in which electrons tightly bond to ion cores, and wave functions do not overlap much with adjacent cores) (2)  If atoms are far apart, potential barrier is weak, energy bands are wide and spaced close together. araswat tanford University 12 EE311/Strained Si 6 E-k diagrams Ge Si GaAs Wave vector k is k= 2" 2 mE = # ! E " k2 Effective mass is ! m* = ! ! h d E dk 2 2 •  Effective mass is dependent on the curvature of a band •  Bands are parabolic at the energy maxima or minima •  Bands are different in different directions araswat tanford University 13 EE311/Strained Si Bandstructure Basics Constant Energy Surfaces E vs. k Eg mlongitudinal Chelikowsky and Cohen mtransverse Silicon is indirect - X valley Three hole bands – LH, HH, SO Si has 6 equivalent valleys (+k and –k) Ellipsoids characterized by ml and mt araswat tanford University 14 EE311/Strained Si 7 Why does strain effect the bandstructure ? ml (z) 001 m t (y) 010 (x)100 araswat tanford University 15 EE311/Strained Si Why does strain effect the bandstructure ? araswat tanford University 16 EE311/Strained Si 8 Effects of Strain Iso-energy surface of upper valance band of GaAs under compressive strain No strain Uniaxial Biaxial •  Under strain spacing of atoms in a crystal is altered and thus the energy band structure changes •  The shape of the constant energy surfaces changes => change in m* A. Nainani et. al, SISPAD 2009 araswat tanford University 17 EE311/Strained Si Desired valley property Silicon 1 1 d2E =2 m* ! dk 2 k = k0 DOS α (m*)3/2 ! araswat tanford University Valley should have the desired property of - Low mtransport along channel - High mwidth along width - High mperpendicular normal to surface 18 EE311/Strained Si 9 Effects of Strain 2 Strain and Semiconductor Crystal Symmetry E-k diagrams of valance band of strained Si 26 ! ! 56678 57768 Heavy hole band 57768 56678 " " ! 56678 57768 LH HH Light hole band !"#$%&'()*''*+ " LH !,#$,-".-"/$$ (*&'-/*$'()*''*+ HH !0#$%&-".-"/$ 0123)*''-4*$'()*''*+ Fig. 2.8. The HH and LH bands for (a) unstressed, (b) biaxial tensile stressed, and •  When spacing of atoms in a crystal Si along the the energy band structure changes (c) uniaxial compressive stressed is altered [110] and [001] directions. and the shape of the E-k diagrams changes   Energy levels, effective mass and density of states (DOS) change Consider next the strain effects on the conduction bands. The conduction band edges are not like the valence bands, and different semiconductors may 2 1 have them 1 different locations in the(Brillouin zone, related to their differat d E v α m*)-1/2 and DOS α (m*)3/2 = ent covalent2and polar k = k0 m* ! dk 2 interactions. For example, the GaAs conduction band edge is located at the Γ point, with L valleys about 290 meV higher. The Si araswat conduction bands are six ∆-valleys located along the ￿100￿ directions at about tanford University X . Ge has its conduction band 19 0.85 edges at the L points. Strain effects EE311/Strained Si vary for different types of conduction valleys. For the Γ valley, since it is singly degenerate, there is no observable splitting. Si and Ge both have multiple ! conduction valleys, and have the star degeneracy. For Si the star degeneracy is 6-fold and for Ge 4-fold. One may wonder why the star degeneracy for Ge is only 4-fold since there are 8 L points. The answer lies in the structure of the FCC Brillouin zone shown in Fig. 2.4. At each L point, only half of the valley lies within the Brillouin zone. In fact, the two L points at the opposite ends are equivalent. They are the same one. Similarly, if the Si conduction valleys on were exactly at the X points, the star degeneracy would be only 3-fold. The trends of splitting of these valleys to obtain. For Silicon Band Structure due to strain are easyvalleys along x example, both biaxial and [110] uniaxial stress affects the 4 and y in an identical way in Si, but affects the other 2 valleys along z differently. (z) and Thus, Si conduction valleys are split into the so-called ∆2 001 ∆4 valleys. ml Biaxial stress affects the 4 L valleys of Ge identically, and thus they do not split. The [110] uniaxial stress distinguishes the L valleys according to their m projection locations in the x-y plane (2 talong [110] and 2 along [¯ 110]), and thus the Ge L valleys split into 2 double-valley groups. Although there is no (y) splitting for the GaAs Γ valley, it has to be noted that due to the energetic010 proximity between the Γ and L valley, strain can alter the electron transport by shifting the gap between these two valleys, since the electron occupation of the L valley cannot usually be neglected. (x)100 Who are the guys responsible for I ? 6 equivalent types of electrons are involved in conduction regime of nMOS 2 types of holes are involved in conduction regime of pMOS : heavy and light araswat tanford University Source: F. Boeuf (ST Microelectronics) 20 EE311/Strained Si 10 What happens in Strained-Si ? Band structure deformation Band structure without strain Band Splitting Iso-energy ellipsoid ml •  Sub-band carrier redistribution -  Carriers occupy valleys with lighter mass •  Less intervalley phonon scattering mt mt < m l mt Mobility is increased araswat tanford University 21 EE311/Strained Si Subband Structure of (001) surface in strained Si? What happens to electrons Si MOS nstrained Silayers Strained Si U inversion 2-fold perpendicular valleys 4-fold in-plane valleys Lower µn <010> 3D 2-fold valle <001> <100> z 2D z (001) Ec Higher µn c E’0 E0 araswat tanford University In strained Si the 6 fold degenerate valleys split into 2 types of valleys. More electrons occupy the 2 fold degenerate valleys where the conductivity effective mass is lower and hence the mobility is higher 22 EE311/Strained Si 11 Holes in Strained Si Tensile strained Si Unstrained Si   2 fold degenerate band split into light and heavy hole bands   Light hole band is at lower energy and has more holes in it   Reduction in conductivity mass   Supression of inter-valley scattering higher µ araswat tanford University 23 EE311/Strained Si Redistribution in subbands and scattering reduction N Fermi-Dirac Unstrained Si Si/Si0.5Ge0.5 Si bulk -0.1 HH -0.2 LH 0.1 LH >80 % in HH -0.3 SO Energy (eV) Energy (eV) Strained-Si 0.2 0.0 E(LH-HH) 0.0 < 1 % in HH -0.1 HH -0.2 -0.4 SO -0.3 -0.5 [100] E Γ [100] [110] Γ [110] In strained Si more holes occupy the band where the conductivity effective mass is lower and hence the mobility is higher Source: F. Boeuf (ST Microelectronics) araswat tanford University 24 EE311/Strained Si 12 Biaxial Strain in Strained Silicon/Germanium Source: J. Hoyt, MIT araswat tanford University 25 EE311/Strained Si NMOS mobility – Biaxial strain Effect of biaxial strain on NMOS mobility saturates at higher strain araswat tanford University (SSOI: Strained Si on insulator) 26 EE311/Strained Si 13 PMOS mobility – Biaxial strain Effect of biaxial strain on PMOS mobility also saturates at higher strain and diminishes at higher Eeff araswat tanford University 27 EE311/Strained Si Strained Silicon – Biaxial strain Device structures Strained Si Relaxed SiGe Strained Si/SiGe Bulk MOSFET Co salicide formed on raised S/D Strained Si Relaxed SiGe Buried Oxide SGOI (SiGe-onInsulator) MOSFET CoSi2 on RSD Strained silicon Strained Si Buried Oxide SSDOI MOSFET Silicide on selective epi SSDOI 16 nm 60nm Strained Si Channel Relaxed SiGe SiGe Buried oxide Buried Oxide K. Rim et al., Symp. VLSI Tech., p. 59, 2001. K. Rim et al., Symp. VLSI Tech., p. 98, 2002. araswat B. Lee et al., IEDM 2002 K. Rim et al., IEDM, 2003. Enabling technology – grow tensile strained Si on relaxed SiGe tanford University 28 EE311/Strained Si 14 Strained Silicon – Why Uniaxial? Biaxial strain studied extensively but…two key problems 1)  Integration difficulties -  Dislocations -  Ge up-diffusion -  Fast diffusion of extensions -  Cost 2) Poor hole mobility gain -  At high Eeff LH-HH separation is reduced -  Hole mobility gain is lost Source: Intel Corp. araswat tanford University 29 EE311/Strained Si Strained Silicon – Uniaxial strain Why uniaxial? Hole mobility vs. Eeff Hole mobility enhancement Biaxial strain C. S. Smith, Phys. Rev. 1954 S. E. Thompson, IEDM Tech. Dig., 2004. araswat tanford University High hole mobility at high Eeff   Hole effective mass reduction (band warping)   Increased valance band splitting   Band separation is not reduced at high Eeff (high quantization effective mass for light holes) 30 EE311/Strained Si 15 PMOS mobility - Uniaxial vs Biaxial strain Eeff=1MV/cm M. Uchida et al, SISPAD (2005) araswat Enhancement retained at high E-field tanford University 31 EE311/Strained Si NMOS mobility - Uniaxial strain [110] [001] No stress Uniaxial tensile Uniax. compr. Biaxial tensile S. Thompson M. Giles Mobility enhancement from: -  Valley repopulation -  Inter-valley scattering suppression -  Similar to biaxial araswat tanford University 32 EE311/Strained Si 16 Strained Silicon – Process induced strain Which is best? Source: Synopsys, AMD Corp. araswat tanford University 33 EE311/Strained Si Strained Silicon – Uniaxial strain Device Technology Intel’s Technology Traditional Approach Source: Intel Corp. araswat tanford University Processed induced strain 34 EE311/Strained Si 17 Additional strain with gate last process araswat tanford University 50% higher stress in channel with replacement gate 35 EE311/Strained Si Ion-Ioff curves – Uniaxial strain Technology scaling Significant performance improvements through 4 generations of CMOS Technology araswat tanford University Source: Intel Corp. 36 EE311/Strained Si 18 How far can we go? – Uniaxial strain Stress scaling Source: Intel Corp. Significant performance gains ~5X still possible for PMOS (due to reduced meff). Performance enhancements for NMOS saturate at ~2X. N/P performance getting comparable with strain: circuit implications. araswat tanford University 232 37 EE311/Strained Si Strain Roadmap: Does strain scale with dimension ? 6 Strain in Electron Devices ! ! !! ! ! ! ! !" " !! ! ! Fig. 6.1. Strain technology for pMOSFETs in different Si technology nodes. The line in the figure is the modeled hole mobility as a function of longitudinal channel strain. •  Dislocations with increased Ge % in SiGe •  induce strain of S/D stressers reduces with pitch scaling Among the two approaches which Volumeto device channels, i.e., the wafer-based global biaxial strain and process-induced uniaxial strain, only •  Volume The global strain s much the latter is applied in real production up to date. of nitride is tresser reduces with pitch scaling 6.1 STRAIN-SI TECHNOLOGY more complex in process and more expensive in cost, while the process-induced strain is easy to adopt. So in this section, we will focus on the technology of the process-induced uniaxial strain. The techniques in production to induce strain araswat include high stress tensile and compressive SiN capping layers and selective epitaxial SiGe deposited University tanford in recessed/raised source and drain (S/D). These techniques induces uniaxial tensile stress to the nMOSFETs and compressive stress to the pMOSFETs. Future generations will bring the SiGe closer to the channel and increase the Ge concentration and increase the stress in the capping layers which is approaching 3 GPa. Other possible future techniques for process stress or mobility enhancement may include tensile shallow trench oxide, embedded SiC for nMOSFETs and hybrid orientated (110) wafers. Together, these techniques are expected to lead to channel stresses of 1-2 GPa approaching the stress level of wafer-based biaxial stress. Strained-Si technology is completely compatible with the traditional Si CMOS technology. There are only slight modifications to a standard CMOS logic technology process flow are needed to insert the longitudinal compressive and tensile stress into the p- and nMOSFETs, respectively, as shown in 38 EE311/Strained Si 19 ...
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