Feng Yuan Ping ( 冯元平 ) Department of Physics National University of Singapore First Principles Studies on High-k Oxides and Their Interfaces with Silicon and Metal Gate

Aug 29 - Sept 1, 2006CCP20062

Aug 29 - Sept 1, 2006CCP

Aug 29 - Sept 1, 2006CCP20064 Outline Introduction Oxygen vacancy in HfO 2 and La 2 Hf 2 O 7 Tuning of metal work function at metal gate and high-k oxide interface Properties of high-k oxide and Si interface Conclusion SD G

Aug 29 - Sept 1, 2006CCP20065 ITRS roadmap shows the expected reduction in device dimensions SD G CMOS Scaling

Aug 29 - Sept 1, 2006CCP20066  1.2 nm (5 atomic layers) physical SiO 2 in production of 90 nm logic technology node; 0.8 nm physical SiO 2 in research of transistors with 15 nm physical Lg  Gate leakage is increasing with reducing physical SiO 2 thickness. SiO 2 layers <1.6 nm have high leakage current due to direct tunneling. Not insulating  SiO 2 running out of atoms for further scaling. Will eventually need high-K Why High-k oxides ? SiO 2 HK Oxide Gate CB Si Rober Chau, Intel

Aug 29 - Sept 1, 2006CCP20067 Choice of High K Oxide

Aug 29 - Sept 1, 2006CCP20068 Growth of ZrO 2 on Si Interface Wang et al. APL 78, 1604 (2001) Wang & Ong, APL 80, 2541 (2002)

Aug 29 - Sept 1, 2006CCP20069 Problems with High K oxides Among other problems, oxide has too many charge traps, and the threshold voltage (V th ) shifts from CMOS standards.

Aug 29 - Sept 1, 2006CCP Dynamic Charge Trapping Time evolution of threshold voltage V th under static and dynamic stresses of different frequencies, for (a) n-MOSFET, and (b) p-MOSFET. The V th evolution has a power law dependence on stress time. C. Shen, H. Y.Yu, X. P. Wang, M. F. Li, Y.-C. Yeo, D. S. H. Chan, K. L. Bera, and D. L. Kwong, International Reliability Physics Symposium Proceedings 2004, 601. Power law shift! Negative-U traps? Oxygen vacancy?

Aug 29 - Sept 1, 2006CCP Hydrogen in HfO 2 Formation energies for (a) interstitial H and H 2 molecules, and (b) the V O -H complex. J. Kang et al., APL, 84, 3894 (2004).

Aug 29 - Sept 1, 2006CCP Bulk HfO 2 J. Kang, E.-C. Lee and K. J. Chang, PRB, 68, (2003) Fm3m Cubic P4 2 /nmc Tetragonal P21/c Monoclinic

Aug 29 - Sept 1, 2006CCP Cubic HfO 2 Vasp Cutoff energy = 495 eV GGA E g = 3.68 eV (direct) (Exp gap ~ 5.8 eV) W L  X W K Valence band = O 2p Conduction band = Hf d Peacock and Robertson, JAP (2002)

Aug 29 - Sept 1, 2006CCP Computational Details DFT, planewave, pseudopotential method (vasp) 2s and 2p electrons of O, 5d and 6s electrons of Hf are treated as valence electrons. Cut off energy: 495 eV 80 atom supercell (3x3x3 primitive cells) Uniform background charge for charged vacancy

Supercell

Aug 29 - Sept 1, 2006CCP Total Energy Charge StateEnergy (eV) V V V V V

Aug 29 - Sept 1, 2006CCP Energetics Excothermic (0.32 eV)Excothermic (0.94 eV)Excothermic (0.38 eV) Negative-U Property!

Aug 29 - Sept 1, 2006CCP electron (a) V g > 0 HKSi sub.n+Poly-Si gate hole (b) V g < 0 HKSi sub. p+Poly-Si gate Charge Trapping Mechanism Positive bias for n-MOSFET Electrons are injected to HK V 0  V - (meta-stable)  V -- Negative bias for p-MOSFET Holes are injected to HK V 0  V + (meta-stable)  V ++ In both cases, when the gate bias is removed, no charges are injected to HK, all charges in the O traps will be de-trapped, the gate dielectric remains neutral

Aug 29 - Sept 1, 2006CCP Frequency Dependence of V th Experimental and simulation results for n-MOSFET

Aug 29 - Sept 1, 2006CCP Formation Energy A. S. Foster, et al. PRB 65, (2002) Formation energy for neutral vacancy: 9.36 eV (O 3 ) & 9.34 eV (O 4 ) Present calculation: 9.33 eV (relative to O atom)

Aug 29 - Sept 1, 2006CCP Band Structures V0V0

Aug 29 - Sept 1, 2006CCP Band Structures V -2 AC plane BC plane

Aug 29 - Sept 1, 2006CCP (a) (b)  11 22 Breathing Mode C 2v Mode Relaxation of NN Hf atoms V V

Aug 29 - Sept 1, 2006CCP Relaxation of NN Hf Atoms Charge State Breathing ModeC2v Mode  ( Å )  1 ( Å )  2 ( Å ) V V-V V0V ̶̶ V+V ̶̶ V ̶̶

Aug 29 - Sept 1, 2006CCP Effect of Lanthanum Charge trapping induced V th shift under constant voltage stress for HfO 2, HfLaO with 15% and 50% La gate dielectric NMOSFETs. X. P. Wang et al. VLSI2006

Aug 29 - Sept 1, 2006CCP Effect of La The formation energies of oxygen vacancies at varies sites in monoclinic HfO 2 and pyrochlore HfLaO, calculated by ab initio total energy calculations. V3 V4 Td C 2V

Aug 29 - Sept 1, 2006CCP Summary Oxygen vacancy in HfO 2 has negative-U property. It is energetically favors trapping two electrons or two holes. Oxygen vacancy is a main source of charge trapping in HfO 2 and the origin for frequency dependence of dynamic charge trapping in HfO 2 MOS transistors. Large lattice relaxation for charged vacancies, due to strong electron-lattice interaction. Oxygen vacancy has higher formation energy at Td site in La 2 Hf 2 O 7.

Aug 29 - Sept 1, 2006CCP Currently polycrystalline silicon (poly-Si) gate electrode is used. Problems: high gate resistance boron penetration Fermi level pinning poor compatibility with high-  gate dielectrics increase of EOT due to gate depletion Need metal gate! Eliminates the gate depletion problem Eliminates boron penetration problem Reduces the gate sheet resistance Generally more compatible with alternative gate dielectric or high-permittivity (high- k) gate dielectric materials than poly-Si. The urgent need for alternative gate dielectrics to suppress excessive transistor gate leakage and power consumption could speed up the introduction of metal gates in complementary metal oxide semiconductor (CMOS) transistors. SD G Gate Material

Aug 29 - Sept 1, 2006CCP Issues The integration of metal gate with high-  gate dielectric requires the metal effective work functions to be within ±0.1 eV of the Si valence- and conduction-band edges for positive- (PMOS) and negative-channel metal-oxide-semiconductor (NMOS) devices, respectively. However, to find two metals with suitable work functions and to integrate them with current semiconductor technology remains a challenge.

Aug 29 - Sept 1, 2006CCP Work Function of Metals Work function of several elemental metals in vacuum, on a scale ranging from the positions of the conduction band to the valence band of silicon. Metal work functions are generally dependent on the crystal orientation and on the underlying gate dielectric.

Aug 29 - Sept 1, 2006CCP Can we tune the metal workfunction?

Aug 29 - Sept 1, 2006CCP Tuning of Workfunction? ZrO 2 Ni Transition Metal Monolayer/half-monolayer Ni-m-ZrO 2 m = Au, Pt, Ni, Ru, Mo, Al, V, Zr and W (for half monolayer) m = Ni, V, and Al (for one monolayer)

Aug 29 - Sept 1, 2006CCP Bulk ZrO 2 Very small lattice mismatch (<2%)

Aug 29 - Sept 1, 2006CCP Models Supercells for the Ni-m- ZrO2 interfaces, The interface is formed using c-ZrO2(001) and fcc Ni(001) surfaces. (a)with one monolayer metal m (m=Ni, V, and Al). (b)with half monolayer metal m (m=Au, Pt, Ni, Ru, Mo, Al, V, Zr and W)

Aug 29 - Sept 1, 2006CCP Computational Details DFT, planewave, pseudopotential method (vasp) Ultrasoft pseudopotential & GGA Cut off energy: 350 eV K points: 8x8x1 In plane lattice constants constrained to that of c- ZrO 2 Electronic energy was minimized using a fairly robust mixture of the blocked Davidson and RMM-DIIS algorithm. Conjugate gradient method for ionic relaxation

Aug 29 - Sept 1, 2006CCP Density of States Spin resolved and atomic site-projected density of states (PDOS) for (a) Ni-Pt- ZrO 2 interface and (b) Ni-Al-ZrO 2 interface, with half monolayer of metal insertion. The PDOS for the Ni in the bulk region (Ni-bulk), interface metal m (Pt or Al), interface oxygen (O-Int.), and oxygen in the bulk region (O-bulk) are shown.

Aug 29 - Sept 1, 2006CCP Schottky Barrier Heights Ni m Oxide Si

Aug 29 - Sept 1, 2006CCP p-type Schottky Barrier Height p-type SBH is obtained using the “bulk plus lineup” procedure, using the average electrostatic potential at the core ( V core ) of ions in the “bulk” region as reference energy  E b the difference between the Fermi energy of Ni and the energy of the valence band maximum (VBM) of the oxide, each measured relative to V core of the corresponding “bulk” ions,  V is the lineup of V core through the interface.  E b is adjusted by quasiparticle and spin-orbital corrections (0.29 eV for Ni, eV to the valence-band maximum of ZrO 2,  overall correction of 0.94 eV).

Aug 29 - Sept 1, 2006CCP V core Average electrostatic potential at the cores (V core ) of Ni (filled dark circle) and Zr (open circle) as a function of the distance from the interface for Ni-m-ZrO2 interfaces (m= Au, Ru, Ti) with half monolayer metal insertion. Breaks were introduced in the vertical axis (V core ) between - 41 eV and -36 eV.

Aug 29 - Sept 1, 2006CCP n-type Schottky Barrier Height where E g is the energy gap of the dielectric The experimental band gap of 5.80 eV was used. The SBH can also be estimated directly from the difference between the Fermi energy and the energy corresponding to the top of the valence band given in the PDOS of oxygen in the bulk region. Results obtained using the two methods are in good agreement (within 0.1~ 0.2 eV).

Aug 29 - Sept 1, 2006CCP Results mθχWFQmQm p-SBHn-SBH Au Pt Ni Ru Mo Al V Zr Ti W Ni V Al

Aug 29 - Sept 1, 2006CCP SBH Tunability Range of tuning: 2.8 eV!

Aug 29 - Sept 1, 2006CCP n-type Schottky Barrier Height n-SBHs of Ni-m-ZrO2 interfaces are shown as a function of electronegativity (Mulliken scale) of m. The straight line is a least-squares fit to data points shown in filled squares (Al and W were not included).

Aug 29 - Sept 1, 2006CCP Workfunction of Ni(001) with m Work functions of Ni(001) with half monolayer of metal m coverage are shown as a function of electronegativity (Mulliken scale) of m. The straight line is a least-squares fit to data points shown in filled squares.

Aug 29 - Sept 1, 2006CCP Mechanism? Contribution from the tails of the metallic wave functions which tunnel into the oxide band gaps or metal induced gap sates can be ruled out, due to short delay length (~0.9Å) which is nearly independent of the interlayer metal. Interface dipole can contribute significantly to band alignment between the metal and oxide. Ionic m-O bonds Charged metal layer and its image Bulk NiBulk ZrO 2 Ni m O

Aug 29 - Sept 1, 2006CCP Gap States Penetration of electronic density of the gap states into the ZrO 2 of Ni-m-ZrO 2 interfaces. Position of the surface oxygen is set to z = 0 Å.

Aug 29 - Sept 1, 2006CCP Interface bonding dependent SBH: experimental evidence (in-situ XPS) MethodStructure  p(eV)  n (eV) DFT-GGA XPS IPE a O-t Zr-t O-v O-rich O-deficient Afanas'ev et al. JAP 91, 3079 (2002).

Aug 29 - Sept 1, 2006CCP Interface bonding dependent SBH: experimental evidence (in-situ XPS)

Aug 29 - Sept 1, 2006CCP Summary A scheme for tuning the Schottky barrier height or workfunction of metal gate – high-k dielectric interface was proposed and has been experimentally confirmed. By including a monolayer or half monolayer of transition metal between the metal gate and high-k dielectric, a tunability as wide as 2.8 eV can be achieved. There exists a linear correlationship between the Schottky barrier heights / workfunction and the electronegativity Preliminary experimental results with m=Al agree with prediction.

Aug 29 - Sept 1, 2006CCP Acknowledgement Y F Dong Physics Department, NUS Y Y Sun Physics Department, NUS S J Wang Institute of Materials Research & Engineering A Huan Institute of Materials Research & Engineering M F Li Dept of Electrical & Computer Engineering, NUS Institute of Microelectronics

Aug 29 - Sept 1, 2006CCP200651