Alain Espinosa Thin Gate Insulators

1. Nanoscale Silicon Technology Presenters Topics Mike Duffy Double-gate CMOS Eric Dattoli Strained Silicon Alain Espinosa Thin Gate Insulators

2. Challenges as CMOS feature sizes decrease • Carrier Mobility reduction • Threshold voltage (VT) control reduction • 3. Off-state leakage increase • 4. Power consumption increase

3. Band diagram when on: A basic MOSFET: Eeff

4. Problem 1: Carrier Mobility Decreases as Channel length decrease and Vertical Electric fields increase Mobility versus technology scaling trend for Intel process technologies. From (Thompson 2004)

5. Problem 2: VT Rolloff as Channel length decreases Substrate-Strained Silicon Technology: Process Integration H. C.-H. Wang, IEDM 2003 One common solution : Increasing Channel Doping reduces Short Channel Effect

6. (Problem 2) VT Rolloff explained by Short Channel Effect This problem is addressed by Double Gate Technology

7. Problem 3: Tunneling Through Gate Oxide (off state current) Eox This problem is addressed by Strained Silicon, and Thin-Insulator technology

8. Problem 4: Wattage/Area increases as density increases MOSFET Scaling Trends, Challenges, and Potential Solutions Peter M. Zeitzoff and James E. Chung. IEEE CIRCUITS & DEVICES MAGAZINE ¦ JANUARY/FEBRUARY 2005 This problem is addressed by Double Gates, Straining, and thin Gate Insulators

9. Double Gate MOSFET • Features: • Upper and lower gates control the channel region • Ultra-thin body acts as a rectangular quantum well at device limits • Directly scalable down to 20 nm channel length

10. Band Structure

11. Layout • Type I : Planar Double Gate • Type II: Vertical Double Gate • Type III: Horizontal Double Gate (FinFET)

12. FinFET Layout

13. Reduced Channel and Gate Leakage • Short channel effects are seen in Standard silicon MOS devices • DGFET offers greater control of the channel because of the double gate • Gate leakage current is prevented by a thick gate oxide

14. Threshold Voltage Control • Silicon MOS Transistor: • Increased body doping used to control VT for short channel • Small number of dopant atoms for very short channel • Lowest VT achievable is .5V • Double Gate FET : • Increased body doping • Asymmetric gate work functions (n+ / p+ gates) • Metal gate • VT of .1V achievable through work function engineering

15. Increased Carrier Mobility • Silicon MOS Transistor: • Carrier scattering from increased body doping • Transverse electric fields from the source and drain reduce mobility • Double Gate FET: • Lightly doped channel in a DGFET results in a negligible depletion charge • Asymmetric gate: experiences some transverse electric fields • Metal gate: transverse electric field negligible with increased channel control

16. Reduced Power Consumption • Double Gate coupling allows for higher drive currents at lower supply voltage and threshold voltage • Energy is a quadratic function of supply voltage • Reduced channel and gate leakage currents in off state translate to huge power savings • Separate control of each gate allows dynamic control of VT : • Simplified logic gates would save power and chip area

17. Power VS Feature Size

18. Challenges Facing Double Gate Technology • Identically sized gates • Self-alignment of source and drain to both gates • Alignment of both gates to each other • Connecting two gates with a low-resistance path

19. Ultimate Double Gate Limits • Thermionic emission above the channel potential barrier: • Short channel effects lower potential barrier • Band-to-band tunneling between body and drain pn junction: • Body-drain electric field increases tunneling probability • Quantum mechanical tunneling directly between source and drain: • Extremely small channel lengths correspond to narrow potential barrier width • 4) Other effects of quantum confinement in the thin body

20. Lattice Constants: Si 5.431 Angstrom Ge 5.658 Angstrom Si/Ge Alloys Alloys are uniform crystal structures containing two different materials which posess the same ordering property. Can create Si1-xGex alloys where x is a number from 0.0 to 1.0 This is possible since both materials create diamond type lattices and their lattice constants are close. Lattice constant of alloy is determined by Vegard’s Law, which is a linear average between the constants of Si and Ge. aalloy = (1-x) • aSi + x • aGe Note: other material parameters change: e.g. bandgap

21. Deviation from Vegard’s Law (Herzog 1993)

22. Si and Si1-xGex Alloy Heterostructures A Heterostructure is a semiconductor structure in which the material composition changes with position. Heterostructure devices are made by using Molecular Beam Epitaxy to grow a different material on a substrate. Performance Projections of Scaled CMOS Devices and Circuits With Strained Si-on-SiGe Channels. Jerry G. Fossum, Fellow, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 4, APRIL 2003

23. Si Ge Si Ge Physics of Semiconductors and their Heterostructures. Jasprit Singh

24. Solid State Electronic Devices. Streetman

25. Required to lay heterolayer within a constrained thickness Substrate-Strained Silicon Technology: Process Integration H. C.-H. Wang, IEDM 2003

26. Scale Picture of Strained Si NMOS Heterostructure Improved Hot-Electron Reliability in Strained-Si nMOS David Onsongo, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 12, DECEMBER 2004 2193

27. Strain Engineering X Y Z Process-Strained Si (PSS) CMOS Technology Featuring 3D Strain Engineering C.-H. Ge, IEDM 2003 <100> Strained-SiGe-Channel p-MOSFET with Enhanced Hole Mobility and Lower Parasitic Resistance v Masashi Shima FUJITSU Sci. Tech. 2003 <100> Orientated Wafer

28. Biaxial tension in Strained Si on SiGe MOSFET Fabrication and Analysis of Deep Submicron Strained-Si N-MOSFET’s Kern (Ken) Rim, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 47, NO. 7, JULY 2000

29. .19 m0 < .98 m0 LH HH Scaling Planar Circuits IEEE Circuits & Device Magazines Jan/Feb 2004

30. Z Rim (2000) X Carriers in channel travel along X-Y plane in k-space Y Carriers move along [010] or [100] direction Applies to common (001) oriented Silicon substrate Same Z or [001] Axis in Real Space

31. Relationship between effective mass and carrier mobility Carrier mobility is given by:μn= q • t mn*Current Density depends on Carrier mobility:Jx = q • n • μn •εxThis decrease in carrier mobility is addressed by Strained Silicon. Specifically, we’ll see that mn*is reduced

32. Bonus: Tunneling through Gate Oxide decreases with Strained Silicon Channel Structure Design, Fabrication and Carrier Transport Properties of Strained-SYSiGe-On-Insulator (Strained-SOI) MOSFETs S. Takagi+ IEDM 2003

33. Problem To Solve: We will use the WKB Approximation to calculate how much the Gate Tunneling Current is reduced by increasing the insulator/channel barrier height. Remember, Straining increases the insulator/channel barrier height. Transmission Probability depends on meff, Electric Field across barrier(Eox) ,and barrier height (Φox)

34. How To Find Si/SiO2 Barrier Height and Eox of Triangular Barrier Eox Unstrained: Φox=3.2 eV Strained: Φox=3.3 eV Device Design for Sub-0.1µm MOSFETs for Sample and Hold Circuits. 2003 Mayank Kumar Gupta

36. Compare 5x difference in Gate Current to difference in Jg (gate current density) at Eox = 8 MV/cm 1/(8 MV/cm) = 0.125 ln (J unstrained) = -12.9 ln (J strained) = -14.6 Their difference is exp(1.7) = 5.5 Which is very close to the theoretical result of 4.7x from the WKB Approximation. This difference isn’t constant, at: • Eox = 7.4 MV/cm, there is about a 7.5x difference • Eox = 9.1 MV/cm, there is about a 4.5x difference

37. Difference in Junstrained/Jstrained as Eox varies is predicted by the theoretical WKB approximation Compares to experimental difference of 4.5x

38. Compares to experimental difference of 7.5x

39. Effects of Eox on Tunneling Current through Gate Improved Hot-Electron Reliability in Strained-Si nMOS David Onsongo, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 12, DECEMBER 2004 2193

40. Better Way to Engineer Strain A 90-nm Logic Technology Featuring Strained-Silicon Scott E. Thompson, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 11, NOVEMBER 2004

41. Strain Applied to NMOSFETs MOSFET Current Drive Optimization Using Silicon Nitride Capping Layer for 65-nm Technology Node. S. Pidin 2004 Symposium on VLSI Tech Digest

42. Advantage over Si on SiGe method

43. This method improves Drain Currents for: PMOS NMOS A 90nm High Volume Manufacturing Logic Technology Featuring Novel 45nm Gate Length Strained Silicon CMOS Transistors. IEDM 2003

44. SiO2 limitations • Scaling • Power Consumption • One solution is using High-k dielectric • material

45. High-k dielectric material • Are used to minimize tunneling current • and the out diffusion of boron from the • gate. • Types • 1) 4 < k < 10 ; SiNx • 2) 10 < k < 100; Ta2O5, Al2O3, TiO2 • 3) 100 < k • What we are looking for in High-k dielectrics?

46. One Example of High-k dielectrics • Al2O3 • I-V Plot for different thicknesses on Si(100)

47. Al2O3 continued • Dielectric Constant (k) • Recent study show Al2O3 tunneling dielectrics <1nm thick are superior to previously used Si3N4 and SiO2

48. Some recent of High-k dielectrics • Al2O3 film have been used to make 1Gbit DRAM • Al2O3 and HfO2 have been used to produce a Vertical Replacement-gate (VRG) n-Mos.

49. -Conclusions on High-k dielectrics

50. Thank you Questions?