Plasma CVD Carbon Nanotubes (CNT’s)

1. Plasma CVD Carbon Nanotubes (CNT’s) Michael .A. Awaah Elec 7730 Advanced Plasma Processing for Microelectronic Fabrication Instructor: Dr. Y. TZENG Fall 2003

2. Outline • CNT’s • CNT’s Properties • Mechanical Properties • Electrical Properties • Growth of CNT’s • Application

3. Questions • Why are carbon nanotubes so strong • What re the current limitation in CNT VLSI fabrication

4. Carbon Nanotubes (CNT’s) • Carbon nanotubes are one of the most fascinating material in recent years, since they show exceptional electronic and mechanical properties that have triggered an ever stronger effort towards application. • The possibilities are promising and range from nanotube composite materials, nanoelectronics, scanning microscope probes, chemical and /or biological sensors, to cold electron sources.

5. CNT’s • Nanotubes have a unique property in that their electronic behavior (semiconducting or metallic) is determined by their structure, which also determines to a great extent the overall properties of devices as wide ranging as field effect transistors, flat panel displays, or chemical sensor • This implies a precise of nanotube diameter and chirality for molecular electronics.

6. Helicity and sp2 bonding

7. Single-walled nanotubes (SWNT)

8. Types of SWNT

9. Types of SWNT • C=n*a1 + m*a2 • m is zero for all zigzag SWNT • m=n for all armchair nanotubes • All other SWNT are chiral, chiral angle q = sin-1 [m(3)1/2/ 2(n2 +nm +m2)1/2]

10. CNT’s Properties Electrical properties depends on geometry of nanotube • Roughly 2/3 are semiconductors and 1/3 are metallic in random growth • Tremendous current carrying capability • 1 billion Amps/cm2 • Excellent heat conductor • twice as good as diamond

11. CNT’s Properties • High strength • much higher than high-strength steel • Young’s Modulus ~1 Pa (DWNT) and 1.25 TPa (MWNT) ( Steel: 230 Gpa) • High Aspect Ratio: 1000 – 10,000 • Density: 1.3 – 1.4 g/cm3 • Maximum Tensile Strength: 30 Gpa • Thermal Conductivity: 2000 W/m.K ( Cu has 400 W/m.K)

12. Thermal Property of CNT

13. Nanotube Conductance • Semiconducting when (m, n) indices: m – n  3 * integer The rest are metallic • Carbon Nanotubes are intrinsically p-type semiconductors.

14. CNT Conductance Variation

15. Quantum Effect • Conductance appears to be ballistic over micron scales, even at room temp. • Ballistic, no dissipation , very high current densities are possible Frank et al., Science 280, 1744(1998)

16. Low turn-on electric field and threshold electric field High field enhancement factor High current density High current stability, low degradation rate Electron Field Emission From CNT’s

17. Synthesis of carbon nanotubes • There are three commonly means by which carbon nanotubes are synthesize • Laser ablation • Arc-discharge method • Chemical vapor deposition (CVD)

18. Arc-discharge method

19. Laser ablation

20. CVD • CVD synthesis is achieved by taking a carbon species in the gas phase and using an energy source , such as plasma or a relatively heated coil, to import energy to a gaseous carbon molecule • The energy source is used to “crack” the molecule into a reactive radical species • Carbon nanotubes are formed if proper parameters are maintained

21. Plasma CVD • Plasma CVD has an advantage of low temperature synthesis over thermal CVD • Carbon nanotubes can be synthesized on soda lime glass. • The power supplies for discharge of plasma are DC and High Frequency • RF(13.56 MHz) and Microwave (2.47 GHz) are typical of high frequency applied are both electrodes.

22. Plasma CVD apparatus

23. Plasma CVD Nanotubes Carbon nanotubes are grown on the metal particles by glow discharge generated from high frequency power • Reaction is supplied to the chamber during the discharge • A substrate is placed on the grounded electrode

24. Plasma CVD Nanotubes • The reaction gas is supplied from the opposite plate • C2H2, CH4, C2H4, C2H4,, CO gases are used for synthesis carbon nanotubes • Catalytic metal, such as Fe, Ni, and Co are used on a Si, SiO2 , or glass substrate using thermal CVD or sputtering

25. SEM, TEM, AFM, STM

26. EELS Images of CNT’s

27. Space elevator

28. CNTFET transistor

29. TUBFET

30. Intramolecular CNTFET Inverter

31. TUBFET

32. Nanotube single electron transistor

33. Conclusion • Carbon nanotubes • exceptional potential to replace Silicon based semiconductor • Tremendous current carrying capability • 1 billion Amps/cm2 • Excellent heat conductor • twice as good as diamond • High strength • much higher than high-strength steel

34. Conclusion Ctd’ • Potential 50A gate length , THz switching speed • The possibilities of CNT are promising and range from nanotube composite materials, nanoelectronics, scanning microscope probes, chemical and /or biological sensors, to cold electron sources.

35. Questions • Strong SP2 bond • CNT VLSI drawbacks: • Large scale replacement, parallel fabrication techniques • Lithography require for source, gate, drain etc

36. Reference • 1 P. J Harris, Carbon Nanotubes and Related Structures, Cambridge Press, (Cambridge, • London, 1999) • 2 M. S. Dresselhaus, G. Dresselhaus, and P. C. Ecklund, Science of Fullerenes and • Carbon Nanotubes, AP, (New York, 1996) • 3 H. O Pierson, Handbook of Chemical Vapor Deposition, Noyes, (Norwich, 1999) • 4 M. Yudasaka, R. Kikuchi,T.Matsui, Y Ohki, S Yoshimura, and E. Ota, Appl. Phys. Lett.

37. Reference • 67, 17 (1995). • 5 Z. F. Ren, Z. P. Huang, D. Z. Wang, J. G. Wen, J. W. Xu, J. H. Wang et al., Appl. Phys. • Lett. 75, 8 (1999). • 6 M. Yukasaka, R. Kikuchi, Y. Ohki, E. Ota, and S. Yoshimura, Appl. Phys. Lett 70, 14 • (1997). • 7 C. J. Lee, D. W. Kim, T. J. Lee et al., Appl. Phys. Lett 75, 12 (1999). • 8 S. B. Sinott, R. Andrews, D. Quian et al., Chem. Phys. Lett. 315, (1999).)

38. Reference • 9 Z. F. Ren, Z. P. Huang, J. W. Xu, J. H. Wang et al., Science. 282, 1105 (1998). • 10 T. Reuckes, K. Kim, E. Joselevich et al., Science 289, 94 (2000). • 11Y. C. Choi, Y. M. Shin, B. S. Lee, et al., Appl. Phys. Lett. 76, 16 (2000). • 12 Y. Y. Wei, Gyula Eres, V. I. Merkulov et al., Appl. Phys. Lett 78, 10 (2001). • 13 M. Ohring, The Matrials Science of Thin Films, AP, (New York, 1992)