Surface Phenomena at Metal-Carbon Nanotube Interfaces

1. Surface Phenomena at Metal-Carbon Nanotube Interfaces Quoc Ngo Dusan Petranovic Hans Yoong Shoba Krishnan Cary Y. Yang Back

2. Outline • Motivation • Multi-wall carbon nanotube (MWNT) architectures • Mechanisms of contact resistance • Characterization of contact resistance - Side-contacted architecture - End-contacted architecture • Conclusion

3. Wire Length: Motivation • Physical limits of copper interconnects and vias will soon be reached if scaling trends continue Chen et al., IEEE Elec. Dev. Lett., 19, 508(1998)

4. Motivation • CNTs provide a feasible alternative due to their superior electrical and mechanical properties • Full understanding of CNT contact resistance has yet to be ascertained • CNT growth processes can be integrated into silicon-based manufacturing

5. Diamond C60 Buckyball Graphite Nanotube

6. MWNT Architectures:Side-contacted geometry • Contacts are either pre-patterned on the substrate, or deposited after the nanotube has been dispersed onto a substrate • Contact is made with the side of the MWNT Wei, et al., Appl. Phys. Lett., 79, 1172(2001) Spacing between electrodes ~2.5m

7. MWNT Architectures:End-contacted geometry* • Nanotubes are grown vertically from a patterned catalyst film • Contact is made with the end of the MWNT 5μm 500nm *Li et al., Appl. Phys. Lett., 82, 2491 (2003) 200nm AFM (current sensing mode) and SEM top view

8. Mechanisms of Side-contact Resistance Copper interconnect: CNT interconnect:

9. Mechanisms of Side-contact Resistance • Direct or Fowler-Nordheim tunneling between two metals through a Schottky Barrier (metal-insulator-metal) • The type of tunneling is dependent on the work function of the metal, and the applied bias • Tunneling in an MIM system is approximated by Simmons (J. Appl. Phys., June 1963)

10. Work Function Dependence of Side-contact Resistance Calculated Contact Resistivity [Ω-cm2]

11. AFM tip to MWNT (contact) • Probe tip/metal to MWNT (contact) • MWNT to metal underlayer • MWNTs to metal underlayer • Metal underlayer sheet resistance • Metal underlayer sheet resistance Mechanisms of End-contact Resistance Tungsten probe tip (on ~10μm chromium pad) AFM probe tip MWNT SiO2 Chromium underlayer Single MWNT Resistance: Parallel MWNT Resistance:

12. A statistical approach is taken for calculating resistance of a single MWNT by measuring many MWNTs in parallel Current [mA] Voltage [V] 10μm End-contact Nantotube Characterization • Nanotube diameters = 50-100nm • ~5-6 MWNT per 1μm2 • 100μm2 contains ~500-600 MWNT • R(single MWNT)  24-29k

13. Metal Underlayer Sheet Resistance • Chromium sheet resistance is a small percentage of overall resistance in four-terminal configuration • Appears to be resistant to high temperature effects of CVD processing

14. (b) (b) R=44Ω (a) R=76Ω Current [mA] Voltage [V] Importance of Quality Contacts • To demonstrate the importance of quality contacts, we conduct two different measurements: • Contacting parallel nanotubes with W probe tip (no contact) • Contacting parallel nanotubes through a deposited Cr contact (a)

15. Conclusion • Two different metal-CNT contact geometries are studied • Side-contact resistance is simulated using MIM tunnel junction theory • End-contact resistance is examined w.r.t. processing effects • Overall resistance for parallel MWNTs demonstrates excellent potential for on-chip interconnect applications

16. Partners • Center for Nanotechnology at NASA Ames Research Center - Drs. Meyya Meyyappan, Jun Li, Alan Cassell, Laura Ye • National Center for Electron Microscopy (Lawrence Berkeley National Laboratory) - Dr. Velimir Radmilovic Publications • Quoc Ngo, et al., “Surface Phenomena at Metal-Carbon Nanotube Interfaces,” IEEE NANO 2003, San Francisco, vol. 1, pp. 252-255, August 11-14, 2003.