S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 Surface Phenomena at Metal-Carbon Nanotube Interfaces Quoc Ngo Dusan Petranovic Hans Yoong Shoba Krishnan Cary Y. Yang Back

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 Outline Motivation Multi-wall carbon nanotube (MWNT) architectures Mechanisms of contact resistance Characterization of contact resistance - Side-contacted architecture - End-contacted architecture Conclusion

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 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) Wire Length:

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 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

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 Diamond C 60 Buckyball GraphiteNanotube

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 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

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 MWNT Architectures: End-contacted geometry* Nanotubes are grown vertically from a patterned catalyst film Contact is made with the end of the MWNT AFM (current sensing mode) and SEM top view 5μm *Li et al., Appl. Phys. Lett., 82, 2491 (2003) 200nm 500nm

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 Mechanisms of Side-contact Resistance Copper interconnect: CNT interconnect:

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 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)

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 Work Function Dependence of Side-contact Resistance Calculated Contact Resistivity [Ω-cm 2 ]

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 Mechanisms of End-contact Resistance Single MWNT Resistance: Chromium underlayer SiO 2 MWNT AFM probe tip Tungsten probe tip (on ~10μm chromium pad) Parallel MWNT Resistance: AFM tip to MWNT (contact) MWNT to metal underlayer Metal underlayer sheet resistance Probe tip/metal to MWNT (contact) MWNTs to metal underlayer Metal underlayer sheet resistance

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 End-contact Nantotube Characterization A statistical approach is taken for calculating resistance of a single MWNT by measuring many MWNTs in parallel 10μm Nanotube diameters = nm ~5-6 MWNT per 1μm 2 100μm 2 contains ~ MWNT R(single MWNT)  24-29k  Voltage [V] Current [mA]

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 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 Measurement Type Bare Cr Resistance (no CVD processing) Bare Cr Resistance (post CVD processing) Two-terminal12-15Ω10-15Ω Four-terminal2-4Ω~6Ω

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 Importance of Quality Contacts To demonstrate the importance of quality contacts, we conduct two different measurements: a)Contacting parallel nanotubes with W probe tip (no contact) b)Contacting parallel nanotubes through a deposited Cr contact (a) (b) Voltage [V] Current [mA] (b) R=44Ω (a) R=76Ω

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 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

S A N T A C L A R A U N I V E R S I T Y Center for Nanostructures September 25, 2003 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 , August 11-14, 2003.