1. Carbon Nanotube Device Fabrication John Gerling EE 235 Introduction to Nanofabrication 3-3-08

2. CNT Device Fabrication Issues  Problem #1  Positon single nanotubes at a predefined location and orientation  Problem #2:  Reproducibility  Problem #3:  Large scale integration Vijayaraghavan, Nano Letters 7, 1556-1560 (2007)

3. CNT Device Fabrication Techniques  “Find ‘em and wire ‘em” technique: scan the area where CNT are located, spin resist, electrode area is exposed with e-beam, resist is washed away, contact metal is deposited. Not scalable.  Random deposition technique: make contacts and deposit CNT, hopefully they make contact. Not scalable.  AFM manipulation. Push them to desired locations. Not scalable.  Directed assembly via chemically modified surfaces. Scalable?  Directed assembly via electric fields. Scalable? a)AFM manipulation technique (www.research.ibm. com/nanoscience)www.research.ibm. com/nanoscience b)“Find ‘em and wire ‘em” technique McEuen – Physics World - June 2000 a)b)

4. Directed Assembly via Electric Fields (Dielectrophoresis) Laser ablation growth of SW CNT Purification in mild acid and suspended in N,N-dimethylformamide (DMF) Au/Ag electrodes on thermal oxide on silicon substrate. Switch on frequency generator/DC voltage, drop of suspension, 1 minute, then drop blown off surface with N2. Krupke Appl. Phys. A 76, 397-400 (2003) V

5. Effect of Voltage Magnitude H.-W. Seo et al. / Microelectronic Engineering 81 (2005) 83–89

6. Mechanism for Directed Assembly  Polarizable nanotubes acquire induced dipole moments along axis. Therefore opposing electrodes align nanotubes along field lines. Additional attraction towards the area between the electrodes.  R_CNT is lower than Rs at contact, field collaspe (2003), alternatively additional CNTs redistribute the electric field, inhibiting further CNTs from contacting (2007). V Rs R_CNT Krupke Appl. Phys. A 76, 397-400 (2003) Vijayaraghavan, Nano Letters 7, 1556-1560 (2007)

7. Observations and Results  More than one CNT bonded for Au and usually 1 CNT bonded for Ag (independent of time and conc.)  Observations: CNT align at f > 1kHz, at lower f, random alignment occurs. Krupke Appl. Phys. A 76, 397-400 (2003) Vijayaraghavan, Nano Letters 7, 1556-1560 (2007)

8. Pros and Cons Pros  Reproducible, self-limiting, directed assembly of nanotubes.  Integration density of several million devices per square cm.  Compatible with back end processes (metalization etc). Cons  Need contact to all devices.  10% of electrodes bridged by mulitple CNTs.  Initial contact R is high > 1M, need annealing or additional lithography (metal layer) to lower contact resistance.

9. Conclusion  Not quite ready for commerical applications.  Useful for lab conditions, identical source and deposition conditions for device testing. Questions?