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Carbon Nanotube Device Fabrication John Gerling EE 235 Introduction to Nanofabrication 3-3-08
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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)
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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)
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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
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Effect of Voltage Magnitude H.-W. Seo et al. / Microelectronic Engineering 81 (2005) 83–89
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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)
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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)
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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.
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Conclusion Not quite ready for commerical applications. Useful for lab conditions, identical source and deposition conditions for device testing. Questions?
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