Nanojoining - An Integration Technology for Nanodevices and Nanosystems

by Professor Norman Zhou

Joining, whether at the nano-, micro- or macro-scale, has been an essential part of manufacturing and assembly of man-made products, providing mechanical coupling and support, electrical connection or insulation, environmental protection, etc. This will still be true with the emerging technology of nanojoining, that is, producing permanent unions or connections between nanosized building blocks, which are typically manufactured using top-down techniques such as nanolithography, or bottom-up methods such as self-assembly, to form functional nanodevices and nanosystems¹.

Nanojoining also allows integrating these nanodevices and nanosystems to the surroundings, i.e. micro- and macro-scale devices and systems. Nanojoining is also referred to as nanobonding, nanowelding, nanobrazing, nanosoldering, etc.

Permanent unions or connections between building blocks or parts to be assembled are produced mainly through the formation of primary (and on occasions secondary) chemical bonds between faying surfaces¹. When the parts are not compatible in atomic structures, an interlayer or intermediate material may be required. In principle, two ideal solid surfaces, e.g., both perfectly clean and atomically flat, will bond together if brought into intimate contact, as they will be drawn together spontaneously by interatomic forces.

However, most engineering surfaces are characterized as rough and contaminated, requiring some form of energy, usually heat and/or pressure, to be applied to overcome these surface impediments to make a joint¹. It is expected that these impediments will be less significant in nanojoining because of the much reduced surface areas and special environments used in most nanojoining processes. On the other hand, other challenges arise because of continuing miniaturization and the associated laws of physics. These would for example cause difficulties in manipulation of parts.

Recently, the development of nanojoining processes is attracting tremendous efforts^2-9. Various methods have been developed for nanojoining, some of which have been at least partially successful. For example, by in-situ e-beam exposure with a transmission electron microscope at high temperatures, Terrone et al.² welded two crossing single wall carbon nanotubes (SWCNT) through the formation of covalent C-C bonds and the creation of seven- or eight-membered carbon rings bridging two nanotubes.

Recently, Professor Norman Zhou and his colleagues at Centre for Advanced Materials Joining, have successfully brazed carbon nanotube bundles to Ni electrodes with Ti-containing brazing alloys at 900°C to 1000°C³, in which nanotubes react with Ti to form Ti-C bonds leading to low Ohmic contacts between carbon nanotubes and Ni electrodes.

Wei et al.⁴ realized nanoconnection between carbon nanotubes and tungsten leads by depositing a Ga layer with a focused Gd+ ion beam (FIB). A small contact resistance of tens to 100 Ohms was achieved. Chen et al.⁵ bonded SWCNTs to Ti electrodes using an ultrasonic bonder to obtain robust bonds with a contact resistance of a few kilo-Ohms. Similar to conventional resistance spot welding, Hirayama et al.⁶ welded two SWCNTs by applying currents through a scanning tunnelling microscope. Obviously, these protocols are effective either in very specific conditions and/or for a very specific material, such as, e-beam or ion beams that require high vacuum, ultrasonic welding which provides less spatial control of joints, Joule heating which is limited to the joining of conductors.

Conversely, two alternate methods are described in detail in the following section which are effective for joining nanomaterials in general:

• femtosecond laser irradiation^7,8 and
• low temperature solid state sintering through surface atomic diffusion and/or partial surface melting⁹.

As the electron-lattice thermal coupling time (about 1 picosecond) is much longer than the femtosecond laser pulse width, the electrons do not have enough time to transfer energy to the lattice. The nature of interaction of femtosecond laser pulses and materials is known as non-thermal processing.

After electrons have been excited by a femtosecond laser, the lattice cohesion is reduced and the binding loosens due to the Coulomb repulsion. This is accompanied by the unique effect known as ultrafast melting, which occurs only over nano-scale dimensions compared to the conventional thermal melting. This opens up exciting possibilities for joining of nano-scale building blocks for micro-electromechanical devices¹⁰. By precisely controlling the laser energy, nanojoining at the atomic level is possible.

On the other hand, the surface atomic migration is dramatically enhanced in nanomaterials due to their high ratio of surface energy to volume condensed energy. Near-surface atoms with a similar mobility as in the liquid state can provide a bonding mechanism through low temperature coalescence and sintering. Recently, we have bonded Cu wires to Cu foils using Ag nanoparticle pastes at 160°C¹⁰.

It is certain that nanojoining is one of the key technologies in industrial success of nanodevices and nanosystems. As Terrones et al. ² pointed out for CNT nanoelectronics and nanodevices, joining "is a key issue because both electronic devices and strong nano-mechanical systems need molecular connections among individual SWCNTs".

Nanojoining will revolutionize various ongoing nano-manufacturing technologies for nano-mechatronics and molecular devices. These nanodevices and nanosystems have the potential to provide distinctive properties and superior sensitivity, and can offer improved integration and reduced operating energy requirements for the next generation of technologies. A current example is the welded Au/Ag nanoparticles used in surface enhanced Raman probes which promise to provide single molecule characterization for modern medical diagnostics, drug development and/or quantum computing^7,8.

References

1. Y. Zhou, "Microjoining and Nanojoining". Woodhead Publishing Ltd, Cambridge, England, CRC Press, 2008
2. M. Terrones, F. Banhart, N. Grobert, J. C. Charlier, H. Terrones, and P. M. Ajayan, Phys. Rev. Lett. 2002, 89, 075505
3. W. Wu, A. Hu, X. Li, J. Wei, Q. Shu, K. L. Wang, M. Yavuz, Y. Zhou, "Vacuum brazing of carbon nanotube bundles", Mater. Lett. 62 (2008) 4486
4. C. Chen, L. Yan, E. Kong, Y. Zhang, Nanotechnology 2006, 17, 2192.
5. B. Wei, R. Spolenak, P. Kohler-Redlich, M. Ruhle, E. Arzt, Appl. Phys. Lett. 1999, 74, 3149.
6. H. Hiyayama, Y. Kawamoto, Y. Ohshima, and K. Takayanagi, Appl. Phys. Lett. 2001, 79, 1169
7. Y. Zhou, A. Hu, M. I. Khan, W. Wu, B. Tam, and M. Yavuz. "Recent progress in micro and nanojoining". J. Phys. Conf. Ser. 2009, 165, 012021.
8. A. Hu, S. K. Panda, M.I. Khan, Y. Zhou, (2009) "Laser Welding, Microwelding, Nanowelding and Nanoprocessing", Chin. J. Lasers Vol.36, no.12, 3149.
9. H. Alarifi, A. Hu, M. Yavuz, Y. Zhou, "Bonding of Cu wires at low temperatures using Ag nanoparticles paste", proceeding of Materials Research Society, 2009 Fall, Boston, USA.
10. A. Hu, M. Rybachuk, Q.-B, Lu and W. W. Duley. "Direct synthesis of sp-bonded carbon chains on graphite surface by femtosecond laser irradiation". Appl. Phys. Lett. 2007, 91, 1319061.

Copyright AZoNano.com, Professor Norman Zhou (University of Waterloo)