Wires made of individual carbon atoms could be used to reduce the size of today’s microchips several-fold. Carbon nanotubes (CNT) were researched in the past few years and used in initial experimental applications. Nano-engineering now has the task of developing production technologies to make CNT applications commonplace even for the mass market.
Sumio Iijima presented the properties of a novel ordered structure of carbon atoms in a paper in Nature in 1991. With their three-dimensional structure and hexagonal arrangement of carbon atoms, the carbon nanotubes (CNT) that he described resemble rolled-up chicken wire. Physicists throughout the whole world were enthusiastic about the material’s promising properties: it was reputed to be stronger than steel and to have a thermal conductivity better than diamond and an electrical conductivity 1000 times higher than copper. Depending on their quantum entanglement, CNT are said to be usable as semiconductors or as conductors.
Via theoretical deductions and individual experiments, fundamental research in recent years has shown that CNT are usable as semiconductors to construct transistors – the basic elements of every computer. CNT could make computer microchips many times smaller. This is attractive particularly against the background that the limits of miniaturisation with conventional chip materials will soon be exhausted and the industry urgently needs alternative technologies for further innovations.
Dimos Poulikakos, Professor at the Laboratory of Thermodynamics in Emerging Technologies (LTNT), says "Although we now understand the properties of CNT relatively well, we are still at the very beginning when it is a question of how to build such systems, which are invisible to the naked eye and extremely vulnerable to external disturbances. That’s why the work we are doing in our laboratory is really basic research in the engineering sciences."
An electrifying, self-organising system
Timo Schwamb, Professor Poulikakos’ doctoral student at the LTNT, published a paper in Nano Letters last November describing possible new ways of positioning CNT in nano-electromechanical systems (NEMS). Dielectrophoresis, a technique well-known from electrical engineering, was used by Schwamb for the first time in his experiments with a high success rate for NEMS with more than two electrodes. This enormously expands the application spectrum of CNTs in nanotechnology. Schwamb applies an inhomogeneous electrical field to a microchip previously treated with a droplet of CNT solution. In this process the electrical circuit has a gap at exactly the place where the CNT is to be positioned. The strongest bipolar forces occur at exactly that point, and ultimately attract and align the CNT.
Schwamb can also use the method described in the paper to incorporate a four-point measuring method into the NEMS. This eliminates results falsifications in the resistance measurement caused by the soldered joints of the CNT that are used. However, because the electrical fields of four electrodes lying in a plane would get in the way, thus disturbing the positioning of the CNT, the doctoral student decided to use a three-dimensional experimental design. Schwamb explains that: "Although we use a little trick to introduce four electrodes for the four-point measuring system, we nevertheless generate an electrical field equivalent to that of only two electrodes and we use this to position the CNT."
This trick works as follows: a gap 0.5 nanometres wide is milled in a three-layer microchip (conductor/insulator/conductor) in such a way that the lowermost layer on both sides projects minimally into the empty space. The two electrodes needed for the four-point measurement can now be “hidden” as it were in the third dimension under the other two electrodes, thus they no longer interfere with the dielectrophoresis.
The LTNT is supported by the EMPA (Swiss Federal Laboratories for Materials Testing and Research) in the difficult work on the chip in the range of a few nanometres. Their experts can use an ion beam to mill gaps and steps in the three-layer silicon chip and can solder tiny contact points between the carrier chip and the CNT in a subsequent step using electron beams. Schwamb summarizes: "For the nanotechnology application we can use the new method to combine two known technologies, dielectrophoresis and the four-point measuring technique, in such a way as to create for the first time the potential to mass-produce nano-devices." This could increase the yield in successfully positioning CNTs across four contact points from approx. 3 percent to 40 percent.
Nano-engineering: an engineering tradition alive in Switzerland
According to Schwamb, the next step will now involve using the new approach to build prototype devices such as transistors and temperature or pressure sensors and to test their properties in a wide variety of ways. However, he says that integrating nano-materials in processes with mass production capability still represents one of the biggest challenges facing nano-technology. However, the most significant benefit of the approach described in the paper is that it can also be used for other nano-particles with interesting properties, and is not limited to CNTs.
Poulikakos also detects a piece of Swiss future in nano-engineering: "Switzerland should not only defend its traditional leading position in the area of engineering achievements in the construction of large machinery but should also position itself as a pioneer in the nano-devices field." However, Poulikakos is still unwilling to make any predictions as to when the first CNT computer with a gigantic computing performance will come onto the market and whether or not this will be in Switzerland.
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