Real-time quality control for nanotechnology

Two publications in the current issue of the IOP journal Nanotechnology report techniques that may bring real-time quality control to two different nanotech fabrication methods. The first author of one of the papers describes their approach for (requires free registration). From “Nanofabrication observed in real-time” by James Vicary:

Conventional nano and microfabrication techniques rely on a patterning process being performed and the sample subsequently imaged to inspect the result of the process. By performing the patterning and imaging simultaneously, defects can be identified immediately and a substantial time saving can be made during the total fabrication cycle. This requires new technology to be developed combining these two attributes and provides a stimulus for the next generation of fabrication tools.

Over the last five years, researchers in the Nanophysics and Soft Matter Group at the University of Bristol, UK, have been developing a novel high-speed atomic force microscope (AFM). Perhaps the most versatile of surface characterization tools, the AFM is capable of nanoscale resolution in three dimensions and can be operated in air, vacuum and under liquid. The high-speed AFM allows video-rate imaging of surfaces to be performed, enabling scientists to observe changes in topography in real-time. A natural progression is the application of the high-speed AFM to nanofabrication.

James Vicary and his coauthor M J Miles present the application of a high speed AFM to the oxidation of silicon surface in the vicinity of the AFM tip (abstract). (The full text (854 KB PDF) is available free for 30 days after publication, and the electronic version includes two movies of the fabrication procedure.) Using a resonant scan stage that they had previously reported, they were able to reach tip vs sample speeds of up to 10 cm per second, fabricate silicon oxide patches on a silicon surface by applying a voltage between tip and sample, and capture images at a rate of 15 frames per second. They were thus able to replicate a standard semiconductor fabrication process, fabricating lines an average of about 100 nm wide and slightly less than 2 nm high, and about 1 micrometer long. The authors note that no damage to the nanostructures formed was observed even though the tip passed over the features more than 250 times. However, the combination of nanofabrication and high speed imaging did lead to some tip degradation. The authors also note that their method is faster than conventional photolithography, which can take several hours to coat a silicon wafer with oxide, while they took only several seconds to pattern individual features and several minutes to pattern larger areas of more than 100 square micrometers, monitoring the fabrication in real time. Furthermore, they note that the oxide patterns could be used a precursors for subsequent chemical modification. The authors conclude:

The successful demonstration of this method for observing the fabrication of nanoscale structures in real-time represents a major advance in the use of scanning-probe techniques for applications in nanotechnology.

The authors briefly discuss improvements to realize the potential for the technique. For those interested in tip-based approaches to productive nanosystems, the most important parameter to improve is the resolution of the fabrication method, since a several-hundred-fold improvement would be necessary to reach atomic precision—a perspective the authors share:

Perhaps more importantly, optimization of the AFM tips and their coating, would greatly improve the patterning resolution attainable, as well as the reliability of this nanofabrication platform.

The second paper (abstract; the 605 KB PDF is available free for 30 days after publication) combines quantum-dot-mediated fluorescence resonance energy transfer (QD-FRET) and microfluidics to monitor the assembly of DNA nanostructures. Although focused on the preparation of nanoparticles to carry DNA through the bloodstream and into the cell for the purpose of gene therapy, the ability to monitor assembly of DNA into nanostructures might also be of use in building DNA frameworks and other devices for the modular molecular composite nanosystems approaches to productive nanosystems. The authors (from Johns Hopkins University and Duke University) demonstrate the combination of QD-FRET and microfluidics to study the formation of nanoparticles with DNA and chitosan, a linear polysaccharide of glucosamine and acetyl-glucosamine derived from the exoskeletons of marine crustaceans. DNA and chitosan were respectively labeled with the donor and acceptor components of a FRET pair so that their interactions could be followed by FRET signals, which are very sensitive to separation between the components of the pair, and thus sensitive to the assembly of the complex. The mixing of DNA and chitosan was controlled in a microfluidic device, which allowed the assembly process to be followed with millisecond resolution. In this case, the DNA and the chitosan were found to follow a two-stage self-assembly process. Custom design of microfluidic devices should allow control over how components are mixed in order to optimize assembly.

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