(Nanowerk Spotlight) The operation principle of atomic force microscopy (AFM) is based on an atomically sharp tip, placed at the end of a flexible cantilever beam, that is brought into physical contact with the surface of a sample. The cantilever beam deflects in proportion to the force of interaction. Scanning across the surface, the sharp tip follows the bumps and grooves formed by the atoms on the surface. By monitoring the deflections of the flexible cantilever beam one can generate a topography of the surface.
For the past 20+ years, this principle has been the basis for one of the most important tools to visualize nanoscale objects where conventional optics cannot resolve them due to the wave nature of light.
One limiting factor of conventional AFM operation is the speed at which images can be acquired. Over the past five years, researchers at the Nanophysics and Soft Matter Group at the University of Bristol have been developing a high-speed AFM capable of video-rate image capture. An AFM with this ability enables nanoscale processes to be observed in real-time, rather than capturing only snap-shots in time.
An obvious application of this instrument is to modify the sample surface while observing changes in the surface topography. Successful demonstration of this would indicate the potential for a new generation of fabrication tools. James Vicary and Mervyn Miles, scientists at the above-mentioned Nanophysics and Soft Matter Group, have now done exactly that.
Frames from a movie capturing the growth of three lines of silicon oxide in real-time. (Source: Dr. James Vicary, University of Bristol)
Here is the entire movie, capturing the fabrication of oxidation lines in real-time at 15 fps:
"Our chosen fabrication process was that of local oxidation, which can be induced by applying an electric field between the AFM tip and a semiconductor or metallic sample" Vicary explains to Nanowerk. "While other scientists have investigated its use for device fabrication and data storage, this has primarily been achieved with AFM operating at conventional tip speeds (1-100 µm/s)."
In essence, what Miles and Vicary demonstrate is an instrument that has the ability to simultaneously image and modify a surface on the nanometer scale. Conventional AFM is already used in the semiconductor industry for routine quality control. With the high-speed AFM reported in this work it is possible not only to perform sample inspection faster, but also to perform lithography at the same time. This represents a first step towards new fabrication technology whereby lithography and inspection can be performed simultaneously.
While the two scientists did not observe any damage to the nanostructures, despite the tip having passed over the features in excess of 250 times, they found that combined high-speed imaging and nanostructuring does, however, lead to degradation of the AFM tip over time.
"High-speed scanning enables the tip to cover several hundred frames in the same time as it would take a conventional AFM to capture a single frame" says Vicary. "With uncoated silicon nitride cantilevers this is not usually a problem, however, combining high-speed imaging with local oxidation nanofabrication requires the tip and cantilever to have a conductive coating. For our work we used a platinum coating, adhered to the tip and cantilever with a thin layer of titanium. We have seen that over the course of several experiments the tip coating and the tip itself can suffer from degradation, as indicated by a broadening of the fabricated oxide features and loss of resolution during imaging."
An immediate solution to this problem would be to lower the electric field strength and accept longer fabrication times. Ultimately, however, an alternative tip composition or tip coating would be favorable.
The obvious potential application for this technique is to replace conventional photolithographic processes for patterning silicon oxide layers on silicon, for example in the semiconductor industry. With its built-in inspection capability, this fabrication tool negates the need for post-patterning inspection saving both time and cost.
Also, the relative simplicity of this approach offers great benefits over multiple-probe systems which require complex manufacturing processes and operation.
Vicary points out that improvements could be made to the scan stage, by using a flexure based scan stage with programmable x–y motion, for example, or using a tip-scanning variation of the high-speed AFM enabling larger samples to be used and, therefore, making the technique more practical for large-scale surface pattering. "Ultimately, however, an alternative tip composition or tip coating would be favorable, in order to improve the patterning resolution and the reliability of this nanofabrication platform" he says.
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