AFM on a chip

New developments from UWA are providing miniaturisation opportunities for one of the most widely used techniques for analysing nanoscale structures.

The UWA team has demonstrated a new approach that provides an alternative way to perform Atomic Force Microscopy (AFM), a characterisation technique used on a daily basis by those investigating the micro/nano world.

AFM relies on tracking the movements of an ultra-sharp tip that is fixed to a cantilever as it encounters nanoscale obstacles when scanned over a sample’s surface.

In conventional AFM, these miniscule movements are followed by observing changes in direction of a laser beam that is reflected off the backside of the cantilever-mounted tip. As the cantilever deflects due to tip interactions with the sample, the beam is reflected in different directions – recording these changes allows the system to map the nanoscale landscape of a sample’s surface, or to infer information about a sample’s intrinsic properties such as stiffness, conductance, or magnetic properties.

This “free space optics” approach relies on optical leveraging and requires the laser beam to travel a significant distance before the tiny changes in path direction can be observed – the system has to be large enough to accommodate this.

The UWA team does away with footprint requirements of free space optics by harnessing the laser beam’s ability to interact with itself.

Light is known to act both like a particle and like a wave. This latter behaviour means, much like waves in the sea, light waves can interfere with each other to cause amplifications or to cancel each other out.

In the UWA system, a laser beam is passed very close to the cantilever in a light-carrying channel called a waveguide.

A portion of the light is redirected out of this path and aimed directly at the back side of the end of the cantilever. The light is reflected straight back into the original path of the laser beam where it’s now becomes out of sync, causing it to interfere with the light that continued on directly – this causes vivid changes in the intensity of the waveguided light that can be easily monitored.

When the cantilever is moving up and down, the time taken for light to reach it and return to the typical path will change – if the cantilever is far away, the light will take longer to return, if it’s closer the light returns sooner – and this causes measurable differences in the interference patterns. By monitoring these changes, the system can track the movements of the tip, and therefore infer the position of the cantilever-mounted tip and the scanned surface topology.

The result is an on-chip integrated system that needs far less space to operate, and that is less susceptible to outside influences. What’s more, the componentry is made using well established fabrication practices making it affordable to produce at scale, and it can be easily retrofitted to existing systems.

The technology is well positioned for commercialisation, and the team’s next steps are to continue to work towards this goal.

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