Jan 5, 2012
Diffraction grating reduces noise of coated AFM cantilevers
In certain atomic force microscopy experiments, the benefits of cantilever coatings may be outweighed by their detrimental side effects – namely, increased detection-, force- and displacement-noise. In their latest work, researchers in Canada propose a special coating pattern to address the issue.
With the advent of atomic force microscopy (AFM), cantilevers have become an essential tool for force sensing at the nanoscale. Commercial cantilevers are produced for a variety of applications and typically fabricated from silicon or silicon nitride. In addition, specialized coatings can be used to increase reflectivity, conductivity, hardness or to make cantilevers magnetic or biochemically functionalizable.
In their study, the scientists unveil a problem with coated cantilevers. The coating converts negligible changes in light power of the detection laser into true measurable bending of the cantilever due to a difference in the thermal coefficient of expansion of the bulk material relative to the coating. In other words, small fluctuations of the laser power can cause significant cantilever vibrations.
For example, 1/f noise of the laser power translates directly into a 1/f movement of the cantilever apex. Also, in the team's AFM set-up, the detection system has zero associated line noise (60 Hz and harmonics), however, small 60 Hz fluctuations of the laser power cause significant cantilever vibrations at 60 Hz. Note that both types of 60 Hz noise are indistinguishable at first glance. Of course, this can affect AFM experiments as the force between the tip and the sample is being modulated by this force noise. Dynamic atomic force microscopy, which is performed at high frequencies, is also prone to this problem. In this case, the 1/f noise and line noise cause apparent fluctuations in the distance between the tip and the sample – a displacement noise.
Specialized patterned cantilever coatings may offset some of these problems (see image). The Fresnel cantilever not only reduces the force and displacement noise, it also reduces detection noise to below the noise floor of a uniformly coated cantilever.
The principle explaining this reduction in detection noise is that of a Fresnel lens (implemented here as a diffraction grating on the cantilever) which focuses the reflected light onto the photodetector to increase the light signal for a given amount of bending.
Further details can be found in the journal Nanotechnology.
About the author
Aleksander Labuda is a PhD student in physics at McGill University, Canada.