Atomic force microscopy (AFM) is a nano to atomic scale imaging tool, which uses a cantilever with a sharp-ended tip, collectively known as an AFM probe. The tip is scanned across the sample surface under investigation and the force between the sample surface and tip is used to create an image of that surface. It is commonly accepted, as an AFM user, that the sharper the tip of an AFM probe, the higher the resolution of the image. But what actually is tip sharpness?
It is, as a matter of fact, frequently used to describe two distinct properties of an AFM probe: the aspect ratio of the tip (the ratio between its height and width) and the radius of curvature of the tip apex (hereafter, the tip radius), as shown in Figure 1, which represent sharpness at two different scales, the nano- and micro-scale, respectively.
To improve imaging resolution, it is essential to understand the difference between the tip radius on the nano-scale and tip aspect ratio on the micro-scale. In doing so the effects of these two properties must be considered in the context of the specific sample surface that we are attempting to image.
When an AFM tip scans over the sample surface under study, the tip radius has to be smaller than the size of the surface features in order to resolve them. Since the tip apex can scan over each point of the feature. As displayed in Figure 2 (a), a tip with a radius of curvature smaller than the surface features traces out the topography of the surface more precisely as the tip apex can scan over each point of the feature.
Although, as shown in Figure 2 (b), when the probe tip has a radius larger than the size of the surface features, the features will seem larger than their actual size in the acquired image. Furthermore, if two features are spaced close together, the probe may resolve them as one large feature.
In situations where the features of the sample surface have edges such as trenches or steep sidewalls, which are frequent in semiconductor device processing, a high aspect ratio AFM tip is particularly convenient. This is due the fact that the height and width of the tip is larger and smaller than the depth and width between the sidewalls of the trench, which allows the tip apex to scan along the sidewalls and bottom of the trench which can be seen in Figure 3 (A).
In the case of a lower aspect ratio tip, its apex does not extend to the bottom of the trench. In this case, the feature is imaged as narrower and shallower than its real profile, as can be seen in Figure 3 (B). High aspect ratio is usually achieved by the removal of part of the tip sidewalls via etching or ion beam milling. The depth of such trenches is commonly around 1 µm so high aspect ratio usually only needs to be achieved in the region of the tip extending from the apex to a distance somewhat greater than the depth of the trench.
In conclusion, tip radius determines absolute imaging resolution at the nanometer or even sub-nanometer scale and aspect ratio governs the ability to resolve features with steep edges or highly 3D structures. As a result, when reflecting on which AFM probe to choose for imaging a sample, the expected topography and dimensions of its surface features need to be taken into account. This will establish whether a tip with a smaller tip apex radius or high aspect ratio is most relevant for your work.
This information has been sourced, reviewed and adapted from materials provided by Nu Nano Ltd.
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