The Superiority of Conical Tips in Atomic Force Microscopy (AFM)

A typical atomic force microscopy (AFM) probe is made up of an almost atomically sharp tip at the end of a micro-cantilever, which is raster scanned over a sample surface. The forces arising between the tip apex and the sample result in the production of an image of the surface.

When a researcher considers AFM tips, it is probably that the primary characteristic they consider is the tip’s sharpness. The sharpness of the tip apex along with the aspect ratio of the tip (microscale sharpness) are both key concerns in surface imaging, as these are the factors which determine image resolution.

Atomic Force Microscopy (AFM) Probe

The tip’s aspect ratio is set by its microscale shape, while the tip’s sharpness apex should be unaffected by this, providing appropriate sharpening procedures have been carried out in the course of manufacture.

Of course, in order to resolve a surface feature, the tip apex must have greater sharpness than the size of the feature due to be imaged. When the tip is used to image a surface, every point imaged is a spatial convolution of the form of the tip and the imaged feature.

If the microscale sharpness of the tip is great, steep edges will seem widened, as every point imaged becomes dictated by the tip’s form. Additionally, the consistency of the tip form will affect whether continuity of imaging can be achieved in differing scan directions. If the tip’s sidewalls are not identical, it will be impossible to achieve symmetrical images.

AFM Tip Shapes

AFM tips come in three common shapes: conical, pyramidal, and tetrahedral.

Trace (black line) of AFM probe with (a) conical and (b) pyramidal tip as it moves along a sample surface

Figure 1. Trace (black line) of AFM probe with (a) conical and (b) pyramidal tip as it moves along a sample surface

The aspect ratio of pyramidal and tetrahedral tips is restricted by the manner in which they are produced, which necessitates etching along the crystallographic planes of the material from which they are created.

Consequently, their sidewalls react to steep edges on the surface of a sample, which can result in artifacts such as shadows in the vicinity of these features. This is clear in the trace of a pyramidal tip as it traverses across a surface with steep-edged features in Fig. 1(b), in which the features seem widened.

Additionally, these tips often have uneven geometries as a result of the use of wet-etching techniques in their manufacture, which leads to a lack of symmetry in varying scan directions.

Conical tips have a number of advantages over pyramidal and tetrahedral types. To begin with, it is possible to fabricate them with a higher aspect ratio, as etching along a particular crystal orientation is not necessary in their manufacture. This make them more suitable than pyramidal and tetrahedral tips for the imaging of trench structures, common in semiconductor processing. The trace of a conical tip over a surface with steep-edged features can be seen in Fig. 1(a). It is clear that the conical tip more accurately demonstrates the true profile of such a surface.

Furthermore, they are produced using dry-etching, which allows for geometrical standardization, ensuring that where conical tips are utilized, images will appear identical, no matter the scan direction.

NuNano AFM Probes

NuNano offers probes with conical tips (Scout 350 and Scout 70) to offer the above-described benefits over probes manufactured with pyramidal or tetrahedral tips. They have been used for high resolution imaging of, for example, semiconducting nanofibers on silicon and C60 on gold.

This information has been sourced, reviewed and adapted from materials provided by Nu Nano Ltd.

For more information on this source, please visit Nu Nano Ltd.

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