Atomic force microscopy can be used to improve image resolution for semiconductor applications.The spatial resolution of surface imaging by atomic force microscopy is influenced by two characteristics of the tip: the aspect ratio and the radius of its apex. This article will focus on the aspect ratio and for which sample types it proves most effective.
The aspect ratio is calculated as the ratio between the width and height of an AFM tip. Conical, tetrahedral, or pyramidal-shaped tips can be seen on standard AFM probes which are made from silicon. Conical shaped tips can be produced to have a comparatively higher aspect ratio.
The topography of highly non-planar attributes like deep and narrow trenches, which are frequently seen in semiconductor device processing, cannot be accurately resolved by these conventional probes.
This is due to the width and height of the probes being shorter than the height of the sidewalls and wider than the spacing between the sidewalls of the structure respectively. The tip apex cannot extend to the bottom of the trench, as demonstrated in Figure 1.
Figure 1. Trace of a low aspect ratio AFM probe scanning over a surface of trench structures. Inset: Obtained AFM image. Image Credit: NuNano
These conventional probes cannot image tall particles, cells, or pillars with high resolution as the tip apex cannot access the bottom corner of the feature next to its steep edge and therefore, cannot accurately trace out the shape of the structure .
It is suggested that the width of the tip should be decreased, and its length extended to gain a higher aspect ratio as a means to fit within the trenches and take high-resolution images of these kinds of characteristics, as shown in Figure 2.
Figure 2. Trace of a high aspect ratio AFM probe scanning over a surface of trench structures. Inset: Obtained AFM image. Image Credit: NuNano
As the height of these aspects is normally not greater than 1 µm, only a section of the tip is required to have high aspect ratio, ranging from the tip apex to slightly longer than the sidewall height of the tall structures or trenches under analysis.
A few different techniques can be used to achieve a high aspect ratio of the tip. Firstly, the silicon can be plasma or chemically etched, or milled by an ion beam to take away a section of the tip sidewalls. This reduces the cone angle of the tip.
Secondly, materials with a high aspect ratio can be attached or grown onto the tip apex for example carbon nanotubes  or silicon nanowires . Carbon nanotubes additionally help to increase the robustness of the tip due to their high strength.
It should be explained that silicon nanowire tips are normally simpler to produce than their carbon nanotube equivalent and that primarily, removing prior material from the tip is preferred over growing or attaching new material on its apex.
NuNano has created high aspect ratio, silicon AFM probes with a cone angle of less than 15° over the last 1 µm section of the tip.
References and Further Reading
 A. Wang and M. J. Butte, “Customized atomic force microscopy probe by focused-ion-beam assisted tip transfer,” Applied Physics Letters, 105(5), 053101, 2014.
 N. R. Wilson and J. V. Macpherson, “Carbon nanotube tips for atomic force microscopy,” Nature Nanotechnology, 4, pp. 483–491, 2009.
 B. A. Bryce, B. R. Ilic, M. C. Reuter, and S. Tiwari, “Silicon nanowire atomic force microscopy probes for high aspect ratio geometries,” Applied Physics Letters, 100(21), 213106, 2012.
This information has been sourced, reviewed and adapted from materials provided by NuNano.
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