High aspect ratio probes can be used to measure the spatial resolution of surface imaging for semiconductors applications.The spatial resolution of surface imaging by atomic force microscopy (AFM) is determined by two properties of the tip, these are the aspect ratio and the radius of its apex. This article outlines the aspect ratio and which sample types are benefited the most by this property.
The ratio between the width and height of an AFM tip is known as the aspect ratio. Standard AFM probes, created from silicon, have pyramidal, tetrahedral, or conical-shaped tips. Conical shaped tips can be created to offer a proportionately higher aspect ratio.
These traditional probes cannot precisely resolve the topography of highly non-planar characteristics, for example the narrow and deep trenches often found in semiconductor device processing.
The probe’s height and width is shorter than the height of the sidewalls and is wider than the spacing between the structure’s sidewalls, creating this challenge. The bottom of the trench cannot be reached by the tip apex, as detailed 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 traditional probes also cannot image tall cells, pillars, or particles with high resolution as the tip apex is unable to access the feature’s bottom corner next to its steep edge and as a result, cannot trace the structure’s shape with accuracy .
It follows that the length of the tip should be increased, and the width should be decreased to create a higher aspect ratio so that it can fit inside of the trenches and create high-resolution images of these types of properties, as illustrated 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
Only a segment of the tip must have a high aspect ratio as the height of these features is frequently not more than 1 µm. The area extends from the tip apex to a little longer than the sidewall height of the tall structures or trenches to be investigated.
High aspect ratio of the tip can be attained by using several different methods. The silicon can firstly be milled by an ion beam to remove a section of the tip sidewalls, or chemically or plasma etched which decreases the cone angle of the tip.
Secondly, high aspect ratio materials, for example, carbon nanotubes  or silicon nanowires  can be fixed or grown onto the apex of the tip. The robustness of the tip is also increased when using carbon nanotubes as a result of their high strength.
It should be added that it is generally simpler to manufacture silicon nanowire tips than their carbon nanotube equivalent and that essentially, attaching or growing new material on the tip apex is not as beneficial as removing existing material from the tip.
NuNano has manufactured high aspect ratio, silicon AFM probes with a cone angle of less than 15° across the final 1 µm 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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