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Effect of Tip Radius on AFM Scans

Guang Li1, K. R. Gadelrab

1, Tewfik Souier

1, Pavel L. Potapov

2, Gang Chen

3 and Matteo Chiesa

1,3*

1 Laboratory for Energy and Nano-Science, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates 2Center of Complex Material Analysis, GLOBALFOUNDRIES, Dresden, Germany

3Department of Mechanical Engineering, Masschusetts Institute of Technology, Cambridge, MA 02139, United States

*Corresponding author: mchiesa@mit.edu

In this study, we try to simulate the effect of Atomic Force Microscope (AFM) tip radius on the captured details of AFM

scans. While such study does not provide a quantification to the errors resulted from using a larger AFM tip radius, it

helps to visualize the influence on the captured details of the scanned surfaces. Furthermore, the extracted results are valid

for the order of magnitude and tip radii commonly utilized in day to day AFM experiments. The study is performed on

real AFM scans with almost no assumptions related to the tip or the surface.

To achieve this, an AFM image is extracted in the form of a matrix that has rows and columns equal to the resolution of

the image. The entries of the matrix are the height values of the topography recorded by the AFM. A scanning process to

the extracted surface is simulated numerically through a MATLAB script. The AFM tip is simulated as a virtual sphere

that sweeps the surface by changing the coordinates of its center while the z value of the region covered by the sphere is

recorded. No mutual penetration condition is fulfilled at every point in the covered zone. To achieve this, the difference in

height between the sphere surface and the sample topography is calculated. The recorded z value in that region is the one

that produces no negative z difference (the topography does not penetrate through the sphere surface). This process is

performed every instance the sphere center is shifted by a step of one pixel.

Figure 1 shows an AFM scan of indentation using a Berkovich tip in fused silica. The 3D scan is viewed along the y-z

plane to be able to compare it with generated surfaces from the simulation. The scan is obtained in tapping mode on MFP

3D Asylum Research with a scan resolution of 512×512.

Figure 1. AFM scan of a fused silica indentation viewed along the y-z plane to be compared with surfaces generated by

the simulation. The figure shows the remaining indent and some structure on the surface probably due to some surface

contamination and pile-up.

The simulation is run with different tip radii that have the values 10nm, 50nm, 100nm and 200nm. In fact, the common

AFM tips commercially available have tip radii in the vicinity of 10nm. During service, the tip gets slightly blunted till it

stabilizes in the range of 20 to 30nm. Furthermore, the range of tip radii in this study is extended to 200nm that is almost

mailto:mchiesa@mit.edu

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never encountered in AFM tips; however, this case may occur when the indenter tip, itself, is employed to get a scan of

the deformed surface after performing an indentation experiment.

Figure 2 shows the simulation results for different tip radii. It is seen that the 10nm tip simulation is able to regenerate the

original surface almost perfectly. Moving to higher values of the tip radius produces surfaces that are dilated laterally

which affects the lateral dimensions of the surface features. In addition the expected loss of small details is observed at

larger value of the tip radius generating smoother surfaces

In the axial direction, there is almost no effect of the tip radius is seen on the height of concave features. On the other

hand, there is an observed difference between the measured indentation depth with a sharp and a blunted tip. However,

even at a tip radius of 200nm, the error in the measured indentation depth is about 6.5%. This can be explained by the

wide angle of the remaining indent due to the angle of the Berkovich tip. Using a smaller angle indenter will result in a

larger error for the same tip radius.

Figure 2. Numerically generated surfaces with different tip radii. The 10nm tip radius is able to regenerate the surface

accurately. Having a larger tip radius produces a lateral expansion in the surface features. The depth of the remaining indent

has about 6.5% error at a tip radius of 200nm.

R=10nm R=50nm

R=100nm R=200nm

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To realize a better understanding of the effect of tip blunting on the obtained AFM scan, a section is cut across the surface

plot to reveal a 2D view to the indentation depth. Figure 3 demonstrates the section cut for different tip radii. Again, the

10nm generated plot cannot be distinguished from the original one. The largest tip radius generates a shallower

indentation depth that is slightly narrower. The error in the obtained depth is about 8%. From figures 2 and 3, it is seen

that even with a tip radius of 50nm (severely blunted AFM tip), the error in the scanned featured is low.

Figure 3. A section cut through the numerically simulated surfaces in comparison to the original AFM scan. The 10nm tip

radius regenerates the surface accurately, while the 200nm tip radius produces shallower indentation depth and slightly

narrower remaining indent.

The same simulation is performed on a scanned Berkovich tip utilized to obtain the previous indentation. Figure 4

illustrates the surface plot generated by the AFM scan viewed along the x-z plane. The indenter tip is quite sharp and the

AFM scan reveals the lapping marks that are clearly observed along the pyramid edge. A feature is also noticed on the

pyramid surface that is probably a lapping remaining.

Figure 4. An AFM scan of a Berkovich tip utilized to generate the previously studied indent. The scan accurately captures

the details of the tip and the lapping marks.

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Running the same simulation previously described with the same range of tip radii, generates the plots in Figure 5. The

most pronounced effect of increasing the tip radius is the clear rounding of the indenter tip radius and indenter tip edges.

In addition, the fine lapping features on the surface of the pyramid are smoothened out.

A section cut through the surface plot again shows that 10nm AFM tip radius is able to regenerate the scanned surface

properly while increasing the AFM tip radius resulted in a clear blunting in the indenter tip shape. Again, the 50nm tip

AFM tip radius shows minimal effect of the scanned features of the indenter tip.

The artificial rounding of the indenter tip apex and edges will result in an error in the calculated tip area function.

However, it is seen that fresh and sharp AFM tips can generates the indenter tip topography accurately.

Figure 5. Numerically generated surfaces for a Berkovich tip with different AFM tip radii. The 10nm tip radius accurately

generated the surface while the 200nm tip radius produced a rounded apex and rounded edges. Such rounding will

introduce errors in the calculated area function of the tip.

R=10nm R=50nm

R=100nm R=200nm

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Figure 6. Comparison between different sections in the Berkovich tip generated by different AFM tip radii. The section

generated by the 10nm AFM tip can’t be distinguished from the original scan. The 200nm tip results in a blunted apex that

is significantly different from the true surface.

Conclusion

Tip radius in AFM affects the accuracy of the scanned images. Sharp tips that have tip radii in the range of 10nm can

accurately generate the topography of the scanned surface. Increasing the tip radius results in a lateral dilation of small

features (comparable to the tip radius) and introduces artificial rounding to sharp regions (Pyramid apex and edges). Blunt

tips that have radii in the range of 20-30nm introduce minimal errors that does not significantly influence the topography.

Using blunted tips with tip radii in the range of 200nm will significantly dilate the scanned topography and result in about

8% error in the estimated depth of deformed surfaces by Berkovich indenters.