2. Other Artifacts


Sample Drift


Sample drift means that your sample is moving as you try to scan it in the AFM.
This artifact occurs in any type of SPM, or indeed any microscopy, but is much more serious in high resolution techniques like SEM, TEM and AFM than optical microscopy. It's known as "drift", because, in general, we see the sample moving slowly, in one direction (or expanding or contracting). You can generally recognize this as a "distortion" of the image, which changes when you change the slow scan direction. beginning of the image, and then the image will start to appear more normal over time. The effect will be particularly pronounced if you move to a new place within your scan range.
See the example below.

Two AFM images showing effects of sample drift

In the example above, the two images of the same cluster of bacteria are not identical because the sample is moving while it is being scanned. It is typical of this effect that you will get different images when scanning in different directions.

How to avoid it.
The best thing to do, is to stop your sample from moving. The way to do this depends on why it's moving.

Is it thermal expansion?
-then keep the temperature constant, turn off external sources of heat(lights), and wait for it to stop moving.
It's usually worth trying to fix the sample down better. The other way to overcome (or at least to reduce) the distortion is to scan quickly.

3. Other Artifacts

3.3 Laser Interference


This effect is seen as broad "stripes" in your AFM images, running along the slow scan direction. They are caused by constructive interference between the reflection of the laser from the tip, and the reflection from the sample. See the example below.

Example of laser interference patterns

In the example above, the near-vertical stripe-like oscillations are cause by laser interference.
How to avoid it.

Sometimes this effect is really hard to avoid. It is more noticeable on flat samples, and large (>3 micron) images, and is worse with small, less reflective cantilevers, and it is particularly pronounced on more reflective samples. It's easy to recognise because usually the stripes will be spaced at double the wavelength of the laser used (i.e. usually around 1.3 microns). The best way to avoid this artifact is to re-align the laser, to ensure as little as possible spreads over the edge of the cantilever. Note: Many newer AFMs use a low-coherence laser, and so are much less prone to this problem.

Where to buy: AFM Probes - Distributors

 

This page lists distributors (as oposed to manufacturers) of AFM and other SPM probes. Currently the list is (very) incomplete.

3. Other Artifacts


3.3 Flying Tip


This artifact shows that the AFM is not properly tracking the surface.
In the example below the effect is obvious, but this is not always the case.
Sometimes it is clearer if you look at some of the other signals, like amplitude or phase or deflection images. In the images shown below, note how all the features have a "tail" to their left.

AFM image showing poor feedback

In the example above, the image of nanoparticles shows "tails" on each particle, to the left. The image was recorded scanning from right to left.Typically, the left to right image would show the tails on the right side of each feature. It is typical of this effect that you will get different images in the two directions.

How to avoid it.
If you see this, your feedback parameters are incorrect.
It is usually easy to fix. You generally should increase integral gain, then the proportional gain, you may need to change the setpoint too, or even scan more slowly. See the answer about adjusting AFM feedback parameters here, in in my book.

AFM Artifacts

 

1.1 Tip-sample convolution

 

This is an inherent feature of AFM and can never be fully removed. Any AFM image is a convolution of the shape of the probe, and the shape of the sample. This has the effect of making protruding features appear wide, and holes appear smaller (both narrower and often less deep, too). Broader (less sharp) probes will enhance the effect, as shown below.

 

 

 Illustration of convolution of AFM probe and sample giving rise to the image (red).

 

Probe-sample convolution tends to have the greatest effect on features of similar or smaller radii than the probe. it can be reduced by deconvolution techniques, discussed further in "Atomic Force Microscopy".

 

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