Probe Artifacts
Images measured with an atomic force microscope are always a convolution of the probe geometry and the shape of the features being imaged. If the probe is much smaller than the features of the images being measured, then the probe-generated artifacts will be minimal and the dimensional measurements derived from the images will be accurate. Avoiding artifacts from probes is achieved by using the optimal probe for the application. For example, if the features that are being imaged have feature sizes of interest in the 100 nanometer range, a probe as large as 10 nanometers in diameter will be adequate for getting good images with no artifacts. In some cases, even if the probe is not as sharp as the object being imaged, it is still possible to get accurate information from the image. Common artifacts are: Features on a surface appear too large  Figure 1: Motion of an AFM probe as it goes over a sphere that is attached to a surface. In such a measurement the side of the probe will cause a broadening of features in the image. Often the size of features on the surface such as nanotubes or nanospheres look larger than expected. However, the height of the feature when measured by a line profile is correct.  Figure 2A-B: 400 × 400 nm AFM image of an 8 nm diameter sphere (A) The line profile of the image shows a diameter of 92 nm and a height of 8 nm. (B). The broadening in the image is caused by the shape of the probe used for measuring this AFM image. Features in an image appear too small  Figure 3: The motion of an AFM probe as it moves over a hole in a surface. Because of the width of the probe, it does not reach the bottom of the hole. If the probe needs to go into a feature that is below the surface, the size of the feature can appear too small. The line profile in these cases is established by the geometry of the probe and not the geometry of the sample. However, it is still possible to measure the opening of the hole from this type of image. Also, the pitch of repeating patterns can be accurately measured with probes that don’t reach the bottom of the features being imaged.   Figure 4A-B: Scanning electron microscope image of a test pattern of squares (NT-MDT TXO1) (A) The sides of the squares are all equal. (B) AFM image of the test pattern. Because the probe is not sharp, the test pattern squares appear much smaller than they should. The features in the AFM image appear as rectangles and not as squares. Strangely shaped objects
If the probe gets broken or chipped before an image is measured, strangely shaped objects may be observed that are difficult to explain. For example, when scanning a semiconductor test pattern, it can appear as though the tip is at a large angle to the surface as described in section 2.1. However, the probe to sample angle would have to be extreme to explain the image artifact.  Figure 5: This “chipped” AFM probe follows the geometry of the sample surface and creates an image with a substantial artifact.   Figure 6A-B: (A) This AFM image of a test pattern appears to have dark right edges. (B) The artifact can be easily seen in the line profile. Although this artifact could be explained by a large angle between the probe and surface, the probe surface angle cannot be this large.
Scan size: 91µm X 91µm. Repeating Strange Patterns in an Image
If the features on a surface are much smaller than the probe, then it is possible to see large numbers of repeating patterns in an image. The patterns will often appear as triangles, especially if silicon probes are used for imaging. Example: Images of colloidal gold particles reflect the shape of the tip rather than their own geometry. Compare the SEM images of tips and related AFM images of spheres in the figures to the right.
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