AFMs can measure nanoscale roughness from sub micron to less than an angstrom. However, there are two main factors which need to be considered when using these instruments for this type of measurement:
1) Vertical Resolution – How the system scans in Z
2) Spatial Resolution – How the system scans in XY
Vertical Resolution. How well the system measures vertically is a function of the noise floor of the system. If the sample feature size is 1nm, the system needs to have a vertical noise floor less than 1nm in order to measure the feature adequately. The largest vertical feature that you can measure will be equal to the noise floor or the Z resolution limit. This noise floor measures all of the mechanical and electrical movement or noise in the system. It is easiest measured by setting the scan size to 0, using the parameters that you will use to measure the sample. When the tip is not moving across the surface any measured variation over time is due to noise int he system and will mask any surface features once the tip starts to move. For measuring the lowest noise floor achievable, many companies use a standard set of parameters so that they compare values.
Spatial Resolution. Since the movement in XY is almost always larger than that in Z. The lateral resolution of the an AFM isn’t limited by system noise, but by the radius of curvature (ROC) of the tip. The sharper the tip, the better the resolution. Super sharp commercial cantilevers have a ROC about 2-3nm. Standard AFM probes have a nominal ROC of 6nm. Any probe used for advanced modes usually has a coating not the to make them sensitive to a particular signal of interest. These probes can have have ROCs 20nm or more.
In addition to the ROC, it is important to keep the tip sharp without blunting the tip to maintain a) the highest resolution possible and b) to maintain a stable and consistent image. If the tip wears during the coarse of imaging. The first image may look dramatically different from the last image on the same sample surface. In cases where the feature size is much smaller than the tip size, a condition called tip imaging (tip self imaging, reverse imaging) will occur where the features surface features will be representative of the end of the tip rather than the surface itself.
What to do? Understand the noise floor of your system. This will determine what are the limits of the machine. Know what the specifications of your probe are and what condition it is in. This will provide you with confidence that the images you are seeing are representative of the sample surface.
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