Like any other scientific tool, an atomic force microscope (AFM) has its limitations. When deciding whether or not analysis with an AFM is suitable, there are numerous advantages and disadvantages that must be considered thoroughly.
Advantages of AFM
AFM has a few notable advantages when compared to the scanning electron microscope (SEM) - unlike the SEM, which can provide a two-dimensional projection or a two-dimensional image of a particular sample, the AFM is able to produce a three-dimensional surface profile. Additionally, samples that are viewed by an AFM don’t require any special treatments (for example, carbon or metal coatings) which would irreversibly alter or damage the sample being analysed.
While an electron microscope requires an expensive vacuum environment for optimal operation, most AFM modes can operate perfectly in ambient air or even in a liquid environment - this makes it possible to conduct studies of biological macromolecules and even living organisms. In principle, AFM can provide higher resolution than SEM. It has been shown to give true atomic resolution in ultra-high vacuum (UHV) conditions and in liquid environments. High-resolution AFM is comparable in resolution terms to scanning tunnelling microscopy and transmission electron microscopy.
Disadvantages of AFM
When comparing AFM to SEM, a disadvantage is the single scan image size. In one pass, SEM can image an area on the order of square millimetres with a depth of field on the order of millimetres. Whereas the AFM is only able to image a maximum height on the order of 10-20 micrometres and a maximum scanning area of about 150×150 micrometres.
Scanning speed is also a considerable limitation of AFM. Traditionally, an AFM is not able to scan images as fast as an SEM, with it requiring several minutes for a typical scan, while an SEM is capable of scanning at near real-time, although this is relatively low quality. The relatively slow rate of scanning during AFM imaging often leads to thermal drift in the image, making the AFM less suited for measuring accurate distances between topographical features on the image.
AFM images can also be affected by hysteresis of the piezoelectric material and cross-talk between the x, y, z axes that may require additional software enhancement and filtering. Such filtering could actually "flatten" out real topographical features. However, newer AFMs utilise closed-loop scanners which practically eliminate these previously noted problems.
As with any other imaging technique, there is the possibility of image artefacts, which could be induced by a poor operating environment, unsuitable tip or even by the sample itself. These image artefacts are unavoidable however, their occurrence and effect on results can be reduced through various methods.
Due to the nature of AFM probes, they are not normally able to measure steep walls or overhangs. Specially made cantilevers and AFMs can be used in order to modulate the probe sideways as well as up and down (as with dynamic contact and non-contact modes) to measure sidewalls, at the cost of more expensive cantilevers, lower lateral resolution and additional artefacts.