The Full Form of SFM is‍ Scanning Force Microscopy.

The scanning force microscopy (SFM), also known as atomic force microscopy (AFM), belongs to the branch of scanning probe microscopy (SPM), which comprises all microscopy techniques that form pictures of surfaces not by optical or electron-optical imaging, but due to interaction of a physical probe with the sample.

The precursor to the SFM, the scanning tunneling microscope (STM), was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s [1, 2] und earned them the Nobel Prize for Physics in 1986. The first SFM was invented by Binnig, Quate and Gerber in 1986 [3]. Its alternative name, AFM, refers to the interactions between probe and sample on the atomic level. The attractive van-der-Waals forces and the Pauli repulsion due to overlapping electron orbitals can be described by the Lennard-Jones-potential. 

The SFM scans surfaces line by line and assembles topographical images. A laser beams onto a cantilever and reflects from it onto a set of position sensitive photodiodes. While the sharp tip of the cantilever moves over the sample, the cantilever itself bends in consistency with the surface and the photodiodes register the resulting position changes of the laser reflection. Two piezos (a piezoelectric element expands or contracts in direct proportion to an applied electric field) generate the scanning movement of cantilever, laser and photodiodes in x- and y-direction. The signal from the photodiodes goes to a z-piezo, that moves the cantilever up or down to compensate the cantilever deflection. The information of the deflection is used to assemble an image.

Since an SFM can image and probe samples in both dry and liquid environments, it is possible to work with living cells under physiological conditions [4, 5]. For our research, we are using the NanoWizard BioAFM (JPK Instruments AG, Berlin), where the x-y-z-scanner is attached to the cantilever mount (see figure). The NanoWizard has the great advantage of using a variety of light microscopy techniques (e.g. phase contrast) together with the SFM-technique, which is especially helpful for biological research.

An SFM image scan visualizes the topography of the surface and can be used to create a three dimensional representation.
There are different imaging modes for SFM, primarily distinguished into static mode (contact mode) and dynamic modes (non-contact and intermittent contact mode) with oscillating probe. The two most commonly used modes for imaging biological samples are the contact and the intermittent contact mode (tapping mode). While in contact mode the tip scratches the surface of the sample, the cantilever in intermittent contact mode vibrates and thus taps point after point of the sample.