Force Modulation Microscopy (FMM) is an extension of AFM imaging that operates in contact Atomic Force Microscopy mode and is used to detect variations in the mechanical properties of the sample surface such as surface elasticity, adhesion, and friction.

In FMM mode, the AFM tip is scanned in contact with the sample surface, and the Z feedback loop maintains a constant cantilever deflection as in constant-force mode AFM. In addition, a periodic signal known as the ‘driving signal’ is applied to the bimorph piezo and vibrates either the tip or the sample. The resulting tip motion is converted to an electrical signal. This electrical signal is separated into AC and DC components for analysis.

The DC signal represents tip deflection as in contact AFM. The Z feedback loop uses this signal to maintain a constant force between the tip and the sample to generate a topographic image.

The AC signal contains the tip response due to oscillation. The amplitude of the AC signal (called ‘FMM Amplitude’) is sensitive to the elastic properties of the sample surface. A hard surface will deflect the oscillation, resulting in a large amplitude response.

On the other hand, a soft surface will absorb the oscillation, resulting in a small amplitude response. The FMM image, which is a measure of the sample’s elastic properties, is generated from variations in the FMM amplitude.