ac Contact techniques

Fig. 1 Mechanical equivalent including lateral and vertical tip-sample interaction forces.

Usage of Scanning Force Microscopy with oscillating cantilever was firstly anticipated by Binnig [1]. This oscillations can take place in Non-Contact, Intermittent-Contact (Semicontact) and Contact (ac Contact) modes. 
The peculiarities of ac Contact modes consist in that simultaneously with cantilever nearest area of the sample surface participates in oscillation. At that sample surface oscillations are not only normal (out-of-plane) but lateral also and cantilever can oscillate in higher harmonics. 
Forcing of the cantilever oscillations can be provided by the scanner, piezodriver, special transducer under sample holder. Cantilever oscillations can be excited also by the tip-sample electric forces. Accordingly frequencies of oscillations can vary between tens kHz to several MHz.
Measurements in ac Contact modes are carried out simultaneously with topography Contact mode measurements and allow to determine contact stiffness, Young's module and other sample parameters. 

References

1. US Pat. 4724318.