Abstract
This study aims at improving the precision of measurements with a frequency modulation (FM) mode atomic force microscope (AFM). While an observation with the FM-AFM is carried out, a vibrating microcantilever probe scans over the sample surface. The measurement accuracy depends on how exactly a change in the eigen-frequency of the microcantilever is obtained. The amplitude suppression enables not only to make the oscillation frequency agree exactly with the eigen-frequency, but also to realize a non-contact observation for easily-deformable or easily-damageable specimens. However, it is difficult to cope with both keeping the microcantilever vibrating firmly and making the amplitude of its vibration as small as possible. To overcome this difficulty, a nonlinear feedback is applied to control the amplitude of the microcantilever probe used in the AFM. As a result, it is achieved that the AFM microcantilever can oscillate with a considerably reduced amplitude in the steady state. The key to the success of this nonlinear feedback control method is that the cantilever vibration can be categorized into the van der Pol-type self-excited oscillation. A prototype AFM was built to demonstrate experimentally the advantage of this amplitude control method. To date, the vibration amplitude less than one nanometer has been achieved in the steady state.
