A method of operating an afm

A method of operating an atomic force microscope (AFM), comprising a probe displaceable relative to a sample along scan lines in an imaging x-y plane and along a z axis perpendicular to the x-y plane, the probe being moved forth and back during respective trace and retrace times of a scan line, and caused to oscillate by an excitation signal, the method comprising:

• a) during trace time, placing the probe in an operating state in which the probe oscillates along the z axis and interacts with the sample,
• b) generating a z feedback signal to displace the probe relative to the sample along the z axis to keep an amplitude of oscillation of the probe constant at a setpoint value Aset, the z feedback signal being generated by a first feedback loop at least on the basis of the setpoint value Aset and having as input a measured amplitude value of oscillation of the probe,
• c) during retrace time, placing the probe in a drift compensation state by changing the setpoint value Aset to a different value Aset+Amock so that the z feedback signal being generated by the first feedback loop causes the probe to move away from the sample and oscillate free of interaction with the sample,
• d) detecting an amplitude of free oscillation of the probe and adjusting with a second feedback loop the excitation signal of the probe to maintain the amplitude of free oscillation of the probe close to a set value Afree,
• e) returning the probe in interaction with the sample for a next scan line by restoring the setpoint value Aset for the generation of the z feedback signal by the first feedback loop while applying the adjusted excitation signal of previous step to the probe.