Dual Actuation of Fast Scanning Axis for High-speed Atomic Force Microscopy

Abstract

In order to overcome the limiting trade-off between the imaging speed and scanning range of an atomic force microscope (AFM), this paper combines two piezoelectric actuators as a dual stage actuator (DSA) for a lateral motion (X axis) of the AFM probe with respect to the sample. As the first actuator, a piezoelectric tube actuator of the commercial AFM is utilized. Although the actuator realizes a relatively large actuation range, it has the first mechanical resonance at a low frequency of 2.5 kHz. In the case of high-speed imaging, this resonance impairs the imaging speed and quality. In order to overcome this, a piezoelectric shear actuator with the first resonance at 19 kHz is selected as the second actuator, combined in the commercial AFM. To generate the X-axis motion by synchronizing those two actuators, this paper proposes a feedforward control design for DSAs in the frequency domain, which takes into account the actuator dynamics. In the proposed approach, triangular raster scan is composed as a Fourier series by individually adjusting the complex Fourier coefficients for each actuator. The effectiveness of the DSA and its control is validated by experimental AFM imaging at a scan rate of 200 Hz, where the lowest frequency component is applied to the tube actuator and the other higher components of the scanning signal to the high-speed shear actuator.