Atomic Force Microscopy Breaking Through the Vertical Range-Bandwidth Tradeoff

This article proposes a mechatronic system design for atomic force microscopes (AFMs) in order to realize a large range and high control bandwidth of the vertical probe motion by a flexure-guided voice coil actuator. The first mechanical resonant frequency of the actuator is decreased to 67 Hz for a large motion, whereas the second resonant frequency is maximized to 1.15 kHz. The frequency band between the resonances enables two-degree-of-freedom control with a position sensor to realize a control bandwidth of 881 Hz, which is 13 times higher than the first resonance that is the limitation of conventional AFMs. The closed-loop actuator achieves a positioning resolution of 1.7 nm and a nonlinearity of 0.02% for a motion range of 700 mu m. Consequently, the actuator realizes a range-bandwidth product of 617 kHz.mu m, breaking through a theoretical limit of piezoelectric actuators (567 kHz.mu m). The high performance of the actuator is further confirmed by 400 mu m topography measurement in the constant force mode with a bandwidth of 197 Hz. The achieved resolution of the topography measurement is 2.9 nm, and it is demonstrated by successfully imaging nanostructures on a CD-ROM disk.