Design of an Inertially Counterbalanced Z-Nanopositioner for High-Speed Atomic Force Microscopy

In many conventional atomic force microscopes (AFMs), one of the key hurdles to high-speed scanning in constant-force contact mode is the low-feedback control bandwidth of the Z-axis loop. This paper presents the design of a fast Z-nanopositioner to overcome this limitation. The Z-nanopositioner has its first resonant mode at 60 kHz and a travel range of 5 mu m. It consists of a piezoelectric stack actuator and a diaphragm flexure. The flexure serves as a linear spring to preload the actuator and to prevent it from getting damaged during high-speed operations. The Z-nanopositioner is mounted to an XY-nanopositioner. To avoid exciting the resonance of the XY-nanopositioner, an inertial counterbalance configuration was incorporated in the design of the Z-nanopositioner. With this configuration, the resonances of the XY-nanopositioner were not triggered. A closed-loop vertical control bandwidth of 6.5 kHz is achieved. High-speed constant-force contact-mode images were recorded at a resolution of 200 x 200 pixels at 10, 100, and 200 Hz line rates without noticeable image artifacts due to insufficient control bandwidth and vibrations. Images were also recorded at 312- and 400-Hz line rates. These images do not show significant artifacts. These line rates are much higher than the closed-loop bandwidth of a conventional AFM in which this nanopositioner was tested.