Control of tip-to-sample distance in atomic force microscopy: A dual-actuator tip-motion control scheme

Abstract

The control of tip-to-sample distance in atomic force microscopy (AFM) is achieved through controlling the vertical tip position of the AFM cantilever. In the vertical tip-position control, the required z motion is commanded by laser reading of the vertical tip position in real time and might contain high frequency components depending on the lateral scanning rate and topographical variations of the sample. This paper presents a dual-actuator tip-motion control scheme that enables the AFM tip to track abrupt topographical variations. In the dual-actuator scheme, an additional magnetic mode actuator is employed to achieve high bandwidth tip-motion control while the regular z scanner provides the necessary motion range. This added actuator serves to make the entire cantilever bandwidth available for tip positioning, and thus controls the tip-to-sample distance. A fast programmable electronics board was employed to realize the proposed dual-actuator control scheme, in which model cancellation algorithms were implemented to enlarge the bandwidth of the magnetic actuation and to compensate the lightly damped dynamics of the cantilever. Experiments were conducted to illustrate the capabilities of the proposed dual-actuator tip-motion control in terms of response speed and travel range. It was shown that while the bandwidth of the regular z scanner was merely a small fraction of the cantilever's bandwidth, the dual-actuator control scheme led to a tip-motion control system, the bandwidth of which was comparable to that of the cantilever, where the dynamics overdamped, and the motion range comparable to that of the z scanner.

Studying biological membranes with extended range high-speed atomic force microscopy

“…These scanners are fast, but are typically limited in range, as they trade scan range for speed by decreasing the size of the actuators. More recently, feedback using two different Z-actuators has been shown to offer a promising increase in speed 24 25 . The tip and the sample are moved with different actuators, where one actuator is lower range but faster.…”

Photothermal cantilever actuation for fast single-molecule force spectroscopy

“…Further improvement of the feedback can be reached by electronically damping the quality factor of the first eigenmode ͑Q control͒ and by different controller types such as field programmable gate arrays. 22 The measurement setup that we presented consists of two cantilevers, one serving as a force sensor, and the other as a displacement actuator. The separation of sensing and actuating gives a more direct insight into the dynamic variables but in principle it is possible to use the same cantilever for both.…”

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