Control of tip position using co-located magnetic actuation
for high-speed AFM
Chia-Hsiang Menq, Younkoo Jeong and G. R. Jayanth
Department of Mechanical Engineering
The Ohio State University
201 West 19th Avenue, Columbus, OH, 43210
E-mail: menq.1@osu.edu
AFM imaging utilizes a sensitive feedback mechanism to achieve a specific control objective by adjusting the tip-to-sample distance through z-motion control. For example, thecontrol objective of contact mode AFM is to maintain a constant deflection of the cantileverwhile the oscillation amplitude or frequency is regulated in usual dynamic mode AFM. Recently, direct control of tip-sample interaction force was also proposed. In AFM imaging, as asample is scanned at higher speed, topographic details present themselves to the z-control
loop as disturbances at higher frequencies. Therefore, the bandwidth of the z-control loop is
one of the key factors that limit imaging rate. The z-control loop in dynamic mode AFM involves various dynamic processes, including z-scanner dynamics, cantilever dynamics, andtapping dynamics. Over the past 15 years, smaller AFM cantilevers as well as smaller piezoelectric actuators, in conjunction with active Q-control, have been proposed, and significant
improvements for high-speed AFM have been made.
In this presentation, a co-located scheme for z-motion control is proposed, in which amagnetic actuator is introduced to work together with the regular z-scanner in a dual-controlloop scheme aiming to directly control the tip position and hence the tip-to-sample distance.
The magnetic actuator overcomes the bandwidth limitations of the z-scanner and does not introduce undesirable under-damped dynamics. Moreover, since the magnetic force is applied
directly at the location where the motion being controlled, it is much easier and reliable to use
a model cancellation method to compensate the dynamics of the cantilever and to elevate thespeed of the tip-position control. This additional magnetic actuator serves to make the entire
cantilever bandwidth available for tracking topographic variations at specified tip-sample interaction force. In high speed imaging, it will pick up high spatial-frequency surface topography and regulate the tip-sample interaction force while the regular z-scanner provides the necessary motion range.
A fast programmable electronics board (Field Programmable Gate Array) was employed
to implement the proposed dual-control-loop scheme, in which model cancellation algorithms
were realized to enhance the bandwidth of the magnetic coil and to replace the lightly damped
dynamics of the cantilever with an over-damped system. It allows the cantilever to position
the tip very rapidly without introducing unwanted transient dynamics. Experimental results
will be presented to illustrate the effectiveness of the propose method. For tip-position control, it is shown that while an ordinary cantilever is excited by the magnetic actuator to oscillate around its resonance frequency (34.8 kHz), the same actuator is actively controlled tomove up the mean position of the tip by 20nm within one cycle. Other preliminary results andpotential issues in relation to high-speed AFM imaging and direct tip-sample interaction force
control will also be discussed.