Controller Speeds Atomic Force Microscopy

Anne L. Fischer

The speed of operation of the atomic force microscope (AFM) is being pushed in biological and other applications where milliseconds count. Obtaining accurate sample profiles is still a limiting factor in the fast operation of AFMs. Now researchers from the University of Illinois at Urbana-Champaign, Iowa State University in Ames and IBM Zurich Research Laboratory in Rüschlikon, Switzerland, have proposed an approach that provides accurate sample topography reconstruction to boost scanning speeds.

AFMs work by scanning the sample surface using a silicon cantilever with an ultrasharp tip. Variations in the sample topography deflect the cantilever, and these deflections are measured using an optical detector. However, the sometimes large variations in force are detrimental both to the sample and to the tip.

For better tip-sample force regulation, current commercial AFMs try to maintain a constant cantilever deflection by moving the sample in the vertical direction using a feedback controller and piezoelectric actuation. In this mode of operation, the control signal from the feedback controller reconstructs the sample topography.

As an AFM operates at faster scanning speeds, the dynamics of the piezo actuator start to play a significant role, and the control signal ceases to be a faithful measure of the sample topography. The researchers tackled this problem with a model-based control design using tools from modern control theory.

The controller exhibits excellent tip-sample force regulation. Moreover, experimental tests showed that its internal signal offers a nearly perfect estimate of the sample topography and promises to enable imaging at high speeds, including in frequency regions where the piezo dynamics are nonnegligible.

Applied Physics Letters, Aug. 1, 2005, 053112.