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Controller Speeds Atomic Force Microscopy

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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.
Cognex Corp. - Smart Sensor 3-24 GIF MR

Published: October 2005
Glossary
atomic force microscope
An atomic force microscope (AFM) is a high-resolution imaging and measurement instrument used in nanotechnology, materials science, and biology. It is a type of scanning probe microscope that operates by scanning a sharp tip (usually a few nanometers in diameter) over the surface of a sample at a very close distance. The tip interacts with the sample's surface forces, providing detailed information about the sample's topography and properties at the nanoscale. Key features and principles of...
atomic force microscopeBasic SciencebiologicalFeaturesMicroscopySensors & Detectorssilicon cantilever

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