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SPM Principles

Contact Error mode

 

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Widely used Constant Force mode has some disadvantage concerned with the usage of the feedback circuit. The scanning speed in Constant Force mode is restricted by the feedback circuit reaction time. This disadvantage can be overcome by the fact that under scanning new value of cantilever deflection (and error signal) is achieved faster than preset value of the cantilever deflection can be reached by the feedback system.

The feedback error signal, emerging when scanning in the Constant Force mode  contains some additional information about the topography. It can be utilized for achieving a more precise recovery of the relief.

Additionally, this mode can be considered as intermediate between the mode of Constant Force mode and Constant Height mode, if the feedback gain factor (i.e. the feedback processing speed) is adjusted for the system to be able to trace subtle changes of the relief and to be too slow to trace the steep changes. Then, when the probe travels over minor irregularities, scanning will be carried out with an almost constant piezoscanner length. As a result, the slow changes of the relief will hardly show up on the images, and the steep changes will appear in high contrast. This may be helpful in finding minor irregularities on large areas against major sloping relief features.

The necessary condition for operation in the feedback error signal registration mode is for the unit to be prepared beforehand for the operation in the Constant Force mode . Upon completion of the unit preparation for operation in the Constant Force mode and after acquiring a surface topographic image, the operator may proceed with operation in the Contact Error mode.

References
  1. Magonov, Sergei N. Surface Analysis with STM and AFM. Experimental and Theotetical Aspects of Image Analysis.VCH 1996.
   
 
 
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