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SPM Principles

Semicontact Error mode

 

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Widely used Semicontact mode has some disadvantage concerned with the usage of the feedback circuit. The scanning speed in Semicontact mode is restricted by the feedback circuit reaction time. This disadvantage can be overcame by the fact that under scanning new value of cantilever oscillation amplitude (and error signal) usually is achieved faster than preset value of the cantilever oscillation amplitude can be reached by the feedback system. Time of the reaching new value of the oscillation amplitude is determined by the oscillation period and Q-quality of the cantilever.
The feedback error signal, emerging when scanning in the Semicontact mode, contains some additional information about the topography. It can be utilized for achieving a more precise recovery of the relief.

Additionally, similarly to the Contact Error mode, which can be considered as intermediate between the Constant Force mode and Constant Height mode, the feedback gain factor (i.e. the feedback processing speed) can be 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. It must be noted that height of the minor irregularities must be less than amplitude of cantilever oscillation.

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 Semicontact mode. Upon completion of the unit preparation for operation in the Semicontact mode and after acquiring a surface topographic image, the operator may proceed with operation in the Semicontact Error mode.

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