Tapping-mode feedback

Figure 8. Panel (A) displays the approach curve of the NSOM tip using tapping-mode feedback. Panel (B) displays the approach curve collect using the optical feedback mode. Comparison of the two simultaneously collected approach curves, indicates that the optical feedback approach curve begins to oscillate within micrometers of the surface, while tapping mode feedback remains unaltered until the tip is very elose to the sample surface Reproduced with permission from (Shiku et al. 1999). Copyright 1999 American Chemical Society.

The surface deformation could be reduced even further with intermittent contact SFM. Tapping mode imaging in liquids has been described by several groups [185-191]. The main focus has been put on biological systems such as DNA, cells, chromosomes and proteins. However, it turned out to be rather tricky to perform the measurements. The resonant frequencies were usually 2-5 times lower than in air and the resonant peak gets strongly dampened and broadened [ 192,193]. Because of acoustic excitation of the cantilever holder and the body of fluid, the spectrum can be superimposed by other resonance s which not sensitive to the surface approach and cannot be used for the feedback control [185]. 

Dynamic plowing lithography (DPL), i.e. the lithography technique in tapping mode in which the force between tip and sample is increased by suddenly increasing the amplitude of the cantilever oscillations, has been developed by Klehn and coworkers [257-260]. The topography of the sample is acquired with a normal, small amplitude of the cantilever oscillations. When the amplitude is increased, the feedback makes the sample approach to the tip, in order to keep constant the oscillation amplitude, and the tip indents the sample. 

The prime differences among the different AFM modes, such as CM (discussed above) and intermittent CM, as elucidated in the following section, are the feedback parameters and the choice of the cantilever. For intermittent contact (tapping) mode AFM, a stiff cantilever (k typically 10—50 N/m) with a resonance frequency of 100—400 kHz is chosen. The cantilever, which is inserted in an identical manner as for CM into the cantilever holder, is excited to vibrate by an integrated piezo actuator. Instead of deflection (contact force), the amplitude of the forced oscillating lever is detected, analyzed, and utilized in the feedback loop (Fig. 2.20). 

AFM generally operates in three different modes contact, noncontact, and tapping mode. In the contact (also called static) mode, the tip is in contact with the sample surface. The force between the tip and the surface is kept constant during scanning by maintaining a constant deflection, which is used as the feedback signal. 

As an example, Shiku et. al. reported an adaptation of the interferometric optical feedback method for use with cantilevered NSOM probes oscillated normal to the sanqile surface.(Shiku et al. 1999 Talley, Cooksey et al. 1996) In contrast to straight fiber optic NSOM probes used in shear-force feedback, cantilevered NSOM probes incorporate a near 90 bend near the aperture. These tips can then be operated in a tapping-mode arrangement for tip-sample gap regulation, much tapping-mode AFM.(Talley et al. 1996)... 

The high lateral forces and concomitant drawbacks of contact mode are circumvented in intermittent contact (also called tapping) mode AFM (Fig. 6.5). This mode utilizes an oscillating tip-cantilever assembly and relies on a feedback from the amplitude (constant amplitude imaging). A typical cantilever for operation in air is much stiffer than a contact mode cantilever (10-100 N/m) and is excited to resonance or near resonance. The forced oscillator is damped upon interaction of the tip with the sample surface (Fig. 6.5b and c). If the cantilever spring constant or the amplitude is too low, the energy in the forced oscillator is not sufficient to overcome the adhesive interactions and the tip remains trapped in contact and is consequently dragged across the surface. 

The tip was first brought into very close proximity to the surface by shear force feedback the approach was stopped when a 3%-5% damping of the fork oscillation was recorded. The tip was then withdrawn by 24nm, before the ECL light intensity transmitted to the substrate was monitored. This tapping mode-like procedure was repeated for every imaging position of the substrate. The... 

---