AFM scanning modes
The Atomic Force Microscopy relies on scanning a very sharp tip across a surface of interest at a very close proximity or even touching it. The tip “sits” on a thin flexible stripe, usually metal-coated silicon, called a cantilever. The back of the cantilever is illuminated with the laser light, and the reflection is observed on a position-sensitive detector (PSD). Due to a large lever of the light path even smallest bends of the cantilever will result in a measurable reflection travel on the detector. Scanning the sample laterally against the cantilever and monitoring the respective displacement of the reflected light on the PSD, one obtains a height map or topography of the sample. The principle of Atomic Force Microscopy is illustrated below.
There are a few different modes of scanning in Atomic Force Microscopy, based on the interaction between the probe (tip) and the sample, a contact mode, a tapping mode and a non-contact mode. These are illustrated in the image below.
In the contact mode the tip is dragged across the sample’ surface being in contact with the latter. The displacement of the laser beam on the position-sensitive photodetector is thus proportional to the force acting on the tip and can be recalculated into the local feature height. There is no feedback loop on the cantilever and this mode allows for the fastest scanning with a high precision in case of solid samples and relatively low height variation.
The disadvantages of the contact mode are
fast wear of the tip
damage to the surface
non-linear response in case of significant height variation, may lead to erroneous readings or breaking of the cantilever’
In the non-contact mode the tip oscillates freely above the surface, at a small amplitude close to the resonant frequency of the cantilever. The tip never comes in the direct contact with the surface, however, as it comes closer to the surface during the scanning, for example on a step or a bump, the long-range attraction forces (mostly VdW-forces) cause the amplitude and the phase of the oscillations to change. This can be sensed by a processor, which monitors the signal from the PSD. A feedback loop then adjusts the position of the probe so that the amplitude/phase of the oscillations were back to the preset values. The change of tip vertical position is proportional to the height change on the sample. The laser reflection though on average always stays in the center of the PSD. The advantages of non-contact mode are the minimum possible damage to the sample (possible to scan, for example, living cells) and an extended life of the cantilever.
The disadvantages are
- low signal/noise ratio
- slow scanning
- sensitive to mechanical noise, etc
- typically worse resolution
Finally, a dynamic contact mode, best known as tapping mode combines the advantages of the aforementioned imaging modes. Here the tip also oscillates close to its resonant frequency, however, typically with larger amplitudes. This allows the tip at the end of each oscillation cycle to touch or “tap” – hence the name – the surface. The feedback loop adjusts the probe position above the sample to keep the oscillation amplitude or phase constant. The advantages of the tapping mode are: high resolution and comparatively low damage to the sample, long life of the probe and the biggest dynamic range of the probe’ response. The disadvantages are slower scanning rates than in contact mode and tapping of the sample, that might be a problem for soft matter samples.