Atomic Force Microscopy

The other method used for characterization of prepared sub-micron optical elements was the Atomic Force Microscopy (AFM). This technique is not very commonly used for in-house investigation and characterization, therefore a brief explanation will be presented here.

Figure 20: Schematic drawing of a dynamic or non-contact Atomic Force Microscopy mode.

It works on principle of force sensing, whereby it tests the surface properties and topography at sub-micron level and sometimes down to atomic dimensions. The application of this technique is very wide, from semiconductors research to cell biology. It can operate under ambient conditions (and in various gases and liquids) as well as under the ultra high vacuum conditions (UHV). The key component is the force sensor: a tiny tip, usually just a few microns long with a defined, sharp angle shape. It is usually made of silicon single crystal. The probe tip is attached to a relatively small, flexible cantilever beam (length is 100-500 $\mu$m), which serves as a spring and is deflected according to the intermolecular forces that exist between the probing tip and the sample. Deflection itself is monitored by a laser beam reflected to a quadrupole (position sensitive) sensor. Sample movement (scanning) is realized by precise piezoelectric actuators. Generally two basic operating modes can be distinguished. In the first, the probe tip is more or less in permanent contact with sample and the mode is called contact AFM or static mode AFM. The second mode, so-called non-contact or dynamic AFM or tapping AFM mode, where the cantilever-tip system is excited by an extra piezoelectric component is sometimes used. This mode is depicted on figure 20. While the tip is kept in close proximity to the sample surface, changes in the oscillation amplitude, phase or frequency due to tip-sample interaction are recorded. The effective contact time is decreased drastically and tip influence on the sample is strongly reduced. This is specially important for the "soft" or biologic samples. In our case, the non-contact AFM mode was entirely used. The AFM used was SIS (Surface Imaging Systems) ultra objective mounted on usual Zeiss Axiotex optical microscope to simplify preliminary sample positioning. The whole system was on active antivibration platform MOD-1. Results are presented as surface topography with false colors and surface profiles.