Atomic Force Microscopy (AFM)

Principle of Operation

The Atomic Force Microscope (AFM) allows to map the morphology of surfaces. The principle of operation is illustrated in the figure (right). A piezo-electric scanner (x-y) moves the sample to be investigated below a fine, pyramid-shaped tip which is sensitive to variations of the thickness of the sample. A third scanner (z) controls the distance between tip and surface. The tip is mounted to a flexible cantilever which reflects infrared light emitted by a LED to a four-area photodetector. If the tip encounters a step at the sample surface during the sample movement the cantilever is bent. This results in a characteristic change of the position of the spot of the reflected light at the photodetector and, therefore, allows for the exact determination of the height of the step. Scanning the sample line-by-line results in a topographic map of the surface. Simultaneously, the lateral forces to the tip can be also obtained from the bending of the cantilever.

In-situ UHV AFM system

An in-situ ultra high vacuum (UHV) atomic force microscopy (AFM) system from Omicron allows to study the interface properties of thin films immediately after the growth process without breaking the vacuum. The lateral scanning range is 5×5 µm² (x-y). Some results are shown below.

SrTiO3 substrate After etching in buffered HF and annealing in O2 atmosphere at 1000°C, the surface becomes TiO2-terminated. AFM shows that the step height is about 0.4 nm corresponding to the height of one unit cell (0.3905 nm).	Sr2CrWO6 thin film on SrTiO3 Using pulsed laser deposition a thin film of Sr2CrWO6 with a thickness of 42 nm was deposited on etched/annealed SrTiO3. Still, the steps of the substrate surface can be detected using AFM proving the high surface quality of the thin film.