TRG SFM Project

Thomas Research Group

Scanning Force Microscopy

While STM was the first scanning probe technique (not counting stylus profilometry), the atomic force microscope (AFM), also called the scanning force microscope (SFM), came a close second and has surpassed STM in its applicability, at least within an industrial setting. The SFM uses a sharpened probe tip attached to the end of a very fine cantilever. A laser is reflected off the back of the cantilever and into a four quadrant position sensitive photodiode. When the cantilever is bent, either up or down, the change in the position of the reflected laser spot is detected by the photodiode. A differential measurement between two sides of the photodiode can be used to detect this change in cantilever deflection. When a sample is brought close to the probe tip, the repulsive forces between the atom at the end of the tip and those on the surface will induce a slight bend in the cantilever. A feedback loop can use this as a signal and control the vertical position of the sample as it is rastered under the probe tip so that the cantilever experiences a constant deflection. The feedback signal produces the data that is used by a computer to generate the topographic image of the surface. Here on the right is an SFM image of porous silicon. The image is about 30 x 30 µm in size. The hillocks on the surface are small agglomerations of crystalline silicon resulting from the unique etching kinetics of silicon in a HNO₃/HF solution.

The feature of the SFM which has popularized it amongst industrial users is that since no electrical current needs flow between the probe tip and sample, non-conductive samples are studied as easily as conductive ones. This stands in contrast to the STM which can only function on conducting and semiconducting substrates. A wide range of applications have been found for the SFM and it is becoming a common surface analysis tool. Below is a series of five images from porous silicon of an increasing magnification. The scan sizes are respectively 70 µm, 35 µm, 14 µm, 7µm, and 3µm. You can see how each subsequent image zooms into the center of the previous image.