Stylus Profilometry
Stylus profilometry is the oldest scanning probe technique. It is akin to rotating an old-fashion record player needle by 45º so that the needle responds to the shape of the bottom of the record groove instead of the sides of the grooves. Because the information obtained by a stylus profilometer is two dimensional-- surface height vs. distance along a horizontal line—the output can be conveniently recorded as a graph with something as simple as a paper chart recorder. Note that this is unlike modern scanning probe techniques, such as Atomic Force Microscopy, which only became practicable once inexpensive computers and graphical display devices were available.
Refer to Figure 1. The stylus shaft of a profilometer is typically about 20 mm in length. The stylus tip in contact with the surface is typically a 60º diamond cone with an end radius of 25 µm. Various sensors may be used to detect the vertical movement of the stylus, including linear variable differential transformers, three plate capacitive sensors, and optical levers. The diagram below shows an optical lever configuration similar to the one used in atomic force microscopy. In operation, as the translation stage drags the sample under the stylus tip the stylus arm rotates about the pivot so that the tip stays in contact with the surface, and as the stylus rotates the laser light reflecting off of a mirror above the pivot tracks this motion across a position sensitive photodetector. The amplified photodetector signal is digitized by a computer for graphical representation of the surface profile and data processing.
The magnetic force solenoid on the opposite side of the pivot has a cylindrical recess to allow free space for a small permanent magnet attached to the end of the stylus shaft. The magnetic field inside the solenoid applies a counter torque to the stylus rotation via the small magnet . The current through the solenoid is varied to raise and lower the stylus tip, and to adjust the contact force between the tip and the sample surface while the sample is scanned.
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