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Author: Ali Kosari Mehr
Four-terminal sensing or 4-point probe is a means of measuring sheet resistance of thin films and coatings; sheet resistance is a measure of the electrical resistance of thin films having a uniform thickness. Actually, the unit of this quantity is the same as electrical resistance; however, owing to its different concept, the unit of Ω/□ is used with the square (i.e., □) meaning a thin film possesses a given sheet resistance regardless of the size of the area on the surface of the thin film. It is noteworthy that the resistivity of a thin film can be calculated by multiplying the film’s thickness and sheet resistance. Thus, given resistivity and sheet resistance, one can calculate the thickness of a thin film.
In 4-point probes, four in-line probe needles with small spacing (e.g., 0.5-2 mm) come into contact with the surface of a thin film in order to measure its sheet resistance. The currents used in this process can be in the order of nanoamperes or picoamperes. Usually, the outer needles function in the mode of the current source while the inner needles examine the voltage drop in the current path on the surface. By this means, the impact of extraneous determinants caused by contact resistance can be considerably reduced.
One of the main problems of utilizing 4-point probes is that the needles can penetrate the surface of thin films to a depth of about 50 ångström. This can culminate in errors pertaining to the sheet resistance of substrates or lower layers in multilayers. Hence, when measuring the sheet resistance of multilayers, one should note that the surface to be measured should possess the lowest resistance among the other layers.
