Measurement
A four point probe is used to avoid contact resistance, which can often be the same magnitude as the sheet resistance. Typically a constant current is applied to two probes and the potential on the other two probes is measured with a high impedance voltmeter. A geometry factor needs to be applied according to the shape of the four point array. Two common arrays are square and in-line. For more details see Van der Pauw method.
Measurement may also be made by applying high conductivity buss bars to opposite edges of a square (or rectangular) sample. Resistance across a square area will equal Ohms/sqr. For a rectangle an appropriate geometric factor is added. Buss bars must make ohmic contact.
Inductive measurement is used as well. This method measures the shielding effect created by eddy currents. In one version of this technique a conductive sheet under test is placed between two coils.
A very crude two point probe method is to measure resistance with the probes close together and the resistance with the probes far apart. The difference between these two resistances will be the order of magnitude of the sheet resistance.
Read more about this topic: Sheet Resistance
Famous quotes containing the word measurement:
“That’s the great danger of sectarian opinions, they always accept the formulas of past events as useful for the measurement of future events and they never are, if you have high standards of accuracy.”
—John Dos Passos (1896–1970)