If measuring a sample with two or more conducting layers, the result will be some type of meaningless average of all connected conductors.Īs mentioned above, since the four point probe technique does not directly measure the thickness of thin films, if one knows two of the following three characteristics for a given sample, a four point probe can be used to determine the third characteristic: 1) the volume resistivity in ohms-cm, 2) the sheet resistance in ohms-per-square, 3) the sample thickness. The four point probe technique is used to measure one single layer or one single homogeneous material. To calculate Ohms-cm using a four point probe, one needs to know the thickness of the wafer (if it is a homogeneous wafer) or the thickness of the top layer that’s being measured, to be able to calculate Ohms-cm. The term “Ohms-per-square” is used when measuring sheet resistance, i.e., the resistance value of a thin layer of a semi-conductive material. Ohms-cm is used for measuring the conductivity of a three dimensional material such as a silicon ingot or a thick layer of a material. The term Ohms-cm (Ohms centimeter) refers to the measurement of the “bulk” or “volume” resistivity of a semi-conductive material. How do I convert from ohms per square to ohms-centimeter? Sheet resistance is the resistance of a thin sheet of material which when multiplied by the thickness (in cm) gives the value of resistivity. It is sometimes called Specific Resistance. RESISTIVITY is the inherent property of the material which gives it electrical resistance. Is sheet resistance an “inherent” property of a material, or is it a function of thickness? Ohms-Centimeter (Ohms-cm): The unit of measurement when measuring the bulk or volume resistivity of thick or homogeneous materials such as bare silicon wafers or silicon ingots, using the four point probe technique. It is equal to the resistance between two electrodes on opposite sides of a theoretical square. Ohms-per-square: The unit of measurement when measuring the resistance of a thin film of a material using the four point probe technique.
Therefore one can calculate the resistivity if the thickness of a film is known, or one may calculate the thickness if the resistivity is known. The thickness of the film (in cm) and its resistivity (in ohm cm) are related to Rs by: If the spacing between the probe points is constant, and the conducting film thickness is less than 40% of the spacing, and the edges of the film are more than 4 times the spacing distance from the measurement point, the average resistance of the film or the sheet resistance is given by:
Four point probe based instruments use a long established technique to measure the average resistance of a thin layer or sheet by passing current through the outside two points of the probe and measuring the voltage across the inside two points.
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