Four point probe
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I = force current
|Four point probe|
Four Point Probe Diagram
|Equipment||Miller FPP-5000 4-Point Probe|
Four point probe is used to measure resistive properties of semiconductor wafers and thin films. If the thickness of a thin film is known, the sheet resistance measured by four point probe can be used to calculate the resistivity of the material; conversely, if the material's resistivity is known, the thickness of the thin film can be calculated.
Miller FPP-5000 4-Point Probe
Main article: Miller FPP-5000 4-Point Probe
Method of operation
- A four point probe is typically used to measure the sheet resistance of a thin layer or substrate in units of ohms per square by forcing current through two outer probes and reading the voltage across the two inner probes. Using this four-terminal configuration avoids measurement error due to the contact resistance between the probe and sample.
- The probes are collinear and are equally spaced.
- Probe spacing for the LNF tools is 1.59mm
- For film thickness ≤ 0.5 x (probe spacing) and diameter or lateral dimensions > 40 x (probe spacing), the sheet resistance calculation simplifies to:
V = measured voltage
I = force current
- Rearranging the equation for resistance of a rectangular thin film resistor helps illustrate the meaning of sheet resistance which is equal to the resistivity of the material divided by its thickness.
- ρ = thin film material resistivity
- L = length of resistor
- W = width of resistor
- t = thickness of thin film material
- If the thickness of a film or material is known, its resistivity can be calculated from the sheet resistance measurement.
- If the resistivity of the film or material is known (or assumed), a thickness can be calculated from the sheet resistance measurement.
- For samples with lateral dimensions or diameter < 40 x (probe spacing), correction factors need to be used to obtain accurate sheet resistance values. For correction factor values, please refer to information at http://four-point-probes.com/finite-size-corrections-for-4-point-probe-measurements 
Four point probes are used in nanofabrication to measure the resistive properties of conducting films which may include substrates, deposited films, and doped regions on a sample surface.