# Four point probe

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Four point probe
Four Point Probe Diagram
Technology Details
Technology Metrology
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.

## 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
Where: • 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.

## Applications

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.