# Four point probe

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Four point probe | |
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Four Point Probe Diagram ^{[1]} | |

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.

## Contents

## Equipment

### 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:

^{[1]}

V = measured voltage

I = force current

Where:

^{[1]}

- 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
^{[2]}

## 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.

## See also

## References

- ↑
^{1.0}^{1.1}^{1.2}[1] PV Education.org Four Point Probe Resistivity Measurements - ↑ Four-point-probes.com