Automated Mapping Four-Point Probe

Overview

Scientists have begun using self-replicating rapid prototyper (RepRap) 3-D printers to manufacture open source digital designs of scientific equipment. This approach is refined here to develop a novel instrument capable of performing automated large-area four-point probe measurements. The designs for conversion of a RepRap 3-D printer to a 2-D open source four-point probe (OS4PP) measurement device are detailed for the mechanical and electrical systems. Free and open source software and firmware are developed to operate the tool. The OS4PP was validated against a wide range of discrete resistors and indium tin oxide (ITO) samples of different thicknesses both pre- and post-annealing. The OS4PP was then compared to two commercial proprietary systems. Results of resistors from 10 to 1 MΩ show errors of less than 1% for the OS4PP. The 3-D mapping of sheet resistance of ITO samples successfully demonstrated the automated capability to measure non-uniformities in large-area samples. The results indicate that all measured values are within the same order of magnitude when compared to two proprietary measurement systems. In conclusion, the OS4PP system, which costs less than 70% of manual proprietary systems, is comparable electrically while offering automated 100 micron positional accuracy for measuring sheet resistance over larger areas.

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Theory

Four needles with equal spacing are placed on the sample to be measured, and a current source is connected to the outer pair of needles. Then the voltage generated between the inner pair of needles is measured. The sheet resistance is then measured with the formula:

Sheet Resistance = Constant * (Voltage measured / Current value)

Where the constant is 4.532 if the following conditions are true:

• The diameter of the sample is large compared to distance between the needles.

• The thickness of the sample is small compared to distance between the needles.

• The location of the needles is not too close to the edge of the sample.

If these conditions are not met, please follow the references in the paper to calculate the correction factor required.