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4-TEST
Offline characterisation unit for sheet resistance
and specific, contact and line resistivity
overview

Situation

Four point probe measurements for the determination of sheet resistances are a versatile tool used in microelectronics, semiconductor, and photovoltaics industry. The primary application is the determination of the sheet resistance of a diffused emitter, and nearly all commercially available four point probers also allow for the calculation of the specific resistivity if the wafer thickness is known. Where the latter value serves for sampling of the incoming material quality, the regular measurement of the sheet resistance after diffusion is useful to identify problems that occur during the diffusion process. By applying the 4-point measurement system to other types of samples, a wide range of further parameters can be determined as derivations from the basic four point measurement. Examples for these applications are line resistance measurements, and contact resistance measurements based on the transfer length method (TLM). Both of these values become important when checking and optimizing the screen printing and firing process: the line resistance indicates the quality of the electrical conductance along the fingers or busbars, whereas the contact resistance shows whether the contact to the silicon has been formed correctly during firing.

Description

The 4-TEST is a four-point-probe tool for reliable testing of all types of resistance measurements, and a wide range of probe heads suits all typical applications in solar cell processing like sheet resistance, specific resistivity, line resistance, and contact resistance. For sheet resistance control in the processing line, an optional 5-fold holder for automatic contacting in 5 points is available. All probe tips inside the sensor head are independently spring mounted. For measurements of sheet resistance and specific resistivity, tips with rhodium coating are available.

As a brand new option, tips made of tungsten carbide are also available. These tips give better contact especially on low-doped material and show lower wear-out. For the measurements after metallisation (finger and contact resistance), all probe tips are rhodium coated. Probe heads from other suppliers can also be used if necessary. The measurement principle for sheet resistance is based on the well-known theory of four-point probing of thin layers with equally spaced tips. A similar model that also considers the wafer thickness is used for resistivity measurements. Line resistance is measured with four tips contacting one finger, where the inner tips have a distance of 30, 40 or 50 mm.
The contact resistance is measured based on the transfer length method, measuring resistance from one finger to adjacent fingers and then extrapolating the fitted curve to zero. The contact resistance head features 6 tip pairs, where each of the pairs contacts adjacent contact fingers. To match different finger distances, standard probe heads with different tip distances can be ordered to cover the full range from 1.5 to 2.8 mm finger distance.

The 4-TEST comes with a 19” rack including an industrial PC and all measurement electronics, and a separate sample stage with glass plate and cover. High quality measurement electronics from Keithley are used for current sourcing, voltage measurement and switching of the tip pairs in contact resistance mode. The operator-friendly control software offers many functions for statistic analysis, production control, and data logging and export, and features a basic MES interface. A loss analysis software will be available to combine the 4-TEST with the SR-TEST. This will give industrial users a fast access to most of the cell data needed to optimise the production processes.