Resistance measurements on wafers and bricks

The resistivity is one of the most important electrical parameters of a material. It is a key parameter for the performance of semiconductor devices as e.g. solar cells and depends on the doping density of the material. Hence, it is necessary to measure the resistivity with a high accuracy and a high resolution, in order to detect inhomogeneity in the doping density.

Fig 1: Emitter sheet resistance map of a typical mc-Si wafer with emitter diffusion and an average sheet resistance of 85.1 Ohm/sq.
Fig. 2: resistivity map of a typical mc-Si wafer for PV applications and an average resistivity of 1.0 Ohm cm
Fig. 3: Resistivity map of a typical mc-Si brick for PV applications and an average resistivity of 1.4 Ohm cm

With MDPmap and MDPingot it is possible to measure the resistivity of wafers or bricks with a high accuracy and a resolution of 1 mm via eddy current measurements. The Eddy current sensor setup has a very good long-term stability, due to a distance correlated internal calibration matrix. Hence, with every resistivity map a geometry map of the surface flatness is measured. The resistivity can be measured simultaneously with the minority carrier lifetime and photoconductivity maps. In case of wafer measurements, the thickness of the sample has to be given by the user.

  • Step width ≥ 1mm

  • Edge exclusion 12 mm

  • Resistivity wafer thickness range 150 ...250 µm

  • Resistivity range can be specified

  • Default setting: 0.5 to 5 Ohm cm

  • Accuracy: < 5 %

  • Repeatability: < 1 % (range of 0.5 until 3 Ohm cm)

It is possible to map the sheet resistance of the emitter, in order to investigate the homogeneity of the emitter diffusion. The resistivity of the base has to be given by the user.

Sheet resistance measurement with range 0.1-200 Ohm/sq

Accuracy at standard sample size,

  • 0.1 - 10 Ohm/sq: < 3 % accuracy

  • 10 - 100 Ohm/sq: < 4 % accuracy

  • 100 – 200 Ohm/sq: < 5 % accuracy

Figure 1 to 3 show examples of resistivity maps measured on mc-Si wafers and bricks.

Freiberg Instruments is currently improving sheet resistance measurements and adapting its devices for measurements of thin layers and stacks of wide-bandgap materials.