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We programmed the first SMU in the Model 4200-SCS (SMU1) in the definition page to sweep current from 0A to 1mA incrementing in 100 micro-amp steps, totaling 11 test points. SMU2 was programmed to measure voltage only and was set to auto-ranging as we were not initially sure what the measured voltage would be.
The sourced current and measured voltages across two of the SMUs are displayed in Table 1. SMU1Volts is the voltage measured using the two-point approach; SMU2Volts is the voltage measured using the fourpoint approach. Notice the differences in the measured voltages between the two columns.
The results of the test are shown in Figure 6, which compares the resistance as measured using the twopoint probe method with the resistance as measured with the four-point probe method. In this configuration, SMU1 was used for sourcing the current as well as measuring the voltage, indicative of a two-point probe measurement.
From this data, we can calculate the resistance. For this calculation, we chose to measure the delta voltage between the two current points of 900 micro-amps and 100 micro-amps. For the two-point measurement, the resulting resistance measurement is ~3.3 ohms. The same current test points are used for the fourpoint measurements, which produce a resistance
4.0E-3 1.1E-3 1.0E-3 900.0E-6
SMU1Volts
800.0E-6 700.0E-6
2.0E-3
600.0E-6
Data Variables 1 Data: SHRES = 5.37988
500.0E-6
1.0E-3
400.0E-6
SMU2Volts
300.0E-6
SMU2 Volts
SMU1 Volts
3.0E-3
200.0E-6
0.0E-3
100.0E-6 0.0E+0 1.0E-3
900.0E-6
800.0E-6
700.0E-6
600.0E-6
500.0E-6
400.0E-6
300.0E-6
200.0E-6
Fit1(Lin): y=a+bx a=47.48e-6 b=1.20 1/b=833.79e-3 xint=39.59e-6
-100.0E-6 100.0E-6
-1.0E-3
SMU1 Curr Figure 6: Two-wire vs. four-wire measurements
The trace indicated by “SMU1Volts” in the graph is the result from the two-point probe measurement. Note that this plot spans voltage measurements from approximately 0 volts to 3.3 millivolts. The trace indicated by “SMU2Volts” in the graph is the result from the fourpoint probe measurement. Notice the difference in the two plots, especially their slopes. These results are as should be anticipated, given that the two-point probe measurement also includes the resistances from the probes, leads, spreading, and contact resistance. Let’s review the numerical results to confirm the errors with the two-point probe approach versus the four-point probe approach and to obtain the values needed to calculate the resistance.
26
Smu1Curr
Smu1Volts Smu2Volts
000.0000E+0
-10.7986E-6
100.0000E-6
327.1888E-6
72.4560E-6
200.0000E-6
654.0748E-6
190.0227E-6
300.0000E-6
986.3097E-6
312.3849E-6
400.0000E-6
1.3099E-3
428.4137E-6
500.0000E-6
1.6413E-3
544.9526E-6
600.0000E-6
1.9737E-3
669.7575E-6
700.0000E-6
2.3035E-3
790.3785E-6
800.0000E-6
2.6299E-3
901.1007E-6
900.0000E-6
2.9664E-3
1.0319E-3
1.0000E-3
3.2921E-3
1.1459E-3
-48.9888E-6
Figure 7: Results from two- and four-point probe approaches to resistance measurements
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