Connection of low current measurement SMU to DUT

When connecting the DUT, in addition to using shielded and guarded cables, it is also very important to connect the appropriate terminals of the Keithley 4200-SCS to the appropriate terminals of the device. Improper connections to the Force HI and Force LO terminals of the SMU can cause current offsets and unstable measurements. These errors are caused by common-mode currents.
In general, always connect the high-impedance terminal (Force HI) of the SMU to the maximum resistance point of the circuit under test. Similarly, always connect the low-impedance terminal (Force LO) of the 4200-SCS to the minimum resistance point of the circuit under test. The minimum resistance point can be a common terminal or ground. If the Force HI terminal is connected to the minimum resistance point, common-mode current will flow through the measurement circuit.
Figure 14 shows the correct and incorrect measurement connections. The connection in Figure 14a is correct because the Force HI terminal of the Keithley 4200-SMU is connected to the gate of the device on the wafer, and the Force LO terminal is connected to the guard chuck. The gate terminal on the wafer is the maximum impedance point and the guard chuck is the low impedance point, so the circuit is connected correctly. Note that the common-mode current flows from the SMUde Force LO terminal to the guard chuck; however, the current does not flow through the ammeter and therefore does not affect the measurement. Figure 14. Connecting the SMU to the device on the guard chuck The connection in Figure 14b is incorrect, as it connects the SMU's Force LO terminal to the high-impedance gate and the SMU's Force HI terminal to the guard chuck. In this case, the common-mode current will flow through the SMU as well as the DUT. This can cause inaccurate or even unstable measurements.