Scenario 1: Sense Line Disconnected from Force Line
In normal operation with the SMU outputting a constant voltage with remote sense enabled, the SMU implements a closed control loop. The Sense (HI_Sense/LO_Sense) lines sense the voltage at DUT. The control loop then uses this measurement as a reference to adjust Force output to keep the DUT voltage fixed. For example, if we want to set the voltage at the DUT to be 1.0 V, the SMU will start generating 1.0 V. Considering the voltage drop across the lead wire, HI_Sense line will tell the SMU that the DUT voltage is 0.1 V lower than the expected 1.0 V. In this case, the control loop will adjust SMU HI_Force to be 1.1 V so that the DUT will see 1.0 V in the end.
Figure 3. The general SMU under normal operation. (Only HI_Sense is shown in this diagram)
When the remote sense line (HI_Sense) is open or disconnected from the force line (HI_Force) on the SMU, the control loop will be broken and will no longer be able to correctly adjust the output voltage at HI_Force. In this case, the remote sense line is left floating, but will likely trend towards 0 V due to internal leakage from SMU ground.
Let’s say that the SMU is configured to output 1.0 V to the DUT. The SMU will start generating 1.0 V at the HI_Force line. Because the HI_Sense line is disconnected from HI_Force, it will tell the control loop of the SMU that the DUT voltage is 0 V, although it is not. The control loop won’t know that the HI_Sense line is disconnected and will think that there is a 1.0 V voltage drop across the HI_Force line. To combat this, the control loop will try it’s best to crank up the output voltage on the HI_Force line by sourcing as much voltage as possible in attempt to output the exact requested voltage. In the end, the SMU will output more than expected voltage, although this is the expected operation of SMU. The sourced current to the DUT, however, should still be in within the specified current limit since the control loop is still closed. Depending on the breakdown mechanism of the DUT, the over-sourced voltage could cause the test fixture, DUT and even the SMU itself to be overstressed under this fault condition and cause damage.
Figure 4. The general SMU when remote sense line is broken
To prevent this high voltage and current output from damaging the DUT, a high impedance resistor can be added between the HI_Force and HI_Sense lines and the LO_Force and LO_Sense lines of the SMU. This will provide open-sense protection by closing the control loop when using remote sense, even though the remote sense line is open. The sense line will sense the voltage across the Force lines through a high impedance open-sense protection resistor and the control loop will drive the SMU to output in the normal range.
Figure 5. The general SMU with a high impedance resistor between HI_Force and HI_Sense
Note: The NI 4135/6/7/8/9 and 4147 SMUs have built-in 1 MΩ open-sense protection resistors between HI_Force and HI_Sense and between LO_Force and LO_Sense, whereas the NI 4162 and 4163 SMUs do not. The use of the NI 416x Open-Sense Protection Accessory is highly recommended for the 4162 and 4163 SMUs to add the open-sense protection resistor between HI_Force and HI_Sense and between LO_Force and LO_Sense to ensure that the lines are connected. Adding the resistor can impact remote sense accuracy. See the System Performance Considerations section below for how to determine application impact.
Figure 6. NI PXIe-416x Remote Sense Resistor Accessory.
Scenario 2: Force Line Disconnected From DUT, But Sense Line Connected to DUT
As intermittent connection can happen at either socket pin, SMU force line may be disconnected from DUT while sense line is connected. Under current output mode, the SMU force may output more than expected voltage if the resistance between force and sense if high. Take for example, the 416x SMU is configured to sink 50mA current with a compliance voltage of 12V and there is a +12 V voltage source at the DUT seen by HI_Sense. When the HI_Force line is disconnected, the HI_Force line will rail to -25.5 V (maximum negative voltage the power plant can reach) in attempt to sink the -50 mA. This potential difference across the Force and Sense lines of the SMU surpasses the absolute maximum voltage rating of the SMU and can lead to damaging the circuitry of the SMU. Figure 8 shows the maximum voltage specifications for the PXIe-416x SMU.
Figure 7. NI 416x SMU with Open-Sense Protection Resistor equipped
Figure 8. Voltage Specifications of NI 4162 SMU.
Designing a connectivity program to catch if the HI_Force line disconnection has occurred can be implemented to detect failures before testing begins. One method of implementing this sort of test would be to configure the SMU to source a very small current to the DUT and measure the voltage. Since the internal circuitry of a typical IC chip contains protection diodes, we can source a small current (10 µA) to the input pin of the DUT and determine if a disconnect has occurred from the measured voltage. If the measured voltage rails or climbs to a voltage that is higher than the acceptable forward-biased voltage drop, then an open circuit/disconnect from HI_Force has occurred. This is a similar test implemented in the example for Opens and Shorts Testing Reference Design.
Figure 9. Sourcing a 10 µA current to the DUT with a 1 MΩ Open-Sense Protection Resistor added between HI_Sense and HI_Force to test for a disconnect of HI_Force.