Kelvin setup consists of four terminals F+,F-,S+,S-. My setup forces current of 100mA into F+,F- and measure voltage at S+,S- . I use keithley 2400 to measure voltage at S+ and then at S-. I will then take difference between S+ and S- and then then divide by the force current to give me the resistance value. But some time I measure S+ lower than S-. What could be the probable reason. One example is 1.479E+0,1.482E+0,-29.760E-3, S+=1.479E+0, S-=1.482E+0, Diff=-29.760E-3 Raw Data. Kelvin setup should give me readins which are consistent, but I measureing resistances for some dies are higher and some dies are negative. I using a probe card,which measures this resistance on a wafer. What could be the probable reasons. Probecard pin, pad interaction with probe card pins? The connections to F+,F-,S+,S- are thus. F+ is connected to + of SMU1,F- is connected to +of SMU2. SMU1 forces current, SMU2 is set to 0 volts to set as return or gnd point. S+, S- are measured with Keithley 2400
I expect the resistance to be around 20-30 milli ohms. I have another probe card which gives me consistent readings. This probecard was repaired a few days ago. Could it be that debris from the pad is deposited over time on the pin(and which is difficult to remove) and cause this variation?. But I am more interested to know why S- reads higher than S+,though it is a kelvin/four-point measurement setup.
Kindly help me to find the answers to my problem.