Contact resistance measurement - observations on technique and test parameters

During testing of a new contact resistance measurement system (ContactSpot), we observed several unexpected results. TLM theory predicts a linear dependence of the resistance measurements vs probe spacing, and from this data set the contact resistance can be extracted. However, we found a non-linearity at wider probe spacing as well as sensitivities to the magnitude of the current, the direction of current flow, the ambient light level, and the choice of using contact pitch or spacing distances in the algorithm. Little appears in the literature concerning these effects and sensitivities. We found acceptable conditions for performing the TLM method in the dark, but we found more consistent results for contact resistance and emitter sheet resistivity values when using a previously developed algorithm that does not depend upon calculating slope and intercept values.