Four-Terminal Measurement and Simulations for Sheet Resistance in Piezoresistive Sensing Elements

We have seen some wafers rejected with abnormally large within-wafer variation for the input resistance (Rin) of the X-ducer in the pressure sensing element. Input resistance consists of $p+,$$p−,$ the resistance of aluminum lines, and its contact resistance to $p+$ and $p−.$ A four-terminal measurement (FTM) is used to characterize piezoresistors for our pressure sensing elements. The purpose in using a FTM is to find which process causes this abnormally large input resistance. Good agreement has been obtained between the finite element analysis (FEA) and experimental measurement results for this FTM technique. The difference between the conventional van der Pauw method and FEA approach is also compared and presented. It is concluded that $p+$ variation is the major process to cause this abnormally large input resistance. This methodology was used to diagnose the process-related problem without using special test patterns. Therefore, the development cost and cycle time was significantly reduced.