Wafer-level calibration of stress sensing test chips

Piezoresistive sensors are a powerful tool for measurement of surface stress states in semiconductor die used within electronic packages. A new wafer-level method for calibrating on-chip piezoresistive stress sensors is presented, in which an entire circular silicon wafer (potentially containing hundreds of fabricated stress sensing chips) is supported on its edge as a simply supported plate and loaded using a uniform pressure. Resistors across the surface of the wafer can then be probed using a standard automated probe station with computer-controlled positioners. The capabilities and limitations of the method have been established and discussed. A typical calibration procedure and the theory needed to determine the piezoresistive coefficients from the raw experimental data have been presented. Finally, the new method has been applied in the laboratory to extract the coefficient /spl pi//sub 44/ of p-type silicon.<>