Application of a Wafer-Level Calibration Technique for Stress Sensing Test Chips

Piezoresistive sensors are a powerful tool for measurement of surface stress states in semiconductor die used within electronic packages. A perennial problem that has prevented the broad application of such sensors has been the calibration process. In this work, applications of a new wafer-level method for calibrating on-chip piezoresistive stress sensors are presented. The demonstrated technique has the potential to alleviate many of the shortcomings of previous calibration procedures. In the wafer-level method, 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 are then probed using a standard automated probe station. Extensive calibration results have been obtained for three different test chips incorporating a variety of stress sensing rosettes. In each case, the theory needed to determine the piezoresistive coefficients from the raw experimental data has been presented. Obtained results for the piezoresistive coefficients have been compared with analogous data from the more tedious four point bending technique. In all cases, values extracted with the wafer-level procedure have correlated well with those from the four-point bending tests.