In-situ calibration of stress chips

A novel procedure employing moire interferometry has been successfully implemented for the accurate in-situ calibration of stress chips. Strains were measured by this accurate and extremely sensitive technique at the locations of the gauges and related to corresponding gauge-resistance changes. The approach is immune from many of the drawbacks of earlier calibration procedures and has eliminated the need to know the piezoresistive properties of the strain-gauge material, which constituted a major difficulty in properly interpreting gauge-resistance changes in terms of strains at the gauge locations. A characteristic gauge factor, S, for each type of strain gauge has been introduced and shown to be the only gauge character to be determined. It reflects how different gauge types react to the same strain. It is suggested that the strain gauges be designed such that the gauge factor is as high as possible, thereby minimizing sensitivity to measurement errors. In addition to being flexible and applicable to new and existing stress chips, the procedure is expected to spread the use of diffused-resistor strain gauges as a realistic monitor of chip strains in addressing reliability concerns