Design and calibration of a 3-D micro-strain gauge for in situ on chip stress measurements

Silicon piezoresistive stress sensors can be used for in situ stress measurements during fabrication and encapsulation of the silicon die. These sensors are fabricated in the silicon die using the conventional silicon processing steps. In this paper we describe a stress sensor or a 3-D micro-stain gauge for such applications. We shall describe the design and fabrication of the test chip, experimental results as well as the detailed calibration procedures using a four-point bending (4PB) fixture. The resistance of stress sensors was found to vary linearly to the applied stress. The piezoresistive coefficient was calculated and found to agree with the reported values for silicon. The problems associated with the calibration process was also discussed