A new technique for calibrating piezoresistive stress sensor chips

Piezoresistive stress sensors are considered to be a powerful tool for experimental stress analysis of electronic packages. The stress sensors are fabricated into the surface of the chip and thus, they are an integral part of the chip to be studied. The chip stresses result in resistance changes in the sensors due to the piezoresistance effect that can be measured. Therefore, the sensors are capable of providing chip surface stress states even in encapsulated packages. To use the stress sensor test chip in estimating the stresses, there are several constants in the equations that need to be determined prior to the stress measurements. The accuracy of the stress measurements depends on the accuracy of these constants. The determination of these constants is termed the calibration of the piezoresistive sensors.One of the well-established calibration procedures is a four- point bending technique. This technique is widely used because a uniform, uni-axial stress state can be easily obtained between two inner supports of the fixture on which the sample rests. Several researchers in the past have developed different four point bend testing fixtures and techniques. Most of the techniques developed involved the use of metal probes to probe the sensors and measure the resistance of the sensors. The probe forces are measured in the current study and it is observed that these forces can be between 5-10 grams in order to obtain a measurement. The probe forces induce uncertainty in the uni-axial stress in the sample that can be greater than 5% depending on the force applied to the sample. The current study establishes a new calibration method that eliminates the use of probes in the measurements. The technique developed completely eliminates the uncertainty in the constants that arise as a result of probing. The reduction of uncertainties in the calibration constants increases the accuracy of the stress measurements.