A micro-actuation mass calibration method for micro-feature enhanced piezoresistive stress sensor

Stress sensor can be used for in-situ and real-time stress measurement on die surface after encapsulation. Typical applications include stress measurement during thermal cycling of different packages, curing of flip chip underfill, curing of encapsulant of chip-on-board packages and for delamination study at die surface. Stress sensor based on (111) silicon have to be calibrated to obtain six piezoresistive coefficients B1, B2 and B3 for both p-type and n-type resistors. Current calibration methods hinder stress sensors to become largely available to experiments in electronic packaging. Stress sensors have to be calibrated in die / strip form and a special design jigs & fixtures are required, which is very tedious and time-consuming. In this study, (111) silicon piezoresistive stress sensors with embedded micro-features were fabricated and a micro-actuation mass calibration method was demonstrated in this study. The calibration mechanism is based on the large difference in coefficients of thermal expansion (CTE) between silicon and aluminum. Stresses in the silicon micro-islands with embedded resistor are produced by aluminum micro-beams. As this calibration is conducted at wafer-level, many resistors can be calibrated concurrently by conventional IC probing and testing equipment. Thermal loading is imposed on the whole wafer during calibration. To calculate the stresses inside the micro-islands, finite element simulation was conducted. Experimental results show that stresses produced by the aluminum micro-beams are in a suitable range for calibration. The piezoresistive coefficient (B2-B3) was obtained. Apart from this, the effect of micro-islands on stress sensitivity was also preliminary investigated. Finite element simulation suggests that sensor with micro-islands is more sensitive to out-of-plane stresses. By using this phenomenon and the fabrication techniques in this study, a new high sensitivity shear stress sensor can be potentially developed