Transferring stress and temperature sensors for stress measurement of wafer level packaging

This paper reports a novel process to fabricate low cost dummy wafers with stress and temperature sensors for wafer level packaging. The polysilicon piezoresistors and Pt resistors are fabricated in 4-in. wafers with the same pad layout as the actual wafers used in wafer level packaging. Further, the sensors are diced and attached in a face-to-face manner at the test points of the large-size dummy wafers by using benzocyclobutene films. After etching away the substrate material, the sensors are transferred to the dummy wafers, and these wafers are then ready for residual stress measurement in wafer level packaging. The polysilicon stress sensors and Pt resistors are fabricated and transferred, and the longitudinal gauge factors before and after the transfer are measured to be 34.2 and 33.0, respectively. The deviation of the longitudinal gauge factors between pre and post transfer is less than −3.5 %, indicating a sufficient level of accuracy of the transferred sensors for stress measurement. The transverse gauge factors are measured to be much lower than the longitudinal gauge factors and can be neglected. The temperature coefficient of the Pt sensor is measured to be 1.73 × 10⁻³/°C.