Integrated CMOS-Based Sensor Array for Mechanical Stress Mapping

This paper reports on a CMOS-based stress sensor array used to characterize the distribution of mechanical in-plane stress in the surface of packaged integrated circuit dies. It consists of an array of 32 stress sensors exploiting the shear piezoresistive effect in silicon. The stress sensor elements are integrated in a mixed-signal system consisting of analog switches, an amplifier stage, a successive-approximation analog-to-digital converter, and a digital logic for overall chip control. The sensors are operated using the current switching method and measure the orthogonal pseudo-Hall voltages. From these signals, the second angular order Fourier components, representing mechanical stress components are calculated by the digital logic block. The response of each sensor to the in-plane stress components (sigma_xx-sigma_xx) and sigma_xy is thus obtained. Data are transmitted to external instrumentation via a serial I²C-bus-like protocol. The measured sigma_xx-stress sensitivity of the sensors is S_xx = 49 muV/V/MPa.