Utilizing avalanche breakdown for stress measurements on micro-structures

This thesis reports on the usage of the breakdown voltage of a p-n junction diode to measure the mechanical stress/strain in micro-resonators. The working principle relies on the dependence of silicon band gap to the mechanical stress which then affects the current-voltage characteristics of the p-n junction. To explore the effects of mechanical stress/strain on breakdown voltage, a flexural-mode micro-resonator is designed by defining a p-n junction at the anchoring region to experience maximum stress during mechanical excitations. An analytical model has been developed for the study and numerical analysis of this phenomenon. The Synopsys Sentaurus TCAD simulations were employed for the investigation of the breakdown voltage dependence to various mechanical stress magnitudes as well as orientations. A micromechanical device with integrated junctions was designed and fabricated for the validation of postulate. Mechanical stress was applied onto the substrate by subjecting it to mechanical vibrations. It is estimated that the breakdown voltage of the device exhibited a high-stress sensitivity of about 240µV/MPa. The mechanical stress can also be measured by monitoring the device current while biased at a constant voltage. In this mode, the steep changes of the junction current in breakdown region led to nearly tenfold higher stress sensitivity compared to a piezoresistive sensor. The high sensitivity, simple measurement, and potential for miniaturization for breakdown voltage sensing make it a promising technique for measurement of stress in micro- and nano-mechanical devices.