Failure Mechanisms at Surfaces and Interfaces

FAILURE mechanisms at surfaces and interfaces in thin film structures have been studied in thin film field effect triodes. These triodes are very suitable for the study of failures at semiconductor-insulator interfaces as the electrical characteristics strongly depend on the state of this interface. The thin film triodes have been subjected to humidity, to elevated temperature, and electrical stress. During the stress tests the failure of the main materials parameters in these devices has been studied as a function of time and stress. The main failure was a change of the threshold or pinch-off voltage in these devices, indicating that the surface potential of the semiconductor changed under stress. The field effect mobility in the semiconductor was rather constant. The capacitance decreased slightly at elevated temperatures and under electrical stress. The rate equation describing the change of the threshold voltage at elevated temperatures could be explained by assuming a desorption process. Experimentally it could be shown that water contributes to this effect. The failure of thin film field effect triodes under electrical stress was more complex. The main cause of failure appeared to be the electrical field between the gate electrode and the semiconductor. A tentative model for the failure mechanisms under electrical stress is presented which is based on a correlation of the observed failure modes to the slow relaxation characteristics of the semiconductor surface conductivity. It was discovered that the experimental devices could be classified in two types which show a distinctly different slow relaxation behavior.