Time and Voltage Dependence of Failure in MNOS Capacitors Containing Point Defects

Time-dependent dielectric breakdown is an important failure mode for silicon integrated circuits. This paper presents a study of such failures in a MNOS capacitor used on a bipolar operational amplifier IC. It is shown that the cause of these failures is point defects in the Si3N4 film. The number of defective capacitors in a given population can be estimated by a silicon "pinhole etch" decoration technique. The failure of these defects is very dependent on voltage and each has a definite threshold for failure. This threshold varies from defect to defect. Aging at a fixed voltage stress causes rapid failure of all those defects with thresholds below the stress. The defects with higher thresholds do not fail. Voltage step-stress data is. presented to show that the defects have a definite distribution of failure thresholds. This distribution is approximately normal with a median of 40 volts. Thus most defects fail well below the 120 volt dielectric breakdown of the MNOS structure. For defects aged above their failure threshold, the time-to-failure distribution is a strong function of the overvoltage (VOV); i.e., the amount the aging voltage exceeds the defect threshold. For VOV of a few volts, the time-to-failure distribution is log-normal with a median of about 10 hours and sigma of about 2. For higher overvoltages, the median life decreases rapidly - approximately as 1/VOV4. Examples of long-term failure rate. calculations are given and compared with experimental data.