Meaning
Physical wear mechanism causes the sudden failure of an insulator after it has been subjected to a constant electric field for a long period. In the study of gate oxide reliability, time dependent dielectric breakdown describes the process of gradual defect accumulation that leads to a catastrophic short circuit. This is a primary factor in determining the maximum lifetime of an integrated circuit.
Defect Accumulation
Individual atomic bonds within the dielectric layer break over time due to the influence of the applied voltage and thermal energy. These defects initially have little impact on the performance of the device, but their concentration increases as the device remains under stress.
Lifetime Prediction
Statistical models are used to estimate the time to failure for a large population of devices based on data from accelerated tests. By applying a higher voltage than normal, engineers can force the time dependent dielectric breakdown to occur more quickly in a controlled setting. This data is then extrapolated to normal operating conditions to ensure that the product will last for its intended service life.
The results are typically plotted on a Weibull distribution to identify the characteristic failure time of the manufacturing lot.
Voltage Acceleration
Rate of failure increases exponentially as the electric field across the dielectric is raised. Small changes in the operating voltage can have a massive impact on the expected lifetime of the transistor. This relationship forces a strict limit on the maximum supply voltage that can be used without risking premature failure of the entire system.
