Q: Why is understanding your chip's age important?
A: Semiconductor devices age over time, we all know that but what is often not well understood are the mechanisms for ageing or the limits that will cause a chip to fail. In addition, there is bound to be a requirement for a minimum lifetime of a device which will depend on application but could be two or three years for a consumer device and up to twenty-five years for a telecommunications device. Given that lifetime requirement and often poorly understood ageing processes, many chips designed today are over designed to ensure reliable operation. If you understand that ageing process or better still can monitor the ageing process then you can reduce the over design and potentially even build chips that react and adjust for the ageing effect, or predict when that chip is going to fail.
Chips at the moment are not getting anywhere near their total lifespan because in most cases there isn’t any in-chip monitoring taking place. I sometimes use the analogy of a rental car which you want to give back with an empty fuel tank. If your chip has a defined lifetime, then you want to run it as hard as you can to just perform within spec for the lifetime, or looking at it the other way, you want to hand your rental car back just as you run out of fuel.
Q: What are the effects and mechanisms of ageing?
A: There are a number of mechanisms which contribute, the most notable ones are electromigration, hot carrier effects, and bias temperature instability. Whilst some of this can be mitigated through design techniques, and CAD tools exist to help with that, they can only go so far. In the case of bias temperature instability, the mechanisms are not fully known. Whilst traditionally only negative bias temperature instability (NBTI) was considered an issue, now, with the