TDDB (Time Dependent Dielectric Breakdown)

Notes

Extrapolation from stress voltage to use voltage is the single largest source of disagreement between reliability engineers and designers asking for more overdrive.

Overview

TDDB, time-dependent dielectric breakdown, is the wear-out mechanism in which an insulating film under a sustained electric field slowly accumulates defects until a conducting path forms through it and it fails. It limits the life of transistor gate dielectrics and of the insulators between closely spaced wires, and so it helps decide how much voltage a process can safely allow over a product's life. Unlike ageing effects that gradually slow a circuit, TDDB ends in an abrupt failure.

How it works

A thin dielectric under voltage carries a small tunnelling current. Over time, defects called traps build up in the film and capture charge, and as more charge is trapped the film's breakdown voltage falls. Eventually the traps link into a percolation path, a chain of defects through the film, and the insulator breaks down. Some traps come from the film's own formation and its interface with silicon; others are created during operation. Moving from silicon dioxide to hafnium-based high-k dielectrics did not remove intrinsic traps. Breakdown can also be extrinsic, triggered by manufacturing defects.

TDDB is measured with constant stress tests. Many test structures are held at a voltage above normal use, often at raised temperature, while leakage is monitored, and the time to breakdown of each is recorded to build a statistical distribution. Lifetime models, such as the E, 1/E and power law forms, then extrapolate from stress conditions to use conditions; which form is valid depends on the film's thickness and material.

In design, aging-aware tools model TDDB alongside bias temperature instability and hot carrier injection as functions of voltage and temperature. The models are not perfect: one RF team found that a static TDDB model predicted immediate failure, while chips survived because very short voltage spikes did less damage than the model assumed. Mission profiles, which describe how long a part spends at each voltage and temperature, therefore matter a great deal.

Examples

• Constant voltage stress on gate dielectric test structures at raised voltage and temperature, extrapolated to use conditions.
• Aging-aware timing libraries that account for TDDB, bias temperature instability and hot carrier injection.
• An RF design where a static TDDB model predicted instant failure but measured chips survived short voltage spikes.
• Automotive mission profiles used to judge dielectric lifetime for parts working in hot climates.
• Comparing E-model and power law extrapolations when setting a process's maximum gate voltage.

Related terms

• PBTI
• Weibull distribution
• Activation energy
• HTOL

Where it appears on the Hub

Used on 10 pages of the Hub. In each group, the pages that mention it most come first.