Reliability

In high-k/metal gate nMOS transistors, what is the degradation called in which the threshold voltage shifts when positive gate voltage and high temperature are applied continuously?

  1. ALatch-up
  2. BPBTI (positive bias temperature instability)
  3. CElectromigration
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Answer: B. PBTI (positive bias temperature instability)

PBTI is degradation in which positive gate voltage and high temperature on an nMOS cause electrons to be trapped at defects in the high-k film, raising the threshold voltage. It was minor with SiO2 gate insulators but became an issue with the introduction of high-k materials. It is the counterpart of NBTI, which occurs in pMOS.

  • A. Latch-up is a high current caused by a parasitic thyristor.
  • B. Correct. It is positive-bias temperature instability in nMOS.
  • C. Electromigration is the migration of wiring metal atoms.
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  1. In leading-edge LSI design, what is the practice called of anticipating transistor degradation from BTI and HCI over the usage period and checking that timing is met even with degraded characteristics?
  2. When estimating field lifetime from temperature-humidity test results, which model is commonly used to express the effect of humidity as a power of relative humidity?
  3. For gate insulator breakdown lifetime, why must the difference between the area of test devices and the total gate area on an actual chip be considered?
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  5. Regarding BTI degradation, part of the threshold voltage shift recovers once the stress is removed. What does this imply for evaluation?