P ro ba bil it y Densi ty Dis tributio n
t =100s
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Table II NBTI failure rates in 100s stress based on two Vt failure criteria.
NBTI Failure Criterion
Poisson
Model This Model Measurement
Vt= 50mV 10.98% 7.62% 9 / 132
Vt= 100mV 0.032% 0.003% N / A
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Chapter 4 Conclusion
A discrete feature in NBTI stress Vt evolutions due to individual trapped charge creations in small-area devices is observed. Single charge creation times and induced Vt shifts are clearly defined. This single charge characterization approach allows us to gain insight into NBTI induced threshold voltage shift distributions in small-area devices. Statistical characterization of individual trapped charge creations in NBTI stress in a large number of nanoscale high-k (HfSiON)/metal gate (TiN) pMOSFETs is performed and an NBTI induced Vt distribution have been investigated.
Two factors are found to influence an NBTI induced Vt distribution. One is the dispersion of single trapped charge induced threshold voltage shift and the other one is the dispersion of trapped charge creation times. The broad distribution of trapped charge creation times is attributed to the dispersion from different local chemistry and 3D electrostatics such as random dopants and edge effects in a nanoscale device.
We develop a statistical model based on measured trap characteristic time distributions to simulate an NBTI induced Vt distribution in nanoscale devices. A correlation between the trapped charge creation distributions and the spread of an NBTI induced Vt distribution has been established. Our model can reproduce a measurement result of an NBTI induced Vt distribution and its stress time evolution well.
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