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Variations in the number and characteristics of charges or traps contributing to transistor degradation lead to a distribution of device “ages” at any given time. This issue is well understood in the study of time dependent dielectric breakdown, but is just beginning to be thoroughly addressed under bias temperature instability (BTI) and hot carrier injection (HCI) stress. In this paper, we present a measurement system that facilitates efficient statistical aging measurements involving the latter two mechanisms in an array of ring oscillators. Microsecond measurements for minimal BTI recovery, as well as frequency shift measurement resolution ranging down to the error floor of 0.07% are achieved with three beat frequency detection systems working in tandem. Measurement results from a 65 nm test chip show that fresh frequency and the stress-induced shift are uncorrelated, both the mean and standard deviation of that shift increase with stress, and the standard deviation/mean ratio decreases with stress time.
Date of Publication: Oct. 2011