Gaussian model for the measured data. His main focus was on modeling rather
than on measuring IC variations.
Ring oscillators spread throughout a test chip were used by Hargreaves et
al. [36] to measure variation on a test chip. The chip design allowed the ring
oscillators could be accessed sequentially. Thus, Hargreaves et al. [36] could
measure each ring oscillator frequency separately. Figure 2.4 shows inverter delays
for four different test ICs. They finally also modeled the variations as a Gaussian
field. Their method differs from the model by Liu [47] in the correlation function
and fitting procedure. Hargreaves et al. used more accurate parameter estimation
method with higher complexity compared to Liu [47].
C⅛p 1 Cħ⅛>2
Figure 2.4: Variation on four test chips measured by Hargreaves et al. [36].
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