9 \times 8.85 \times 10^{-14} \, F/cm$ is the permittivity of the oxide and $t_{ox} = 18 \, nm = 1800 \, Å$ is the thickness of the oxide. Substituting these values, we get
\[C_{ox} = \frac{3.9 \times 8.85 \times 10^{-14} \, F/cm}{1800 \, Å} = 1.9175 \times 10^{-7}
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