Power Spectral Density (PSD)
Together with the line edge roughness (3*σ) or contact edge roughness (3*σ), two other important parameters, the correlation length (ξ) and the roughness exponent (H), can also be calculated from its power spectral density.
The graph on the left plots the PSD of left edges (the odd numbered edges).
The power spectral frequency is defined as the square of the magnitude of the Fourier transform of the edge residuals.
using Fast Fourier Transform (FFT)