Fitting of a robust reference profile for separation of form and waviness from surface profiles

Establishing a reference profile representing wavelength components of waviness and form of measured surface profile is an important step in surface metrology. This paper describes a new method of fitting a reference profile by taking a combination of sinusoidal functions and a second degree polynomial using least absolute deviations (LAD) criterion. The reference profile so obtained is found to exhibit form approximation capability and robustness against outliers such as deep valleys in the surface profile. Few examples of practical and simulated surface profiles are taken to bring out the nature of the reference profiles and a comparison with robust Gaussian regression filtering method is also presented to show the effectiveness of the proposed fitting scheme in separating waviness and form.