An Improved Robust Multibasin Profiling Framework with Physics-Protected Selective Structured-Residual Refinement for Thin-Film Thickness Inversion

Recovering thin-film thickness from reflectance spectra becomes unreliable when measurements contain sparse outliers, competing thickness minima, or structured mismatch with the nominal optical model. We use a robust multibasin framework in which a Tukey profile first establishes a thickness anchor. A local structured-residual correction is then allowed only in directions that do not reproduce the main thickness-sensitive spectral variation. First-order sensitivity is protected explicitly, and second-order protection is adjusted with an observation-specific curvature-overlap coefficient C2. A frozen gate decides whether the refined candidate replaces the anchor; otherwise, the anchor is returned unchanged. The method was tested in an independent-seed numerical study with 7200 observations spanning three optical systems, three thicknesses, and four contamination conditions. Unprotected residual flexibility did not improve accuracy, whereas adaptive protection reduced the pre-gating mean absolute error to 0.216601 nm. Selective gating reduced harmful accepted refinements from 923 to 495 while retaining 93.28% of beneficial refinements. We then applied the frozen procedure, without experiment-specific retuning, to five physical SiO2/Si specimens measured at three spatial positions each. Relative to independent model-based ellipsometric references, the specimen-level mean absolute error was 0.374 nm, with relative errors of 0.01–0.15% and within-specimen spatial SDs of 0.16–0.34 nm. These results show that structured residual correction can improve thickness recovery when thickness-sensitive directions are protected and refinement is not forced on every spectrum.