Theoretical modelling and experimental verification of gradient thickness function for thin film deposition of linear variable filter

This study proposes a theoretical model for a gradient thickness function for thin film deposition of a linear variable filter (LVF), which is used for order sorting for a diffraction grating. To achieve linear variable filtering, the thin film must be fabricated with a gradient of increasing thickness along the direction of wavelength dispersion. The thickness gradient is created using a local mask on top of the substrate during thin film deposition inside the evaporation chamber. A theoretical model and ray tracing simulation are developed to obtain the thin film thickness distribution and the parameters for the fabrication of LVFs. Within the 25%–75% thickness range, the profile distribution exhibits a high degree of linearity (coefficient of determinations are greater than 0.9939). This study shows that the proposed theoretical model and ray tracing predict a thin film profile for an LVF with high precision (root–mean–square deviations are less than 4.52%).