Fabrication of Al2O3/SiO2 multilayer reflective filters with excellent uniformity for demanding optical interference filters

In this work, we report the fabrication of a high reflective filter with excellent uniformity for demanding optical interference filters. To achieve this, films were deposited in different batches at three different angular positions using electron beam deposition technique. The coatings were carried out using indigenously designed flat jig holder without shapers. Fourteen alternating layers of high and low refractive index films were deposited on BK7 substrates of dimensions 186 x 150 mm by using aluminum dioxide (Al2O3) and silicon dioxide (SiO2) evaporation grade granuals. Individual layers in the stack were deposited at quarter wave optical thickness at a wavelength of 550 nm. To examine the optical properties of multilayer optical films, Gonio-spectrophotometer was used. Roughness and thickness of all the samples were evaluated using Stylus Profilometer and Coherence Correlation interferometer. Surface properties such as wettability, adhesion and hardness were studied. Additionally, to investigate the change in chemical bonding and structural properites of the coating materials used for evaporation before (bulk powdered material) and after deposition of thin films (material left in the crucible after deposition process), FTIR and XRD studies were carried out.