Stress relaxation in dual ion beam sputtered Nb₂O₅ and SiO₂ thin films: application in a Fabry–Pérot filter array with 3D nanoimprinted cavities

Miniaturized spectrometers can be implemented using Fabry–Pérot (FP) filter arrays. Such filters are defined by two parallel mirrors with a resonance cavity in between. For high optical quality, ion beam sputtered distributed Bragg reflectors (DBRs), with alternating high and low refractive index material pairs, can be used as the FP mirrors; while 3D nanoimprint technology provides an efficient way of implementing multiple organic FP cavities of different heights in a single step. However, the high residual stress in ion beam sputtered films results in poor adhesion between the DBR films and the organic polymer cavities, causing debonding of the DBR. Therefore, the residual stress of the ion beam sputtered films forming the DBRs must be reduced. Niobium pentoxide (Nb₂O₅) and silicon dioxide (SiO₂) are used as the DBR materials in this work due to their high index contrast, resulting in high reflectivity for only a few alternating pairs. Stress relaxation in ion beam sputtered Nb₂O₅ and SiO₂ films is achieved in this work by deposition under simultaneous high energy ion bombardment (oxygen and argon gas mixture) from a second ion source. Using this technique, the film density and hence compressive film stress for both Nb₂O₅ and SiO₂ films is reduced without introducing any additional optical absorption in the films. FP filter arrays fabricated with stress reduced Nb₂O₅ and SiO₂ as DBR films exhibit high optical and mechanical performance, with good adhesion between the films and the polymer cavity. (paper)