Influence of the involuntary underetching on the mechanical properties of tunable Fabry-Perot filters for optical communications

We present in this paper the mechanical analysis of tunable InP-based Fabry-Perot filters. These filters are composed by a top and a bottom InP/air Bragg mirrors with a central air microcavity. The tunability is achieved by changing electrostatically the cavity thickness. We have developed a theoretical model using finite elements analysis (FEA) calculation method, which permits to evaluate the mechanical deformation of the central cavity. This model considers the electrostatic force between the two freestanding layers delimiting the central cavity. Using this model, we have analyzed the tuning performance of two kinds of device: a bridge structure and a square membrane. We have evaluated the influence on the mechanical performances of the involuntary underetching of the clamped ends at the freestanding layers extremities. The theoretical results have been compared to experimental ones obtained by optical reflectivity measurements.