Charge control of electrostatically actuated micromechanical infrared Fabry-Pérot filters

In this work, the applicability of charge controlled electrostatically tuneable optical filters is investigated. The filters are based on a Fabry-Pérot architecture, fabricated in a bulk micromachining process. Compared to surface micromachined devices, this design opens a path to higher optical performance due to the high planarity and low roughness of substrates but also introduces the drawback of acceleration sensitivity because of a moving mass. The common way of tuning those electrostatic actuators by applying constant voltages decreases the effective stiffness of the system and thus further increases this sensitivity for large deflections. In addition, the tuning range is limited to one third of the initial electrode spacing due to the pull-in effect. Therefore, designing voltage-controlled electrostatic actuators of such optical filters result in tough tradeoffs between initial electrode spacing, spring stiffness, supply voltage and chip area. In order to overcome the limitation of the tuning range and relax these tradeoffs, controlling the charge instead of voltage by using a switched capacitor amplifier is examined. Experiments have shown that it is possible to obtain a stable relative displacement of up to 60% limited by reflector tipping. Measuring gravity impact confirmed the expected reduced deflection dependency. Thus, it is possible to downsize the initial electrode spacing by 45% and the spring stiffness by 40% while achieving the same optical tuning range and acceleration sensitivity as in voltage mode. However, because of reflector tilting and the associated filter bandwidth degradation, a further tradeoff arises when using relative deflections greater 40 %.