A recently introduced new CMOS compatible actuator class, called nano electrostatic drive (NED), uses electrostatic actuation to provide significant deflections of elastic structures. The behavior of such actuators can be dominated by nonlinear phenomena, if the nonlinearities are not understood and not considered in the design. One of the main nonlinearity sources is the electrostatic actuation, which results in the well-known instability named pull-in. Additionally, due to large deflections provided by NED technology, stress stiffening and large deformation significantly influence the system, shifting the eigenfrequencies, altering the pull-in voltage, or even introducing geometrical buckling. All these effects together characterize static and dynamic behavior and can be tailored to partially counterbalance each-other by specific designs. In following, we use finite element method (FEM) to analyze the static and dynamic behavior of MEMS based on NED technology. Owing to coupled-field FEM technique, we observe effects like static pull-in, electromechanical eigenfrequency shift and transient phenomena in detail. The numerical results are validated during optical experiments, which supports the conclusions arose from the FEM. Finally, characterizing of the nonlinearities grants the ability to tailor and minimize them during the MEMS design process.
A translatory MOEMS actuator with extraordinarily large stroke - especially developed for fast optical path-length modulation in miniaturized FT-spectrometers (FTS) designed for NIR spectral region (800 nm – 2500 nm) - is presented. A precise translational out-of-plane oscillation at 260 Hz with a stroke of up to 700 μm and minimized dynamic mirror deformation of 80 nm is realized by means of an optimized MEMS design. The MOEMS device is driven electro-statically near resonance and is manufactured in a CMOS-compatible SOI process. Due to the significant viscous gas damping, dominated by the drag resistance of the comparatively large mirror plate with 5mm diameter, the resonant MEMS device has to operate under reduced pressure. A mirror stroke of 700 μm at a driving voltage of 4V is achieved by hermetic encapsulation of the actuator at at a maximal pressure of 3.2 Pa. For FTS system integration the MOEMS actuator has been encapsulated in an optical vacuum wafer-level package (VWLP) to guarantee a long-term stable vacuum pressure of 0.1 Pa and lifetime t ≥ 10a.
Electrostatic actuation is highly efficient at micro and nanoscale. However, large deflection in common electrostatically driven MEMS requires large electrode separation and thus high driving voltages. To offer a solution to this problem we developed a novel electrostatic actuator class, which is based on a force-to-stress transformation in the periodically patterned upper layer of a silicon cantilever beam. We report on advances in the development of such electrostatic bending actuators. Several variants of a CMOS compatible and RoHS-directive compliant fabrication processes to fabricate vertical deflecting beams with a thickness of 30 μm are presented. A concept to extend the actuation space towards lateral deflecting elements is introduced. The fabricated and characterized vertical deflecting cantilever beam variants make use of a 0.2 μm electrode gap and achieve deflections of up to multiples of this value. Simulation results based on an FE-model applied to calculate the voltage dependent curvature for various actuator cell designs are presented. The calculated values show very good agreement with the experimentally determined voltage controlled actuation curvatures. Particular attention was paid to parasitic effects induced by small, sub micrometer, electrode gaps. This includes parasitic currents between the two electrode layers. No experimental hint was found that such effects significantly influence the curvature for a control voltage up to 45 V. The paper provides an outlook for the applicability of the technology based on specifically designed and fabricated actuators which allow for a large variety of motion patterns including out-of-plane and in-plane motion as well as membrane deformation and linear motion.
In this paper the authors report about the six inch wafer level vacuum packaging of electro-statically driven two dimensional micro-mirrors. The packaging was done by means of two types of wafer bonding methods: anodic and glass frit. The resulting chips after dicing are 4 mm wide, 6 mm long and 1.6 mm high and the residual pressure inside the package after dicing was estimated to be between 2 and 20 mbar. This allowed us to reduce the driving voltage of the micro-mirrors by more than 40% compared to the driving voltage without vacuum packaging. The vacuum stability after 5 months was verified by measurement using the so called “membrane method”. Persistence of the vacuum was proven. No getter materials were used for packaging.