We studied the fluid transport by a bionically inspired micro-flapper fabricated in piezoelectric thin-film technology. The undulatory, wave-like motion of the proposed design is supposed to generate vortex chains in the surrounding fluid resulting in a directed jet stream and, hence, enhanced mass convection and heat transport inside the fluid. Fully-coupled finite element (FE) simulations have been carried out to investigate the fluid transport induced by such an excitation in order to assess the efficiency of the concept. The results show that there is a significant higher net flow for undulation compared to the simple, resonant-like up-and-down motion of the flap, which corroborates the feasibility of the concept.
Strong competition within the consumer market urges the companies to constantly improve the quality of their devices. For silicon microphones excellent sound quality is the key feature in this respect which means that improving the signal-to-noise ratio (SNR), being strongly correlated with the sound quality is a major task to fulfill the growing demands of the market. MEMS microphones with conventional capacitive readout suffer from noise caused by viscous damping losses arising from perforations in the backplate [1]. Therefore, we conceived a novel microphone design based on capacitive read-out via comb structures, which is supposed to show a reduction in fluidic damping compared to conventional MEMS microphones. In order to evaluate the potential of the proposed design, we developed a fully energy-coupled, modular system-level model taking into account the mechanical motion, the slide film damping between the comb fingers, the acoustic impact of the package and the capacitive read-out. All submodels are physically based scaling with all relevant design parameters. We carried out noise analyses and due to the modular and physics-based character of the model, were able to discriminate the noise contributions of different parts of the microphone. This enables us to identify design variants of this concept which exhibit a SNR of up to 73 dB (A). This is superior to conventional and at least comparable to high-performance variants of the current state-of-the art MEMS microphones [2].
We present a system-level model for fast and efficient investigations of distributed electrostatic effects in state-of-the-art silicon microphones. Combining lumped and distributed submodels it accounts for electrostatic forces and capacitive read-out, including non-linearities, fringing fields and parasitics. The derived model is calibrated using electrostatic finite element (FE) simulations and validated by measurements. The non-linearities caused by electrostatic effects have a decisive impact on the sensitivity of the microphone and the distortion of the transduced acoustical signal. Hence, the proposed model provides important insights into the operation of the device, which can be employed to optimize the microphone characteristics.
A new process kit for a SPTS Pegasus DRIE Si-Etch tool has been developed and tested for several different process regimes, e.g. bulk-Si cavity etching and TSV (through-Silicon-Via) etching with high aspect ratios <10:1, using the socalled Bosch process. Additionally, Si-etch back (recess etching) with a single step process has been tested as well. The especially developed "edge protection kit", consisting of Al2O3 material and optionally of PEEK material, covers the edge of a wafer, preventing it from being etched or even being etched away. However, placing such a part on top of the cathode, results in changes of the electric field distribution and the gas flow behavior compared to the standard process kit supplied by SPTS. The consequences may be altered Si-etch rates combined with changes of the tilt and side wall taper of the etched structures, mainly near the outside regions of the wafer. To this end, extensive investigations on the mask and bulk-Si etch rates, the tilt and taper angle of various MEMS test structures and their respective uniformity over the wafer surface have been performed. Additionally, simulations applying Comsol Multiphysics have been carried out to visualize the potential impact of the new process kit on the electrical field distribution. A simplex-optimization was carried out, varying the platen power and source power, in order to improve the tilt and to maintain the proper taper angle. One major advantage of the new process kit design compared to the original one is the reduction of movable parts to a minimum.
We present investigations on acoustic effects occurring inside the sound port of capacitive silicon microphones, which exert additional frequency-dependent damping forces on the sensing membrane and, thus, affect the overall performance of the microphone. Extensive FE simulations have been carried out in order to study the airflow inside the package, to identify relevant impact factors, and to optimize an existing acoustic network model intended for implementation in a fully coupled, multi-energy domain system-level model of the device.
Three different multi-energy domain coupled system-level models are used to simulate the closing and opening
transients of a respective ohmic contact type RF-MEMS switch. The comparison of simulated and measured data shows
that, due to the presence of multiple structural modes, none of the system-level models is able to capture exactly the
initial closing and contact phase whilst dynamic pull-in. The system-level model, that uses a mechanical submodel based
on modal superposition, produces the result closest to the real situation. Notably, the effective residual air gap, assumed
whilst contact between the membrane with high surface roughness and the contact pads of the switch, is the most
influential parameter in the simulation of the closing transient, as this parameter strongly affects the air damping on the
device during pull-in. This finding demonstrates that a reliable model of air damping is a vital prerequisite for the
predictive simulation of pull-in and pull-out transients.
The polymer Parylene has proved to be very suitable as membrane material in many applications, because it exhibits a
low Young's modulus, is biocompatible and non-conducting. A drawback, however, is that intrinsic stress reduces the
flexibility of the membrane. In order to minimize the intrinsic stress and to extract the proper material parameters as
inevitable input for reliable FE simulations, we investigated two Parylene derivatives (Parylene C and Parylen HT)
fabricated by two different releasing procedures (plasma etching and KOH etching). To this end, we produced teststructures,
measured the deflection under various pressure loads applying white light interferometry (load-deflection
measurement) and extracted the Young's modulus and the intrinsic stress of the Parylene layers by fitting the
measurement results to both an analytical model and FE (finite element) simulations. The results were then verified by
detailed measurements of the bending lines. Our investigations revealed that Parylen C membranes released via KOH
etching show a nearly unchanged Young's modulus and nearly no intrinsic stress. Plasma etched Parylene C and
Parylene HT, however, exhibit a not negligible modification of the Young's modulus of 30% and 20%, respectively, and
a noticeable amount of tensile intrinsic stress of 14.4 MPa and 1 MPa, respectively. By thoroughly comparing the results
obtained for the different Parylene variants, we were able to identify the change in crystallinity induced by the
temperature load during plasma etching as the primary cause for intrinsic stress formation in Parylene membranes.
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