Table of Contents

Abstract: When I joined a course on microsensors given by Steve Senturia, Martin Schmidt, and Roger Howe at MIT in 1988, I saw the first video of the earliest rotating silicon micromotor. It was the beginning of a high-time of microelectromechanical systems (MEMS): everything we did was new [1], and the MEMS community was sure that we not only reached a new frontier (which might be true), but that overcoming this boundary would lead to solutions for the most pressing problems of human kind [2] (which might not be true). With the demonstration of a rotating micromachine as a key element in MEMS, we had the impression that basically all problems in our young field could be solved. In the past 20 years, we saw the explosive development of the field. We now have a good idea of the value of MEMS, which is in many respects different to what we anticipated 20 years ago, and which includes many new developments.

Abstract: The performance of micro-mixers is evaluated in terms of deviations from perfectly mixed state and mixing length (i.e., device length required to achieve perfect mixing). Different variations of T-mixer are reported for improved mixing performance, including geometric constrictions/obstacles embedded in the channel wall, heterogeneously charged walls, grooves on channel base, etc. Most of the reported designs provide improved mixing at the expense of reduced flow rate; there exists therefore a tradeoff between mixing and transport. The reduced flow rate, which affects species residence time, is unfortunately not taken into account in most micro-mixing performance analyses. This issue is addressed by the comparative mixing index (CMI), which evaluates mixing performance more appropriately by normalizing the effect of residence time among different designs. In this study, the performance of several mixing strategies are evaluated based on the CMI; these are mixer designs that incorporate (a) physical constrictions, (b) induced charge electro-osmotic (ICEO) effects, and (c) heterogeneously charged walls. The present analysis clearly identifies conditions under which a given mixer design is superior to a T-mixer.

Abstract: Shuffle motors are electrostatic stepper micromotors that employ a built-in mechanical leverage to produce large output forces as well as high resolution displacements. These motors can generally move only over predefined paths that served as driving electrodes. Here, we present the design, modeling and experimental characterization of a novel shuffle motor that moves over an unpatterned, electrically grounded surface. By combining the novel design with an innovative micromachining method based on vertical trench isolation, we have greatly simplified the fabrication of the shuffle motors and significantly improved their overall performance characteristics and reliability. Depending on the propulsion voltage, our motor with external dimensions of 290 μm × 410 mm displays two distinct operational modes with adjustable step sizes varying respectively from 0.6 to 7 nm and from 49 to 62 nm. The prototype was driven up to a cycling frequency of 80 kHz, showing nearly linear dependence of its velocity with frequency and a maximum velocity of 3.6 mm/s. For driving voltages of 55 V, the device had a maximum travel range of ±70 μm and exhibited an output force of 1.7 mN, resulting in the highest force and power densities reported so far for an electrostatic micromotor. After five days of operation, it had traveled a cumulative distance of more than 1.5 km in 34 billion steps without noticeable deterioration in performance.

Abstract: A gap with variable geometry is presented for both cantilever beam and fixed-fixed beam actuators as a method to reduce the pull-in voltage while maintaining a required displacement. The method is applicable to beams oriented either in a plane parallel to or perpendicular to a substrate, but is most suitable for vertically oriented (lateral) beams fabricated with a high aspect ratio process where variable gap geometry can be implemented directly in the layout. Finite element simulations are used to determine the pull-in voltages of these modified structures. The simulator is verified against theoretical pull-in voltage equations as well as previously published finite element simulations. By simply varying the gap in a linear fashion the pull-in voltage can be reduced by 37.2% in the cantilever beam case and 29.6% in the fixed-fixed beam case over a structure with a constant gap. This can be reduced a further 4.8% by using a polynomial gap shape (n = 4/3) for the cantilever beam and 1.2% for the fixed-fixed beam by flattening the bottom of the linearly varying gap.