The operational range of electrostatic MEMS parallel plate actuators can be extended beyond pull-in in the presence of an intermediate dielectric layer, which has a significant effect on the behavior of such actuators. Here, we study the behavior of cantilever beam electrostatic actuators beyond pull-in using a beam model along with a dielectric layer. The results from the simple beam model are validated with 3D simulations performed in CoventorWare TM . Three possible static configurations of the beam are identified over the operational voltage range. We call them floating, pinned and flat; the latter two are also called arc-type and S-type in the literature. We compute the voltage ranges over which the three configurations can exist and the points where transitions occur between these configurations. Voltage ranges are identified where bi-stable and tri-stable states exist. A classification of all possible transitions (pull-in and pull-out as well as transitions we term pull-down and pull-up) is presented based on the dielectric layer parameters. Dynamic stability analyses are presented for the floating and pinned configurations. For high dielectric layer thickness, discontinuous transitions between configurations disappear and the actuator has smooth predictable behavior, but at the expense of lower overall tunability.

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Two dimensional (2D) trap Boundary element method (BEM) Electric field in the vicinity of aperture Multipole a b s t r a c t This paper presents two approximate analytical expressions for nonlinear electric fields in the principal direction in axially symmetric (3D) and two dimensional (2D) ion trap mass analysers with apertures (holes in case of 3D traps and slits in case of 2D traps) on the electrodes. Considered together (3D and 2D), we present composite approximations for the principal unidirectional nonlinear electric fields in these ion traps. The composite electric field E has the formwhere E noaperture is the field within an imagined trap which is identical to the practical trap except that the apertures are missing and E aperture is the field contribution due to apertures on the two trap electrodes. The field along the principal axis of the trap can in this way be well approximated for any aperture that is not too large.To derive E aperture , classical results of electrostatics have been extended to electrodes with finite thickness and different aperture shapes.E noaperture is a modified truncated multipole expansion for the imagined trap with no aperture. The first several terms in the multipole expansion are in principle exact (though numerically determined using the BEM), while the last term is chosen to match the field at the electrode. This expansion, once computed, works with any aperture in the practical trap.The composite field approximation for axially symmetric (3D) traps is checked for three geometries: the Paul trap, the cylindrical ion trap (CIT) and an arbitrary other trap. The approximation for 2D traps is verified using two geometries: the linear ion trap (LIT) and the rectilinear ion trap (RIT). In each case, for two aperture sizes (10% and 50% of the trap dimension), highly satisfactory fits are obtained. These composite approximations may be used in more detailed nonlinear ion dynamics studies than have been hitherto attempted.