Electrostatic actuators and tunable micro capacitors/switches with expanded tuning range due to intrinsic stress gradients

Novel zipper varactors with the potential for achieving large tuning ranges have been fabricated and characterized. These varactors have a curved cantilever electrode that is actuated by a single pull-down electrode. The shape of the cantilever is designed such that its local stiffness is tailored to enable extended stable zipping. In a series-mounted varactor, the measured capacitance ratio was 16.5 for actuation voltages between 0 and 46 V. However, the presence of an unexpected tuning instability at 32 V bias limited the practical tuning range in this varactor. This behaviour was attributed to fabrication imperfections. The first electrical self-resonant frequency of the same varactor was extrapolated to be 72.6 and 17.2 GHz at 20 and 329 fF, respectively. In a different shunt-mounted varactor, the quality factor (Q) was measured to be 91 (60 fF) and 176 (600 fF) at 2 GHz. Including the anchor, the varactors have a small device footprint and fit within an area of 500 by 100 μm.

Section: Varactor Designmentioning

confidence: 99%

Stable zipping RF MEMS varactors

Holmes²,

Yeatman³

et al. 2010

“…A conventional MEMS parallel-plate capacitor is made of two electrodes, one of which is usually patterned on the substrate and the other one is suspended by a set of supporting beams (also known as mechanical springs). Assuming that the moving electrode is rigid and the beams have linear stiffness coefficients, it has been shown that for electrostatically actuated capacitors the characteristic capacitance-voltage (C-V) response is highly nonlinear and leads to structural instability at pull-in, when the electrostatic force overcomes the balancing force of the beams [11]. The tunability of a conventional capacitor (defined as the percentage of capacitance increase [(C max − C min )/C min ] × 100) is limited to 50% due to pull-in.…”

Section: Introductionmentioning

confidence: 99%

Linearization and tunability improvement of MEMS capacitors using flexible electrodes and nonlinear structural stiffness

Shavezipur

Nieva

Hashemi

et al. 2012

This paper proposes solutions for high nonlinearity and structural instability in electrostatically actuated MEMS capacitors. The proposed designs use the flexibility of the moving electrode and nonlinear structural stiffness to control the characteristic capacitance–voltage (C–V) response. The moving plate displacements are selectively constrained by mechanical stoppers to prevent sudden jumps in the capacitance and to eliminate the pull-in. A symmetric double-humped electrode shape is utilized which results in a fairly constant sensitivity in the C–V curve and therefore a linearized response. An analytical and a finite-element coupled-field model are developed to study the behavior of the proposed capacitors and to optimize their design for maximum linearity. The experimental results verify that the designs introduced in this paper improve the linearity of the C–V response and increase the maximum tunability by three times compared to conventional MEMS parallel-plate capacitors. At the same time, they also eliminate the pull-in hysteresis of the response.

“…The tuning range in conventional designs (i.e. with rigid and flat moving electrode and beams with linear stiffness coefficients) is limited to 50% at g = 2/3g 0 (refer to figure 1(a)) at which structural instability and pull-in occurs [8]. At this point, the moving electrode collapses on the fixed one, resulting in a sudden jump in the capacitance if the two electrodes are separated with an insulation layer or a short circuit if the two electrodes directly touch one another.…”

Section: Introductionmentioning

confidence: 99%

“…Improvement in the tuning range of MEMS parallelplate tunable capacitors has been the center of attention in the past decade and exploiting different design techniques, novel devices have been developed that offer tunabilities up to 1700% [6][7][8]. However, in many cases, the resulting C-V responses display an intensified nonlinearity at pull-in where the device behaves like a switch, jumping from a small capacitance value to a much larger one for a small change in tuning voltage [9].…”

Section: Introductionmentioning

confidence: 99%

Development of parallel-plate-based MEMS tunable capacitors with linearized capacitance–voltage response and extended tuning range

Shavezipur

Nieva

Khajepour

et al. 2009

This paper presents a design technique that can be used to linearize the capacitance-voltage (C-V) response and extend the tuning range of parallel-plate-based MEMS tunable capacitors beyond that of conventional designs. The proposed technique exploits the curvature of the capacitor's moving electrode which could be induced by either manipulating the stress gradients in the plate's material or using bi-layer structures. The change in curvature generates a nonlinear structural stiffness as the moving electrode undergoes out-of-plane deformation due to the actuation voltage. If the moving plate curvature is tailored such that the capacitance increment is proportional to the voltage increment, then a linear C-V response is obtained. The larger structural resistive force at higher bias voltage also delays the pull-in and increases the maximum tunability of the capacitor. Moreover, for capacitors containing an insulation layer between the two electrodes, the proposed technique completely eliminates the pull-in effect. The experimental data obtained from different capacitors fabricated using PolyMUMPs demonstrate the advantages of this design approach where highly linear C-V responses and tunabilities as high as 1050% were recorded. The design methodology introduced in this paper could be easily extended to for example, capacitive pressure and temperature sensors or infrared detectors to enhance their response characteristics.