Pull-down Electrodes Research Articles

Design, simulation and analysis of a digital RF MEMS varactor using thick SU-8 polymer

This paper presents the design, simulation and analysis of a 5-bit RF MEMS digital variable capacitor using thick SU-8 polymer to achieve both high capacitance ratio (varvec{C}_{varvec{r}}) and Q-factor. The proposed varactor design consists of five capacitive shunt switches loaded over a co-planar waveguide (CPW) line. This results 32 capacitance steps ranging from 102.23 fF to 3.57 pF (varvec{C}_{varvec{r}} = 35). The high capacitance ratio of the varactor is achieved by implementing SU-8 polymer as the base structure of the capacitor due to its low dielectric constant (varvec{varepsilon}_{varvec{r}} = 4) hence reducing the overall minimum capacitance (varvec{C}_{{varvec{min}}}). The optimization of the SU-8 thickness has been carried out to get the optimum capacitance ratio and Q-factor. Moreover, the inclusion of semi-elliptical slot at the ground of CPW and variable sizes of CPW line further reduce the parasitic capacitance. To improve the reliability of the variable capacitor design, a new aluminium (Al) stopper design is used to prevent contact between the beams and the pull-down electrodes. Mechanical simulations and analysis have been carried out to investigate the effects of in-plane residual stress and stress gradient on the spring constant and the initial displacement of the proposed horizontal truss fixed–fixed beam structure. Compared to the conventional doubly clamped solid beam design, the proposed beam has less sensitivity to in-plane residual stress. This makes the pull in voltage to be less affected by the stress due to the fabrication. The simulated pull-in and lift-off voltages are 30 and 25 V respectively while the estimated switching time for the beam is 4.64 μs. The overall size of the varactor is 740 μm × 653 μm. The proposed RF MEMS varactor could be integrated in reconfigurable filters, phase shifters and matching networks targeting future multi-standard wireless communication systems.

Electrostatic fringing-field actuation for pull-in free RF-MEMS analogue tunable resonators

This paper presents the design, fabrication and measurement of the first pull-in free tunable evanescent-mode microwave resonator based on arrays of electrostatically actuated fringing-field RF-MEMS tuners. Electrostatic fringing-field actuation (EFFA) is the key on achieving a wide tunable frequency range that is not limited by the conventional pull-in instability. Furthermore, total lack of dielectric layers and no overlap between the pull-down electrode and movable beams significantly enhance the robustness of our proposed tuning mechanism by making it devoid of dielectric charging and stiction and amenable to high-yield manufacturing. The proposed electrostatic fringing-field tuners are demonstrated in a highly loaded evanescent-mode cavity-based resonator. The measured unloaded quality factor is 280–515 from 12.5 to 15.5 GHz. In addition, a 10× improvement in switching time is demonstrated for the first time for EFFA tuners in a tunable microwave component by employing dc-dynamic biasing waveforms. With dynamic biasing, the measured up-to-down and down-to-up switching times of the resonator are 190 and 148 μs, respectively. On the other hand, conventional step biasing results in switching times of 5.2 and 8 ms for up-to-down and down-to-up states, respectively.

Stable zipping RF MEMS varactors

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.

High-Reliability Miniature RF-MEMS Switched Capacitors

This paper presents a radical departure from the standard RF microelectromechanical systems (MEMS) devices and introduces novel miniature MEMS switched capacitors for RF to millimeter-wave applications, which are around 150 times smaller in lateral dimensions than standard MEMS designs. The measured capacitance of a single device (l,w,t of 20, 9.5, 0.37 mum), suspended 0.25 mum above the pull-down electrode, is 3.9 and 11.7-12.1 iF (Cr = 3.0-3.1) in the up- and down-state positions, respectively. The measured capacitance ratio of a 4 X 4 array fabricated on quartz substrates and in a coplanar waveguide (CPW) configuration is 3.0 with an electrical Q > 90 at 20 GHz. The miniature MEMS spring constant is very high, and is much less sensitive to residual stress or temperature variations than standard MEMS devices. Dielectric charging simulations show that these devices can withstand a charge density 6.25 times higher than the standard MEMS devices. The mechanical resonant frequency for a gold device is 2.6 MHz and results in a switching time of 200 ns under a 32-V actuation voltage. Preliminary reliability tests at 13 GHz using bipolar actuation (plusmn30 V) and hot power switching have been performed on five 4 times 4 devices at 100 mW for 20 billion cycles, and at 500 mW for 5 billion cycles with no failures. A 4 times 4 array has also been used in the design of a tunable CPW resonator at 19.3 GHz with a 21% tuning range and 1.6-dB insertion loss. The authors believe that miniature MEMS devices will be very useful in the future for high-reliability MEMS and reconfigurable networks.

A DC-contact MEMS shunt switch

This paper presents the design, fabrication, and performance of a metal-to-metal contact micro-electro-mechanical (MEMS) shunt switch. The switch is composed of a fixed-fixed metal beam with two pull-down electrodes and a central DC-contact area. The switch is placed in an in-line configuration in a coplanar waveguide transmission line. This topology results in a compact DC-contact shunt switch and high isolation at 0.1-18 GHz. The isolation at MM-wave frequencies is limited by the inductance to ground and is -20 dB at 18 GHz. The application areas are in wireless communications and high-isolation switching networks for satellite systems.

All-metal high-isolation series and series/shunt MEMS switches

This paper presents a novel all-metal series switch with several different pull-down electrode geometries. The switch results in an up-state capacitance of 5-9 fF and an isolation of -25 to -30 d8 at 10 GHz. The fabrication process is completely compatible with the standard capacitive (or dc-contact) shunt switch, A dc-30 GHz series/shunt switch is also presented with an isolation of -60 dB at 5 GHz and -42 dB at 10 GHz. This is the highest isolation switch available to-date. The performance is limited by radiation in the CPW lines and not by the series/shunt switch characteristics. The application areas are in high-isolation switches for basestations and satellite systems.

Inline capacitive and DC-contact MEMS shunt switches

This paper presents inline capacitive MEMS shunt switches suitable for X/K-band and Ka/V-band applications. The inline switch allows for a low- or high-inductance connection to the ground plane without changing the mechanical characteristics of the MEMS bridge. Excellent isolation and loss are achieved with this design, and the performance is very similar to the standard capacitive MEMS shunt switch. Also, a new metal-to-metal contact MEMS shunt switch is presented. A novel pull-down electrode is used which applies the electrostatic force at the same location as the metal-to-metal contact area. A contact resistance of 0.15-0.35 /spl Omega/ is repeatable, and results in an isolation of -40 dB at 0.1-3 GHz. The measured isolation is still better than -20 dB at 40 GHz. The application areas are in high-isolation/low-loss switches for telecommunication and radar systems.