Jeremy B. Muldavin

RF MEMS switches and switch circuits

MEMS switches are devices that use mechanical movement to achieve a short circuit or an open circuit in the RF transmission line. RF MEMS switches are the specific micromechanical switches that are designed to operate at RF-to-millimeter-wave frequencies (0.1 to 100 GHz). The forces required for the mechanical movement can be obtained using electrostatic, magnetostatic, piezoelectric, or thermal designs. To date, only electrostatic-type switches have been demonstrated at 0.1-100 GHz with high reliability (100 million to 10 billion cycles) and wafer-scale manufacturing techniques. It is for this reason that this article will concentrate on electrostatic switches.

High-isolation CPW MEMS shunt switches. 1. Modeling

This paper, the first of two parts, presents an electromagnetic model for membrane microelectromechanical systems (MEMS) shunt switches for microwave/millimeter-wave applications. The up-state capacitance can be accurately modeled using three-dimensional static solvers, and full-wave solvers are used to predict the current distribution and inductance of the switch. The loss in the up-state position is equivalent to the coplanar waveguide line loss and is 0.01-0.02 dB at 10-30 GHz for a 2-/spl mu/m-thick Au MEMS shunt switch. It is seen that the capacitance, inductance, and series resistance can be accurately extracted from DC-40 GHz S-parameter measurements. It is also shown that dramatic increase in the down-state isolation (20/sup +/ dB) can be achieved with the choice of the correct LC series resonant frequency of the switch. In part 2 of this paper, the equivalent capacitor-inductor-resistor model is used in the design of tuned high isolation switches at 10 and 30 GHz.

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.

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.

High-isolation W-band MEMS switches

This paper presents the design, fabrication and measurement of single, T-match and /spl pi/-match W-band high-isolation MEMS shunt switches on silicon substrates. The single and T-match design result in -20 dB isolation over the 80-110 GHz range with an insertion loss of 0.25/spl plusmn/0.1 dB. The /spl pi/-match design results in a reflection coefficient lower than -20 dB up to 100 GHz, and an isolation of -30 to -40 dB from 75 to 110 GHz (limited by leakage through the substrate). The associated insertion loss Is 0.4/spl plusmn/0.1 dB at 90 GHz. To our knowledge, this is the first demonstration of high-performance MEMS switches at W-band frequencies.

Millimeter-wave tapered-slot antennas on synthesized low permittivity substrates

This paper presents 30-GHz linear-tapered slot antennas (LTSA) and 94-GHz constant-width slot antennas (CSWA) on synthesized low dielectric constant substrates (/spl epsiv//sub r/=2.2). The performance of tapered-slot antennas (TSA) is sensitive to the effective thickness of the substrate. We have reduced the effective thickness by selectively machining holes in the dielectric substrate. The machined substrate antenna radiation patterns were significantly improved independent of the machined hole size or lattice as long as the quasi-static effective thickness remained the same, even if the hole/lattice geometry is comparable to a wavelength. The method was applied at 94 GHz on a CSWA with excellent radiation pattern improvement, making it suitable for f/1.6 imaging array applications.

30 GHz tuned MEMS switches

This paper demonstrates the use of resonant tuning in high-isolation reflective MEMS electrostatic switches. Tuned switches can achieve higher isolation and a lower pulldown voltage than a comparable single element switch. An equivalent circuit model was developed for individual shunt capacitive membrane switches and then implemented in tuned circuits. The novel cross switch was developed on a high resistivity silicon. The cross switch attained an insertion loss of less than 0.6 dB and a return loss below -20 dB from 22-38 GHz in the up-state, and a down-state isolation of 50 dB with only 1.1 pF of down-state capacitance (Cd) per element. The pulldown voltage is 15-20 V, which is much better than typical industry numbers of 28-50 V. Application areas are low-loss high-isolation communication switches at 28 GHz and automotive switches at 77 GHz.

Power handling and linearity of MEM capacitive series switches

This paper presents the power handling and linearity of a capacitive series MEMS switch. The switching time as a function of incident RF power is also discussed. The MIT Lincoln Laboratory series capacitive MEMS switch handled nearly 10 Watts of RF power under cold switching conditions and up to 1.7 Watts of RF power under hot switching conditions. The power handling is a function of the pull-down voltage of the switch and the frequency of the RF signal.

Large tuning range analog and multi-bit MEMS varactors

This paper presents a variety of analog and multi-bit RF MEMS varactors for use in tunable circuits and filters. Novel use of multiple pull-down electrodes covered with thick dielectrics allow for analog and digital-type control and long-hold down times. An analog varactor with nearly 7:1 usable tuning range and better than 80:1 switching range, a multi-state varactor-switch and a 4 bit capacitor bank are presented.

Wafer-Scale Packaged RF Microelectromechanical Switches

This paper presents results of fully packaged RF microelectromechanical (RF-MEM) switches including capacitive series, series-shunt, and single-pole-four-throw (SP4T) switch nodes. The RF-MEM capacitive switches are packaged using recently developed wafer scale low-loss and broadband packaging technology developed at MIT Lincoln Laboratory, Lexington, MA. A packaged series capacitive switch with 0.11-dB insertion loss and better than 19-dB isolation, a series-shunt packaged capacitive switch with 0.3-dB insertion loss and better than 54 dB isolation, and an SP4T switch with less than 0.26-dB insertion loss and better than 25-dB isolation at 20 GHz are reported. Detailed reliability, radiation, cryogenic, and power-handling data are also presented.