Sumit Majumder

Micromechanical switches fabricated using nickel surface micromachining

Micromechanical switches have been fabricated in electroplated nickel using a four-level surface micromachining process. The simplest devices are configured with three terminals, a source, a drain, and a gate and are 30 /spl mu/m wide, 1 /spl mu/m thick, and 65 /spl mu/m long. A voltage applied between the gate and source closes the switch, connecting the source to the drain. Devices switch more than 10/sup 9/ cycles before failure and exhibit long-lifetime hot switching currents up to 5 mA. The initial contact resistance is less than 50 m/spl Omega/. The breakdown (stand-off) voltage between the source and the drain is greater than 100 V and the off-current is less than 20 fA at 100 V.

Study of contacts in an electrostatically actuated microswitch

Surface micromachined, electrostatically actuated microswitches have been developed at Northeastern University. Microswitches have an initial contact resistance of 0.5-1 /spl Omega/, and current handling capability of about 20 mA. Typically, contact resistance degrades progressively when the switches are cycled beyond approximately 10/sup 6/ cycles. In this work, the microswitch contact resistance is studied on the basis of a simple, clean metal contact resistance model. Comparison of measured contact resistance (measured as a function of contact force) with the characteristics predicted by the model shows the measured resistance to be higher than the prediction, approximately by an order of magnitude, suggesting that insulating films at the contact interface need to be taken into account. Microswitches with a large number of parallel contacts have also been developed, and measurement data is presented showing that these devices have a current handling capability greater than 150 mA.

A packaged, high-lifetime ohmic MEMS RF switch

An electrostatically actuated broadband ohmic microswitch has been developed that has applications from DC through the microwave region. The microswitch is a 3-terminal device based on a cantilever beam and is fabricated using an all-metal, surface micromachining process. It operates in a hermetic environment obtained through a wafer-bonding process. Characteristics of the wafer-level packaged switch include DC on-resistance of less than 1 ohm with an actuation voltage of 80 V, lifetime of greater than 10/sup 10/ cycles with on-resistance variation of less than 0.2 Ohm and current handling capability of 1 A. Key RF characteristics at 2 GHz include an insertion loss of 0.32 dB and isolation of 33 dB for our 4-contact microswitch. Preliminary measurements at higher microwave frequencies are extremely promising with full characterization and planned product improvements underway.

Electrostatically actuated micromechanical switches

Micromechanical switches and relays realized by a simple surface micromachining process are presented. The devices use a nickel cantilever beam and gold-to-gold electrical contact, and are electrostatically actuated. They have lifetimes exceeding 109 cycles, and a current handling capability of 150 mA.

Measurement and modelling of surface micromachined, electrostatically actuated microswitches

Electrostatically actuated micromechanical switches have been reported in earlier work by the authors. In the present work, a simple contact resistance model of the microswitch is discussed. Preliminary contact resistance measurements are presented, and compared with the modeled contact resistance characteristics.

Lifetime characteristics of ohmic MEMS switches

In the future, MEMS switches will be important building blocks for designing phase shifters, smart antennas, cell phones and switched filters for military and commercial markets, to name a few. Low power consumption, large ratio of off-impedance to on-impedance and the ability to be integrated with other electronics makes MEMS switches an attractive alternative to other mechanical and solid-state switches. Radant MEMS has developed an electrostatically actuated broadband ohmic microswitch that has applications from DC through the microwave region. The microswitch is a 3-terminal device based on a cantilever beam and is fabricated using an all-metal, surface micromachining process. It operates in a hermetic environment obtained through a wafer-bonding process. We have developed PC-based test stations to cycle switches and measure lifetime under DC and RF loads. Best-case lifetimes of 1011 cycles have been achieved in T0-8 cans (a precursor to our wafer level cap) while greater than 1010 cycles have been achieved in the wafer level package. Several switches from different lots have been operated to 1010 cycles. Current typical lifetime exceeds 2 billion cycles and is limited by contact stiction resulting in stuck-closed failures. Stuck-closed failures can be intermittent with a large number of switches continuing to operate with occasional sticks beyond several billion cycles. To eliminate contact stiction, we need to better control the ambient gas composition in the die cavity. We expect lifetime to improve as we continue to develop and optimize the wafer capping process. We present DC and RF lifetime data under varying conditions.

Adhesion and contact resistance in an electrostatic MEMS microswitch

A multi-asperity model of the contact resistance in a MEMS microswitch has been developed which includes the effects of elastic and plastic deformation, adhesion, and constrictive resistance. The number of asperities in contact is small enough that a discrete distribution of asperity heights is used. Adhesion produces significant hysteresis in the contact resistance vs. contact force characteristics. Due to the combined effects of plasticity and adhesion, the surface profile changes during the first load/unload cycle. Measurements of the contact resistance as a function of actuation voltage show good qualitative agreement with the model.

Finite element analysis of the thermal characteristics of MEMS switches

Electrostatically actuated microswitches and relays have been developed at Northeastern University and Analog Devices, Inc. Here, we report a steady-state thermal-electrical finite element model of microswitches with gold-gold contacts. The modeling results show that in a microswitch with a typical geometry, the thermal constriction occurs in the thin film trace leading up to the contact, and not at the contact interface. The model correctly predicts the switch voltage at which the drain trace melts, but underestimates the switch resistance, and therefore overestimates the failure current. SEM images indicate that the contact area increases significantly with current.

Low-loss, MEMS based, broadband phase shifters

Broadband, true time delay phase shifters based on MEMS switches provide the high instantaneous bandwidth desired for applications such as multi-frequency antenna arrays with multiple functions such as radar, communications, and ECM. MEMS based phase shifters offer significant performance advantages compared to technologies using PIN diode or MESFET switches including high linearity that does not limit the dynamic range of the receiver in a crowded signal environment. Furthermore, MEMS phase shifters offer a combination of low RF insertion loss and low switch control power consumption that is unmatched by PIN diode or MESFET technologies, giving the MEMS phase shifters a decided advantage at reducing prime power consumption in airborne or space based systems. Radant MEMS Inc., employing its high power 10 Watt MEMS switch with a demonstrated 1 trillion cycle lifetime, designed, fabricated and tested a 6-bit, 5 Watt, true time delay, 1–6 GHz, MEMS phase shifter that, with the exception of the three longest delay lines, was integrated on a single silicon die. This compact, low cost phase shifter is hermetically sealed using the same wafer capping process now employed for Radants MEMS switches.

Contact Resistance and Adhesion in a MEMS Microswitch

A multi-asperity model of the contact resistance in a MEMS microswitch has been developed. This model includes the effects of elastic and plastic deformation, adhesion, and scale-dependent constrictive resistance. The number of asperities in contact is small enough that a discrete, rather than a continuous, distribution of asperity heights is used. Due to the combined effects of plasticity and adhesion, the surface profile changes with repeated load and unload cycles. Furthermore, adhesion produces significant hysteresis in the contact resistance vs. contact force characteristics. Measurements of the contact resistance as a function of actuation voltage show good qualitative agreement with the model.Copyright