Richard H. Morrison

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.

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.

Study of tunneling noise using surface micromachined tunneling tip devices

This paper reports the measurement and theoretical analysis of noise in surface micromachined tunneling tip devices. The devices are fabricated using the surface micromachining technology. The bending beams and proof masses are formed by electroplating of nickel and gold. The tips are made by plating metal on the partially etched sacrificial layer. Noise spectra are obtained and compared for different electrode materials and different operating pressures. The results show that 1/f noise dominates in the low frequency region and the 1/f noise level in the vacuum is lower than that in air ambient. A mathematical model is established to simulate the 1/f noise related to surface adsorption-desorption process.

Backside preparation and failure analysis for packaged microelectromechanical systems (MEMS)

Failure analysis tools and techniques that identify root cause failure mechanisms are key components to improving MEMS technology. Failure analysis and characterization are relatively simple at the wafer and die level where chip access is straightforward. However, analysis and characterization of packaged parts or components encapsulated with covers, caps, etc may be more cumbersome and lead to problems assessing the root cause of failure. This paper will discuss two methods used to prepare the backside of the package/device to allow for failure analysis and inspection of different MEMS components without removing the cap, cover, or lid on the device and/or the package. One method for backside preparation was grinding and polishing the package for IR inspection. This method involved backfilling the package cavity with epoxy to hold the die in place. The other method involved opening a window through the backside of the package, exposing the die for IR inspection. Failure analysis results showed both methods of backside preparation were successful in revealing the failure mechanisms on two different MEMS technologies.

Contact Resistance Measurements and Modeling of an Electrostatically Actuated Microswitch

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 µm wide, 1 µm thick, and 65 µm long. A voltage applied between the gate and the source closes the switch, connecting the source to the drain. Devices operate for more than 109 cycles in a nitrogen ambient before failure and have an initial contact resistance of less than 50 mΩ. The threshold voltages for these devices are between 30 V and 200 V. Long lifetimes are achieved with a current of 30 mA. The breakdown (stand-off) voltage between the source and the drain is greater than 100 V and the off-impedance exceeds 1012Ω. The modeling of these switches includes a structural model of the beam and a contact resistance model for the electrical contact. Preliminary contact resistance measurements are presented and compared with the modeled contact resistance characteristics.