High-Cycle Life Testing of RF MEMS Switches

Air Force Research Laboratory

Monday, December 01 2008

MEMS passive circuits are used as phase shifters or tunable filters in phased antenna arrays.

The electromechanical, RF, and charging properties of an “air-gap” capacitive switch enable it to be utilized in high-cycle life testing. Monitoring both high-speed and low-speed switching characteristics provides insight into quantifying the lifetime properties of the switch, and enable estimation of switching lifetime under a variety of operating conditions.

The figure illustrates a metal-dielectric-metal RF MEMS capacitive switch fabricated on a glass substrate. The top electrode is a 0.3-μm-thick flexible aluminum alloy membrane that is tied to DC and RF ground potential. The bottom switch electrode is composed of chromium/gold and serves as the center conductor of 50-Ω coplanar waveguide for the RF signal. Thick copper posts, approximately 3 μm tall, serve as the anchor points for the suspended MEMS membrane as well as the RF transmission line conductors. Without applied electrostatic force, the membrane is normally suspended in air 2.2 μm above the switch insulator. A control voltage in the range of 25-35 V, applied to the bottom electrode, pulls the membrane into contact with the dielectric, thus forming a 120 × 80 μm capacitor to shunt the RF signal to ground. When the control voltage is removed, the membrane springs back to its fully suspended position due to the restoring force of the membrane, resulting in little capacitive loading of the RF line.

Within this switch, the layer of switch insulator is composed of sputtered silicon dioxide 0.28 μm thick (εr~5.5). The switch insulator is not a continuous sheet of dielectric, but patterned into a series of insulating, hexagonal posts approximately 4 μm across on an 8 μm pitch. The patterned dielectric bumps create an “air-gap” or “proximity” switch, in which a larger percentage of the switch area utilizes air insulator rather than silicon dioxide. This use of patterned dielectric posts reduces the contact area accessible to dielectric charging. Trading off on-capacitance for reduced charging provides the opportunity for switching of moderate capacitance ratios (more like a switched reactance than a high isolation switch) with increased switch longevity.

Prior to the start of life testing, a thorough characterization of the electromechanical, RF, and dielectric charging performance was completed. The bistable switching characteristics of the MEMS devices were tested by probing the switches with a swept voltage through a capacitance meter. These measurements were taken at multiple locations across the wafer. The average pull-in voltage was 30.1V with a standard deviation of 3.8V. The release voltage of the switches was measured to be approximately 17V.

From the switch operating curves and appropriate test structures, the RF capacitive characteristics of the MEMS switch were extracted. The switch itself averages 44 fF of shunt off-capacitance, of which approximately 30 fF is plate capacitance and 14 fF is fringing capacitance. This switch capacitance is partially compensated with inductive feedlines in and out of the switch, yielding an effective shunt capacitance of ~15-20 fF in off-state. In the on-state, the proximity switch described above possesses 0.28 pF to 0.34 pF of on-capacitance. In a 50-ohm system, this switch provides a reactance ratio, approximately (280 ff to 340 ff)/(15 ff to 20 ff), in the range of 15:1 to 20:1. The RF properties of these switches at microwave and millimeter-wave frequencies were measured on a vector network analyzer. The RF insertion loss of the MEMS switch is typical of most modern MEMS switches, with less than 0.1 dB insertion loss through 40 GHz.

Transient current measurements were made over a voltage range of 30-60 volts to quantify the change density as a function of both time and voltage. This characterization was only completed for negative polarity, as this material commonly exhibits significantly higher charging for a positive polarity drive voltage. The test devices used for this characterization were MIM capacitors fabricated on the same wafers as the MEMS switches.

The most effective methods for reducing dielectric charging within MEMS switches are to reduce the operating voltage, reduce the dielectric area, and/or reduce the operating duty cycle. In order to achieve high cycle lifetime, all three of these techniques were utilized to minimize the amount of dielectric charging present in the switch.