High-Cycle Life Testing of RF MEMS Switches

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

December 1, 2008

Air Force Research Laboratory

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.

A prototype RF MEMS Air-Gap Switch using patterned dielectric posts.

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.

The lower limit of operating voltage is determined by the minimum restoring force necessary to ensure that surface forces do not induce stiction of the switch membrane. Further, the minimum control voltage is also determined by the ability to achieve a repeatable tensile stress of the switch membrane during fabrication. Residual stress of aluminum films is dependent on the temperature history of the membrane prior to release. To fabricate these switches, tensile stresses in the 100-150 MPa range are the minimum repeatable values. With the given switch dimensions, this yields switches with approximately 30V pull-in voltage.

By changing from a continuous sheet of dielectric beneath the switch to an array of posts, the active area for charging can be reduced. The dielectric posts of 4-μm-diameter/8-μm pitch used in this switch has a fill factor of 25%, thereby reducing dielectric charging by 25%. This trades off switch capacitance for reduced charging and increased longevity.

In order to obtain high cycle counts in switch lifetime measurements, it is necessary to operate the switch as quickly as possible. With typical switching times of ~5-8 microseconds, operating at cycling frequencies above 50 kHz yields short onor off-times, with most of the period spent in transit between the two states. These switches were operated at 60 kHz, with a cycle period of 16.7 μS. In this case, the effective duty cycle for on-time was on the order of 10%.

The switch was operated at -30V bias using a trapezoidal waveform at a repetition frequency of 60 kHz. The effective duty cycle of the switch was 10%. At this rate, the switch accumulated cycles at the rate of 216 million cycles/hr. The switch was run for a total of 476 hours without failure, accumulating a total of 102.8 billion cycles.

This work was done by C.L. Goldsmith and D.I. Forehand of MEMtronics Corp.; Z. Peng and J.C.M. Hwang of Lehigh University; and John L. Ebel of the Air Force Research Laboratory.

AFRL-0105