Stress bimorph MEMS switches and methods of making same

Abstract

A micro-electromechanical system (MEMS) switch formed on a substrate, the switch comprising a transmission line formed on the substrate, a substrate electrostatic plate formed on the substrate, and an actuating portion. The actuating portion comprises a cantilever anchor formed on the substrate and a cantilevered actuator arm extending from the cantilever anchor. Attraction of the actuator arm toward the substrate brings an electrical contact into engagement with the portions of the transmission line separated by a gap, thus bridging the transmission line gap and closing the circuit. In order to maximize electrical isolation between the transmission line and the electrical contact in an OFF-state while maintaining a low actuation voltage, the actuator arm is bent such that the minimum separation distance between the transmission line and the electrical contact is equal to or greater than the maximum separation distance between the substrate electrostatic plate and arm electrostatic plate.

Description

BACKGROUND [0001] 1. Field [0002] The present invention relates to micro-electromechanical systems (MEMS) and, in particular, to a micromachined electromechanical radio frequency (RF) switch that can preferably function over a range of signal frequencies from 0 Hz to approximately 100 GHz. [0003] 2. Description of Related Art [0004] MEMS (micro-electromechanical system) switches have a wide variety of uses in both military and commercial applications. For example, electrostatically actuated micro-electromechanical switches can conduct RF current in applications involving the use of antenna phase shifters, in the tuning of reconfigurable antenna elements, and in the fabrication of tunable filters. [0005] A representative example of a prior art MEMS switch is disclosed in Yao, U.S. Pat. No. 5,578,976, issued Nov. 26, 1996. Typically, this type of MEMS switch is fabricated on a semi-insulating substrate with a suspended micro-beam element as a cantilevered actuator arm. The cantilever ...

AI Technical Summary

Benefits of technology

[0015] Embodiments of electromechanical switches according to the present invention minimize the OFF-state capacitance of the electrostatically actuatable micro-electromechanical switch formed on a substrate, without a corresponding increase in the voltage required to actuate the switch. Embodiments of the present invention achieve minimization of the OFF-state capacitance by utilizing an actuator arm bent such that the minimum separation distance between an electrical contact formed on the actuator arm and a transmission line formed on the substrate is equal to or greater than the maximum separation distance between a substrate electrostatic plate formed on the substrate and an arm electrostatic plate formed on the actuator arm. The bilaminar cantilever structure of the preferred embodiments enable a large separation (up to approximately 300 micrometers) to be achieved between the transmission line formed on the substrate and the electrical contact formed on the actuator arm, while maintaining a very low actuation voltage (approximately 20 V). This large separation can be used to reduce the capacitance of the RF MEMS switch in the OFF state, thus providing high isolation at high frequencies.

[0016] The desired minimization of the OFF-state capacitance is achieved without a corresponding increase in the actuation voltage by forming the arm electrostatic plate at a point on the actuator arm that allows the distance between the arm electrostatic plate, formed on the actuator arm, and the substrate electrostatic plate, formed on the substrate, to be precisely and repeatably controlled, thus allowing the actuation voltage to be correspondingly controlled.

[0017] The tendency of the beam to stick to the substrate during drying is reduced by forming the bend in the actuator arm through the generation of unbalanced residual stresses in either the polycrystalline silicon comprising the actuator arm or the metallic layer formed on the actuator arm, this metallic layer comprising the arm electrostatic plate. The unbalanced residual stresses can be generated by manipulation of deposition process parameters during formation of the actuator arm structure. Due to these residual stresses in the actuator arm structure, the actuator arm is in a stressed condition prior to release from the sacrificial layer and will tend to bend away from the substrate when released. This counters the tendency of the arm to deflect toward the substrate in response to surface tension forces exerted by the release solution.

Problems solved by technology

Thus, the distance between the metal contact and the RF signal line when the RF MEMS switch is in the open position is limited to the distance between the cantilever actuator arm and the substrate along nearly the entire length of the cantilever actuator arm.

However, integrated RF MEMS switches are difficult to implement.

Due to the proximity of the electrical contact formed on the cantilever arm to the signal line formed on the substrate, these switches tend to exhibit poor electrical isolation at high frequencies.

These losses, which increase with signal frequency, limit the use of MEMS switches in high frequency applications.

However, in the MEMS switch described above, there is a design tradeoff between the OFF-state capacitance and the switch actuation voltage.

Another problem with the conventional cantilever switch described above stems from the methods used to manufacture the switch.

One problem with the use of this release process for a beam in relatively close proximity to the substrate is that surface tension forces exerted by the release agent tend to pull the beam toward the substrate as the device is immersed in and pulled out of the solutions.

The unbalanced residual stresses can be generated by manipulation of deposition process parameters during formation of the actuator arm structure.

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