Folded spring based micro electromechanical (MEM) RF switch

Details Folded spring based micro electromechanical (MEM) RF switch Folded spring based micro electromechanical (MEM) RF switch

1999-09-16

2001-10-23

Bettendorf, Justin P. (Department: 2817)

Wave transmission lines and networks

Long line elements and components

Switch

C361S233000

Reexamination Certificate

active

06307452

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to micro electromechanical systems (MEMS) and, in particular, to a micro machined switch having a folded spring structure that functions with signal frequencies from DC up to at least 50 GHz.
BACKGROUND OF THE INVENTION
Micro machined, integrated radio frequency (RF) switches are favored for telecommunications applications because they provide a large dynamic range between on-state and off-state impedances in the RF regime while avoiding bulk and high cost. However, because of the proximity of the electrodes in such micro machine switches, there is a problem with arcing induced stiction. This problem becomes significantly increased in MEMS in which the RE switches are mechanically operated at high speed and must handle high power signals at frequencies of above 5 GHzs.
MEMS systems described in U.S. Pat. No. 5,578,976 are only able to handle low power RF signals at signal frequencies only up to 4 GHzs while maintaining a relatively minimum insertion loss in the “on” state and relatively high electrical isolation in the “off” state. The patented micro machined switches contain a micro machined armed cantelever actuator which is fixed at one end as shown in
FIG. 1
reproduced from he '976 patent. In operation, as the cantelever actuator is switched on, only a line contact is made between the shorting bar and fixed electrical contacts of the switch. Therefore, the load carrying capacity of the micro switch is quite limited due to arcing and welding problems (See FIG.
2
).
Micro switches that are based on a membrane structure and are fixed on a plurality of sides to the substrate are described in U.S. Pat. No. 5,619,061. The membrane structure is susceptible to thin-film stresses and has little tolerance for deflections which may be produced during the in-situ processing of the membrane. Therefore, functional failures of the devise are commonplace.
Thus, there still exists a need in telecommunication systems for micro electromechanical switches that provide a wide dynamic impedance range from on to off at signal frequencies from DC up to at least 50 GHz and that have a large load carrying capacity.
SUMMARY OF THE INVENTION
The present invention comprises a micro machined micro electromechanical switch which is capable of handling up to 50 GHz signal frequencies while maintaining excellent electrical isolation in the “off” state and minimal insertion loss in the “on” state while carrying a large load.
In a preferred embodiment, the RE switch is fabricated on an electrical insulating substrate having a suspended micro platform connected by a plurality of folded spring suspensions to the substrate through anchor structures. The micro platform structure provides a simultaneous plane-to-plane electrical contact in an air gap formed by the signal line on the substrate. A top electrode on the platform forms a capacitor structure above the signal line on the substrate and the capacitor's structure may preferably include a grid of holes extending through the op electrode and the platform. The holes, preferably have dimensions comparable to the gap between the platform and the bottom electrode and are also useful to reduce the bulk of the device. The switch is actuated by application of a voltage to the top electrode. When voltage is applied, electrostatic forces attract the capacitor structure on the platform toward the signal line to close the gap in the signal line. The switch functions from DC to at least 50 GHz with an electrical isolation of 35 dB and an insertion loss of 0.5 dB at 20 GHz.
The process of forming the micromachined switches uses a low temperature (250° C.) process with a photodash mask so that the switch may be integrated with microwave and radio frequency integrated circuits. Applications for the micromachined switch are in the telecommunications industry, particularly in wireless cellular phones.
As demonstrated in the prototype of the present invention, the micro electromechanical RF switch can be switched from the normally off-state (open circuit) to the on-state with 25 volts and maintained in either state with nearly zero power. In low pressure ambient helium, closure time and disclosure time in the switch are on the order of 20 microseconds. The switch can handle a current of at least 50 mA.
An object of the RF switch is to precisely control the area of the signal line gap for desired RF isolation and to provide a low RF insertion loss with large power handling capability.


REFERENCES:
patent: 5578976 (1996-11-01), Yao
patent: 5619061 (1997-04-01), Goldsmith et al.
patent: 5880921 (1999-03-01), Tham

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