Overdrive structures for flexible electrostatic switch

Details Overdrive structures for flexible electrostatic switch Overdrive structures for flexible electrostatic switch

2002-05-06

2004-05-04

Donovan, Lincoln (Department: 2832)

Electricity: electrical systems and devices

Electric charge generating or conducting means

Use of forces of electric charge or field

C200S181000, C361S207000

Reexamination Certificate

active

06731492

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to microelectromechanical switch and relay structures, and more particularly to overdrive structures to be used in conjunction with electrostatically activated switch and relay structures.
BACKGROUND OF THE INVENTION
Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices. One such device which leverages thermal expansion to move a microdevice is found in U.S. Pat. No. 5,475,318, entitled “Microprobe”, issued on Dec. 12, 1995, in the name of inventors Marcus et. al. In that device a micro cantilever is constructed from materials having different thermal coefficients of expansion; When heated, the bimorph layers arch differently, causing the micro cantilever to move accordingly. A similar mechanism is used to activate a micromachined thermal switch as described in U.S. Pat. No. 5,463,233, entitled “Micromachined Thermal Switch”, issued on Oct. 31, 1995, in the name of inventor Norling.
Electrostatic forces have also been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or Mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339, entitled “Method for Making Rolling Electrode for Electrostatic Device”, issued on May 12, 1981, in the name of inventor Kalt. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
Micromachined MEMS electrostatic devices have been created which use electrostatic forces to operate electrical switches and relays. Various MEMS relays and switches have been developed which use relatively rigid cantilever members separated from the underlying substrate in order to make and break electrical connections. Typically, contacts at the free end of the cantilever within these MEMS devices move as the cantilever deflects, so that electrical connections may be selectively established. As such, when the contacts are connected in these MEMS devices, most of the cantilever remains separated from the underlying substrate. For instance, U.S. Pat. No. 5,367,136, entitled “Non-Contact Two Position Microelectronic Cantilever Switch”, issued on Nov. 22, 1994, in the name of inventor Buck; U.S. Pat. No. 5,544,001, entitled “Electrostatic Relay”, issued on Aug. 6, 1996, in the name of inventors to Ichiya, et al., and U.S. Pat. No. 5,278,368, entitled “Electrostatic Relay”, issued Jan. 11, 1994, in the name of inventors Kasano, et al. are representative of this class of microengineered switch and relay devices.
Another class of micromachined MEMS switch and relay devices include curved cantilever-like members for establishing electrical connections. For instance, U.S. Pat. No. 5,673,785, entitled “Micromechanical Relay”, issued on Oct. 7, 1997, in the name of inventors Schlaak, et al., describe a microcantilever that curls as it separates from the fixed end of the cantilever and then generally straightens. The electrical contact is disposed at the generally straight free end of the microcantilever. When electrostatically attracted to a substrate electrode, the Schlaak devices conform substantially to the substrate surface except where the respective electrical contacts interconnect. In addition, a technical publication by Ignaz Schiele et al., titled Surface-Micromachined Electrostatic Microrelay, 1198, Sensors and Actuators, also describes micromachined electrostatic relays having a curled cantilever member. The Schiele cantilever initially extends parallel to the underlying substrate as it separates from the fixed end before curling away from the substrate. While the cantilever member having a contact comprises a multilayer composite, flexible polymer films are not used therein. As such, the Schiele devices do not describe having the cantilever member conform substantially to the underlying substrate in response to electrostatic actuation thereof.
MEMS electrostatic switches and relays are used advantageously in various applications because of their extremely small size. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations inherent in MEMS devices. However, problems may arise when these miniaturized devices are used in high voltage applications. Because MEMS devices include structures separated by micron scale dimensions, high voltages can create electrical arcing and other related problems. In effect, the close proximity of contacts within MEMS relays and switches multiplies the severity of these high voltage problems. Further, relatively high electrostatic voltages are required to switch high voltages. The air gap separation between the substrate electrode and moveable cantilever electrode affects the electrostatic voltage required to move the cantilever electrode and operate the switch or relay. A relatively large air gap is beneficial for minimizing high voltage problems. However, the larger the air gap, the higher the voltage required to operate the electrostatic switch or relay. As such, traditional MEMS electrostatic switch and relay devices are not well suited for high voltage switching applications.
Recent innovations have led to MEMS switches and relays that leverage the benefits of electrostatic forces and provide for devices capable of switching high voltages with relatively low electrostatic voltages. Additionally, these devices have shown to be instrumental in overcoming at least some of the arcing and high voltage operational problems. See for example, U.S. patent application Ser. No. 09/345,722, entitled “High Voltage Micromachined Electrostatic Switch”, filed on Jun. 30, 1999, in the name of inventor Goodwin-Johansson and assigned to the same assignee, MCNC, as the present invention. That application is expressly incorporated by reference as if fully setforth herein. A key attribute to the structures discussed in the aforementioned application is the availability of large electrostatic forces due to the flexible metallized polymer film coming into direct contact with the substrate that contains the stationary electrode.
In the switches and relays discussed in the Goodwin-Johansson '722 Application the switch contacts that are disposed in the substrate are typically designed as posts that extend slightly above the surface of the substrate structure. The release layer operation employed during switch fabrication generally results in the posts having a flat plan view topography. As such, the majority of the area of the contact in the flexible composite is generally the same spacing

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