Microfabricated double-throw relay with multimorph actuator...

Details Microfabricated double-throw relay with multimorph actuator... Microfabricated double-throw relay with multimorph actuator...

2001-10-29

2003-01-07

Barrera, Ramon M. (Department: 2832)

Electricity: circuit makers and breakers

Electrostrictive or electrostatic

C310S307000, C310S309000, C310S330000, C310S332000

Reexamination Certificate

active

06504118

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
None of the research and development leading to the present invention was Federally sponsored.
BACKGROUND OF THE INVENTION
This invention pertains to the general field of switching devices, and more specifically, to the field of microfabricated relays. Since the original concept of a microfabricated switching device was created by Petersen in 1979, many attempts have been made to develop switches and relays for applications of low power and high frequency. The goal of this work is to improve the cost-effectiveness and performance of switching technologies by using miniature, batch-fabricated, photolithographically-defined, moveable structures as part of a mechanical device.
Microfabricated electromechanical systems (MEMS) promise high lifetimes, low cost, small sizes, and faster speeds than switching devices manufactured by conventional means, and offer higher performance than solid-state devices. In many applications, particularly those in high performance instrumentation, automated test equipment, radar, and communication systems, switching devices with certain qualities are required or preferred. Specific values vary by application and are quantified where appropriate in the detailed description of the invention:
1) Relay rather than switch functionality, to isolate control signals from load signals
2) Low resistance Ohmic-contacts between the relay electrodes
3) Low power usage to toggle relay open/close states
4) Zero or very low power to maintain a particular relay open/close state
5) High precision, low cost manufacturing
6) High speed, high force mechanical closure of relay contacts
7) High speed, high force mechanical opening of relay contacts
8) Easily achieved control signals and operating requirements
Many switching device development efforts have been undertaken to obtain some of these advantages, but none have succeeded in attaining all. The switching device designs of prior art can be largely discussed in terms of two major categories of devices: those employing electrostatic actuating mechanisms and those employing bimorph actuating mechanisms. Each type of actuating mechanism has intrinsic qualities and advantages, as well as physical limitations preventing prior designs from obtaining every desirable quality listed above. These devices and mechanisms are described below, with the majority of prior microfabricated relay devices featuring single-throw actuation. Single throw actuation refers to the making and breaking of a single electrical contact when actuated, whereas double throw actuation refers to the breaking of one electrical contact and the making of a second contact when actuated.
Electrostatically actuated devices employ two (or more) bias electrodes across which a voltage is applied. Opposite charges are generated on the surfaces of the facing electrodes, and an electrostatic force is generated. If the bias electrodes are allowed to deflect towards each other, actuation is enabled. The switch or relay contact electrodes in an electrostatically actuated device would be mechanically coupled to these moving bias electrodes, so that the contact electrodes would mate together or separate as the voltage was applied and removed.
Electrostatic actuation intrinsically supports a number of the operating qualities described, and, as a result, is the most widely examined MEMS actuation mechanism for switches and relays. Electrostatic actuators enable Ohmic-contact relays and switches, although low resistances are difficult to achieve. They require effectively zero power to toggle states and effectively zero power to maintain states. A designer can employ microfabrication techniques to develop precise, low-cost electrostatic actuators. These actuators can provide high speeds, but high closure force is difficult to achieve, and they are not amenable to developing high opening forces. These actuators are difficult to design with low drive voltages (less than 10 V) typical of modem integrated circuits, though drive currents are typically negligible (less than 1 &mgr;A).
The literature contains numerous examples of electrostatic MEMS switches and relays demonstrating low force actuation with very low power usage. Loo, et al., U.S. Pat. No. 6,046,659, describes a typical example of a single-throw, double-contact cantilever MEMS relay, employing an insulator-metal-insulator stack for stress compensation. Other cantilever MEMS devices employ different contact metals for improved performance, such as a relay by Yao, et al., U.S. Pat. No. 5,578,976, and a switch by Buck, U.S. Pat. No. 5,258,591. James, et al., U.S. Pat. No. 5,479,042, has double contact relays incorporating bumps to improve manufacturing. Zavracky, U.S. Pat. No. 5,638,946, adds a novel element for actuation, using separate fixed electrodes for biasing, after his early work in solid metal switches. The literature includes switching device work by Milanovi, et al. wherein devices are transferred from one substrate to another for improved high-frequency signal switching.
Several notable attempts have been made to improve performance at larger signal loads, typically by increasing device size and force at the expense of size, speed, and, reliability. A typical example is that of Lee, U.S. Pat. No. 6,054,659, with a copper device an order of magnitude larger and more forceful than the efforts previously noted. Komura et al. and Sato et al. have also developed millimeter-sized two-contact electrostatic MEMS relays for moderate signal loads. A device by Goodwin-Johansson, U.S. Pat. No. 6,057,520, reduces arcing under hot-switch conditions by varying the contact resistance of electrodes as the device opens and closes.
A few electrostatic MEMS switching devices have been designed to lower drive voltage requirements at the expense of device size, contact force, and, often, manufacturing disadvantages. Shen et al. and Pacheco have reduced voltage requirements by increasing bias electrode size and armature flexibility. Ichiya, et al., U.S. Pat. No. 5,544,001, incorporates novel use of stepped and sloped substrate bias electrodes for reducing drive voltage.
A few electrostatic MEMS devices have been designed with sets of bias electrodes to open the device with increased speed and force as compared to the passive restoring forces of deflected springs more typically found in MEMS devices. Hah, et al. is a typical example, combining torsional spring restoring forces with opposing bias electrodes to drive relays open. Kasano, et al., U.S. Pat. No. 5,278,368, describes a double-contact MEMS relay with drive-open electrodes as well as novel embedded electrets to reduce overall voltage requirements.
Bimorph actuators, unlike electrostatic actuators, transduce the control signals into mechanical deformation within the actuator itself. Bimorph (or, more generally, multimorph) actuators are comprised of layers demonstrating different physical responses to a particular stimulus. A thermal bimorph, for example, might have a first layer with a high coefficient of thermal expansion (above 10 ppm/° C.) and a second layer with a low coefficient of thermal expansion (below 5 ppm/° C.). When this bimorph is exposed to an increase in temperature, the relative expansion of the first layer is constrained by the intimate contact to the second layer, and the actuator curls in response. Devices employ this curl to perform work, and the forces generated by bimorphs can be much higher than those attainable by electrostatic actuators.
Bimorph actuation also intrinsically supports a number of the operating qualities described above, and, as a result, is the second most widely examined MEMS actuation is mechanism for switches and relays. They can be used in Ohmic-contact devices, and the high forces generated by bimorph actuators result in low contact resistances. They can be designed to actuate with low power to toggle states, though only certain types of bimorphs allow for low power state latching. Bimorph actuators can be made to provide high speeds and high closure force, and can be designed

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