Bi-stable micro switch

Details Bi-stable micro switch Bi-stable micro switch

2000-03-03

2001-10-16

Sircus, Brian (Department: 2839)

Electricity: circuit makers and breakers

Electrostrictive or electrostatic

C310S307000

Reexamination Certificate

active

06303885

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO MICROFICHE APPENDIX
Not applicable.
FIELD OF THE INVENTION
The present invention is generally related to switches for use in micro systems, and more particularly to a MEMS switch capable of latching in either of two switch positions.
BACKGROUND OF THE INVENTION
Optical switches can be used in a variety of applications, such as optical fiber transmission networks, to route optical signals along various signal paths. An optical switch typically has an optical element, such as a mirror or a filter, that is switched into and out of a path of an optical signal beam. Switches are typically characterized by the number of input and output port, referred to as N×N. For example, a 1×2 switch would switch one input between two outputs.
Switches can often be described as “latching” or “non-latching”. A latching switch reliably remains in a known position, even if the power is removed or lost. A non-latching switch may revert to an unknown position, or even a position intermediate between switch states, when the power is lost, for example if current provided to an electro-magnetic solenoid or thermal actuator is lost. One type of latching switch reverts to a known default position (state), no matter what state the switch was in when power was lost. Another type of latching switch preserves the switch state, no matter what that state was. The latter case is known as a “bi-stable” switch.
Bi-stable optical switches are desirable for use in optical telecommunication systems because they preserve the network configuration associated with the position of the switch(es) when the power was lost. Various approaches have been used to produce bi-stable optical switches. One approach uses a permanent magnet in conjunction with a piece of magnetic material to hold the switch element in the desired position. Other approaches use a mechanical latch to hold the switch element in the desired position.
In a particular application, as illustrated and described in U.S. Pat. No. 5,994,816 entitled THERMAL ARCHED BEAM MICROELECTROMECHANICAL DEVICES AND ASSOCIATED FABRICATION METHODS by Dhuler et al., issued Nov. 30, 1999, a mechanical latch is used in a micro-electro-mechanical system (“MEMS”) (See, e.g. FIG. 11, ref. nums. 69 and 68
c
). A thermal arched beam actuator is used to move a switch element back and, with a thermally activated latch holding the switch element in the desired position(s). However, having contact surfaces between the latch and the switch element can result in the mechanism sticking or produces “sticktion (i.e. sticking friction), thus altering the force required to change switch states. This sticking or sticktion can not only affect the reliability of switch operation, but also affect the timing of the switch, particularly with fast (i.e. ≦1 ms) in light of the need to time the operation of the latch with the operation of the electrostatic comb.
U.S. Pat. No. 5,994,816 also describes a latching mechanism that uses an electrostatic field to clamp a movable portion of the switch to the switch body (substrate). Clamping allows the relatively high current flow to the thermal beam actuator to be removed without losing the clamped switched state, thus conserving power. However, if the voltage to the electrostatic clamping circuit is removed the switch may revert to a state other than what was previously held.
Accordingly, it is desirable to provide a bi-stable MEMS switch without mechanical contacting surfaces between moving and static surfaces of the switch. It is further desirable that the optical switch be repeatable and have a high switching lifetime, and maintain a present switch state when power to the switch is removed.
SUMMARY OF THE INVENTION
The present invention provides a bi-stable MEMS switch without contact surfaces between the moving and static portions of the switch. In a preferred embodiment, a switch element or center body is movable in relation to the switch body or substrate (i.e. MEMS chip). The switch element is suspended between portions of the switch body by a plurality of spring arms attached at walls of hollow body portions of a center beam and can serve to operate in a relay, a valve, or an optical switch, for example. An actuator, such as an electrostatic comb drive motor, thermal beam actuator, or magnetic motor, provides a motive force to the switch element according to an electronic switch signal. The spring arms hold the switch element in place in relation to the switch body and in one of two switch positions, whether or not the electronic switch signal continues to be applied.
The switch is cycled between states by appropriate electronic switch signals. For example, if an electrostatic comb drive motor is used, a first electronic switch signal, such as a pulse or series of pulses, causes the switch to assume a first switch position. A second electronic switch signal causes the switch to assume a second switch position. The electrostatic comb drive motor uses two electrostatic arrays, one array configured to move a movable portion of the switch in a first direction, and the other array configured to move the movable portion of the switch in a second direction in response to applied electrical signals. In the event of a thermal beam motor, an electric current might be temporarily applied to a first set of heaters to set the switch in a first position, and a temporary electric current might be applied to a second set of heaters to set the switch in a second position. Using either type of actuator, the switch will remain in its present state if power is lost, but typically power is removed to conserve power consumption of the switch.
The spring arms and hollow beam walls deform in response to the motive force of the actuator, and attain an equilibrium position in either of the switch states. In a preferred embodiment, one switch position has a lower potential energy than the other switch position; however, the switch is bi-stable.


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Marxer and de Rooij, Micro-Opto-Mechanical 2×2 Switch for Single-Mode Fibers Based on Plasma-Etched Silicon Mirror and Electrostatic Actuation, IEEE J. of Lightwave Technology, vol. 17, No. 1, 2-8 (Jan. 1999).
Lee et al., Bi-Stable Planar Polysilicon Microactuators with Shallow Arch-Shaped Leaf Springs, SPIE Conference on Micromachined Devices and Components V, 274-279 (Sep. 1999), Santa Clara, California.
Chen et al., A High-Speed Low-Voltage Stress-Induced Micromachined 2×2 Optical Switch, IEEE Photonics Technology Letters, vol. 11, No. 11, 1396-1398 (Nov. 1999).

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