Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Physical deformation
Reexamination Certificate
2000-09-14
2003-07-08
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Physical deformation
C257S414000
Reexamination Certificate
active
06590267
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to microelectromechanical system (MEMS) valve structures, and more particularly to low-power, high speed electrostatically actuating MEMS valve structures and the associated fabrication methods.
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 been 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 is found in U.S. Pat. No. 5,475,318 entitled “Microprobe” issued Dec. 12, 1995 in the name of inventors Marcus et al., which leverages thermal expansion to move a microdevice. 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 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 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.
MEMS electrostatic devices 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. An example of these devices can be found in U.S. patent application Ser. No. 09/345,300 entitled “ARC resistant High Voltage Micromachined Electrostatic Switch” filed on Jun. 30, 1999 in the name of inventor Goodwin-Johansson and U.S. patent application Ser. No. 09/320,891 entitled “Micromachined Electrostatic Actuator with Air Gap” filed on May 27, 1999 in the name of inventor Goodwin-Johansson. Both of these applications are assigned to MCNC, the assignee of the present invention.
Typical MEMS valves have employed thermal actuation/activation methods to control valves with high flow rates (i.e. large apertures and large clearance areas around the aperture). Thermal actuation has been preferred because it is able to provide the large forces necessary to control the valve over the requisite large distances. However, these valves have relatively slow operation rates due to the thermal time constraints related to the valve materials. Additionally, thermally activated MEMS valves use resistive heating where the power consumed is calculated by the current squared times the resistance and considerable power is consumed in operating the valve.
It would be advantageous to construct a MEMS valve device using electrostatic actuation that is capable of both large displacements and large forces. The electrostatic nature of the MEMS valve will allow for relatively low power consumption and, therefore, no unwarranted heating of the flowing gas or fluid would occur. Additionally, the electrostatic valve will provide for relatively fast operation, allowing for more precise control of the open and closed states of the valve. In addition, it would be advantageous to develop a MEMS valve that forms a secure valve seat to valve cover interface to assure low leakage rates are realized. It would also be beneficial to provide for a MEMS valve that minimizes the occurrence of stiction between the substrate and moveable membrane. Stiction, which is a well-known concept in microelectronics, is defined as the tendency for contacting MEMS surfaces to stick to one another. Stiction is especially a concern in valve devices in which a pressure differential exists across the closed valve. It would be beneficial to devise a MEMS valve that relieves the pressure differential prior to opening the valve.
As such, MEMS electrostatic valves that have improved performance characteristics are desired for many applications. For example, micromachined valves capable of fast actuation, large valve force and large valve flap displacements that utilize minimal power are desirable, but are currently unavailable.
SUMMARY OF THE INVENTION
The present invention provides for improved MEMS electrostatic valves that benefit from large valve force, fast actuation and large displacement of the moveable membrane to allow for the efficient transport of increased amounts of gas or liquid through the valve. Further, methods for making the MEMS electrostatic valve according to the present invention are provided.
A MEMS valve device driven by electrostatic forces according to the present invention comprises a planar substrate having an aperture formed therein and substrate electrode disposed on the planar substrate. Further, the MEMS valve device of the present invention includes a moveable membrane that overlies the aperture and has an electrode element and a biasing element. The moveable membrane is defined horizontally as having a fixed portion attached to the substrate and a distal portion that is moveable with respect the substrate. Additionally, at least one resiliently compressible dielectric layer is provided to insure electrical isolation between the substrate electrode and electrode element of the moveable membrane. In operation, a voltage differential is established between the substrate electrode and the electrode element of the moveable membrane to move the membrane relative to the aperture to thereby controllably adjust the portion of the aperture that is covered by the membrane.
In one embodiment of the MEMS valve device according to the present invention the resiliently compressible dielectric layer is formed on the substrate electrode and provides for the valve seat surface. In another embodiment of the present invention the resiliently compressible dielectric layer is formed on the moveable membrane and provides for the valve seal surface. In yet another embodiment resiliently compressible dielectric layers are formed on both the substrate electrode and the moveable membrane and provide for both the valve seat surface and the valve seal surface. The resiliently compressible nature of the dielectric layer allows for a secure closed valve to form that benefits from a low leakage rate.
In yet another embodiment the resiliently compressible dielectric layer has a textured surface; either at the valve seat, the valve seal or at both surfaces. By texturing these surfaces the va
Goodwin-Johansson Scott H.
McGuire Gary E.
Alston & Bird LLP
MCNC
Meier Stephen D.
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