Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-10-30
2002-05-28
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S291000, C359S230000, C359S227000
Reexamination Certificate
active
06396620
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electromagnetic radiation shutter, and more particularly to a microelectromechanical system (MEMS) dual electrostatic flexible membrane shutter capable of deflecting electromagnetic radiation.
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 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. Referring to
FIG. 1
shown is a MEMS flexible membrane electrostatic device
10
as described in detail in U.S. patent application Ser. No. 09/464,010, entitled “Electrostatically Controlled Variable Capacitor”, filed on Dec. 15, 1999, in the name of inventor Goodwin-Johansson and assigned to MCNC, the assignee of the present invention. That application is herein incorporated by reference as if set forth fully herein. The MEMS flexible membrane device comprises in layers a substrate
20
, a first insulating layer
30
, a substrate electrode
40
, a substrate insulator
50
and a flexible membrane
60
. The flexible membrane is generally planar and overlies a portion of the substrate and, generally, the entirety of the substrate electrode. The flexible membrane typically comprises multiple layers including at least one electrode layer
62
and at least one biasing/insulating layer
64
.
The flexible membrane may be defined as having two portions; referred to as the fixed portion
70
, and the distal portion
80
. The portions are defined horizontally along the length of the moveable composite. The fixed portion is substantially affixed to the underlying substrate or intermediate layers at the flexible membrane to substrate attachment point. The distal portion is released from the underlying substrate or intermediate layers during fabrication of the MEMS device.
Referring to
FIG. 2
, shown is an alternate embodiment of a MEMS moveable membrane device
100
having a predefined air gap
110
underlying a medial portion
120
of the moveable composite. The medial portion extends from the fixed portion
70
and is held in position or biased without the application of electrostatic force. The air gap results from the release operation employed during fabrication of the MEMS device. During operation the distal portion is free to move, characteristically curling away from the underlying planar surface in the absence of electrostatic forces. The medial portion maintains a non-increasing separation (i.e. the separation is either constant or decreasing) with respect to the underlying planar surface until the flexible membrane begins to bend toward the substrate. As shown an auxiliary biasing layer
130
overlies the electrode layer and structurally constrains the medial portion. By predefining the shape of the air gap, recently developed MEMS electrostatic devices can operate with lower and less erratic operating voltages. For a more comprehensive discussion of MEMS moveable membrane devices having a predetermined air gap see 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 and assigned to MCNC the assignee of the present invention. That application is herein incorporated by reference as if set forth fully herein.
Optical displays have been formed that utilize metallized polymer films as one electrode and a second rigid electrode. In application, when voltage is applied between the two electrodes the metallized polymer electrode deflects and is attracted toward the fixed electrode. In particular, the metallized polymer film is typically a rolled up (fully curled) structure prior to application of the voltage as a means of minimizing the overall size of the electrode. Typical prior art optical displays will employ polymer films ranging from 1-4 micrometers in thickness and metal films ranging from 300 to 1000 angstroms in thickness. The display shutters are typically greater than 2 millimeters on a side such that the shutter rolls up to less than 10 percent of the total area. In addition, these shutters have benefited from the use of optically transparent conductive films, such as indium tin oxide (ITO), fabricated on transparent substrates, such as glass, to form an optically transparent fixed electrode.
The present problem is that no film currently exists that is both conductive and completely transparent to a wide frequency range of RF electromagnetic radiation. For any conductive material and frequency of RF electromagnetic radiation, there can be calculated a skin depth of &dgr;=sqrt(2/((&ohgr;&mgr;&sgr;)). A layer of conductive material more than a few skin depths in thickness will severely attenuate and reflect incident electromagnetic radiation. The skin depth for a gold film with 40 GHz radiation is 0.37 micrometers. Thus, it is not presently feasible to construct a shutter that has an RF transparent fixed electrode. What is desired is a structure that can serve as a shutter for electromagnetic radiation and, more specifically, a MEMS electromagnetic radiation shutter. A MEMS structure is highly preferred because it offers ease in fabrication, thus minimal cost, and can be operated with relatively low electrostatic power. Such a device would be capable of being implemented in a single MEMS device or in larg
Alston & Bird LLP
Ben Loha
Hindi Omar
MCNC
LandOfFree
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