Patent Application
20060238852
Microelectromechanical systems (MEMS) devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching, which have been developed for the fabrication of integrated circuits. Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of MEMS device used in projection displays by controlling light through reflection. Other types of MEMS devices include accelerometers, pressure and flow sensors, and gears and motors.
A conventional DMD 100 is illustrated in
One problem associated with a conventional MEMS device, such as the DMD 100, is “stiction”, which occurs when the yoke 114 rotates on the torsion hinges 118 and the yoke landing tips 116 come in physical contact with landing sites 120 located within the underlying bottom layer 106. In some cases, when surface adhesion forces are high enough, the yoke landing tips 116 may stick to the landing sites 120 in the underlying bottom layer 106, and thereby adversely affect the response time of the pixel mirror 108 and the overall device performance. In other cases, the landing tips 116 may adhere to the landing sites 120 and remain stuck if an applied mechanical restoring force is not strong enough to overcome the existing surface adhesion forces. The pixel mirror 108 will then be considered permanently defective because it will remain fixated at only one angle.
Stiction has heretofore been addressed by applying lubrication or passivation layers to the yoke landing tips 116 and the landing sites 120 in the hopes of making these metal surfaces slippery enough to minimize sticking. In addition, reset electronics 122 have been employed to pump additional electrical energy into the yoke 114 in order to help it break free from the constraining surface adhesion forces between the yoke landing tips 116 and the landing sites 120. These techniques require extra fabrication processes and additional cost.
The present disclosure relates to a microelectromechanical system (MEMS) device, and more particularly, to an electrostatically-driven digital micromirror device (DMD) that prevents or at least reduces stiction. A central electrode includes interspersed extensions initially formed on a substrate. Two outer electrodes with interspersed extensions are subsequently formed on the substrate such that the two outer electrodes flank the central electrode. The extensions of the central and outer electrodes are interdigitated whereby a low bias voltage applied to the outer electrodes generates an electrostatic force upon the central electrode enabling a pixel mirror that is formed on top of the central electrode to freely move, rotate, and tilt.
Referring to the conventional digital micromirror device (DMD) of
Reference is now made to
The middle layer 204, disposed beneath the top layer 202, includes a yoke 212 supported by a plurality of yoke support posts 214. The yoke support posts 214 may be formed according to the same or similar materials and methods as the mirror support posts 210. Furthermore, the yoke support posts 214 may also have the same or similar thickness as that of the mirror support post 210. The middle layer 204 also includes a post-receiving hole 211, which may be formed using known materials and methods.
The bottom layer 206, situated below the middle layer 204, includes a yoke address electrode 216 and mirror address electrodes 220. The bottom layer 206 further includes contact pads 224, which are provided for receiving the yoke support posts 214. Still further, the bottom layer 206 includes a pair of metal contact openings 217 separated by the yoke address electrode 216. Of course, other metal contact opening arrangements are contemplated, such as additional metal contact openings and alternatively configured metal contact openings. Electrical signals and connections from an integrated circuit (not shown) positioned beneath the bottom layer 206 may be sent through the pair of metal contact openings 217 into either the yoke address electrode 216 or the mirror address electrodes 220. The integrated circuit may be a static random access memory (SRAM) cell or an integrated complementary metal oxide semiconductor (CMOS) device. In other embodiments, the integrated circuit may be a multi-chip module (MCM) where many devices are assembled together by stacking one on top of another into a single module for faster electronic devices with added functionalities.
The yoke address electrode 216 generally resides in a middle portion of the bottom layer 206 and is flanked by two outer mirror address electrodes 220. The yoke address electrode 216 includes a plurality of interspersed extensions 218, thereby defining a plurality of interspersed grooves 221. In one embodiment, the pluralities of interspersed extensions 218 are situated at opposing lateral sides of the yoke address electrode 216. Disposed within the plurality of grooves 221 are a plurality of corresponding interspersed extensions 222 of the laterally disposed mirror address electrodes 220. Accordingly, the extensions 218, 222 are substantially interdigitated to form a comb-like structure. In some embodiments, the yoke address electrode 216 and the two mirror address electrodes 220 have a thickness of about 500 to about 3,000 Å. Preferably, the thickness of the yoke address electrode 216 and the two mirror address electrodes 220 within the presently disclosed embodiment is about 1,500 Å. Additionally, the interspersed extensions 218, 222 may have a corresponding width and length of about 20 μm and a thickness of about 500 to about 3,000 Å. Preferably, the thickness of the interspersed extensions 218, 222 within the presently disclosed embodiment is about 1,500 Å. Still further, the spacing between the interspersed extensions 218, 222 can vary from about 5 to 10 μm. Preferably, the spacing between the interspersed extensions 218, 222 within the presently disclosed embodiment is about 7.5 μm.
Although the interspersed extensions 218, 222 are depicted as being square in shape, they can take on a variety of polygonal shapes and sizes. For example, the interspersed extensions 218, 222 may be in the shape of a rectangle, a triangle, a parallelogram, a diamond, a trapezoid or any other suitable shape. In addition, the interspersed extensions 218, 222 may also take on plane-curve shapes such as circles, semi-circles, ellipses, semi-ellipses, lines, parabolas, or hyperbolas. Furthermore, the interspersed extensions 218, 222 may be uniformly spaced or non-uniformly spaced and uniform in shape and size or non-uniform in shape and size. Uniform and non-uniform combinations of shapes and sizes are also contemplated.
One benefit of the DMD 200 is realized through the amount of electrostatic force that can be generated between the extensions 218, 222. In particular, an electrostatic force F acting upon a charged object Q1 as a result of the presence of another charged object Q2 can be calculated by Coulomb's law (F=k×Q1×Q2/d2), where k is a constant and d is the distance between the objects. The magnitude of a charged object Q can be calculated by multiplying the surface density σ with the surface area of the charged object A (Q=σA). Accordingly, the electrostatic force F scales proportionally with the surface area of the charged object A (FαA). The DMD 200 has a larger surface area when compared with conventional DMDs, such as DMD 100 of
In practice, an electrostatic field is generated by pulsing the mirror address electrodes 220. The generated electric field in turn generates an electrostatic force that causes the pixel mirror 208 to tilt or rotate. Unlike a conventional DMD 100, wherein the pixel mirror 108 can experience stiction during tilting or rotation, the DMD 200 can generate much greater electrostatic forces thereby eliminating or at least reducing the chance that the pixel mirror 208 will stick to underlying layers of the DMD 200. In addition, the increased electrostatic force eliminates the need for reset electronics.
It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. For example, the DMD 200 may be manufactured by surface micromachining, where the structures are built up in layers of thin film on the surface of a silicon wafer or any other suitable substrate. Another technique of manufacturing a DMD is bulk micromachining. In addition, the presently disclosed embodiments may also be applied to MEMS devices for useful applications in the study and understanding of biological proteins and gene functions. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and ranges of equivalents thereof are intended to be embraced therein.
Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. § 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. A description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(s) set forth in the claims found herein. Furthermore, any reference in this disclosure to “embodiment” in the singular should not be used to argue that there is only a single point of novelty claimed in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims associated with this disclosure, and the claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification, but should not be constrained by the headings set forth herein.
