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STRUCTURE OF A MICRO ELECTRO
MECHANICAL SYSTEM AND THE
MANUFACTURING METHOD THEREOF
 This application is a divisional of U.S. application Ser. No. 10/810,660, filed Mar. 29, 2004, which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
 The present invention relates to a structure of an optical interference display cell and the manufacturing method thereof, and more particularly, to the structure of a movable electrode in an optical interference display cell and the manufacturing method.
BACKGROUND OF THE INVENTION
 In a micro electro mechanical system (MEMS), the development of a sacrificial layer technique has become a key factor for manufacturing a suspended structure, such as a cantilever, a beam, a membrane, a channel, a cavity, a joint or hinge, a link, a crank, a gear or a rack, to name a few. A structure release etching process is adapted for removing a sacrificial layer, so a structure of a structure release in a micro electro mechanical system has a critical influence on the process of removing the sacrificial layer.
 A conventional structure release etching process is first introduced with an optical interference display cell as an example. The optical interference display cell, a kind of a micro electro mechanical system, is used to fabricate a planar display. Planar displays are popular for portable displays and displays with space limits because they are light and small in size. To date, planar displays in addition to liquid crystal displays (LCD), organic electro-luminescent displays (OLED), plasma display panels (PDP), a mode of optical interference displays is another option for planar displays.
 U.S. Pat. No. 5,835,255 discloses an array of display units of visible light that can be used in a planar display. Please refer to FIG. 1, which depicts a crosssectional view of a display unit in the prior art. Every optical interference display unit 100 comprises two walls, 102 and 104. Posts 106 support these two walls 102 and 104, and a cavity 108 is subsequently formed. The distance between these two walls 102 and 104, that is, the length of the cavity 108, is D. One of the walls 102 and 104 is a hemitransmissible/hemi-reflective layer with an absorption rate that partially absorbs visible light, and the other is a light reflective layer that is deformable when voltage is applied. When the incident light passes through the wall 102 or 104 and arrives in the cavity 108, in all visible light spectra, only the visible light with the wavelength corresponding to the formula 1.1 can generate a constructive interference and can be emitted, that is,
where N is a natural number.
 When the length D of cavity 108 is equal to half of the wavelength times any natural number, a constructive interference is generated and a sharp light wave is emitted. In the meantime, if the observer follows the direction of the
incident light, a reflected light with wavelength XI can be observed. Therefore, the display unit 100 is "open".
 FIG. 2 depicts a cross-sectional view of a display unit in the prior art after applying a voltage. As shown in FIG. 2, while driven by the voltage, the wall 104 is deformed and falls down towards the wall 102 due to the attraction of static electricity. At this time, the distance between wall 102 and 104, that is, the length of the cavity 108 is not exactly zero, but is d, which can be zero. If we use d instead of D in formula 1.1, only the visible light with a wavelength satisfying formula 1.1, which is X2, can generate a constructive interference, and be reflected by the wall 104, and pass through the wall 102. Because wall 102 has a high light absorption rate for light with wavelength X2, all the incident light in the visible light spectrum is filtered out and an observer who follows the direction of the incident light cannot observe any reflected light in the visible light spectrum. The display unit 100 is now "closed".
 FIG. 3A to FIG. 3B illustrate a method for manufacturing a conventional display cell. Referring to FIG. 3A, a first electrode 110 and a sacrificial layer 111 are formed in sequence on a transparent substrate 109, and opening 112, which is suitable for forming a supporter therein, is formed in the first electrode 110 and the sacrificial layer 111. Then, a supporter 106 is formed in the opening 112. Next, an electrode 114 is formed on the sacrificial layer 111 and the supporter 106. Subsequently, referring to FIG. 3B, the sacrificial layer 111 shown in FIG. 3A is removed by a release etching process to form a cavity 116, which is located in the position of the sacrificial layer 111, and the length D of the cavity 116 is the thickness of the sacrificial layer 111.
 In a micro electro mechanical process, a micro suspended structure is fabricated by using a sacrificial layer. A suspended movable microstructure is fabricated by a selective etching between a device structure layer and the sacrificial layer to remove the sacrificial layer and leave the structure layer, and this process is called a structure release etching. The difference between the structure release etching process and an IC process is that in the structure release etching process, the selective etching is an isotropic etching, so that an undercut or an under etching is formed in the structure layer for smooth separation of the structure layer and the substrate.
 No matter the wet structure release process or the dry structure release process is used, the choices of the material of the sacrificial layer and the micro suspended structure should be restricted, that is, the material used should have high etching-selectivity in the etching process or else the goal of removing the sacrificial layer without etching the micro suspended structure can not be achieved. Therefore, it is impossible to use the same material in both sacrificial layer and the micro suspended structure in the present process.
 In the process of the optical interference display cell, some materials, such as molybdenum, are very suitable to form the sacrificial layer and the movable electrodes. However, in the structure and the process of the conventional optical interference display cell, molybdenum cannot be the material of forming both sacrificial layer and the micro suspended structure at the same time. Thus results in enormous limits in choosing the materials.