Abstract:
A hydrophobic layer covers the cavity-side surface of the bottom electrode of the interferometric modulation pixel. Consequently, the hydrophobic layer prohibits the hydrophilic surface of the bottom electrode from the adsorption of water molecules, thereby preventing the top electrode from being pulled toward the bottom electrode when the interferometric modulation pixel is active.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to a planar panel display and a manufacturing method thereof. More particularly, the present invention relates to an interferometric modulation pixel and a manufacturing method thereof.  
         [0003]     2. Description of Related Art  
         [0004]     Planar displays are extremely popular in the portable and limited-space display market because they are lightweight and small. To date, in addition to liquid crystal display (LCD), organic light-emitting diode (OLED) and plasma display panel (PDP) display panels, a module of the optical interference display has been investigated.  
         [0005]     The features of an interferometric modulation pixel of the optical interference display include low electrical power consumption, short response time and bi-stable status. Therefore, the optical interference display can be applied in planar display panels, especially in portable products such as mobile phones, personal digital assistants (PDA), and portable computers.  
         [0006]     U.S. Pat. No. 5,835,255 discloses a modulator array for visible light, and an interferometric modulation pixel of the modulator array can be used in a planar display panel.  FIG. 1A  illustrates a cross-sectional diagram showing an interferometric modulation pixel in the prior art. Every interferometric modulation pixel  100  comprises a bottom electrode  102  and a top electrode  104 . The bottom electrode  102  and the top electrode  104  are separated by supports  106 , thus forming a cavity  108 . The distance between the bottom electrode  102  and the top electrode  104 , that is, the depth of cavity  108 , is D and is usually less than 1 μm. The bottom electrode  102  is a light-incident electrode and partially absorbs visible light according to absorption rates of various wavelengths. The top electrode  104  is a light-reflection electrode which is flexed toward the bottom electrode  102  when a voltage is applied to it.  
         [0007]     A white light is usually used as an incident light source for the interferometric modulation pixel  100  and represents a mixture of various wavelengths (represented by λ) of light in the visible light spectrum. When the incident light shines through the bottom electrode  102  and enters the cavity  108 , only the visible light with wavelength (λ 1 ) corresponding to the formula 1.1 is reflected back, that is, 
 
2 D=Nλ   1   (1.1), 
 
 wherein N is a natural number. 
 
         [0009]     When twice the cavity depth, 2D, equals one certain wavelength λ 1  of the incident light multiplied by any natural number, N, a constructive interference is produced, and a light with the wavelength λ 1  is reflected back. Thus, an observer viewing the panel from the direction of the incident light will observe light with the certain wavelength λ 1  reflected back at him. The display unit  100  here is in an “open” state, i.e. a “bright” state.  
         [0010]      FIG. 1B  illustrates a cross-sectional diagram of the interferometric modulation pixel  100  in  FIG. 1A  after a voltage is applied to it. Under the applied voltage, the top electrode  104  is flexed by electrostatic attraction toward the bottom electrode  102 . At this moment, the distance between the walls  102  and  104 , the depth of cavity  108 , becomes d and may equal zero. The D in the formula 1.1 is hence replaced with d, and only the visible light with another certain wavelength λ 2  satisfying the formula 1.1 produces constructive interference and reflects through the top electrode  102 . However, in the interferometric modulation pixel  100 , the bottom electrode  102  is designed to have a high absorption rate for the light with the wavelength λ 2 . Thus, the incident visible light with the wavelength λ 2  is absorbed, and the light with other wavelengths is annulled by destructive interference. The incident visible light of all wavelengths is thereby filtered, and the observer is unable to see any reflected visible light when the top electrode  104  is flexed. The interferometric modulation pixel  100  is now in a “closed” state, i.e. a “dark” state.  
         [0011]     As described above, under the applied voltage, the top electrode  104  is flexed by electrostatic attraction toward the bottom electrode  102  such that the interferometric modulation pixel  100  is switched from the “open” state to the “closed” state. When the interferometric modulation pixel  100  is switched from the “closed” state to the “open” state, the voltage for flexing the top electrode  104  is removed, and the top electrode  104  elastically returns to the original state, i.e. the “open” state as illustrated in  FIG. 1A .  
         [0012]     In light of foregoing, the interferometric modulation pixel  100  is obtained by combining thin film interference principles of optics with the reflective plate and microelectromechanical system (MEMS) processes. In a MEMS process, the cavity  108  is formed by etching a sacrificial layer between the bottom electrode  102  and the top electrode  104 . After removing the sacrificial layer, water vapor can be easily adsorbed within the cavity  108 , creating an undesired electrostatic attractive force between the bottom electrode  102  and the top electrode  104 . The electrostatic attractive force created by the water molecules can switch the “open” state of the display unit to its “closed” state. Hence, a display unit using interferometric modulation and a manufacturing method thereof are needed to avoid the adsorption of water molecules within the cavity  108  and thereby eliminate the possibility of forming an undesired electrostatic attractive force.  
       SUMMARY OF THE INVENTION  
       [0013]     In one aspect, the present invention provides an interferometric modulation pixel and a manufacturing method of which a hydrophobic layer is formed on the bottom electrode to protect the upper surface of the bottom electrode from adsorbing water molecules.  
         [0014]     In another aspect, the present invention provides an interferometric modulation pixel and a manufacturing method of which a hydrophobic layer is formed on the bottom electrode to maintain the distance between the bottom electrode and the top electrode such that the top electrode is not pulled toward the bottom electrode due to adsorbed moisture in the cavity.  
         [0015]     In yet another aspect, the present invention provides an interferometric modulation pixel and a manufacturing method that enhances the image display quality of the planar optical interference display.  
         [0016]     In accordance with the foregoing and other aspects of the present invention, the present invention provides a method of manufacturing an interferometric modulation pixel. A first electrode layer and a sacrificial layer are sequentially formed on a transparent substrate, wherein an uppermost layer of the first electrode layer is an insulating layer. At least two first openings are formed in the sacrificial layer and the first electrode layer to demarcate and define a first electrode. A photosensitive material is formed on the sacrificial layer and within the first openings and is then partially removed to leave supports in the first openings. A second electrode layer is formed on the sacrificial layer and the supports. Then, at least two second openings are formed in the second electrode layer to demarcate and define a second electrode such that the two second openings perpendicularly crisscross the two first openings. The sacrificial layer is then removed, and a hydrophobic layer is formed on the insulating layer.  
         [0017]     In the foregoing, the hydrophobic layer is formed by adsorbing a layer of a hydrophobic organic compound having at least a hydrogen atom being capable of forming hydrogen bonds with oxygen or nitrogen atoms. The hydrophobic organic compound comprises silanes including hexamethyl disilane or silanols including trimethyl silanol.  
         [0018]     In accordance with the foregoing and other aspects of the present invention, the present invention provides another method of manufacturing an interferometric modulation pixel. A first electrode layer, a hydrophobic layer and a sacrificial layer are sequentially formed on a transparent substrate, wherein an uppermost layer of the first electrode layer is an insulating layer. At least two first openings are formed in the sacrificial layer, the hydrophobic layer and the first electrode layer to demarcate and define a first electrode. A photosensitive material is formed on the sacrificial layer and in the first openings and is then partially removed to leave supports in the first openings. A second electrode layer is formed on the sacrificial layer and the supports. Then, at least two second openings are formed in the second electrode layer to demarcate and define a second electrode such that the two second openings perpendicularly crisscross the two first openings. The sacrificial layer is then removed.  
         [0019]     In the foregoing, the hydrophobic layer may comprise a hydrophobic resin.  
         [0020]     In accordance with the foregoing and other aspects of the present invention, the present invention provides an interferometric modulation pixel. The interferometric modulation pixel comprises a first electrode, a movable second electrode situated above the first electrode, two supports between the first electrode and the second electrode for forming a cavity between the first and second electrodes, and a hydrophobic layer on the cavity-side surface of the bottom electrode. Materials for use as the hydrophobic layer include a hydrophobic resin and a hydrophobic organic compound having at least a hydrogen atom being capable of forming hydrogen bonds with oxygen or nitrogen atoms. The hydrophobic organic compound comprises silanes including hexamethyl disilane or silanols including trimethyl silanol.  
         [0021]     In light of the preferred embodiments of the present invention described above, a hydrophobic layer covers the insulating layer of the bottom electrode to prevent adsorption of water molecules. Hence, the distance between the bottom electrode and the top electrode is not decreased due to the adsorption of water molecules and thereby provides a high-quality image display.  
         [0022]     It is to be understood that both the foregoing general description and the following detailed description are made by use of examples and are intended to provide further explanation of the invention as claimed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The accompanying drawings are included to provide a better understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
         [0024]      FIG. 1A  illustrates a cross-sectional diagram showing an interferometric modulation pixel in the prior art;  
         [0025]      FIG. 1B  illustrates a cross-sectional diagram of the interferometric modulation pixel  100  in  FIG. 1A  after a voltage is applied to it;  
         [0026]      FIGS. 2A-2D  are cross-sectional diagrams showing a process of manufacturing an interferometric modulation pixel according to a preferred embodiment of this invention; and  
         [0027]      FIGS. 3A-3D  are cross-sectional diagrams showing a process of manufacturing an interferometric modulation pixel according to another preferred embodiment of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     The bottom electrode of the prior art interferometric modulation pixel is made of a transparent conductive layer, a light-absorption layer and a silicon-based insulation layer. The silicon-based insulation layer is usually a silicon oxide layer or a silicon nitride layer, both of which are hydrophilic. The cavity depth of the interferometric modulation display unit is the distance between the bottom electrode and the top electrode after a sacrificial layer therebetween is etched away by a structural release etching process. The cavity depth is usually on the order of one micrometer or even smaller. Therefore, water vapor in the air is very easily adsorbed within the cavity to create an undesired electrostatic attractive force between the bottom and the top electrodes that permanently forces the interferometric modulation pixel to appear as the “closed” state and consequently produces image defects.  
         [0029]     Therefore, this invention provides an interferometric modulation pixel and a manufacturing method thereof to solve the prior art problem of the adsorption of water molecules onto the bottom electrode. In a preferred embodiment of the present invention, the bottom electrode is covered by a hydrophobic layer in order to prohibit the bottom electrode from adsorbing water molecules.  
         [0030]     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
         [heading-0031]     Embodiment 1  
         [0032]      FIGS. 2A-2D  are cross-sectional diagrams showing a process of manufacturing an interferometric modulation pixel according to a preferred embodiment of this invention. In  FIG. 2A , a transparent conductive layer  205 , a light-absorption layer  210 , an insulating layer  215 , and a sacrificial layer  220  are sequentially formed on a transparent substrate  200 .  
         [0033]     The transparent conductive layer  205  is preferably made of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide or indium oxide. The light-absorption layer  210  is preferably made of aluminum, silver or chromium. The insulating layer  215  may be comprised of silicon oxide or silicon nitride. The sacrificial layer  220  is made of metal, amorphous silicon, polysilicon or other suitable material.  
         [0034]     In  FIG. 2B , at least two first openings  225  are formed in the sacrificial layer  220 , the insulating layer  215 , the light-absorption layer  210  and the transparent conductive layer  205  by a process such as photolithography and etching to define a bottom electrode. The first openings  225  are substantially oriented perpendicularly to the diagram surface such that the openings can be likened to channels, and only the cross-sections of the channels are visible in the diagram. The bottom electrode of the interferometric modulation pixel is located between the two first openings  225  and is formed by stacking the transparent conductive layer  205 , the light-absorption layer  210 , and the insulating layer  215 .  
         [0035]     Then, a photosensitive material  230  is coated on the sacrificial layer  220  and inside of the first openings  225 . The photosensitive material  230  comprises positive photoresist, negative photoresist, or various kinds of photosensitive polymers such as polyimide, acrylic resins, or epoxy resins.  
         [0036]     In  FIG. 2C , supports  235  in the first openings  225  are formed by exposing and developing the photosensitive material  230 . A reflective conductive layer  245  is formed on the sacrificial layer  220  and the supports  235 . Then, at least two second openings (not shown in  FIG. 2C ) are formed in the reflective conductive layer  245  by a process such as photolithography and etching to demarcate and define a top electrode between the two second openings. The orientation of the second openings is parallel to the diagram surface. The top electrode is formed from the reflective conductive layer  245  and is a light-reflection electrode. The top electrode can be flexed to move up and down. The material used as the reflective conductive layer  245  must be reflective so as to reflect the incident light from the bottom electrode. The reflective conductive layer  245  preferably comprises metal.  
         [0037]     In  FIG. 2D , the sacrificial layer  220  is removed by a structural release etching process such as remote plasma etching. The precursor of the remote plasma includes a fluorine-based or chlorine-based etchant, such as xenon difluoride, carbon tetrafluoride, boron trichloride, nitrogen trifluoride, sulfur hexafluoride, or combinations thereof.  
         [0038]     In a moisture-free environment or in a vacuum, a hydrophobic layer  250  is formed on the surface of the insulating layer  215 . The method used for forming the hydrophobic layer  250  includes introducing a gas of a hydrophobic organic compound into a reaction chamber such that the gas condenses and adsorbs onto the insulating layer  215 . The hydrophobic organic compound must have at least a hydrogen atom that can form a hydrogen bond with the lone pair electrons of oxygen or nitrogen atoms on the surface of the insulating layer  215 . Consequently, the oxygen or nitrogen atoms in the insulating layer  215  are unable to form hydrogen bonds with water molecules, preventing adsorption of water molecules. The hydrophobic organic compound includes silanes, such as hexamethyl disilanes, or silanols, such as trimethyl silanol.  
         [heading-0039]     Embodiment 2  
         [0040]      FIGS. 3A-3D  are cross-sectional diagrams showing a process of manufacturing an interferometric modulation pixel according to another preferred embodiment of this invention. In  FIG. 3A , a transparent conductive layer  305 , a light-absorption layer  310 , an insulating layer  315 , a hydrophobic layer  320  and a sacrificial layer  325  are sequentially formed on a transparent substrate  300 .  
         [0041]     The transparent conductive layer  305  is preferably comprised of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide or indium oxide. The light-absorption layer  310  is made of a metal such as aluminum, silver or chromium. The insulating layer  315  is preferably comprised of silicon oxide or silicon nitride. In this embodiment, the hydrophobic layer  320  is made of a hydrophobic resin. The sacrificial layer  325  preferably comprises metal, amorphous silicon or polysilicon.  
         [0042]     In  FIG. 3B , at least two first openings  330  are formed in the sacrificial layer  325 , the hydrophobic layer  320 , the insulating layer  315 , the light-absorption layer  310  and the transparent conductive layer  305  by a process such as photolithography and etching to demarcate and define a bottom electrode. The first openings  330  are substantially oriented perpendicularly to the diagram surface such that the openings can be likened to channels, and only the cross-sections of the channels are visible in the diagram. The bottom electrode of the interferometric modulation pixel is located between the two first openings  330  and is formed by stacking the transparent conductive layer  305 , the light-absorption layer  310 , and the insulating layer  315 . Then, a photosensitive material  335  is coated on the sacrificial layer  325  and inside the first openings  330 . The photosensitive material  335  comprises positive photoresist, negative photoresist, or various kinds of photosensitive polymers such as polyimide, acrylic resins, or epoxy resins.  
         [0043]     In  FIG. 3C , supports  340  in the first openings  330  are formed by exposing and developing the photosensitive material  335 . A reflective conductive layer  345  is formed on the sacrificial layer  325  and the supports  340 . Then, at least two second openings (not shown in  FIG. 2C ) are formed in the reflective conductive layer  345  by a process such as photolithography and etching to define a top electrode between the two second openings. The orientation of the second openings is parallel to the diagram surface. The top electrode is formed from the reflective conductive layer  345  and is a light-reflection electrode. The top electrode can be flexed to move up and down. The material used as the reflective conductive layer  345  must be reflective so as to reflect the incident light from the bottom electrode. The material of the reflective conductive layer  345  preferably comprises metal.  
         [0044]     In  FIG. 3D , the sacrificial layer  325  is removed by a structural release etching process, such as remote plasma etching. The precursor of the remote plasma includes a fluorine-based or chlorine-based etchant, such as xenon difluoride, carbon tetrafluoride, boron trichloride, nitrogen trifluoride, sulfur hexafluoride, or combinations thereof.  
         [0045]     In light of the preferred embodiments of the present invention described above, a hydrophobic layer covers the insulating layer of the bottom electrode to prohibit adsorption of water molecules. Hence, the distance between the bottom electrode and the top electrode is not decreased by the adsorption of water molecules and thereby provides a high-quality image display.  
         [0046]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.