Abstract:
An embodiment of the present invention relates to a digital micromirror device. More particularly, an embodiment of the present invention relates to a method of manufacturing a digital micromirror device having a perfectly flat mirror surface, wherein a hole of a mirror surface from which light is reflected is obviated by forming a mirror support post portion using an electro-plating process, unlike the related art digital micromirror device in which the hole is formed at the center of the mirror surface, thereby degrading the reflection efficiency of light.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a digital micromirror device, and more particularly, to a method of manufacturing a digital micromirror device having a perfectly flat mirror surface, in which the hole of a mirror surface from which light is reflected is obviated by forming a mirror support post portion using an electro-plating process, unlike the related art digital micromirror device in which a hole is formed at the center of the mirror surface so that the reflection efficiency of light is degraded.  
         [0003]     2. Background of the Related Art  
         [0004]      FIG. 1  is a cross-sectional view of a micromirror device in the related art, which is cut on a center line.  
         [0005]     As shown in  FIG. 1 , the digital micromirror device was first developed by Texas Instruments Incorporated (U.S.). An example of the prior art micromirror device is disclosed in U.S. Pat. No. 5,535,047 (issued on Jul. 09, 1996 entitled “Active Yoke Hidden Hinge Digital Micromirror Device” granted to Larry J. Hornbeck. The micromirror device is driven by electrostatic force and adopts a method in which the path of incident light is changed by reflecting the light according to a driving angle. The micromirror device is generally used in cantilever display fields.  
         [0006]     Referring to  FIG. 1 , the related art micromirror device compises a mirror  11  for reflecting incident light, a mirror support post  13  for supporting the mirror surface, a twisting hinge  14  to operate the bi-directional tilting of the micromirror device, a conducting layer  15  for electrical connection, and a yoke  16  that connects the twisting hinge  14 , the mirror support post  13  and so on.  
         [0007]     A hole  12  exists at the center of the mirror surface due to the structure of the mirror  11  that reflects the incident light and the support post  13  that supports the mirror surface. The mirror  11  and the mirror support post  13  are simultaneously fabricated using a deposition method such as sputtering. As a result, the hole  12  is inevitably formed at the center of the mirror surface. Reflected light is lost due to the hole  12  of the mirror surface, which results in a reduction in the contrast ratio when displaying images. Furthermore, a central portion within one pixel is always dark.  
         [0008]     It is therefore necessary to form a perfectly flat mirror surface by removing the concave hole  12  existing at the center of the mirror surface to enhance the light use efficiency and the contrast ratio, to remove a dark region in an image, to save power consumption when displaying images and to implement images with a high quality.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention has been made in view of the above problems, and it is an object of the present invention to enhance light reflection efficiency and the contrast ratio by forming a perfectly flat mirror surface by fabricating mirror support posts of a micromirror device using a plating process.  
         [0010]     To achieve the above object, a method of manufacturing a mirror support post of a micromirror device using a plating process according to an embodiment of the present invention compises the steps of forming a seed electrode on a substrate, forming a lower electrode, a driving unit of a micromirror and a sacrificial layer comprising a mirror support post formation region on the seed electrode, and forming a mirror support post in the mirror support post formation region using an electro-plating process.  
         [0011]     In the step of forming the mirror support post, the mirror support post may have the same height as the sacrificial layer.  
         [0012]     In the step of forming the mirror support post, the mirror support post may have a flat surface.  
         [0013]     To achieve the above object, a method of manufacturing a mirror support post of a micromirror device using a plating process according to another embodiment of the present invention comprises the steps of forming a lower electrode, a sacrificial layer and a cantilever support post of a driving unit of a micromirror on a substrate, forming a seed electrode on the cantilever support post of the driving unit of the micromirror, forming a cantilever of the micromirror and forming a sacrificial layer comprising a mirror support post formation region on the seed electrode, and forming the mirror support post in the mirror support post formation region using an electro-plating process.  
         [0014]     In the step of forming the mirror support post, the mirror support post may have the same height as the sacrificial layer.  
         [0015]     In the step of forming the mirror support post, the mirror support post may have a flat surface.  
         [0016]     To achieve the above object, a method of manufacturing a mirror support post of a micromirror device using a plating process according to still another embodiment of the present invention comprises the steps of forming a conducting layer on a substrate, forming a lower sacrificial layer, a hinge and a yoke on the conducting layer, forming an upper sacrificial layer comprising a mirror support post formation region on the yoke and the hinge, and forming a mirror support post in the mirror support post formation region by an electro-plating process.  
         [0017]     In the step of forming the mirror support post, the mirror support post may have the same height as the upper sacrificial layer.  
         [0018]     In the step of forming the mirror support post, the mirror support post may have a flat surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Further objects and advantages of the invention can be more completely understood from the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0020]      FIG. 1  is a cross-sectional view of a micromirror device in the related art, which is cut on a center line;  
         [0021]      FIG. 2  is a dismantled perspective view schematically showing the construction of a micromirror device according to the present invention;  
         [0022]      FIG. 3  is a plan view of the micromirror device shown in  FIG. 2 , which is vertically viewed downward from the substrate after the mirror is removed according to the present invention;  
         [0023]      FIG. 4  is a flowchart schematically illustrating a plating process of a mirror support post of a micromirror device according to the present invention;  
         [0024]      FIG. 5  is a flowchart illustrating an electro-plating process of a mirror support post portion of a micromirror device according to an embodiment of the present invention;  
         [0025]      FIG. 6  is a flowchart illustrating an electroplating process of a mirror support post portion of a micromirror device according to another embodiment of the present invention;  
         [0026]      FIG. 7  is a flowchart illustrating an electro-plating process of a mirror support post portion of a micromirror device according to still another embodiment of the present invention; and  
         [0027]      FIG. 8  shows a Scanning Electron Microscope (SEM) photograph of micromirror devices that are actually fabricated according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
         [0029]     The present invention will now be described in detail in connection with preferred embodiments with reference to the accompanying drawings.  
         [0030]      FIG. 2  is a dismantled perspective view schematically showing the construction of a micromirror device according to the present invention.  
         [0031]     As shown in  FIG. 2 , the micromirror device compises a mirror  20 , a substrate  21 , a plurality of electrodes  22 , a plurality of cantilever support posts  23 , a plurality of cantilevers  25  and a plurality of mirror support posts  24 .  
         [0032]     In the micromirror device, an addressing circuit (not shown) is formed in the substrate  21 . The electrodes  22  are formed on the substrate  21 . Each of the three cantilever support posts  23  is attached to the substrate  21 . Each of three cantilevers  25  has a flat plate and has its one end attached on each of the three cantilever support posts  23 . The mirror  20  is disposed on the mirror support posts  24 , each attached on the other end of each of the cantilevers  25 .  
         [0033]     The cantilever support posts  23  that support the neighboring cantilevers  25  intersect each other on the substrate  21 . The three mirror support posts  24  are adhered to the mirror  20  at locations opposite to that of the cantilever support posts  23  of the cantilevers  25 .  
         [0034]     The cantilever  25  that connects the cantilever support post 23  and the mirror support post 24  is bent up and down under expansion and contraction stress by electrostatic force generated by a voltage applied to the electrodes  22  on the substrate  21  and thus rotates the mirror  20 .  
         [0035]     In the three mirror support posts  24  that support the mirror  20 , a location at which the substrate  21  is fixed and a location at which the mirror  20  is attached intersect each other. Therefore, the mirror  20  has two kinds of rotation states where it is inclined either right or left at a predetermined angle depending on a direction in which electrostatic force is applied. The rotation angle of the mirror can also be controlled according to an amount of applied electrostatic force.  
         [0036]     A pair of the electrodes  22  for driving the mirror  20  is connected to an addressing circuit (not shown).  
         [0037]      FIG. 3  is a plan view of the micromirror device shown in  FIG. 2 , which is vertically viewed downward from the substrate after the mirror is removed according to the present invention.  
         [0038]     Referring to  FIG. 3 , the cantilever support posts  23  for supporting the cantilevers  25  and the mirror support posts  24  for supporting the mirror  20  are symmetrical to each other on the basis of the horizontal center line (a-a′) of the substrate. The electrodes  22  for addressing the mirror are symmetrical to each other on the basis of the vertical center line (b-b′) of the substrate.  
         [0039]     It has been shown in  FIG. 3  that the electrodes  22  are not overlapped with the cantilever support posts  22 . However, the present invention can be implemented even if the electrodes  22  and the cantilever support posts  23  do not overlap.  
         [0040]     Through the above structure, the mirror is applied with some degree of force in two directions along which the mirror is rotated. Therefore, the mirror can have the two kinds of rotation states.  
         [0041]      FIG. 4  is a flowchart schematically illustrating a plating process of a mirror support post of the micromirror device according to the present invention.  
         [0042]     The plating process applied to the present invention comprises a general electro-plating method.  
         [0043]     In the general electro-plating method, if a sample  31  becoming a conducting layer and an electrode  33  are dipped into a plating solution  30  and are then applied with a voltage, metal ions  32  within the plating solution  30  form the conducting layer in the sample  31  according to a pattern shape of a pattern layer  34 .  
         [0044]     In the plating process of forming the mirror support posts according to the present invention using the electro-plating method, a seed metal  35 , i.e., the conducting layer is previously formed on the substrate  36  so that electricity can conduct on the surface of the substrate  36 .  
         [0045]     For such electro-plating to be performed only in a selected region of the surface of the substrate  36 , it is necessary to block a non-conductive material using the pattern layer  34 . A photoresist film used in the pattern layer  34  can be formed of an organic material such as polymer. The organic material is not conductive and can be thus used as an electro-plating mask. After the completion of the electro-plating, the photoresist film that is no longer necessary is removed using an organic solvent such as acetone. The photoresist film can be removed easily using an organic solvent since it is an organic material.  
         [0046]     Where the electro-plating metal (refers to the mirror support posts of the present invention) is used without being separated from the substrate  36 , it is preferred that the seed metal  35  of the selective region be removed to prevent the entire substrate from being electrically by the seed metal  35 .  
         [0047]     In this case, the seed metal  35  below the electro-plating metal is not removed, but only the seed metal  35  in the region where the electro-plating metal is not formed is removed.  
         [0048]     Therefore, the plating process can deposit the conducting layer without limitation in height using an external power supply source that is electrically connected and the plating solution.  
         [0049]     If the method is used, a metal pole with a high vertical ratio can be formed in fully filled form. It is therefore possible to fabricate the mirror support posts that support the mirror in fully filled form in the micromirror manufacturing method of the present invention.  
         [0050]     The present applicant proposed Korean Patent Publication No. 2003-0023300 entitled “Micromirror Device Using Interdigitated Cantilevers and Its Applications.” A formation method of mirror support posts will be described based on the structure disclosed in the above patent.  
         [0051]      FIG. 5  is a flowchart illustrating an electro-plating process of a mirror support post portion of a micromirror device according to an embodiment of the present invention.  FIG. 5  is a cross-sectional view of the micromirror device taken along line a-a′ in  FIG. 3 .  
         [0052]     As shown in  FIG. 5 , to fabricate mirror support posts, a mirror support post formation region  41  is required.  
         [0053]     As shown in  FIG. 5 ( a ), the mirror support post formation region  41  is formed using a photolithography process.  
         [0054]     A seed electrode  44  is first formed on a substrate  45 .  
         [0055]     A lower electrode  48  for applying a voltage to rotate the mirror, a driver  43  having a cantilever and a cantilever support post, for driving the micromirror device, and a sacrificial layer  42  comprising the mirror support post formation region  41  are formed on the seed electrode  44  by means of a photolithography process.  
         [0056]     The photolithography process is a process that has been widely known to those skilled in the art. Therefore, description thereof will be omitted. The region  41  in which the mirror support post will be formed is patterned by the process.  
         [0057]     Referring to  FIG. 5 ( b ), the region  41  formed in  FIG. 5 ( a ) is filled with metal by means of the electro-plating method described with reference to  FIG. 4 . In this case, the metal  46  becomes the mirror support post.  
         [0058]     In this case, it is preferred that the metal  46  filled by electro-plating has the same height as the sacrificial layer  42 .  
         [0059]     Referring to  FIG. 5 ( c ), after the mirror support post  46  is formed, a mirror  47  is formed by a deposition process and a photolithography process.  
         [0060]     In this case, if the mirror support post  46  is formed to have the same height as the sacrificial layer  42  by the electro-plating process, the region to be used as the mirror support post  46  is completely filled with a metal material.  
         [0061]     If the mirror  47  is formed by a method, such as the deposition method, by forming the mirror support post  46  whose hole is fully filled, a region on which the mirror  47  will be deposited when forming the mirror  47  becomes perfectly flat. Therefore, the mirror  47  having a perfectly flat can be formed.  
         [0062]     Therefore, it is preferred that the mirror support post be formed to have the same height as the sacrificial layer and to have a perfectly flat surface, by accurately controlling the plating method.  
         [0063]     Thereafter, the sacrificial layer  42  is removed and the seed electrode  44  formed for the plating process is removed, if needed, for the purpose of avoiding an electrical short circuit.  
         [0064]      FIG. 6  is a flowchart illustrating an electro-plating process of a mirror support post portion of a micromirror device according to another embodiment of the present invention.  FIG. 6  is a cross-sectional view of the micromirror device taken along line a-a′ in  FIG. 3 .  
         [0065]     The mirror support post portion shown in  FIG. 6  is different from that of  FIG. 5  in that a seed electrode  54  for electrical connection of an electro-plating process is formed not on a substrate  56 , but formed in a sacrificial layer  52 .  
         [0066]     Referring to  FIG. 6 ( a ), a mirror support post formation region  51  is formed using a photolithography process.  
         [0067]     A lower electrode  55  to which a voltage is applied to rotate the mirror, the seed electrode  54  formed in the sacrificial layer  52  and a cantilever support post of a driving unit  53  of the micromirror, the driving unit  53  of the micromirror having a cantilever and a cantilever support post, and a sacrificial layer  52  comprising the mirror support post formation region  51  are formed by a photolithography process.  
         [0068]     In the same manner as  FIG. 5 , the photolithography process is a process that has been widely known to those skilled in the art. Therefore, description thereof will be omitted. The region  51  in which a mirror support post will be formed is patterned by the process.  
         [0069]     Referring to  FIG. 6 ( b ), the region  51  formed in  FIG. 6 ( a ) is filled with metal by means of the electro-plating method that has been described with reference to  FIG. 4 . The mirror support post  57  that is fully filled can be formed without limitation in height. In this case, the metal  57  filled by electro-plating becomes the mirror support post.  
         [0070]     In this case, it is preferred that the electroplated region  51  is formed to have the same height as the sacrificial layer  52 .  
         [0071]     Referring to  FIG. 6 ( c ), after the mirror support post  57  is formed, a mirror  58  is deposited on the mirror support post  57 .  
         [0072]     If the mirror  58  is formed by a method, such as the deposition method, by forming the mirror support post  57  as described above, the mirror  57  having a perfectly flat surface can be fabricated.  
         [0073]     The region  51  that is previously defined by a photolithography process, which is one of processes of forming a semiconductor pattern, is filled using a plating process.  
         [0074]     The conducting layer  54  for electrical connection of the electro-plating process can be formed between-the-sacrificial layer  52 . The micromirror structure comprises a conductive material such as metal for the purpose of voltage application.  
         [0075]     In the electro-plating process, since deposition is performed beginning from an electrically connected portion, the hole is sequentially filled. The mirror support post  57  that is fully filled can be formed without limitation in height.  
         [0076]     The plated region can be formed to have the same height as the sacrificial layer  52 .  
         [0077]     By forming the mirror support post  57  as described above, the mirror  58  with a perfectly flat surface can be formed if the mirror  58  is formed using a method such as the deposition method.  
         [0078]     Thereafter, the sacrificial layer  52  is removed and the metal layer  54  formed for the plating process is then removed, as necessary, for the purpose of avoiding an electrical short circuit.  
         [0079]      FIG. 7  is a flowchart illustrating an electro-plating process of a mirror support post portion of a micromirror device according to still another embodiment of the present invention.  FIG. 7  is a flowchart illustrating an electro-plating process of the mirror support post portion of the micromirror device shown in  FIG. 1 .  
         [0080]      FIG. 7 ( a ) is a cross-sectional view of the micromirror device in which an upper sacrificial layer  61  and a lower sacrificial layer  62  are formed to form a conducting layer  65 , a hinge  64 , a yoke  66  and a mirror support post  63 . The conducting layer  65  for electrical connection is first formed on a substrate  70 . The lower sacrificial layer  62  for forming the hinge  64  is then formed. The lower sacrificial layer  62  is removed by a subsequent process. A space from which the lower sacrificial layer  62  is removed remains as an air gap. The hinge  64  is formed on the lower sacrificial layer  62 .  
         [0081]     The yoke  66  is then formed on the hinge  64 . The upper sacrificial layer  61  comprising a mirror support post formation region  60  is formed on the yoke  66  and the hinge  64 .  
         [0082]     Referring to  FIG. 7 ( b ), the mirror support post  63  is formed in the mirror support post formation region  60  by an electro-plating process as described above. The mirror support post  63  may have the same height as the upper sacrificial layer  61  and may have a perfectly flat surface.  
         [0083]     Referring next to  FIG. 7 ( c ), the mirror  64  is deposited on the upper sacrificial layer  61  and the mirror support post  63 . As shown in  FIG. 7 ( d ), the upper sacrificial layer  61  and the lower sacrificial layer  62  are removed.  
         [0084]     In this case, if the micromirror device of  FIG. 1  is fabricated by completely filling the mirror support post  63  using the electro-plating process, a perfectly flat mirror surface cam be formed as described above.  
         [0085]     At this time, the conducting layer  65  may be used as a seed electrode (refers to  44  in  FIG. 5 ) for electrical connection for the purpose of the electro-plating process. Alternatively, the conducting layer  65  may be formed on the hinge  64  and may be used as the seed electrode, as shown in  FIG. 6 . On the other hand, after the upper sacrificial layer  61  and the lower sacrificial layer  62  are removed, the conducting layer  65  may be removed, if necessary, for avoiding an electrical short circuit.  
         [0086]      FIG. 8  is a SEM photograph of micromirror devices that are actually fabricated according to the present invention.  
         [0087]     As described above, according to the present invention, a pole that supports a mirror surface is formed by a plating method. Therefore, a hole of a mirror surface that reflects light can be obviated. Therefore, the present invention is advantageous in that a micromirror device can have a perfectly flat mirror surface.  
         [0088]     Furthermore, a perfectly flat mirror surface can be formed. Therefore, the present invention is advantageous in that it can improve the reflection efficiency of light, obtain a high contrast ratio of display images with a high quality and save power consumption.  
         [0089]     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.