US20070199823A1

US20070199823A1 - Method of manufacturing mirror support post of micromirror device using electro-plating process

Info

Publication number: US20070199823A1
Application number: US11/363,846
Authority: US
Inventors: Jin-Wan Jeon; Dae-Hyun Kim; Jun-Bo Yoon; Koeng Lim
Original assignee: Korea Advanced Institute of Science and Technology KAIST
Current assignee: Korea Advanced Institute of Science and Technology KAIST
Priority date: 2006-02-28
Filing date: 2006-02-28
Publication date: 2007-08-30

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

1. Field of the Invention
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.
2. Background of the Related Art
FIG. 1 is a cross-sectional view of a micromirror device in the related art, which is cut on a center line.
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.
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.
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.
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

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.
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.
In the step of forming the mirror support post, the mirror support post may have the same height as the sacrificial layer.
In the step of forming the mirror support post, the mirror support post may have a flat surface.
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.
In the step of forming the mirror support post, the mirror support post may have the same height as the sacrificial layer.
In the step of forming the mirror support post, the mirror support post may have a flat surface.
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.
In the step of forming the mirror support post, the mirror support post may have the same height as the upper sacrificial layer.
In the step of forming the mirror support post, the mirror support post may have a flat surface.

BRIEF DESCRIPTION OF THE DRAWINGS

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:
FIG. 1 is a cross-sectional view of a micromirror device in the related art, which is cut on a center line;
FIG. 2 is a dismantled perspective view schematically showing the construction of a micromirror device according to the present invention;
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;
FIG. 4 is a flowchart schematically illustrating a plating process of a mirror support post of a micromirror device according to the present invention;
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. 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;
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
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

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.
The present invention will now be described in detail in connection with preferred embodiments with reference to the accompanying drawings.
FIG. 2 is a dismantled perspective view schematically showing the construction of a micromirror device according to the present invention.
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.
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.
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.
The cantilever 25 that connects the cantilever support post23 and the mirror support post24 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.
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.
A pair of the electrodes 22 for driving the mirror 20 is connected to an addressing circuit (not shown).
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.
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.
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.
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.
FIG. 4 is a flowchart schematically illustrating a plating process of a mirror support post of the micromirror device according to the present invention.
The plating process applied to the present invention comprises a general electro-plating method.
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.
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.
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.
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.
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.
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.
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.
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.
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.
As shown in FIG. 5, to fabricate mirror support posts, a mirror support post formation region 41 is required.
As shown in FIG. 5(a), the mirror support post formation region 41 is formed using a photolithography process.
A seed electrode 44 is first formed on a substrate 45.
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.
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.
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.
In this case, it is preferred that the metal 46 filled by electro-plating has the same height as the sacrificial layer 42.
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.
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.
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.
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.
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.
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.
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.
Referring to FIG. 6(a), a mirror support post formation region 51 is formed using a photolithography process.
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.
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.
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.
In this case, it is preferred that the electroplated region 51 is formed to have the same height as the sacrificial layer 52.
Referring to FIG. 6(c), after the mirror support post 57 is formed, a mirror 58 is deposited on the mirror support post 57.
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.
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.
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.
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.
The plated region can be formed to have the same height as the sacrificial layer 52.
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.
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.
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.
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.
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.
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.
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.
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.
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.
FIG. 8 is a SEM photograph of micromirror devices that are actually fabricated according to the present invention.
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.
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.
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.

Claims

1. A method of manufacturing a mirror support post of a micromirror device using a plating process, the method comprising 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.

2. The method as claimed in claim 1, wherein in the step of forming the mirror support post, the mirror support post has the same height as the sacrificial layer.

3. The method as claimed in claim 1, wherein in the step of forming the mirror support post, the mirror support post has a flat surface.

4. A method of manufacturing a mirror support post of a micromirror device using a plating process, the method comprising 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 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.

5. The method as claimed in claim 4, wherein in the step of forming the mirror support post, the mirror support post has the same height as the sacrificial layer.

6. The method as claimed in claim 4, wherein in the step of forming the mirror support post, the mirror support post has a flat surface.

7. A method of manufacturing a mirror support post of a micromirror device using a plating process, the method comprising 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.

8. The method as claimed in claim 7, wherein in the step of forming the mirror support post, the mirror support post has the same height as the upper sacrificial layer.

9. The method as claimed in claim 7, wherein in the step of forming the mirror support post, the mirror support post has a flat surface.