Abstract:
A method for fabricating an interference display unit is disclosed. A first wall and a sacrificial layer are formed in order on a substrate and an opening is formed in the first wall and the sacrificial layer. A first photoresist layer is spin-coated on the sacrificial layer and fills the openings. A post having a first arm is formed through patterning the first photoresist layer. At least a second photoresist is formed by spin-coating. A second arm is formed on the first arm through patterning the second photoresist layer. A second wall is formed on the sacrificial layer and posts. The first and the second arms&#39; stress is released through a thermal process. The position of the arm is shifted and the distance between the first wall and the second wall is therefore defined. Finally, the sacrificial layer is removed.

Description:
CLAIM OF PRIORITY  
       [0001]    This application claims benefit to the Taiwanese Application No. 92109265 filed Apr. 21, 2003, which is hereby incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method for manufacturing an optical interference display. More particularly, the present invention relates to a method for manufacturing an optical interference display with posts of arms.  
           [0004]    2. Description of the Related Art  
           [0005]    Planar displays are extremely popular in the portable and limited-space display market because they are lightweight and small. To date, planar displays including liquid crystal display (LCD), organic electro-luminescent display (OLED), plasma display panel (PDP) and so on, as well as a mode of the optical interference display have been investigated.  
           [0006]    U.S. Pat. No. 5,835,255 discloses an array of display units of the visible light that can be used for a planar display. Reference is made to FIG. 1, which depicts a cross-sectional 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 semi-transmissible/semi-reflective layer with an absorption rate that partially absorbs visible light, and the other one is a light reflective layer that is deformable when a voltage is applied. When the incident light passes through the wall  102  or  104  and arrives in the cavity  108 , in all the visible light spectrum, only visible light with a wavelength corresponding to formula 1.1 can generate a constructive interference and be emitted, that is, 
           2D=Nλ  (1.1) 
           [0007]    where N is a natural number.  
           [0008]    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 mean time, if the observer follows the direction of the incident light, a reflected light with wavelength λ 1  can be observed. Therefore, the display unit  100  is “on”.  
           [0009]    The first wall  102  is a semi-transmissible/semi-reflective electrode that comprises a substrate, an absorption layer, and a dielectric layer. An incident light passing through the first wall  102  is partially absorbed by the absorption layer. The substrate is made from conductive and transparent materials, such as ITO glass or IZO glass. The absorption layer is made from metal such as aluminum, chromium or silver and so on. The dielectric layer is made from silicon oxide, silicon nitrite or metal oxide. The metal oxide can be obtained by directly oxidizing a portion of the absorption layer. The second wall  104  is a deformable reflective electrode. It shifts up and down by applying a voltage. The second wall  104  is typically made from dielectric materials/conductive transparent materials, or metal/conductive transparent materials.  
           [0010]    [0010]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 d is used instead of D in formula 1.1, only visible light with a wavelength satisfying formula 1.1, which is λ 2 , can generate a constructive interference, and be reflected by the wall  104 , and pass through the wall  102 . Due to the wall  102  with the high light absorption rate for the light with wavelength λ 2 , all the incident light in the visible light spectrum is filtered out; therefore 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 “off”.  
           [0011]    Reference is made to FIG. 1 again, which shows that the posts  106  of the display unit  100  are generally made from negative photoresist materials. Reference is also made to FIGS. 3A to  3 C, which depict a method for manufacturing a display unit in the prior art. In FIG. 3A, the first wall  102  and a sacrificial layer  110  are formed in order on a transparent substrate  109 , and then an opening  112  is formed in the wall  102  and the sacrificial layer  110 . The opening  112  is suitable for forming posts therein. Next, a negative photoresist layer  111  is spin-coated on the sacrificial layer  110  and fills the opening  112 . The objective of forming the negative photoresist layer  111  is to form posts between the first wall  102  and the second wall (not shown). A backside exposure process is performed on the negative photoresist layer  111  in the opening  112 , in a direction indicated by arrow  113 . The sacrificial layer  110  must be made from opaque materials, typically metal materials, to meet the requirements of the backside exposure process.  
           [0012]    Reference is made to FIG. 3B, which shows that posts  106  remain in the opening  112  after removing the unexposed negative photoresist layer. The wall  104  is then formed on the sacrificial layer  110  and posts  106 . Reference is made to FIG. 3C, in which the sacrificial layer  110  is removed by a release etch process to form a cavity  114 . The length D of the cavity  114  is the thickness of the sacrificial layer  110 . Therefore, the different thicknesses of the sacrificial layers must be used in different processes of the different display units to achieve the objective of controlling reflection of light with different wavelengths.  
           [0013]    An array comprising the display unit  100  controlled by voltage operation is sufficient for a single color planar display, but not for a color planar display. A method in the prior art is to manufacture a pixel that comprises three display units with different lengths of the cavities as shown in FIG. 4, which depicts a cross-sectional view for a matrix color planar display in the prior art. Three display units  302 ,  304  and  306  are formed as an array on a substrate  300 , respectively. Display units  302 ,  304  and  306  can reflect an incident light  308  to colors of light with different wavelengths, for example, which are red, green and blue lights, due to the different lengths of the cavities of the display units  302 ,  304  and  306 . Use of different reflective mirrors for the display units arranged in the array is not required. More important is that good resolution is provided and the brightness among all colors of light is uniform. However, three display units with different lengths of cavities need to be manufactured separately.  
           [0014]    Reference is made to FIGS. 5A to  5 D, which depict cross-sectional views of a method for manufacturing the matrix color planar display in the prior art. In FIG. 5A, the first wall  310  and the first sacrificial layer  312  are formed in order on a transparent substrate  300 , and then openings  314 ,  316 ,  318 , and  320  are formed in the first wall  310  and the sacrificial layer  312  for defining predetermined positions wherein display units  302 ,  304 , and  306  formed. Subsequently, the second sacrificial layer  322  is conformally formed on the first sacrificial layer  312  and in the openings  314 ,  316 ,  318 , and  320 .  
           [0015]    In FIG. 5B, after the second sacrificial layer  322  in and between the openings  314  and  316 , and in the openings  318  and  320  is removed by a photolithographic etch process, the third sacrificial layer  324  is conformally formed on the first sacrificial layer  312  and the second sacrificial layer  322  and in the openings  314 ,  316 ,  318  and  320 .  
           [0016]    Reference is made to FIG. 5C, in which the third sacrificial layer  324  in the openings  318  and  320  is left but the remainder of the third sacrificial layer  324  is removed by a photolithographic etching process. Next, a negative photoresist is spin-coated on the first sacrificial layer  312 , the second sacrificial layer  322 , and the third sacrificial layer  324 , and in the openings  314 ,  316 ,  318  and  320 , and fills the all openings to form a negative photoresist layer  326 . The objective of the negative photoresist layer  326  is to form posts (not shown) between the first wall  310  and the second wall (not shown).  
           [0017]    Reference is made to FIG. 5D, which shows that a backside exposure process is performed on the negative photoresist layer  326  in the openings  314 ,  316 ,  318  and  320 , in a direction of the transparent substrate  300 . For the requirement of the backside exposure process, the sacrificial layer  110  at least must be made from opaque materials, and typically metal materials. Posts  328  are left in the openings  314 ,  316 ,  318  and  320  after removing the unexposed negative photoresist layer  326 . Subsequently, the second wall  330  conformally covers the first sacrificial layer  312 , the second sacrificial layer  322 , the third sacrificial layer  324  and posts  328 .  
           [0018]    Afterward, the first sacrificial layer  312 , the second sacrificial layer  322 , and the third sacrificial layer  324  are removed by a release etch process to form the display units  302 ,  304 , and  306  shown in FIG. 4, where the lengths d 1 , d 2 , and d 3  of three display units  302 ,  304 , and  306  are the thicknesses of the first sacrificial layer  312 , the second sacrificial layer  322 , and the third sacrificial layer  324 , respectively. Therefore, different thicknesses of the sacrificial layers must be used in different processes of the different display units to control reflection of different wavelengths of light.  
           [0019]    At least three photolithographic etching processes are required for manufacturing the matrix color planar display in the prior art to define the lengths of the cavities of the display units  302 ,  304 , and  306 . In order to cooperate with the backside exposure for forming posts, metal materials must be used for making the sacrificial layer. The cost of the complicated manufacturing process is higher, and the yield cannot be raised due to the complicated manufacturing process.  
           [0020]    Therefore, it is an important subject to provide a simple method of manufacturing an optical interference display unit structure, for manufacturing a color optical interference display with high resolution, high brightness, simple process and high yield.  
         SUMMARY OF THE INVENTION  
         [0021]    It is therefore an objective of the present invention to provide a method for manufacturing an optical interference display unit structure, which method is suitable for manufacturing a color optical interference display and provides high resolution and high brightness.  
           [0022]    It is another objective of the present invention to provide a method for manufacturing an optical interference display unit structure suitable for manufacturing a color optical interference display, which method has a simple and easy manufacturing process and high yield.  
           [0023]    It is still another objective of the present invention to provide a method for manufacturing an optical interference display unit structure suitable for manufacturing a color optical interference display with posts.  
           [0024]    In accordance with the foregoing objectives of the present invention, one preferred embodiment of the invention provides a method for manufacturing an optical interference display unit structure. The first wall and a sacrificial layer are formed in order on a transparent substrate, and then an opening is formed in the first wall and the sacrificial layer. The opening is suitable for forming posts therein. Next, the first photoresist layer is spin-coated on the sacrificial layer and fills the opening. A photolithographic process patterns the photoresist layer to define a support with an arm, in which the support and the arm are used for a post, and to define the length of the first supporting layer. Subsequently, at least a second photoresist layer is spin-coated on the first photoresist layer and the sacrificial layer for defining the second supporting layer, in which the first and second supporting layers form an arm. Due to the exposure of the photoresist layer with the help of a mask, the sacrificial layer no longer must be made of opaque materials such as metal and the like; common dielectric materials are also used for making the sacrificial layer.  
           [0025]    The second wall is formed on the sacrificial layer and posts, and then the posts are baked. The arm may generate displacement as the pivot of the support caused by stress action, in which an end of the arm adjacent to the support has less displacement, but another end of the arm has more displacement. The displacement of the arm may change the position of the second wall. Afterward, the sacrificial layer is removed by a release etch process to form a cavity, and the length D of the cavity may not be equal to the thickness of the sacrificial layer due to the displacement of the arm.  
           [0026]    The arms with the ratios of various lengths to thicknesses have various amounts of stress due to the difference between thicknesses of arms, and displacements and directions generated by arms are variable during baking. Therefore, the arms with the ratios of various lengths to thicknesses may be used for controlling the length of the cavity, instead of the various thicknesses of the sacrificial layers used in the various processes of the display units to control various wavelengths of light reflected in the prior art. There are many advantages in the above way. First of all, the cost drops drastically. The thickness of the cavity in the prior art is the thickness of the sacrificial layer, and the sacrificial layer needs to be removed at the end of the process. However, the length of the cavity is increased by using an upward displacement of the arms in the present invention, so that the length of the cavity is greater than the thickness of the sacrificial layer, even when the thickness of the sacrificial layer is substantially decreased while forming the same length of cavities. Therefore, the material used for manufacturing the sacrificial layer is substantially reduced. Second, the process time is shortened. The release etch process of the metal sacrificial layer in the prior art consumes lots of time, because the sacrificial layer is removed by an etching gas that must permeate into spaces between the posts. The present invention utilizes the mask for a front exposure, so the sacrificial layer may be made of transparent materials such as dielectric materials, instead of opaque materials such as metal and the like in the prior art. Besides, the thickness used by the sacrificial layer can be substantially reduced, so the time required for the release etch process can be also drastically decreased. Moreover, the use of dielectric materials also speeds up the release etch process, such that the time required for the release etch process is decreased. Third, the length of the arms may decrease the effective reflection area of the optical interference display unit. If the color optical interference display is formed only with posts having arms of various lengths, because the effective reflection areas of the optical interference display units are different, variation may occur in the intensity of the reflected light. Furthermore, if the posts are made from photoresist materials, the thickness of the photoresist layer that is generally formed by spin-coating is limited. After a thermal process and displacement, the structural strength for supporting the second wall may not be enough. Thus, the variation in the thickness of arms of posts changes the ratios of lengths to thicknesses of arms for changing the stress of arms. It makes the effective reflection areas of optical interference display units with different colors of light closer to each other, and also strengthens the structural strength of arms. After baking, various optical interference display units have various lengths of the cavities due to the displacement of arms, such that reflected light is changed with various wavelengths, such as red (R), green (G), and blue (B) lights, so as to obtain various colors of light.  
           [0027]    In accordance with another an objective of the present invention, one preferred embodiment of the invention provides a method for manufacturing a matrix color planar display structure. Each matrix color planar display unit has three optical interference display units. The first wall and a sacrificial layer are formed in order on a transparent substrate, and then an opening is formed in the first wall and the sacrificial layer. The opening is suitable for forming posts therein, and posts are used for defining the first, the second, and the third optical interference display units. Next, the first photoresist layer is spin-coated on the sacrificial layer and fills the opening. A photolithographic process patterns the photoresist layer to define a support with the first supporting layer. The support with the first supporting layer is used for a post, and defines the length of the arm. Then, the second photoresist layer is spin-coated on the first photoresist layer and the sacrificial layer and fills the opening. The second photoresist layer disposed on the first supporting layer of the second and the third optical interference display units is left for forming a second supporting layer by a photolithographic process. Later, the third photoresist layer is spin-coated on the first photoresist layer, the second photoresist layer, and the sacrificial layer and fills the opening. The third photoresist layer disposed on the second supporting layer of the third optical interference display unit, is left for forming a third supporting layer by a photolithographic process. The first supporting layer forms the first arm of the first optical interference display unit, the first and the second supporting layers form the second arm of the second optical interference display unit, and the first, the second and the third supporting layers form the third arm of the third optical interference display unit. The arms of three optical interference display units are the same in length but different in thickness. Due to the exposure of the photoresist layer with the help of a mask, the sacrificial layer no longer must be opaque materials such as metal and the like; common dielectric materials are also used for making the sacrificial layer.  
           [0028]    The second wall is formed on the sacrificial layer and posts, and then the posts are baked. The arms of three optical interference display units are different in the ratio of length to thickness, and thus different in stress. After a thermal process, the arms of three optical interference display units are different in displacement. The arm may generate displacement as the pivot of the support caused by stress action, where an end of the arm adjacent to the support has less displacement, but another end of the arm has more displacement. The displacement of the arm may change the position of the second wall. Afterward, the sacrificial layer is removed by a release etch process to form a cavity, and the length D of the cavity may not be equal to the thickness of the sacrificial layer due to the displacement of the arm.  
           [0029]    The first wall is the first electrode, and the second wall is the second electrode. Each arm of the optical interference display unit is different in length and stress. Therefore, after baking, each optical interference display unit has various lengths of the cavities due to the various displacements of arms, such that reflected light is changed with different wavelengths, such as red, green, and blue light, so as to obtain various colors of light, thus to obtain a matrix color planar display structure.  
           [0030]    In accordance with the color planar display consisting of an array of optical interference display units disclosed by the present invention, high resolution and high brightness are obtained, and each optical interference display unit is similar in the effective reflection area, as well being simple in process and high in yield. It is understood that the present invention discloses the optical interference display unit which not only has uniform color tones, high resolution, high brightness, a simple process and high yield during forming arrays, but also increases the abundance during processing and raises the yield of the optical interference color planar display.  
           [0031]    It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    These and other features, aspects, and advantages of the present invention will be more fully understood by reading the following detailed description of the preferred embodiment, with reference made to the accompanying drawings as follows:  
         [0033]    [0033]FIG. 1 depicts a cross-sectional view of a display unit in the prior art;  
         [0034]    [0034]FIG. 2 depicts a cross-sectional view of a display unit in the prior art after applying a voltage;  
         [0035]    [0035]FIGS. 3A to  3 C depict a method for manufacturing a display unit in the prior art;  
         [0036]    [0036]FIG. 4 depicts a cross-sectional view of a matrix color planar display in the prior art;  
         [0037]    [0037]FIGS. 5A to  5 D depict cross-sectional views of a method of manufacturing a matrix color planar display in the prior art;  
         [0038]    [0038]FIGS. 6A to  6 C depict a method for manufacturing an optical interference display unit according to one preferred embodiment of this invention;  
         [0039]    [0039]FIG. 6D depict a cross-sectional view of an optical interference display unit according to one preferred embodiment of this invention; and  
         [0040]    [0040]FIGS. 7A to  7 F depict a method of manufacturing a matrix color planar display structure according to the second preferred embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0041]    In order to provide more information of the optical interference display unit structure, the first embodiment is provided herein to explain the optical interference display unit structure in this invention. In addition, the second embodiment is provided to give further description of the optical interference color planar display formed with an array of the optical interference display unit.  
         [0042]    [0042]FIGS. 6A to  6 C depict one embodiment of a method for manufacturing an optical interference display unit according to a preferred embodiment of the invention. Reference is made to FIG. 6A first, in which a first electrode  502  and a sacrificial layer  506  are formed in order on a transparent substrate  501 . The sacrificial layer  506  may be made of transparent materials such as dielectric materials, or opaque materials such as metal materials. An opening  508  is formed in the first electrode  502  and the sacrificial layer  506  by a photolithographic etch process. The opening  508  is suitable for forming a post therein.  
         [0043]    Next, a first material layer  510  is formed in the sacrificial layer  506  and fills the opening  508 . The first material layer  510  is suitable for forming posts, and the first material layer  510  generally uses photosensitive materials such as photoresists, or a non-photosensitive polymer materials such as polyester, polyamide or the like. If the non-photosensitive materials are used for forming the material layer  510 , a photolithographic etch process is required to define posts in the first material layer  510 . In this embodiment, the photosensitive materials are used for forming the first material layer  510 , so merely a photolithographic etch process is required for patterning the first material layer  510 .  
         [0044]    Reference is made to FIG. 6B, in which the posts  512  are defined by patterning the first material layer  510  during a photolithographic process. The post  512  has a support  514  disposed in the opening  508 , and the post  512  has the first supporting layers  5121  and  5122 . The same photolithographic process also defines the lengths of the first supporting layers  5121  and  5122 . Next, a second material layer (not shown) is formed on the sacrificial layer  506  and the first supporting layers  5121  and  5122 . Then, the second material layer on the sacrificial layer  506  is patterned and removed by a photolithographic process, for forming the second supporting layers  5123  and  5124  on the first supporting layers  5121  and  5122 . Thus, the first supporting layer  5121  and the second supporting layer  5123  form the first arm  516 , and the first supporting layer  5122  and the second supporting layer  5124  form the first arm  518 . A second electrode  504  is formed on the sacrificial layer  506  and the post  512 .  
         [0045]    Reference is next made to FIG. 6C. A thermal process, such as baking, is performed. The first arm  516  and the second arm  518  of the post  512  may generate displacement as the pivot of the support  514  caused by stress action, where ends of the first arm  516  and the second arm  518  adjacent to the support  514  have less displacement, but another ends of the first arm  516  and the second arm  518  have more displacement. The displacement of the first arm  516  and the second arm  518  may change the position of the second electrode  504 . Thereafter, the sacrificial layer  506  is removed by a release etching process to form a cavity  520 .  
         [0046]    If the first material layer  510  is made from photoresist materials, the spin-coated photoresist layer is limited in thickness; thus the first supporting layers  5121  and  5122  may have less structural strength. By forming the second supporting layers  5123  and  5124 , the first supporting layers  5121  and  5122  are increased in thickness to strengthen their structural strength.  
         [0047]    The optical interference display unit made as illustrated by FIGS. 6A to  6 C is shown in FIG. 6D, which depicts a cross-sectional view of an optical interference display unit of one preferred embodiment of this invention. An optical interference display unit  500 , which may be a color changeable pixel unit, at least comprises a first electrode  502  and a second electrode  504 , with the first electrode  502  and the second electrode  504  are arranged approximately parallel to each other. The first electrode  502  and the second electrode  504  can be narrowband mirrors, broadband mirrors, non-metal mirrors or the combination thereof.  
         [0048]    Posts  512  support the first electrode  502  and the second electrode  504 . The first arm  516  and the second arm  518  of the posts  512  are raised upwards. The length of the cavity is the thickness of the sacrificial layer in the optical interference display unit structure in the prior art. If the thickness of the sacrificial layer is D, the length of the cavity is D, too. In this embodiment, a cavity  520  is formed between the first electrode  502  and the second electrode  504  supported by posts  512 . The posts  512  have the first arm  516  and the second arm  518 . The ratio of lengths to thicknesses of the first arm  516  and the second arm  518  decide stress thereof, and a dotted line  516 ′ and a dotted line  518 ′ label the positions prior to performing a thermal process of the first arm  516  and the second arm  518 . After performing the thermal process, the first arm  516  and the second arm  518  may generate displacement; therefore the position of the second electrode  504  changes from the original position labeled by the dotted line  504 ′, and the length D′ of the cavity  520  between the first electrode  502  and the second electrode  504  changes from the original length D. Since the length of the cavity  520  is changed, the frequency of a reflected light changes following the length of the cavity  520 . In general, when post  512  is made from polyamide compounds, the ratio of lengths to thicknesses of the first arm  516  and the second arm  518  is  5  to  50 , and the length D′ of the cavity  520  is approximately 1.5 to 3 times the length D of the thickness of the sacrificial layer. Of course, the ratio of lengths to thicknesses of the first arm  516  and the second arm  518  can be changed to make the length D′ of the baked cavity  520  smaller than the thickness of the sacrificial layer.  
         [0049]    In one aspect of this invention, the materials suitable for forming posts  512  include positive photoresists, negative photoresists, and all kinds of polymers such as acrylic resins and epoxy resins.  
         [0050]    [0050]FIGS. 7A to  7 F depict another embodiment of a method for manufacturing a matrix color planar display structure according to the second preferred embodiment of this invention. Reference is first made to FIG. 7A, in which the first electrode  602  and a sacrificial layer  604  are formed in order on a transparent substrate  601 . The sacrificial layer  604  can be made of transparent materials such as dielectric materials, or opaque materials such as metal materials. Openings  606 ,  608 ,  610 , and  612  are formed in the first electrode  602  and the sacrificial layer  604  by a photolithographic etch process, and openings  606 ,  608 ,  610 , and  612  are suitable for forming posts therein.  
         [0051]    Next, a material layer  614  is formed on the sacrificial layer  604  and fills the openings  606 ,  608 ,  610 , and  612 . The optical interference display unit  630  is defined by openings  606  and  608 , the optical interference display unit  632  is defined by openings  608  and  610 , and the optical interference display unit  634  is defined by openings  610  and  612 . The material layer  614  is suitable for forming posts, and is generally made from photosensitive materials such as photoresists or a non-photosensitive polymer materials such as polyester, polyamide or the like. If non-photosensitive materials are used for forming the first material layer  614 , a photolithographic etch process is required to define posts on the first material layer  614 . In this embodiment, the photosensitive materials are used for forming the first material layer  614 , so merely a photolithographic etch process is required for patterning the first material layer  614 .  
         [0052]    Reference is made to FIG. 7B. A photolithographic process patterns the first material layer  614 , so as to define posts  616 ,  618 ,  620 , and  622 . The posts  616 ,  618 ,  620 , and  622  have supports  6161 ,  6181 ,  6201 , and  6221  disposed in the openings  606 ,  608 ,  610 , and  612 , respectively. The posts  616 ,  618 ,  620 , and  622  also have the first supporting layers  6162 ,  6182 ,  6183 ,  6202 ,  6203 , and  6222 . The first supporting layers  6162 ,  6182 ,  6183 ,  6202 ,  6203 , and  6222  are the same in length. Subsequently, a second material layer  624  is formed on the sacrificial layer  604  and the first supporting layers  6162 ,  6182 ,  6183 ,  6202 ,  6203 , and  6222 .  
         [0053]    Reference is made to FIG. 7C. A photolithographic process patterns the second material layer  624 , for keeping the second material layer  624  on the first supporting layers  6162 ,  6182 ,  6183 ,  6202 ,  6203 , and  6222 , so as to form the second supporting layers  6241 ,  6242 ,  6243 , and  6244 . Further, a third material layer  626  is formed on the sacrificial layer  604  and the second supporting layers  6241 ,  6242 ,  6243 , and  6244 .  
         [0054]    Reference is made to FIG. 7D. A photolithographic process patterns the third material layer  626 , for keeping the third material layer  626  on the second supporting layers  6241 ,  6242 ,  6243 , and  6244 , so as to form the third supporting layers  6261  and  6262 . The first supporting layers  6162  and  6182  form the arms  646  and  648  of the optical interference display unit  630 . The first supporting layers  6183  and  6202 , and the second supporting layers  6241  and  6242  respectively, form the arms  636  and  638  of the optical interference display unit  632 . The first supporting layers  6203  and  6222 , the second supporting layers  6243  and  6244 , and the third supporting layers  6261  and  6262  respectively, form the arms  640  and  642  of the optical interference display unit  634 . Next, a second electrode  644  is formed on the sacrificial layer  604  and the arms  646 ,  648 ,  636 ,  638 ,  640 , and  642 .  
         [0055]    Reference is made to FIG. 7E. A thermal process, such as baking, is performed. The arms  646 ,  648 ,  636 ,  638 ,  640 , and  642  of the optical interference display units  630 ,  632 , and  634  may generate displacement as the pivot of the supports  6161 ,  6181 ,  6201 , and  6221  caused by stress action. There is less displacement at the ends of the arms  646 ,  648 ,  636 ,  638 ,  640 , and  642  adjacent to the supports  6161 ,  6181 ,  6201 , and  6221 , but more displacement at the other ends of the arms  646 ,  648 ,  636 ,  638 ,  640 , and  642 . The displacements of the arms  646  and  648  are the same, the displacements of the arms  636  and  638  are the same, and the displacements of the arms  640  and  642  are the same. But there are various displacements among three above pairs of the arms. Therefore, the amount of change in positions of the second electrode  644  caused by the arms  646  and  648 , the arms  636  and  638 , and the arms  640  and  642  is also varied.  
         [0056]    Thereafter, reference is made to FIG. 7F. The sacrificial layer  604  is removed by a release etch process to form the cavities  6301 ,  6321 , and  6341  of the optical interference display units  630 ,  632 , and  634 . The cavities  6301 ,  6321 , and  6341  have various lengths d 1 , d 2 , and d 3 , respectively. In the state that the optical interference display units  630 ,  632 , and  634  are “on”, as shown as the formula 1.1, the design of lengths d 1 , d 2 , and d 3  of the cavities  6301 ,  6321 , and  6341  can generate the reflected light with different wavelengths, such as red (R), green (G), or blue (B) light.  
         [0057]    The lengths d 1 , d 2 , and d 3  of the cavities  6301 ,  6321 , and  6341  are not decided by the thickness of the sacrificial layer, but by the lengths of the arms  646  and  648 ,  636  and  638 , and  640  and  642 , respectively. Therefore, a complicated photolithographic process as seen in the prior art where various lengths of the cavities are defined by forming various thicknesses of the sacrificial layers is unnecessary.  
         [0058]    Although the present invention has been described in considerable detail with reference certain preferred embodiments thereof, other embodiments are possible. Therefore, their spirit and scope of the appended claims should no be limited to the description of the preferred embodiments container herein. 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.