Abstract:
A liquid crystal display (LCD) substrate and a fabrication method thereof are provided. The LCD substrate comprises a substrate, a spacer definition layer formed on the substrate comprising a first step, and a spacer formed along a profile of the first step of spacer definition layer and adjacent to the first step, thereby forming a second step on the spacer. The invention utilizes a single photolithographic process to form spacers with steps, thus, effectively lowering the probability of mura defects caused by gravity, contact, or an uneven cell gap.

Description:
BACKGROUND 
   The invention relates to a liquid crystal display (LCD) substrate, and more particularly, to a LCD substrate having spacers with steps and a fabrication method thereof using a photolithographic process. 
   Liquid crystal displays (LCDs) typically comprise a pair of opposing substrates and a liquid crystal layer interposed therebetween. And a plurality of photo spacers is defined the distance between the opposing substrates. (i.e., cell gap). In order to extend the category of the LCD application, the cell gap of the LCD shall be shrunk and cell gap uniformity control will be a key issue in manufacturing. An uneven cell gap may cause luminance variation over a line or a region of the LCD panel, hereinafter referred to as mura defects. 
   Mura defects are related to the density of photo spacers or contact areas of the substrate with photo spacers. When external force temporarily applied, such as finger wiping, the photo spacers are deformed, causing photo spacer deviation. However, as the density of the photo spacer is large, the friction force increases. The spacer deviation cannot recover even if force removed, thereby causing a wipe mura defect. 
   If the density of the spacers decreases to ameliorate the wipe mura defect, other problems will occur. For example, when normal force is exerted on the substrate, the spacer deforms. When the density of the photo spacer is reduced, however, the support provided thereby is insufficient to withstand the force such that deformation cannot recover even if the force is removed, resulting in a push mura defect. 
   U.S. Patent. No. 2002/0075443, the entity of which is fully incorporated by reference herein, Shimizu et al. disclose two different height column-shaped spacers to solve the aforementioned problems. 
   Two different height column-shaped spacers are formed on the color filter substrate. One spacer contacts the TFT substrate, while the other does not.  FIG. 1  is a cross section illustrating two different height column-shaped spacers on the color filter substrate. A TFT substrate  100 A comprises signal lines  103  and  104 , an insulating layer  150 , a passivation layer  108 , and an alignment layer  111  thereon. A color filter substrate  100 B comprises a substrate  205 , a black matrix (BM)  203 , a passivation layer  204 , spacers  1   b  and  1   c , and an alignment layer  208 . A liquid crystal layer  900  is interposed between the TFT substrate  100 A and the color filter substrate  100 B. 
   Spacer  1   b  disposed on the signal line  104  contacts the TFT substrate  100 A, thereby creating a specific gap between the TFT substrate  100 A and the color filter substrate  100 B. The spacer  1   c  is not disposed on the signal line  104  and often kept a small distance away from the TFT substrate  100 A. When a normal force is applied on the LCD substrate, the spacer  1   b  can be elastically deformed while the spacer  1   c  can contact the TFT substrate  100 A. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
     FIG. 2  is a cross section illustrating another embodiment of two different height column-shaped spacers according to U.S. Patent. No. 2002/0075443. Only a portion of the color filter substrate  100 B is shown for the sake of simplicity. Numeral  205  denotes a substrate,  202  denotes a color filter,  203  denotes a black matrix (BM),  204  denotes a passivation layer, and  311  denotes a base pattern. The spacer  1   b  is disposed on the base pattern  311 . Similarly, the spacer  1   b  contacts the TFT substrate (not shown), while the spacer  1   c  is kept a small distance from the TFT substrate. When a normal force is applied on the LCD substrate, the spacer  1   b  can be elastically deformed while the spacer  1   c  can contact the TFT substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   Shimizu et al. also disclose a spacer with a step on top of the spacers capable of preventing push mura defects. A spacer with a step is formed on the color filter substrate. The step on the spacer partially contacts the TFT substrate.  FIGS. 3   a - 3   c  schematically depict procedures for manufacturing the spacer with a step. Referring  FIG. 3   a , a black matrix  203  and a color filter  202  are sequentially formed on the substrate  205 . A passivation layer  204  is formed on the substrate  205  covering the black matrix  203  and the color filter  202 . A photoresist layer  410  is formed on the passivation layer  204 . 
   Referring to  FIG. 3   b , the photoresist layer is lithographically exposed using a half-tone mask  510 . The center region  413  is exposed to a higher dosage than the peripheral region  411 , thus forming a spacer  420  with a step comprising a protrusion  425  and a recess  426 , as shown in  FIG. 3   c.    
     FIGS. 4   a - 4   c  schematically depict other procedures for manufacturing the spacer with a step using dual exposure steps. Referring  FIG. 4   a , a black matrix  203  and a color filter  202  are sequentially formed on the substrate  205 . A passivation layer  204  is formed on the substrate  205  covering the black matrix  203  and the color filter  202 . A photoresist layer  410  is formed on the passivation layer  204 . A portion  415  of the photoresist layer  410  is exposed using a mask  510 . 
   Referring to  FIG. 4   b , the photoresist layer  410  is then exposed using a second mask  510   b  with a smaller exposed region such that a portion  417  of the photoresist layer  410  is shielded. The region  415  is exposed to a higher dosage than the region  417 , thus forming a spacer  420  with a step comprising a protrusion  425  and a recess  426 , as shown in  FIG. 4   c.    
   According to the spacers with a step as disclosed in both  FIGS. 3   a - 3   c  and  FIGS. 4   a - 4   c , the protrusion  425  contacts the TFT substrate, while the recess  426  does not. When a normal force is applied on the LCD substrate, the protrusion  425  can be elastically deformed while the recess  426  can contact the TFT substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   The conventional methods of forming spacers with a step require half-tone exposure or dual exposure steps, thereby creating technical hurdles, process complexity, and cost barriers. 
   SUMMARY 
   Embodiments of the invention substantially overcome the disadvantages associated with the related art and achieve other advantages not realized by the related art. 
   Embodiments of the invention provide a LCD substrate comprising a structure having a first step. A spacer with a second step can be formed on the first step. Consequently, only one photo mask step is required to form a spacer with a step and is simpler than the conventional half-tone masking method. 
   One aspect of the invention is directed to a LCD substrate comprising a substrate, a spacer definition layer formed on the substrate comprising a first step, and a spacer formed along a profile of the first step adjacent to the first step, thereby forming a second step on the photo spacer. It is noted that the spacer definition layer comprises a light shield array or a color filter. 
   Another aspect of the invention is directed to a method for fabricating a LCD substrate comprising forming a spacer definition layer on a substrate having a first step, forming a spacer layer on the spacer definition layer, thereby forming a second step along a profile of the first step on the spacer layer, and defining the spacer layer into a spacer by a lithographic development step remaining from the second step. 
   In accordance with a first embodiment of the invention, a LCD substrate comprises a substrate, a light shield array formed on the substrate comprising a first opening and a second opening, thereby the first opening defines an active region and the second opening defines a first step, a color filter formed on the active region of the substrate, and a spacer formed along a profile of the first step adjacent to the first step, thereby forming a second step on the photo spacer. 
   The fabrication method for the LCD substrate in accordance with the first embodiment comprises forming a light shield array on a substrate comprising a first opening and a second opening, thereby the first opening defines an active region and the second opening defines a first step, forming a color filter in the active region, forming a spacer layer on the light shield array, thereby forming a second step along a profile of the first step, and defining the spacer layer into a spacer with the forgoing second step by a lithographic process. 
   In accordance with a second embodiment of the invention, a LCD substrate comprises a substrate, a light shield array formed on the substrate comprising a first opening defining an active region, a color filter formed in the active region of the substrate having an edge defining a first step, and a spacer formed along a profile of the first step adjacent to the first step, thereby forming a second step on the spacer. 
   The fabrication method for the LCD substrate in accordance with the second embodiment comprises forming a light shield array on a substrate comprising a first opening defining an active region, forming a color filter in the active region of the substrate having an edge defining a first step, forming a pacer layer on the color filter, thereby forming a second step along a profile of the first step, and defining the spacer layer into a spacer by a lithographic development step remaining from the second step. 
   In accordance with a third embodiment of the invention, a LCD substrate comprises a substrate, a light shield array formed on the substrate comprising a first opening defining an active region, a color filter formed in the active region and non-active region of the substrate, wherein the color filter comprises a third opening defining a first step in the non-active region, and a spacer formed along a profile of the first step adjacent to the first step, thereby forming a second step on the spacer. 
   The fabrication method for the LCD substrate in accordance with the third embodiment comprises forming a light shield array on a substrate comprising a first opening defining an active region, forming a color filter in the active region of the substrate, wherein the color filter comprises a third opening defining a first step in the non-active region, forming a spacer layer on the color filter, thereby forming a second step along a profile of the first step, and defining the spacer layer into a spacer with the forgoing second step by a lithographic process. 
   In accordance with a fourth embodiment of the invention, a LCD substrate comprises a substrate, a light shield array formed on the substrate comprising a first opening defining an active region, a color filter formed in the active region and non-active region of the substrate, and a first spacer and second spacer, wherein the first spacer is formed in the region without color filter, and the second spacer is formed in the non-active region with color filter, wherein a height difference is between the first spacer and the second spacer. 
   Embodiments of the invention additionally provide a liquid crystal display comprising a first substrate, a second substrate, a liquid crystal layer interposed between the first substrate and the second substrate, wherein a spacer definition layer formed on the first or the second substrate having a first step, a plurality of spacers formed along a profile of the first step adjacent to the first step, thereby forming a second step on the spacer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein: 
       FIG. 1  is a cross section illustrating two different height column-shaped spacers on the color filter substrate; 
       FIG. 2  shows a cross section illustrating another embodiment of two different height column-shaped spacers according to the related art; 
       FIGS. 3   a - 3   c  schematically illustrate procedures for manufacturing a spacer with a step; 
       FIGS. 4   a - 4   c  schematically illustrate other procedures for manufacturing a spacer with a step using dual exposure steps to form the spacer; 
       FIG. 5  is a cross section illustrating a LCD substrate according to one aspect of the invention; 
       FIG. 6  is a cross section illustrating a LCD substrate according to another aspect of the invention; 
       FIG. 7   a  is a top view illustrating a LCD substrate of the first embodiment of the invention; 
       FIG. 7   b  is a cross section taken along line  7   b - 7   b  of  FIG. 7   a;    
       FIG. 8   a  is a partial top view of  FIG. 7   b  illustrating an arrangement of the light shield array and spacer within the region A; 
       FIGS. 8   b  and  8   c  are partial top views illustrating an alternative illustrative embodiment of the invention; 
       FIG. 9   a  is a top view illustrating a LCD substrate in which the spacer definition layer is a color filter layer in accordance with a second illustrative embodiment of the invention; 
       FIG. 9   b  is a cross section taken along line  9   b - 9   b  in  FIG. 9   a;    
       FIG. 10   a  is a top view illustrating a LCD substrate in which the spacer definition layer is a color filter layer in accordance with a third illustrative embodiment of the invention; 
       FIG. 10   b  is a cross section taken along line  10   b - 10   b  in  FIG. 10   a;    
       FIG. 11   a  is a top view illustrating a LCD substrate in accordance with a fourth illustrative embodiment of the invention; 
       FIG. 11   b  is a cross section taken along line  11   b - 11   b  in  FIG. 11   a ; and 
       FIGS. 12   a - 12   c  are cross sections illustrating parts of a liquid crystal display in which the spacer definition layer comprises a capacitor, a TFT, or a metal line of illustrative embodiments of the invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 5  is a cross section illustrating a LCD substrate according to one aspect of the invention. The LCD substrate comprises a substrate  12 , a spacer definition layer  14  formed on the substrate  12 , a photo spacer PS formed on the spacer definition layer  14 . The spacer definition layer  14  comprises an opening with a first step S 1  along the opening. The photo spacer PS is formed along the profile of the first step S 1  on the spacer definition layer  14  adjacent to the first step S 1 , thereby forming a second step S 2  on the photo spacer PS. 
     FIG. 6  is a cross section illustrating a LCD substrate according to another aspect of the invention. The LCD substrate comprises a substrate  12 , a spacer definition layer  16  formed on the substrate  12 , a photo spacer PS formed on the spacer definition layer  16 . The edge of the spacer definition layer  16  comprises a first step S 1 . The photo spacer PS is formed along the profile of the first step S 1  on the spacer definition layer  16  adjacent to the first step S 1 , thereby forming a second step S 2  on the photo spacer PS. 
   Accordingly, the spacer definition layer can be a light shield array or a color filter. The spacer definition layer also can be conductive line, such as M 1 , M 2 , and the likes, semiconductor, insulator layer or passivation. Further, the spacer definition layer can be a stacked structure of above-mentioned layers. Excepted, the substrate can be color filter substrate or TFT array substrate. 
   First Embodiment 
     FIG. 7   a  is a partial top view illustrating a LCD substrate of a first illustrative embodiment of the invention, wherein the spacer definition layer is a light shield array.  FIG. 7   b  is a cross section taken along line  7   b - 7   b  of  FIG. 7   a . In  FIG. 7   b , a liquid crystal display comprises a color filter substrate  1 , a TFT array substrate  2 , and a liquid crystal layer  50  interposed between the color filter substrate  1  and the TFT array substrate  2 . 
   Referring to  FIGS. 7   a  and  7   b , the color filter substrate  1  comprises a first glass substrate  10 , a light shield array  30 , a plurality of color filters CF, a passivation layer  40 , and a photo spacer PS. A fabrication method of the color filter substrate  1  comprises forming a light shield array  30  on the first glass substrate  10 , wherein the light shield array comprises a first opening  31  and a second opening  32 . The first opening  31  defines an active region AR. The second opening  32  defines a first step S 1 . The second opening, as shown in  FIGS. 7   a  and  7   b , is a slit. Sequentially, a plurality of color filters CF are formed on the active region AR of the first glass substrate  10 . The color filters CF comprise strip-type red R, green G, and blue B color layers. Next, a passivation layer  40  is formed on the color filters CF and the light shield layer  30  along the profile of the color filters CF and the light shield layer  30 . 
   A photo spacer layer (not shown) is formed on the passivation layer  40  such that the photo spacer layer creates a second step S 2  along the profile of the first step S 1 . The thickness of the photo spacer layer is approximately 2.5-5 μm. Afterward, an exposure using a mask and at least one development step are sequentially performed to remove a portion of the photo spacer layer creating a photo spacer PS with a second step S 2 . The second step S 2  comprises a protrusion  61  and a recess  62 . And the TFT array substrate  2 , depicted in  FIG. 7   b , comprises a second glass substrate  20 , a metal line  22 , and an insulating layer  24 . 
   According to embodiments of the invention, the photo spacer PS with a second step S 2  is formed on the light shield array  30  utilizing the profile of a second opening  32  with a first step. To prevent light leakage, a metal line  22 , such as a signal line of a gate line or a data line, is formed on the TFT array substrate  2  corresponding to the second opening  32  of the light shield array  30 . 
     FIG. 8   a  is a partial top view of  FIG. 7   b  illustrating an arrangement of the light shield array  30  and photo spacer PS within the region A. Referring to  FIGS. 7   b  and  8   a , after exposure and development, photo spacer PS is formed across the second opening  32  of the light shield array  30 , thereby forming two second steps S 2  on the photo spacer PS. 
     FIGS. 8   b  and  8   c  are partial top views illustrating an alternative embodiment of the invention. Referring to  FIG. 8   b , light shield array  30  comprises a second opening  32  in the form of a slit.  FIG. 8   b  is different form  FIG. 8   a  in that the photo spacer PS is disposed adjacent to only one step S of the second opening  32  of the light shield array  30  instead of the other step S 1 , thereby forming a second step S 2  on the photo spacer PS. 
   Referring to  FIG. 8   c , light shield array  30  comprises a second opening  32  in the form of a circular hole. After exposure and development, the photo spacer PS is formed covering the circular hole  34  of the light shield array  30 , thereby forming two circular second steps (not shown) on the photo spacer PS. 
   The feature of this embodiment of the invention is that the photo spacer PS is formed on a structure with a step, thereby substantially forming a step on the photo spacer PS. For example, according to the first illustrative embodiment, photo spacer PS is formed on the light shield array with a first step S 1 , thereby substantially forming a second step S 2  on the photo spacer PS along the first step S 1  of the light shield array  30 . Therefore, the invention requires only one photo mask step to from a photo spacer with a spacer and is simpler than the conventional half-tone masking step. 
   Accordingly, the height of the second step S 2  of the photo spacer PS, such as the distance between protrusion  61  and recess  62  as shown in  FIG. 7   b , is preferably between approximately 0.05 and 0.3 μm. After assembling the color filter substrate  1  and the TFT array substrate  2 , the protrusion  61  of the photo spacer PS normally contact the array substrate  2 , while the recess  62  of the photo spacer PS does not contact the TFT array substrate  2 . When a normal force is applied on the substrate, the protrusion  61  can be elastically deformed while the recess  62  can contact the TFT substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   When a black matrix resin is introduced to the light shield array  30 , the height of the first step S 1  of the second opening  32  is approximately between 1.2-1.5 μm, because the thickness of the black matrix resin is approximately between 1.2-1.5 μm. After the passivation layer  40  is formed, the height of the second step S 2  formed by the photo spacer layer is slightly less than that of the first step S 1  but still cannot reach the desired range of 0.05-0.3 μm. If the second step S 2  is too high, when a normal force applied, the recess  62  cannot contact the TFT substrate and the entire density of the spacer cannot effectively increase such that push mura defects cannot prevented. Accordingly, a step of reflow is performed to appropriately adjust the height of the second step S 2  prior to exposure and development, thereby reducing the height of the second step S 2  such as within the desired range of 0.05-0.3 μm. 
   Additionally, when chromium (Cr) is introduced, the height of the first step S 1  of the second opening  32  is approximately between 0.2-0.3 μm, because the thickness of the chromium layer is approximately between 0.2-0.3 μm. After the passivation layer  40  is formed, the height of the second step S 2  formed by the photo spacer layer can reach the desired range of 0.05-0.3 μm without requiring additional reflow. Reflow, however, can also be performed to adjust the height of the second step S 2  dependent on design requirements. 
   Second Embodiment 
     FIG. 9   a  is a partial top view illustrating a LCD substrate in which the spacer definition layer is a color filter layer in accordance with a second illustrative embodiment of the invention.  FIG. 9   b  is a cross section taken along line  9   b - 9   b  in  FIG. 9   a . In  FIG. 9   b , a liquid crystal display comprises a color filter substrate  1 , a TFT array substrate  2 , and a liquid crystal layer  50  interposed between the color filter substrate  1  and the TFT array substrate  2 . 
   Referring to  FIGS. 9   a  and  9   b , the color filter substrate  1  comprises a first glass substrate  10 , a light shield array  30 , a plurality of color filters CF, a passivation layer  40 , and a photo spacer PS. A fabrication method of the color filter substrate  1  comprises forming a light shield array  30  having a first opening  31  on the first glass substrate  10 , thereby defining an active region AR. A plurality of color filters CF are sequentially formed on the active region AR of the first glass substrate  10 . The color filters CF comprise strip-type red R, green G, and blue B color layers. Next, a passivation layer  40  is formed on the color filters CF and the light shield layer  30  and along the profile of the color filter CF and the light shield layer  30 . 
   A photo spacer layer (not shown) is formed on the passivation layer  40  such that the photo spacer layer creates a second step S 2  along the profile of the first step S 1 . Afterward, an exposure using a mask and at least one development step are sequentially performed to remove a portion of the photo spacer layer creating a photo spacer PS with a second step S 2 . The second step S 2  comprises a protrusion  63  and a recess  64 . And the TFT array substrate  2 , depicted in  FIG. 9   b , comprises a second glass substrate  20 , a metal line  26 , and an insulating layer  24 . 
   In the second embodiment, the photo spacer PS is formed on the color filters CF with a first step S 1 , thereby forming a second step S 2  on the photo spacer PS along the first step S 1  of the color filters CF. Therefore, the invention requires only one lithographic process to form a photo spacer with a step. 
   Similarly, in the second embodiment, the height of the second step S 2  of the photo spacer PS, i.e., the distance between protrusion  63  and recess  64 , is preferably between approximately 0.05 and 0.3 μm. After assembling the color filter substrate  1  and the TFT array substrate  2 , the protrusion  63  of the photo spacer PS normally contacts the TFT array substrate  2 , while the recess  64  of the photo spacer PS does not contact the TFT array substrate  2 . When a normal force is applied on the LCD substrate, the protrusion  63  can be elastically deformed while the recess  64  can contact the TFT substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   Moreover, if the height of the second step S 2  of photo spacer PS cannot reach the desired range simply using the profile of the first step S 1  of the color filters and passivation layer  40 , a step of reflow can be performed to appropriately adjust the height of the second step S 2  prior to exposure and development. 
   Third Embodiment 
     FIG. 10   a  is a partial top view illustrating a LCD substrate in which the spacer definition layer is a color filter layer in accordance with a third illustrative embodiment of the invention.  FIG. 10   b  is a cross section taken along line  10   b - 10   b  in  FIG. 10   a . In  FIG. 10   b , a liquid crystal display comprises a color filter substrate  1 , a TFT array substrate  2 , and a liquid crystal layer  50  interposed between the color filter substrate  1  and the TFT array substrate  2 . 
   Referring to  FIGS. 10   a  and  10   b , the color filter substrate  1  comprises a first glass substrate  10 , a light shield array  30 , a plurality of color filters CF, a passivation layer  40 , and a photo spacer PS. A fabrication method of the color filter substrate  1  comprises forming a light shield array  30  having a first opening  31  on the first glass substrate  10 , thereby defining an active region AR. Sequentially, a plurality of color filters CF are formed on the active region AR and non-active region NAR of the first glass substrate  10 . A third opening C 3  is formed within the color filters CF, thereby defining a first step S 1 . The color filters CF comprise strip-type red R, green G, and blue B color layers. Next, a passivation layer  40  is formed on the color filters CF and the light shield layer  30  and along the profile of the color filters CF and the light shield layer  30 . 
   A photo spacer layer (not shown) is formed on the passivation layer  40  such that the photo spacer layer creates a second step S 2  along the profile of the first step S 1 . Afterward, an exposure using a mask and at lease one development step are sequentially performed to remove a portion of the photo spacer layer creating a photo spacer PS with a second step S 2 . The second step S 2  comprises a circular protrusion  65  and recess  66 . And the TFT array substrate  2 , depicted in  FIG. 10   b , comprises a second glass substrate  20 , a metal line  26 , and an insulating layer  24 . 
   In the third embodiment, the photo spacer PS is formed on the color filters CF with a first step S 1 , thereby forming a second step S 2  on the photo spacer PS along the first step S 1  of the color filters CF. Therefore, the invention requires only one lithographic process to form a photo spacer with a step. 
   Similarly, in the third embodiment, the height of the second step S 2  of the photo spacer PS, i.e., the distance between protrusion  65  and recess  66  as shown in  FIG. 10   b , is preferably between approximately 0.05 and 0.3 μm. After assembling the color filter substrate  1  and the TFT array substrate  2 , the protrusion  65  of the photo spacer PS normally contact the array substrate  2 , while the recess  66  of the photo spacer PS does not contact the TFT array substrate  2 . When a normal force is applied on the substrate, the protrusion  65  can be elastically deformed while the recess  66  can contact the TFT substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   Moreover, if the height of the second step S 2  of photo spacer PS cannot reach the desired range simply using the profile of the first step S 1  of the color filters and passivation layer  40 , a step of reflow can be performed to appropriately adjust the height of the second step S 2  prior to exposure and development. 
   Fourth Embodiment 
     FIG. 11   a  is a partial top view illustrating a LCD substrate in accordance with a fourth illustrative embodiment of the invention.  FIG. 11   b  is a cross section taken along line  11   b - 11   b  in  FIG. 11   a . In  FIG. 11   b , a liquid crystal display comprises a color filter substrate  1 , a TFT array substrate  2 , and a liquid crystal layer  50  interposed between the color filter substrate  1  and the TFT array substrate  2 . 
   Referring to  FIGS. 11   a  and  11   b , the color filter substrate  1  comprises a first glass substrate  10 , a light shield array  30 , a color filter CF, a passivation layer  40 , a first photo spacer PS 1 , and a second photo spacer PS 2 . Light shield array  30  having a first opening  31  is formed on the first glass substrate  10 , thereby defining an active region AR. Sequentially, a plurality of color filters CF are formed on the active region AR and non-active region NAR of the first glass substrate  10 . The color filters CF comprise strip-type red R, green G color, and blue B layers. Next, a passivation layer  40  is formed on the color filters CF and the light shield layer  30  along the profile of the color filters CF and the light shield layer  30 . 
   A photo spacer layer (not shown) is formed on the passivation layer  40  such that the photo spacer layer creates a second step S 2  along the profile of the first step S 1 . Afterward, an exposure using a mask and at least one development step are sequentially performed to remove a portion of the photo spacer layer creating a first photo spacer PS 1  and second photo spacer PS 2 . And the TFT array substrate  2 , depicted in  FIG. 11   b , comprises a second glass substrate  20 , a metal line  26 , and an insulating layer  24 . 
   In the fourth illustrative embodiment, the first photo spacer PS 1  is formed in the region without color filters CF, and the second photo spacer is formed in the non-active region NAR with color filters CF. The distance difference H between the first photo spacer PS 1  and the second photo spacer PS 2  is preferably between approximately 0.05 and 0.3 μm. After assembling the color filter substrate  1  and the TFT array substrate  2 , the second photo spacer PS 2  normally contact the array substrate  2 , while the first photo spacer PS 1  does not contact the TFT array substrate  2 . When a normal force is applied on the substrate, the second photo spacer PS 2  can be elastically deformed while the first photo spacer PS 1  can contact the TFT substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   Fifth Embodiment 
     FIG. 12   a  is a cross section illustrating a part of a liquid crystal display in which the spacer definition layer is a capacitor in accordance with a fifth illustrative embodiment of the invention. In  FIG. 12   a , a liquid crystal display comprises a color filter substrate  1 , a TFT array substrate  2 , and a liquid crystal layer  50  interposed between the color filter substrate  1  and the TFT array substrate  2 . 
   Referring to  FIG. 12   a , the TFT array substrate  2  comprises a glass substrate  20 , a thin film transistor T, a capacitor C, a passivation layer  214 , and a photo spacer PS. The capacitor C comprises a first electrode  211   b , a dielectric layer  212  and a second electrode  213 , thereby defining a first step S 1 . 
   A photo spacer layer (not shown) is formed on the passivation layer  214  such that the photo spacer layer creates a second step S 2  along the profile of the first step S 1 . Afterward, an exposure using a mask and at least one development step are sequentially performed to remove a portion of the photo spacer layer creating a photo spacer PS with a second step S 2 . 
   In the fifth embodiment, the photo spacer PS is formed on the TFT array substrate  2  with a first step S 1 , thereby forming a second step S 2  on the photo spacer PS along the first step S 1  of the capacitor C. Therefore, the invention requires only one lithographic process to form a photo spacer with a step. 
   Similarly, in the fifth embodiment, the height of the second step S 2  of the photo spacer PS, i.e., the distance between protrusion  63  and recess  64  as shown in  FIG. 12   a , is preferably between approximately 0.05 and 0.3 μm. After assembling the color filter substrate  1  and the TFT array substrate  2 , the protrusion  63  of the photo spacer PS normally contact the color filter substrate  1 , while the recess  64  of the photo spacer PS does not contact the color filter substrate  1 . When a normal force is applied on the substrate, the protrusion  63  can be elastically deformed while the recess  64  can contact the color filter substrate  1 . The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   Sixth Embodiment 
     FIG. 12   b  is a cross section illustrating a part of a liquid crystal display in which the spacer definition layer is a thin film transistor in accordance with a sixth illustrative embodiment of the invention. In  FIG. 12   b , a liquid crystal display comprises a color filter substrate  1 , a TFT array substrate  2 , and a liquid crystal layer  50  interposed between the color filter substrate  1  and the TFT array substrate  2 . 
   Referring to  FIG. 12   b , the TFT array substrate  2  comprises a glass substrate  20 , a thin film transistor T, a capacitor C, a passivation layer  214 , and a photo spacer PS. The thin film transistor T comprises a gate electrode  211   a , a dielectric layer  212 , a channel  213 , and a source/drain  215   a . An ohmic contact layer  215   b  is disposed between the channel  213 , and the source/drain  215   a . The passivation layer  214  covers the thin film transistor T. The edge of the ohmic contact layer  215   b  and the source/drain  215   a  defines a first step S 1 . 
   A photo spacer layer (not shown) is formed on the passivation layer  214  such that the photo spacer layer creates a second step S 2  along the profile of the first step S 1 . Afterward, an exposure using a mask and at least one development step are sequentially performed to remove a portion of the photo spacer layer creating a photo spacer PS with a second step S 2 . The second step S 2  comprises a circular protrusion  65  and recess  66 . 
   In the sixth embodiment, the photo spacer PS is formed on the thin film transistor T with a first step S 1 , thereby forming a second step S 2  on the photo spacer PS along the first step S of the thin film transistor T. Therefore, the invention requires only one lithographic process to form a photo spacer with a step. 
   Similarly, in the fifth embodiment, the height of the second step S 2  of the photo spacer PS, i.e., the distance between protrusion  65  and recess  66  as shown in  FIG. 12   b , is preferably between approximately 0.05 and 0.3 μm. After assembling the color filter substrate  1  and the TFT array substrate  2 , the protrusion  65  of the photo spacer PS normally contact the color filter substrate  1 , while the recess  66  of the photo spacer PS does not contact the color filter substrate  1 . When a normal force is applied on the substrate, the protrusion  65  can be elastically deformed while the recess  66  can contact the color filter substrate  1 . The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   The spacer definition layer on the TFT array substrate is not limited to a capacitor C and a thin film transistor T. Other structures, such as metal lines  211   c , providing a first step S 1  can also act as the spacer definition layer, as shown in  FIG. 12   c.    
   Furthermore, a LCD structure of color filter on array (COA) could be also introduced into the foregoing invention. Accordingly, the invention improves on the related art in that the photo spacer PS is formed on the spacer definition layer, such as light shield array, color filter, conductive line, semiconductor, passivation or insulator layer with a first step, thereby substantially forming a second step on the photo spacer along the first step. Therefore, only one photo mask step is required to from a photo spacer with a step and is simpler than the conventional half-tone masking step. After assembling the color filter substrate and the TFT array substrate, the protrusion of the photo spacer normally contacts the opposite substrate, while the recess of the photo spacer does not contact the surface of the opposite substrate. When a normal force is applied on the substrate, the protrusion can be elastically deformed while the recess can contact the opposite substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   Additionally, the invention also provides two photo spacers. One photo spacer is formed in the region without spacer definition layer, and the other photo spacer is formed in the non-active region with spacer definition layer. A height difference is between the first photo spacer and the second photo spacer. After assembling the color filter substrate and the TFT array substrate, the second photo spacer normally contacts the opposite substrate, while the first photo spacer does not contact the surface of the opposite substrate. When a normal force is applied on the substrate, the second photo spacer can be elastically deformed while the first photo spacer can contact the opposite substrate. The entire density of the spacer increases such that more load can be sustained, thereby preventing push mura defects. 
   While the invention has been particularly shown and described with reference to preferred embodiments, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above, and all equivalents thereto.