Abstract:
The present disclosure provides a contact window structure. In the contact window structure, a first insulating layer, having a first opening, is positioned on a first metal layer, wherein the first opening exposes a part of the first metal layer. A second metal layer covers the first opening and contacts with the first metal layer via the first opening. A second insulating layer, having a second opening, is positioned on the first insulating layer, wherein the second opening exposes a part of the second layer and the first insulating layer. The projection area of the second opening on the first metal layer covers the projection area of the first opening on the first metal layer. A pixel structure containing the contact window structure and a manufacturing method thereof are also provided herein.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Taiwanese Application Serial Number 103110897, filed Mar. 24, 2014, which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a contact window structure. More particularly, the present invention relates to a pixel structure having the contact window structure and a method for manufacturing the contact window structure. 
         [0004]    2. Description of Related Art 
         [0005]    In conventional pixel structures of a display device, a contact window is generally used to vertically connect an upper metal with a lower metal. For example, a pixel electrode at an upper layer is electrically connected to a lower source/drain conductive layer via the contact window. 
         [0006]    A conventional method for forming the contact window includes forming an insulating layer on a metal layer, and then forming the contact window on the insulating layer by a lithography process, in which the contact window exposes a part of the metal layer. However, when the contact window is formed on the metal layer, since the metal layer may reflect light, a developing resolution may be decreased such that an aperture of an opening is too small or the opening cannot be even formed. 
         [0007]    Therefore, a novel contact window structure is needed to solve problems that conventional contact window structures may have. 
       SUMMARY 
       [0008]    The invention provides a contact window structure used to solve lithography problems occurring in conventional contact window structures so as to increase a resolution of a lithography process. 
         [0009]    One aspect of the invention is to provide a contact window structure. The contact window structure includes a first metal layer, a first insulating layer, a second metal layer, a second insulating layer and a third metal layer. 
         [0010]    The first insulating layer is positioned on the first metal layer, and the first insulating layer has a first opening exposing a part of the first metal layer. The second metal layer covers the first opening, and is in contact with first metal layer via the first opening. The second insulating layer is positioned on the first insulating layer, and the second insulating layer has a second opening exposing the second metal layer and a part of the first insulating layer, in which a projection area of the second opening on the first metal layer covers a projection area of the first opening on the first metal layer. The third metal layer covers the second insulating layer and the second opening, and is in contact with the second metal layer via the second opening. 
         [0011]    According to an embodiment of the invention, the first opening has a first width ( 1 ) and the second opening has a second width (W 2 ) along a first direction of the substrate, and the second width (W 2 ) is greater than the first width (W 1 ). 
         [0012]    According to an embodiment of the invention, a ratio (W 2 /W 1 ) of the second width to the first width is greater than 1 and less than or equal to 2. 
         [0013]    According to an embodiment of the invention, the first opening has a third width (W 3 ) and the second opening has a fourth width (W 4 ) along a second direction of the substrate, and the fourth width (W 4 ) is greater than the third width (W 3 ). 
         [0014]    According to an embodiment of the invention, a ratio (W 4 /W 3 ) of the fourth width to the third width is greater than 1 and less than or equal to 2. 
         [0015]    According to an embodiment of the invention, a first distance (D 1 ) between a side of the second metal layer and an edge of the first opening at the side is not greater than a second distance (D 2 ) between the an edge of the second opening at the side of the second metal layer and the edge of the first opening. 
         [0016]    According to an embodiment of the invention, a ratio (D 2 /D 1 ) of the second distance to the first distance is greater than 1 and less than or equal to 1.5. 
         [0017]    According to an embodiment of the invention, any side edge of the second opening is in contact with the second metal layer. 
         [0018]    According to an embodiment of the invention, any two side edges of the second opening are in contact with the second metal layer. 
         [0019]    According to an embodiment of the invention, the any two side edges of the second opening are two adjacent side edges or two opposite side edges. 
         [0020]    According to an embodiment of the invention, any three side edges of the second opening are in contact with the second metal layer. 
         [0021]    According to an embodiment of the invention, the contact window structure further includes an upper gate transistor structure electrically connected to the first metal layer. 
         [0022]    According to an embodiment of the invention, the contact window structure further includes a lower gate transistor structure electrically connected to the first metal layer. 
         [0023]    Another aspect of the invention is to provide a pixel structure. The pixel structure includes a substrate, a first metal layer, a semiconductor layer, a gate insulating layer, a second metal layer, a protection layer and a pixel electrode. 
         [0024]    The first metal layer is positioned on the substrate, and the first metal layer includes a source, a drain, and a first electrode. The semiconductor layer is positioned between the source and the drain, and is in contact with the source and the drain. The gate insulating layer covers the semiconductor layer and the first metal layer, and the gate insulating layer has a first contact window exposing a part of the first electrode. 
         [0025]    The second metal layer is positioned on the gate insulating layer. The second metal layer includes a gate, a second electrode and a contact pad. The second electrode and the first electrode form a storage capacitor, and the contact pad is in contact with the first electrode. 
         [0026]    The protection layer covers the gate insulating layer and the second metal layer. The protection layer has a second contact window exposing the contact pad and a part of the gate insulating layer, in which a projection area of the second contact window on the substrate covers a projection area of the first contact window on the substrate. A pixel electrode covers the protection layer and is formed in the second contact window to contact with the contact pad. 
         [0027]    According to an embodiment of the invention, the first contact window has a first width (W 1 ) and the second contact window has a second width (W 2 ) along a first direction of the substrate. The second width is greater than the first width. 
         [0028]    According to an embodiment of the invention, a ratio of the second width (W 2 ) to the first width (W 1 ) is greater than 1 and less than or equal to 2. 
         [0029]    According to an embodiment of the invention, the first contact window has a third width (W 3 ) and the second contact window has a fourth width (W 4 ) along a second direction of the substrate. The fourth width (W 4 ) is greater than the third width (W 3 ). 
         [0030]    According to an embodiment of the invention, a ratio of the fourth width (W 4 ) to the third width (W 3 ) is greater than 1 and less than or equal to 2. 
         [0031]    Another aspect of the invention is to provide a method for manufacturing a pixel structure. The method includes following steps. A substrate is provided. A first metal layer is formed on the substrate, in which the first metal layer includes a source, a drain and a first electrode. A semiconductor layer is formed between the source and the drain, and the semiconductor layer is in contact with the source and the drain. A gate insulating layer is formed on the semiconductor layer, the first metal layer and the substrate, in which the gate insulating layer has a first contact window to expose a part of the first electrode. A second metal layer is formed on the gate insulating layer, in which the second metal layer includes a gate, a second electrode and a contact pad. The second electrode and the first electrode form a storage capacitor, and the contact pad is in contact with the first electrode in the first contact window. A protection layer is formed on the second metal layer and the gate insulating layer. A lithography process is performed such that the protection layer forms a second contact window exposing the contact pad and a part of the gate insulating layer, and a projection area of the second contact window on the substrate covers a projection area of the first contact window on the substrate. A pixel electrode is formed to cover the protection layer, and the pixel electrode is formed in the second contact window to contact with the contact pad. 
         [0032]    According to an embodiment of the invention, the lithography process includes following steps. A photoresist layer is formed on the protection layer. The first contact window is covered with a mask, in which a projection area of the mask on the gate insulating layer covers a part of the gate insulating layer and a part of the contact pad. The photoresist layer is developed to form a patterned photoresist layer. The protection layer is etched to form the second contact window. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
           [0034]      FIG. 1A  is a top view illustrating a contact window structure  100  according to an embodiment of the invention; 
           [0035]      FIGS. 1B and 1C  are cross-sectional views illustrating the contact window structure  100  in  FIG. 1A ; 
           [0036]      FIG. 2  is a top view illustrating a contact window structure  200  according to an embodiment of the invention; 
           [0037]      FIG. 3  is a top view illustrating a contact window structure  300  according to an embodiment of the invention; 
           [0038]      FIG. 4  is a top view illustrating a contact window structure  400  according to an embodiment of the invention; 
           [0039]      FIG. 5  is a top view illustrating a contact window structure  500  according to an embodiment of the invention; 
           [0040]      FIG. 6A  is a top view illustrating a pixel structure  600  according to an embodiment of the invention; 
           [0041]      FIGS. 6B and 6C  are cross-sectional views illustrating the pixel structure  600  in  FIG. 6A ; 
           [0042]      FIG. 7  is a cross-sectional view illustrating a pixel structure  700  according to a comparative example of the invention; and 
           [0043]      FIGS. 8A to 8I  are cross-sectional views illustrating manufacturing steps of a pixel structure  800  according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    Specific embodiments of the present invention are further described in detail below with reference to the accompanying figures. In the descriptions or the figures, similar or identical parts use identical symbols or numbers. In the figures, shapes or thickness of the embodiments may be expanded for simplicity or marking convenience, and a part of components in the figures is described by words. It should be understood that components that are not shown or described can be any form known by those skilled in the art. 
         [0045]    The terminology used herein is for describing particular embodiments but not for limiting the invention. For example, a using of singular forms of “a”, “an” and “the” intends to cover plural forms except that it is clearly indicated in the specification. It should be understood that, terms of “comprise” and “include” used in the specification are for indicating particular features, integers, steps, operations, units and/or components, but not limited to add one or more other features, integers, steps, operations, units and/or components. Cross-sectional views are referenced to provide exemplary descriptions (and structure inside) of the embodiments of the invention, in this way, it is expected that shapes may deviate from the descriptions due to changes of manufacturing technology and/or tolerances. Therefore, the embodiments of the invention should not be understood as being limiting into particular shapes, but as that the described shapes include change due to, for example, shape changes of manufacturing. In addition, described areas are essentially exemplary, and the shapes thereof do not intent to describe practical shapes of the areas of a device and not intent to limit the invention. 
         [0046]    In conventional lithography processes of forming contact windows, since metal layers may reflect light, a developing resolution may be decreased such that an aperture of an opening is too small or the opening cannot be even formed. Embodiments of the invention provide a contact window structure including a first metal layer, a first insulating layer, a second metal layer, a second insulating layer and a third metal layer. In the embodiments of the invention, the contact window structure can overcome the problems of bad developing resolutions in the lithography process so as to improve the developing resolution, and cause the contact window having an expected aperture. 
         [0047]    Referring to  FIGS. 1A  to AC,  FIG. 1A  is a top view illustrating a contact window structure  100  according to an embodiment of the invention;  FIG. 1B  is a cross-sectional views of  FIG. 1A  along a line segment A-A′; and  FIG. 1C  is a cross-sectional views of  FIG. 1A  along a line segment B-B′. 
         [0048]    The contact window structure  100  includes a first metal layer  110 , a first insulating layer  120 , a second metal layer  130 , a second insulating layer  140  and a third metal layer  150 . In  FIG. 1A , the contact window structure  100  has a concentric square structure including the first metal layer  110 , a first opening  121 , the second metal layer  130 , a second opening  141 , the third metal layer  150  and the second insulating layer  140  from the inside out. 
         [0049]    Along the line segment A-A′, referring to  FIG. 1B , the first metal layer  110  is positioned on the substrate  102 , and the first insulating layer  120  is positioned on the first metal layer  110 . The first insulating layer  120  has the first opening  121  exposing a part of the first metal layer  110 . The second metal layer  130  covers the first opening  121 , and is in contact with the first metal layer  110  via the first opening  121 . According to an embodiment of the invention, the first opening  121  has a first width (W 1 ) along a first direction (line segment A-A′) of the substrate  102 . 
         [0050]    The second insulating layer  140  is positioned on the first insulating layer  120 , and the second insulating layer  140  has the second opening  141  exposing the second metal layer  130  and a part of the first insulating layer  120 . A projection area of the second opening  141  on the first metal layer  110  covers a projection area of the first opening  121  on the first metal layer  110  (see  FIG. 1A ). 
         [0051]    According to an embodiment of the invention, the second opening  141  has a second width (W 2 ) along the first direction (line segment A-A′) of the substrate  102 . According to an embodiment of the invention, the second width (W 2 ) is greater than the first width (W 1 ). According to an embodiment of the invention, a ratio (W 2 /W 1 ) of the second width to the first width is greater than 1 and less than or equal to 2. 
         [0052]    The third metal layer  150  covers the second insulating layer  140  and the second opening  141 , and is in contact with the second metal layer  130  via the second opening  141 . According to an embodiment of the invention, an upper date transistor structure (not shown) is electrically connected to the first metal layer  110  of the contact window structure  100 . According to an embodiment of the invention, a lower gate transistor structure (not shown) is electrically connected to the first metal layer  110  of the contact window structure  100 . 
         [0053]    In  FIG. 1B , there is a first distance (D 1 ) between a side of the second metal layer  130  and an edge of the first opening  121  at the side, and there is a second distance (D 2 ) between the an edge of the second opening  141  and the edge of the first opening  121 . The second distance (D 2 ) is greater than the first distance (D 1 ). According to an embodiment of the invention, a ratio (D 2 /D 1 ) of the second distance to the first distance is approximately greater than 1 and less than or equal to 1.5. 
         [0054]    Along the line segment B-B′ in  FIG. 1A , referring to  FIG. 1C , the first opening  121  has a third width (W 3 ) along the second direction (line segment B-B′) of the substrate  102 ; the second opening  141  has a fourth width (W 4 ) along the second direction (line segment B-B′) of the substrate  102 . According to an embodiment of the invention, the fourth width (W 4 ) is greater than the third width (W 3 ). According to an embodiment of the invention, a ratio (W 4 /W 3 ) of the fourth width to the third width is approximately greater than 1 and less than or equal to 2. 
         [0055]      FIG. 2  is a top view illustrating a contact window structure  200  according to an embodiment of the invention. The contact window structure  200  includes a first metal layer  210 , a first insulating layer  220 , a second metal layer  230  and a second insulating layer  240 . Different from the contact window structure  100  in  FIG. 1A , one side of a second opening  241  is in contact with the second metal layer  230  in  FIG. 2 . Since first metal layer  210  is in contact with the second metal layer  230  via the first opening  221 , when the second metal layer  230  is further connected to other circuit components (not shown), the first metal layer  210  can be indirectly and electrically connected to other circuit components via the second metal layer  230 . In one embodiment of the invention, the contact window structure  200  may further include a third metal layer (not shown). The third metal layer may cover the second insulating layer and the second opening, and may contact with the second metal layer via the second opening. 
         [0056]      FIG. 3  is a top view illustrating a contact window structure  300  according to an embodiment of the invention. The contact window structure  300  includes a first metal layer  310 , a first insulating layer  320 , a second metal layer  330  and a second insulating layer  340 . Different from the contact window structure  100  in  FIG. 1A , two opposite side edges of a second opening  341  are in contact with the second metal layer  330  in  FIG. 3 . Since the first metal layer  310  is electrically connected to the second metal layer  330  via the first opening  321 , when the second metal layer  330  is further connected to other circuit components (not shown), the first metal layer  310  can be electrically and indirectly connected to other circuit components via the second metal layer  330 . In one embodiment of the invention, the contact window structure  300  may further include the third metal layer (not shown). The third metal layer may cover the second insulating layer and the second opening, and may contact with the second metal layer via the second opening. 
         [0057]      FIG. 4  is a top view illustrating a contact window structure  400  according to an embodiment of the invention. The contact window structure  400  includes a first metal layer  410 , a first insulating layer  420 , a second metal layer  430  and a second insulating layer  440 . Different from the contact window structure  100  in  FIG. 1A , two adjacent side edges of the second opening  441  are in contact with the second metal layer  430  in  FIG. 4 . Since the first metal layer  410  is electrically connected to the second metal layer  430  via the first opening  421 , when the second metal layer  430  is further connected to other circuit components (not shown), the first metal layer  410  can be electrically and indirectly connected to other circuit components via the second metal layer  430 . In one embodiment of the invention, the contact window structure  400  may further include the third metal layer (not shown). The third metal layer may cover the second insulating layer and the second opening, and may contact with the second metal layer via the second opening. 
         [0058]      FIG. 5  is a top view illustrating a contact window structure  500  according to an embodiment of the invention. The contact window structure  500  includes a first metal layer  510 , a first insulating layer  520 , a second metal layer  530  and a second insulating layer  540 . Different from the contact window structure  100  in  FIG. 1A , three side edges of the second opening  541  are in contact with the second metal layer  530  in  FIG. 5 . Since the first metal layer  510  is electrically connected to the second metal layer  530  via the first opening  521 , when the second metal layer  530  is further connected to other circuit components (not shown), the first metal layer  510  can be indirectly and electrically connected to other circuit components via the second metal layer  530 . In one embodiment of the invention, the contact window structure  500  may further include the third metal layer (not shown). The third metal layer may cover the second insulating layer and the second opening, and may contact with the second metal layer via the second opening. 
         [0059]      FIG. 6A  is a top view illustrating a pixel structure  600  according to an embodiment of the invention.  FIG. 6B  is a cross-sectional view of  FIG. 6A  along a line segment A-A′.  FIG. 6C  is a cross-sectional view of  FIG. 6A  along a line segment B-B′. 
         [0060]    The pixel structure  600  includes a substrate  603 , a first metal layer  610 , a semiconductor layer  620 , a gate insulating layer  630 , a second metal layer  640 , a protection layer  650 , and a pixel electrode  660 . In  FIG. 6A , the top view of the pixel structure  600  can be roughly divided into a thin-film transistor structure  601  and a contact window structure  602 . 
         [0061]    Along the line segment A-A′ in  FIG. 6A , referring to  FIG. 6B , the first metal layer  610  is positioned on the substrate  603 . The first metal layer  610  includes a source  611 , a drain  612 , and a first electrode  613 . The semiconductor layer  620  is positioned between the source  611  and the drain  612 , and is in contact with the source  611  and the drain  612 . The gate insulating layer  630  covers the semiconductor layer  620  and the first metal layer  610 , and the gate insulating layer  630  has a first contact window  631  exposing a part of the first electrode  613 . According to an embodiment of the invention, the first contact window  631  has a first width (W 1 ) along the first direction (line segment A-A′) of the substrate  603 . 
         [0062]    The second metal layer  640  is positioned on the gate insulating layer  630 , and the second metal layer  640  includes a gate  641 , a second electrode  642  and a contact pad  643 . The second electrode  642  and the first electrode  613  form a storage capacitor, and the contact pad  643  is in contact with the first electrode  613 . 
         [0063]    The protection layer  650  covers the gate insulating layer  630  and the second metal layer  640 . The protection layer  650  has a second contact window  651  exposing the contact pad  643  and a part of the gate insulating layer  630 , in which a projection area of the second contact window  651  on the substrate  603  covers a projection area of the first contact window  631  on the substrate  603  (see  FIG. 6A ). According to an embodiment of the invention, the second contact window  651  has a second width (W 2 ) along the first direction (line segment A-A′) of the substrate  603 . According to an embodiment of the invention, the second width (W 2 ) is greater than the first width (W 1 ). According to an embodiment of the invention, a ratio (W 2 /W 1 ) of the second width to the first width is approximately greater than 1 and less than or equal to 2. 
         [0064]    The pixel electrode  660  covers the protection layer  650 , and is formed in the second contact window  651  to contact with the contact pad  643 . 
         [0065]    In  FIG. 6B , there is a first distance between a side of the contact pad  643  and an edge of the first contact window  631  at the side. There is a second distance between an edge of the second contact window  651  and the edge of the first contact window  631 . The second distance (D 2 ) is greater than the first distance (D 1 ). According to an embodiment of the invention, a ratio (D 2 /D 1 ) of the second distance to the first distance is approximately greater than 1 and less than or equal to 1.5. 
         [0066]    Along the line segment B-B′ in  FIG. 6A , referring to  FIG. 6C , the first contact window  631  has a third width (W 3 ) along the second direction (line segment B-B′) of the substrate  603 , and the second contact window  651  has a fourth width (W 4 ) along the second direction (line segment B-B′) of the substrate  603 . According to an embodiment of the invention, the fourth width (W 4 ) is greater than the third width (W 3 ). According to an embodiment of the invention, a ratio (W 4 /W 3 ) of the fourth width to the third width is approximately greater than 1 and less than or equal to 2. 
         [0067]      FIG. 7  is a cross-sectional view illustrating a pixel structure  700  according to a comparative example of the invention, in  FIG. 7 , the pixel structure  700  includes a substrate  703 , a first metal layer  710 , a semiconductor layer  720 , a gate insulating layer  730 , a second metal layer  740 , a protection layer  750  and a pixel electrode  760 . 
         [0068]    The first metal layer  710  is positioned on the substrate  703 , and the first metal layer  710  includes a source  711 , a drain  712  and a first electrode  713 . The semiconductor layer  720  is positioned between the source  711  and the drain  712 , and is in contact with the source  711  and the drain  712 . The gate insulating layer  730  covers the semiconductor layer  720  and the first metal layer  710 . The gate insulating layer  730  has a first contact window  731  exposing a part of the first electrode  713 . According to an embodiment of the invention, the first contact window  731  has a first width (Wa). 
         [0069]    The second metal layer  740  is positioned on the gate insulating layer  730 , and the second metal layer  740  includes a gate  741 , a second electrode  742  and a contact pad  743 . The second electrode  742  and the first electrode  713  form a storage capacitor, and the contact pad  743  is in contact with the first electrode  713 . 
         [0070]    The protection layer  750  covers the gate insulating layer  730  and the second metal layer  740 . The protection layer  750  has a second contact window  751  exposing a part of the contact pad  743 . According to an embodiment of the invention, the second contact window  751  has a second (Wb), and the second width (Wb) is less than the first width (Wa). The pixel electrode  760  covers the protection layer  750 , and is formed in the second contact window  751  to contact with the contact pad  743 . 
         [0071]    In the comparative example, the second width (Wb) of the second contact window  751  is less than the first width (Wa) of the first contact window  731 , and the contact pad  743  is below the second contact window  751 . Since the second contact window  751  is formed by a lithograph process, the contact pad  743  under the second contact window  751  is likely to reflect light and resulting in bad developing resolutions, which may cause a problem of the second width (Wb) of the second contact window  751  being too narrow or failing to form an opening. 
         [0072]    Therefore, in the embodiments of the invention, the second contact window of the protection layer exposes the contact pad and a part of the gate insulating layer, and a projection area of the second contact window on the substrate covers a projection area of the first contact window on the substrate. The first contact window has the first width along the first direction of the substrate or the first metal layer; and the second contact window has the second width along the first direction of the substrate or the first metal layer, in which the second width is greater than the first width. Since the second contact window covers a part of the gate insulating layer and light penetrates the gate insulating layer, the problem of bad developing resolutions due to the light rejection is avoid to improve resolutions of the lithograph process. 
         [0073]      FIGS. 8A to 8I  are cross-sectional views illustrating manufacturing steps of a pixel structure  800  according to an embodiment of the invention. 
         [0074]    In  FIG. 8A , a substrate  803  is provided, and a first metal layer  810  is formed on the substrate  803 . The first metal layer  810  includes a source  811 , a drain  812 , and a first electrode  813 . Then, referring to  FIG. 8B , a semiconductor layer  820  is formed between the source  811  and the drain  812 , and is in contact with the source  811  and the drain  812 . 
         [0075]    In  FIG. 8C , a gate insulating layer  830  is formed on the semiconductor layer  820 , the first metal layer  810  and the substrate  803 . The gate insulating layer  830  has a first contact window  831  to expose a part of the first electrode  813 . According to an embodiment of the invention, the first contact window  831  has a first width (W 1 ). Then, as shown in  FIG. 8D , a second metal layer  840  is formed on the gate insulating layer  830 . The second metal layer  840  includes a gate  841 , a second electrode  842  and a contact pad  843 . The second electrode  842  and the first electrode  813  form a storage capacitor, and the contact pad  843  is in contact with the first electrode  813  in the first contact window  831 . 
         [0076]    In  FIG. 8E , a protection layer  850  is formed on the second metal layer  840  and the gate insulating layer  830 . Next, a lithograph process is performed, such that the protection layer  850  forms a second contact window  851  exposing the contact pad  843  and a part of the gate insulating layer  830 , as shown in  FIG. 8H . A projection area of the second contact window  851  on the substrate  803  covers a projection area of the first contact window  831  on the substrate  803 . According to an embodiment of the invention, the first contact window  831  has a first width (W 1 ), and the second contact window  851  has a second width (W 2 ). According to an embodiment of the invention, the second width (W 2 ) is greater than the first width (W 1 ). According to an embodiment of the invention, a ratio (W 2 /W 1 ) of the second width to the first width is approximately greater than 1 and less than or equal to 2. According to an embodiment of the invention, there is a first distance between a side of the contact pad  843  and an edge of the first contact window  831  at the same side, and there is a second distance between an edge of the second contact window  851  and the edge of the first contact window  831 . The second distance (D 2 ) is greater than the first distance (D 1 ). According to an embodiment of the invention, a ratio (D 2 /D 1 ) of the second distance to the first distance is approximately greater than 1 and less than or equal to 1.5. In  FIG. 8I , a pixel electrode  860  is formed to cover the protection layer  850 , and is formed in the second contact window  851  to contact with the contact pad  843 . 
         [0077]    The lithograph process in  FIG. 8H  further includes steps in  FIGS. 8F to 8G . In  FIG. 8F , a photoresist layer  870   a  is formed on the protection layer  850 . The first contact window  831  is covered with a mask  880 , and a projection area of the mask  880  on the gate insulating layer  830  covers a part of the gate insulating layer  830  and the contact pad  843 . When the photoresist layer  870   a  is developed by light C, the light is not reflected by the contact pad  843  due to the projection area contact pad  843  of the mask  880  on the gate insulating layer  830  and thus a developing resolution is not decreased. 
         [0078]    Then, as shown in  FIG. 8G , the photoresist layer  870   a  is developed (see  FIG. 8F ) to form a patterned photoresist layer  870   b . In  FIG. 8G , since a width of an opening of the patterned photoresist layer  870   b  is greater than the contact pad  843 , after forming the second contact window  851 , the second contact window  851  exposes a part of the gate insulating layer  830  (see  FIG. 8H ). 
         [0079]    In the embodiments of the invention, the contact window structure includes a first metal layer, a first insulating layer, a second metal layer, a second insulating layer and a third metal layer. The contact window structure provided in the embodiments of the invention can overcome the problem of bad developing resolutions in lithograph processes, so as to increase the developing resolution and cause the contact window having an expected aperture. 
         [0080]    On the other hand, in the pixel structure provided in the embodiments of the invention, the second contact window exposes the contact pad and a part of the gate insulating layer, and a projection area of the second contact window on the substrate covers a projection area of the first contact window on the substrate. In other words, the width of the second contact window is greater than the width of the first contact window. Since the second contact window covers a part of the gate insulating layer and light penetrates the gate insulating layer, the problem of bad developing resolutions due to the light reflection is avoid, and thus the resolution of the lithograph process is improved. 
         [0081]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims.