Abstract:
The invention discloses an LTPS LCD comprising a plurality of NMOS elements and PMOS elements on a substrate. Each element comprises a SiN x  layer underlying or capping a gate electrode. The SiN x  layer features an appropriate length extending from the bottom edge of the gate electrode. The SiN x  layer can be replaced with a SiO x N y  layer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a flat panel display, and in particular relates to a low temperature polysilicon thin film transistor liquid crystal display (LTPS TFT-LCD). 
         [0003]    2. Description of the Related Art 
         [0004]    In conventional fabrication processes for an LTPS LCD, switching elements are typically treated with an high pressure anneal (HPA) process to improve uniformity thereof. However, existing devices suffers from problems such as so-called “threshold voltage shift” (as shown in  FIG. 3B ) for a P-type thin film transistor (also PTFT) after the HPA treatment. Simultaneously, an N-type thin film transistor (also NTFT) can not be turned off normally, referring to  FIG. 3A . As a result, an electric circuit of a panel may not work. Also, remaining oxide charges, possibly induced by the HPA treatment, may diffuse into active regions of the device. 
         [0005]    Accordingly, an LTPS TFT-LCD capable of preventing such problems is desirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In view of the problems in related art, several embodiments are disclosed as the following. 
         [0007]    One embodiment of a system for displaying images comprising: a low temperature poly-silicon liquid crystal display panel which comprises a substrate; an active layer, overlying the substrate; a gate insulating layer comprising a first extended portion, a second extended portion and a central portion therebetween, overlying the active layer; and a gate electrode, overlying the central portion of the gate insulating layer, wherein the active layer, the gate insulating layer and the gate electrode constitute a switching element; wherein the first extended portion and the second extended portion are uncovered by the gate electrode, and wherein each of the first extended portion and the second extended portion has a length larger than about 0.5 μm to prevent oxide charges from diffusing into the active layer. 
         [0008]    Another embodiment of a system for displaying images comprising: a low temperature poly-silicon liquid crystal display panel which comprises a substrate; an active layer, overlying the substrate; a gate insulating layer, overlying the active layer; a., overlying the gate insulating layer; a gate electrode opposite to the active layer, overlying the gate insulating layer, wherein the active layer, the gate insulating layer and the gate electrode constitute a switching element; and a passivation layer comprises a first extended portion, a central portion covering the gate electrode and a second extended portion, wherein the first and second extended portions are in contact with the gate insulating layer, and wherein each of the first and second extended portions has a length larger than about 0.5 μm to prevent oxide charges from diffusing into the active layer. 
         [0009]    Another embodiment of fabricating such a system for displaying images is also provided. The method comprise providing a low temperature poly-silicon thin film transistor, comprising: providing a substrate; forming an active layer overlying the substrate; forming a gate insulating layer overlying the active layer; forming a dielectric layer with a first extended portion, a second extended portion and a first central portion therebetween, overlying the gate insulating layer; and forming a gate electrode, overlying the central portion of the dielectric layer; and performing an HPA process on the low temperature poly-silicon thin film transistor. 
         [0010]    According to embodiments of the invention, oxide charges induced by the subsequent HPA process are prevented from diffusing into switching elements by the extended SiN x  layer or SiO x N y  layer underlying or capping a gate electrode. As a result, uniformity of switching elements is enhanced such that electric circuits of a display can be operated normally. 
         [0011]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0013]      FIGS. 1A to 1L  are cross sections of one embodiment of a method of fabricating an LTPS LCD in accordance with the invention. 
           [0014]      FIGS. 2A to 2F  are cross sections of another embodiment of a method of fabricating an LTPS LCD in accordance with the invention. 
           [0015]      FIGS. 3A to 3B  are schematic diagrams of drain current vs. gate voltage for NMOS elements and PMOS elements of a conventional LTPS LCD, respectively. 
           [0016]      FIGS. 4A to 4B  are schematic diagrams of drain current vs. gate voltage for NMOS elements and PMOS elements of a LTPS LCD according to one embodiment of the invention, respectively. 
           [0017]      FIGS. 5A to 5B  are schematic diagrams of drain current vs. gate voltage for NMOS elements and PMOS elements of a LTPS LCD according to another embodiment of the invention, respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0019]    In a first embodiment of an LTPS LCD shown in  FIG. 1L , a buffer layer  102  is on the substrate  100 . An active layer layer is on the buffer layer  102 , and comprises at least a first active layer including channel region  104   c , LDDs  104   d , source/drain electrodes  104   b  or a second active layer including channel region  105   a , source/drain electrodes  105   c , or both. A gate insulating layer  114  is on the active layer layer and the buffer layer  102 . A dielectric layer layer is on the gate insulating layer  114 , and comprises at least a first dielectric layer  116 ′ or a second dielectric layer  116 ″, or both. A first gate electrode  118  and a second gate electrode  118 ′ are on the first dielectric layer  116 ′ and the second dielectric layer  116 ″, respectively. An interlayer dielectric layer  126  is on the first gate electrode  118 , the second gate electrode  118 ′, the dielectric layer layer, and, the gate insulating layer  114 . A passivation layer  129  is on the interlayer dielectric layer  126 . The first active layer, the gate insulating layer  114 , the first dielectric layer  116 ′, and the first gate electrode  118  constitute an NMOS element. The second active layer, the gate insulating layer  114 , the second dielectric layer  116 ′, and the second gate electrode  118 ′ constitute a PMOS element. Each conductive line  130  is in contact with the source/drain electrodes  104   b  of the NMOS element and the source/drain electrodes  105   c  of the PMOS element, respectively, through the passivation layer  129 , the interlayer dielectric layer  126  and the gate insulating layer  114 . 
         [0020]    In addition, the first dielectric layer  116 ′ includes a first extended portion  117   a  and a second extended portion  117   b  which are not covered by the first gate electrode  118 , each extended portion has a length large than 0.5 μm. The second dielectric layer  116 ″ includes a third extended portion  117   c  and a fourth extended portion  117   d  which are not covered by the second gate electrode  118 ′, each extended portion has a length large than 0.5 μm. The dielectric layer layer can be a SiN x  layer or a SiO x N y  layer. The length of the first extended portion  117   a  can equals that of the second extended portion  117   b  while the length of the first extended portion  117   a  may be not equal to that of the second extended portion  117   b  in other embodiments. Also, the length of the third extended portion  117   c  can equals that of the fourth extended portion  117   d  while the length of the third extended portion  117   c  may be not equal to that of the fourth extended portion  117   d  in other embodiments. 
         [0021]    Processes of fabricating such an LTPS LCD are briefly described with accompanying drawings,  FIGS. 1A to 1L . In  FIG. 1A , a substrate  100  with a buffer layer  102  thereon is provided. An active layer, such as a poly silicon layer, is formed on the buffer layer  102 . The active layer includes a first active layer  104  and a second active layer  105 . 
         [0022]      FIG. 1B , the second active layer  105  is covered by a photoresist material  106 . A channel doping process  108  is conducted on the first active layer  104 . 
         [0023]    In  FIG. 1C , the doped first active layer  104   a  is partially covered by a photoresist material  110 , and the exposed portion of the doped first active layer  104   a  is subjected to an N+ doping process  112 . Source/drain electrodes  104   b  are thus available. Subsequently, the photoresist material  106  and  110  are removed. 
         [0024]    In  FIG. 1D , a gate insulating layer  114  is formed on the first active layer  104 , the second active layer  105 , and the buffer layer  102 . 
         [0025]    In  FIG. 1E , a dielectric material  116  is deposited on the gate insulating layer  114 . After a conventional patterning process, a patterned dielectric layer including a first dielectric layer  116 ′ and a second dielectric layer  116 ″ is obtained, as shown in  FIG. 1F . Specifically, each dielectric layer is extended to a desired length. 
         [0026]    In  FIG. 1G , the first, second gate electrodes  118 ,  118 ′ are formed on the first dielectric layer  116 ′ and the second dielectric layer  116 ″, respectively. It is noted that the first dielectric layer  116 ′ includes a first extended portion  117   a  and a second extended portion  117   b ; the second dielectric layer  116 ″ includes a third extended portion  117   c  and a fourth extended portion  117   d.    
         [0027]    In  FIG. 1H , an LDD doping process  120  is performed, thus, a channel region  104   c  and LDDs  104   d  are formed. In  FIG. 1I , the first gate electrode  118 , the first dielectric layer  116 ′ and the first active layer  104  are covered by a photoresist material. A P+ doping process is conducted on the second active layer  105 , forming source/drain electrodes  105   c.    
         [0028]    In  FIG. 1J , an interlayer dielectric layer  126  is formed on the first gate electrode  118 , the first dielectric layer  116 ′, the second gate electrode  118 ′, the second dielectric layer  116 ″, and the gate insulating layer  114 . 
         [0029]    In  FIG. 1K , a water atmosphere HPA treatment followed by the formation of a capping layer is performed. Other well known processes such as metallization are subsequently progressed, as shown in  FIG. 1L . 
         [0030]    According to the first embodiment, oxide charges induced by the HPA process are prevented from diffusing into active layer by the extended SiN x  layer or SiO x N y  layer underlying a gate electrode. As a result, uniformity of switching elements is enhanced, as shown in  FIGS. 4A and 4B  for NMOS elements and PMOS elements, respectively, so that electric circuits of a display can be operated normally. 
         [0031]    In a second embodiment of an LTPS LCD shown in  FIG. 2F , a buffer layer  202  is on the substrate  200 . An active layer layer is on the buffer layer  202 , and comprises at least a first active layer including channel region, LDDs  204   d , source/drain electrodes  204   a  or a second active layer including channel region  205   b , source/drain electrodes  205   c , or both. A gate insulating layer  214  is on the patterned active layer and the buffer layer  202 . A patterned dielectric layer is on the gate insulating layer  214 , and comprises at least a first dielectric layer  216 ′ or a second dielectric layer  216 ″, or both. A first gate electrode  218  and a second gate electrode  218 ′ are on the first dielectric layer  216 ′ and the second dielectric layer  216 ″, respectively. A first patterned passivation layer is on the first gate electrode  218 , the second gate electrode  218 ′, the patterned dielectric layer, and the gate insulating layer  214 , and it comprises a first passivation layer  226  and a second passivation layer  226 ′ which overly the first gate electrode  218  and the second gate electrode  218 ′, respectively. An interlayer dielectric layer (not shown) is on the first patterned passivation layer, the patterned dielectric layer, and the gate insulating layer  214 . A capping layer (not shown) is on the interlayer dielectric layer. The first active layer, the gate insulating layer  214 , the first dielectric layer  216 ′, and the first gate electrode  218  constitute an NMOS element. The second active pattern, the gate insulating layer  214 , and the second dielectric pattern  216 ″, and the second gate electrode  218 ′ constitute a PMOS element. Similarly, each conductive line (not shown) is in contact with the source/drain electrodes  204   a  of the NMOS element and the source/drain electrodes  205   c  of the PMOS element, respectively, through the passivation layer, the interlayer dielectric layer and the gate insulating layer. 
         [0032]    Specifically, the first passivation layer  226  includes a first extended portion  217   a  and a second extended portion  217   b  which are in contact with the first dielectric layer  216 ′ and the gate insulating layer  214 , each extended portion has a length large than 0.5 μm. The second passivation layer  226 ′ includes a third extended portion  217   c  and a fourth extended portion  217   d  which are in contact with the second dielectric layer  216 ″ and the gate insulating layer  214 , each extended portion has a length large than 0.5 μm. The first passivation layer  226  can be a SiN x  layer or a SiO x N y  layer. The length of the first extended portion  217   a  can equals that of the second extended portion  217   b  while the length of the first extended portion  217   a  may be not equal to that of the second extended portion  217   b  in other embodiments. Also, the length of the third extended portion  217   c  can equals that of the fourth extended portion  217   d  while the length of the third extended portion  217   c  may be not equal to that of the fourth extended portion  217   d  in other embodiments. 
         [0033]    Fabrication processes of the second embodiment are similar to the first embodiment. An additional patterned passivation layer is formed. 
         [0034]    In  FIG. 2A , a buffer layer  202 , a patterned active layer, a gate insulating layer  214  and a dielectric material  216  is formed on a substrate  200  in sequence. The patterned active layer includes a second active pattern  205  and a first active pattern comprising a doped region  204   b , source/drain electrodes  204   a.    
         [0035]    In  FIG. 2B , a patterned dielectric layer including a first dielectric layer  216 ′ and a second dielectric layer  216 ″ is formed after exposure and development processes. In  FIG. 2C , gate electrodes  218  and  218 ′ are formed on the first dielectric layer  216 ′ and the second dielectric layer  216 ″, respectively. In  FIG. 2D , an LDD doping process  220  is performed, forming LDDs  204   d.    
         [0036]    In  FIG. 2E , the first gate electrode  218 , the first dielectric layer  216 ′ and portions of the gate insulating layer  214  are covered by a photoresist material  222 . Subsequently, a P+ doping process  224  is performed, and then the photoresist material  222  is removed. 
         [0037]    In  FIG. 2F , a patterned passivation layer comprising a first passivation layer  226  and a second passivation layer  226 ′ which respectively overly the first gate electrode  218  and the second gate electrode  218 ′ is formed. The subsequent processes such as the formation of a capping layer and metallization are well known. 
         [0038]    According to the second embodiment, oxide charges induced by the HPA process are prevented from diffusing into switching elements by the extended SiN x  layer or SiO x N y  layer capping a gate electrode. As a result, uniformity of switching elements is enhanced, as shown in  FIGS. 5A and 5B  for NMOS elements and PMOS elements, respectively, so that electric circuits of a display can be operated normally. 
         [0039]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.