Patent Publication Number: US-6908780-B2

Title: Laser repair facilitated pixel structure and repairing method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of, and claims the priority benefit of U.S. application Ser. No. 10/248,404, filed on Jan. 16, 2003 now U.S. Pat. No. 6,664,568, which claims the priority benefit of Taiwan application serial no. 91102060, filed Feb. 6, 2002. 

   BACKGROUND OF INVENTION 
   1. Field of Invention 
   The present invention relates to a thin film transistor liquid crystal display (TFT-LCD) pixel structure. More particularly, the present invention relates to a laser repair facilitated pixel structure and repairing method. 
   2. Description of Related Art 
   A thin film transistor liquid crystal display (TFT-LCD) mainly includes a thin film transistor (TFT) array substrate board, a color filter array substrate board and a liquid crystal layer. The TFT array substrate board comprises an array of thin film transistors and a pixel electrode for each thin film transistor. The thin film transistor further comprises a gate electrode, a channel layer, a source terminal and a drain terminal. The thin film transistor serves as a switching element for each liquid crystal display cell. 
     FIG. 1  is a schematic top view of a conventional pixel structure. As shown in  FIG. 1 , a pixel structure mainly comprises a thin film transistor  101  and a pixel electrode  110 . The pixel is controlled through a scan line  102  and a data line  104 . The thin film transistor  101  of the pixel structure further includes a gate terminal  102   a , a source terminal  104   a  and a drain terminal  104   b . The drain terminal  104   a  connects electrically with the data line  104 . The gate electrode  102   a  of the thin film transistor  101  connects electrically with the scan line  102 . The source terminal  104   b  of the thin film transistor  101  connects electrically with the pixel electrode  110 . Each pixel electrode  110  corresponds with a thin film transistor  101 . 
   When a break  120  on the data line  104  occurs, a repairing step needs to be conducted so that the ends of the data line  104  at the break region  120  are electrically connected back together. Several methods of repairing a severed data line have been developed. One of the methods is explained with reference to  FIGS. 2A  to  2 C below. 
     FIGS. 2A  to  2 C are schematic cross-sectional views along line I-I″ of  FIG. 1  showing the steps for repairing a broken data line using a laser beam. A data line  104  having a broken region  120  on the dielectric layer  106  of a substrate board  100  is shown in  FIGS. 1 and 2A . The dielectric layer  106  and the gate insulation layer of the thin film transistor  101  are formed together. The data line  104  further includes another dielectric layer  108  formed in the same process of depositing a protective layer between the thin film transistor  101  and the pixel electrode  110 . 
   To carry out a laser repair, openings  200   a  and  200   b  are formed in the dielectric layer  108  above the data line  104  near each end of the broken region  120  using a laser as shown in  FIG. 2B  so that a portion of the data line  104  is exposed. Since the openings  200   a  and  200   b  are formed by a laser burning operation, some material from the dielectric layer  108  piles up to form protruding ledges  201  near the upper corners. 
   As shown in  FIG. 2C , a laser chemical vapor deposition (laser CVD) is carried out to form a conductive layer  202  over the interior surface of the openings  200   a  and  200   b  and the exposed dielectric layer  108 . Through the conductive layer  202 , broken ends of the data line  104  within the broken region  120  are reconnected electrically. 
   Due to the formation of protruding ledges  201  near the upper corners of the openings  200   a  and  200   b , the conductive layer  202  formed by laser CVD also includes a prominent peak or spike there. The pointed peak or spike in the protruding area  201  is electrically conductive and hence may contact with color filter to form a short circuit route. Ultimately, performance of the device is affected. Occasionally, the protrusion  201  may even lead to a short circuit between the upper and lower panel of a liquid crystal display. In addition, if the broken region within the data line  104  is too long, a conventional laser CVD may not bridge the gap reliably. Hence, yield of the laser repair is often compromised. 
   SUMMARY OF INVENTION 
   Accordingly, one object of the present invention is to provide a laser repairing method capable of reconnecting a broken data line without leading to possible subsequent short-circuiting between the upper and lower substrate board of a liquid crystal display panel. 
   A second object of this invention is to provide a laser repair facilitated pixel structure capable of tackling low yield problem resulting from the appearance of a long broken section on the data line being repaired. 
   To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a pixel structure on a substrate board. The pixel structure includes a thin film transistor, a scan line, a pixel electrode, a data line, a conductive line, a first dielectric layer and a second dielectric layer. The thin film transistor is formed over the substrate board. The thin film transistor further includes a gate electrode, a source terminal and a drain terminal. The scan line is formed over the substrate board and electrically connected to the gate electrode. The pixel electrode is formed over the substrate board and positioned next to the thin film transistor. The data line is formed over the substrate board and electrically connected to the pixel electrode via the source/drain terminals. The conductive line is formed underneath the data line. The conductive line has a connective region and a repair section at each end of the connective region. The repair sections protrude over the data line covered range. The repair section at each end of the conductive line may form on the same side as the data line or on the opposite side of the data line. Alternatively, the conductive line may be designed to have a width greater than the data line so that the conductive line outside the data line covered area can also serve as part of the repair section. The first dielectric layer is formed over the substrate board for electrically isolating the conductive line from the data line. The second dielectric layer is also formed over the substrate board to cover the data line. The two repair sections at each end of the conductive line within the pixel structure serve as areas for laser repair according to this invention. 
   When a break in the data line occurs, the broken data line is repaired by carrying out the following steps. First, the first dielectric layer and the second dielectric layer above the two repair sections are burnt away using a laser beam to form a first opening and a second opening that expose the data line and repair section. Thereafter, a laser chemical vapor deposition is carried out to form a conductive layer over the exposed surface inside the first opening and the second opening. Hence, the two repair sections and the data line are electrically connected. Through the special structural design between the conductive line and the repair sections, the data line can be easily repaired. 
   This invention provides an alternative pixel structure on a substrate board. The pixel structure includes a thin film transistor, a scan line, a pixel electrode, a data line, a conductive line, a conductive structure, a first dielectric layer and a second dielectric layer. The thin film transistor is formed over the substrate board. The thin film transistor further includes a gate electrode, a source terminal and a drain terminal. The scan line is formed over the substrate board and electrically connected to the gate electrode. The pixel electrode is formed over the substrate board and positioned next to the thin film transistor. The data line is formed over the substrate board and electrically connected to the pixel electrode via the source/drain terminals. The conductive line is formed underneath the data line. The conductive line has a connective region. Each end of the connective region of the conductive line has a contact section and a repair section. The contact section and the repair section protrude over the data line covered range. The contact section and the repair section at each end of the conductive line may form on the same side as the data line or on the opposite side of the data line. Alternatively, the conductive line may be designed to have a width greater than the data line so that the conductive line outside the data line covered area can also serve as part of the repair section or contact section. The conductive structure is formed over the contact section of the conductive line so that the contact section and the data line are electrically connected. The first dielectric layer is formed over the substrate board for electrically isolating the conductive line from the data line. The second dielectric layer is also formed over the substrate board to cover the data line. The repair section at each end of the conductive line within the pixel structure serves as an area for laser repair according to this invention. 
   When a break in the data line occurs, the broken data line is repaired according to the following steps. First, the first dielectric layer and the second dielectric layer above the repair section is burnt away using a laser beam to form an opening that exposes the data line and repair section. Thereafter, a laser chemical vapor deposition is carried out to form a conductive layer over the exposed surface inside the opening. Hence, the repair section and the data line are electrically connected. Through the special structural design between the conductive line, the contact section, and the repair section, the data line can be easily repaired. 
   This invention also provides a laser repair method. First, a substrate board is provided. The substrate board includes a distributing wire having a broken region and a dielectric layer that covers the distributing wire. To repair the broken distributing wire, an opening is formed in the dielectric layer within the broken region by laser burning. The opening exposes not only the broken region, but also exposes a portion of the distributing wire at each end of the broken region. Thereafter, a laser chemical vapor deposition is carried out to form a conductive line over the exposed broken region and the broken ends of the distributing wire so that the broken distributing wire is electrically reconnected through the conductive line. 
   The laser repair method according to this invention is capable of preventing the formation of spikes that may lead to point electric discharge or short-circuiting between the upper and lower substrate board of a liquid crystal display panel. 
   In a first embodiment of the laser repair facilitated pixel structure and repairing method according to this invention, the repair conductive line and the scan lines are formed in the same process and both ends of the conductive line are designed to be a laser repair region. Hence, any broken data line can be repaired within the small area of the two laser-repair regions. For a larger broken wire, the repairing process will not directly affect production. 
   In a second embodiment of the laser repair facilitated pixel structure and repairing method according to this invention, the repair conductive line and the scan lines are formed in the same process. Moreover, one end of the conductive line is electrically connected to the data line through a conductive structure while the other end of the conductive line is designed to be a laser-repair region. Hence, any broken data line can be repaired within the small area of a single laser-repair region. For a larger broken wire, the repairing process will not directly affect production. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a schematic top view of a conventional pixel structure. 
       FIGS. 2A  to  2 C are schematic cross-sectional views along line I-I″ of  FIG. 1  showing the steps for repairing a broken data line using a laser beam. 
       FIGS. 3A  to  3 C are schematic cross-sectional views along line I-I″ of  FIG. 1  showing the steps for repairing a broken data line using a laser beam according to a first embodiment of this invention. 
       FIG. 4  is a schematic top view of a laser-repair facilitated pixel structure according to a second embodiment of this invention. 
       FIG. 5A  is a cross-sectional view along line II-II″ of FIG.  4 . 
       FIG. 5B  is a cross-sectional view along line III-III″ of FIG.  4 . 
       FIGS. 6A and 6B  are cross-sectional views along line II-II″ and line II-II″ of  FIG. 4  showing a method of repairing the pixel structure shown in FIG.  4 . 
       FIG. 7  is a schematic top view of an alternative laser-repair facilitated pixel structure according to the second embodiment of this invention. 
       FIG. 8  is a schematic top view of a laser-repair facilitated pixel structure according to a third embodiment of this invention. 
       FIGS. 9A and 9B  are cross-sectional views along line II-II″ and line II-II″ of FIG.  8 . 
       FIG. 10  is a cross-sectional view along line III-III″ of  FIG. 8  showing a method of repairing the pixel structure shown in FIG.  8 . 
       FIG. 11  is a schematic top view of an alternative laser-repair facilitated pixel structure according to the third embodiment of this invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIGS. 3A  to  3 C are schematic cross-sectional views along line I-I″ of  FIG. 1  showing the steps for repairing a broken data line using a laser beam according to a first embodiment of this invention. As shown in  FIGS. 1 and 3A , a data distributing line  104  having a broken region  120  thereon is on a dielectric layer  106  above a substrate board  100 . The dielectric layer  106  and gate insulation layer of a thin film transistor  101  are formed in the same fabrication process. Another dielectric layer  108  covers the data distributing line  104 . The dielectric layer  108  and the passivation layer between the thin film transistor  101  and a pixel electrode  110  are formed in the same fabrication process. 
   To repair the data distributing line  104 , an opening  300  is formed in the dielectric layer  108  within the broken region  120  as shown in FIG.  3 B. The opening  300  exposes the broken region  120  and a portion of the data line  104  at each end of the broken region  120 . The opening  300  in the dielectric layer  108  is formed, for example, by burning with a laser beam. In this embodiment, since the opening  300  is formed through a laser burning operation, some protruding material  301  piles up near the upper ledges of the opening  300 . 
   As shown in  FIG. 3C , a conductive layer  302  is formed on the exposed surface of the opening  300  so that the broken data distributing line  104  within the broken region  120  is electrically reconnected through the conductive layer  302 . The conductive layer  302  is formed, for example, by conducting a laser chemical vapor deposition (laser CVD). A laser CVD is carried out by first providing a reactive gas over the substrate  100  and then activating the gas by laser pulses so that the reactive gas deposits into the opening  300 . 
   In the first embodiment, a relatively large opening  300  is formed within the broken region  120  and around the broken ends of the data line  104 . Hence, subsequent deposition of conductive material into the opening  300  to form a conductive layer  302  immediately reconnects the broken data line  104 . Unlike a conventional repair method, this invention does not require any deposition of conductive material on a dielectric layer and the two data line exposed openings. Since the conductive layer  302  is not formed over the dielectric layer  108 , the presence of protruding material  301  near the upper corners of the opening due to laser burning will not result in the formation of conductive spikes. Consequently, short circuit due to direct contact with a color filter or between the upper and lower substrate board of a liquid crystal display panel is prevented. 
     FIG. 4  is a schematic top view of a laser-repair facilitated pixel structure according to a second embodiment of this invention.  FIG. 5A  is a cross-sectional view along line II-II″ of FIG.  4 .  FIG. 5B  is a cross-sectional view along line III-III″ of FIG.  4 . In  FIGS. 4 ,  5 A and  5 B, the method of fabricating a pixel structure according to the second embodiment is shown. First, a substrate board  400  such as a transparent glass panel is provided. A scan line  402 , the gate electrode  402   a  of a thin film transistor  401  and a conductive line  406  are formed on the substrate  400 . The scan line  402  and the gate electrode  402   a  are electrically connected. The conductive line  406  is formed in a region where a data distributing line  404  is subsequently laid. 
   The conductive line  406  has a connective section  406   a  and a repair section  406   b  at each end of the connective section  406   a . The two repair sections  406   b  at each end of the conductive line  406  have a width greater than the connective section  406   a . Thus, the area covered by the repair section  406   b  of the conductive line  406  is greater than the data line  404  covered area after the data line  404  is formed over the conductive line  406 . In other words, a portion of the repair section  406   b  is outside the data line  404  covered area. 
   A first dielectric layer  408  that covers the scan line  402 , the gate electrode  402   a  of the thin film transistor  401 , and the conductive line  406 , is formed over the substrate  400 . The first dielectric layer  408  is a silicon nitride layer, for example. The dielectric layer  408  covering the gate electrode  402   a  serves also as an insulator. 
   A channel layer  412 , for example an amorphous silicon layer, is formed over the first dielectric layer  408  above the gate electrode  402   a . Source/drain terminals  404   b / 404   a  are formed over the channel layer  412 . In the meantime, a data distributing line  404  is formed over the first dielectric layer  408 . The source terminal  404   b  and the data distributing line  404  are electrically connected. The data distributing line  404  covers the connective section  406   a  and a portion of the repair sections  406   b  of the conductive line  406 . The conductive line  406  and the data distributing line  404  are electrically isolated from each other through the first dielectric layer  408 . 
   A second dielectric layer  416  is formed over the substrate  400 . The second dielectric layer  416  covers the data distributing line  404 , the source/drain terminals  404   b / 404   a  and the channel layer  412 . The second dielectric layer  416  is a passivation layer that protects the source/drain terminals  404   b / 404   a  and the channel layer  412 . 
   An opening  409  is formed in the passivation layer above the drain terminal  404   a . The opening  409  exposes a portion of the drain terminal  404   a . Finally, a pixel electrode  410  is formed over the substrate board  400 . The pixel electrode  410  and the drain terminal  404   a  are electrically connected through a bridge of material through the opening  409 . The pixel electrode  410  is made from a material such as indium-tin oxide or indium-zinc oxide. 
   In the pixel structure, the thin film transistor  401  is controlled by signals on the scan line  402  and the data distributing line  404 . The pixel electrode  410  and the thin film transistor  401  are positioned next to each other. In particular, the conductive line  406  lies underneath the data distributing line  404  and the two repair sections  406   b  at each end of the conductive line  406  exceed the data line  404  covered area. The two repair sections  406   b  also serve as laser repair regions  418   a ,  418   b  for conducting a laser repair. 
     FIGS. 6A and 6B  are cross-sectional views along line II-II″ and line III-III″ of  FIG. 4  showing a method of repairing the pixel structure shown in FIG.  4 . As shown in  FIG. 4 , the data distributing line  404  within the pixel structure may be broken occasionally. To repair the broken distributing data line  404 , a first opening  420   a  and a second opening  420   b  are formed in the first dielectric layer  408  and the second dielectric layer  416  within the laser repair regions  418   a  and  418   b  as shown in  FIGS. 6A and 6B . Hence, the repair section  406   b  and a portion of the data distributing line  404  are exposed. The first opening  420   a  and the second opening  420   b  are formed, for example, by conducting a laser burning operation. 
   Thereafter, a conductive layer  422  is formed over the exposed surface of the first opening  420   a  and the second opening  420   b  so that the repair section  406   b  and the data distributing line  404  are electrically connected. The conductive layer  422  is formed, for example, by conducting a laser chemical vapor deposition. A laser CVD is carried out by first providing a reactive gas over the substrate  400  and then activating the gas by laser pulses so that the reactive gas deposits into the first opening  420   a  and the second opening  420   b.    
   Since the repair sections  406   a ,  406   b  have a special design that permits the data distributing line  404  to connect electrically with the conductive line  406  through a laser repair operation, any broken data distributing line  404  inside the pixel structure may be repaired. 
     FIG. 7  is a schematic top view of an alternative laser-repair facilitated pixel structure according to the second embodiment of this invention. As shown in  FIG. 7 , the repair sections  406   a  and  406   b  at each end of the conductive line  406  may be on the opposite side of the data distributing line  404 , rather than on the same side of the data distribution line  404  (as shown in FIG.  4 ). In addition, the conductive line  406  may be designed to have a width greater than the data distributing line  404  so that the excess portion of the conductive line  406  may provide more repair area in the repair section. 
     FIG. 8  is a schematic top view of a laser-repair facilitated pixel structure according to a third embodiment of this invention.  FIGS. 9A and 9B  are cross-sectional views along line II-II″ and line III-III″ of FIG.  8 . In  FIGS. 8 ,  9 A and  9 B, the method of fabricating a pixel structure according to the third embodiment is shown. First, a substrate board  400  such as a transparent glass panel is provided. A scan line  402 , the gate electrode  402   a  of a thin film transistor  401  and a conductive line  406  are formed over the substrate board  400 . The scan line  402  and the gate electrode  402   a  are electrically connected. The conductive line  406  is formed in a position underneath a subsequently formed data distributing line  404 . 
   The conductive line  406  has a connective section  406   a  and, at the ends of the connective section  406   a , a repair section  406   b  and a contact section  406   c . The repair section  406   b  and the contact section  406   c  at each end of the conductive line  406  have a width greater than the connective section  406   a . Thus, the width of the repair section  406   b  and the contact section  406   c  of the conductive line  406  is greater than the data line  404  covered area after the data line  404  is formed over the conductive line  406 . In other words, a portion of the repair section  406   b  and the contact section  406   c  is outside the data line  404  covered area. 
   A first dielectric layer  408  that covers the scan line  402 , the gate electrode  402   a  of the thin film transistor  401  and the conductive line  406  is formed over the substrate  400 . The first dielectric layer  408  is a silicon nitride layer, for example. The dielectric layer  408  covering the gate electrode  402   a  serves also as an insulator. 
   A channel layer  412 , for example an amorphous silicon layer, is formed over the first dielectric layer  408  above the gate electrode  402   a . Source/drain terminals  404   b / 404   a  are formed over the channel layer  412 . In the meantime, a data distributing line  404  is formed over the first dielectric layer  408 . The drain terminal  404   a  and the data distributing line  404  are electrically connected. The data distributing line  404  covers the connective section  406   a  and a portion of the repair section  406   b  and the contact section  406   c  of the conductive line  406 . The conductive line  406  and the data distributing line  404  are electrically isolated from each other through the first dielectric layer  408 . 
   A second dielectric layer  416  is formed over the substrate  400 . The second dielectric layer  416  covers the data distributing line  404 , the source/drain terminals  404   b / 404   a  and the channel layer  412 . The second dielectric layer  416  is a passivation layer that protects the source/drain terminals  404   b / 404   a  and the channel layer  412 . 
   An opening  409   a  that exposes a portion of the drain terminal  404   a  is formed in the second dielectric layer  416  above the drain terminal  404   a . At the same time, another opening  409   b  that exposes the data distributing line  404  and the contact section  406   c  of the conductive line  406  is formed in the first dielectric layer  408  and the second dielectric layer  416  above the contact section  406   c . Finally, a pixel electrode  410  is formed over the substrate board  400  and a pixel electrode material layer  410   a  is formed inside the opening  409   b . The pixel electrode  410  and the drain terminal  404   a  are electrically connected through a bridge of material through the opening  409   a . Similarly, the contact section  406   c  of the conductive line  406  and the data distributing line  404  are electrically connected through the pixel electrode material layer  410   a  inside the opening  409   b . The pixel electrode  410  is made from a material such as indium-tin oxide or indium-zinc oxide. 
   In the pixel structure, the thin film transistor  401  is controlled by signals on the scan line  402  and the data distributing line  404 . The pixel electrode  410  and the thin film transistor  401  are positioned next to each other. In particular, the conductive line  406  lies underneath the data distributing line  404  and the width of the repair section  406   b  and the contact section  406   c  at each end of the conductive line  406  exceeds the data line  404  covered area. Furthermore, the contact section  406   c  and the data distributing line  404  are electrically connected through the pixel electrode material layer  410   a . The repair section  406   b  also serves as a laser repair region  418  for conducting a laser repair. 
     FIG. 10  is a cross-sectional view along line III-III″ of  FIG. 8  showing a method of repairing the pixel structure shown in FIG.  8 . As shown in  FIG. 8 , the data distributing line  404  within the pixel structure  404  may be broken occasionally. To repair the broken data distributing line  404 , an opening  420  is formed in the first dielectric layer  408  and the second dielectric layer  416  within the laser repair region  418  as shown in FIG.  10 . Hence, the repair section  406   b  and a portion of the data distributing line  404  are exposed. The opening  420  is formed, for example, by conducting a laser burning operation. 
   Thereafter, a conductive layer  422  is formed over the exposed surface of the opening  420  so that the repair section  406   b  and the data distributing line  404  are electrically connected. The conductive layer  422  is formed, for example, by conducting a laser chemical vapor deposition. A laser CVD is carried out by first providing a reactive gas over the substrate  400  and then activating the gas by laser pulses so that the reactive gas deposits into the opening  420 . 
   Since the contact section  406   c  at one end of the conductive line  406  and the data distributing line  404  have already been electrically connected through the pixel electrode material layer  410   a , only a single laser repair operation in the laser repair region  408  is required to rejoin the broken data line  404 . After a laser repair, the repair region  406   b  of the conductive line  406  is electrically connected to the data distributing line  404 . Hence, using the conductive line  406  and the special contact section  406   c  and repair section  406   b  design, any broken data distributing line  404  inside the pixel structure is easily repaired. 
     FIG. 11  is a schematic top view of an alternative laser-repair facilitated pixel structure according to the third embodiment of this invention. As shown in  FIG. 11 , the repair sections  406   b  and the contact section  406   c  at each end of the conductive line  406  may be on the opposite side of the data distributing line  404  rather than on the same side of the data distribution line  404  (as shown in FIG.  8 ). In addition, the conductive line  406  may be designed to have a width greater than the data distributing line  404  so that the excess portion of the conductive line  406  may provide more repair area in the repair section  406   b  or the contact section  406   c.    
   In conclusion, major advantages of this invention includes: 1. The laser repair method is capable of preventing the formation of spike discharge and short circuit between the upper and lower substrate board of a liquid crystal display panel. 2. In the laser repair facilitated pixel structure, the scan line and the conductive line for repairing a broken data line are formed concurrently. Moreover, the two ends of the conductive line are designed to be a laser repair region so that laser repair is carried out within the relatively small area of the two laser repair regions. In other words, the repair operation will not directly affect yield even if the broken region is large. 3. In an alternative laser repair facilitated pixel structure, the scan line and the conductive line for repairing a broken data line are formed concurrently. Furthermore, one end of the conductive line is electrically connected to the data distributing line through a conductive structure while the other end of the conductive line is designed to be a laser repair region. Hence, repair can be carried out within the small laser repair region when the data distributing line is broken somewhere. 
   In the embodiment of this invention, the common five-mask process of fabricating the thin film transistor is used to form the pixel structure. However, other thin film transistor processes such as a four-mask thin film transistor process or a thin film transistor process that uses an etching top layer may also be employed to form the pixel structure. 
   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 and their equivalents.