Abstract:
In an active matrix organic light emitting diode (AMOLED) display panel having an improved OLED circuit layout in the TFT back panel, the AMOLED pixels in the AMOLED pixel array are arranged to have the TFT circuit portions of the AMOLED pixels in clustered regions so that each pulse of laser beam during laser annealing of the amorphous silicon film irradiates mostly TFT circuit portions, thus, allowing more efficient laser annealing process.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an active matrix organic light emitting diode (AMOLED) display panel, and more particularly, to an improved layout of thin film transistor (TFT) circuits on the TFT back panel.  
       BACKGROUND  
       [0002]     In a typical AMOLED display panel, the TFT device circuits are formed on a TFT back panel of the display panel. The TFT devices, which generally include a polycrystalline silicon film as a semiconductor layer, may be a bottom gate type or a top gate type, such as low temperature polysilicon thin film transistor. The polycrystalline silicon film requires high electron mobility in order for the TFT device to function optimally. In general, the polycrystalline silicon film is formed from an amorphous silicon film. One way to form the polycrystalline silicon film from the amorphous silicon film is to crystallize the amorphous silicon film by irradiating it with laser light, such as a high-power excimer laser. An excimer laser is a pulsed laser having KrF, ArF, or XeCl as a light source. The amorphous silicon film is generally crystallized over its entire surface by irradiating the substrate from one end to the other with excimer laser light that has been processed to have a linear shape. The linear shaped laser beam generally spans a portion or the whole length of a TFT back panel and is scanned in a lateral direction.  
         [0003]     Illustrated in  FIG. 1  is a 4×4 pixel array portion of a conventional AMOLED&#39;s TFT back panel  100 . As illustrated, pixel region  110  comprises a TFT circuit portion  112  and an OLED circuit portion  114 . The amorphous silicon film layer is initially deposited over the entire TFT back panel  100  and crystallized into polycrystalline form using the excimer laser annealing process. A linear-shaped excimer laser beam  120  is scanned over the entire surface of the TFT back panel  100  by irradiating a portion of the TFT back panel  100  at a time. Since the size of the laser beam is limited, many pulses of laser beams are required to cover the entire TFT back panel  100 .  
         [0004]     After the amorphous silicon film is laser annealed into polycrystalline film, subsequent photolithographic process steps remove unnecessary portions of the polycrystalline film except for the polycrystalline islands that are required for the source, drain and channel regions of the TFT devices in the TFT circuit portion  112 . But, as illustrated in  FIG. 1 , the width W L  of the laser beam  120  is wider than the TFT circuit portion  112  and irradiates more than just the TFT circuit portion  112  of the amorphous silicon film covering the TFT back panel  100 . For example, the width W L  of excimer laser beam  120  commonly used in this application is about 400 micrometers, whereas, the width of the TFT circuit portion  112  is about 100 micrometers. Thus, the laser annealing process crystallizes the amorphous silicon film covering the OLED circuit portion  110  of the TFT back panel  100  as well. Although the polycrystalline silicon film is subsequently removed from the OLED portion  110 , this often results in undesirable line mura defects in the finished AMOLED display panel.  
         [0005]     Mura defects are defects that exhibit as non-uniform contrast regions on an LCD or an OLED display panel and are attributed to pulse-to-pulse variations in the laser beam energy that is used to crystallize the amorphous silicon film. These defects are more pronounced when a constant gray value image or pattern is displayed. In AMOLED display panels, the laser anneal irradiation of the non-TFT regions, such as the OLED circuit portion  110 , on the TFT back panel often results in line-shaped mura defects. The non-uniform laser beam energy caused by pulse-to-pulse variations in the laser beam energy results in non-uniform performance of polycrystalline silicon. And because the TFT characteristic is sensitive to the performance of the polycrystalline silicon and the TFT devices drive the OLED devices, the non-uniform TFT characteristics result in non-uniformity in OLED&#39;s brightness. This non-uniformity causes the line mura defects.  
         [0006]     To eliminate the line mura defect problem, conventional laser annealing process for crystallizing the amorphous silicon film calls for overlapping each pulse of the laser beam to minimize the effects of the pulse-to-pulse variations in the laser beam energy. Furthermore, the silicon film is scanned with the laser beam twice to further minimize the effects of the pulse-to-pulse variations in the laser beam energy. But these conventional solutions are expensive because the processing time is increased and the life of the laser is shortened because of the increased duty cycle.  
         [0007]     Also, because substantial portion of the laser beam energy is spent in irradiating unnecessary portions of the amorphous silicon thin film areas, the conventional AMOLED circuit layout results in an inefficient use of manufacturing resources. And the unnecessary expenditure of the laser beam energy attributes to unnecessarily shortening the life of the excimer laser tool.  
       SUMMARY  
       [0008]     According to an aspect of the present invention, an improved active matrix organic light emitting diode (AMOLED) circuit layout for a thin film transistor back panel that addresses the problems associated with the conventional AMOLED display panel is disclosed.  
         [0009]     According to an aspect of the present invention, an improved AMOLED display panel comprises a TFT back panel and an array of AMOLED pixels on the TFT back panel. Each of the AMOLED pixels has a TFT circuit portion and an OLED circuit portion. The TFT circuit portion comprises at least one layer of polycrystalline silicon film formed by laser annealing an amorphous silicon film. The array of AMOLED pixels are arranged to have the TFT circuit portions of the AMOLED pixels in clustered regions wherein each of the clustered regions of the AMOLED pixels constitute substantially the area irradiated by a pulse of laser beam during the laser annealing of the amorphous silicon film. This allows the laser beam to irradiate mostly the amorphous silicon film in the TFT circuit portions rather than any of the amorphous silicon film in the OLED circuit portions which do not require laser annealing. Thus, the improved AMOLED circuit layout allows more efficient use of the laser.  
         [0010]     The TFT circuit portion may comprise at least one TFT device and the polycrystalline silicon film forms source, drain and channel regions of the at least one low temperature polysilicon thin film transistor device.  
         [0011]     According to another aspect of the present invention, an improved AMOLED display panel comprises a TFT back panel and an array of AMOLED pixels on the TFT back panel. One of the AMOLED pixels has a TFT circuit portion and an OLED circuit portion, and the TFT circuit portion comprises at least one layer of polycrystalline silicon film formed by laser annealing a layer of amorphous silicon film. The array of AMOLED pixels comprises at least one pair of columns of the AMOLED pixels and the AMOLED pixels are arranged to have the TFT circuit portions of the AMOLED pixels in one column opposing the TFT circuit portions of the AMOLED pixels in the other column. This arrangement of the AMOLED pixel circuits allow the TFT circuit portions from both columns to be irradiated at the same time with a single pulse of laser beam during the laser annealing process. By arranging the TFT circuit regions into clustered regions formed by columns of the AMOLED pixels in this manner, the overall laser annealing process time may be reduced because unlike in the case of laser annealing the conventional TFT back panel, any time and laser resources spent in irradiating the unnecessary OLED circuit portions.  
         [0012]     Again, the TFT circuit portion may comprise at least one TFT device and the polycrystalline silicon film forms source, drain and channel regions of the at least one low temperature polysilicon thin film transistor device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic illustration of AMOLED circuit layout on a TFT back panel of a conventional AMOLED display panel.  
         [0014]      FIG. 2  is a schematic plan view illustration of the AMOLED circuit layout according to an aspect of the present invention.  
         [0015]      FIG. 3   a  is a schematic illustration of a TFT back panel of an AMOLED display panel having the AMOLED circuit layout of  FIG. 2 .  
         [0016]      FIGS. 3   b  and  3   c  are schematic illustrations of a portion of a TFT back panel of an AMOLED display panel according to various embodiments of the present invention.  
         [0017]      FIG. 4  is a schematic plan view illustration of a pixel region of a typical AMOLED panel.  
         [0018]      FIG. 5  is a schematic circuit diagram of a pixel of a typical AMOLED panel. 
     
    
       [0019]     The features shown in the above-referenced drawings are schematic only and are not drawn to scale. Like reference numbers represent like elements.  
       DETAILED DESCRIPTION  
       [0020]     Referring to  FIGS. 2 and 3   a , an example of an AMOLED&#39;s TFT back panel  200  having an improved AMOLED circuit layout according to an aspect of the present invention is disclosed. As shown in  FIG. 3   a , the AMOLED circuits on the improved TFT back panel  200  are arranged so that the TFT circuit portions  212  of the AMOLED pixels  210  are in clustered regions  225  that fit substantially within an area covered by a linear-shaped excimer laser beam  220 . In this example, the TFT circuit portions  212  of two neighboring AMOLED pixels  210  are oriented toward one another. As shown, the TFT circuit portions  212  of the AMOLED pixels  210  in the first pixel column A of the TFT back panel  200  are located on the right side of the AMOLED pixels  210  while the TFT circuit portions  212   a  of the AMOLED pixels  210  in the second pixel column B are located on the left side of the AMOLED pixels  210 . Same arrangement of the TFT circuit portions is repeated for the third and the fourth pixel columns C and D.  
         [0021]     This innovative arrangement of the TFT circuit portions  212  of the AMOLED pixels  210 , where the TFT circuit portions  212  of two adjacent columns of the AMOLED pixels  210  are facing each other, allows the linear-shaped excimer laser beam  220  to capture the TFT circuit portions  212  of two adjacent columns in one irradiation. The TFT circuit portion  212  occupies a substantial area irradiated by the excimer laser beam  220 . Thus, unlike the conventional AMOLED TFT back panel  100  of  FIG. 1 , the energy of the laser beam  220  is more efficiently utilized for irradiating the desired TFT circuit portions  212  rather than being wasted on the OLED circuit portions  214 .  
         [0022]     In addition to each irradiating shot of the laser beam  220  being more efficiently utilized, because the TFT circuit portions  212  are clustered together between adjacent pairs of columns of the AMOLED pixel  210 , the overall laser annealing process may be more efficiently conducted in the improved AMOLED TFT back panel  200 . To crystallize the amorphous silicon film in the TFT circuit portion  212  of the improved AMOLED TFT back panel  200 , the laser beam  220  may irradiate the first pair of columns AB of the TFT circuit portions and then skip over to the second pair of columns CD of the TFT circuit portions. No time is spent irradiating the OLED circuit portions  214  of the TFT back panel  200 . Because the TFT circuits are arranged more efficiently, allowing only the amorphous silicon film in the TFT circuit portions to be laser annealed, the overall laser annealing process time is substantially reduced.  
         [0023]     The 4×4 array OLED pixel layout of the TFT back panel  200  illustrated in  FIG. 3   a  is only an exemplary illustration only and the present invention is equally applicable to TFT back panels having different number of columns and rows of OLED pixels.  
         [0024]     Referring to  FIG. 3   b , according to an embodiment of the present invention, the width W T  of the TFT circuit portions AB or CD may be substantially equal to the width W L  of the laser beam. Then, one of the TFT circuit portions AB or CD may be irradiated with a single laser beam to crystallize the amorphous silicon film in those regions. Further, in order to minimize the pulse-to-pulse laser beam energy variations, one of the TFT circuit portions AB or CD may be irradiated with additional pulses of the laser beams as necessary.  
         [0025]     Referring to  FIG. 3   c , according to another embodiment of the present invention, the width W T  of the TFT circuit portions AB or CD may be larger than the width W L  of a laser beam, the laser beam may scan across the width W T  of one of TFT circuit portions AB or CD within the TFT circuit portions AB or CD. Each subsequent pulse of the laser beam may be overlapped as the laser beam scans across the width of one of the TFT circuit portions AB or CD. When the laser scanning of the first TFT circuit portion AB is completed, the laser is skipped over to the next TFT circuit portion CD without irradiating the unnecessary OLED portions in between the regions AB and CD. If necessary, one of the TFT circuit portions AB or CD may be scanned twice in order to minimize the pulse-to-pulse variations in the laser beam energy.  
         [0026]     Detailed illustrations of a pixel  210  are shown in  FIGS. 4 and 5 .  FIG. 4  is an illustration of a plan view illustration of a pixel  210  showing the detailed structures of the TFT circuit portion  212 . In this example, the OLED pixel includes top-gate type TFTs  230  and  240 , a capacitor  270  and an OLED  280 . The channel regions  232  and  242  of the TFTs  230  and  240  respectively are formed from the polycrystalline silicon film that was crystallized from amorphous silicon film via the excimer laser annealing process. Referring to the circuit diagram of the AMOLED pixel  210  illustrated in  FIG. 5 . A high voltage level on a gate line (scan line)  250  turns the TFT  230  ON, thus providing a voltage from a data line  260  to the capacitor  270 . After a period of time, the gate voltage of the TFT  240  is the same as the voltage on data line  260 , and voltage on gate line  250  is set low. The TFT  240  operates as a voltage follower to drive the OLED  280 . Current through the OLED  280  is sourced from a supply voltage Vdd and returned to a supply voltage Vss. As the OLED  280  is driven, a threshold voltage of the TFT  240  changes with time.  
         [0027]     While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention.