Patent Publication Number: US-7714589-B2

Title: Array test using the shorting bar and high frequency clock signal for the inspection of TFT-LCD with integrated driver IC

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is related to and claims benefit under 35 USC 119(e) of U.S. provisional Application No. 60/737,090, filed Nov. 15, 2005, entitled “Array Test Using The Shorting Bar And High Frequency Clock Signal For The Inspection Of TFT-LCD With Integrated Driver IC”, the content of which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to inspection of thin film transistor (TFT) arrays, and more specifically to inspection of TFT arrays that have integrated circuit (IC) drivers. 
   In a finished liquid crystal flat panel, a thin layer of liquid crystal (LC) material is disposed between two sheets of glass. On one sheet of glass, a two-dimensional array of electrodes has been patterned. Each electrode may be on the order of 100 microns in size and can have a unique voltage applied to it via multiplexing transistors positioned along the edge of the panel. In a finished product, the electric field created by each individual electrode couples into the LC material and modulates the amount of transmitted light in that pixelated region. This effect when taken in aggregate across the entire two dimensional array results in a visible image on the flat-panel. 
   A significant part of the manufacturing cost associated with LCD panels occurs when the LC material is injected between the upper and lower glass plates. It is therefore important to identify and correct any image quality problems prior to this manufacturing step. The problem with inspecting LCD panels prior to deposition of the liquid crystal (LC) material is that without LC material, there is no visible image available to inspect. Prior to deposition of LC material, the only signal present at a given pixel is the electric field generated by the voltage on that pixel, if driven by an external electrical source. Means of testing such panel arrays typically take advantage of an electrical property of the pixel (such as electrical field or pixel voltage as a function of changing drive voltages on the transistor gates or data lines). Array testers devised by Photon Dynamics use a voltage image optical system (VIOS), as described by U.S. Pat. No. 4,983,911, for example. Array testers sold by Applied Komatsu use an electron beam and imaging system to detect defects. Both these array test machines require a means to electrically drive the sample in conjunction with their respective detection methodologies. 
   U.S. Pat. No. 5,081,687, issued to Henley et al. and incorporated herein by reference in its entirety, describes an array test method according to which a pattern of electrical driving signals are applied to the panel under test. Referring to  FIG. 1 , a typical active matrix LCD panel segment  10  is shown as including, an array of pixels  12 . Each pixel  12  is activated by addressing simultaneously an appropriate drive line  14  and gate line  16 . A drive element  18  is associated with each pixel. The drive lines  14 , gate lines  16 , pixels  12  and pixel drive elements  18  are deposited on a clear glass substrate by a lithographic or other processes. Odd numbered gate lines may be addressed simultaneously via shorting bar  30 , which joins every other gate line  16 . Even numbered gate lines may be addressed by a second shorting bar (not shown). Similarly, odd numbered data lines may be addressed via shorting bar  28 , which joins every other data line  14 . Even numbered data lines may be addressed by a second shorting bar (not shown). Different drive patterns may be applied to the gate and data lines to determine which pixels may be defective. 
   Typically, electrical drive circuitry of the final display panel is added during manufacturing and assembly of the panel into its final form (for example, computer monitor, cell phone display, television, etc.)  FIG. 2  shows a panel  200  that is in electrical communication with printed circuit board  204  using a multitude of connectors  204 . Panel  200  of  FIG. 2  is assumed to include the circuitry shown in  FIG. 1 . A gate driver integrated circuit (IC) (not shown) is mounted on printed circuit board  204  which is then brought into electrical contact with panel  200  for driving the pixel gate lines. 
   Recently, however, with the increased application of amorphous silicon material and associated processes and designs, integrated circuit (IC) gate drivers are being formed on the panel, as shown in simplified  FIG. 3 . See for example Kim et al, “High-Resolution Integrated a-Si Row Drivers,”  SID  05  Digest , page 939; Lebrun et al “Design of Integrated Drivers with Amorphous Silicon TFTs for Small Displays, Basic Concepts”  SID  05  Digest , page 950. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a first shorting bar drives the data lines of a TFT array having integrated gate driver circuitry. Another set of shorting bars drive the corresponding terminals of the gate driver circuitry. The pixel voltages are measured after all the pixels are charged by the driving signals applied to the shorting bars. Gate voltages are progressively applied to the gate lines by the gate driver integrated circuit (IC) via the set of shorting bars that, in turn, are driven by clock signals received from one or more pattern generators. Voltages are concurrently applied to the data lines which are connected together by the first shorting bar. The application of voltages generates a display pattern that is subsequently compared to an expected display pattern. By comparing the resulting display pattern and the expected display pattern, possible defects are detected. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a typical active matrix LCD panel segment, as known in the prior art. 
       FIG. 2  shows a partially assembled panel in electrical contact with a printed circuit board that includes an integrated circuit gate driver, as known in the prior art. 
       FIG. 3  shows a partially assembled panel with an integrated circuit adapted to drive the gate lines of the pixels formed on the panel. 
       FIG. 4A  shows a multitude of shift registers disposed in a gate driver IC integrated onto the TFT panel. 
       FIG. 4B  is a timing diagram of a number of input signals applied to the gate driver circuit of  FIG. 4A . 
       FIG. 4C  is a timing diagram of a number of the output signals generated by the gate driver circuit of  FIG. 4A . 
       FIG. 5  is a simplified high level block diagram of a flat panel being tested using a multitude of shorting bars, in accordance with one embodiment of the present invention. 
       FIG. 6  is an exemplary timing diagram of the various signals used in testing of the flat panel of  FIG. 5 . 
       FIG. 7A  is a table showing the number of input signals of another exemplary gate driver IC. 
       FIG. 7B  is an exemplary timing diagram of the input signals shown in  FIG. 5A . 
       FIG. 8  shows a number of exemplary circuit blocks used in generating signals that drive shorting bars of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In accordance with the present invention, a first shorting bar drives the data lines of a TFT array having integrated gate driver circuitry, i.e., a TFT array having a substrate on which the integrated circuit is formed. Another set of shorting bars drive the corresponding terminals of the gate driver circuitry. The pixel voltages are measured after the pixels are charged by the driving signals. Gate voltages are progressively applied to the gate lines by the gate driver IC via the set of shorting bars that, in turn, are driven by clock signals received from one or more pattern generators. Voltages are concurrently applied to the data lines which are connected together by the first shorting bar. The current invention generates arbitrary waveform with high frequency for the gate driver IC and with low frequency for the data lines. In some embodiments, a first multitude of shorting bars may be used to supply signals to the data lines and a second multitude of shorting bars may be used to supply signals to the gate lines. 
     FIG. 4A  illustrates a gate driver IC  404  shown as including a multitude of shift registers  406   1  . . .  406   N  (collectively and alternatively referred to herein as  406 ) each receiving a pair of clock signals that are 180 out-of-phase, and an enabling signal Vst. Each register  406  is configured to output a pulse when its associated enable signal Vst is asserted.  FIG. 4B  is a timing diagram of the signals applied to the gate driver IC  404 , and  FIG. 4C  is a timing diagram of the signals generated by gate driver IC  404 . As seen from these timing diagrams, when signal Vst applied to an input terminal of shift register  406   1  makes a low-to-high transition, shift register  406  generates an output pulse, synchronously with respect to the clock signal CK 1  and CK 2 , that is shown as being supplied to gate 414 1  (not shown). In other words, signal Vst enables the start of the driving pattern. The output pulse of shift register  406   1  is used as an enabling signal to shift register  406   2 , which, in turn, supplies its output signal to gate  414   2  (not shown), etc. Accordingly, output pulses  414  are generated in a stepwise fashion in time, corresponding to the stream of input clock signal CK 1  and CK 2 . In accordance with the present invention, a first shorting bar  450  is used to supply clock signal CK 1  to shift registers  406 , a second shorting bar  452  is used to supply clock signal CK 2  to shift registers  406 , and a third shorting bar  454  is used to supply voltage Vdd. The two-phase clock design, i.e., a pair of complementary clock signals that are 180° out-of-phase, allows any signal distortions from clock feed-through and high parasitic capacitances to be compensated by the opposing clock. 
   To electrically test a TFT array, a pattern of electric driving signals is applied and a means of detection, such as Photon Dynamics&#39; voltage imaging system, (VIOS) scans over the panel observing optically or electrically any pixels that are not responding to the pattern of signals. The pattern of electric driving signals is applied to the IC gate drivers as described above, and also to the data lines through the data shorting bars or individual data lines. The generated display pattern is compared to an expected display pattern to detect defects. 
     FIG. 5  is a highly simplified top level view of panel  400 . As shown, panel  400  includes, in part, pixel array  402 , and gate driver IC  404 . Gate driver IC  404  includes a multitude of shift registers as shown in  FIG. 4A . In the example of  FIG. 5 , IC gate driver  404  requires three input signals, namely signals Vst, CLK 1 , CLK 2 , and a supply voltage VDD. Signals CLK 1  and CLK 2  are respectively driven by shorting bars  450  and  452 . Voltage Vdd is supplied using shorting bar  454 . 
   The data lines are driven through shorting bars  608   1  and  608   2 . The data lines are separated into a set of “odd” lines and “even” lines, which are connected respectively via shorting bars  608   1  and  608   2  to contact pads DO (“data odd”)  610  and DE (“data even”)  612 . In accordance with the test method of the present invention, pixels which are connected together with the same shorting bar are turned on concurrently.  FIG. 6  is an exemplary timing diagram of the various signals shown in  FIG. 5 . As shown in  FIG. 6 , the data lines (“Data even” and “Data odd”) are typically driven at a lower frequency relative to the gate lines (“CK 1 ” and “CK 2 ”). 
   Each flat panel manufacturer designs the IC gate drivers differently, and may have different input signal definitions as well as different number of required input signals.  FIG. 7A  is a table illustrating another example of a gate driver IC (not shown) having ten input terminals and thus requiring ten input signals to operate.  FIG. 7B  shows an example of a timing diagram of the input signals corresponding to the table shown in  FIG. 6A . In accordance with the present invention, 6 shorting bars supplying signals Reset, CLK 1 , CLK 2 , CLK 3 , CLK 4 , and Vg 1 , are used with each shorting bar supplying a signal to a different one of the ten input terminals of such a gate driver IC. Three more shorting bars supply drive voltages Vdd, Vdd 1  and Vdd 2  to the transistors. 
   One example of the system configuration to test TFT array with an integrated gate driver circuit is shown in  FIG. 8 . Pattern generator  802  generates arbitrary waveforms and voltage amplifier  804  amplifies the generated waveforms. Multiplexer  806  selects the panel to test and delivers the required signals to the IC gate driver and data line shorting bar. The gate driver IC may be designed to operate at a frequency of 60 Hz or 75 Hz in one embodiment. The typical pulse width of the clock signal with 60 Hz driving for XGA resolution panel is 20 μs. If the design parameter for safety factor is 2, the pulse width should be bigger than 10 μs to drive the gate driver IC. In the example shown in  FIG. 6 , the clock pulse width is 16 μs which is smaller than the typical pulse width of 60 Hz driving for XGA. However, this can properly turn on the pixels. The present invention may be used to test both types of TFT array, a conventional TFT array and a TFT array with gate driver IC implemented, with the same system. 
   The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. The invention is not limited by the type of flat panel display, nor is it limited by the type of gate driver circuit integrated in with the flat panel. The invention is not limited by the number of input signals of the integrated gate driver circuit. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.