Abstract:
A system for inspecting at least a portion of a display panel having thin film transistors (TFTs) and light emitting devicxes (OLEDs), during or immediately following fabrication, so that adjustments can be made to the fabrication procedures to avoid defects and non-uniformities. The system provides bonding pads connected to signal lines on at least portions of the display panel, and probe pads along selected edges of the display panel. The probe pads are coupled to the bonding pads through a plurality of multiplexers so that the number of probe pads is smaller than the number of bonding pads.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/861,614, filed Aug. 2, 2013, and U.S. Provisional Application No. 61/814,580, filed Apr. 22, 2013, both of which are hereby incorporated by reference herein in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to OLED displays and, more particularly, to inspection systems for detecting defects and non-uniformities in displays such as active matrix organic light emitting diode displays. 
       BACKGROUND 
       [0003]    Display panels can be created from an array of light emitting devices each controlled by individual circuits (i.e., pixel circuits) having transistors for selectively controlling the circuits to be programmed with display information and to cause the light emitting devices to emit light according to the display information. Thin film transistors (“TFTs”) fabricated on a substrate can be incorporated into such display panels. Both OLEDs and TFTs can demonstrate non-uniform behavior across display panels due to production problems. Such problems can be corrected if the defects and non-uniformities can be identified at the time the panels are produced, e.g., during or immediately following fabrication. 
       SUMMARY 
       [0004]    A system is provided for inspecting at least a portion of a display panel having thin film transistors (TFTs) and light emitting devicxes (OLEDs), during or immediately following fabrication, so that adjustments can be made to the fabrication procedures to avoid defects and non-uniformities. The system provides bonding pads connected to signal lines on at least portions of the display panel, and probe pads along selected edges of the display panel. The probe pads are coupled to the bonding pads through a plurality of multiplexers so that the number of probe pads is smaller than the number of bonding pads. 
         [0005]    The foregoing and additional aspects and embodiments of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. 
           [0007]      FIG. 1  is a diagrammatic perspective illustration of a display panel adapted to receive a probe card. 
           [0008]      FIG. 2  is a diagrammatic front elevation of the display panel shown in  FIG. 1 , showing the locations of probe pads for receiving probe cards. 
           [0009]      FIG. 3  is a diagram of a pair of probe pads connected to a multiplexer used to supply probe signals to the probe pads. 
           [0010]      FIG. 4  is a schematic circuit diagram of one of the probe pads illustrated in  FIG. 3  connected to receive the display signals. 
           [0011]      FIG. 5  is a diagrammatic illustration of an inspection system for taking measurements of an AMOLED panel, and various corrective actions that can be taken to fix defects identified by analysis of the measurements. 
           [0012]      FIG. 6  is a schematic circuit diagram of a pixel circuit having a signal WR. 
       
    
    
       [0013]    While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0014]      FIG. 1  illustrates a system for inspecting an OLED display panel  10  at one or more stages of the fabrication of the panel  10  (e.g., a TFT backplane, a fully fabricated panel, or a fully completed and sealed panel). The display panel  10  is coupled to a computer  14  through measurement electronics  11  and a probe card  12 , to provide the capability of testing and verifying the panel at each processing step. For example, after finishing the TFT backplane, the probe card system can be used to measure the performance of the TFT backplane by itself. If the TFT backplane is acceptable, then the panel  10  is passed to the next step which can be the OLED deposition stage. After the OLED deposition is completed, the panel  10  can be measured for proper OLED deposition before sealing. After sealing, the panel  10  can be measured again before it is sent to an assembly process. 
         [0015]    As can be seen in  FIG. 2 , the illustrative display panel  10  has probe pads  20  formed along three of the four edges of the panel. Probe pads can also be formed inside the panel, preferably before the OLED deposition stage. The probe pads  20  are used to supply test signals to the numerous pixel circuits on the display panel  10 , via bonding pads  30  formed at the outer ends of the various signal lines leading to the pixel circuits. 
         [0016]      FIG. 3  illustrates the connection of the probe pads  20  to the bonding pads  30  through a multiplexer (MUX)  40 , to reduce the required number of probe pads, which in turn permits the pad pitch to be increased. To ensure that the other signals connected to the probe pads  20  are biased properly, the MUX  40  needs to be capable of connecting each probe pad  20  to a common signal (Vcom) for each group of signals (e.g., source signals, gate signals, etc.). 
         [0017]      FIG. 4  illustrates a MUX  40  with common signal control and two or more panels signals for each probe pad  20 .  FIG. 4  illustrates h panel signals connected to one probe pad  20 , thus requiring 2 h controlling signals for connection to the probe pads  20  or connection to the common signals. The connections of the panel signals to the probe pad  20  are controlled by first switches  41  and  42 , and the connections of the common signals Vcom to the panel signal lines are controlled by second switches  43  and  44 . 
         [0018]    The proper pad pitch for full panel probing is typically about 150 μm. As illustrated by the data in Table 1, the pad pitch for most conventional configurations meets the minimum pad pitch requirements. However, using multiplexing ratios of 2:1 or greater permits the pad pitch to be increased, resulting in much simpler probe cards, as also illustrated by the data in Table 1: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Pad pitch for different display sizes and resolutions. 
               
             
          
           
               
                   
                   
                   
                 Gate Pad 
                 Source Pad 
                 EIC Pad 
               
               
                   
                   
                   
                 Pitch 
                 Pitch 
                 Pitch 
               
               
                   
                 Panel 
                 MUX 
                 (μm) 
                 (μm) 
                 (μm) 
               
               
                   
                   
               
             
          
           
               
                   
                 55″ HD 
                 1:1 
                 295 
                 330 
                 330 
               
               
                   
                 55″ HD 
                 2:1 
                 592 
                 661 
                 661 
               
               
                   
                 55″ HD 
                 8:1 
                 2370 
                 2645 
                 2645 
               
               
                   
                 55″ UD 
                 1:1 
                 148 
                 165 
                 165 
               
               
                   
                 55″ UD 
                 2:1 
                 295 
                 330 
                 330 
               
               
                   
                 55″ UD 
                 8:1 
                 1185 
                 1132 
                 1132 
               
               
                   
                 78″ UD 
                 1:1 
                 222 
                 222 
                 222 
               
               
                   
                 78″ UD 
                 2:1 
                 444 
                 445 
                 445 
               
               
                   
                 78″ UD 
                 8:1 
                 1777 
                 1781 
                 1781 
               
               
                   
                   
               
             
          
         
       
     
         [0019]    As depicted in  FIG. 5 , an electronic measuring system  13  mounted on the probe card  12  can measure the electrical characteristics of every TFT and every OLED device in a display panel  10  and identify defects and non-uniformities. This data is supplied to a GUI  14 , where the data can be used to fine-tune every process step, to achieve higher yields, faster process ramp-up, and lower line monitoring costs. Examples of the various process steps that can be fine tuned are illustrated in  FIG. 5 , namely, a sputtering and PECVD module  50 , a process annealing module  51 , a patterning module  52 , a laser repair module  53 , an inkjet printing module  54  and an evaporation module  55 . The end result is a complete display panel  56 . 
         [0020]    The circuitry depicted in  FIG. 5  takes data from the measurement electronics  13 , analyzes that data, and displays it in a wide variety of reports, tables, and pictures. Some of the views are described in the following table: 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 View 
                 Description 
               
               
                   
               
             
             
               
                 TFT Absolute 
                 View the absolute measurement replacement values 
               
               
                 LUT 
                 for each pixel on the panel. 
               
               
                 TFT Filtered 
                 View the filtered replacement values used to 
               
               
                 LUT 
                 calculate delta values. 
               
               
                 TFT Base 
                 View the factory shipment values of the panel 
               
               
                 LUT 
                 to determine how much the pixels have aged 
               
               
                   
                 (baseline). 
               
               
                 TFT Delta 
                 View the difference between the current average 
               
               
                 LUT 
                 measured value and the baseline values (used to 
               
               
                   
                 determine compensation). 
               
               
                 TFT Histogram 
                 View the number of times a pixel has been measured 
               
               
                 LUT 
                 since the last time the histogram was reset. This 
               
               
                   
                 lookup table is primarily used for priority scan 
               
               
                   
                 algorithm. 
               
               
                 TFT Pixel 
                 View either the current state of the measurement 
               
               
                 State LUT 
                 state machine or the last comparison values for 
               
               
                   
                 each pixel. 
               
               
                 TFT Region 
                 Show the priority of each region in the priority 
               
               
                 Priority 
                 scan algorithm. 
               
               
                 OLED Absolute 
                 View the absolute measurement replacement values 
               
               
                 LUT 
                 for each pixel on the panel OLED layer. 
               
               
                 OLED Filtered 
                 View the filtered replacement values used to 
               
               
                 LUT 
                 calculate delta values. 
               
               
                 OLED Base LUT 
                 View the factory shipment values of the panel to 
               
               
                   
                 determine how much the pixels have aged (baseline). 
               
               
                 OLED Histogram 
                 View the number of times a pixel has been measured 
               
               
                   
                 since the last time the histogram was reset. 
               
               
                 OLED Pixel 
                 View either the current state of the measurement 
               
               
                 State 
                 state machine or the last comparison values for 
               
               
                   
                 each pixel. 
               
               
                 OLED Region 
                 Show the priority of each region in the priority 
               
               
                 Priority 
                 scan algorithm. 
               
               
                 Dead Pixels 
                 Show which pixels were either dead at point of 
               
               
                 LUT 
                 manufacture or have since been determined to be 
               
               
                   
                 unresponsive. Note that dead pixels are not 
               
               
                   
                 compensated. 
               
               
                 Combine Delta 
                 The combined TFT and OLED delta values used to 
               
               
                 LUT 
                 determine the final compensation. 
               
               
                 Scratch LUT 
                 A temporary LUT View to allow users to manipulate 
               
               
                   
                 the data without making modifications to the 
               
               
                   
                 system tables or to simply “backup” a table. 
               
               
                 Statistics 
                 Reports the performance statistics and the current 
               
               
                   
                 frame rate. Use these statistics to compare the 
               
               
                   
                 time required to process and display the current 
               
               
                   
                 data in MaxLife Viewer vs. the time it would 
               
               
                   
                 actually take if the display was not required. 
               
               
                 Pixel Trends 
                 View the pixel state to determine if there are 
               
               
                   
                 unsettled pixels and view comparative levels. 
               
               
                   
                 Allows you to obtain a visual representation of 
               
               
                   
                 uncertainty zones and look at specific defective 
               
               
                   
                 pixels. 
               
               
                   
                 Use this option to obtain active measurements 
               
               
                   
                 over time; determine how many times a pixel was 
               
               
                   
                 measured before it settled. 
               
               
                 Uniformity 
                 Select a LUT table, then analyze the uniformity. 
               
               
                 Report 
               
               
                 Offset 
               
               
                 Character- 
                 Plots the V-to-I voltage DAC code to the probability 
               
               
                 ization 
                 of a comparator result flip. 
               
               
                 Hardware 
                 View the current hardware configuration parameters. 
               
               
                 Configuration 
               
               
                 Display 
                 Adds the Display Controls to the bottom of the 
               
               
                 Controls 
                 current tab. 
               
               
                   
                 These options allow you to set the frame refresh 
               
               
                   
                 rate in frames per second. 
               
               
                   
                 Drag the slider to the left or right to speed up or 
               
               
                   
                 slow down the refresh rate. Slower speeds are more 
               
               
                   
                 visible to the naked eye. 
               
               
                 CLI View 
                 Issues commands from MaxLife Viewer to the system. 
               
               
                   
               
             
          
         
       
     
         [0021]    A wide variety of different circuitry and algorithms may be used for extracting measurements of different parameters from the display panel at different stages of its fabrication, such as the extraction systems described in U.S. patent application Ser. No. 13/835,124 filed Mar. 15, 2013 and entitled “Systems and Methods for Extrraction of Threshold and Mobility Parameters in AMOLED Displays,” which is incorporated by reference herein in its entirety. 
         [0022]    The inspection system can identify many potential defects and problems (e.g., with sputtering and PECVD steps, that can be used to identify the likely cause of the defect or problem so that the fabricating process can be immediately fine-tuned to correct the problem). Examples of such issues and their likely causes are the following: 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 Issue identified by MaxLife ™ 
                   
               
               
                 inspection system 
                 Likely cause 
               
               
                   
               
             
             
               
                 Line defect (open circuit 
                 Particle defect during deposition, 
               
               
                 on metal lines) 
                 poor adhesion, contaminated substrate 
               
               
                   
                 and poor step coverage. 
               
               
                 High resistance or non- 
                 Non-uniform sputtering process, 
               
               
                 uniform resistance on metal 
                 contaminated sputter gas or process 
               
               
                 lines 
                 chamber. 
               
               
                 Non-uniform TFT contact 
                 Problem with n+ layer PECVD step, 
               
               
                 resistance 
                 incomplete via etch or photoresist 
               
               
                   
                 stripping process. 
               
               
                 Vt or mobility of TFTs out 
                 Problem with a-Si layer PECVD step 
               
               
                 of specification 
                 (contamination during deposition, 
               
               
                   
                 process parameter drift or film stress) 
               
               
                 Open TFT channel 
                 Problem with a-Si or n+ layer deposi- 
               
               
                   
                 tion (particle contamination during 
               
               
                   
                 deposition/contaminated substrate) 
               
               
                 Gate shorted to fixed 
                 Incomplete metal patterning and/or 
               
               
                 voltage 
                 damaged dielectric layers from 
               
               
                   
                 particles or ESD 
               
               
                 Source or gate shorted to 
                 Damaged dielectric layers (pinholes) 
               
               
                 drain 
                 from particles or ESD 
               
               
                 Open or high resistance 
                 Incomplete via etching. 
               
               
                 contacts 
               
               
                 Out of spec capacitance 
                 Non-uniform dielectric layer deposi- 
               
               
                   
                 tion or drift in deposition process 
               
               
                   
                 parameters. 
               
               
                 Line defect (crossover 
                 Pinhole in the dielectric layers from 
               
               
                 short) 
                 particles or ESD 
               
               
                   
               
             
          
         
       
     
         [0023]    For defects that cannot be directly identified by a single measurement of the inspection system, the first measurement can reveal that a problem exists, and specify additional tests that will conclusively identify the exact defect. One example is the identification of line defects, which can be detected by any of the following procedures:
       1. Measuring the current of different lines: if the current is higher than a threshold, the pixel is shorted.   2. Applying pulse to measure the charge transfer: if the amount of charge transfer is smaller than a threshold, the line is open.   3. For a signal with connection to DC current (e.g., Vdd and Vmonitor), the current can be measured to detect the open defect       
 
         [0027]    Defects in the thin film transistors (TFTs) can also be detected. For example, in the situation where the pixel circuit in  FIG. 6  has a signal WR measured as high (while Vdata=high, and also while Vdata=low, and Vdd=high), an additional test needs to be performed. Table 1 shows the different conditions and what the results mean. 
         [0028]    To detect problems with process annealing, the exact Vt and mobility of each TFT can be used to adjust process annealing parameters, as follows: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Issue identified by MaxLife ™ 
                   
               
               
                   
                 inspection system 
                 Likely cause 
               
               
                   
                   
               
             
             
               
                   
                 Vt and/or mobility of TFTs is higher 
                 Laser power drift 
               
               
                   
                 or lower than specification 
               
               
                   
                 Small scale non-uniformity of Vt 
                 Intermittent laser power 
               
               
                   
                 and/or mobility of TFTs 
                 output 
               
               
                   
                 Large scale non-uniformity of Vt 
                 Laser repeatability 
               
               
                   
                 and/or mobility of TFTs 
               
               
                   
                   
               
             
          
         
       
     
         [0029]    The number and types of defects can be used to identify problems in patterning (particles, under/over exposure, etc.), as follows: 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 Issue identified by MaxLife ™ 
                   
               
               
                 inspection system 
                 Likely cause 
               
               
                   
               
             
             
               
                 High-resistance metal lines 
                 Pattern definition or metal etch 
               
               
                   
                 process. Poor line width control. 
               
               
                 Open or high resistance 
                 Poor via pattern definition/photo- 
               
               
                 contacts 
                 resist residue 
               
               
                 Gate shorted to fixed 
                 Pinholes in the dielectric layers. 
               
               
                 voltage 
               
               
                 Abnormal capacitance or 
                 Mask alignment error (rotation), 
               
               
                 resistance in corners of 
                 photoresist thickness non-unifor- 
               
               
                 panel 
                 mity. 
               
               
                 Large scale capacitance or 
                 Pattern alignment error or exposure 
               
               
                 resistance, non-uniformity 
                 power fluctuation. 
               
               
                 Adjacent metal lines shorted 
                 Particles in photoresist/pattern 
               
               
                   
                 definition. 
               
               
                 Pattern stitching defects 
                 Stepper alignment failure 
               
               
                 Repetitive defect 
                 Exposure masks damage or contami- 
               
               
                   
                 nated. 
               
               
                   
               
             
          
         
       
     
         [0030]    The defect location and defect type can be used to pinpoint areas suitable for laser repair (removing material) or ion beam deposition (adding material), as follows: 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 Issue identified by MaxLife ™ 
                   
               
               
                 inspection system 
                 Repair Step 
               
               
                   
               
             
             
               
                 Gate shorted to fixed 
                 Give exact pixel location to laser 
               
               
                 voltage 
                 repair system 
               
               
                 Short on metal lines 
                 Identify the metal lines that are 
               
               
                   
                 shorted. 
               
               
                 Open circuit on metal lines 
                 Identify the metal lines that are 
               
               
                   
                 open. 
               
               
                 Open or high resistance 
                 Quickly identify the number and 
               
               
                 TFT contacts 
                 location of the defective pixels. 
               
               
                   
               
             
          
         
       
     
         [0031]    The uniformity data can also be used to continuously calibrate each print head used for inkjet printing, in real-time. The system knows which print head was used to print each pixel, and thus problems with individual print heads can be detected. The print head used to print those pixels can then be immediately adjusted, as follows. 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 Issue identified by MaxLife ™ 
                   
               
               
                 inspection system 
                 Likely cause 
               
               
                   
               
             
             
               
                 Dead pixels 
                 Printhead occasionally putting down 
               
               
                   
                 too little material, causing shorts 
               
               
                 Stuck-on pixel 
                 Printhead occasionally putting down 
               
               
                   
                 too little material 
               
               
                 High-resistance pixels 
                 The printhead printing those pixels 
               
               
                   
                 may be putting down too much material 
               
               
                 Uniformity of OLED&#39;s 
                 Flow control of printhead malfunc- 
               
               
                 voltage is poor 
                 tioning 
               
               
                   
               
             
          
         
       
     
         [0032]    The exact failure mode of every OLED device can be used to tune the evaporation process, as follows: 
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 Issue identified by MaxLife ™ 
                   
               
               
                 inspection system 
                 Likely cause 
               
               
                   
               
             
             
               
                 All pixels from one printhead 
                 Problem with calibration of 
               
               
                 are too high (or too low) 
                 printhead 
               
               
                 resistance 
               
               
                 Short-circuit OLED 
                 Too little organic material being 
               
               
                   
                 deposited, causing shorts 
               
               
                 High-resistance pixels 
                 Too much organic material being 
               
               
                   
                 deposited 
               
               
                 OLED voltage too high 
                 Too much organic material being 
               
               
                   
                 deposited 
               
               
                 Long-range Uniformity of 
                 Problem with substrate rotation or 
               
               
                 OLED&#39;s voltage is poor 
                 evaporator too close to substrate 
               
               
                 Short-range uniformity of 
                 Problem with thermal evaporation 
               
               
                 OLED&#39;s voltage is poor 
                 temperature control 
               
               
                 Open-circuit OLED 
                 Particles during evaporation 
               
               
                 Short to cathode or anode 
                 Particles during evaporation 
               
               
                 Partial short (low resistance) 
                 Too little organic material being 
               
               
                   
                 deposited 
               
               
                   
               
             
          
         
       
     
         [0033]    The electrical characteristics (collected during TFT and OLED inspection) can be loaded into a lookup table, and used to correct for all TFT and OLED non-uniformities. 
         [0034]    Additional defects can be identified once both the OLEDs and TFTs have been deposited. The first measurement can identify that a problem exists, and specify additional tests that will conclusively identify the exact defect. 
         [0035]    If test samples are created around the periphery of the panel, then more details about the global process parameters can be extracted. Typically this is done by cutting off the test samples from a small percentage of displays and putting them in a separate characterization system. However, with the present inspection system, this can be done as part of panel characterization, for every panel, as follows:
       Metal lines can be created and resistance measured. This can test both metal deposition steps and etching.   Semiconductor layers to be annealed can have their characteristics and uniformity tested.   Structures can be used at different locations around the panel to test alignment.   OLED structures can be used to test evaporation and inkjet printing steps.       
 
         [0040]    While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.