Patent Publication Number: US-7211452-B2

Title: Method and apparatus for uniformity and brightness correction in an OLED display

Description:
FIELD OF THE INVENTION 
     The present invention relates to the manufacture of OLED displays and, in particular, a method for the calibration, grading, and correction of OLED displays. 
     BACKGROUND OF THE INVENTION 
     Organic Light Emitting Diodes (OLEDs) have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and may employ a plurality of pixels. The pixels are typically arranged in two-dimensional arrays with a row and a column address for each pixel and having a data value associated with the pixel value. However, such displays suffer from a variety of defects that limit the quality of the displays. In particular, OLED displays suffer from non-uniformities in the pixels. These non-uniformities can be attributed to both the light emitting materials in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the light emitting elements. 
     Referring to  FIG. 2 , in a current manufacturing and grading process, the OLED devices are first manufactured. This manufacturing step  100  involves, e.g., the preparation of a substrate, typically glass, the formation of electrodes and other electronic components on the substrate, the deposition of organic material layers, the addition of a second electrode, the encapsulation of the device, followed by singulation, packaging, and the attachment of electrical connectors. After the OLED device is manufactured, its performance is measured  110  to ensure that the light-emitting elements of the OLED device are working properly. Some faults may be present, for example stuck-on or stuck-off pixels, dark or bright pixels, and other non-uniform pixels. The OLED device may or may not meet the standards of the application for which it is intended so it is graded  120 . If the OLED device does not meet the specification standards of the application, a repair  130  may be attempted. If the repair is not possible, the display is discarded  140 . If it can be repaired, the repair is performed and the device tested  110  again. 
     If the OLED device does meet the application standards, it is a good device, and is subsequently burned-in  150  by illuminating the OLED device over a period of time with a burn-in pattern, for example a flat-field image. This burn-in process is necessary to ensure a stable operation of the device when it is first used in an application. Following burn-in, the device performance is again measured  160  and re-graded  170 . If the device does not meet the specification at this point, it is discarded  180 . If it does meet the specification, it may be shipped to a customer  190 . 
     This process is effective but suffers from a high rejection rate. Some faults in light emitters may be compensated using a variety of means taught in the art. For example, copending, commonly assigned U.S. Ser. Nos. 10/858,260, 10/869,009 and 10/894,729 describe various means to detect and correct for some faults found in OLED devices. Other methods, for example, U.S. Pat. No. 6,414,661 B1 entitled “Method and apparatus for calibrating display devices and automatically compensating for loss in their efficiency over time” by Shen et al issued 20020702 describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes in an OLED display device by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. The compensation system is best used after the display device has been calibrated to provide uniform light output. This patent provides a means for correcting the non-uniformities through the use of a look-up table. 
     U.S. Pat. No. 6,473,065 B1 entitled “Methods of improving display uniformity of organic light emitting displays by calibrating individual pixel” by Fan issued 20021029 describes methods of improving the display uniformity of an OLED. In order to improve the display uniformity of an OLED, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. 
     All of these correction schemes require uniformity and/or performance calibration information to be effective. However, the art does not teach manufacturing processes that provide a means to obtain the uniformity and/or performance calibration information in order to optimize the manufacturing process and thereby reducing the cost and improving the yield of the manufactured product. 
     There is a need, therefore, for an improved method of providing uniformity and reducing manufacturing costs in an OLED display manufacturing process. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment, the invention is directed towards a method for manufacturing and grading OLED devices, comprising the steps of: 
     a) manufacturing OLED devices having a plurality of pixels; 
     b) measuring pixel brightness and uniformity variation of each of the OLED devices prior to burning-in the OLED devices; 
     c) correcting the pixel brightness and uniformity variation of each of the OLED devices prior to burning-in the OLED devices; 
     d) grading each of the corrected OLED devices prior to burning-in the OLED devices; 
     e) burning-in OLED devices graded as acceptable prior to burning-in the OLED devices; 
     f) measuring burned-in pixel brightness and uniformity variation of each of the burned-in OLED devices; 
     g) re-correcting the pixel brightness and uniformity variation of each of the burned-in OLED devices; and 
     h) grading each of the re-corrected, burned-in OLED devices. 
     Advantages 
     The present invention has the advantage of providing improved yields in manufacture of OLED displays having acceptable uniformity and thereby reducing the cost of manufacturing an OLED display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram illustrating the method of the present invention; 
         FIG. 2  is a flow diagram illustrating a currently practiced OLED device manufacturing method; 
         FIG. 3  is a perspective view of OLED device manufacturing equipment; 
         FIG. 4  is a perspective view of an OLED device performance measurement tool; 
         FIG. 5  is a perspective view of a circuit for correcting brightness and uniformity variations of OLED devices; 
         FIG. 6  is an illustration of a system for grading an OLED device; and 
         FIG. 7  is a photograph of a measurement and calibration system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a method for manufacturing an OLED device and providing improved correction of brightness and uniformity variations, comprises the steps of manufacturing  100  an OLED device having a plurality of pixels; measuring  110  the performance of the OLED device, including pixel brightness and uniformity variations prior to burning-in the OLED device; correcting the pixel brightness and uniformity variations of the OLED device prior to burning-in the OLED device by compensating  115  for the brightness and uniformity variations; grading  120  the corrected OLED devices prior to burning-in the OLED device; burning in  150  the OLED device which has been graded as acceptable prior to burning-in the OLED device; measuring  160  the burned-in pixel brightness and uniformity variation of the burned-in OLED device; re-correcting the pixel brightness and uniformity variations of the OLED device by compensating  165  for the brightness and uniformity variations; and grading  170  the re-corrected, burned-in OLED device for a second time. If the OLED device meets the required standards, it is shipped  190  to a customer. If it does not meet the required standards, it is discarded  180 . 
     In a further embodiment of the present invention, after the first grading step  120 , OLED devices that do not meet a specification, but that are graded as repairable, may be sent out for repair  130 , for example by using laser repair techniques known in the art. Such techniques can repair some, but not all OLED device problems. If the device cannot be repaired, it is discarded  140 . If it is repaired, the device performance may be measured  110  again and continue through the manufacturing process as described above. Alternatively, the repaired device may be burned in  150  without being re-measured in step  110 . 
     The information gained by measuring the performance of the OLED device initially and after burn-in is used to form correction parameters employed to compensate the OLED device for non-uniformities. The information is typically stored in a controlling device, such as an integrated circuit controller or computer. The controlling device then employs the information to create signals that compensate the OLED device for non-uniformities. The information can include, but is not limited to, the light output from each light-emitting element of each pixel of the OLED device, brightness information for the OLED display as a whole, an identifier for the OLED device, the size, type, resolution, color, pixel patterns, materials, control signal, and display type information. As is known in the art, OLED devices also tend to age and decrease their light output over time as the OLEDs are used. In a further embodiment, the information from the initial measurement step  110  and the burned-in measurement step  160  are combined to form a record of the aging characteristics of the OLED device. This aging characteristic information may also be stored in and used by a controller to provide aging compensation to the OLED device in an application. 
     Burn-in may be the same for every pixel in an OLED device. That is, every pixel may receive the same instructions to illuminate the same amount. Alternatively, the burn-in process may be selective. Specific pixels may be burned-in at different rates than others, thereby providing uniformity in output without requiring external compensation. For example, brighter pixels may be burned-in at a higher current than dimmer pixels, so that after a period of time the brighter pixels will have aged more and will have the same brightness after aging as the other pixels. 
     OLED devices may have light emitting elements of different colors. The color elements may have their own performance characteristics, for example brightness, uniformity variation, and aging characteristics. The process described herein may be applied to each color plane of an OLED device separately. That is, the performance characteristics of, for example, the performance characteristics of red light emitters may be measured and compensated before grading, followed by the performance characteristics of green light emitters, followed by the performance characteristics of blue light emitters in an OLED device. 
     The present invention reduces the costs of the manufacturing process by improving yields. OLED devices may include non-uniformities that do not meet required standards. As is described in the prior art, by correcting the non-uniformities, devices that would otherwise be unusable, are made usable. However, simply performing the measurement and correction after a device has been burned-in does not optimize the manufacturing process flow. Some faults, such as stuck-on or stuck-off faults may not be correctable through a specific uniformity correction scheme. Some of these non-correctable faults, but not all, may be corrected by repair step  130 . Those OLED devices that can neither be corrected through uniformity correction or repaired are discarded  140 . Hence, using the process of the present invention, those OLED devices that cannot be repaired or corrected for non-uniformities do not pass any further through the manufacturing process. Since the initial repair and burn-in processes may be the most time-consuming and expensive processes, removing the non-correctable OLED devices from the process before the initial repair and burn-in processes reduces the cost of manufacturing the products. Not only will costs be reduced by improving the manufacturing process, they are reduced by performing the compensation described, since OLED devices that would not otherwise meet a specification will meet the specification after correcting the brightness and uniformity variations of the OLED device. 
     The method of manufacturing an OLED device, as described in step  100 , is known in the art and may include the steps of providing a substrate, forming electronic circuitry including signal and power connections on the substrate, forming an electrode on the substrate, depositing layers of organic materials over the first electrode, forming a second electrode over the layers of organic materials, encapsulating the OLED device, singulating the OLED device, and attaching electrical connections to the signal and power conductors on the OLED device. 
     Referring to  FIGS. 3–6 , the method of the present invention may be implemented by a system for manufacturing an OLED device and providing improved correction of brightness and uniformity variations, comprising equipment  210  for manufacturing an OLED device; a measurement tool  220  for measuring the initial performance of the OLED device  200 , including brightness and uniformity variations; a circuit  230  for correcting the brightness and uniformity variations of the OLED device; means  240  for grading the OLED device; a controller  250  for burning in the OLED device; a measurement tool  220  for measuring the burned-in performance of the OLED device, including brightness and uniformity variations; a circuit  230  for correcting the brightness and uniformity variations of the OLED device; and means for grading  240  the OLED device after burn-in and correction. 
     Referring to  FIG. 3 , manufacturing equipment  210  for manufacturing an OLED device is available from a variety of commercial vendors and may include, e.g., silicon deposition and photo-lithography equipment and organic layer deposition by means of evaporation or other coating technologies. A measurement tool  220  ( FIG. 4 ) for measuring the performance characteristics of an OLED device  200  may include a digital camera  225  and controlling computer  245 . The manufactured OLED device  200  may also be connected via a connector  247  to the controlling computer  245  to drive the OLED device  200  and digital camera  225  to measure the performance characteristics of the OLED device. Means to control an OLED device  200 , a digital camera  225 , programs for a computer  245  and suitable image and signal processing techniques are all known in the art and suitable designs are described in the references cited below. 
     Referring to  FIG. 5 , the correction of uniformity and brightness variations in an OLED device  200  may be accomplished with commercially available integrated circuits  255 , for example ASICs, memories, signal processors, and digital-to-analog convertors. Such circuits may be integrated onto a printed circuit board  215  together with a display controller  250 . The controller and circuits may perform both the correction and burn-in tasks, since both tasks are accomplished by illuminating the OLED device  200  with signals. 
     Referring to  FIG. 6 , means  240  for grading an OLED device  200  may include an operator  260  manually reviewing an OLED display connected to a computer  245 . As used here, grading refers to reviewing the performance of an OLED device and selecting or rejecting the OLED device on the basis of whether the performance of the OLED device meets one or more required specifications, or is repairable. The performance of the OLED device can be compared to a variety of specifications associated with a variety of applications for which different performance levels are specified and, as part of the grading process, the OLED device may be assigned to an application whose specifications are met by the OLED device. The grading means may alternatively be automated and include a digital image acquisition system (e.g., such as that shown in  FIG. 4 ) with software for numerically calculating the OLED device performance and comparing the performance with required specifications. Such image and signal processing techniques are known in the art and include, for example, morphological processing, histogram techniques, thresholding, and filtering. 
       FIG. 7  is a photograph of a measurement and calibration system including a digital camera  225  and a fixture  227  for holding an OLED device  200 . 
     Specific means for measuring the performance characteristics of an OLED device which may be employed in the present invention are known in the art (e.g., U.S. Pat. No. 6,414,661 B1 and U.S. Pat. No. 6,473,065 B1 referenced above), and also include those described, for example, in copending U.S. Ser. No. 10/858,260 referenced above. Means for correcting the uniformity of an OLED device which may be employed in the present invention are also known in the art (e.g., U.S. Pat. No. 6,414,661 B1 and U.S. Pat. No. 6,473,065 B1 referenced above), and also include those described, for example, in copending U.S. Ser. Nos. 10/869,009 and 10/894,729 referenced above. The disclosures of each of such patents and copending applications referenced in this paragraph are hereby incorporated by reference. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST  
     
         
           100  manufacture step 
           110  measure step 
           115  compensate step 
           120  grading step 
           130  repair step 
           140  discard step 
           150  burn-in step 
           160  measure step 
           165  compensate step 
           170  grading step 
           180  discard step 
           190  ship step 
           200  OLED device 
           210  manufacturing equipment 
           215  printed circuit board 
           220  measurement tool 
           225  digital camera 
           227  fixture 
           230  circuit 
           240  grading means 
           245  computer 
           247  connector 
           250  controller 
           255  integrated circuits 
           260  operator