Patent Publication Number: US-7911432-B2

Title: Methods and apparatus for repairing inoperative pixels in a display

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/748,623, filed 22 Dec. 2000, now U.S. Pat. No. 7,280,090. 
    
    
     BACKGROUND 
     The present invention provides methods and apparatus for repairing inoperative pixels in a display. In particular, the present invention provides methods and apparatus for improving the effective yield rates of displays, such as liquid crystal micro-displays, by disconnecting inoperative pixels from their defective drive circuitry and tying such pixels to the working drive circuit of a nearby pixel. 
     Manufacturing of displays, such as liquid crystal (LC) micro-displays, with very large numbers of pixels is hampered by low yield rates due to the large area of semiconductor material (e.g., Complementary Metal Oxide Semiconductor (CMOS)) required for each device. Nevertheless, displays can actually tolerate a certain level of various types of defects and still be considered acceptable. Additionally, there are various techniques that have been developed and successfully applied to other display technologies that can reduce the visibility of some types of defects and dramatically increase the yield of acceptable displays. 
     For example, on a device size suitable to hold 8 million pixels (approximately 32×58 mm), without any repair strategy, the yield of “perfect” display devices is estimated at less than 2% (assumed defect rate of 0.2/cm2). 
     Defects in the microelectronic circuitry can cause a variety of types of malfunctions in the resulting display, such as:
     Stuck-On Pixel These pixels are always on. In a three-chip RGB (red, green, blue) system, they would be visible as a constant red, green, or blue dot in the display. Such stuck-on pixels are much more visible in dark areas of the display than bright areas.   Stuck-Off Pixel These pixels are always off. In a three-chip RGB system, they would be visible as a constant cyan, magenta, or yellow dot in the display. Such stuck-off pixels are much more visible in the bright areas of the display than the dark areas.   Stuck-Intermediate Some pixels can get stuck in an intermediate state. Depending on the intended color of the pixel and the surrounding area of the display, they will sometimes appear as red, green, or blue, and at other times will appear as the complement of the color channel affected.   Partial Response Some pixels may have a portion of their circuitry affected such that they partially respond to the intended color. Perhaps they are of reduced intensity or contrast, but they still track the intended value to some degree.   Defective Clump A significant portion of the circuitry in the visible area may be shared between clumps of pixels. A defect in this area may effect the entire clump of pixels and may take any of the above forms.   Dead Column/Row Defects in signals generated by the drive circuitry which are fed to every pixel in a column or row will manifest themselves in the entire column or row. The manifestation may take a variety of forms.   

     Although a display having any of the foregoing defects may still be useable, all such defects are significant enough that once noticed, the user will be unhappy. The only exception being perhaps a few isolated stuck-off pixels in the display used for the blue channel, due to the low contrast sensitivity of human vision to blue light. 
     A previously known method for dealing with such defects is to provide redundant circuitry in the display that can be selected to drive a pixel or column of pixels in the event of a defect in the primary circuitry. As it is already difficult to fit all required circuitry in the space available in such micro-displays, the inclusion of redundant circuitry becomes problematic. 
     It would be advantageous to provide a method of repairing inoperative pixels in a display without requiring redundant circuitry in order to increase the yield rate of such displays. 
     The methods and apparatus of the present invention provide the aforementioned and other advantages. 
     SUMMARY 
     The present invention provides methods and apparatus for repairing inoperative pixels in a display. In particular, the present invention provides methods and apparatus for improving the effective yield rates of displays, such as liquid crystal micro-displays, by disconnecting inoperative pixels from their defective drive circuitry and connecting such pixels to the working drive circuit of a nearby pixel. 
     The “repaired” pixel will then display the same value as the nearby pixel that it is connected to. However, since in most images, neighboring pixels are highly likely to be displaying similar values, the resulting slight error in the image is very well below acceptable limits for almost all applications. Most viewers find it difficult to detect such an artifact at all even when it is pointed out to them. As the display resolution goes higher, this type of repair becomes even more difficult to detect. 
     In an illustrated embodiment of the present invention, an electronic display is provided which is capable of repairing inoperative pixels. The display comprises a plurality of pixels with drive circuitry for controlling the pixels. Means for disconnecting an inoperative pixel from its defective drive circuitry are provided. The inoperative pixel is then provided with a connection to a working drive circuit of a nearby pixel. 
     The means for connecting the inoperative pixel to a working drive circuit may comprise additional circuitry associated with each pixel in the display. This additional circuitry connects the inoperative pixel to the working drive circuit of a nearby pixel. 
     In one embodiment, the additional circuitry may comprise a bypass bit latch. When the bypass bit is set, the defective drive circuitry is bypassed and the inoperative pixel is driven from the working drive circuit of a nearby pixel. The bypass bit may be loaded from an external memory after the display is turned on. In addition, multiplexing circuitry associated with the bypass bit latch may be provided. 
     Alternatively, the additional circuitry may further comprise a tri-state transistor associated with each pixel connected to the bypass bit latch and a resistor coupling neighboring pixels. When the bypass bit is set, the transistor is switched to bypass the defective drive circuitry so that the inoperative pixel is driven from the working drive circuit of a nearby pixel through the resistor. 
     In a further embodiment, the additional circuitry may comprise a resistive connection between neighboring pixel metal layers. The defective drive circuitry can be disconnected from the inoperative by severing a via connecting the defective drive circuitry to the inoperative pixel, such that the inoperative pixel is driven by a nearby pixel through the resistive connection. 
     Alternatively, the additional circuitry may comprise a capacitive connection between neighboring pixel metal layers. The defective drive circuitry can be disconnected from the inoperative pixel by severing a via connecting the defective drive circuitry to the inoperative pixel, such that the inoperative pixel is driven by a nearby pixel through the capacitive connection. 
     The via may be severed, for example, by laser ablation, melting or ablation of a fusible link by passing sufficient current through it, selective chemical etching using a photoresist or other selection means, melting or ablation with an electron beam, melting or ablation with a focused microwave or other electromagnetic beam, electro-ionic erosion, physical cutting or removal of the metal using a sharp or abrasive implement or probe, or by any other suitable technique. 
     The pixels may be repaired individually or in groups (i.e., groups of up to 8 adjacent inoperative pixels may be repaired without significantly impacting the quality of the display). 
     In a further embodiment, the display may also comprise test circuitry to identify the defective drive circuitry. 
     The pixel drive circuitry associated with each pixel may be located adjacent to each pixel or located separately from each pixel. 
     The display may be a liquid crystal micro-display or a similar display device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same elements throughout, and in which: 
         FIG. 1  illustrates an embodiment of a liquid crystal micro-display assembly in accordance with the present invention; 
         FIG. 2  shows a schematic representation of an embodiment of repair circuitry in accordance with the present invention; 
         FIG. 3  shows a schematic representation of a further embodiment of repair circuitry in accordance with the present invention; 
         FIG. 4  shows an alternate embodiment of repair circuitry in accordance with the present invention; 
         FIG. 5  shows another alternate embodiment of repair circuitry in accordance with the present invention; and 
         FIG. 6  shows examples of various pixel repair configurations. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to methods and apparatus for repairing inoperative pixels in a display. In particular, the present invention provides methods and apparatus for improving the effective yield rates of displays, such as liquid crystal micro-displays, by disconnecting inoperative pixels from their defective drive circuitry and connecting such pixels to the working drive circuit of a nearby pixel. 
     The invention is particularly adapted for use with a traditional reflective liquid crystal on complimentary metal oxide semiconductor (FLC-on-CMOS) micro-display assembly with the LC material being placed on top of a CMOS control chip. Pads on the top layer of the CMOS chip provide both electrical control of the LC material as well as the optical mirror for the light. 
     Such a display may have 8.3 million pixels arranged as 3840 pixels by 2160 pixels. The pixels may be on a 15 μm pitch. The size of the active area of this exemplary display is thus 57.6×32.4 mm. Additional control circuitry and frame buffer memory (e.g., Dynamic Random Access Memory (DRAM)) along the two long sides of the circuit may be 61.4×3 mm. Input/output (I/O) pads and an LCD seal ring would add another couple few millimeters to the overall dimensions of the chip, but these areas would be relatively immune from the very small defects typical of the CMOS process. The invention is also applicable to displays of varying sizes and types. 
     A significant amount of functionality in the circuitry under each pixel is desired. The basic modulation for each pixel may be, for example, a 10-bit counter running on a chirped clock to produce a log or gamma-corrected response. Generation of the pulse timing required by the LC material is also required under each pixel. Diagnostic capabilities may be included to verify correct operation of the control circuit for each pixel. Finally, an additional 10 bits of storage may be desired to allow loading of the data for a next frame to be distributed over the entire frame time. 
     Fitting this circuitry in the available space has proven challenging and drives the design towards a larger pixel pitch compared to existing designs. The smallest feasible CMOS feature sizes are also utilized. Even still, for the desired complex pixel control functionality, typical prior art redundant repair circuitry is problematic to provide in the available space. 
       FIG. 1  illustrates the arrangement of a liquid crystal-micro display in accordance with the present invention. Display area  10  is divided into several sections due to the differing impacts that defects in each section will have on the display yield. Pixel drive and repair circuitry  12  covers the majority of the device area (e.g., the pixel drive circuitry comprises 92% of the active area or approximately 17.2 cm2 and the pixel repair circuitry comprises 4% of the active area or approximately 0.07 cm2). By providing the repair mechanisms described herein, most types of defects in this circuitry can be tolerated and still result in acceptable devices. 
     Signal distribution buffer circuitry  14  may comprise 4% of the active area of the display or approximately 0.07 cm2. Signals in the display are routed to every pixel in a column. Distribution buffer circuitry  14  will be required to fan these signals out over the large number of loads and distances and still meet timing requirements. Defects in these buffers will render large portions of the display inoperable and render the display unusable. 
     Column signal generation logic (1.2 cm2) and Buffer DRAM (1.8 cm2) are provided at  16 . Column signal generation logic generates the signals for each column. Defects in this area are highly likely to render the entire display unusable since the error is likely to impact a large area of the display such as an entire column or more. Buffer DRAM is similar to conventional DRAM. If yield is a problem in this area, traditional DRAM techniques of including spare rows and columns can be employed. 
     Input/output (I/O) pads  18  are provided. These features typically do not contribute to low yield. Cover Glass Glue-Down Area  20  also consumes a portion of the wafer, but typically does not have any features that would contribute to low yield. 
     In an illustrated embodiment of the present invention as shown in  FIG. 1 , an electronic display  10  is provided which is capable of repairing inoperative pixels. The display comprises a plurality of pixels with drive circuitry  12  for controlling the pixels. Means for disconnecting an inoperative pixel from its defective drive circuitry are provided. The inoperative pixel is then provided with a connection to a working drive circuit of a nearby pixel. 
     Using the drive circuitry  12  of a neighboring pixel to drive an otherwise inoperative pixel will result in the two pixels displaying the same intensity. Visual artifacts will typically be visible in the case of high contrast textures and edges moving across the two tied-together pixels at a slow rate. 
     The means for connecting the inoperative pixel to a working drive circuit may comprise additional circuitry associated with each pixel in the display that connects the inoperative pixel to the working drive circuit of a nearby pixel. The additional circuitry may comprise discrete or integrated circuitry. 
     In one embodiment as shown in  FIG. 2 , the additional circuitry may comprise bypass bit latches  34 ,  44  which can select the source of the drive signal for pixels  30 ,  40 . When a bypass bit is set, the defective drive circuitry is bypassed and the inoperative pixel is driven from the working drive circuit of a nearby pixel. The bypass bit may be loaded from an external memory after the display is turned on. For example, in  FIG. 2 , assume that pixel  30  is inoperative due to defective drive circuitry  32 . After the display is turned on, bypass bit  34  associated with the inoperative pixel  30  is set such that defective drive circuitry  32  is bypassed and inoperative pixel  30  is driven by working drive circuitry  42  of a nearby pixel  40 . In addition, multiplexing circuitry  36 ,  46  associated with bypass bit latches  34 ,  44  may be provided. An advantage of such an arrangement is that testing and repair can be accomplished at any time in the manufacturing process, even after assembly of the LCD. 
     As shown in  FIG. 3 , the additional circuitry may alternatively comprise tri-state transistors  38 ,  48  associated with each respective pixel  30 ,  40 . Transistors  38 ,  48  are connected to respective bypass bit latches  34 ,  44  and resistor  50  couples neighboring pixels  30 ,  40 . Resistor  50  may be any suitable resistor, such as a discrete resistor, an integrated circuit resistor fabricated in metal, polysilicon, or any other suitable resistive material. As an example, assume that pixel  30  is inoperative due to defective drive circuitry  32 . When bypass bit  34  is set, the transistor  38  is switched to bypass defective drive circuitry  32  so that inoperative pixel  30  is driven from working drive circuit  42  of nearby pixel  40  through the resistor  50 . An advantage of such an arrangement is that testing and repair can be accomplished at any time in the manufacturing process, even after assembly of the LCD. 
     In a further embodiment as shown in  FIG. 4 , the additional circuitry may comprise a resistor  52  coupled between pixel metal layers of nearby pixels  31 ,  41 . Resistor  52  may comprise a discrete or integrated circuit resistor fabricated in metal, polysilicon, or any other suitable resistive material. Assuming that pixel  31  is inoperative due to defective drive circuitry  32 , defective drive circuitry  32  can be disconnected from the inoperative pixel  31  by severing a via  55  connecting defective drive circuitry  32  to inoperative pixel  31 , such that inoperative pixel  31  is driven by a nearby pixel  41  through the resistor  52 . Because the pixel drive waveform typically consists of pulses of very low duty cycle, resistor  52  typically will not contribute significantly to the power dissipation of the device. 
     Additionally, since the current flowing for each pixel typically is very small, resistor  52  typically will be sufficient to drive the pixel  31  with essentially the same waveform as pixel  41 . 
     As shown in  FIG. 5 , the additional circuitry may alternatively comprise a capacitor  54  coupled between pixel metal layers of nearby pixels  31 ,  41 . Assuming pixel  31  is inoperative due to defective drive circuitry  32 , the defective drive circuitry  32  can be disconnected from the inoperative pixel by severing a via  55  connecting the defective drive circuitry  32  to the inoperative pixel  31 , such that the inoperative pixel  31  is driven by a nearby pixel  41  through the capacitor  54 . Capacitor  54  may be formed from a pixel metal layer normally used for light-blocking or may comprise any other suitable capacitive connection. 
     The via  55  may be severed, for example, by laser ablation, melting or ablation of a fusible link by passing sufficient current through it, selective chemical etching using a photoresist or other selection means, melting or ablation with an electron beam, melting or ablation with a focused microwave or other electromagnetic beam, electro-ionic erosion, physical cutting or removal of the metal using a sharp or abrasive implement or probe, or by any other suitable technique. The via  55  may comprise a fuseable link between the drive circuitry and the pixel, and the link may be “blown”, e.g., by applying a suitable short circuit voltage across the link, or by any other suitable technique. 
     The pixels may be repaired individually or in groups. In one embodiment of the invention, the amount of circuitry per pixel is reduced by sharing the bypass bit latch amongst a group of pixels. If the drive circuit for any pixel in the group fails, the bypass bit is set for that group, and each pixel in the group is then driven by the circuitry of a nearby pixel. The drive circuit used for each repaired pixel may be different or the same for each pixel in the group. Even though some working pixels are bypassed in the group, the overall quality of the display is still improved by removing the stuck-ON or stuck-OFF defect(s). Groups may be any size depending on the demands of the application, but typically groups larger than 8 pixels begin to have diminishing returns in terms of space saved in relation to quality of the repair. 
       FIG. 6  shows examples of various pixel repair configurations, including examples of individual pixel repair and examples of various pixel repair groupings. In the repair example  100 , the defective pixels  101  may be driven by an adjacent pixel located either to the left or the right of the defective pixel (e.g., pixels  102  and  103 ). In repair example  200 , a 4 pixel group (2×2) of defective pixels is repaired by driving the defective pixels  201  using the drive circuitry of pixels located to the left ( 202 ,  203 ) or the right ( 204 ,  205 ) of the 4 pixel group. In repair example  300 , an 8 pixel group (4×2) of defective pixels  301  is repaired using pixels located above ( 302 ,  303 ), below ( 304 ,  305 ), to the left ( 306 ,  307 ) and to the right ( 308 ,  309 ). In repair example  400 , a 16 pixel group (4×4) of defective pixels  401  is repaired using pixels above ( 402 ,  403 ), below ( 404 ,  405 ), to the left ( 406 ,  407 ,  408 ,  409 ), and to the right ( 410 ,  411 ,  412 ,  413 ). Pixels  402 ,  403 ,  404 , and  405  are shown as driving a respective adjacent pixel as well as a respective pixel located in the interior of the 4×4 defective pixel block  401 . It will be appreciated that any of the other pixels ( 406 - 413 ) can be used to drive an adjacent pixel and an interior pixel. 
     The repair examples shown in  FIG. 6  are illustrative examples, and it will be appreciated that a defective pixel may be driven by any of the adjacent pixels (including for example, pixels located above, below, or diagonally from the defective pixels). Further, it will be appreciated that defective pixels may be repaired by connecting the defective pixels to the drive circuitry of non-adjacent pixels (as shown, for example, in repair example  400 ). 
     In a further embodiment, the display may also comprise test circuitry to identify the defective drive circuitry. For example, all the pixels in a row may be tested in parallel using wired-OR and wired-AND circuitry. Rows with an inoperative pixel will then be subject to an additional series of tests to identify the exact pixel in the row that is inoperative. 
     The pixel drive circuitry associated with each pixel may be located adjacent to each pixel or located separately from each pixel. Moving the drive circuitry for each pixel so it is not necessarily physically located under the pixel that it drives allows for nearby repair partner pixels with non-adjacent drive circuitry. Depending on the size of defects that may occur, the drive circuitry for repair partners can be located one, two, or more pixels away from the pixels that they drive. Since the total number of defects typically is small, the probability of two independent defects damaging any given pair of repair partner pixels becomes negligible. 
     The display may be a liquid crystal micro-display or other suitable displays that comprise integrated circuit manufacturing techniques. 
     It should now be appreciated that the present invention provides an improved method and apparatus for repairing inoperative pixels in a display. In particular, the invention provides an improved method and apparatus for repairing defective pixels in a liquid crystal micro-display without the need to provide redundant drive circuitry underneath each pixel. 
     Although the invention has been described in connection with various preferred embodiments, it should be appreciated that numerous adaptations and modifications can be made thereto without departing from the scope of the invention as set forth in the claims. In particular, the invention is not limited to liquid crystal micro displays, and can be used in connection with the many different types of display technology.