1. Field of the Invention
The present invention relates to the field of automatic testing of active plates for liquid crystal displays.
2. Prior Art
LCD panels typically are formed with a liquid crystal material sandwiched between an active plate and a ground plate. Polarizers, colorizing filters and spacers may also be included between the plates. During fabrication, many active panels may be formed on a single glass plate. In each area of the glass plate that is to form an active panel, pixel areas, drive lines, gate lines and drive elements are formed. Typically, thin-film transistors are used for the drive elements.
Because of the relative complexity of the active plate in comparison to the ground plate, most LCD display defects may be traced to some form of defect in the active plate. Because of the substantial additional expense to fabricate operative LCD displays from the active plates, various techniques have been developed for inspecting the active plates alone so that defective active plates can be identified and repaired or discarded at that stage of the fabrication process.
An exemplary portion of an active plate 20 for a monochrome display is schematically illustrated in FIG. 1. As may be seen therein, a plurality of conductive areas 14 are arranged in an XY matrix, each conductive area defining a pixel in the final display. Associated with each conductive area is a thin film transistor 16 having its input terminal connected to a respective row line in the matrix, and its control terminal or gate connected to a respective column line. In the particular matrix shown in FIG. 1, adjacent row lines are coupled to opposite sides of the matrix, and adjacent column lines are coupled to the top and bottom of the matrix, respectively. A color display may be similar, though each pixel of the display is comprised of 3 pixels on the active plate, each for a different color.
In operation, each conductive area is charged to a voltage on the respective array lines when the respective thin film transistor is turned on by the voltage on the respective column line during each scan of the array. However, while the active plate may be electrically exercised without the ground plate and the liquid crystal material there between, no visibly perceivable changes occur during that exercise.
Various techniques are well known in the prior art for inspecting and testing LCD active plates before proceeding with the further fabrication of the full LCD panel. Each of these techniques is well suited to the detection of certain faults, but not suited to the detection of certain other faults.
One inspection technique that is commonly used is inspection under a visible light using a digital camera and computer based image analysis. Because the active plate is comprised of an array of a large number of pixel areas and thin film transistors, one convenient form of optical inspection is to form a difference image between repetitive patterns in the image. This is commonly called Automatic Optical Inspection (AOI). Without defects, the difference image is zero. With defects, the difference image is positive or negative. In this way, various faults may be detected such as shorts and opens and other faults in geometry that would cause failure of the display to operate, or unacceptable variations in the image intensity across the pixel array. However, other potential faults are not detectable in this manner, such as some faults which cause one or more transistors to not operate properly and/or which prevent pixel conductive areas from maintaining a charge until refreshed on the next scan.
Visible light systems may be either transmissive or pass through (the illumination and camera being on opposite sides of the plate), or reflective (camera and lighting on the same side of the plate), with the active plate fixed and the camera on a transport system for step and repeat operation, particularly with a lighting system moving with the camera to assure uniform lighting for each camera view. Normally the camera is also provided with the Z axis transport system for greater flexibility.
Other known systems for evaluating active plates at that stage of the LCD fabrication process comprise methods of electrically testing the active plate to enable accept/reject decisions based on reasonably accurate projections on how the plate will perform in the finished LCD display. One such technique utilizes a voltage imaging sensor, such as that described in U.S. Pat. No. 4,983,911, assigned to Photon Dynamics, Inc., assignee of the present invention. These systems provide a two-dimensional image of the voltage distribution across a surface of the active plate, thereby allowing the image to be digitized by an appropriate camera. The systems illuminate the active plate with a collimated beam of optical energy of known polarization through an electro-optic modulator disposed sufficiently close to the plate so as to be affected by the voltages on the active plate. Thus the voltage imaging sensor simulates the top half of the LCD panel and converts charge (voltage) on the active plate to visible light. Accordingly, such a system requires at least the electro-optic modulator to be positioned very close to the substrate. This approach can detect faults in operation, such as defective transistors, but has a resolution limit higher than an AOI system.
Systems of the voltage imaging type are manufactured by Photon Dynamics, Inc., assignee of the present invention. Such systems may be plate scale in size or smaller, normally with a camera operating in a step and repeat mode. Both reflective and pass-through systems are known, though normally the reflective mode is used.
Another well known testing technique is the electron beam scanning or e-beam scanning technique. In these testing systems, the active plate is placed in a vacuum chamber and the plate scanned with an electron beam, with the secondary electrons being detected by a scintillation camera. The camera output versus electron-beam position provides the image data for the plate voltage. Typically, a small part of the active plate is scanned at any one time, with the entire plate being scanned in a step and repeat process. Insufficient secondary electrons are indicative of a plate defect.
Finally, active plate testing systems based on charge sensing are also known. These systems are based on the concept of turning on each transistor on the active plate to charge the respective pixel conductive area to a specific voltage, then turning the transistor off, and then turning the transistor on again to short the pixel conductive area to ground while measuring the charge returned from the conductive pixel area. An insufficient charge is indicative of a plate defect. Typically, the transistor on periods while charging the pixel conductive area is on the order of the time the transistor would be on for such purpose in the completed display, and the time before shorting out the pixel conductive area for charge measurement is on the order of the time between scans in the completed LCD.
Thus, in an AOI system using visible light, the camera is typically spaced substantially away from the active plate being inspected, whereas in the voltage imaging sensor type systems, at least the electro-optic modulator must be positioned very close to the active plate to obtain a voltage image of a useful resolution. In an electron beam system, a vacuum environment must be provided, though a camera for providing a two-dimensional image is not required. Instead, only a light sensor is needed, preferably with a photomultiplier to increase the light intensity, with the XY information for a two-dimensional image being provided by the electron beam scan control system. Finally, in the charge sensing type equipment, no camera at all is used, and no mechanical transport system is required beyond that used for loading and unloading the active plate to be tested, such as generally also needed in some form for all of the other testing and inspection systems.