Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1999-02-26
2001-05-08
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S761010, C324S1540PB, C445S063000, C445S062000
Reexamination Certificate
active
06229325
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to field emission displays. More particularly the invention relates to monitoring of electrical and optical performance of one or more field emission display devices during burn-in of the devices.
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for monitoring the electrical and optical performance of one or more field emission display (FED) devices during a burn-in period. More specifically the video driver circuitry, the field emission tips and the phosphor screen of such devices all require testing and burn-in prior to sale for the purpose of obtaining proper steady state operation, detecting faults in parts which may fail prematurely and for diagnostic purposes.
Known techniques in the manufacture of semiconductor devices involve the burning in of such devices in order to accelerate the failure of those devices having defects. Such testing is conventionally performed at elevated temperatures.
Additionally, image display devices such as cathode ray tubes, although known to have good characteristics with respect to color, brightness, contrast and resolution, use phosphors which generally degrade in performance upon initial use under prolonged cathode ray excitation. Thus, the phosphors are generally aged until stable luminance conditions are obtained along with a uniform screen appearance. Although cathode ray tube technology has been applied in various applications including desktop computer screens with good results, such devices are bulky and consume relatively large amounts of power.
More recently flat panel displays have become increasingly important in a variety of applications where lightweight portable screens with good display characteristics are required. One type of flat panel display device which is well suited for such applications is the thin film field emission display device. Such flat panel displays seek to combine the cathodoluminescent-phosphor technology of cathode ray tubes with integrated circuit technology to obtain thin high resolution displays wherein each pixel is activated by its own electron emitter or set of emitters. Such field emission displays in elementary form include a generally planar substrate having an array of integral projecting emitters which are typically conical projections grouped into emitter sets. Depending upon the size and type of display, a conductive extraction grid is positioned above the emitters and driven at a positive voltage with the emitters selectively activated by providing a current path to ground with appropriate voltage differential between the emitters and extraction grid. The resulting electric field extracts electrons from the emitters. Moreover, the field emission display device additionally includes a display screen-anode formed from a glass plate coated with a transparent conductive material forming a relatively high positive voltage differential with respect to the cathode emitters. The display screen additionally includes a cathodoluminescent layer covering the conductive anode surface whereby emitted electrons are attracted by the anode and strike the phosphor layer to thus cause the emission of light at the impact site which in turn passes through the anode and glass plate.
The luminescent level of the produced light is dependent upon the magnitude of the current flow to the emitters which is selectively controlled to produce a desired image. Field emission devices and various manners of driving circuits for use therein are known in the art, examples of which are found in commonly assigned U.S. Pat. No. 5,357,172 issued Oct. 18, 1994 to Lee et al, U.S. Pat. No. 5,387,844 issued Feb. 7, 1995 to Browning and U.S. Pat. No. 5,459,480 issued Oct. 17, 1995 to Browning et al. These patents are hereby incorporated by reference in their entirety.
In the operation of such field emission display devices there is a relatively high voltage differential generally above 200 volts between the cathode emitters and the display screen, and it is important to prevent electrical breakdown between the cathode electron emitting surface and the anode or extraction grid by maintaining an evacuated cavity between the emitters, the extraction grid and the anode. However, it is additionally desired to maintain relatively narrow spacings to obtain structurally thin flat panel displays. Moreover, since the manufacture of FEDs is relatively recent, methods and apparatus for continual monitoring of the electrical and optical performance of such displays are unknown.
Accordingly, a primary object of the present disclosure is that of providing a practical method and apparatus whereby the video driver circuitry, the field emission tips and the phosphor screen included in field emission display devices can be appropriately tested for electrical and optical performance during burn-in before the displays are sold or incorporated in a variety of applications.
BRIEF SUMMARY OF THE INVENTION
In accordance with the herein disclosed test method and apparatus, field emission display devices are loaded into an oven which is designed to allow operation of the display devices at elevated temperatures while simultaneously making measurements of relevant operating parameters of the display devices. In order to provide electrical connections for operation of the devices and testing, a plurality of such devices are electrically connected to a burn-in board by way of display carriers wherein a plurality of burn-in boards are placed in the oven for providing an interface between the displays and a control and measurement system. In this regard the oven in combination with the use of plural burn-in boards can be designed to hold hundreds of large display devices or thousands of small displays.
Once the display devices are placed in carriers, which provide appropriate sockets on the burn-in board, the preferred embodiment of the method includes placing the burn-in boards and display devices in an oven which is heated to an appropriate temperature for burn-in and testing. Thereafter, the display devices are powered up with the driver circuitry turned on and the current drawn by each display device is measured. This current is monitored to make certain that there are no failures of the driver devices. Moreover, a fuse can be placed between the power supply and the circuitry to isolate any display device drawing excessively high current due to an electrical short to thus prevent other displays in the oven from being affected by the failure of one display device. Thereafter the high voltage anode and extraction grid are energized with resistors being placed in series between the grid and its power source as well as between the high voltage anode and its power source. In this manner other display devices are protected if a failure or arc occurs by limiting the current drawn during the arc and thus preventing the failure or arcing of one display from damaging another.
Once the display devices are powered, they may be driven with video gain to cause field emission to occur. Care must be taken, however, to assure that all of the display pixels or at least one color are energized at the same level to obtain uniform aging of the phosphor screen. In this regard, the display drive is initially turned on at a low luminance level and gradually increased over many hours until the maximum luminance and anode current drive conditions are reached. In order to determine the status of the display units during burn-in, it is beneficial to measure certain display performance parameters, such as luminance, anode current and rapid grid voltage decreases.
Such measurements are used to determine the state of aging of the display screen phosphors, the driver circuit operation and field emitter failures. Moreover, the herein disclosed system and method provide clear benefits as to quality control, circuit diagnostics and reduced production costs.
REFERENCES:
patent: 5151061 (1992-09-01), Sandhu
patent: 5186670 (1993-02-01), Doan et al.
patent: 5205770 (1993-04-01), Lowrey et al.
patent: 5210472 (
Browning Jim
Xia Zhongyi
Metjahic Safet
Micro)n Technology, Inc.
Nguyen Jimmy
Nixon & Vanderhye P.C.
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