Abstract:
A method for determining functionality of pixels comprising the addressing of at least one but not more than ten pixels at a time, thereby producing a current. The pixels are comprised of cathode emitters. The current is collected on an anode disposed opposite the pixels, and the current is measured.

Description:
This application is a continuation of U.S. patent application Ser. No. 08/377,320, filed Jan. 24, 1995 now abandoned. 
     This invention was made with Government support under Contract No. DABT63-94-C-0012 awarded by the Advanced Research Projects Agency (ARPA). The Government has certain rights in the invention. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with United States Government support under contract No. DABT63-94-C-0012 awarded by the Advanced Research Projects Agency (ARPA). The United States Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to matrix addressable display devices, and more particularly to a method for testing the emission of the displays. 
     BACKGROUND OF THE INVENTION 
     Quality assurance requires the testing of manufactured products to determine if they will function over a given set of parameters. Testing is performed on a random sampling of products from the production lines, or in some industries, on every product which is manufactured. 
     Optical measurements are currently performed on display devices at the final stages of display testing. Optical measurement is accomplished by turning on all the cathode emitters in a pixel, and visually observing them. This method is extremely subjective, as it relies on the judgement of the individual peering at the screen. 
     Photon Dynamics of Milipitas, Calif. employs a method in which photons from a full display (i.e., substantially all of the pixels are turned “on” at the same time) are measured. A completed display device is optically tested by measuring the light emitted from the display. 
     There are several drawbacks to the above described optical method. For example, the process is very expensive, as it requires sophisticated optics and processing algorithms in order to determine the relative number of functional pixels. 
     Additionally, the optical systems require closely spaced anodes with respect to the cathode in order for the inspection optics to obtain accurate measurements. Therefore, the optical systems are not practical for testing display base-plates prior to their assembly in a field emission device. 
     Other methods used to measure current emitted by field emitter devices employ a positively biased electrode. However, these methods also measure the total current emitted by substantially all of the pixels in the completed unit. Therefore, these methods are also limited to measuring the operation of the device as a whole. 
     There still exists a need for a method to measure the current emitted by individual pixels in the display in order to determine their functionality, as well as the total number of functioning pixels (also known as the “yield”). Additionally, manufacturing realities require that the functionality of the individual pixels is measured in a rapid fashion. 
     SUMMARY OF THE INVENTION 
     The functionality of individual pixels in an addressable emissive cathode is determined by measuring the emitted current from a single pixel or from a small group of pixels. The current is measured on an anode screen disposed above the cathode emitters, thereby enabling the anode to collect electrons emitted from the cathodes. The collected current is compared to known parameters in order to determine whether the individual pixel site is functional. 
     The method of the present invention allows for the testing of cathode emitters in a vacuum chamber through the use of probe cards or sockets, and an anode. This format allows for the testing of approximately 100,000 pixels in less than 30 seconds, thereby expanding testing capabilities. Not only does the present technique permit faster testing, but also a less expensive alternative to the functionality tests used for individual pixels. 
     One advantage of the test method of the present invention is that it enables rapid determination of individual pixel functionality of the cathode emitters used in field emission type displays, or other similarly addressed matrix displays. For example, the present test method is suitable for displays that are actively or passively addressed. 
     The method of the present invention permits testing of the cathode emitters either as a baseplate or an assembled display device. Additionally, the use of an anode which luminesces under electron bombardment permits the measurement of emitted photons to determine pixel functionality. 
     The present invention is employed to sequence through all of the pixels, one or more at a time. Additionally, there is no required spacing for the anode in order to achieve accurate measurement. Further the present invention can be used with current optical methods to enhance the value of those methods. 
     One embodiment of the present invention is a method of testing cathode emitters by measuring the current emitted from the emitters. A group of cathode emitters comprises a pixel. 
     Another embodiment of the present invention is a method for measuring functionality in an addressable matrix used in a display device by addressing a single pixel, thereby creating an output. The output of the single pixel is then measured to determine functionality of that particular pixel. 
     A further embodiment of the present invention is a method for determining functionality of cathode emitters in which at least one pixel is addressed, thereby producing a current. The pixel is comprised of at least one cathode emitter. Collecting the current using an anode which is disposed opposite said at least one pixel. Measuring the current, and comparing the current to an expected value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from reading the following description of nonlimitative embodiments, with reference to the attached drawings, wherein below: 
     FIG. 1 is a schematic cross-section of a pixel of an illustrative field emission display having emitter tips; 
     FIGS. 2A-C are schematic top-view of the field emission display illustrating the relative location of the pixels of FIG. 1, illustrating a testing sequence, according to the present invention; 
     FIG. 3 is a schematic cross-section of the pixels of FIG. 2, illustrating an electrical method used to test the current of the pixels, according to the present invention; and 
     FIG. 4 is a schematic cross-section of the pixels of FIG. 2, illustrating an optical method used to test the photons emitted by the pixels, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a representative field emission display employing a display segment  22  is depicted. Each display segment  22  is capable of displaying a pixel of information, or a portion of a pixel, as, for example, one green dot of a red/green/blue full-color triad pixel. 
     Preferably, a single crystal silicon layer serves as a substrate  11 . Alternatively, amorphous silicon deposited on an underlying substrate comprised largely of glass or other combination may be used as long as a material capable of conducting electrical current is present on the surface of a substrate so that it can be patterned and etched to form micro-cathodes  13 . 
     At a field emission site, a micro-cathode  13  has been constructed on top of the substrate  11 . The micro-cathode  13  is a protuberance which may have a variety of shapes, such as pyramidal, conical, or other geometry which has a fine micro-point for the emission of electrons. Surrounding the micro-cathode  13 , is a grid structure  15 . When a voltage differential, through source  20 , is applied between the cathode  13  and the grid  15 , a stream of electrons  17  is emitted toward a phosphor coated screen  16 . Screen  16  is an anode. 
     The electron emission tip  13  is integral with substrate  11 , and serves as a cathode. Gate  15  serves as a grid structure for applying an electrical field potential to its respective cathode.  13 . 
     A dielectric insulating layer  14  is deposited on the S conductive cathode  13 , which cathode  13  can be formed from the substrate or from one. or more deposited conductive films, such as a chromium amorphous silicon bilayer. The insulator  14  also has an opening at the field emission site location. 
     Disposed between said faceplate  16  and said baseplate  21  are located spacer support structures  18  which function to support the atmospheric pressure which exists on the electrode faceplate  16  as a result of the vacuum which is created between the baseplate  21  and faceplate  16  for the proper functioning of the emitter tips  13 . 
     The baseplate  21  of the invention comprises a matrix addressable array of cold cathode emission structures  13 , the substrate  11  on which the emission structures  13  are created, the insulating layer  14 , and the anode grid  15 . 
     The process of the present invention provides a method for testing cathodes emitters, which cathodes are frequently used in field emitter display devices. The testing procedure of the present invention is discussed in terms of field emission displays. However, one having ordinary skill in the art, upon being apprised of the present invention would realize its applicability to other matrix addressable displays or devices having emissive cathodes. 
     The field emitter displays have cathode emission structures  13  which emit electrons into a vacuum environment. For purposes of display devices, the electrons are collected on an anode  16  which is coated with phosphors  19 . The phosphors  19  luminesce under electron  17  bombardment, thereby creating the colored light which forms the displayed image. 
     During the operation of the display, individual pixels  22  are turned on, so that the pixels  22  emit a controlled amount of current. Each pixel contains one or more cathode emitter structures  13 . FIGS. 2A-C illustrate that the control of the pixels  22  is accomplished by a matrix addressable system of rows R 1  . . . R m  and columns C 1  . . . C n . Pixel sites  22  are disposed at the intersection of the rows and columns, and are thereby addressable by their location. In this manner, individual pixels  22  are addressable one or more at a time. 
     The test method of the present invention is performed by turning on all of the pixels of the cathode  12  for “burn in,” and to establish an equilibrium. “Burn in” refers to stressing the devices through electrical cycling, thereby causing any defective parts to fail. 
     The drive cycle is started in which individual pixels are turned on one at a time or in small groups. This application will refer to the pattern of addressing the pixels  22  as “sequencing,” and it is to be understood that the pixels  22  are addressable in a variety of patterns or in a row or column. Sequencing through the pixels  22  enables all of the pixels  22  to be addressed and tested for functionality. 
     The drive cycle is illustrated in FIGS. 2A-C. A column (C 1 ) is turned on, as well as a row (R 1 ) , thereby causing the pixel  22  located at their intersection to emit electrons  17 . The emitted electrons create a current which is collected on an anode plate  16  disposed above the cathode emitters  13 . The anode  16  has a positive bias with respect to the cathodes  13 , thereby enabling the anode  16  to collect the electrons  17 . 
     For purposes of the test method of the present invention, the anode used may or may not be the screen  16 , depending on whether the cathode emitters are being tested before or after complete assembly. 
     The collected current is measured and compared with known data for good pulse current. See, for example, FIG.  3 . The current magnitude is measured to determine whether the current is above or below the known value. Alternatively, the shape of the pulse (rise time, fall time, etc.) is measured to determine whether the pixel  22  responds properly in a given time frame. 
     The measurement techniques include coupling the current through a capacitor; measuring the voltage across a resistor; or directly measuring the current through a power source. The method chosen varies with the amount of anode voltage used, the size of the current pulse, and the speed with which the measurement is taken. 
     After the current from the first pixel is measured, a second pixel in the same or another column is addressed. The current from the second pixel is measured, and the process is repeated until all of the pixels have been measured. 
     A delay time between each measurement is incorporated to allow for data processing, and for decay of the pulse when the time response of the system is slow. For example, pixel (P 1 ) is turned on for 100 μsec. After which no pixel is turned on for 100 μsec. Then pixel (P 2 ) is turned on for 100 μsec, and so on. Using this 200 μsec. total time (i.e., pulse plus delay time), the system is capable of measuring approximately 100,000 pixels in about 20 seconds. This is especially useful in displays having more than 10,000 pixels. 
     Of course, depending on the size of the baseplate  21  and the number of pixels thereon, the timing of the individual pixel test may vary. The preferable range for pulse plus delay is approximately 10 μsec to 10 msec. 
     This is quite a significant advancement over the visual inspection methods currently used, as the present method enables the electrical testing of substantially every pixel  22  individually, and in a rapid fashion. 
     Non-functional pixels appear as black spots in the display. It only takes a small number of non-functional pixels to cause a display to be non-marketable. Hence, it is worthwhile to know whether the baseplate  21  has defective pixels before completing the assembly of the display device. 
     The amount of time required for the test varies with the amount of time needed to measure the pulse, the time needed for the pulse to reach equilibrium and decay, and the time needed to process the data. However, the test method of the present invention is independent of address method (e.g., passive or active circuitry) or current regulation schemes. 
     In an alternative embodiment, multiple pixels are turned on at the same time. For example, one desires to test a group of five pixels. In such a case, four of the pixels are turned on at the same time (e.g., P 1 , P 2 , P 3 , P 4 ), and one pixel (P 5 ) remains off, and the current emitted is measured. Then, a different subset of the same five pixels is turned on (e.g., P 1 , P 2 , P 3 , P 5 ), and a different one of the pixels remains off (P 4 ), and the current is measured. Preferably, the number of pixels  22  tested at one time is less than 10. 
     The current emitted from the first subset is then compared with that of the second subset, or with the current from a subset known to contain working pixels. Then as different combinations of pixel groups are turned on, the functionality of those pixels is determined from the expected value. 
     This approach is especially useful in those situations where the current emitted from one pixel  22  is so small that accurate measurement is difficult due to background noise in the system. In this embodiment, the total current measured is larger because several pixels are tested at once. 
     Additionally, using the above-described approach, it is possible to measure the total current emitted from a group of pixels. So, a bulk value is obtained for a specified number of pixels. The total value is compared to a known or expected value. The measurement determines whether a particular group of pixels is non-functional. This is useful when the there is a low current emitted per pixel  22 , but it does not involve the sorting of the combination measurement as in the above described approach. 
     The measurement techniques of the present invention are suitable for both fully assembled display devices, and for baseplates  21  which are disposed in a vacuum environment with a separate anode or an integrated anode  16 . 
     The testing scheme of the present invention is repeatable with optical, as opposed to electrical measurements. The anode  16  is coated with a material, preferably a phosphor  19 , that luminesces under electron bombardment  17 . An optical detector, such as a photodiode, is disposed over the anode  16 . When the electrons from a pixel  22  or group of pixels cause the anode  16  to luminesce, the photons  23  are measured by the detector, and the information is recorded, as illustrated in FIG.  4 . 
     As with the electrical measurements previously described, the magnitude, shape, rise time, and duration of the pulse are parameters which can be compared to those of a known good pulse, thereby allowing the pixel functionality to be determined. As with the other methods, this embodiment is useful in assembled displays, or with cathode emitters disposed in a vacuum. However, a difference in the timing sequence is present, as it would depend on the decay time of the luminescing material, as well as the frequency response time of the detector used. 
     All of the U.S. Patents cited herein are hereby incorporated by reference herein as if set forth in their entirety. 
     While the particular process as herein shown and disclosed in detail is fully capable of obtaining the objects and advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims. 
     For example, one having ordinary skill in the art will realize that the current from the cathode can also be measured to determine the functionality of the pixels. In such an embodiment, the current from the power supply to the pixel or baseplate is measured, and compared to a known value.