Testing flat panel display plates using high frequency AC signals
Methods of and apparatus for detecting pixel element defects in flat panel display (FPDs). Floating pixel elements (fpes) of uncompleted active plates in a manufacturing process are activated with high frequency AC test signals. In response to the activation signal, a high frequency output signal is produced by a voltage divider formed by an impedance of the fpe under test and an impedance presented by high frequency elements (e.g. stray capacitances) associated with the fpe under test. A signal characteristic (e.g. the amplitude) of the output signal is compared to an expected characteristic to determine the presence of pixel element defects. The methods of the present invention may be performed prior to completion of the active plate, e.g., prior to forming a liquid crystal between plates of a passive matrix LCD and prior to coating a partially formed OLED active plate with light emitting organic material layers. Use of high frequency activation signals allows detection of pixel element defects that are invisible to DC test methods. Additionally, because the methods and apparatus of the present invention allow testing prior to FPD plates being completely manufactured and prior to FPD final assembly, pixel defects can be detected early in the display manufacturing process, thereby resulting in a substantial reduction in production costs.
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The present invention relates generally to testing flat panel displays (FPDs). More specifically, the present invention relates to testing FPDs using high frequency alternating current (AC) signals.
BACKGROUND OF THE INVENTIONFlat panel displays (FPDs) are increasingly replacing the conventional cathode ray tube (CRT) as the display type of choice. FPDs are electronic displays in which a flat screen is formed by a two-dimensional array of display elements (or “pixels”). They can be manufactured from a variety of different display technologies. One common display technology utilizes an array of light emitting diodes (LEDs) to form the FPD. An LED is a solid-state electronic device, more specifically a p-n junction or “diode”, which emits photons (i.e. light) when forward biased. The light emitting effect is referred to as injection electroluminescence, a light emitting phenomenon that occurs when minority charge carriers generated by an applied electric field recombine with charge carriers of the opposite type in the diode. The energy of the emitted photon, which determines the wavelength of the emitted light, varies with the band gap of the semiconductor material used (e.g., GaP, GaAs, GaN, etc.) to form the LED.
Typically, control of the LEDs in an FPD is performed using one of two approaches. According to the first approach, the LEDs are controlled by a row-column grid control pattern and associated row and column drivers/controllers. This approach is known as the “passive matrix” approach. The second approach, known as the “active matrix” approach, uses one or more control transistors at each pixel site to control pixel emission. Because each pixel is controlled by its own associated control transistor(s), active matrix LED FPDs consume less power than passive matrix FPDs, and are able to turn pixels on and off faster than passive matrix displays.
Another display technology of recent interest is based on the so-called Organic Light Emitting Diode (OLED). Operation of an OLED is similar to that of an inorganic semiconductor LED described above. When two organic materials, one with an excess of mobile electrons the other with a deficiency, are place in close contact, a junction region is formed. When a small forward bias is applied across the diode, electron-hole pairs are created, which upon recombination produce photons as described above. OLEDs are attractive for use in FPDs since they provide excellent display and viewing characteristics, can be manufactured on a flexible substrate (e.g. plastic), do not require high-temperature processing to dope them, and have fast element response times.
OLED FPDs are formed by etching an array of pixel elements into a substrate. In the array, portions of the active pixel elements, including thin film transistor (TFT) devices, storage capacitors and ITO patterns are formed on the substrate. The substrate is then coated with organic materials that form the light emitting portion (i.e. the diode) of the OLED. Further details concerning the manufacturing of OLED FPDs may be found in U.S. Pat. No. 5,688,551, which describes the first application of organic materials for OLED FPD manufacturing.
Turning now to the topic of defects in FPDs, it is well known that vast majority of defects in FPDs are found in the active plates of the FPDs. Because of this, during the manufacturing of FPDs, the active plates are typically tested prior to finally assembling the displays. By testing prior to final assembly, pixel defects can be detected early in the display manufacturing process, thereby resulting in a reduction in production costs.
Defects also commonly arise in the active plate of OLED displays. Accordingly, it would be desirable to test the active plates used in OLED displays prior to final assembly (e.g. prior to application of organic layer 108 and metal layer 109) as well. This desire is increased when it is recognized that organic layer 108 contributes substantially to the total display manufacturing costs. Besides material costs, a primary reason for the high cost is that atmospheric sealing methods must be employed to protect currently available organic emissive layers. Without proper protection from the atmosphere, the expected lifetime of organic layers can be substantially compromised. For more information on this topic, see the article “Microdisplays Based Upon Organic Light-Emitting Diodes,” W. E. Howard, et al., IBM J. RES. & DEV., vol. 45 no. 1, January 2001.
Unfortunately, testing the active plates prior to applying the organic and metal layers presents significant challenges since each pixel element output is in essence electrically floating, as shown schematically in
Methods of and apparatus for detecting pixel element defects in flat panel display (FPDs) are disclosed. Floating pixel elements (fpes) of uncompleted active plates in a manufacturing process are activated with a high frequency test signal. In response to the activation signal, a high frequency output signal is produced by a voltage divider formed by an impedance of the fpe under test and an impedance presented by high frequency elements (e.g. stray capacitances) associated with the fpe under test. A signal characteristic (e.g. the amplitude) of the output signal is compared to an expected characteristic to determine the presence of pixel element defects. The methods of the present invention may be performed prior to completion of the active plate, e.g., prior to forming a liquid crystal between plates of a passive matrix LCD and prior to coating a partially formed OLED active plate with light emitting organic material layers. Use of high frequency activation signals allows detection of pixel element defects that are invisible to DC test methods. Additionally, because the methods and apparatus of the present invention allow testing prior to FPD plates being completely manufactured and prior to FPD final assembly, pixel defects can be detected early in the display manufacturing process, thereby resulting in a substantial reduction in production costs.
Further aspects of the invention are described and claimed below, and a further understanding of the nature and advantages of the inventions may be realized by reference to the remaining portions of the specification and the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention are described herein in the context of testing uncompleted FPDs. Those of ordinary skill in the art will realize that the following detailed description of the present invention is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to implementations of the present invention as illustrated in the accompanying drawings. Unless indicated otherwise, the same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts.
In some types of FPDs, such as, for example, passive matrix LCDs and passive or active matrix OLED displays, at least one of the plates contain pixel elements during the manufacturing process that exhibit an open circuit condition at the intended frequency of operation or lower. For example, as discussed above, prior to coating the substrate with the organic material layers during the manufacture of an OLED FPD, the absence of the LED renders the pixel elements of the display as open circuits. In accordance with embodiments of the present invention, alternating current (AC) activation signals, of a frequency greater than, for example, the frequency of operation of an OLED FPD are applied to detect variations possibly corresponding to pixel element defects.
The stray capacitances encountered in the active plates of FPDs can be extremely low. Accordingly, it is necessary to measure the amplitude of the signal output using a very high input impedance measuring instrument, so that the instrument itself does not affect the measurement. According to an embodiment of the invention, one such instrument that can be used is the electron beam probe, which in practice provides nearly infinite input impedance. The basic operation of the electron beam probe is very well described in the available prior art literature and will not be explained in detail here. Reference may be made to, for example, U.S. Pat. Nos. 6,075,245 and 5,982,190 by the same inventor, both of which disclose systems and methods for testing FPD arrays using electron beams.
Referring to
Floating pixel element 402 is activated with at least one high frequency signal Va provided by an AC signal generator 403. The generated SE are collected by an electron detector 404 and amplified. An output signal Vo′ of electron detector 404 is a signal that corresponds to Vo, which is a signal that results from voltage dividing the input activation waveform Va between the equivalent pixel impedance Z of fpe 402 and the associated impedance of stray capacitance Cst. Output signal Vo′ of electron detector 404 is fed into an amplitude or envelope detector 406. Detector 406 may be any one of several types of detectors, such as, for example, a peak-to-peak detector, a synchronous detector, or a matched filter.
Referring now to
According to an embodiment of the present invention, the electrical conduction characteristics of the rows and columns 504 and 506 of plates 500 and 502 may be tested prior to be assembled into a complete display containing the liquid crystal, polarizers and other components.
The test methods described above in connection with the test apparatus shown in
It should be emphasized here that the fpe shown in
Optimum selection of the activation frequency of activation signal Va depends on the values of the stray capacitances present in the particular design of the OLED plate.
According to another embodiment of the invention, it may also be desirable to test each pixel element at the range of transconductances encountered during normal operation of the finished OLED display. In a typical case the current Id required to activate the OLED to full scale light emission is in the range of 10 μA with a required minimum gray level requirement of 1/64 times or less. For a Vdd value of 10 V, that range corresponds to 1/Gm values ranging from approximately 930 kΩ to 59 MΩ. From
According to an alternative embodiment of the invention, it may be possible to achieve similar results, as to those already described, by applying a high frequency signal to the Vcs line, rather than to the Vdd line. This approach is shown schematically in
FIGS. 10A-E shows some examples of testing sequences applied to an OLED plate having fpes similar to the fpe shown in
The foregoing detailed description describes methods of and apparatus for testing unfinished FPD plates, according to various embodiments of the present invention. Whereas the description is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. For example, whereas the design implementation of the pixel element driving circuit shown in
Claims
1. A method of determining pixel defects in an FPD plate, comprising:
- activating an FPD plate with an AC test signal;
- at a point on the FPD plate corresponding to a location of a pixel element, measuring the amplitude of a signal that is responsive to the AC test signal;
- comparing the amplitude of the measured signal to a predetermined amplitude; and
- determining whether the difference between the measured and predetermined amplitudes represents that the pixel element is defective.
2. The method of claim 1 wherein the FPD plate is an OLED FPD plate.
3. The method of claim 2 wherein the OLED FPD plate is part of a down-emitting OLED FPD structure.
4. The method of claim 2 wherein the OLED FPD plate is part of an up-emitting OLED FPD structure.
5. The method of claim 2 wherein the OLED FPD plate comprises a substrate upon which one or more pixel elements are disposed.
6. The method of claim 5 wherein the substrate is a polymer or a plastic.
7. (canceled)
8. (canceled)
9. (canceled)
10. The method of claim 1 wherein the step of measuring is performed using an electron beam probe.
11. The method of claim 10, further comprising collecting secondary electrons emitted from the point on the FPD plate corresponding to a location of a pixel element.
12. The method of claim 10 wherein the energy of an electron beam from the electron beam probe is about 500 eV or less.
13. The method of claim 10 wherein a current of an electron beam from the electron beam probe is about 5 nA or less.
14. A method of testing an electrically floating pixel element of an FPD plate, comprising:
- applying an AC test signal to an input of an electrically floating pixel element (fpe);
- directing an electron beam to a surface of said fpe;
- collecting electrons emitted from the surface of said fpe;
- forming an electrical signal from the collected electrons; and
- comparing a measured characteristic of the electrical signal to an expected characteristic.
15. The method of claim 14, further comprising determining whether said fpe is defective, based on results obtained from the step of comparing.
16. The method of claim 14 wherein the measured characteristic is an amplitude of the electrical signal and the expected characteristic is an expected amplitude of the electrical signal.
17. The method of claim 14 wherein said fpe comprises one of a plurality of fpes of the FPD plate.
18. The method of claim 14 wherein the FPD plate is an OLED FPD plate.
19. The method of claim 18 wherein the OLED FPD plate is part of a down-emitting OLED FPD structure.
20. The method of claim 18 wherein the OLED FPD plate is part of an up-emitting OLED FPD structure.
21. The method of claim 18 wherein the OLED FPD plate comprises a substrate upon which said fpe and other fpes are formed.
22. The method of claim 21 wherein the substrate is a polymer or a plastic.
23. The method of claim 14 wherein the energy of said electron is about 500 eV or less.
24. The method of claim 14 wherein a current of said electron beam is about 5 nA or less.
25. A method of testing a floating pixel element (fpe) of an FPD, comprising:
- applying an AC signal to a power input of an fpe of a FPD;
- providing a first activation signal to a current controlling device of said fpe, to activate said fpe to a first activation level; and
- measuring characteristics of a first output signal provided at an output of said fpe.
26. The method of claim 25 wherein the step of measuring is performed using an electron beam probe.
27. The method of claim 25, further comprising:
- providing a second activation signal to said controlling device of said fpe, to activate said fpe to a second activation level; and
- measuring characteristics of a second output signal provided at the output of said fpe.
28. The method of claim 27, further comprising comparing the characteristics of said first and second output signals, to determine a transconductance of said current controlling device.
29. The method of claim 25, further comprising:
- applying a second AC signal to the power input of said fpe, the amplitude of said second AC signal having a different value than the amplitude of said first AC signal; and
- measuring characteristics of a second output signal provided at the output of said fpe.
30. The method of claim 29, further comprising comparing the characteristics of said first and second output signals, to determine a drain conductance of said current controlling device.
31. An apparatus for determining pixel defects in an FPD plate, comprising:
- means for activating an FPD plate with an AC test signal;
- means for measuring the amplitude of a signal that is responsive to the AC test signal, at a point on the FPD plate corresponding to a location of a pixel element;
- means for comparing the amplitude of the measured signal to a predetermined amplitude; and
- means for determining whether the difference between the measured and predetermined amplitudes represents that a pixel element is defective.
32. The apparatus of claim 31 wherein the FPD plate is an OLED FPD plate.
33. The apparatus of claim 32 wherein the OLED FPD plate is part of a down-emitting OLED FPD structure.
34. The apparatus of claim 32 wherein the OLED FPD plate is part of an up-emitting OLED FPD structure.
35. The apparatus of claim 32 wherein the OLED FPD plate comprises a substrate upon which one or more pixel elements are disposed.
36. The apparatus of claim 35 wherein the substrate is a polymer or a plastic.
37. A test apparatus for testing an electrically floating pixel element of an FPD plate, comprising:
- means for applying an AC test signal to an input of an electrically floating pixel element (fpe);
- means for directing an electron beam to a surface of said fpe;
- means for collecting electrons emitted from the surface of said fpe;
- means for forming an electrical signal from the collected electrons; and
- means for comparing a measured characteristic of the electrical signal to an expected characteristic.
38. The apparatus of claim 37 wherein the measured characteristic is an amplitude of the electrical signal and the expected characteristic is an expected amplitude of the electrical signal.
39. The apparatus of claim 37 wherein the FPD plate is an OLED FPD plate.
40. The apparatus of claim 39 wherein the OLED FPD plate is part of a down-emitting OLED FPD structure.
41. The apparatus of claim 39 wherein the OLED FPD plate is part of an up-emitting OLED FPD structure.
42. The apparatus of claim 39 wherein the OLED FPD plate comprises a substrate upon which said fpe and other fpes are formed.
43. The apparatus of claim 42 wherein the substrate is a polymer or a plastic.
44. An apparatus for testing a floating pixel element (fpe) of an FPD, comprising:
- means for applying an AC signal to a power input of an fpe of a FPD;
- means for providing a first activation signal to a current controlling device of said fpe, to activate said fpe to a first activation level; and
- means for measuring characteristics of a first output signal provided at an output of said fpe.
45. The apparatus of claim 44 wherein said means for measuring characteristics of a first output signal is an electron beam probe.
46. An apparatus for testing floating pixel elements (fpes) of an FPD plate, comprising:
- an AC signal generator operable to provide an AC test signal to an input of an fpe of an active plate of an FPD; and
- a measuring instrument operable to measure an amplitude of an fpe output signal that is responsive to said AC test signal.
47. The apparatus of claim 46 wherein the measuring instrument comprises an electron beam probe.
48. The apparatus of claim 46 wherein the FPD plate is an OLDED FPD plate.
49. The apparatus of claim 48 wherein the OLED FPD plate comprises a substrate upon which said fpe and other fpes are formed.
50. The apparatus of claim 49 wherein the substrate is a polymer or a plastic.
51. The apparatus of claim 46 wherein the measured amplitude of the output signal of the fpe is compared to an expected amplitude, to determine whether the fpe is defective.
Type: Application
Filed: Aug 30, 2005
Publication Date: Mar 23, 2006
Applicant:
Inventor: Guillermo Toro-Lira (Sunnyvale, CA)
Application Number: 11/217,085
International Classification: G01R 31/00 (20060101);