Inspection methods for defects in electrophoretic display and related devices

This application claims priority to U.S. provisional application No. 60/790,098, filed Apr. 7, 2006, the content of which is incorporated herein by reference in its entirety.

The present invention provides methods for inspection of defects in an electrophoretic display and related devices.

The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably in a dielectric solvent. More specifically, an electrophoretic fluid comprising charged pigment particles dispersed in a dielectric solvent is enclosed between two electrode plates. At least one of the electrode plates is transparent and such a transparent plate is usually the viewing side. When a voltage difference is imposed between the two electrode plates, the charged pigment particles migrate by attraction to the electrode plate of polarity opposite that of the charged pigment particles. Thus, the color showing at the viewing side may be either the color of the dielectric solvent or the color of the charged pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite electrode plate, thereby reversing the color. Alternatively, two types of pigment particles of different colors and polarities may be dispersed in a solvent. In this case, when a voltage difference is imposed between the two electrode plates, the color showing at the viewing side would be one of the two colors of the pigment particles. Reversal of plate polarity will cause the two types of pigment particles to switch positions, thus reversing the color.

Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.

EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD [M.A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)], the microencapsulated EPD (U.S. Pat. Nos. 5,961,804, 5,930,026, and 7,184,197. and the total internal reflection (TIR) type of EPD using microprisms or microgrooves as disclosed in M.A. Mossman, et al, SID 01 Digest pp. 1054 (2001); SID IDRC proceedings, pp. 311 (2001); and SID'02 Digest, pp. 522 (2002).

An improved EPD technology was disclosed in U.S. Pat. Nos. 6,930,818, 6,859,302 and 6,788,449, the contents of all of which are incorporated herein by reference in their entirety. The improved electrophoretic display comprises isolated display cells formed from microcups which are filled with charged pigment particles dispersed in a dielectric solvent. To confine and isolate the electrophoretic fluid in the microcups, the filled microcups are top-sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.

The U.S. patents identified above also disclose a roll-to-roll process for manufacturing electrophoretic displays. With a roll-to-roll manufacturing process, in-line testing and inspection of the elelctrophoretic display panel produced is highly desirable.

Currently, inspection of an electrophoretic display panel is often carried out by applying a temporary conductive layer to the display panel. The temporary conductive layer is on the opposite side of one of the two electrode plates already in place. When a voltage difference is applied between the temporary conductive layer and the electrode plate, the performance of the display panel (i.e., switching of the charged pigment particles) can be visually inspected. The temporary conductive layer, however, has to be removed before the second electrode plate is applied, to complete the assembly. The use of a temporary conductive layer therefore is not an efficient and cost-effective way for testing and inspection.

The present invention is directed to methods for inspection of defects in an electrophoretic display and related devices.

The first aspect of the invention involves the use of a pair of testing electrodes for in-line or off-line inspection of defects of a display panel.

The second aspect of the invention involves the use of a single testing electrode which, in combination with a common electrode layer laminated to a display panel, for in-line or off-line inspection of defects of the display panel.

It is noted that the whole content of each document referred to in this application is incorporated by reference into this application in its entirety.

FIGS. 1 a and 1 b show the cross-section view of a display panel which can be inspected by the methods of the present invention.

FIG. 2 shows an inspection method with two testing electrodes.

FIG. 3 shows two testing electrodes in the shape of plates.

FIG. 4 is the elevation view of an alternative design of two testing electrodes.

FIG. 5 exemplifies one of the inspection methods.

FIG. 6 shows the elevation view of an alternative design of two pairs of testing electrodes.

FIGS. 7 a and 7 b show further alternative designs of testing electrodes.

FIG. 8 shows an inspection method with one testing electrode.

FIG. 9 exemplifies a driving waveform suitable for the inspection methods of the present invention.

The present invention is directed to an inspection method for inspecting defects of a display panel, wherein said display panel comprises a layer of display cells filled with an electrophoretic fluid. The method comprises applying a voltage difference to two testing electrodes which are in contact with the display panel, and identifying defects of the display panel.

The present inspection methods may be used on a display panel in a variety of forms. For example, FIG. 1 a shows a display panel comprising a layer of display cells ( 10 ) which are filled with an electrophoretic fluid ( 11 ) comprising charged pigment particles ( 14 ) dispersed in a dielectric solvent. The display panel may be tested directly with a testing method of the present invention. However it is preferred that the display panel is protected by a contact film ( 12 ) during testing as shown in the figure.

Suitable materials for the contact film may include, but are not limited to, polyimide, polysulfone, polyarylether, polycarbonate (PC), polyethylene terephthalate (PET), polyethylene terenaphthalate (PEN), poly(cyclic olefin), polypropylene, polyethylene, and composites thereof.

Alternatively, the display panel may further comprise an electrode layer (i.e., ITO) ( 13 ) coated or laminated to one side of the display panel as shown in FIG. 1 b . In this case, the display panel may be tested directly by a method of the present invention; however, it is also preferred that a contact film ( 12 ) is used to protect the display panel and the contact film is preferably placed on the opposite side of the electrode layer ( 13 ). It is noted that while the display panel may have an electrode layer as shown, the presence of such an electrode layer is not always needed.

In one embodiment of the present invention, the inspection method is applied to a microcup-based display panel. In this embodiment, the display panel may comprise the microcup-based display cells formed on a substrate layer or on an electrode layer. The display cells are filled with an electrophoretic fluid and sealed with a polymeric sealing layer. The microcup-based display panel may further optionally comprise a primer layer and/or an adhesive layer. The methods of the present invention may also be applied to any of the display devices previously known, such as those described in the Background section.

While the electrophoretic display panel is extensively discussed in this application, it is noted that the inspection methods of the present invention are also applicable to other types of display panel, such as liquid crystal display panel or the like, as long as the display panel is driven by an electric field which is generated, for example, by two electrode plates.

In the first aspect of the invention, a pair of testing electrodes is used. This method may be applied to the display panel of FIG. 1 a or 1 b . The display panel comprises a layer of display cells ( 10 ) and a contact film ( 12 ) as shown in FIG. 2 . The two testing electrodes may be placed on the opposite sides of a display panel. It, however, is preferred to have the two testing electrodes (A & B) on the same side of the display panel as shown. The surface of the two testing electrodes in contact with the display panel may be coated with a dielectric layer ( 25 ). The dielectric layer may also appear in the gap. A voltage generator ( 26 ) is connected to both testing electrodes, which voltage generator can generate constant voltages or a specific waveform for inspection of the display panel.

The dimension of the two testing electrodes and the gap ( 27 ) between them may vary, depending on the testing conditions (e.g., the size of the display panel or speed of the moving web, etc.) The gap is preferably filled with an electrically insulating material.

The side opposite from the testing electrodes would be the viewing side (i.e., the inspection side).

If there is an electrode layer already laminated to the display panel, the two testing electrodes are preferably placed on the opposite side of the electrode layer. In this case, the side of the electrode layer would be the inspection side. No voltage is applied to the electrode layer during testing.

The two testing electrodes may be of any shapes. For example, they may be in the shape of plates as shown in FIG. 3 . To ensure full area coverage in the inspection process, the length (l) of the two testing electrodes (A and B) is preferably the same as the width (w) of the display panel ( 30 ).

The two testing electrodes are in close contact with the display panel via the electrostatic force. A soft flat plate may be optionally placed on the surface of the display panel. The soft flat plate needs to have a reasonable amount of weight and its purpose is to ensure close contact between the display panel and the testing electrodes by the gravity force.

FIG. 4 shows the elevation view of an example of two testing electrodes which are concentric. In the figure, one (A) of the two testing electrodes is an inner square whereas the other testing electrode (B) has a square shape surrounding the inner square testing electrode A. The testing electrode A is not in physical contact with the testing electrode B. There may be an electrically insulating gap ( 27 ) between the two testing electrodes and such a gap is formed of an electrically insulating material. To ensure full coverage for the inspection, the dimension of the inner testing electrode has a length which is the same as, or slightly shorter than, the width of the display panel ( 40 ) whereas the dimension of the outer testing electrode may slightly exceed, or the same as, the width of the display panel.

In practice, when a voltage difference is applied to the pair of testing electrodes, the charged pigment particles in areas corresponding to the testing electrodes may move to one side or the other (as shown in FIG. 5 ), causing either the color of the charged pigment particles or the color of the dielectric solvent to be seen from the inspection side. For example, if the pigment particles are positively charged, while the testing electrode A is applied a positive voltage potential and the testing electrode B is applied a negative voltage potential, the color of the charged pigment particles will be seen in the area corresponding to the testing electrode A and the color of the dielectric solvent will be seen in the area corresponding to the testing electrode B, from the inspection side. When the voltages applied to the two testing electrodes are reversed, the colors would be reversed too. For a complete inspection of the display panel, each section should be inspected for both contrasting colors (i.e., the color of the charged pigment particles and the color of the dielectric solvent). This is accomplished by reversing the voltages applied to the two testing electrodes or turning the display panel by 180 degrees while keeping the voltages unchanged. The display panel is inspected by switching to the two color states. In each color state, the defects may be identified either by color difference or by the difference of the optical density of the defected areas from that of the non-defected areas.

FIG. 6 shows a further alternative design. The display panel 60 is moving in a stop-and-go mode in the direction shown. In this design, two pairs of testing electrodes are used. When the display panel is over or near the first pair of testing electrodes (A and B), voltages, +V and −V, are applied to the testing electrodes A and B, respectively. When the display panel moves to be near or over the second pair of testing electrodes (A′ and B′), voltages, +V and −V, are applied to the testing electrodes B′ and A′, respectively. Following these steps, both color states in each section may be inspected. During this process, the voltages applied to the first pair of testing electrodes (A and B) must be removed (i.e., electrodes grounded) to allow dissipation of the electrostatic force holding the testing electrodes to the display panel, before the display panel moves to the second pair of testing electrodes.

The inspection may be carried out visually by an operator. It is also possible to have an automated inspection system which would comprise a camera and a computer to identify the defects (i.e., areas, locations and counts). The operator is located, or the automated inspection system is installed, on the inspection side.

The voltages applied to the two testing electrodes may vary. If no contact film is present, lower voltages (e.g., less than 300V) are sufficient. However, when the contact film is present, higher voltages (e.g., above 1000V) may be required.

For in-line roll-to-roll inspection, the two testing electrodes may be face-to-face as shown in FIGS. 7 a and 7 b . In FIG. 7 a , the two testing electrodes are in a flat form and very close to each other. In this design, the two testing electrodes are on the opposite sides of a display panel to be tested. The gap between the two testing electrodes is controlled to allow the display panel passing through without touching the testing electrodes. FIG. 7 b is the cross section view of the two testing electrodes and in this case, the testing electrode A is a rotatable cylinder and the testing electrode B can be a curved plate or bar. The curvature of the testing electrode B that faces the electrode A should match the curvature of the cylinder-like testing electrode A. During the roll-to-roll inspection process, one side of the display panel will be in contact with electrode A while the other side will be very close to electrode B.

Alternatively, FIGS. 7 a and 7 b can be used in a stop-and-go mode with a lower voltage difference between the two testing electrodes for inspection. In this case, the two testing electrodes will move toward each other to contact (sandwich) the display panel.

In the second aspect of the present invention, only one testing electrode is needed. In this aspect, the invention is directed to an inspection method for a display panel, wherein said display panel comprises a layer of display cells filled with an electrophoretic fluid and an electrode layer. The method comprises applying a voltage difference to a testing electrode and said electrode layer, and identifying defects of the display panel.

This method is particularly suitable for the display panel of FIG. 1 b where an electrode layer is present. The electrode layer ( 83 ) has at least one area ( 81 ) (i.e., edge) which is not covered by the layer of display cells ( 80 ). The testing electrode C (shown in FIG. 8 ) preferably has a length which is substantially the same as, or slightly shorter than, the width of the display panel. A voltage potential difference is applied to the testing electrode C and the electrode layer (via the edge) to cause the charged pigment particles in the area corresponding to the testing electrode C to switch. While the voltages applied to the testing electrode C and the electrode layer are reversed, a contrast color may be displayed. Therefore by alternating the voltages, both contrasting colors can be inspected. The inspection may also be carried out by an operator or by an automated inspection system as described above.

It is also noted that in either one of the two methods disclosed in the present application, arbitrary waveforms may be applied to the two testing electrodes (in the first method) or to the one testing electrode and the electrode layer (in the second method). FIG. 9 illustrates a driving waveform which may be applied in the testing methods. Such a waveform may be used to test an electrophoretic display panel in a gray state where the pigment particles are in an intermediate state (i.e., between the two extreme states). Some defects may show in such an intermediate state, not in any of the extreme states. In practice, the voltage and duration in each phase of the waveform may vary, depending on the characteristics of display panel tested.

While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation, materials, compositions, processes, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

It is therefore wished that this invention to be defined by the scope of the appended claims as broadly as the prior art will permit, and in view of the specification.