The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent.  It was first proposed in 1969.  The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers.  One of the electrodes is usually transparent.  A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates.  When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference. 
There are several different types of EPDs.  In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans.  Electr.  Dev.,  26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of particles such as sedimentation.  The microcapsule type EPD (as described in U.S. Pat.  Nos.  5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent.  Another type of EPD (see U.S. Pat.  No.  3,612,758) has electrophoretic cells that are formed from parallel line reservoirs.  The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors.  A layer of transparent glass from which side the panel is viewed overlies the transparent conductors. 
An improved EPD technology was disclosed in co-pending applications, U.S. Ser.  No.  09/518,488 (corresponding to WO01/67170), filed on Mar. 3, 2000, U.S. Ser.  No.  09/759,212, filed on Jan. 11, 2001, U.S. Ser.  No.  09/606,654, filed on Jun. 28, 2000 (corresponding to WO02/01280) and U.S. Ser.  No.  09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference.  The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. 
All of these EPDs may be driven by a passive matrix system.  For a typical passive matrix system, there are row electrodes on the top side and column electrodes on the bottom side of the cells.  The top row electrodes and the bottom column electrodes are perpendicular to each other.  However, there are two well-known problems associated with EPDs driven by a passive matrix system: cross talk and cross bias.  Cross talk occurs when the particles in a cell are biased by the electric field of a neighboring cell.  FIG.  1 provides an example.  The bias voltage of the cell A drives the positively charged particles towards the bottom of the cell.  Since cell B has no voltage bias, the positively charged particles in cell B are expected to remain at the top of the cell.  However, if the two cells, A and B, are close to each other, the top electrode voltage of cell B (30V) and the bottom electrode voltage of cell A (0V) create a cross talk electric field which forces some of the particles in cell B to move downwards.  Widening the distance between adjacent cells may eliminate such a problem; but the distance may also reduce the resolution of the display. 
The cross talk problem may be lessened if a cell has a significantly high threshold voltage.  The threshold voltage, in the context of the present invention, is defined to be the maximum bias voltage that may be applied to a cell without causing movement of particles between two electrodes on opposite sides of the cell.  If the cells have a sufficiently high threshold voltage, the cross-talk effect is reduced without sacrificing the resolution of the display. 
Unfortunately, the cells in EPDs made using the typical electrophoretic materials and techniques currently available typically do not have a sufficiently high driving threshold voltage to prevent the undesired movement of particles.  As a result, the EPDs constructed from these materials usually cannot achieve high resolution. 
Cross bias is also a well-known problem for a passive matrix display.  The voltage applied to a column electrode not only provides the driving bias for the cell on the scanning row, but it also affects the bias across the non-scanning cells on the same column.  This undesired bias may force the particles of a non-scanning cell to migrate to the opposite electrode.  This undesired particle migration causes visible optical density change and reduces the contrast ratio of the display.  A system having gating electrodes was disclosed in U.S. Pat.  Nos.  4,655,897 and 5,177,476 (assigned to Copytele, Inc.) to provide EPDs capable of high resolution at relative high driving voltage using a two layer electrode structure, one of which layers serves as a gating electrode.  Although these references teach how the threshold voltage may be raised by the use of gating electrodes, the cost for fabricating the two electrode layers is extremely high due to the complexity of the structure and the low yield rate.  In addition, in this type of EPD, the electrodes are exposed to the solvent, which could result in an undesired electroplating effect. 
In many cases it is desirable to be able to display shades and colors other than the main display colors.  For example, in a display in which the main colors are white and black, such as an electrophoretic display in which the cells may be shifted between a white state and a black state, it may be desirable to be able to display shades of gray.  This is referred to in the art as a grayscale display. 
There are various means to achieve a grayscale display.  Spatial modulation creates grayscale by dithering, a process by which a certain proportion of pixels within a localized area of the array of pixels (or cells) that comprise the display are set to a first color and the remainder of pixels in the localized area are set to a second color, giving the visual effect to one viewing the display of a shade in between the first and second colors.  For example, to achieve a shade of gray, every other pixel in the localized area may be set to white and the remainder set to black.  To achieve a lighter shade of gray, a higher proportion of pixels in the localized area may be set to white.  To achieve a darker shade of gray, a higher proportion of pixels in the localized area may be set to black.  Other shades and colors may be displayed or approximated by combining pixels set to two or more different colors.  Another way to achieve a grayscale display is to use frame rate control to quickly switch one or more pixels on and off in such a way that the eye perceives the pixels as gray. 
Both of the methods described above for achieving a grayscale display are commonly used in twisted nematic (TN) and super-twisted nematic (STN) liquid crystal displays (LCD""s).  However, there are major deficiencies in those systems.  Spatial modulation effectively reduces the image resolution and also can generate a visible dithering dot effect, in which the eye perceives dots or unevenness in the dithered areas, especially in very light gray areas (sometimes called the highlight area).  Frame rate control requires a higher driving frequency to quickly switch the pixels on and off, and also certain color areas may appear to be in motion. 
The fact that EPD cells may be set to one of multiple stable states makes EPD technology very suitable for use as a grayscale display.  FIGS.  2A-2D show the side view of an EPD cell in which white charged particles are suspended in a black solvent.  FIG.  2A shows a white state in which all of the white particles have been driven to the top.  FIG.  2B shows a black state in which all of the white particles have been driven to the bottom.  FIG.  2C shows a 60% gray state in which approximately 60% of the white particles have been driven away from the top with the remaining 40% of the white particles remaining at the top of the cell.  FIG.  2D shows a 25% gray state in which approximately 25% of the white particles have been driven away from the top of the cell. 
Those skilled in the art will recognize there are at least three ways to drive an EPD as a grayscale display:
1.  Voltage Amplitude Modulation: By applying bias voltages of different amplitudes to different EPD cells, the charged particles in the EPD cells are driven to various distributions, depending on the amplitude of the respective bias voltage applied to each cell, which cause various gray level perceptions. 
2.  Pulse Width Modulation (PWM): By applying driving pulses of different pulse widths to different EPD cells, the charged particles are driven to various distributions, which also cause various gray level perceptions. 
3.  Pulse Rate Modulation: By applying different numbers of driving pulses to different EPD cells, in a finite driving period that is the same for each cell, the charged particles are driven to various distributions, which cause various gray level perceptions. 
The three driving pulse modulation algorithms described above can also be combined to achieve optimal grayscale quality.  The same grayscale driving algorithms can also be applied to a color EPD, which greatly improves the color quality of the display. 
However, when the modulation algorithms described above are applied to an EPD in a typical passive matrix, with either up/down switching (particles switched between a top electrode and a bottom electrode) or in-plane switching (particles switched between being dispersed in the solvent and being held at side electrodes), the cross bias and cross talk effects described above make the gray level very difficult to control.  Unless the cells have an adequately high threshold voltage, the cross talk and cross bias effects may cause undesired particle movement, which causes the gray level to change.  It is difficult, however, to achieve such a high threshold voltage using the materials, processes and techniques available currently. 
Therefore, there is a need for an electrophoretic display in which the cross talk and cross bias effects will not cause a degradation of display performance, even if cells having a relatively low intrinsic threshold voltage are used.  In addition, there is a need to provide such a display in which a grayscale may be achieved without a degradation of performance due to cross talk and/or cross bias. 
A typical EPD has a top electrode layer, which in one embodiment may have one or more row electrodes and a bottom electrode layer, which in one embodiment may have one or more column electrodes.  In the absence of a holding electrode, such as those described herein, in one embodiment the electric field generated by the row and column electrodes would control the up/down movement of the charged particles in EPD cells positioned between the top electrode layer and the bottom electrode layer at the locations at which the row and column electrodes intersect.  Such an electrode structure is very sensitive to the cross talk and cross bias effect.  An improved design having at least one holding electrode is disclosed.  In one embodiment, the holding electrode is in the bottom electrode layer and parallel to the column electrode.  The holding electrode and the top row electrode are used to hold the particles during non-scanning cycles. 
In one embodiment, the electrophoretic display comprises a plurality of electrophoretic cells filled with charged particles dispersed in a dielectric solvent.  Each said cell is positioned between a top electrode layer and a bottom electrode layer.  The top electrode layer comprises at least one driving electrode positioned over more than one cell.  The bottom electrode layer comprises at least one driving electrode positioned under more than one cell.  The display further comprises a holding electrode located in the bottom electrode layer. 
An electrophoretic grayscale display is disclosed.  In one embodiment, the display comprises a plurality of electrophoretic cells filled with charged particles dispersed in a dielectric solvent.  Each said cell is positioned between a top electrode layer and a bottom electrode layer.  The top electrode layer comprises at least one driving electrode positioned over more than one cell.  The bottom electrode layer comprises at least one driving electrode positioned under more than one cell.  The display further comprises a holding electrode located in the bottom electrode layer.  The display further comprises a colored background layer located under and visible through the bottom electrode layer. 
In one embodiment, the EPD cells have positively charged particles and the voltage applied to the holding electrode and top row electrode is lower than the voltage applied to any other electrode during non-scanning cycles.  Therefore, the positively charged particles attracted at these electrodes will not migrate. 
The holding electrode(s) provide a holding effect, which prevents undesired movement of the charged particles in the cells.  This eliminates the need to provide cells with a threshold voltage high enough to avoid the cross talk and/or cross bias effects described above.  In addition, the design of the present invention can be manufactured using low cost materials by efficient processes. 
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures, which illustrate by way of example the principles of the invention.