WO2008010163A2 - Arrays of particle containing cells - Google Patents

Abstract

An array device comprises an array of rows and columns of device cells, each device cell comprising a sealed region containing a fluid in which particles are suspended, wherein the movement of particles within each cell is controlled to define a cell state, the cell states of all device cells together defining an output of the device. The device comprises an array of orthogonal addressing conductors comprising at least a first set of addressing conductors (2) for addressing lines of cells and a second set of addressing conductors (4) for addressing perpendicular lines of cells. The first set of addressing conductors comprises, for each line of cells, at least a first conductor and a second conductor which are electrically connected together and are provided at opposite cell boundaries, wherein between the second conductor of one line and the first conductor of the next line there is defined an overflow channel (32). The overflow channel (32) enables the sealing of the cells can be conducted with excess cell fluid having a passageway to drain to. The electrode design also enables short and/or open circuits to be tolerated.

Description

Arrays of particle containing cells

This invention relates to devices in the form of arrays of cells, with the cells containing particles, and in particular in which the movement of particles within the cell defines an output condition of the device.

There a few examples of device of this type, for example shutters, sun blinds and diaphragms. However, the application of most interest is in electrophoretic display devices.

Electrophoretic display devices are one example of bistable display technology, which use the movement of charged particles within an electric field to provide a selective light scattering, subtractive or absorption function.

In one example, white particles are suspended in an absorptive liquid, and the electric field can be used to bring the particles to the surface of the device. In this position, they may perform a light scattering function, so that the display appears white. Movement away from the top surface enables the colour of the liquid to be seen, for example black. In another example, there may be two types of particle, for example black negatively charged particles and white positively charged particles, suspended in a transparent fluid. There are a number of different possible configurations.

It has been recognised that electrophoretic display devices enable low power consumption as a result of their bistability (an image is retained with no voltage applied), and they can enable thin display devices to be formed as there is no need for a backlight or polariser. They may also be made from plastics materials, and there is also the possibility of low cost reel-to-reel processing in the manufacture of such displays.

If costs are to be kept as low as possible, passive addressing schemes are employed. The most simple configuration of display device is a segmented reflective display, and there are a number of applications where this type of display is sufficient. A segmented reflective electrophoretic display has low power consumption, good brightness and is also bistable in operation, and therefore able to display information even when the display is turned off. However, improved performance and versatility is provided using a matrix addressing scheme. An electrophoretic display using passive matrix addressing typically comprises a lower electrode layer, a display medium layer, and an upper electrode layer. Biasing voltages are applied selectively to electrodes in the upper and/or lower electrode layers to control the state of the portion(s) of the display medium associated with the electrodes being biased.

Another type of electrophoretic display device uses so-called "in plane switching". This type of device uses movement of the particles selectively laterally in the display material layer. When the particles are moved towards lateral electrodes, an opening appears between the electrodes, through which an underlying surface can be seen. When the particles are randomly dispersed, they block the passage of light to the underlying surface and the particle colour is seen. The particles may be coloured and the underlying surface black or white, or else the particles can be black or white, and the underlying surface coloured. An advantage of in-plane switching is that the device can be adapted for transmissive operation, or transflective operation. In particular, the movement of colour subtractive particles creates a passageway for light, so that both reflective and transmissive operation can be implemented through the material. This enables illumination using a backlight rather than reflective operation. The in-plane electrodes may all be provided on one substrate, or else both substrates may be provided with electrodes, although this raises costs due to alignment needs.

Active matrix addressing schemes are also used for electrophoretic displays, and these are generally required when bright full colour displays with high resolution greyscale are required. Such devices are being developed for signage and billboard display applications, and as (pixellated) light sources in electronic window and ambient lighting applications. Colours can be implemented using colour filters and the display pixels then function simply as greyscale devices, or else colour subtractive particles may be used (YMCK).

To explain some of the issues associated with the design of electrophoretic display devices, Figure 1 shows an example of the electrode layout for a passive matrix in- plane switching device.

Figure 1 shows an array having row conductors 2 and column conductors 4. The column conductors are for providing data signals to the pixels, with selection voltages applied to electrodes coupled to the row conductors 2. The row conductors include a flag part, which is an area where particles can be collected out of view as a part of a reset operation.

To enable only two electrodes per pixel, the particles are required to have a threshold property in addition to bistability, so that the applied voltages for controlling one row of pixels do not undo the pixel state of the previously addressed rows. Alternatively, for particles having no threshold, a control electrode, usually called a gate electrode, may be positioned between the row electrode 2 and the display electrode 4, preferably as close as possible next to the flag part of the row electrode 2. In this case, column- wise oriented gate electrodes perform an electronic gate function. Pixel data is then applied to the row electrodes 2, and depending on the potential at the selected gate column (not shown in Figure 1) particles may either cross or not cross over the gate towards the viewing area, i.e. towards the display electrode 4. This particle movement then depends on the potential applied to the row electrodes 2.

For small sized displays, the possibility of developing open and/or short circuits during device processing is small, but increases substantially (depending on clean room class) for (much) larger sized displays. These two risk scenario are schematically depicted in Figure 1, where an open circuit in the row conductor is shown as 10, and a short is shown as 12.

Clearly, when an open circuit develops, the remainder of the pixels after that open circuit in that given line become non- functional, or worse, become cross-talk driven. Of course, when only one open circuit has developed, electrical contact may be made from both sides of the row, but in effect this option provides no solution for larger sized displays. Also, connecting a display from two sides increases its cost.

Similarly, for a short circuit, the short may be removed by laser repair. However, when the short coincides with the driveline, the risk of developing an open circuit after laser repairing is substantial.

Another problem with electrophoretic displays is that when the local concentration of pigments differs from one pixel to the next, for example due to different driving states, inter-pixel diffusion will occur. Thus when no precautions are taken, pigments will move from one pixel to the next until the impact of the applied electric field has been balanced. Hence, when the next image is to be displayed, the maximum light modulation per pixel (even for those having equal drive states) will be quite different. In other words, a non- sealed display will suffer from a complex form of image ghosting, and non-uniformity, and hence individual sealed pixel cells are essential. Figure 2 shows a cross section through a pixel, and shows a lower substrate 20, an upper sealing substrate 22, and cell walls 24. As shown, there can be leakage 26 over the cell walls. The electrode array is represented as 28.

The leakage is through a thin hydrodynamic suspension layer between the top sealing substrate and the top of the pixel walls. Thus pigments may diffuse from one pixel to the next.

Well-known sealing methods include phase separation of a monomer from the electrophoretic suspension and subsequent sealing by surface polymerisation, for example in thermo-embossed micro-cups, or encapsulation in the bulk of the suspensions thereby forming self-enclosing microcapsules. Other methods include fill and seal by (micro perforated) sheets, thereby enclosing the pixels.

Some of these methods are not suitable for in-plane switching designs, in which the electrodes need to line up exactly with the pixel cells. This means it is necessary to develop the backplane (electrode array) in parallel with the electrophoretic medium and the sealing method. There is also a black mask located at the particle collection area and at the cell walls, to increase the contrast ratio. Alignment of the black mask layer with the cell walls is therefore also an issue.

The preferred sealing method for in-plane switching devices is to use a top sealing method by means of sheet lamination onto the top of a filled cell array, with the sheet carrying no technology layer or only a blanket (nonstructured) film or films, for example a reflective layer.

The problem with sheet sealing is that there is excess liquid per pixel, resulting from drop or bulk overfilling, and this liquid must escape somehow from each of the pixels, until the top sheet hits the tops of the enclosing walls everywhere within the display. This is extremely difficult, and is often only achieved for the peripheral pixels of the display. For pixels located towards the centre of the display, removing the thin fluid layer between the sheet and the top of the cell walls not only becomes progressively more difficult, but also is achieved rarely.

Several methods to resolve this issue have been reported such as centrifuging whilst sealing, roll sealing with or without adhesive on the seal sheet, or by means of a first perforated sealing sheet followed by a second sealing layer. Other methods include pixel filling by vacuum breaking of a submerged display, ink-jet filling, optionally combined with a doctor blade. However, whatever filling and sealing method is chosen, uniformity remains a problem, as well as the removal of the small amounts of excess liquid between the top of the cell walls and the cover sheet. It is this thin layer that forms the hydrodynamic barrier explained above.

According to the invention, there is provided an array device, comprising: an array of rows and columns of device cells, each device cell comprising a sealed region containing a fluid in which particles are suspended, wherein the movement of particles within each cell is controlled to define a cell state, the cell states of all device cells together defining an output of the device, wherein the device comprises an array of orthogonal addressing conductors comprising at least a first set of addressing conductors for addressing lines of cells and a second set of addressing conductors for addressing perpendicular lines of cells, wherein the first set of addressing conductors comprises, for each line of cells, at least a first conductor and a second conductor which are electrically connected together and are provided at opposite cell boundaries, wherein between the second conductor of one line and the first conductor of the next line there is defined an overflow channel.

The invention thus provides an electrode design for a sealed array of cells containing particles. The electrode design provides an overflow channel so that the sealing of the cells can be conducted with excess cell fluid having a passageway to drain to. The electrode design also enables short and/or open circuits to be tolerated.

The invention is applicable to moving particles devices generally, but in particular for in-plane switching electrophoretic displays, and provides improved cup-sealing characteristics and redundancy and/or repair options for non-intentional open and short circuits. Each line of cells preferably comprises a row of cells, the first set of addressing conductors comprise row conductors and the second set of addressing conductors comprise column conductors.

In a preferred arrangement, the first and second conductors for each line are connected in a ladder pattern, with the first and second conductors and adjacent ladder rungs together being located at all boundaries of a respective device cell.

The electrodes can thus be used in a self-alignment process to define the cell walls, which may be are aligned with the first set of addressing conductors.

Preferably, the overflow channels each extend to opposite ends of the array area, so that excess liquid can be drained away. The first set of addressing conductors can be opaque and substantially black, so that they can be used as the black mask layer. This means there is automatic alignment between the black mask layer and the cell walls.

The overflow channels can contain air, and this reduces optical cross talk between neighbouring cells.

The invention also provides a method of fabricating an array device, comprising an array of rows and columns of device cells, the method comprising: defining an array of orthogonal addressing conductors comprising at least a first set of addressing conductors for addressing lines of cells and a second set of addressing conductors for addressing perpendicular lines of cells, wherein the first set of addressing conductors comprises, for each line of cells, at least a first conductor and a second conductor which are electrically connected together, and are provided at opposite cell boundaries; defining cell walls aligned with the first set of addressing conductors and thereby defining an overflow channel between the second conductor of one line and the first conductor of the next line; filling the cells with a fluid in which particles are suspended; covering the cells, thereby causing fluid to overflow into the overflow channels; and sealing the device cells.

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows a known electrode layout for a passive matrix electrophoretic display;

Figure 2 is used to explain a problem encountered in the manufacture of electrophoretic displays;

Figure 3 compares the known electrode layout with an electrode layout of the invention; Figure 4 shows how the electrode layout of the invention provides fault tolerance;

Figure 5 is used to explain how the invention improves the filling and sealing process;

Figure 6 shows in greater detail the cell structure of the invention; and Figure 7 is used to show how a reservoir electrode pattern can be defined within the cell internal area.

The same references are used in different Figures to denote the same layers or components, and description is not repeated.

The invention provides a ladder shaped electrode layout for controlling the cells of an array device. This ladder electrode design can be used to enable self-aligned pixel cells with a black mask, to provide improved cell sealing characteristics and to provide electrical redundancy and/or repair options for open and short circuits in the electrode array.

Figure 3 shows part of a conventional electrophoretic back-plane on the left (corresponding to Figure 1) and part of an improved in-plane electrophoretic back-plane of the invention to the right.

For the conventional layout, it can be seen that the external connection, for each row or column of pixels, depends entirely on the continuity of a single row conductor 2.

The layout of this example of the invention has two row conductors 2a,2b per row, with interconnecting rungs 30 defined by the storage electrodes. It can be seen that the two row conductors 2a and 2b and the rungs together surround all four sides of the cell shape, so that the two row conductors are provided at opposite cell boundaries (top and bottom), and the rungs define the other two opposite cell boundaries. As will be explained below, this means the conductors can be used for a self-alignment process and can be used as a black mask layer.

Figure 3 also shows that a gap 32 is provided between the adjacent rows of cells, and this functions as an overflow channel in the filling and sealing processes used to fabricate the device.

The ladder arrangement provides inherent rerouting in the event of open circuits, giving improved redundancy is ensured. Figure 4 shows how the arrangement of the invention can tolerate open and short-circuits.

The left part of Figure 4 shows four open circuit faults 40. The second row is unaffected, so that the conductive path 42 is along both row conductors. The third row has three open circuit faults 40, and the conductive path 44 meanders along the ladder.

The right part of Figure 4 shows five possible short circuit faults 46. One of these, 46a, does indeed cause a short circuit between two adjacent rows. By applying a laser repair, the fault is converted into an open circuit, and the two affected rows (the bottom two rows) function again in the same way as for an open circuit. For open circuits, no action is required as re-routing is automatically effected by the ladder branches. In the case of a short, however, action may be required, but only in the case that branches of adjacent ladders would be interconnected. Only in that case the branch is best cut left and right of the short, isolating it from the rest of the device.

As mentioned above, another benefit of the ladder shaped electrode layout is that it may serve as a self-alignment mask with respect to the pixel walls. In other words, by using image reversal techniques or alternatively negative photo-resist (with the ladder structure made of a material that substantially blocks part of the wavelength at which the photo-resist is exposed), perfect alignment between cell walls and the ladder electrode pattern is obtained.

This self alignment capability relaxes the manufacturing tolerances. This issue becomes very important when moving towards flexible carriers for simplifying the manufacturing process. Relaxation of electrical and other manufacturing tolerances is one way to reduce manufacturing costs.

Furthermore, the ladder shaped pattern can be made of a black coloured material, for example black Titanium or a dye-stained conductor, carbon black (CB) or a CB filled (conductive) polymer.

In this way, the ladder electrode pattern simultaneously acts as (i) a conducting row or column electrode,

(ii) a black-mask, and

(iii) a self-alignment mask for the pixel walls

This provides a further opportunity to reduce manufacturing costs.

As shown in Figure 3, channels 32 are defined between the ladder electrode patterns of adjacent rows, so that the overall electrode pattern comprises parallel spaced ladders electrodes. These channels 32 give two significant advantages.

Firstly, the pattern provides drain channels to both ends of each row or column of pixels, and all the way to the exterior of the display. Thus, during the fill and seal process, excess fluid can much more easily be drained to the display exterior. In addition, the hydrodynamic barrier layer between the top of the pixel walls and a top-sealing sheet is broken down much more rapidly, resulting in substantial improvement in the sealing yield.

Figure 5 shows a cross section of a conventional grid on the left, showing a lower substrate 50, an upper sealing sheet 52 and the cell walls 54. A small gap 56 remains between the top of the pixel walls and the sealing sheet. This hydrodynamic layer must be removed during sealing to prevent the formation of the gap. Otherwise, concentration driven diffusion will occur.

The right side of Figure 5 shows two cell walls 54 with the channel 32 between them, which enables the sealing sheet to be fully in contact with the tops of the channel walls.

The invention thus enables the hydrodynamic film between the sealing sheet and the top of the walls to be minimized, because excess fluid can be drained through the channels. The fluid that remains in the channels, once dried out by evaporation, also stains the channels with the same optical density as that in the pixels in the dark state. The dried-out suspension no longer responds to the drive fields at the pixels, and, hence, the black mask area is stationary as a whole. Figure 5 shows the base of the channel 32 as dark, and this represents dried out cell fluid.

The second significant advantage is that the dried-out channels contain air. Thus the refractive index in the channels has been changed dramatically. This causes light that enters the pixel walls at angles less than the critical angle to be reflected back into the pixel cells by total internal reflection. Thus the risk of colour cross talk is reduced, whilst simultaneously a possible loss in pixel brightness is minimized.

Figure 6 shows more clearly the arrangement at the channel, and is the other way up to Figure 5. Figure 6 shows more clearly one end of the channel 32 stained, and the remainder of the channel 32 is filled with air. The cell walls 54 are built on top of and self- aligned with the row electrodes, and the row electrodes are not shown in Figure 6 for clarity.

Figure 6 also shows an optional in-cell diffuse reflector 60, located at the sealing sheet. Thus, the same basic technology may be used for a reflective or a transmissive display. Only the sealing sheet needs to be different. It may accommodate a reflective layer (plus an optional protective film) for a reflective display, but will be transparent for a transmissive display.

The reflector 60 sits over the cell walls, at the sealing sheet 52. Thus, light that enters the cell walls at their bases, from the viewing side of the display opposite the reflector, is not lost but reflected. Below the critical angle, this light cannot pass into the channel 32 and be absorbed by the dried-out suspension.

The dried out suspension may stain the bottom of the channel 32 where it is closed by the lower substrate and/or the top of the channel where it is closed by the sealing sheet. As shown in Figure 6, reflections below the critical angle do not cross the channel 32, and this enables cross talk between adjacent rows of pixels to be reduced. In a conventional configuration light from one pixel may enter the adjacent pixel(s) because the refractive index difference between the electrophoretic medium and the walls is small. The air can enable the refractive index difference to be in the range 0.3 - 0.5.

The use of the ladder electrode pattern as a self-alignment mask for the cell walls means that no part of the ladder electrode pattern projects into the cell area. However, a reservoir electrode may be required within the cell area at which particles collect, for the reset operation mentioned above. This reservoir electrode can be defined by a conductive spur which projects into the pixel cell space.

Figure 7 is used to explain this and shows a region of 2x2 cells. The first layer over the substrate is an ITO layer, and this is patterned to define the transparent conductive spurs 70. The ladder electrode pattern overlies the spurs so that the spurs become electrically connected to the ladder electrode pattern. A blanket dielectric insulation layer is provided over the spurs 70 and ladder electrode pattern, but openings are defined over the pixel cell areas to reduce absorption losses. These openings expose at least part of the spurs 70 as well as defining providing a clear pixel aperture. The insulation layer is essentially required to enables column conductors 72 to be defined which cross over the ladder electrode pattern. Figure 7 shows only one column electrode per pixel, but there may be two or more.

The material used to form the column conductors 72 is also used to define reservoir electrode areas 74, and is a transparent conductor such as ITO. The reservoir electrode areas 74 thus connect to the spurs 70 through the opening in the dielectric layer. As a transparent conductor is used, the opaque ladder electrode pattern can still be used as a self- alignment mask for the cell walls.

The reservoir electrodes 74 may be left transparent, or else they may be stained, for example by a electrochemical plating process, with the ladder electrodes providing the potential for the plating process. For example, they may be stained black.

Thus, it can be seen that a light-shielding reservoir electrode can be provided within the cell area while still using the self-alignment of the cell walls with the ladder row electrode pattern.

The invention has been described in connection with an in-plane switching arrangement, but the concepts can be extended to other configurations. One example of display has been given with row and columns in a particular orientation. The orientation is however somewhat arbitrary. The row is in the example given the conductor to which the pixel address signal is applied and the column is the conductor to which the data signal is applied. These may be switched around, and it should therefore be understood that a "row" may run from top to bottom, and a "column" may run from side to side. The scope of the claims should be understood accordingly.

The invention can be applied to devices with a variety of pixel shapes, but it has for simplicity been described above in connection with an in-plane switching electrophoretic displays having rectangular (or square) shaped pixel cups, with a row or a column of cups forming a ladder shaped like section.

The electrode layouts shown are suitable for passive matrix devices. However, the same advantages can be obtained by applying the invention to an active matrix device. In this case, the electrodes are associated with an array of switching devices, such as transistors and diodes. However, the same manufacturing and fault tolerance issues arise. In this case it may be even more beneficial (as other lithography is used), to have channels enclosing the pixel walls at all sides. In the passive matrix design shown, this is not possible as the second set of electrodes is made from a transparent conductive material (such as ITO or a polymer conductor). so cannot be used as an alignment mask.

The row and column electrode have been described as for "addressing lines (rows or columns) or cells. Different applications will use the rows and columns differently. Typically, the row conductors are for selecting rows of cells and the column conductors are for providing cell control data to a selected row of cells in parallel. However, other arrangements are possible, and these are all intended to be within the scope of the term "addressing". For example, there may be one or more additional control and/or display electrodes to perform electronic gating functions, and/or evolution functions. This is particularly the case for particles having no bi-stability and/or no threshold.

The application of most interest is for pixel cells in an in-plane switching electrophoretic display. However, the processes could be particularly suitable in a range of areas where a particular substance must be (continuously or temporarily) confined into a limited area. Examples of such applications may be shutters, sun blinds, diaphragms, drug delivery systems, fragrance delivery systems, chemical reagent delivery systems, biochips or other micro-fluidic devices. One important class of device where individual pixel sealing is of extreme importance is so-called lab-on-chip devices, where qualitative amounts are of extreme importance with respect to a medical treatment procedure to be chosen.

Various modifications will be apparent to those skilled in the art.

Claims

CLAIMS:

1. An array device, comprising: an array of rows and columns of device cells, each device cell comprising a sealed region containing a fluid in which particles are suspended, wherein the movement of particles within each cell is controlled to define a cell state, the cell states of all device cells together defining an output of the device, wherein the device comprises an array of orthogonal addressing conductors comprising at least a first set of addressing conductors (2) for addressing lines of cells and a second set of addressing conductors (4) for addressing perpendicular lines of cells, wherein the first set of addressing conductors comprises, for each line of cells, at least a first conductor (2a) and a second conductor (2b) which are electrically connected together and are provided at opposite cell boundaries, wherein between the second conductor (2b) of one line and the first conductor (2a) of the next line there is defined an overflow channel (32).

2. A device as claimed in claim 1, wherein each line of cells comprises a row of cells, the first set of addressing conductors (2a,2b) comprise row conductors and the second set of addressing conductors (4) comprise column conductors.

3. A device as claimed in claim 1 or 2, comprising an electrophoretic device, in which the particles comprise electrophoretic particles.

4. A device as claimed in claim 3, comprising an in-plane switching electrophoretic display device.

5. A device as claimed in any preceding claim, wherein the first and second conductors for each line are connected in a ladder pattern, with the first and second conductors (2a,2b) and adjacent ladder rungs together being located at all boundaries of a respective device cell.

6. A device as claimed in any preceding claim, wherein the overflow channels (32) each extend to opposite ends of the array area.

7. A device as claimed in any preceding claim, wherein cell walls (54) are aligned with the first set of addressing conductors (2a,2b).

8. A device as claimed in any preceding claim, wherein the first set of addressing conductors (2a,2b) are opaque and substantially black.

9. A device as claimed in any preceding claim, wherein the overflow channels

(32) contain air.

10. A method of fabricating an array device, comprising an array of rows and columns of device cells, the method comprising: defining an array of orthogonal addressing conductors comprising at least a first set of addressing conductors (2) for addressing lines of cells and a second set of addressing conductors (4) for addressing perpendicular lines of cells, wherein the first set of addressing conductors comprises, for each line of cells, at least a first conductor (2a) and a second conductor (2b) which are electrically connected together, and are provided at opposite cell boundaries; defining cell walls (54) aligned with the first set of addressing conductors (2a,2b) and thereby defining an overflow channel between the second conductor of one line and the first conductor of the next line; filling the cells with a fluid in which particles are suspended; covering the cells, thereby causing fluid to overflow into the overflow channels; and sealing the device cells.

11. A method as claimed in claim 10, wherein each line of cells comprises a row of cells, the first set of addressing conductors (2a,2b) comprise row conductors and the second set of addressing conductors (4) comprise column conductors.

12. A method as claimed in claim 10 or 11 for fabricating an electrophoretic display device, in which the particles comprise electrophoretic display particles.

13. A method as claimed in any one of claims 10 to 12, wherein defining the first set of addressing conductors comprises defining a ladder pattern, with the first and second conductors and adjacent ladder rungs together being located at all boundaries of a respective device cell.

14. A method as claimed in any one of claims 10 to 13, wherein defining cell walls comprises self-aligning cell walls with the first set of addressing conductors (2a,2b) using the first set of addressing conductors having a ladder shape as an alignment mask.

15. A method as claimed in any one of claims 10 to 14, wherein the first set of addressing conductors are opaque and substantially black.