Through connection of a display device

An electrowetting display device comprising at least one picture element with a first support plate including a first electrical element, a second electrical element and a through-connection electrically connecting the first electrical element to the second electrical element. The through-connection is arranged outside a projected display area of the at least one picture element.

BACKGROUND

An electrowetting display is known which comprises a plurality of picture elements having a first support plate, a second support plate and a space between the first support plate and the second support plate, the space including at least one first fluid and a second fluid immiscible with each other. Each of the plurality of picture elements includes an electrode for applying an electric field in the element. The electrode may be connected to underlying picture element circuitry by a through-connection, also known as a via, formed in the first support plate. A display effect may be created in a display area of each picture element by applying the electric field.

It is desirable to provide an improved display device.

DETAILED DESCRIPTION

The entire contents of the following patent documents are incorporated by reference herein:1. GB 1116093.4 filed on 16 Sep. 20112. PCT/EP2012/068175 filed Sep. 14, 2012

Embodiments described herein relate to improvements in relation to a method of manufacturing an electrowetting device.

Before describing detailed embodiments referencing the Figures, embodiments will be described in summary form.

In accordance with first embodiments, there is provided an electrowetting display device comprising at least one picture element, a first support plate, a second support plate and a space between the first support plate and the second support plate, the space including a first fluid and a second fluid immiscible with each other, the first support plate including a first electrical element, a second electrical element and a through-connection electrically connecting the first electrical element to the second electrical element, the at least one picture element comprising at least part of the second electrical element and a display area, wherein the through-connection is arranged such that, when the display area is projected through the first support plate, the through-connection is outside the projected display area.

A new problem has been identified which is unknown.

A through-connection formed in a first support plate of an electrowetting picture element is known. In order to control motion of a first and second fluid, a through-connection is used for creating a height difference in a surface of the first support plate such that the first fluid forms a non-uniform thickness layer on the first support plate when no voltage is applied between an electrode and the second fluid. Thus, the height difference causes differences in the strength of electric field applied across the layer of the first fluid when a voltage is applied between an electrode and the second liquid. The applied field is greater where the thickness of the first fluid is less. Therefore, at the position of the through-connection, which forms a depression in the surface of the first support plate, the first fluid is thicker and thus the applied electric field is less. This creates a location where the first fluid preferentially moves towards, thereby forming a pinning location for the first fluid.

It has been identified that in known systems the through-connection causes undesirable motion of the first and second liquids. At the surface of the first support plate, an edge of the depression formed by the through-connection experiences a fringe field upon application of the voltage. This causes, undesirably, initiation of fluid motion at the edge, resulting in uncontrolled motion of the first fluid and agglomeration of the first fluid at several locations in the picture element rather than at one preferred location. This impairs the quality of the display effect in the picture element and therefore in an electrowetting display; this is manifested in a still or moving image as low brightness of the image, retention of the previous image or non-uniform switching behavior across the display.

Moreover, it is known that the presence of the through-connection and electrical elements in the first support plate cause insulating layers and a hydrophobic layer formed over the through-connection and electrical elements, to have a non-uniform thickness. This is due to limitations in the manufacture, meaning the insulating layers and a hydrophobic layer may be thinner at the edge of the depression formed by the through-connection than elsewhere without an underlying through-connection. This may lead to delamination or breakdown of the insulating layers, or may accelerate electrolysis between the fluids and the insulating layers at the edge, thereby reducing the lifetime of the display. Further, thinner insulating layers and/or a hydrophobic layer at the edge results in a higher electric field being applied at the edge compared with another part of the surface without the through-connection underneath, and therefore initiation of fluid motion at the edge.

It has now been realized that the through-connection does not need to be arranged in a part of the picture element which is defined by projecting the display area through the first support plate. Therefore, in embodiments to be described, the through-connection is arranged outside of a projected display area of the picture element.

The term display area used herein is an area, when viewed from a viewing side of the picture element, within which the first fluid is confined and over which a display effect is provided. Further details of the display area will be explained below.

The term through-connection, also referred to as a via, used herein is defined as an electrical connection between the first electrical element which for example is part of a transistor for an active matrix display device, and the second electrical element such as an electrode for applying an electric field in the picture element. To apply a voltage to the second electrical element, a voltage signal applied to the first electrical element is conducted to the second electrical element via the through-connection, i.e. through the through-connection.

The term projecting is used herein in the context of a display area and a border area of a picture element, to describe clearly the arrangement of the through-connection with respect to the display area and the border area. In embodiments described below, a display area and a border area are two dimensional and planar. Projecting the display area through the first support plate means projecting the extent of the display area through the first support plate in a direction perpendicular the plane of the display area, for example towards a rear side of the picture element, thus defining a part of the picture element with the extent of the display area. The term projected display area may be used below to refer to such a part. Similarly, projecting the border area through the first support plate means projecting the extent of the border area through the first support plate in a direction perpendicular the plane of the border area, and for example towards a rear side of the picture element, thus defining a part of the picture element with the extent of the border area. The term projected border area may be used below to refer to such a part.

Embodiments described herein overcome the problems described. With the through-connection being arranged outside the projected display area of the picture element, the optical performance of the picture element such as brightness may be significantly enhanced because motion of the first fluid is now more controllable over the display area, compared with known systems, since as an example the thickness of insulating layers and a hydrophobic layer, beneath the display area may now be uniform. With the through-connection arranged outside the projected display area, a surface of the first support plate may be free of height differences and therefore a fringe field associated with an edge of the through-connection may no longer exist. Further, as an example, insulating layers and a hydrophobic layer may now have a uniform thickness within the projected display area and the lifetime of the display may therefore also be improved.

Further, manufacturing the picture element and display device of embodiments described herein is simple, without requiring complex modifications to current manufacturing methods for an electrowetting display device.

Arranging the through-connection outside of the projected display area as in embodiments described herein is contrary to the teaching of known systems to position the through-connection underneath a display area.

In some embodiments, the at least one picture element may include a first and a second picture element, wherein when the display area of the first and second picture elements is projected through the first support plate the through-connection is arranged between the projected display area of the first picture element and the projected display area of the second picture element. One or more through-connections may therefore be arranged in a display device between the projected display area of the first picture element and the projected display area of the second picture element. An area between the display areas of adjacent picture elements may not be used for creating a display effect; therefore, by locating the through-connection between and outside of the projected display areas, there may be no detrimental impact on the display effect created in the display area. In some embodiments where the picture element has more than one second electrical element for creating the display effect, more than one through-connection may be arranged outside the projected display area, between the projected display areas of the first and second picture elements, for example.

The first and second picture elements may be neighboring picture elements. The through-connection may therefore be arranged between the projected display areas of two picture elements that are next, i.e. adjacent, to each other. One through-connection may be associated with the first picture element and a further through-connection may be associated with the second picture element. In some embodiments, both of these through-connections may be arranged between the projected display areas of the first and second picture elements.

In a further embodiment, the at least one picture element may include a third and a fourth picture element, wherein when the display area of the third and fourth picture elements is projected through the first support plate the through-connection is arranged between the projected display areas of the first, second, third and fourth picture elements. Therefore, the through-connection may be arranged at a junction between four picture elements. This junction may give a larger space for locating the through-connection since, when viewed from a viewing side of the picture element, and when compared with locating the through-connection between the projected display areas of two adjacent picture elements, the through-connection is not constrained between the sides of the display areas of the two picture elements. Instead, in this embodiment, the through-connection is less constrained by four picture element corners, one for each of the four picture elements; where the dimensions of the through-connection are fixed, being determined by the manufacturing technology, arranging the through-connection between the projected display areas of the first, second, third and fourth picture elements gives greater freedom of design for arranging the through-connection outside of the projected display area, without affecting the display effect of the display area. This is illustrated inFIG. 5.

In further embodiments, the at least one picture element may comprise a border area outside of the display area, wherein when the border area is projected through the first support plate at least part of the through-connection is arranged in the projected border area. In such embodiments, the at least one picture element comprises a display area and a border area outside the display area, which border area may be adjacent a side of the display area. The border area may surround the display area of a picture element. In a display device comprising a matrix of picture elements, each picture element may lie adjacent another picture element, with an outer side of each of their border areas adjoining each other. At least part of the through-connection may be arranged in the projected border area of the picture element; for example, part of the through-connection may be arranged in a projected border area of a first picture element, and the other part of the through-connection may be arranged in a projected border area of a second picture element. In embodiments comprising also a third and fourth picture element, each of the first, second, third and fourth picture elements may comprise a border area, outside of the projected display areas. Therefore, when the border area of the third and fourth picture elements is projected through the first support plate at least part of the through-connection may be arranged in the projected border areas of the third and fourth picture elements, as well as in those of the first and second picture elements. Alternatively, the whole of the through-connection may be arranged in the projected border area of one picture element, for example in a display device having only one picture element. There are many alternatives envisaged for arranging the through-connection outside the projected display area. This gives increased design freedom for arranging a through-connection in the picture element.

In a further embodiment, the border area of the at least one picture element comprises at least part of a wall arranged to confine the first fluid to the display area. The wall may be in the form of a wall structure or a wettability treatment, formed of a wall material, and may surround the display area of the at least one picture element. In some embodiments, the wall may separate the display areas of adjacent picture elements. Thus, at least part, for example half, of the wall may lie in the border area of one picture element and at least part, for example half, of the wall may lie in the border area of an adjacent picture element. Alternatively, the whole of the wall may be arranged in the border area of one picture element. The wall material may be selected so as not to contribute to or interfere with the display effect.

In further embodiments, at least part of the wall is arranged to partially or fully cover the through-connection. The through-connection may therefore be arranged at least partly under a wall of the picture element. In some embodiments, a wall may be formed by filling a depression caused by an underlying through-connection with wall material. This can reduce or eliminate undesirable surface topography in close proximity to the display area, compared with other configurations of forming a wall on a surface of the first support plate. Therefore, surface irregularities which may affect fluid motion can be avoided, and the controllability of the fluid motion in the display area may be improved.

In yet further embodiments, the second electrical element is an electrode for applying an electric field in the at least one picture element and the first electrical element is arranged to provide a voltage signal to the electrode, via the through-connection. The first electrical element may be part of, or may be connected to, any of a transistor, a capacitor or an electrically conducting layer or line, such as a control line. Such electronic components are commonly used in active matrix displays, and can aid display performance owing to their compact size.

In further embodiments, the through-connection is arranged such that when a voltage is applied to the second electrical element a fringe field is outside the display area. Thus, when a voltage is applied to the second electrical element, a configuration of the first and second fluids may not be undesirably affected by any fringe field from the through-connection.

In a further embodiment, the through-connection is arranged through at least one layer of the first support plate the at least one layer separating the first electrical element and the second electrical element. The through-connection may therefore enable connection of the first and second electrical elements through other functional layers of the first support plate.

In accordance with further embodiments, there is provided a first support plate for an electrowetting display device comprising at least one picture element having a display area, the first support plate comprising a first electrical element, a second electrical element and a through-connection electrically connecting the first electrical element to the second electrical element, wherein the through-connection is arranged such that when the first support plate is assembled in the electrowetting display device and the display area is projected through the first support plate the through-connection is outside the projected display area. The first support plate may for example be provided pre-assembled or partly assembled for a manufacturing process of the display device of embodiments. Therefore, the first support plate may be provided, for example, with all components composing the circuitry of an active matrix and the arrangement of through-connections for a matrix of picture elements, each through-connection connecting the corresponding first and second electrical element and each through-connection arranged to be outside of the projected display area of the picture element when in the assembled display device. With the provision of such a support plate, subsequent stages of assembling a complete electrowetting display device, i.e. step-wise processing of the support plate, may be simplified. Such steps may include for example forming walls at predetermined locations, applying the first and second fluids, providing the second support plate and sealing the first and second support plates together.

In certain embodiments, wall material at least partly covers the through-connection. The first support plate may therefore be arranged to confine the first fluid within the display area of the picture element of a display device, when fully assembled. In examples, where the wall material partly or completely covers the through-connection, unwanted surface topography near or in the display area may be reduced or eliminated, thus effecting controllable fluid motion and improving optical performance of the display device.

Embodiments will now be described in detail.

FIG. 1shows a diagrammatic cross-section of part of an electrowetting display device1. The display device includes at least one picture element2, also known as a pixel, one of which is shown in the Figure. The lateral extent of the picture element is indicated in the Figure by two dashed lines3,4. The electrowetting display device comprises a first support plate5and a second support plate6. The support plates may be separate parts of each picture element, and the support plates may be shared in common by the plurality of picture elements. The support plates may include a glass or polymer substrate7and may be rigid or flexible. The display device has a viewing side8on which an image or display formed by the display device can be viewed and a rear side9. In the Figure the first support plate5faces the rear side; the second support plate6faces the viewing side; alternatively, the first support plate may face the viewing side.

The display device may be of the reflective, transmissive or transflective type. The display device may be of a segmented display type in which the image may be built up of segments, each segment may include one or more picture elements. The display device may be an active matrix driven display type or a passively driven display device. The plurality of picture elements may be monochrome. Alternatively, for a full color display device, the picture elements may be sub-pixels, each sub-pixel having a different color; alternatively, an individual picture element may be able to show different colors.

A space10between the support plates is filled with two fluids: a first fluid11and a second fluid12, wherein the fluids may, for example, be liquids. The second fluid is immiscible with the first fluid. The second fluid is electrically conductive or polar, and may be, for example, water or a salt solution such as a solution of potassium chloride in a mixture of water and ethyl alcohol. The second fluid may be transparent, but may be colored, white, absorbing or reflecting. The first fluid is electrically non-conductive and may for instance be an alkane like hexadecane or (silicone) oil. The first fluid absorbs at least a part of the optical spectrum. The first fluid may be transmissive for a part of the optical spectrum, forming a color filter. For this purpose the first fluid may be colored by addition of pigment particles or dye. Alternatively, the first fluid may be black, i.e. absorb substantially all parts of the optical spectrum, or reflecting. A reflective layer may reflect the entire visible spectrum, making the layer appear white, or part of it, making it have a color.

A hydrophobic layer13is arranged on the support plate5, the former may be transparent or reflective. The hydrophobic layer may be an uninterrupted layer extending over a plurality of picture elements2or it may be an interrupted layer, each part extending only over a plurality of picture elements2, as shown in the Figure. The layer may be for instance an amorphous fluoropolymer layer such as AF1600 or another low surface energy polymer. The hydrophobic character causes the first fluid to adhere preferentially to the first support plate5, since the first fluid has a higher wettability with respect to the surface of the hydrophobic layer13than the second fluid. Wettability relates to the relative affinity of a fluid for the surface of a solid.

The first support plate includes a first electrical element and a second electrical element. The second electrical element is for example an electrode14arranged in the support plate5. The electrode14is separated from the fluids by an electrically insulating cover layer, which may be the hydrophobic layer13. Further layers may be arranged between the hydrophobic layer and the electrode. The electrode14can be of any desired shape or form and in this example is planar. The electrode14is supplied with voltage signals by a signal line15, which may be part of a first electrical element arranged in the first support plate5. A second signal line16is connected to an electrode which is in contact with the conductive second fluid12. This electrode may be common to all elements, when they are fluidly interconnected by and share the second fluid, uninterrupted by walls. The picture element2can be controlled by a voltage V applied between the signal lines15and16. The electrodes14of the support plate5each are coupled to a display driving system. In a display device having the elements arranged in a matrix form, the electrodes can be coupled to a matrix of printed control lines on the first support plate.

The first fluid11is confined to one picture element by walls17that follow the extent of the picture element. Although the walls are shown as structures protruding from the first support plate5, they may instead be a surface layer on the first support plate that repels the first fluid, such as a hydrophilic layer. The walls may extend from the first to the second support plate but may also extend partly from the first support plate to the second support plate. The extent of the picture element, indicated by the dashed lines3and4, is defined by the center of the walls17. The area of the hydrophobic layer16between the walls of a picture element, indicated by the dashed lines18and19, is a display area20over which a display effect occurs. The display area20lies in the plane of a surface23of the hydrophobic layer13.

When no voltage is applied between the electrodes, the first fluid11forms a layer between the walls17, as shown in the Figure. Application of a voltage to the electrode14will contract the first fluid, for example against a wall as shown by the dashed shape21in the Figure. The shape of the first fluid is controllable by controlling the applied voltage, and is used to operate the picture element as a light valve, providing a display effect over the display area20. Further details of features of the display device are described in International patent publication no. WO2003/071346, the contents of which is incorporated herein by way of reference.

FIG. 2shows a schematic view along the line A-A inFIG. 1.FIG. 2shows five adjacent picture elements of the display device1, in a plan view of the first support plate. The picture elements are arranged in a matrix. For clarity, not all picture elements of the matrix are shown; instead, an arbitrary selection of five picture elements is illustrated. A plurality of display areas20, each being the display area of a picture element, together form a display area22of the display on which an image is created. The walls17are arranged in the areas around the display areas20of the picture elements. The dashed line25indicates the extent of each picture element. Parts of the dashed line25therefore correspond to the dashed lines3and4defining the extent of the picture element inFIG. 1. Line26indicates the border of the display area20. Parts of line26therefore correspond with the dashed lines18,19inFIG. 1.

FIG. 3shows schematically a plan view of part of the first support plate of an embodiment, taken in the same plane asFIG. 2. Features described previously are referred to using the same reference numerals, incremented by 100; corresponding descriptions should be taken to apply here also. For explanation purposes, a first picture element and a second picture element are referred to below. Features of these two picture elements are similar and are referred to using the same reference numerals, incremented by a prime character, i.e. ′, for the second picture element.

The surface of a first support plate105comprises a plurality of display areas120, each display area being part of a picture element which is similar to that described above, except that for at least one of the picture elements the first support plate comprises a through-connection28, arranged outside a projection of the display area120, which electrically connects a first electrical element with a second electrical element, as will be described in more detail below. At least one of the picture elements, and in this embodiment each picture element, comprises a border area30outside of the display area120, and at least part of the through-connection28is arranged in a projection of the border area. The border area30of a first picture element102adjoins the border area30′ of at least a second picture element102′ which neighbors, i.e. is adjacent to, the first picture element. In this embodiment the border area of each picture element is square and surrounds the display area of that picture element. In other embodiments, the display area may be differently shaped, as may be the border area, for example to surround the display area. Referring toFIG. 1, the border area30may be considered to correspond to an area having an extent between dashed lines18and3, and19and4. The border area of at least one of the picture elements, and in this embodiment each of the picture elements, comprises at least part of a wall arranged to confine the first fluid to the display area of each picture element. In this embodiment, the wall fills the border area of each picture element, such that one wall covers the border area at one side of the first picture element and the border area at one side of the second picture element. In other embodiments, only part of the border area of each picture element may comprise at least part of a wall. Such walls are similar to the walls described previously in relation toFIG. 1; further details will be explained later.

Referring toFIG. 3, the through-connection28is arranged between a projected display area of the first picture element102and a projected display area of the second picture element102′. Further, at least part of the wall is arranged to partially or to fully cover the through-connection28. In this embodiment the wall fully covers the through-connection28. A dashed circular line represents the position of the through-connection28beneath the walls located in the border area30. In this embodiment, the through-connection28for the first picture element102is arranged on the opposite side of its display area120from the through-connection28′ for the second picture element102′. Similarly, the through-connection for a third picture element (not indicated) would be arranged on an opposite side of the display area120′ for the second picture element102′. Each through-connection is positioned at the midpoint of a side of the associated display area, as illustrated, but in other embodiments may be offset from the midpoint. The through-connections of picture elements in other rows of the picture element matrix may be arranged similarly.

FIG. 4shows a further embodiment. Features described previously are referred to using the same reference numerals, incremented by 200; corresponding descriptions should be taken to apply here also. In this embodiment, part of the first support plate205is illustrated and the through-connections228are positioned in an alternative arrangement in the border area230. This arrangement is similar to that ofFIG. 2, however, each picture element202comprises two second electrical elements, for example electrodes (not indicated) for applying a voltage over the display area, and one or two first electrical elements, for example control lines. Each picture element therefore is associated with two through-connections: a first through-connection228aconnecting one of the electrodes and one of the control lines, and a second through-connection228bconnecting the other of the electrodes to the same or a second control line. Both through-connections228a,228bare arranged in the projected border area230, outside of the projected display area220. In this example, both through-connections for one picture element are arranged at one side of the display area220, between the display areas of two adjacent picture elements. In other embodiments the two through-connections may be arranged at different sides of the display area. An example of a two electrode picture element is described in International patent publication no. WO2008/026179 or WO2004/104671, the contents of each of which is incorporated herein. In one example, one electrode drives the picture element and the other electrode may be used for collecting the first fluid, when contracted, or a voltage may be applied to the other electrode to achieve fast switching from the contracted state of the first fluid to a state when the first fluid is not contracted and adjoins the hydrophobic layer.

FIG. 5shows an alternative embodiment. Features described previously are referred to using the same reference numerals, incremented by 300; corresponding descriptions should be taken to apply here also. For explanation purposes, a first, second, third and fourth picture element are referred to below. Features of these four picture elements are similar and are referred to using the same reference numerals, incremented by a prime character, i.e. ′, ″ and ′″ for the second, third and fourth picture elements, respectively.

In this embodiment, there are at least four picture elements, namely a first, second, third and fourth picture element302,302′,302″ and302′″ which are arranged adjacent each other, as illustrated in the Figure. There is a junction32where the border areas330,330′,330″,330′″ of the first, second, third and fourth picture elements meet. The through-connection328for one picture element, for example the first picture element, is arranged outside the projected display areas of the four picture elements, as viewed inFIG. 5, and at the junction32, i.e. between the projected display areas first, second, third and fourth picture elements. Therefore, at least part of the through-connection328is arranged in the projected border area of each of the first, second, third and fourth picture elements. Similarly, the through-connections for further picture elements in the matrix of picture elements are each arranged at a junction between four of the picture elements.

FIG. 6shows a cross-section of part of a reflective electrowetting display, according to an embodiment. This cross-section may be of the embodiment illustrated inFIG. 3, taken in a plane perpendicular that of theFIG. 3and which bisects one of the through-connections. Features described previously are referred to using the same reference numerals, incremented by 400; corresponding descriptions should be taken to apply here also.

A stack of layers of insulating and conducting layers forming electronic components of a circuit of the display device is arranged on a substrate (not indicated) of the first support plate37, the substrate being made of, for example, glass or plastic. An example of such a circuit is described in International patent publication no. WO2009/071694, the contents of which is incorporated herein by way of reference. Although the example therein describes two transistors in a picture element, an alternative single transistor design, as illustrated inFIG. 6, would be readily understood by the skilled person.

A first electrical element may be an electronic component, a part of an electronic component, or connected to an electrical component, such as a thin film transistor (TFT)34, a capacitor36or metal or insulating layers thereof, or a control line, formed in the first support plate37. A through-connection428electrically connects the first electrical element38explained in more detail below with a second electrical element which is for example an electrode40, similar to the electrode14ofFIG. 1, for applying an electric field in the picture element. The first electrical element is arranged to provide a voltage signal to the electrode40, via the through-connection, i.e. by passing current through the through-connection. The electrode40is electrically insulated from the corresponding electrode42of an adjacent picture element by insulating material of the insulating layer60and the barrier layer68described below, so the voltage signal applied to the second electrical element does not interfere with the adjacent picture element display effect.

The first electrical element is for example connected to the drain45of the transistor34in order to control the voltage on electrode40, thereby influencing the display effect created in the display area of the electrowetting picture element. The electrode40is made of an electrically conductive material. The transistor34comprises a source46, gate48and drain45, which are arranged in the first support plate37. It will be readily appreciated by the skilled person that the source46of the transistor is connected to a source line (not shown) which may lie in a plane perpendicular to the cross-section ofFIG. 3. The gate48is covered by an insulating layer56, e.g., made of silicon nitrite, SiNx, typically having a thickness between 150 nm and 300 nm. The drain45of the TFT34is connected to the first electrical element layer38which also forms the top plate of storage capacitor36. The transistor34has an active area52overlaying the gate48. The storage capacitor36has a lower electrode54. The active area52is arranged between an insulating layer56and the source46and drain45of the transistor34. The active layer of the thin film transistor, commonly used in display devices, may be made of, for example, an amorphous silicon layer or Low Temperature Poly-silicon (LTPS).

The transistor34and the storage capacitor36are covered by an insulating layer58, made of e.g. SiNx or SiOx, which may have a thickness between 200 and 800 nm. The insulating layer58is covered by a further layer60which may be made of an organic material, such as epoxy or acrylate polymer having a thickness of about 2 to 4 micrometer. The electrode40is arranged on the further layer60.

In the reflective electrowetting display ofFIG. 6, a hydrophobic layer62, made of e.g. AF1600, having a thickness between 200 nm and 1000 nm, may be arranged on a barrier layer68having a thickness between 100 nm and 150 nm and arranged between a reflective layer64described below and the hydrophobic layer62. The barrier layer may be made of an inorganic material like silicon oxide or silicon nitride or a stack of these (silicon oxide-silicon nitride-silicon oxide) or an organic material like polyimide or parylene. Moreover, the barrier layer68may comprise multiple layers having different dielectric constants. The reflective layer64for example made of Aluminum (Al), and a layer66such as Molybdenum (Mo) are each applied over the electrode40made of for example indium-tin-oxide (ITO), and therefore Mo does not significantly contribute to reflection in the display. The reflective and Mo layers64and66do not continue uninterrupted into adjacent picture elements, and their extent ends under the wall70, in the depression of the through-connection428. This arrangement avoids electrical connection between adjacent picture elements. Direct contact of Al with the electrode40should be avoided as this can lead to unwanted electrochemical reactions; therefore, the layer of Mo is deposited between the Al layer64and electrode40. When the display device is of a reflective type, the electrode may alternatively be made of separate layers of Al or Neodymium (Nd) or an alloy of Al and Nd or separate layers of Al or Mo or an alloy of Al and Mo, with a thickness between 100 nm and 150 nm.

The display area420has a lateral extent as illustrated usingFIG. 1.FIG. 6illustrates the right hand side of a picture element of an embodiment, and therefore shows only the right hand extent404of the picture element, and the right hand extent419of the display area420. It is to be understood that the left hand extent of the picture element and display area in this embodiment are similar to those described usingFIG. 1. A surface423of the hydrophobic layer62defines the plane of the display area420. As explained previously, for clarity of description, a projected display area and a projected border area is defined. The projected display area corresponds with a part of the first support plate having the extent of the display area, projected through the first support plate in a direction D perpendicular the plane of the display area, by a distance73taken between the surface423and in this example the rear side409. The projected border area corresponds with a part of the first support plate having the extent of the border area430, which lies in the same plane as the display area420for example, and is projected through the first support plate in the direction D by distance73. The border area430′ of the adjacent picture element may similarly be projected by distance73.

The electrode40is connected to the first electrical element layer38by the through-connection428through the insulating layers58and60of the first support plate, which layers separate the first and second electrical elements. Therefore, there is a height difference between the first and second electrical elements. The through-connection is a local depression of the electrode40, such that it contacts the layer38of the first electrical element. The layers applied on the electrode40, namely the Mo layer66, the Al layer64, the barrier layer68and the hydrophobic layer62, are similarly depressed at the position of the through-connection428. Walls70, similar to those described inFIG. 1, are formed on the hydrophobic layer62to at least partly, in this embodiment completely, cover the through-connection428with wall material, filling the depression caused by the through-connection.

When a voltage is applied to the second electrode40, via the through-connection428, there may be an electric fringe field. The fringe field may be caused for example by a point74where the second electrode40starts to depress to form the through-connection. The change in shape of the electrode at the point facing the fluids may cause the fringe field. By arranging the through-connection outside the projected display area, the effects of the fringe field at the display area, and therefore the effect on the configuration of first and second fluids, is reduced or eliminated. For the fringe field to be outside of the display area, in further embodiments, the through-connection may be arranged further away from the display area than illustrated. For example, the extent of the display area may be less, having thicker walls, and/or the depth of the through-connection in the first support plate may be greater.

In a further embodiment the display device may be a transmissive display device, similar to that described usingFIG. 6, but without the reflective layer.

The manufacture of an electrowetting display device is now described with reference to the structure described inFIG. 6.

During manufacture of such an electrowetting display device, layers of different materials may be arranged on a substrate36of e.g. plastic or glass. The first support plate37is provided at least partly pre-assembled with the first electrical element, in this embodiment the layer38connected to the drain45. The through-connection428is formed in the first support plate and connects the first electrical element with the second electrical element, such as electrode40, for each picture element. The at least partly pre-assembled first support plate provided is for a display device comprising at least one picture element each having a display area, the through-connection for each picture element being arranged outside the projected display area of the corresponding picture element. The Mo and the reflective Al layers66,64may be applied over the electrode40using Physical Vapor Deposition (PVD), such as sputter deposition or electron beam evaporation in which temperature, pressure and power are the main deposition parameters to control the deposition of the layer. Such deposition processes are well known to the skilled person.

Subsequently, an insulating layer68is arranged over the reflective layer; the insulating layer68is a barrier layer such as Si—O which may be applied using Chemical Vapor Deposition (CVD) or PVD. The main process parameters for PVD are as described above, while CVD requires consideration of at least the combination of deposition gases and flow rates thereof, temperature and pressure in the deposition chamber, which parameters are well known to the skilled person

In a next step the hydrophobic layer62, which may be an amorphous fluoropolymer layer such as AF1600, is applied on the barrier layer68, for example by a wet coating process, such as flexoprinting, spin coating, slit coating or dip coating which are well known to the skilled person.

Part of the surface of the hydrophobic layer62may be prepared for application of the wall70by a process step reducing the hydrophobicity of the surface. This step may include reactive ion etching and/or a plasma treatment. The part of the surface423inside the display area of the picture element may be screened off from this step. The walls may be arranged on the hydrophobic layer62to cover the through-connection428, and filling the depression, using a known method, which may involve spinning the wall material, e.g. SU-8, onto the surface72, prebaking the layer of wall material, patterning the layer using photo-lithography and removing the wall material from the display area. In this embodiment, when forming walls outside the display area, the through-connections are completely filled with wall material.

The resulting first support plate is filled with the first and second fluids and subsequently the first and second plates are appropriately coupled and sealed. For example, the first fluid is applied to the first support plate using a known method, as disclosed for example in International patent publication no. WO2005/098797. The first support plate and second support plate are mounted together using e.g. pressure-sensitive adhesive, after the space has been filled with the second fluid in a known manner, as disclosed e.g. in International patent publication no. WO2009/065909.

The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. For example, a through-connection for a picture element may be arranged outside the projected display area but with an alternative positioning from those described above.

Further, the shape of the through-connection may be different from that described above, for example the circular shape illustrated inFIG. 3. Such different shapes can include, for instance, an elongated shape as viewed in the plane ofFIG. 3, lying within and along a side of the border area. Such an elongated shape may facilitate positioning of the wall of the picture element, such that it fully covers the through-connection.

In another envisaged embodiment, the wall shape may be chosen to deviate from a straight sided shape. For instance, when viewed in the plane of FIG.3, the wall may be thicker at the position covering the underlying through-connection, to ensure that the entire through-connection is covered by the wall. In this way, an effect of a fringe field in the display area may be eliminated, for example, whilst increasing the size of the display area at thinner parts of the wall which do not cover the through-connection.

In embodiments described above, the walls may be transparent. In such embodiments, where the reflective layer is underneath the wall, the contrast-ratio of the display may be reduced. To reduce or avoid this effect, a part of the reflective layer that is situated underneath the wall could instead be transparent. In another embodiment, the part of the reflective layer underneath the wall can be designed in such a way that the area of that part underneath the wall is minimized, for instance by only using a thin strip of reflective layer.

Further, as in embodiments described above, the second electrical element is within the extent of a picture element. In other embodiments, a picture element may comprise a part of the second electrical element, which part may be an electrode corresponding with a display area of the picture element; another part of the second electrical element may be outside that picture element, which other part may for example be a connector connecting the electrode part to the through-connection. Further, the first electrical element may be at least partly within a picture element, or outside that picture element. The through-connection may be at least partly within a picture element as in embodiments above, or may be entirely within or outside a picture element it is associated with. The skilled person would readily understand possible configurations of the first electrical element, the second electrical element and the through-connection, within the scope of the claims.