WO2009106546A1

WO2009106546A1 - Fluid dispensing method

Info

Publication number: WO2009106546A1
Application number: PCT/EP2009/052232
Authority: WO
Inventors: Robert A. Hayes
Original assignee: Liquavista B.V.
Priority date: 2008-02-27
Filing date: 2009-02-25
Publication date: 2009-09-03
Also published as: GB0803585D0; KR101596059B1; TWI499853B; TW200947093A; CN101960356A; CN101960356B; KR20100134616A

Abstract

The present invention relates to a method of providing a first fluid on a surface, for manufacturing an electrowetting device. The method includes providing the surface; providing a volume of a second fluid immiscible with the first fluid; providing a dispenser for dispensing the first fluid; and dispensing the first fluid such that the first fluid moves through the second fluid onto the surface in a direction opposite to a direction of a gravitational force. The invention further relates to α method including placing a first fluid in contact with a surface whilst providing the surface in a position located relative to the first fluid in a direction opposite to a direction of a gravitational force. The present invention relates also to apparatus arranged to provide such methods, and to an electrowetting device.

Description

Fluid Dispensing Method

Field of the Invention

The present invention relates to a method of providing a first fluid on a surface, particularly for manufacturing an electro wetting display device.

Background of the Invention

International application WO 2005/098797 discloses a method of providing an oil layer on a surface of a substrate, particularly suitable for making electrowetting displays. The surface is initially covered by a layer of water. A dispenser has its opening in the water layer and above the surface. Oil is fed into the dispenser and a drop of oil is formed between the opening and the surface. The surface includes first hydrophobic areas surrounded by second hydrophilic areas. When the dispenser moves over the surface, the drop of oil is dragged over the first and second areas and replaces the water on the first areas by a layer of oil and leaves the water on the second areas.

It is an object of the present invention to provide an alternative method of providing an oil layer on a surface.

Summary of the Invention

The object of the present invention is achieved by a method of providing a first fluid on a surface, for manufacturing an electrowetting device, the method including: a) providing the surface; b) providing a volume of a second fluid immiscible with the first fluid; c) providing a dispenser for dispensing the first fluid; and d) dispensing the first fluid such that the first fluid moves through the second fluid onto the surface in a direction opposite to a direction of a gravitational force. With the gravitational force, being for example a downward direction of force, and the dispenser being arranged below the surface, the second fluid moves towards the surface once dispensed, to touch the surface. Advantageously, the dispenser can therefore be positioned remotely from the surface, since the dispensed first fluid moves towards the surface of its own accord. Thus, careful positioning of a dispensing needle to bring the needle into close proximity of the surface, perhaps within a small area, as known from the prior art, may thus be obviated. Accordingly, the dispenser may be of a simple construction. Further, by dispensing the first fluid in the opposite direction to the gravitational force, with the surface positioned accordingly, the surface helps to stabilise a volume of the first fluid provided to it, thus providing accurate dispensing onto the surface. Thus, the method of the present invention is simple and efficient.

Preferably, the first fluid has a lower density than said second fluid. When the first fluid is dispensed, it will therefore tend to rise through the second fluid towards the surface without the need to bring dispensing apparatus within close proximity of the surface to apply the first fluid to the surface. In electrowetting devices the first and second fluids may be provided with equal densities. However, when manufacturing such devices in bulk, providing first and second fluids with exactly matched densities at all operating temperatures may be difficult to achieve. The method of the present invention advantageously may utilise a density difference between the fluids to provide the first fluid on the surface in a simple manner. Moreover, since the fluid is provided on the surface utilising inherent properties of the fluids, complex dispensing apparatus is not required. Plus, accurate density matching of different fluids is not required, providing greater tolerances for manufacture.

In further embodiments of the invention, the dispenser is arranged to dispense a mixture of the first fluid and further of the second fluid, the method including dispensing the mixture in step d). The mixture may be an emulsion of the first and second fluids. The surface may have a greater wettability for the first fluid than the second fluid, meaning that when the first fluid is in a suitable proximity of the surface, possibly touching the surface, it may separate out from the second fluid to form a layer of first fluid on the surface; thus simply coating the surface with the first fluid using the different wetting behaviour of the fluids with respect to the surface. For example, the surface may be a hydrophobic surface, the first fluid non-polar and the second fluid polar or conductive.

In other embodiments of the invention, the first fluid has a density substantially equal the density of the second fluid. Substantially equal means that the first and second fluids have densities such that neither the first or second fluid would tend to rise in a mixture of the first and second fluids. Equal densities of the first and second fluids may be obtained by using additives in the fluids to tune their densities. An electrowetting device including first and second fluids of equal densities may have advantageous performance. For example, the display if shaken may resist disruption of configurations of the first and second fluids. With the first and second fluids having substantially equal densities, the method of the present invention allows the first fluid to be provided on the surface easily. For example, in preferred embodiments, the dispenser may be arranged to dispense the first fluid with a velocity for passing through the second fluid to the surface. Therefore, by ejecting the first fluid from the dispenser with an appropriate force, the first fluid may reach the surface by overcoming a tendency of the first fluid to remain at a fixed position in the second fluid, due to the density matching.

Advantageously, the monitoring apparatus and the dispenser may be arranged on opposite sides of the surface, to avoid any distortion of the monitoring caused by effects of a meniscus between the first and second fluids which occur when monitoring from the same side as dispensing the first fluid. Further, monitoring from the same side as dispensing makes it difficult to observe the point of applying the first fluid on the surface, which is desirable for effective monitoring. Accordingly, with the dispenser arranged below the surface, the monitoring apparatus is arranged above the surface. With the surface being transparent, monitoring can be conducted by observing through the surface provision of the first fluid on the surface. Moreover, with the monitoring apparatus being arranged above the surface, there is more freedom to move the apparatus three dimensionally without hindrance, than if the apparatus was positioned below the surface, and especially if the apparatus was positioned between the surface and part of the manufacturing apparatus, such as a base of the bath of the second fluid described later.

In preferred embodiments of the invention, the dispenser dispenses the first fluid with the assistance of a further fluid immiscible with the first fluid. The further fluid may be air and allows a layer of the first fluid to be applied on the surface in an effective manner. In further embodiments of the invention, the method includes providing a third fluid on a second surface, the method including: e) providing the second surface; f) providing a further dispenser for dispensing the third fluid; and g) dispensing the third fluid such that the third fluid moves through the second fluid onto the second surface in a direction opposite to the direction of the gravitational force. Advantageously, the method of the invention can be used for manufacturing electrowetting display devices having two or more switchable display layers. By using the method of the invention to manufacture at least one display layer, two separately assembled display layers may be bonded together without needing to flip one of the display layers over, which would be an awkward and inefficient manufacturing step.

In preferred embodiments of the invention, the method includes monitoring formation of a layer of first fluid on the surface using monitoring apparatus, and controlling the dispensing of the first fluid to provide the layer with a predetermined thickness. This allows providing of the first fluid on the surface to be conducted in a controlled manner.

In preferred embodiments of the invention, the method of the invention is adapted for a roll-to-roll process for manufacturing the electrowetting device. Roll-to-roll processes may be used for manufacturing electrowetting devices simply and efficiently, in bulk quantities and at low cost. The nature of roll-to- roll processing, otherwise known as web processing, means that device assembly steps may be performed continuously, without needing to slow or stop the process to perform steps. Electro wetting devices may be assembled using laminar and/or flexible materials which are well suited for roll-to-roll processing. Thus, the method of the present invention is fully compatible for roll-to-roll processes, enabling electrowetting devices to be manufactured in an efficient manner.

Further aspects of the present invention are defined in the appended claims.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings Figure 1 shows schematically an electrowetting display device manufactured using the method of the present invention;

Figure 2 shows schematically apparatus for providing a roll-to-roll method of manufacturing an electrowetting device using the method of the present invention; Figures 3 and 4 show schematically a dispensing method in accordance with an embodiment of the present invention;

Figures 5 a, 5b and 5 c show schematically a dispensing method according to a further embodiment of the present invention;

Figure 6 shows schematically an alternative electrowetting display device manufactured using the method of the present invention.

Detailed Description of the Invention

Figure 1 shows schematically a cross-section of a series of electrowetting elements of an electrowetting display device made using the dispensing method of the present invention. The series of elements is shown inverted compared with the orientation of a surface on which a layer of first fluid is formed, explained later. A first substrate 2 is provided with electrodes 4, deposited as a thin-film conductor on the substrate. Each electrode is connected to a signal line 6 for providing a voltage. The electrodes are covered by a thin hydrophobic layer 8 of an amorphous fluoropolymer, for example AF 1600. A pattern of a thin hydrophilic layer 10 of e.g. SU8 divides the surface of the substrate in hydrophobic first areas 12 between the hydrophilic second areas 10. The hydrophilic layer is of a material which is less hydrophobic, i.e. more hydrophilic than the thin hydrophobic layer 8. Therefore, whilst the term hydrophilic is used herein, the hydrophilic layer may exhibit hydrophobic characteristics. In other words, the hydrophobic layer is more wettable for the first fluid and the hydrophilic layer is more wettable for the second fluid. In this example, the size of the first areas is approximately 20 to 500 micrometres square, for example 160 micrometres square, and the second areas have a width of approximately 1 to 50 micrometres, for example 10 micrometres and a height of approximately 3 to 6 micrometres. The first substrate 2, provided on top with the electrodes, hydrophobic and hydrophilic layers 4, 8 and 10, is subjected to the dispensing method according to the present invention using oil as the first fluid, water as the second fluid and air as the further fluid. Another combination of fluids may be used in further embodiments. The dispensing method will be explained in detail below. After carrying out the dispensing method the first areas 12 are uniformly covered by an oil layer 14 having a thickness of between 3 and 6 micrometres, for example 5 micrometres. The thickness of the oil layer is determined at least in part by the height of the second areas, since the height determines a position on the second areas at which an interface between the first and second fluids is pinned. The second areas 10 and the oil layer 14 are covered by water 16. The water may contain salt or other additives to increase its electrical conductivity and/or to enlarge the temperature window for device operation. The second fluid, being water in this example, used during the dispensing method is preferably the same fluid used in the product that includes the substrate, which avoids changing the second fluid with another fluid after the execution of the dispensing method. A second substrate 18 forms a closed space between the first and second substrate. A seal, not illustrated in Figure 1, is provided between the first and second substrates 2, 18 to separate the first and second substrates 2, 18 from each other, and to form the closed space. Such a seal may be provided around a perimeter of the display device. The seal therefore determines a distance between the first and second substrates, and defines a display area of the device.

The pattern of the hydrophilic layer 10 defines elements on the substrate to which the oil layer 14 is confined. Each element has an electrode 4. Another electrode 17, connected to a signal line, is in contact with the water 16, forming a common electrode for a plurality of elements. When a voltage is applied between the common electrode 17 and the electrode 4 of an element, the oil layer 14 in that element moves to the side of an element or breaks up and the first surface will at least partly be covered by the water 16. This so-called electrowetting effect is more fully described in international patent application no. WO03/071346. When the oil and / or the water has specific optical properties for absorption, reflection and / or transmission of light, the element can operate as a light valve in e.g. a display.

The electrowetting elements may be used in a display apparatus, in which a plurality of electrowetting elements forms a display device. A display driving system in the apparatus provides the voltages for setting the elements in the desired state.

Figure 2 shows schematically an apparatus for making electrowetting devices using a roll-to-roll process. The roll-to-roll process is a continuous process involving feeding, processing and assembling component layers together using a series of rollers. The process may be conducted at room temperature and atmospheric pressure. In this example, the device being assembled is the device illustrated using Figure 1.

Component layers of the device to be described hereafter are similar to parts of the device described in Figure 1, and will be referred to herein using the same reference numerals incremented by 100; corresponding descriptions should be taken to apply here also. In this example, each component layer of the device is provided on a separate roller Rl . However, in further embodiments, at least two of the component layers may be adjoined to each other, and provided on one roller. The component layers are fed around and manipulated by further rollers R2 between each step of the manufacture process. The direction of rotation of each of the rollers Rl, R2 is indicated in Figure 2 using arrows. The layers may be fed through the roll-to-roll process at a speed in the range of for example approximately 0.1 to 100 millimetres per second, and preferably approximately 0.5 to 5 millimetres per second.

Details of the component layers will now be described for device manufacture. Given the use of rollers to provide and manipulate the layers during the manufacturing process, each layer is suitably flexible and robust to avoid being damaged by the rollers. At least one of the component layers may be punched with a series of notches down each of two opposite sides of the layer, each for co-operating with a series of protrusions around the perimeter of each end of a roller which the layer passes around. Using this notch and protrusion arrangement, contact between an active area of a component layer sensitive and prone to damage and the surface of the roller may be reduced to minimise damage to the active area. A modified roller may be used in conjunction, for example so that the component layer is suspended over the roller by the notches and protrusions. Further, the rollers may be positioned appropriately to avoid sensitive areas of the layers coming into contact with the rollers. Plus, slipping of a component layer on a roller may be avoided.

The first substrate 102 and the second substrate 118 are component layers of the device and may be formed of for example polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulphone (PES), or polyimide (PI). If the display to be manufactured is a reflective display, the first substrate 102 may be a reflective substrate. The electrodes 104 are provided as a component layer, and may be formed of poly(3,4- ethylenedioxythiophene) (PEDOT) or carbon nanotubes which has been printed according to the desired pattern of the electrodes 104. Electro wetting devices such as that shown in Figure 1 operate as a capacitor. The electrode material need not therefore be highly conductive, i.e. the material may have a high ohmic resistance. Thus, there is greater design freedom to select a suitable electrode material, and/or treatment method of the material, for roll to roll processing. Such an electrode layer may for example be used for manufacturing a direct drive type of electrowetting display device. For providing a more complex electrode pattern on one of the substrates, for example for use in manufacturing an active matrix type of display device, the layered structures manufactured by for example Polymer Vision or Plastic Logic may be used. The hydrophobic layer 108 may be manufactured using a printing method, for example gravure printing in roll-to-roll processing, similar to a flexoprinting, flat-bed, technique. The component layer of the pixel walls 110 may be formed for example using a printing, embossing, lamination or lithographic technique. In the case of embossing, a pixel wall material may be provided as a sheet and passed over a roller with an embossing pattern so that the pixel wall material is stamped with the desired pixel wall pattern. Printing of pixel walls may be a screen printing method. A seal layer 20 may be provided on a roller Rl. In further embodiments, the seal layer may be combined with another component layer, for example with the pixel wall layer, as one layer. Also, the hydrophobic layer and the pixel wall layers may be provided as a combined layer, for example by embossing a hydrophobic material.

The first substrate 102, the electrode layer 104, the hydrophobic layer 108 including the hydrophobic first areas, the pixel walls layer 110 and the seal layer 20, each being a separate component layer, are fed from the rollers Rl, via further rollers R2, with the first substrate 102 being in this embodiment the uppermost layer, and are aligned and coupled to each other using aligning and coupling apparatus 22. The component layers may be manipulated by the rollers using a carrier, or the component layers may be suspended between rollers. The alignment may use a mechanical or an optical technique. A mechanical technique, for example using pins or rails, may be used for a low resolution alignment of for example +/- 0.1 millimetres. An optical technique may be required for a higher resolution alignment of for example +/- 5 micrometres, for example for pixel wall alignment with an electrode pattern, or for aligning of an optional colour filter layer in the device. Optical alignment may use marks at the edges of the layer areas being processed; these marks could for example be provided in the electrode layer, and may be removed in the final stages of device assembly.

Following alignment, the layers are coupled to each other simultaneously. Coupling may be a lamination process, which may use adhesive to bond the component layers to each other. The adhesive may be provided on the component layers as a further step in the roll-to-roll process, prior to the alignment and coupling. Or, alternatively, the component layers may be provided on the rollers Rl already with the adhesive applied, protected by a peel layer which is removed during the roll-to-roll process prior to aligning and coupling of the layers. Coupling is performed at appropriate conditions to enable effective bonding between the layers, for example at room temperature and at atmospheric pressure. Local heating and pressure may be applied to parts of the layers during coupling. The adhesive may be curable, to form a suitable bond.

After coupling, a layer of the first fluid is dispensed to form a layer of the first fluid on the surface of the hydrophobic first areas 112 using dispensing apparatus 23. For this dispensing, the coupled layers 21 are immersed in a volume of the second fluid 116 held in a bath 26 and moved over the dispensing apparatus whilst the first fluid is dispensed. The dispensing will be described in detail later.

Dispensing of the first fluid may optionally be monitored, using monitoring apparatus such as a radiation detector 28 and a radiation beam source 29. When manufacturing the device, it is important that the layer of first fluid has a suitable thickness for correct operation of the completed display. When monitoring the dispensing, a monitoring signal from the monitoring apparatus can be used to control the dispensing of the first fluid accordingly, to provide a first fluid layer with a predetermined thickness. For example, the speed of moving the surface of the hydrophobic first areas over the dispensing apparatus, or the rate of dispensing the first fluid 114 could be controlled.

Monitoring is conducted for instance by measuring an intensity of radiation passing through the first fluid layer formed on the surface of the hydrophobic first areas. The radiation beam source 29 may be arranged on the same or opposite side of the surface as the detector 28, for a reflective or a transmissive display device under manufacture, respectively, to emit radiation through the first fluid layer once dispensed on the surface. If the first fluid is coloured, the first fluid layer absorbs a proportion of the emitted radiation in dependence on the thickness of the first fluid layer. The radiation detector 28 measures the intensity of radiation after passing through the first fluid layer. If the detected intensity deviates from a predetermined amount of intensity indicative of a desired layer thickness, the dispensing is controlled accordingly. In further embodiments, the monitoring apparatus may alternatively comprise a video camera for imaging formation of the first fluid layer. Processing of the images captured may be used to control the dispensing.

It is advantageous to monitor formation of the first fluid layer from an opposite side of the surface of the hydrophobic first areas as the side the dispensing apparatus is positioned on. Therefore, since the first fluid is dispensed from underneath the surface, the monitoring would be conducted from above the surface. This is advantageous since it avoids distortion and possibly blocking of the radiation or image being monitored, which is caused by a curved meniscus between the first fluid layer and the second fluid. Whilst in alternative embodiments the monitoring may be conducted below the surface, and required movement of at least part of the monitoring apparatus in three dimensions may be hindered for example by the base of the bath 26.

After dispensing the layer of first fluid on the surface, its thickness may be levelled using a leveller, for example a scraper or a blade, positioned at a suitable distance from the surface for obtaining a predetermined first fluid layer thickness. The thickness is taken in a direction from the surface to a meniscus between the first and second fluids, in a perpendicular direction from the plane of the surface. In embodiments to be described using Figures 3 and 4, the dispensing apparatus may be adapted to form such levelling. In other embodiments such as that described using Figure 5, the leveller may be separate from the dispensing apparatus, and may level the first fluid after dispensing the first fluid onto the surface. For accurate levelling, the speed of moving the first fluid layer with respect to the leveller is controlled appropriately.

Pockets of air may get trapped on the hydrophobic surface because of the low wettability of the surface for the second fluid. Such air pockets are preferably removed during assembly of the device. This may be performed simultaneously as dispensing the first fluid, using dispensing apparatus described below using Figures 3 or 4. Alternatively, air removal may be a separate process to the dispensing, for example by passing a globule of a further fluid such as air along the hydrophobic surface before dispensing the first fluid so that trapped air pockets merge with the air globule. Sonication or dissolution of air in a fluid undersaturated with air may also be used. Separate air removal may be applied prior to dispensing when using the dispenser of Figures 5a, 5b and 5 c, described below.

Once the layer of first fluid has been dispensed, the coupled layers 21 are fed by rollers R2 to further alignment and coupling apparatus 30 for aligning and coupling with the second substrate. The second substrate 118 is fed via rollers Rl, R2 to the further alignment and coupling apparatus 30. The further alignment and coupling apparatus 30 aligns the second substrate with the coupled layers 21, and couples them together using a sealing material such as an adhesive using alignment and coupling techniques similar to those described previously for the alignment and coupling apparatus 22. The second substrate 118 and the coupled layers 21 are immersed in the second fluid 116 when they are coupled together. This facilitates simple filling of the closed space of the device being manufactured with the second fluid 116, without introducing air into the closed space. After the further aligning and coupling, the coupled layers 21, now including the second substrate 118, are withdrawn from the bath 26. To complete manufacture of the electrowetting device, various further manufacturing steps may be performed using suitable apparatus 32; for example, applying integrated circuits to the device such as components for driving operation of the device, and installing the signal lines and common electrodes, cutting the coupled layers 21 into segments to form multiple electrowetting devices, and trimming the coupled layers 21 to remove any parts of the layers used for manufacture not required for the final device, for example the notches at the sides of any of the component layers such as the substrates.

The dispensing of the first fluid will now be described using Figure 3. Features similar to those of the device described using Figure 1 will be referred to using the same reference numerals incremented by 200; corresponding descriptions should be taken to apply here also.

Figure 3 shows in cross-section an embodiment of the dispensing apparatus for providing the method of the present invention. The dispensing apparatus comprises a dispenser 34 for providing the first fluid 214 on a surface 36 of the thin hydrophobic layer and thus the hydrophobic areas of the coupled layers 21 shown in Figure 2. The coupled layers 21 may include a plurality of such surfaces, each being of one first hydrophobic area. The dispenser 34 is arranged below the surface 36 which in this embodiment is immersed in and covered with the second fluid, prior to the first fluid layer being applied to it. The dispenser 34 is in the form of a syringe needle 38, having a central channel 40, and the surface 36 is moved over the dispenser 34, which has a fixed position, in a direction 42 during the roll-to-roll process described above. This direction 42 lies in a plane of the substantially planar surface 36. The first fluid 214 is supplied through the channel 40. The needle has an opening 44, which in operation is located within the second fluid 216 and below the surface 36. A further fluid 45 is located partly within the channel 40 and partly between the opening 44 and the surface 36. The further fluid is immiscible with the first fluid and forms a globule 46 between the opening 44 and the surface 36. The globule 46 may locally adjoin the surface 36. The first fluid 214 moves as a relatively thin layer along the interface 50 between the first fluid 214 and the further fluid 45 from the syringe 38 to the surface 36, where it settles as a layer 51. A leading interface 52 between the second fluid 216 and the first fluid 214 pushes the second fluid away and replaces it by the first fluid. The thickness of the layer 51 that remains on an area of the surface after passage of the dispenser depends, among others, on the width of the opening 44, its shape, the speed of movement of the dispenser, the distance between the opening 44 and the surface 36, viscosities of the fluids and the size of the globule, amount of first and further fluid, interfacial tensions of the various interfaces, and the chemical contrast, i.e. the difference in hydrophobicity between the various combinations of fluids and the surface and the dispenser. By dispensing from below the surface, the surface helps to stabilise from above the volume of first fluid and further fluid during and after dispensing. Such an advantage applies for the further embodiments described later.

The first fluid 214 may be non-polar, for example an alkane, such as hexadecane, or an oil, such as a hydro-carbon oil, and in this embodiment is a silicone oil. The second fluid 216 may be any fluid that is non-miscible with the first fluid. The second fluid may be polar or electroconductive, which is useful in some applications of the first substrate covered with the first and second fluid. The embodiment shown uses water as second fluid. The further fluid 45 is advantageously immiscible with both the first fluid and the second fluid to stabilise the globule. The further fluid may be a gas, such as air, nitrogen or argon. This embodiment uses air as further fluid. Other immiscible fluids that can be used are a fluorocarbon and liquid metals such as mercury.

The deposition of the first fluid on the surface will be facilitated if the surface has a greater wettability for the first fluid than for the second fluid. In the embodiment shown, the surface is a hydrophobic layer, e.g. an amorphous fluoropolymer such as AF 1600. The hydrophobic layer increases the tendency of the oil to adhere to the surface and repel the water. The application of the further fluid as shown in Figure 3 can be achieved by the following subsequent steps: filling the syringe with the first fluid, pulling a quantity of air into the syringe, inserting the syringe into the layer of the second fluid 216, and pushing air out of the syringe. The size of the globule is determined by the amount of air in the syringe and the properties of the first and second fluids. The syringe may be replaced by a reservoir filled with first liquid and a pump mechanism for dispensing the desired amount of first liquid.

During the dispensing of the first fluid using the dispenser 34, the globule of air between the dispenser and the surface will merge with any trapped air pockets, thereby releasing the pocket of air from the surface of the hydrophobic area and making the entire area available for the first fluid. When the dispenser moves over the surface, the globule operates as a cleaner for trapped air pockets.

In this embodiment of the present invention and those described using Figures 4, 5a, 5b and 5c, the first fluid has a lower density than the second fluid. A difference between the density of the first fluid and the second fluid may for example be at least approximately 0.01 grams per cubic centimetre, and preferably at least approximately 0.05 grams per cubic centimetre, or greater. With an increasing difference between the density of the first fluid and the second fluid density, the tendency of the first fluid to rise through the second fluid to touch the surface increases. Therefore, a high density difference, of for example approximately 0.35 grams per cubic centimetre or greater may be preferable. The density difference however should be selected so as not to adversely affect operation of the completed device. The densities of the first and second fluids may be tuned using additives. For example, the second fluid being water may have its density adjusted using salt, and may have a density greater than 1 grams per cubic centimetre, and in some examples greater than 1.1 grams per cubic centimetre. Given that the first fluid has a lower density than the second fluid, the first fluid once dispensed moves in a direction opposite the direction of the force of gravity, towards the surface, until it touches the surface. In other words, the first fluid rises through the second fluid to reach the surface, with the surface being arranged in the volume of the second fluid. Upon touching the surface, the first fluid wets the surface to form a layer of first fluid adjoining the surface. This is aided by the hydrophobicity of the surface. Figure 4 shows dispensing apparatus, according to a further embodiment of the invention, having an elongate dispenser for providing a layer of the first fluid oil through the second fluid water onto the hydrophobic surface and assisted by the further fluid air. Features similar to those described previously will be described using the same reference numerals incremented by 300; corresponding descriptions should be taken to apply here also.

Figure 4 shows a cut through a dispenser 55 and the coupled layers 321. The dispenser may be closed at the short sides by two vertical walls. The dispenser 55 has the shape of a U with its opening 56 facing the surface 336. The opening is below the surface of the second fluid 316. The interface between the second fluid and the environment is not shown in the Figure. The dispenser has a first input in the form of a tube 58 for feeding the oil into the dispenser and a second input in the form of a tube 60 for controlling the air. A long dispenser may have two or more first inputs and/or second inputs, regularly spaced of the length of the dispenser to improve the control of the fluids. The air forms an elongate globule 62 surrounded by a layer of oil 314. The width of the opening 56 is preferably smaller than 10 mm, e.g. 2 mm. When the opening is wider than 10 mm, oil tends to escape from the opening and move up in the water. The distance of the opening above the surface is preferably smaller than 2 mm, and is 0.1 mm in a special embodiment. A small distance between the opening and the surface facilitates filling of the dispenser by the first fluid through capillary forces.

The dispenser has a hydrophobic surface on the inside walls 64 and on the wall parts 66 adjacent the opening 56. The hydrophobic character pins the oil to the dispenser. The outer walls 68 of the dispenser, which are at least partly adjoined by water, are hydrophilic to avoid contamination with oil. The dispenser may be made of PMMA, which is slightly hydrophobic. This material has the advantage of a high contact angle hysteresis, which improves the positional stability of the oil. Such a material can be either oil or water wetting by simply controlling the direction of oil/water interface movement; interfacial pinning at corners of the dispenser adds to the utility of this effect. Although the first fluid in the figure pins on the two outer edges of the wall part, it may also pin on the two inner edges of the wall part. Hysteresis refers to the difference in contact angle after advancing and receding motion of the fluid boundary.

During the deposition process the coupled layers 321 are moved in a direction 70, substantially perpendicular to the long axis of the opening 56, thereby moving a leading interface 72 between the water and the oil over the surface 336. The surface is in this embodiment one of a plurality of surfaces corresponding with a pattern of hydrophobic areas 74, each of which may be a first area or a sub-pattern of first areas and second areas. For example, each sub-pattern may be for a display device, and two or more display devices are arranged on the surface in a direction parallel to the long axis of the dispenser. Furthermore, a pattern or sub-pattern may have a shape that conveys a meaning to an observer, such as a logo. Such a sub-pattern may be combined with another sub-pattern for a display function, together providing a signage function. In a display device the pattern or sub-pattern may be switchable as a common display element or permanent, i.e. non- switchable. In another embodiment, such a pattern or sub-pattern provides a decorative effect, for example to enhance the viewer experience.

The length of the opening is substantially the same as or larger than the dimension of the pattern parallel to the long axis of the opening. The length should be at least so large that any deposition irregularities caused at the boundary of the globule 62 occur outside the pattern. The length of the opening may by substantially equal to the size of the surface in the direction perpendicular to the scan direction. The area 73 between the areas 74 is preferably hydrophilic, e.g. covered with SU8, to avoid deposition of oil in this area of the surface; similar to the hydrophilic areas described using Figure 1. In the embodiment of the apparatus shown in Figure 4, the leading interface 72 of the oil 314 runs parallel to borderlines 76 between the first areas and the second areas. Since the oil does not want to adjoin second areas, the shape of the oil below the dispenser is disturbed along the contact line from hydrophobic to hydrophilic areas. The pinning of the oil on the borderlines causes a stick-slip motion during the movement of the dispenser over the surface, which may result in a striped deposited oil layer.

Figures 5 a, 5b and 5 c show schematically an alternative dispenser for dispensing the first fluid, and the forming of a first fluid layer on a target hydrophobic surface TS of a series of such surfaces, according to a further embodiment of the invention. Only a portion of the coupled layers are illustrated for the sake of clarity. Features are similar to those described previously, and will be referred to herein and in Figures 5 a, 5b and 5 c using the same reference numerals incremented by 400; corresponding descriptions should be taken to apply here also.

The dispenser of this embodiment is a tube 80 fixed in the bath 426 of second fluid 416. The dispenser has an opening 82 facing upwards, towards the coupled layers 421 which pass over the dispenser in the direction 442 lying in the plane of the coupled layers 421, similar to passing of the coupled layers over the dispenser described using Figure 3 or 4. The dispenser is connected to a first fluid supply, not illustrated for the sake of clarity.

Referring to Figure 5 a, the target surface TS is not yet covered by a layer of the first fluid. Corresponding surfaces further ahead of the target surface TS in the direction of passing 442 are already covered by a layer of the first fluid 414. Corresponding surfaces further behind the target surface TS, i.e. in a direction opposite the direction of passing 442, are not covered by the first fluid 414. The dispenser 80 dispenses through the opening 82 first fluid 414 into the second fluid 416 in the form of droplets 84. The volume of the droplets may be controlled in dependence on the size and shape of the opening 82, and a flow rate of first fluid supplied to the dispenser 80. A distance between the dispenser and the target surface TS is predetermined, in accordance with the speed of the passing of the coupled layers 421 over the dispenser, so that droplets of first fluid rise and touch and wet the target surface TS, and each successive target surface, correctly.

In Figure 5b, the droplet 84 of first fluid has risen through the second fluid 416 to touch and begin to wet the target surface TS. A further droplet 85 is being dispensed, for wetting the surface which is to pass over the dispenser after the target surface TS. In Figure 5c, the droplet 84 has spread out to wet the entire target surface TS, forming a layer 86 on the target surface TS of first fluid. The further droplet 85 has touched and begun to wet the next surface, whilst a yet further droplet 88 has been dispensed.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. For example, alternative dispensing apparatus may be used to dispense the first fluid. Embodiments of dispensers have been described above using Figures 3 and 4. Further details of such dispensers, and of further embodiments which may be used in accordance with the present invention are described in GB patent application no. 0707201.0, the content of which is incorporated herein by reference. Further, the dispenser described above using Figure 5a may instead of dispensing the first fluid as separate droplets, dispense the first fluid as a continuous flow. In alternative embodiments, the first fluid may be placed in close proximity or directly in contact with the surface by the dispensing apparatus, for dispensing of the first fluid in the opposite direction to gravity. This contrasts with other embodiments where the dispensing apparatus dispenses first fluid remotely from the surface. The dispensing apparatus may also contact the surface during dispensing. In other embodiments, it may be desirable to cover different target surfaces with differently coloured first fluid. This may be achieved by providing a plurality of dispensers, each for example being similar to that described using Figure 5 a, for dispensing first fluid of a different colour, and controlling dispensing of the differently coloured fluids accordingly. An ink-jet type of dispenser, loaded with differently coloured first fluids, may also be used to dispense differently coloured first fluid. In further embodiments, the dispensing apparatus may be positioned outside of the second fluid and be arranged to dispense first fluid into the second fluid, the first fluid then moving towards the surface.

The embodiments described above relate to dispensing of the first fluid where the first fluid has a lower density than the second fluid. In further embodiments of the invention, the first and second fluid may have densities which are substantially equal to each other. This may be achieved by tuning the densities of the fluids using additives. In such embodiments, the dispenser may be arranged to dispense the first fluid with a velocity for passing through the second fluid to touch the surface. The force of ejecting the first fluid, for example using pressure, from the dispenser may therefore be controlled in dependence on the volume of the second fluid between the dispenser and the surface, to overcome any gravitational forces and dragging forces from the second fluid, and a preference of the first fluid to remain at a fixed position in the second fluid. In one example, the dispenser and the surface are separated from each other by a distance of less than approximately 5 millimetres, preferably 2 millimetres or less. In other envisaged embodiments, the first fluid may be more dense than the second fluid and may be applied to the surface by ejecting it under pressure, as described above, or by dispensing the first fluid in close proximity to the surface to allow the surface hydrophobicity to attract the first fluid to it.

In further embodiments of the invention, the dispenser may be arranged to dispense a mixture of the first fluid and further of the second fluid, for example as an emulsion. If the first fluid has a lower density than the second fluid, then the first fluid would rise through the emulsion to touch and wet the surface, thus separating the first fluid from the second fluid. In other embodiments, the first and second fluids may be substantially density matched. Then, the emulsion may be dispensed suitably close to the surface so that the affinity of the first fluid for wetting the hydrophobic surface, in preference to the second fluid, will attract the first fluid to wet the surface, resulting in separation of the first fluid and the second fluid from the emulsion. In the methods of manufacturing the device described above, the component layers are simultaneously coupled prior to dispensing of the first fluid. Alternatively, for the start of the roll-to-roll process, pre-coupled component layers may be provided on one roller, ready for dispensing of the first fluid thereon. The already coupled layers may be created for example by building up each layer separately on top of each other, starting with the first substrate. The underlying layer may be processed to create the adjacent layer above, for example by printing a suitable material on the underlying layer. Or, the next layer may have already been fabricated and may therefore simply be laminated to the underlying layer.

The method of manufacturing the electrowetting device using the present invention has been described for manufacturing a display device having a single switchable display layer on the first substrate. The present invention is also advantageous for manufacturing an electrowetting display device having two or more switchable display layers. An exemplary display device with two display layers is illustrated in Figure 6. Features similar to those previously described are referred to herein and in Figure 6 using the same reference numerals incremented by 500, and corresponding descriptions should be taken to apply here also. A second display layer is provided on the second substrate 518. The second display layer has a similar construction as the first display layer on the first substrate, and therefore comprises electrodes 90, a signal line 92, a hydrophobic layer 94, a hydrophilic layer 96, and a pattern of hydrophilic layer dividing the surface of the substrate into hydrophobic areas 98, which are similar to the corresponding features of the first display layer described previously. A layer of a third fluid 99 covers each hydrophobic area 98, the third fluid being immiscible with the second fluid and with similar, maybe the same properties as the first fluid. The third fluid may be switched using electrowetting forces, similarly to switching of the first fluid. Details of exemplary electrowetting displays with two or more display layers are more fully described in international patent application no. WO03/071346. To manufacture the two display layer device using the roll-to-roll process described above, the first display layer is manufactured as described above using Figure 2. The second display layer may be manufactured, separately, using any of the methods described above for manufacturing the first display layer. The first and second display layers may be provided in the same bath of second fluid, for dispensing the first and third fluids, respectively. The method of dispensing the third fluid includes providing at least one second surface, being at least one hydrophobic area 98, providing a further dispenser for dispensing the third fluid, and dispensing the third fluid so that the third fluid touches the second surface having moved through the second fluid in a direction opposite to a direction of the gravitational force..

Once the first and second display layers have been manufactured separately, they are aligned and coupled together, for example using the further alignment and coupling apparatus described previously. It is preferable to couple the first and second display layers together along a plane which bisects the second fluid rather than the first fluid, for example at a plane approximately midway between the first substrate and the second substrate. Alternatively, an oil resistant sealing material could be used to couple the first and second display layers along a plane bisecting the layer of the first or the third fluid. The sealing material may be applied prior to, or after, dispensing of the first and third fluids.

By dispensing the first and the third fluids from below the coupled layers, and then using rollers to manipulate the first and second display layers so they are coupled along an approximately vertical plane, in a similar manner as shown in Figure 2 for the one display layer and the second substrate, there is no need to flip one of the manufactured display layers over through 180°, which is an inefficient and difficult manoeuvre which could result in damage of the manufactured display.

It is envisaged that the present invention may be used advantageously to manufacture display devices having more than two switchable display layers, similar to the first and second display layers described, using a roll-to-roll process. Alternative materials and/or component layers from those described above for the manufacture of the electrowetting device may be used. For roll- to-roll processing, it is envisaged that any material or component layer which is flexible and laminar, and therefore suitable for manipulating by rollers may be used. For example, for an electrowetting display device which utilises a backlight, the backlight may be attached to the display device during the final stages of assembly, after the second substrate has been coupled to the display layer. Alternatively, a backlight layer may be provided as a component layer for coupling with the other component layers described above. Examples of such a flexible backlight layer include an organic light emitting diodes (OLED) or more conventional architectures using polymeric lightguides. Additional component layers from those described may be included in the device by roll-to- roll processing, for example colour filter layers.

The methods of manufacturing a display device, involving the method of the present invention, have so far been described in the context of a roll-to-roll process, and include adaptations for such. The present invention is not limited for use in a roll-to-roll type process, and may be adapted for other manufacturing processes. For example, an electrowetting display device could be manufactured without rollers, by applying each component layer, or a combination of the component layers, on top of the underlying layer, stepwise, or processing the underlying layer to form the layer above, and then flipping the resulting coupled layers, before application of the second substrate, so that the first fluid can be applied to the hydrophobic surface using the method of the present invention. Or alternatively, one of the switchable display layers could be manufactured using the method of the present invention, and the second display layer could be manufactured using a method of dispensing the first fluid from above.

In non-roll-to-roll processes using the present invention, the coupled layers may pass back and forth over the dispensing apparatus, rather than continuously passing in one direction over the dispensing apparatus. Indeed, this can be advantageous for levelling the layer of first fluid applied to the hydrophobic surfaces, by filling in parts of the layer thinner than desired, and smoothing thicker parts of the layer.

Methods have been described above for dispensing the first and third fluid using dispensing apparatus at a fixed position below the coupled layers passing overhead. In such embodiments the dispensing apparatus may instead be provided through an opening of the bath. In alternative embodiments, the coupled layers may be fixed, and the dispensing apparatus may pass along underneath to dispense the first and/or third fluid.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments.

Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A method of providing a first fluid on a surface, for manufacturing an electro wetting device, the method including: a) providing the surface; b) providing a volume of a second fluid immiscible with the first fluid; c) providing a dispenser for dispensing the first fluid; and d) dispensing the first fluid such that the first fluid moves through the second fluid onto the surface in a direction opposite to a direction of a gravitational force.

2. A method according to claim 1, wherein said first fluid has a lower density than said second fluid.

3. A method according to claim 2, wherein a difference between the density of the first fluid and the density of the second fluid is at least approximately 0.01 grams per cubic centimetre.

4. A method according to claim 3, wherein the difference is at least approximately 0.35 grams per cubic centimetre.

5. A method according to any preceding claim, wherein the dispenser is arranged to dispense a mixture of the first fluid and further of the second fluid, the method including dispensing the mixture in step d).

6. A method according to claim 1, wherein said first fluid has a density substantially equal to the density of the second fluid.

7. A method according to claim 6, wherein the dispenser is arranged to dispense the first fluid with a velocity for passing through the second fluid to the surface.

8. A method according to any preceding claim, wherein the surface is provided in the volume of the second fluid.

9. A method according to any preceding claim, wherein an area of the surface has a greater wettability for the first fluid than for the second fluid, the first fluid wetting the surface upon touching the surface to form a layer of first fluid adjoining the surface.

10. A method according to claim 9, wherein an area of the surface is a hydrophobic surface, the first fluid is non-polar and the second fluid is polar or conductive.

11. A method according to claim 9 or 10, including monitoring formation of the layer on the surface using monitoring apparatus, and controlling the dispensing of the first fluid to provide the layer with a predetermined thickness.

12. A method according to claim 11, wherein the monitoring apparatus and the dispenser are arranged on opposite sides of the surface.

13. A method according to any preceding claim, wherein the dispenser is arranged below the surface in a direction of the gravitational force, and the first fluid is dispensed to rise through the second fluid towards the surface.

14. A method according to any preceding claim, wherein the dispenser dispenses the first fluid with the assistance of a further fluid immiscible with the first fluid.

15. A method according to claim 14, wherein the dispenser provides the further fluid to form a globule of the further fluid between a dispensing opening of the dispenser and the surface.

16. A method according to any preceding claim, wherein the surface is substantially planar and the method includes moving the surface and/or the dispenser with respect to each other in a direction lying in the plane of the surface.

17. A method according to any preceding claim, including providing a third fluid on a second surface, the method including: e) providing the second surface; f) providing a further dispenser for dispensing the third fluid,; and g) dispensing the third fluid such that the third fluid moves through the second fluid onto the second surface in a direction opposite to the direction of the gravitational force.

18. A method according to any of claims 1 to 16, including providing a third fluid on a second surface, the method including: e) providing the second surface; f) providing a further dispenser for dispensing the third fluid,; and g) dispensing the third fluid such that the third fluid moves through the second fluid onto the second surface in the direction of the gravitational force.

19. A method according to any preceding claim, adapted for a roll-to- roll process for manufacturing the electrowetting device.

20. A method of providing a first fluid on a hydrophobic surface, for manufacturing an electrowetting device, the method including placing the first fluid in contact with the surface whilst providing the surface in a position located relative to the first fluid in a direction opposite to a direction of a gravitational force.

21. A method according to claim 20 including using a volume of a second fluid to place the first fluid in contact with the surface.

22. Apparatus arranged to provide the method according to any of the preceding claims.

23. Apparatus according to claim 22, adapted for a roll-to-roll process.

24. An electrowetting device including a closed space defined by at least a surface and a substrate, said closed space enclosing a first fluid and a second fluid immiscible with each other, the first fluid having been provided on the surface using the method of any of claims 1 to 21.