Patent Application
20070163875
The invention relates to an electrowetting cell comprising a body section provided with a substrate and at least one side wall, which substrate and side wall jointly define a cavity containing electrowetting fluids and further comprising at least one end section secured to the body section.
The invention also relates to a method of manufacturing an electrowetting cell comprising a body section provided with a substrate and a side wall, which substrate and side wall jointly define a cavity containing a first and a second electrowetting fluid.
Electrowetting cells are cells in which light is refracted by a meniscus between two immiscible fluids. One of the two fluids is electrically insulating and the other is electrically conducting. The shape of the meniscus is variable under the influence of a voltage between two electrodes, one of which is connected to the electrically conducting fluid and the other to a surface of the body. Such cells are known and can for instance be applied as lenses or displays. In the case of the application of a cell, there is a light-path through the body section.
Such an electrowetting cell is for instance known from WO-A 03/069380. This patent application discloses a lens in which an inner surface of the at least one side wall is covered by a hydrophobic fluid contact layer. When no voltage is applied, the wettability of the fluid contact layer with respect to the electrically insulating fluid differs from the wettability of the fluid contact layer with respect to the electrically conducting fluid. Under influence of the applied voltage a change of the wettability occurs. This leads to a change of a contact angle of the meniscus at a line of contact between the fluid contact layer and the fluids, whereby the shape of the meniscus is adjusted. Hence, the shape of the meniscus is dependent on the applied voltage.
As the electrowetting cell has optical properties and includes fluids, it is of primary importance for an adequate operation that a complete filling of the cell is achieved and that no fluid can leak out of the cell after closing the cell. The cell disclosed in the prior art does not give any solution how to achieve such a complete filling and a prevention of fluid leakage.
It is therefore a first object of the invention to provide an electrowetting cell of the kind mentioned in the opening paragraph, which can be completely filled and that is protected against fluid leakage after closing of the cell.
It is a second object to provide a method of the kind mentioned in the opening paragraph, with which the complete filling can be achieved in an industrially viable manner.
The first object is achieved in that the at least one side wall of the body section comprises an inner wall and an outer wall, a portion of which end section is part of the inner wall, thus securing the end section and the body section, and which outer wall is provided with extensions extending at a first side and a second, to the first opposed side of the inner wall.
The second object is achieved in a cell with an inner and an outer wall, said outer wall having a first and second extension extending at opposite sides of the inner wall, which method comprises the steps of:
The solution of the present invention is a cell with a double wall. The outer wall provides stability and adequate sealing, whereas the inner wall is built up from the constituent elements that are assembled consecutively. Due to the consecutive assembly and the double wall, the electrowetting fluids can be provided before the end section. As a result, the end section can be immersed in the fluid, practically the fluid with the lowest density. Some fluid will flow to above the end section, but is nevertheless kept within the outer wall. The immersion leads to the required complete filling. An adequate sealing is then arrived at, in that the outer wall is present at the opposite sides of the inner wall. The outer wall of the package will then comprise only two major materials, that can be attached to each other in a good manner. A suitable combination is metal and glass.
The substrate could be part of the outer wall, but is preferably a separate part of which an portion is part of the inner wall. A spacer may be present between the portions of the substrate and the end section. This is suitable but not necessary. The advantage of such spacer is that it can be provided with any desired surfacial coating, such as the above mentioned fluid contact layer. Advantageously, the spacer is provided with a gap at its bottom side. This allows a reduction of the height of the cell.
Several embodiments can be envisaged for the first extension that is provided on top of the end section. It may for instance be a flexible extension of the outer wall. It may alternatively be a metal layer that is positioned through a deposition technique, probably with the help of a tool exerting pressure.
In a preferred embodiment, the first extension is a clamping body that is assembled on top of the end section as a separate component and—simultaneously or consecutively—connected to the outer wall. This allows an efficient assembly method. Reliable connection can be realized in both mechanical and chemical means, such as locking features, additional clamps, adhesive layers and sealing layers. Advantageously, use is made of a joining and protective layer that is grown on said surfaces. The grown layer is bonded to the surfaces chemically. It has the same coefficient of thermal expansion and it is not sensitive for inelastic and irreversible expansion under the influence of temperature. The grown layer can be grown to a desired thickness, therewith smoothening out gaps and height differences. Moreover, such a grown layer is quite inert, particularly if provided with an oxide, that may be a native oxide. Finally, the grown layer is not sensitive to attack by any of the electrowetting fluids and does not have an open or porous structure that would allow diffusion of molecules. With such a sealing, particularly together with a substrate and end section of glass, the resulting cell is closed hermetically.
A particularly suitable technique hereto is electroplating. This technique has the advantage that it can be applied at bulk level, by immersing the complete cell into a bath. Such an immersion of the complete cell moreover has the advantage that the grown layer extends on the complete outer wall and on both extensions at either side of the inner wall. Herewith the clamping character of the outer wall is strengthened.
In a further embodiment, a first and a second electrode that are in contact with at least one of the electrowetting fluids are defined at opposed sides of the cavity at the substrate and at the end section. At least two electrodes are needed in the electrowetting cell so as to apply the voltage needed to set the shape of the meniscus. Such electrodes can be applied at the inner walls, at opposite sides of the meniscus. In this embodiment however, the electrodes are present at the end section and at the substrate. This actually allows the use of a metal encapsulation without an isolated connection for one of the electrodes.
Preferably, the substrate is provided with an aperture, at an end of which said electrode is present. The electrode may be present both at a surface of the substrate facing the meniscus and at an opposite surface facing away from the meniscus. The latter modification is most preferred, as there is no need to fill said aperture with metal herein: the fluid fills the aperture. Moreover, the manufacture of this electrode can be integrated with the provision of metal strips that are used for the attachment of the outer wall to the substrate, and hence provide an adequate sealing.
It is preferred that the package is provided with a volume expansion member. Such an expansion member is for instance known from JP-A 2002/162506. This known package comprises a specific chamber that is present adjacent to the substrate, and is separated from the cell with a flexible cover. On increase of the pressure in the cell, the flexible cover may be deformed or even be stretched out, so as to form a curved surface extending into the said chamber. The invention offers the possibility to locate such a chamber between the inner wall and the outer wall.
A more preferable embodiment of the volume expansion member is a flexible membrane that is part of the end section. In this manner, a larger part of the end section—that is: the part that is present in the light path—is moved. Hence, a substantial volume increase can be compensated. The flexible membrane is particularly ring-shaped. The end section is thus divided into an outer edge, a membrane and an inner portion. The standard position of the end section need not to be planar herein. As will be explained with respect to the Figures, it is particularly such that the outer edge is pushed downwards under the pressure of the clamping body. This moreover results in a pressurized package. Although not preferred, it is not excluded that the outer edge portion of the end section comprises an other material than the inner portion.
The membrane is made of metal by far preference. Such a material withstands the fluids effectively. It has the additional advantage that the membrane can be used as one of the electrodes of the cell. If the connection between clamping body and outer wall is provided by electroplating or the like, the membrane will be provided with a metal layer too. A suitable thickness of the membrane layer that has sufficient stability on the one hand and sufficient flexibility on the other hand, is in the order of 5-30 microns, and more preferably between 15 and 25 microns. This is also dependent of the material of the membrane. In order to limit the thickness of the membrane, it may be provided with an insulating coating.
The substrate and the end section are preferably glass plates. Glass is inert against the electrowetting fluids. It can be treated with techniques such as powder blasting, and metal layers can be attached to it. If desired, one or both glass plates may be provided with coatings and surface layers, including IR-coatings, UV-absorption coatings, antireflection coatings, but also lenses. Such lenses can be made of surface layers with the replica technique. Alignment features may be included in such replica made surface layers. This allows a proper alignment of the cell with further lenses at the bottom and the top side.
If the cell of the invention is used as a lens, it is suitable assembled with further lenses to obtain a desired path. It is not excluded that two electrowetting lenses are part of the assembly; in fact this provides zoom properties. An alternative embodiment is however a stacked electrowetting lens. This stacked electrowetting lens can be suitable made with the method of the invention.
In a first embodiment of stacked cell manufacture, the substrate is present located in the middle of the stacked cell construction. A first assembly and filling takes then place on the one side of the substrate, and a second assembly and filling takes place on the opposed second side after finalizing the assembly and filling at the first side. Preferably both opposite end section are provided with flexible membrane for compensation of volume expansion. The outer wall is provided in this construction with an extension to support the substrate, half way the stacked cell.
In a second embodiment of the stacked cell manufacture, the two cells are provided within the outer wall one after the other. The end section of the first cell may be the substrate of the second cell, but that is not necessary. Most simply, a spacer is provided instead of the clamping body. On this spacer, that may well be connected to the outer wall by electroplating, a further substrate and the further elements of the second cell are provided consecutively. Such a construction moreover allows that the electrodes are located in the substrates and the end sections without the need for mutual coupling.
Instead for stacking of several electrowetting cells, the outer wall may be applied for stacking of one electrowetting cell and further optical and/or non-optical elements. In addition to separate lenses and filters, it is envisaged that an image sensor is assembled in this manner, separated from the electrowetting lens at a desired distance with any spacer or the like. Such an integration is most effectively with an image sensor in which the bond pads are located at a surface opposite to the optically active surface. Alternatively, the image sensor can be assembled on top of an interposer substrate, that is provided with through-holes so as to bring the contacts to a side opposite from the optically active surface. The use of interposer substrates is known per se in the art of semiconductor packaging.
It is an advantage of the method of the invention that it can be carried out at a wafer-level or a bulk level. The outer wall may be part of a plate, such as a printed circuit board with apertures. It can be separated into individual packages only after the filling and encapsulating steps. However, even if the manufacture of the cells is carried out individually, the electroplating steps can occur in a bath, allowing the simultaneous electroplating of a plurality of devices.
In a further preferred embodiment, at least one of the overlapping surfaces of the outer wall and the inner wall or the substrate or end section comprises a sealant for sealing the overlapping part under pressure. An advantage thereof is that the cell is being sealed during the assembly thereof. In other words, the cell is sealed while the closing extension is applied to the cell. It is to be kept in mind that leakage of fluids has to be prevented all around the overlapping surfaces. The sealing of this embodiment prevents leakage of fluids through passages that are formed by irregularities in the adjacent surfaces. A further advantage of this embodiment is that, because leakage is prevented after assembly, the surface of the exterior of the cell will remain free of liquids migrating from the inside after cleaning of the cell. Therefore, the application of the, preferably, metal layer as described in the above is not hampered by newly leaking fluids.
Advantages of a sealant comprising an organic composite such as plastic or rubber is that such materials are easily deformed and or pressed into areas wherein the adjacent surfaces are further away from each other, which areas are most prone to leakage. Preferably the sealant has electrically conductive properties which enables the process of electroplating by means of the positing technique such as electro galvanizing directly onto the sealant. Applying the electroplating directly on the sealant speeds of up the electroplating process over the gap between adjacent wall parts.
The invention is further related to a manufacturing method and a cell of the kind mentioned in the opening paragraph, in which a hermetic sealing is provided. This is achieved in that the end section is attached to the body section with a metal layer that is applied on a surface of both the end section and the joint section. As is explained above, the application of a metal layer will lead to a hermetic sealing. A clamping is achieved if the metal layer, or the construction of which the metal layer is part extends on both sides of the cell. A preferred application method is electroplating, but other methods such as sol-gel deposition of metal, sputtering or chemical vapor deposition, or combinations of such deposition methods, are not excluded. The metal layer will suitably be attached to metal parts at the surface of the end section and at the surface of the body section.
The cell may be used as a lens for use in a camera, in an optical recording apparatus or any other optical equipment. The cell may be assembled with further lenses, to obtain an optical path as needed, or even to obtain a zoom lens. Alternatively, the cell is used as a display, in which case only one of the substrate and the end section needs to be optically transparent. The cell may be further used as a sensor.
These and other aspects of the cell and the method of the invention will be further elucidated with reference to the Figures, in which:
The Figures are diagrammatic and not drawn to scale. The same reference numbers in different Figures refer to like parts.
According to this embodiment, the body section 17 comprises an inner wall 80 and an outer wall 90, and—at the second side 112 of the cell 15—the second cover plate 6. The inner wall 80 comprises an electrically insulating member 8 that is coated with a fluid contact layer 10. The inner wall 80 also comprises a portion of the end section 4. This end section (or first cover plate) 4 comprises a ring-shaped glass member 81 that is through an expandable joint 45 connected to an inner portion. In a preferred embodiment, this ring-shaped glass member 81 and the inner portion of the end section 4 are manufactured from a single glass plate, as will be explained with reference to FIGS. 2 to 7. The inner wall, 80 further comprises the end 61 of the second cover plate 6. This second cover plate 6 is provided with a through hole 62, an electrode 2, and a metallization 63. In an alternative embodiment, the plate 6 may be replaced by a construction similar or identical to that at the first side 111 of the cell 15, i.e. a ring-shaped glass member, an expandable joint and a cover plate.
These three sections of the inner wall 80—the ring-shaped glass member 81, the—also ring-shaped—insulating member 8 and the end 61—are clamped between a protrusion 85 of the outer wall 90 and a ring-shaped closing member 86. The closing member 86 is herein a piece of metal, but can be anything with an electrically conducting surface. The outer wall 90 comprises an inner core of plastic or other material 92 that is provided with a metallized surface 91. This metallized surface 91 also circumferes the metallization 63 of the second cover plate 6. In this manner, a mechanically stable connection is provided.
The inner wall 80 and the outer wall 90 are attached to each other, as well as to the joint 45 and the end section 4, in that a sealing layer 95 is present around it. The sealing layer 95 can be made of a suitable material. Polymeric coating of rubber, epoxy or the like, as are known per se as protective coating may be used. It is however preferred that the sealing layer 95 comprises a metal. This allows the provision of a package that is hermetical and not prone to diffusion of air, water or fluid. A particularly preferred method for the provision of this metal sealing layer 95 is electroplating. This method can be carried out at three-dimensional surfaces, e.g. in a bath.
FIGS. 2 to 7 show in schematic cross-sectional views consecutive steps in a method of manufacturing of the substrate 200 with an integrated membrane 210, to be applied as expandable joint 46.
FIGS. 8 to 16 show diagrammatical cross-sectional views of the method of manufacturing the electrowetting cell 15. Shown here is the manufacture of the embodiment of
The first cover plate 4 is herein a continuous plate that is chosen to be sufficiently thin so as to be bendable under stress. In this example, use is made of a plate of glass. A cavity 41 is present between the body member 8 and the first cover plate 4. Under expansion of at least one of the fluids 51,52—particularly at a temperature increase—the first cover plate 4 will bend so as to enlarge the volume in the cell. Under shrinkage of at least one of the fluids 51,52 the first cover plate will bend so as to diminish the volume in the cell. Since the electrically insulating fluid 51 is preferably an oil and has a larger coefficient of thermal expansion than the aqueous second fluid 52, it is preferably that the bendable first cover plate 4 is present at the side of the electrically insulating fluid 51.
Before the assembly of this embodiment, the cylindrical wall part 92 is provided with a layer 91 comprising niflon. Niflon is a composite of Teflon and nickel which composite is electrically conductive. Another example of such a composite comprises Teflon and copper. Because such a layer is electrically conductive, and the final metallic layer 95, which is also applied in the other embodiments, can directly grow onto the niflon layer 91 when the use is made of a plating technique based on electricity such as electro galvanizing. Also the annular members 12,86 are provided with niflon layers 11,13.
The embodiment is assembled as follows. The assembly is started with just the cylindrical wall part 92. The ring shaped closing member 12 is inserted in to the cylindrical wall part. On top of the ring shaped closing member 12, the glass plate or substrate 6 is attached by means of an adhesive. On top of the glass plate or substrate 6, the annular or ring shaped insulating member 8 is placed. Then the first and second liquids are provided into the cavity and the cover plate is placed on top of the liquids in a manner similar as described with the earlier embodiments.
The sealant or niflon layer has advantages in that the materials are pressed into uneven parts of the overlapping surfaces, which spreading can be improved by moving two adjacent surfaces relative to each other during or after the joining process. Such a relative movements can be achieved by means of rotating and/or repeatedly sliding surfaces relative to each other. The sealant is preferably applied onto either of the adjacent surfaces by means of a depositing or growth process such as a galvanizing process. Such a process allows for accurately depositing a thin layer or a labyrinth of material suitable for the purpose of sealing a very narrow closure, which is present between the adjacent surfaces in the embodiment.
Several additional features described in connection with other embodiments are also applicable in this embodiment.
Although not shown here, very good results have been obtained with the electroplated sealing layer. It is not excluded that this electroplated sealing layer is also applicable in other configurations of fluid containing cells, and that not for all applications a cell with both an inner wall 80 and an outer wall 90 and an extension 85,86.
It is further understood, that the extensions 85, 86 of the outer wall 80 extending at the first and second side 111, 112 are not separate elements, but either protrusions to parts or connections grown or deposited. Particularly, an electroplated connection may be effective as such an extension 86.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04101996.9 | May 2004 | EP | regional |
| 04105940.3 | Nov 2004 | EP | regional |
| 05102398.4 | Mar 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/51435 | 5/3/2005 | WO | 11/2/2006 |
