WO2005109392A1

WO2005109392A1 - Display device and electro magnetic wave modulating device

Info

Publication number: WO2005109392A1
Application number: PCT/IB2005/051495
Authority: WO
Inventors: Roy Van Dijk; Theodorus J. P. Van Den Biggelaar; Hubertus W. J. J. Van De Laar
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-05-12
Filing date: 2005-05-09
Publication date: 2005-11-17
Also published as: TW200625264A

Abstract

The electrostatic behavior of a (sub) pixel in an electrostatic (roll blind) display (1) can be controlled by reducing the capacitance per unit of area of the device (the electrostatic force during use) on several places on the surface area of the picture elements. The following parameters can be changed to influence the electrostatic force, such as making holes (8) in the structure of a fixed electrode (3), increasing or decreasing thickness of the dielectric layer (4) or using different dielectric material (differentE).

Description

Display device and electro magnetic wave modulating device

The invention relates to a display device having picture elements, each of the picture elements comprising on a substrate, a first electrode, a dielectric layer between said first electrode and a second electrode, the second electrode being movable in response to an electric field between a first position corresponding to an edge region of the picture element and a second position in which the second electrode at least partly covers the further surface region of the picture element. Wherever in this Patent Application reference is made to a picture element (pixel) it may either be a full picture element or a sub-pixel such as the red, green or blue sub-pixel in a picture element. Wherever in this Patent Application reference is made to a dielectric layer a layer is meant having such a high resistance that the mobility of the second electrode between the two positions, which positions, in the case of a display device, are related to electro- optical states of the display device (fully transmissive, fully reflecting or fully opaque (back)). The invention also relates to a method of driving such a display device. The invention further relates to an electro magnetic wave-modulating device based on the same principle. The display device can be used, dependent on the pixel size in micro-projector applications, large screen applications such as wallpaper but also in window applications. The electro magnetic wave-modulating device may be used as a light valve, but also may provide a controllable capacitor. A display device and electro magnetic wave-modulating device of the kind mentioned above are known from e.g. USP 5,519,565. The second electrode here is rollable in response to an electric field between a first position in which the rolled electrode is present at the edge region of the picture element and a second position in which the second electrode is unrolled and covers the further surface region of the picture element. One of the problems encountered in such a display is that the switching of the picture elements (pixels) is quite a slow process (of the order of tenth of milliseconds or even slower). This makes it not suitable to address line at a time in e.g. a matrix-display, unless it can be driven in a bistable way, i.e. picture elements that are switched on will stay on and all picture elements that are switched off stay switched off unless addressed a second time. A further problem encountered is that the bistability (the difference between the voltages for switching on and off respectively) in such display devices is small and the variation per picture element may vary a lot, which makes it impossible to drive the panel reliably. Especially the voltage to start the second electrode rolling to the first (rolled) position may vary extremely A main problem however is that the known device offers no possibility to obtain gray-values in a display device. Corresponding problems are encountered when using the electro magnetic wave-modulating device in applications outside the display area. The invention has as its purpose to overcome at least partly the above- mentioned problems. It further has as its purpose to provide a display device of the above mentioned kind in which gray-values can be displayed. To this end in a display device (and also an electro magnetic wave modulating device) according to the invention the capacitance per unit of area in the second position is variable over the surface region of the picture element (the electro magnetic wave modulating device). To make it possible to discriminate between the variation introduced by measures of the invention and normal variations due to e.g. processing the capacitance per unit of area in the second position (maximum value / minimum value) over the surface region of the picture element (the electro magnetic wave modulating device) for instance varies at least 5 %. By giving the capacitance per unit of area in the second position a controlled variation the electrostatic force per unit of area between the first and second electrode during use is given a controlled variation too, which makes it possible to control switching parameters. The controllable switching parameters are for instance the threshold voltages of a bistable display device, the voltages associated with certain gray-values. On the other hand the control of said variation in capacitance is used to reduce sticking of the second (rollable) electrode to the dielectric layer, which leads to a more constant behavior (both in time within the same device as when comparing a device to other devices). In a first embodiment of the invention, particularly suited in display applications for realizing gray- values the capacitance per unit of area decreases in the direction away from the edge region of the picture element over the further region of the picture element. The decreasing capacitance per unit of area causes a decreasing electrostatic force, so increasing voltages, to unroll the second (rollable) electrode. Decrease of the capacitance per unit of area can be either step-wise (discrete gray levels or in continuous way (giving a continuous variation within a gray-scale). One way to vary the capacitance per unit of area is providing the first electrode with openings. Such openings may also be provided in the dielectric layer. To prevent shorts between the electrodes in the latter embodiment small openings are spread over the area. Another way to vary the capacitance per unit of area is varying the thickness of the dielectric layer or the relative dielectric constant. To obtain gray-values in a display the thickness of the dielectric layer (the value of the relative dielectric constant of the dielectric layer) increases in the direction away from the edge region of the picture element over the further surface region of the picture element. A further device according to the invention has at least one opening at its end away from the edge region of the picture. This prevents the end of the second (rollable) electrode from completely sticking to the dielectric layer due to "van der Waals" forces and residual electrical forces and enhances switching back to the rolled up position.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Figure 1 schematically shows a part of a device according to the invention, Figure 2 shows a plan view of the part of the device of Figure 1, Figures 3 and 4 show transmission voltage characteristics of the device of Figure 1, Figures 5 and 6 show plan views of a first and second embodiment of a device according to the invention, Figures 7 to 11 show further embodiments of devices according to the invention, in which gray-levels are realized, while Figures 12 to 14 show related driving forms for the devices of Figures 7 to 11 and Figures 15 to 18 show embodiments of other devices according to the invention The Figures are diagrammatic and not to scale; corresponding components are generally denoted by the same reference numerals. Figures 1 and 2 schematically show a part of a device 1 according to the invention, in this particular embodiment a transparent display device. A transparent substrate 2 is covered with transparent first electrodes 3 e.g. ITO -electrodes. The electrodes 3 are covered with a thin dielectric layer 4. A foil 6, which is covered with a conductive electrode part 5, forms together with said conductive electrode part 5 a second, rollable electrode. The thin dielectric layer 4 electrically isolates the electrodes 3 (e.g. parts of column electrodes) from the rollable electrode parts 5 (e.g. parts of column electrodes). Figure 1 shows three (sub) picture elements, two of which are in an open, transparent state (rollable electrode 5, 6 rolled up to a first position), the other one being in a closed, opaque (black) state (rollable electrode 5, 6 unrolled to a second position). In this example the foil 6 is glued to the dielectric layer 4 on one side part 7 of every picture element. The rollable electrode 5, 6 are switchable between a transmissive (open) state and an opaque (closed) state, e.g. by choosing aluminum for the electrode parts 5. The device 1 further comprises e.g. driving means and for example a backlighting system. On the other hand, in a reflective device the foil 6 or the rollable electrode 5, 6 may be covered with a white layer to reflect the ambient light, while the substrate now is opaque by covering it with an opaque layer at one of its sides. A further possibility is making the substrate 2 reflective and the rollable electrode 5, 6 black. It is assumed that in the device 1 three (or four) forces determine the switching behavior, an elastic force, an electrostatic force, the "van der Waals" force and to a minor extend the gravitational force. The elastic force is the force present in the rollable electrode 5, 6 and is the result of e.g. shrinkage during manufacturing and this force is directed at rolling up the rollable electrode 5, 6 of a (sub) picture element or (sub) pixel. The electrostatic force is the attractive force between the conductive electrode part 5 and the (ITO) on the substrate by applying a voltage. The "van der Waals" force is the force between the (sub) pixel foil 6 and the substrate 2. This force depends on the distance between the two media, the roughness of the media and the material properties. The smaller the distance is, the larger the "van der Waals" force is. The electrostatic force depends strongly on the distance, the surface area, dielectric constant of the materials and the voltage difference between the foil and the substrate. The gravitational force acts upon the rolled up electrode 5, 6 which also depends on the orientation of this foil. It is very thin and has therefore a very low mass, so it is probably negligible. The elastic force is directed at rolling up the rollable electrode 5, 6 , while the electrostatic force and the "van der Waals" force are directed at keeping the rollable electrode 5, 6 closed. To keep a picture element open (the left two picture elements in Figures 1, 2, the elastic force must be larger than the "van der Waals" force, since the picture element (pixel) in the device 1 is open if no or little electrostatic force is present. When a picture element or pixel is closed, the "van der Waals" force and the electrostatic force keep it closed, whereas the elastic force wants to open it. If no voltage is applied the rollable electrode 5, 6 is in a rolled up state, giving a transparent picture element in transmissive mode or a dark pixel in reflective mode. When applying a certain voltage V₂, in matrix-display devices the difference between the column voltages and the row voltages, the electrostatic forces rolls down the rollable electrode 5, 6 on to the substrate 2, covering the pixel area and creating a dark pixel in transmissive mode or a white pixel in reflective mode. This switching behavior is shown by means of the transmission voltage characteristic of Figure 3, which shows the transmission T of the device of Figure 1 as a function of the voltage V. At a first threshold voltage Vi (which may be presented as a voltage difference between a row and column in a matrix display) a picture element is opened (the rollable electrode 5, 6 rolls up), if it was not open already. At the second threshold voltage, V₂ a pixel is closed (the rollable electrode 5, 6 becomes flattened), if it was not already closed. The polarity of the voltages is not important, only the absolute value is important. In between these values a pixel that was open, will remain open and a pixel that was closed will remain closed. The threshold voltages are determined by the material parameters, i.e. the elastic forces, thickness of the foil, material properties, and surface properties, etcetera. In practice it is difficult to maintain these threshold voltages the same throughout the lifetime and for all (picture) elements in a single device. One problem is that the hysteresis ( I V₂ - V| | ) of the picture elements in e.g. a passive matrix device is too small and the variations in the display are too large to drive the display. To drive such a passive matrix display, the pixels in a row that is being addressed are first set to one state for all pixels in that row. This can be done by putting the voltage differences between the row and column voltages at 0 V for all pixels in that row so that all rollable electrodes in said row roll up. In the other rows the voltage differences between the row and column voltages remain at a voltage, V_hoi_d, so that the pixels that were addressed in a previous state remain in the same state. After that the voltage differences between the row and column voltages are set to a voltage, V_add_r, -which is slightly below V₂. Dependent on whether a pixel should remain open or must be closed, a column voltage remains at 0 V (pixel remains open), or is set to the voltage V₂ or higher (pixel will close). The voltage differences between the pixels in the rows that are not being addressed must remain between Vi and V₂ since the pixels in these rows must remain in the state they were in. In a practical situation, not all pixels change state at exactly these voltages but this varies as shown in Figure 4, which shows possible variation of Vi and V₂. The maximum of voltage Vi and minimum voltage V₂ determine the minimum hysteresis or the margin of the device, i.e. the voltage difference between, the lowest voltage V₂ when no pixel will switch from open to closed minus the maximum voltage Vi when no pixel changes from closed to open. Said margin must always be larger than zero. In Figure 4a there is enough margin to drive the display device. There is a voltage difference I V₂ - Vi | , which is larger than the margin | V_addr - Vhoid I of the display device. In Figure 4b however, there is not enough margin to drive the panel, since the voltage difference I V₂ - Vi I is smaller than the margin | V_addr - hoid I of the display device. According to one aspect of the invention said voltage difference | V₂ - Vi | is increased by reducing the capacitance per unit of area of the picture clement in the second position of the picture clement and consequently reducing the electrostatic force needed for closing the picture element in a part of the picture element. In Figure 5, the first electrodes 3 (ITO layer) are not covering the entire surfaces of the picture elements, but have openings 8. This reduces the electrostatic force per unit width of a picture element (pixel) for the associated part and a higher voltage to close said picture element (the voltage V₂) is needed. At the other end of the picture element the ITO is fully covering the surface so the voltage to keep a pixel closed (Vi) remains the same. In a further embodiment, instead of providing openings 8, the thickness of the dielectric layer 4 is made larger near the side part 7 of the pixel. Other methods to obtain the same effect include structuring the electrode metal on the foil, varying the dielectric material in choosing at least two different materials or mixtures of material in which the dielectric constant of the material varies. Figure 6 shows a further embodiment, in which the dielectric layer 4 is provided with an opening 9. Figures 7 - 1 1 show embodiments according to the invention in which gray values are realized by reducing (the local variation of) the electrostatic force (decreasing the capacitance per unit of area) of the device on several places on the surface area of the pixel. The capacitance per unit of area can be decreased by, making holes 8 of a fixed size in the structure of electrode 3, see Figure 7 or by making holes of a variable size in the structure of electrode 3, see Figure 8. Figure 9 shows an embodiment in which the capacitance per unit of area decreases and hence the electrostatic force needed increases via a staircase structure. Reference numerals have the same meaning as in Figures 7, 8. Figure 10 shows a further embodiment, in which the dielectric layer 4 is provided with small openings 9, the number of openings increasing towards the further end of the picture element. The small openings are spread over the area to prevent shorts between the electrodes. Figure 11 shows an embodiment in which the thickness of dielectric layer material increases (indicated as di, d₂, d₃,). Reference numerals again have the same meaning as in Figures 7, 8. As mentioned in the introduction, instead of making holes in the ITO structure, variation in different dielectric material is possible (different ε). Switching behavior for the embodiment of Figure 7 is shown in Figure 12. The sub-pixels, as defined by the separate holes 8, are closed successively by driving them by a number of pulses 10, having an absolute voltage value of at least | V₂ 1 (3 in this example), while they are opened successively by driving them by a number of pulses 10, having an absolute voltage value of below | Vi | . | Vi I and | V₂ 1 are defined by the hysteresis ( I V₂ - Vi I ) of Figure 3. A (sub) pixel can also be driven the other way around, viz. starting from a closed sate and ten opening it step by step. On the other hand several (sub) pixels can be closed (or opened) simultaneously by using a single pulse, the duration of which determines the gray -level Switching behavior for the embodiments of Figures 8, 9. 11 is shown in Figure

13. The sub-pixels, as defined by the (separate) hole(s) 8 are now closed successively by driving them with absolute voltage values | V₃ 1 , | V₅ 1 and | Vβ I - corresponding to positions in which the holes are 66% open, 33% open and fully open (so defining a grayscale having 4 gray-levels including black and white) while they are opened successively by driving them with absolute voltage values I V | , | V₂ 1 and | Vi | . The hysteresis of the device is now defined by ( | Vό - Vi | ). Now a "staircase" voltage controls the sub-pixels signal see Figure

14. Absolute voltage values | V₃ 1 , | V₅ 1 and | V₆ I are presented to close the first, second and third sub-pixel, while absolute voltage values I V | , | V₂ 1 and | Vi | are presented to open them in the reverse order. A gray-scale having more gray-values may be obtained by introducing more openings 8. A substantially continuous gray-scale may be obtained by using a rounded opening 8 or by structuring in a continuous way the openings 9 in embodiment of Figure 10. The embodiments of Figures 15 -17 solve the problem of so-called pixel sticking, which is the fact that rollable electrodes 5, 6 don't unroll anymore. This happens sometimes only for one cycle, but sometimes it leads to pixel failure. A solution is obtained by reducing or eliminating the electrostatic force at the end of a picture element (pixel), again by reducing the capacitance per unit of area. In Figures 15 and 16, the rollable electrodes 5, 6 are not rolled out completely, but remain partly rolled up. If this part is mall, it will hardly or not visible. In Figure 15 this is done by removing (by means of an opening 8) the electrode 3 (ITO) at the end of a pixel at the area 10, see Figure 15. If the electrode 3 forms part of e.g. an ITO column this interrupts the ITO column and further pixels in said column cannot be driven anymore. Making several holes in the electrode 3 (ITO), as shown in Figure 16 solves this. Now the electrode 3 (ITO) is not completely open but there are slits in the ITO to make connection to another pixel in the column. The electrostatic force is reduced enough to make the rollable electrodes 5, 6 not fully close, so no pixel sticking occurs In Figure 17 the electrostatic force in the middle of the rollable electrode 5, 6 is reduced, so that it will not stick easily there and the part at the area 10 is the first to roll up. In Figure 17 an opening 8 is provided in electrode 3 (ITO) in the middle end of a pixel. This eliminates the electric field so that there is not force in the middle of a pixel, but there is enough force at the corners of a pixel so that the rollable electrodes 5, 6 will not fully close, so no pixel sticking occurs. In Figure 18 a similar embodiment is shown but slits 8' are made in the ITO so that there remains an electrostatic force in the middle, but the force is lower, so a rollable electrode 5, 6 detaches itself earlier in the middle than in the corners. This can also be achieved by other methods as mentioned above, i.e. locally removing the metal from the rollable electrode 5, 6 at the end of a pixel, making the dielectric thicker or the dielectric constant lower. The invention is not restricted to the embodiments shown. As mentioned in the introduction it s also applicable to electro magnetic wave-modulating devices of the kind mentioned in e.g. USP 5,519,565. A display can be made with near 100% aperture. The rollable electrodes 5, 6 have two sides. In a reflective display device the top side is reflective (white, red, green or blue). The bottom side is non-reflective, preferably black. When a pixel is open (rollable electrodes 5, 6 unrolled) the topside is shown over the full pixel (except for the black matrix, which is obtained by using said black substrate). When a pixel is closed, the pixel rolls up and shows the bottom side of the roll and simultaneously, the non-reflective (black) substrate is shown. The same can be obtained by reversing all the layers and using a reflective (white, red, green or blue) substrate. Although in this Application the reduction of the capacitance per unit of area has been focused on, other applications may use an increase in said capacitance in a certain direction. The invention resides in each and every novel characteristic feature and each and every combination of features. Reference numerals in the claims do not limit the protective scope of these claims. The use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. The use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A display device (1) having picture elements, each of the picture elements comprising on a substrate a) a first electrode b) a dielectric layer between said first electrode and c) a second electrode, the second electrode being movable in response to an electric field between a first position corresponding to an edge region of the picture element and a second position in which the second electrode at least partly covers the further surface region of the picture element, the capacitance per unit of area of the picture element in the second position being variable over the surface region of the picture element.

2. A display device according to Claim 1, the capacitance per unit of area of the picture clement in the second position varying over at least 5 %.

3. A display device according to Claim 1, the capacitance per unit of area of the device decreasing in the direction away from an edge region of the picture element over the further region of the picture element.

4. A display device according to Claim 1 in which the first electrode has at least one opening.

5. A display device according to Claim 4 for driving the display device from a state in which the second electrode is in the first or second position to a state related a gray- level, the first electrode having at least two openings of substantially the same size, which display device has a driver for supplying patterns of first pulses of substantially the same absolute value, the number of first pulses determining the gray-level and for resetting the display device from said gray-level to the state in which the second electrode is in the first or second position.

6. A display device according to Claim 5, in which resetting is obtained by supplying patterns of reset pulses of substantially the same absolute value, the first pulses and the reset pulses having opposite values with respect to a mean value, the number of reset pulses being equal to the number of openings or the number of reset pulses being equal to the number of first pulses.

7. A display device according to Claim 4 in which the size of the openings in the first electrode increases in the direction away from an edge region of the picture element over the further surface region of the picture element.

8. A display device according to Claim 1 in which the dielectric layer has at least one opening.

9. A display device according to Claim 8 in which the size of the openings in the dielectric layer increases in the direction away from an edge region of the picture element over the further surface region of the picture element.

10. A display device according to Claim 1 in which the dielectric layer has at least two different thicknesses.

1 1. A display device according to Claim 1 or Claim 10 in which the thickness of the dielectric layer increases in the direction away from an edge region of the picture element over the further surface region of the picture element.

12. A display device according to Claim 1 in which the material of the dielectric layer has at least two different values of the relative dielectric constant.

13. A display device according to Claim 1 or Claim 12 in which the value of the relative dielectric constant of the dielectric layer increases in the direction away from an edge region of the picture element over the further surface region of the picture element.

14. A display device according to Claims 7, 9, 11 or 13 for driving the display device from a state in which the second electrode is in the first or second position to a state related to a gray-level, which display device has a driver for supplying a staircase voltage pattern, a step of the staircase determining a gray-level and for resetting the display device from said gray- level to said state in which the second electrode is in the first or second position.

15. A display device according to Claim 15 in which a reversed staircase voltage pattern is applied for resetting.

16. A display device according to Claim 1 in which the first electrode has at least one opening at its end away from the edge region of the picture.

17. A display device according to Claim 1 in which the dielectric layer has at least one opening at the area of the picture element away from the edge region of the picture element.

18. A display device according to Claim 1 in which the first electrode has a reflective top side, the bottom side of the first electrode and the substrate being non- reflective.

19. A display device as claimed in Claim 1 in which the first electrode has a non- reflective top side, the bottom side of the first electrode and the substrate being reflective.

20. A method for displaying gray-levels in a display device according to Claim 4, which method comprises the steps of a) resetting, if necessary, the display device to the state in which the second electrode is in the first or second position and b) supplying patterns of first pulses of substantially the same absolute value, the pattern of first pulses determining the gray-level

21. A method according to Claim 20, in which resetting is obtained by supplying patterns of reset pulses of substantially the same absolute value, the first pulses and the reset pulses having opposite values with respect to a mean value, the number of reset pulses being equal to the number of openings or the number of reset pulses being equal to the number of first pulses.

22. A method for displaying gray-levels in a display device according to Claims 7, 9, 1 1 or 13, which method comprises the steps of a) resetting, if necessary, the display device to the state in which the second electrode is in the first or second position and b) supplying a staircase voltage pattern, a step of the staircase determining a gray- level.

23. A method according to Claim 22 in which a reversed staircase voltage pattern is applied for resetting.

24. An electro magnetic wave modulating device comprising on a substrate a) a first electrode b) a dielectric layer between said first electrode and c) a second electrode, the second electrode being movable in response to an electric field between a first position corresponding to an edge region of the device and a second position in which the second electrode at least partly covers the further surface region of the device, the capacitance per unit of area of the device in the second position being variable over the surface region of the device.

25. An electro magnetic wave modulating device according to Claim 24, the capacitance per unit of area of the device in the second position varying over at least 5 %.

26. An electro magnetic wave modulating device according to Claim 24, the capacitance per unit of area of the device decreasing in the direction away from an edge region of the device over the further surface region of the device.

27. An electro magnetic wave modulating device according to Claim 24 in which the first electrode has at least one opening.

28. An electro magnetic wave modulating device according to Claim 24 in which the dielectric layer has at least one opening.

29. An electro magnetic wave modulating device according to Claim 24 in which the dielectric layer has at least two different thicknesses.

30. An electro magnetic wave modulating device according to Claim 24 or Claim 30 in which the thickness of the dielectric layer increases in the direction away from an edge region of the device over the further surface region of the device.

31. An electro magnetic wave modulating device according to Claim 24 in which the material of the dielectric layer has at least two different values of the relative dielectric constant.

32. An electro magnetic wave modulating device according to Claim 24 or Claim 30 in which the value of the relative dielectric constant of the dielectric layer increases in the direction away from an edge region of the device over the further surface region of the device.

33. An electro magnetic wave modulating device according to Claim 24 in which the first electrode has at least one opening at its end away from the edge region of the picture.

34. An electro magnetic wave modulating device according to Claim 24 in which dielectric layer has at least one opening at the area of the device away from an edge region of the device.