WO2003056367A1

WO2003056367A1 - Reflective flat panel display

Info

Publication number: WO2003056367A1
Application number: PCT/FI2001/001143
Authority: WO
Inventors: Jyrki Kimmel
Original assignee: Nokia Corporation
Priority date: 2001-12-21
Filing date: 2001-12-21
Publication date: 2003-07-10
Also published as: AU2002225061A1

Abstract

The invention discloses a direct view reflective flat panel display device comprising an array of pixels (P), said pixels each comprising an electrically tunable optical resonant reflector cavity (C). A resonant reflector cavity (C) in a single pixel comprises at least a layer of dielectric and viscoelastic transparent material (G) applied onto a light absorber layer (AL) and an transparent front filter plate (FL) arranged opposite said viscoelastic layer (G). Transparent row electrodes (RE) and transparent column electrodes (CE) are arranged in a manner that said electrodes (RE,CE) are capable of receiving voltages for generating an electric field to alter the thickness of said viscoelastic layer (G) in the area between said row (RE) and column (CE) electrodes in single pixel. This makes it possible to change the reflection and absorption of incident light (L) in the individual pixels of the display device.

Description

REFLECTIVE FLAT PANEL DISPLAY

The present invention relates to a reflective flat panel display device according to the preamble of the appended claim 1.

Portable electronic devices such as mobile phones, notebook computers or digital cameras etc. place exceptional demands on the display devices. In particular, displays in portable devices must have low power consumption and they must perform well in ambient light conditions.

Flat panel displays based on LCD (Liquid Crystal Display) technology are predominantly used in such applications today. LCDs are based on the use of certain organic molecules, liquid crystals, that can be reoriented by an electric field and thus the transmission of light through a layer containing the liquid crystal material can be altered. LCDs, which do not generate light by themselves, can be categorized into reflective and transmissive displays. Reflective LCDs utilize ambient light (or frontlights when necessary) to be visible, whereas transmissive LCDs require the use of additional backlights. Due to the power consumption limitations in portable devices, reflective LCDs are preferred in such applications because the use of power consuming frontlights is necessary only when ambient light level (daylight or artificial light) is not adequate.

The major shortcomings of the reflective LCDs are related to their limited brightness and colour reproduction. This has triggered research on several alternative display techniques. Some of these display devices are discussed shortly hereinbelow.

Field emission displays (FEDs) have many similarities with conventional cathode ray tubes (CRTs). In FEDs electrons are accelerated in vacuum towards phosphors which become excited and emit glow. Different phosphor materials are used to create primary red, green, and blue (RGB) colours, respectively. The main difference compared to the CRTs is that the electrons are generated by field emission rather than by thermal emission so FED consumes less power than a CRT and does not require any substantial warming up time before it can be viewed. Instead of one single electron gun, each pixel comprises several thousands of sub-micrometer tips from which electrons are emitted. The major shortcoming of the FEDs is related to problems achieving operating voltages low enough which would allow FEDs to be used in portable devices. Due to the complicated manufacturing process, FEDs are also expensive display devices.

A plasma display panel (PDP) can be characterized as being essentially a matrix of tiny fluorescent tubes which are controlled in a sophisticated fashion. In a pixel in PDP a plasma discharge is first induced by an electric field. The discharge creates a plasma containing ions and electrons which gain kinetic energy from the presence of the electric field. These particles collide at high speed with neon and xenon atoms, which thereby are brought to higher excited states and upon de- excitation to lower states emit ultraviolet radiation. This radiation, in turn, excites phosphor material, which emits glow. Different phosphor materials are used to create red, green, and blue (RGB) colours, respectively. The major shortcomings of the PDPs are related to high power consumption and limited possibilities to manufacture display devices thin enough and with pixels small enough to be used in small- size portable devices. Despite of somewhat less stringent requirements on manufacturing technology than for example in the case of FEDs, the price of PDPs is at the moment relatively high.

US 6040937 and US 6055090 disclose direct view flat panel displays, which consist of arrays of light modulator cells. A single light modulator cell modulates light by electrostatically varying the spacing of a cavity comprising two walls, one of which is a reflector and the other one is an induced absorber. Thus, in general, a single light modulator cell forms an interferometric cavity, where the cavity spacing is adjusted using an electrostatically driven MEMS (Microelectromechanical System) structure. Silicon-surface micromachining is a recent and rapidly developing technology for fabricating optical MEMS devices, but it is still a rather demanding manufacturing technology. WO 01/48531 discloses a display panel (see especially Figs 49 to 55) in the form of an array of cells each of which comprises two deformable dielectric layers which meet at an interface, at least one of which is a relief forming gel, the other one being typically air. A first electrode is arranged on one side of said layers and a signal electrode on the other side of said layers, there being means for providing signals to the signal electrodes to create reliefs in the cells on the gel surfaces. The periodical, typically sinusoidal reliefs created on the gel surface in each of said cells allows to create images on the display panel, which images are viewable by the naked eye under the effect of light from a scattered source. The major shortcomings of the display panels described in WO 01/48531 may be associated with the practical difficulties in producing desired profiles for the gel reliefs in the cells. This further impairs the light modulating capabilities of the individual cells.

With the goal of bringing display quality closer to that of a paper print, brightness, contrast, and colour saturation of the displays must be further improved. In order to allow the use of display devices in small- size portable devices, the power consumption and thickness of the display devices should also be further reduced. To make mass production of portable devices possible, the manufacturing technology of the display devices should be simple in order to allow lower prices.

The main purpose of the present invention is to produce a novel direct view reflective flat panel display device which has high image quality, low power consumption and is also easy to manufacture. These properties make the display device according to the invention especially suitable to be used in modern mass-produced portable electronic devices, where the display technologies according to the prior art have not yet proved to provide completely satisfactory solutions.

To attain this purpose, the display device according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1. The reflective display device according to the invention is based on creating an electrically tunable resonant reflector cavity, i.e. an interferometric cavity within each pixel in a display matrix consisting of several individually addressable pixels. According to the invention the optical properties of a resonant reflector cavity, i.e. the reflectivity and absorption of incident light, are arranged to be tuned by altering the thickness of a layer of dielectric and viscoelastic material inside the resonant reflector cavity with the aid of electric fields.

Compared to the prior art disclosed in WO 01/48531 , the basic and main difference in the current invention is that no reliefs are produced to the surface of the dielectric and viscoelastic material, but instead the thickness of said material within a pixel is altered in order to simultaneously maintain a substantially flat surface of the viscoelastic layer inside a certain area defined by the electrode structures. Due to the fact that the viscoelastic material has a different index of refraction compared to the medium forming the necessary gap between the viscoelastic layer and at least one of the opposite electrode structures, the light passing through the viscoelastic layer and said gap experiences a different optical path length depending on the thickness of the viscoelastic layer. This allows to tune the optical properties of the resonant reflector cavity inside a pixel to produce the desired optical function.

According to the present invention, the display matrix consisting of several electrically tunable resonant reflector cavities is realized in the following manner.

A layer of dielectric and transparent viscoelastic material is disposed on a light absorber layer. A transparent front filter plate is located adjacent to the viscoelastic layer, leaving a gap between the surface of the viscoelastic layer and the front filter plate. A transparent row electrode structure is formed on the absorber layer below the viscoelastic layer. A transparent column electrode structure is formed on the front filter plate towards the viscoelastic layer. The row electrode structure and the column electrode structure both consist of separate electrodes arranged in such a manner that said electrodes are capable of being activated by receiving voltages from a voltage source. The activation of the row and column electrodes generates electric field/s between said electrodes in those areas where the row and column electrodes are overlapping, i.e. crossing each other. This provides a pixel-like structure, where pixels are located within the crossing area of a row and a column electrode, and where each pixel can thus be addressed individually by activating corresponding row and column electrodes. In response to the electric field within a pixel, the layer of viscoelastic material is deformed and the distance between the surface of the viscoelastic layer and the front filter plate is changed. When a range of frequencies of light is incident through the front filter plate towards the viscoelastic layer, the front filter plate, the viscoelastic layer and the absorber layer together form a resonant reflector cavity. Thus depending on the applied voltage and corresponding distance between the surface of the viscoelastic layer and the front filter, said resonant reflector cavity can reflect or absorb a range of frequencies of the incident light.

In an advantageous embodiment of the invention, proper selection of materials and dimensions allows the fabrication of pixels, of which each pixel can be electrically switched between reflecting all or any colour to absorbing all colours. Thus the device according to the invention can be used as a direct view reflective flat panel display, where every single pixel is capable of reproducing a wide range of colours. The advantage of this embodiment is that it can be used to create full colour flat panel displays with high effective colour pixel density.

In a second embodiment of the invention, the selection of materials and dimensions allows the fabrication of pixels, of which each pixel can be electrically switched between reflecting all colours to absorbing all colours. Thus the device according to invention can be used as a reflective black-and-white-type flat panel display. The advantage of this embodiment is the simpler structure of individual pixels and also the simpler control of the electrode voltages because only a few discrete voltage levels need to be generated instead of continuously adjustable electrode voltages. In a third embodiment of the invention, the selection of materials and dimensions allows the fabrication of a matrix of pixels, so that said matrix contains three or more different types of pixels, each type of pixels capable of being electrically switched between reflecting a certain primary colour to absorbing all colours. Thus, a group of adjacent pixels can be used to create a range of colours in similar manner than in a RGB colour system, where colours are created as combinations of the primary red, green and blue colours. An advantage of this approach is the simpler structure and control of individual pixels, but at the same time because of several adjacent pixels are required to form a single "virtual" colour pixel, the effective colour pixel density is somewhat reduced.

The device according to the aforementioned third embodiment can be realized by simply adding a mosaic colour filter in front of the previously described black-and-white flat panel display. Alternatively, the creation of primary colours can involve arranging the different types of pixels (corresponding to different primary colours) to be activated with different electrode voltage levels, and/or to have different selection of materials and dimensions in the resonant reflector cavity.

In all of the aforementioned embodiments of the invention, the on-off duty cycle of voltages activating the row and column electrodes can be used to adjust the level of brightness of the pixels. The duty cycle can also be used to adjust the relative amount of primary colours when creating a desired colour as a combination of the primary colours.

The device according to the invention is significantly more advantageous than prior art devices in providing a possibility to achieve high image quality (brightness, contrast, and colour saturation). The manufacturing of such devices also promises to be relatively easy and economical compared to prior art technologies allowing, for example, the use of a wider variety of substrate materials and simpler manufacturing processes. The reflective display device according to the invention also has a low power consumption and it can be made thin to suit portable devices. The preferred embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given hereinbelow, and also through the appended claims.

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig.1 illustrates the behaviour of dielectric liquid in an electric field between electrode plates of a field capacitor,

Fig. 2 shows schematically the row and column electrode structures in a reflective display device according to the invention,

Fig. 3 shows schematically the formation of a pixel in the crossing area of the row and column electrodes,

Fig. 4 is a schematical cross-section diagram showing the structure of a single pixel according to one embodiment of the invention, and

Fig. 5 is a schematical cross-section diagram showing the structure of a single pixel according to another embodiment of the invention.

It is to be understood that the drawings presented hereinbelow are designed solely for purposes of illustration and thus not, for example, for showing the various structures and components of the device in their correct relative scales and/or shapes. For the sake of clarity, the components and details which are not essential in order to explain the spirit of the invention have been omitted from the drawings.

Fig.1 illustrates a general principle of physics, which can be observed in connection with dielectric substances. Dielectric substance can be defined as a substance in which an electric field may be maintained with zero or near zero power dissipation, i.e. the electrical conductivity is zero or near zero. In an electric field, the surface of two dielectrics with different dielectric constants is known to experience a force which is proportional to the square of the electric field strength. In Fig. 1 where an electric field is formed between electrode plates 10 and 11 of a field capacitor by applying suitable voltages on said electrodes, dielectric liquid 12 is drawn between the electrode plates because of the aforementioned force effect.

Figures 2-5 illustrate schematically the construction of the reflective display device according to the invention.

Figure 2 illustrates the row and column electrode structures consisting of a number of separate row RE and column electrodes CE. The row RE and column electrodes CE are arranged in such a manner that said individual electrodes are capable of being activated by receiving suitable voltages.

Figure 3 shows a close-up view of area A from Fig. 2. In each crossing of the overlapping row RE and column CE electrodes a pixel P is formed. Each pixel P can be addressed individually by activating the corresponding row RE and column CE electrodes.

It should be understood that the number, orientation and width of the individual electrodes in the row and column electrode structures is not limited to that shown in Figs 2 and 3. Instead, the number of the separate electrodes in the row and column electrode structures is arbitrary, and the number of said electrodes and thus the number of individual pixels can therefore be increased or decreased depending on the particular application. When the row and column electrodes are arranged in right angles with respect to each other and also the column and row electrodes are arranged to have equal electrode widths, this results in the forming of substantially quadratic pixels. However, the angle between the row and column electrodes and their relative widths can be altered to achieve different pixel sizes and pixel shapes.

Further, the pixel sizes, shapes and other properties can also be different in different parts of a single flat panel display. In addition, suitable black matrix regions or other structures defining further the shape of the pixel area can be included without departing from the scope of the present invention.

Figure 4 is a schematical cross-section diagram showing the structure of a single pixel P in a reflective display device according to the invention.

A dielectric and viscoelastic transparent material G is applied onto a light absorber layer AL. A transparent front filter plate FL is located adjacent to the viscoelastic layer G, leaving a gap AG between the surface of the viscoelastic layer G and the front filter plate FL. The light absorber layer AL, the viscoelastic layer G and the front filter plate FL form a resonant reflector cavity together.

A transparent row electrode RE is arranged on the absorber layer AL below the viscoelastic layer G. A transparent column electrode CE is arranged on the surface of the front filter plate FL towards the viscoelastic layer G. The activation of the row RE and column CE electrodes by applying a voltage V between said electrodes generates an electric field between said electrodes and through the viscoelastic layer G. This causes the dielectric viscoelastic layer G to deform from a substantially flat surface (marked with a broken line in Fig. 4) in such manner that the thickness of the viscoelastic layer G in the area between the row RE and column CE electrodes is changed. Depending on the applied voltage, the distance of the surface of the viscoelastic layer G and the front filter plate FL is thus changed. Within the overlapping area of the row RE and column CE electrodes the viscoelastic material G maintains a substantially flat surface and thus the front filter plate FL, the viscoelastic layer G and the absorber layer AL form a resonant reflector cavity together, which can reflect or absorb a range of frequencies of the incident light L.

Suitable transparent viscoelastic material G includes, for example, silicone gel, oil, various polymer materials or other viscous substances that have a tendency to deform when placed in the presence of an electric field, and said materials relax towards their original form or shape after the aforementioned effect ceases.

The transparent row RE and column CE electrode structures are preferably made of indium tin oxide (ITO), as is known in the art, and the front filter plate FL and the absorber layer AL are preferably made of suitable glass materials. Said glass materials are arranged to have suitable optical properties by, for example, adding impurities to the bulk material (colour glass filter materials) and/or by using optical coatings (thin film coatings), both aforementioned methods being well known in the art. Other methods for creating substantially transparent electrode structures on other suitable materials, for example on plastic materials or on silicon based materials, can also be employed without departing from the scope of the present invention.

The gap AG is left between the surface of the viscoelastic layer G and the front filter plate FL and column electrode structure CE in order to allow the viscoelastic layer G to deform without contacting said opposite structures. The gap AG can be for example air, gas or vacuum.

The absorber layer AL can provide the mechanical support for the display structure shown in Fig. 4 by itself, or an additional substrate material S can be used to provide the necessary mechanical rigidity and/or to provide means for supplying voltage to the row electrodes RE. If substrate material S is used, the absorber layer AL can be formed as a thin layer above the substrate material S. Silicon wafer may be used as substrate material S. The substrate S may be also for example glass, plastic, ceramic or metal. The light absorber layer AL may formed on the substrate S with any suitable method known as such.

In a similar manner, the front filter plate FL can provide mechanical support and protection against the ambient environment by itself, or an additional top layer TL can be used together with the front filter plate FL. Figure 5 is a schematical cross-section diagram describing another possible embodiment of the invention. In this embodiment a partially reflective mirror layer M is arranged below the column electrode CE and facing towards the viscoelastic material G. This allows to affect in a more detailed manner the optical properties of the resonant reflector cavity.

In the following, the operation of the reflective display device according to invention is explained further in order to clarify the formation of different colours and level of brightness observed by a person viewing the display.

In an advantageous embodiment of the invention, proper selection of the materials and dimensions affecting the properties of the aforementioned resonant reflector cavity allows each pixel to be electrically switched between reflecting all (white) or any colour/s to absorbing all colours. The properties of the resonant reflector cavity comprising the front filter plate FL and/or the mirror layer M, the viscoelastic layer G and the absorber layer AL, are tuned by applying different voltages between the row RE and the column CE electrodes and thus changing the thickness of the viscoelastic layer G and the distance between the surface of the viscoelastic layer G and the opposite front filter plate FL.

The on-off duty cycle of voltages activating the row RE and column CE electrodes can be used to adjust the level of brightness of a pixel. If the resonant reflector cavity is arranged to reflect substantially all incident light L when no voltages are present, and a certain voltage V is arranged to tune the resonant reflector cavity to reflect, for example, only green light and absorb other colours, then the adjustment of the relative time when voltage V is applied in comparison with the time when no voltage is applied, changes the brightness of the pixel observed as a green pixel by a person viewing the display. When the voltages and the pixel is activated/deactivated at frequencies which are sufficiently high, for example at > 25 Hz, the human visual perception is not able to distinguish the flickering between the maximum brightness (of green) and black, but instead observes an pixel with a certain brightness of green between black and the maximum brightness.

In another embodiment of the invention, the selection of materials and dimensions affecting the properties of the resonant reflector cavity within a pixel are selected to allow the switching of a pixel between absorbing all colours (black) to reflecting all colours (white). This provides a black-and-white display structure. The main benefit of this approach is that the control of the electrode voltages is simplified because only a single, or a few discrete voltage levels need to be generated instead of continuously adjustable electrode voltages.

In another embodiment of the invention, the selection of materials and dimensions affecting the properties of the resonant reflector cavity allows the fabrication of matrix of pixels, so that said matrix contains three or more different types of pixels, each type of pixels capable of being electrically switched between reflecting a certain primary colour to absorbing or reflecting all colours. Thus, a group of adjacent pixels can be used to create a range of colours in a manner similar to a RGB colour system, where colours are created as combinations of the primary red, green and blue colours. An advantage of this approach is the simpler structure and control of the individual pixels, but at the same time because of several adjacent pixels are required to form a single "virtual" colour pixel, the effective colour pixel density is somewhat reduced.

The device according to the last mentioned embodiment can be realized by simply adding a mosaic colour filter in front of a black-and- white flat panel display as is generally known in the art. Alternatively, the creation of primary colours can involve arranging the different type of pixels (corresponding to different primary colours) to be activated with different electrode voltage levels, and/or to have different selection of materials and dimensions.

It is obvious for a person skilled in the art, that the invention can also be used to create reflective display devices where all pixels can be switched between a certain single colour and white or black. It is also obvious that a single display matrix can contain pixels with different sizes and/or shapes.

In all of the aforementioned embodiments of the invention, the on-off duty cycle of voltages activating the row and column electrodes can be used to adjust the level of brightness of the pixels. The duty cycle can also be used to adjust the relative amount of primary colours when creating a desired colour as a combination of the primary colours. In addition, it is not the intention of the present invention to restrict the application of the pixels in passive-matrix addressing. Active-matrix backplanes can be used while remaining within the scope of this invention.

In a situation where ambient light level (daylight or artificial light) is not adequate for viewing the reflective display device according to the invention, additional frontlights can be used to illuminate the pixel matrix through the top layer TL and/or the front filter plate FL. The frontlights can be realized in any manner known in the art.

The properties of the front filter plate FL, mirror layer M or the light absorber layer AL may vary from one pixel to another. Also the total length of the resonant cavity may be selected to be different in different pixels.

Even if the main applications of the present invention can be found in the portable personal devices such as for example mobile communication devices, pocket computer games, digital still/video cameras and pocket video players, the invention can also be utilized in many other types of applications. These include, for example, ruggedized portable instruments, automotive devices, aircraft devices, desktop and laptop computers and even large screen displays and TV sets.

Even though the invention has been shown and described above with respect to selected types of embodiments, it should be understood that these embodiments are only examples and that a person skilled in the art could construct other reflective display devices utilizing techniques other than those specifically disclosed herein while still remaining within the spirit and scope of the present invention. It should, therefore, be understood that various omissions and substitutions and changes in the form and detail of the display devices illustrated, as well as in the operation of the same, may be made by those skilled in the art without departing from the spirit of the invention.

For example, it is expressly intended that all combinations of those elements which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to restrict the invention only in the manner indicated by the scope of the claims appended hereto.

Claims

Claims :

1. A direct view reflective flat panel display device comprising an array of pixels (P), said pixels each comprising an electrically tunable optical resonant reflector cavity (C), characterized in that said resonant reflector cavity (C) within a single pixel comprises at least a layer of dielectric and viscoelastic transparent material (G) applied onto a light absorber layer (AL), an transparent front filter plate (FL) arranged opposite to said viscoelastic layer (G) leaving a gap (AG) between the free surface of said viscoelastic layer (G) and said front filter plate (FL), said light absorber layer (AL), said viscoelastic layer (G) and said front filter plate FL forming a resonant reflector cavity (C) for incident light (L) entering said cavity through said front filter plate (FL), and a transparent row electrode (RE) formed on said absorber layer (AL) below said viscoelastic layer (G), - a transparent column electrode (CE) formed on the surface of said front filter plate (FL) towards the viscoelastic layer (G), said row electrode (RE) and column electrode (CE) being arranged in a manner that said electrodes (RE,CE) are capable of receiving voltages for generating an electric field to alter the thickness of said viscoelastic layer (G) in the area between said row (RE) and column (CE) electrodes in order to change the reflection and absorption of incident light (L) in said resonant reflector cavity (C).

2. The device according to claim 1 , characterized in that said resonant reflector cavity (C) comprises further a partially reflective mirror layer (M) arranged between said column electrode CE and said viscoelastic layer (G).

3. The device according to claim 1 or claim 2, characterized in that said resonant reflector cavity (C) is designed to be electrically switched between reflecting a single colour to absorbing all colours.

4. The device according to claim 1 or claim 2, characterized in that said resonant reflector cavity (C) is designed to be electrically switched between reflecting all colours to absorbing all colours.

5. The device according to claim 1 or claim 2, characterized in that said resonant reflector cavity (C) is designed to be electrically switched between reflecting a primary colour, for example a RGB-colour, to absorbing all colours.

6. The device according to claim 5, characterized in that a group of adjacent pixels each capable of reflecting a single primary colour are arranged to form together a larger multi-colour pixel, the colour of said multi-colour pixel corresponding to the combination of said primary colours.

7. The device according to any of the foregoing claims, characterized in that a mosaic colour filter is arranged in front of the device.

8. The device according to any of the foregoing claims, characterized in that the on-off duty cycle of voltages activating said row (RE) and column (CE) electrodes is arranged to be used to adjust the level of brightness of said pixel.