Active matrix electro-optic display device with storage capacitors and projection color apparatus employing same

Abstract: The capacitance of a pixel of an active matrix liquid crystal valve is increased without reducing its optical transparency by adding polysilicon islands (301) under the adjacent gate lines (12), degeneratively doping the islands (301) to render them conductive under operating conditions, thermally oxidizing the islands to form dielectric layers (302), depositing gate lines (12) over the dielectric layers to form extra capacitors (300), and connecting the islands (301) to the adjacent pixel electrodes (10). Such active matrix displays are advantageously employed in a color projection TV to produce the separate red, blue and green components of a full color display.

Description: The invention relates to an active matrix electro-optic display device comprising a pair of walls, at least one of which is optically transparent, an array of pixel electrodes, an array of switches associated with the pixel electrodes, and an array of orthogonally related row and column electrodes interconnecting the array of switches and pixel electrodes, the arrays arranged on the inner surface of one of the walls, a counter electrode on the inner surface of the other wall, orienting layers overlying the arrays and the counter electrode on the walls, and an electro-optic material filling the space between the walls and contacting the orienting layers, the display including an array of storage capacitors comprised of islands of a first material, each in electric contact with a pixel electrode, the islands underlying the row electrodes adjacent the pixel, and a layer of a dielectric material between the islands of the first material and the overlying row electrodes. A two-dimensional array of such cells, each cell individually addressable through a matrix of row and column electrodes, can be used to build up a display such as a video image, where each cell constitutes a peel of information. During the repetitive "scanning" (sequential addressing) of the pixel rows, e.g., ion response to a video signal, it has been found advantageous to reduce cross-talk between the pixels by providing a separate switch for each pixel. This so-called "active matrix" addressing scheme depends for its success on the ability of the pixels to hold their charge received during addressing, by virtue of their inherent capacitances, until they are readdressed. However, charge can leak off the pixels, e.g., through the liquid crystal material or through the switch. It is thus common in the design of the pixel to add extra capacitance in order to improve its charge storage capacity. One way of introducing the extra capacitance that has been used is to extend the transparent pixel electrode to overlap the adjacent row electrode above the cross-over dielectric between the row and column electrodes. Since the row electrode remains at a fixed potential except when being addressed, it becomes a convenient base plate for the extra capacitor. The amount of extra capacitance that can be obtained in this manner is about the same as the capacitance of the liquid crystal pixel. In projection television, for example, this may not be adequate since the pixels are typically very small (for example 250,000 pixels may be arranged in a 2 square inch area), and hence so is the LC capacitance of each pixel. A device as mentioned in the opening paragraph is known from JP-A-63 070 832. In the example shown the material of the islands is similar to that used for the switching elements viz. semiconductive (amorphous or polycrystalline) silicon. Since this material is semiconductive the capacitances are voltage dependent and too low for e.g. projection television applications. The voltage dependency also implies that the pixel voltage changes after selection. It is an object of the invention to provide an active matrix electro-optic display device having increased capacitance. It is another object of the invention to provide such a display device having constant capacitance. It is another object of the invention to provide such a display device with increased capacitance without substantially reducing the optical transmission of the display. It is another object of the invention to provide a colour projection apparatus incorporating one or more of such active matrix electro-optic devices. A device according to the invention is characterized in that the first material consists of a conductive material. In accordance with the invention, the capacitance of a pixel of an active matrix electro-optic display device is increased in a structure having a conductive island of a conductive material which is conducting under all operating conditions underlying the row electrode adjacent the pixel, the conductive island electrically connected to the pixel, and a thin dielectric layer between the conductive island and the overlying row electrode, to form a capacitor outside the perimeter of the pixel electrode so that the transmission of the pixel electrode is substantially unaffected thereby. According to a preferred embodiment of the invention, the dielectric comprises an upper surface region of the conductive island, this region converted to a dielectric state by surface treatment, for example thermal oxidation, of the island. According to a particularly preferred embodiment of the invention, the elements defined by electrodes of a transparent conductive material such as ITO is associated with an array of TFT switches whose sources are connected to column electrodes, whose gates are connected to orthogonally related row electrodes and whose drains are connected to the pixel electrodes. According to another aspect of the invention, the capacitance of a pixel of an active matrix electro-optic display device is increased by a method comprising the steps of: a) providing a conductive island of a conductive material which is conducting under all operating conditions of the display device, b) providing a dielectric layer between the conductive island and the overlying row electrode, and c) electrically connecting the conductive island to the pixel electrode, at least above the islands of conductive material. According to another preferred embodiment of the method of the invention, the active matrix electro-optic display device is a TFT switched LCD, the conductive island is provided by first depositing polysilicon and then degeneratively doping it either n or p type, and the dielectric layer is provided by thermally oxidizing the surface of the doped polysilicon island to convert it to a silicon oxide layer. According to another aspect of the invention, a color projection apparatus incorporates one or more of such display devices with increased capacitance. Preferably the apparatus is a television, and one display device is provided for each of the separate red, blue and green channels of a television signal, whereby a full color display is formed by superimposing the three separate images obtained on a projection screen. The invention will be described with reference to the drawing, in which Figure 1 is a diagrammatic perspective view showing an embodiment of an arrangement for providing increased capacitance to a pixel of an active matrix TFT switched LCD according to the invention; Figure 2 is a diagrammatic cross-section of an LCD of the type shown in Figure 1; and Figure 3 is a diagrammatic plan view of one embodiment of an LCD color PTV system employing three LCDs of the invention. Figure 1 shows an arrangement for providing added capacitance to such a pixel in accordance with the invention. The nth pixel electrode in a column of an array of such electrodes 10 is addressed through TFT 18 by means of its electrical connection to drain region 28 of polysilicon layer. A capacitor 300 for this pixel is formed by providing a conductive island 301 under the adjacent gate electrode 12, and by providing a thin dielectric layer 302 on the surface of the conducting layer 301 under the gate electrode. Electrical connection of the nth pixel eelctrode 10 to the capacitor 300 is provided at area 303 by conventional means. Preferably, the conducting island 301 is a thin layer of polysilicon which is deposited on the underlying substrate at the same time that polysilicon channel region 20 is deposited for TFT 18. Since the polarities of the voltages in the pixel during operation would be such as to keep such a thin polysilicon island 301 depleted and hence non-conducting, the island is degeneratively doped either n or p type in order to render it conducting under all operating conditions. A principle advantage of using such a polysilicon island 301 is that the dielectric for the capacitor may be formed by thermal oxidation of the surface of the island 301 at the same time that the surface of the polysilicon channel region 20 is thermally oxidized to form the gate dielectric for the TFT 18. As is known, the dopant for the island 301 may be diffused or implanted before or after thermal oxidation is carried out. One way of achieving the desired structure is to deposit a thin polysilicon layer uniformly over the entire substrate, deposit a thick photoresist layer on the thin polysilicon layer, pattern the photoresist layer to cover the regions where the TFTs will be formed, and implant the exposed areas of polysilicon to achieve degenerative doping, wherein the photoresist acts as an implant mask for the TFT regions. The resist is then stripped and the polysilicon layer is etched to define the channel regions 20 for the TFTs and the conducting islands 301 for the capacitors. After this definition, the polysilicon structures remaining are thermally oxidized, during which the implants of the island 301 are activated. Because the thin polysilicon island 301 for the capacitor 300 is degeneratively doped, it cannot be biased into depletion during operation and thus is able to function as a capacitor electrode. Gate line 12, which may also be of polysilicon, are then formed over the dielectric layer 302 to complete the capacitor 300. In this construction, the capacitors do not occupy any of the normally transparent regions of the pixels defined by the perimeters of the pixel electrodes 10, and thus do not effect transparency of the pixels. A cross-section of the device of the type shown in Figure 1 is shown in Figure 2. Of two glass supporting plates 41 and 42, the supporting plate 41 carries on its inner surface a pattern of row and column electrodes, and an array of pixel electrodes and associated TFTs, illustrated schematically as layer 43. Supporting plate 42 carries a transparent counter electrode 44. Orienting layers 45 and 46 are present on the inner surface of the layer 43 and the counter electrode 44, these irienting layers formed for example by vapor deposition of silicon oxide (SiO) at an oblique angle to the normal of the surfaces. A nematic liquid crystal material 47, such as the proprietary formulation known as ZLI 84-460, manufactured by E. Merck of Darmstadt, West-Germany, containing a chiral addition of for example n cyano 4'-(2-methyl)-isobutyl-biphenyl (obtainable as CB15 from DBH Chemicals, Poole, England), is present between the supporting plates. The spacing beween the plates is typically about 6 to 8 microns and is maintained by spacer beads which are distributed regularly over the surfaces of the plates, and are not shown in the drawing. A sealing edge 48 joins the plates together at their circumference. As a result of the obliquely vapor deposited silicon oxide layers, the liquid crystal molecules undergo a rotation of approximately 90° over the cell thickness. The chiral additive determines the direction of twist of the liquid crystal molecules, CB15 imparting a counter clockwise twist. The outer surfaces of the supporting plates 41 and 42 furthermore each carry a linear polarizer. Polarizer 49 intercepts the incident light indicated by the arrow in the drawing, and analyzer 50 intercepts the light transmitted by the device. The display cell can be switched from a first stable state to a second stable state differing optically from the first by driving the electrodes in a suitable known manner. With a parallel orientation of polarizer and analyzer, a selected display cell or pixel in the off-state (no voltage applied) will be dark (light absorbing). By rotating one of the polarizers 90° with respect to the other, (crossed orientation) a white (light transmitting) off-state can be achieved. Referring now to Figure 3, there is shown a diagrammatic representation of an LCD color PTV system employing a front projection screen. In the arrangement shown, light from a tungsten halogen lamp 51 is variously transmitted and reflected through a cross pair of dichroic mirrors 52 and adjacent mirrors 53 to pass through three active matrix TFT switched LCDs 54, 55 and 56. These three LCDs, acting as light valves in the transmissive mode, modulate three channels to create the red, green and blue portions of a TV picture. The dichroic mirrors 52 break the white light from source 51 into red, blue and green portions of the spectrum. These three color portions are then recombined by way of a dichroic prism 57. The recombined light is projected by way of a projection lens 58 along a projection axis P to a projection screen 59. This arrangement also includes a condenser arrangement consisting of a condenser lens 60 and a reflector 61. Condenser lens 60 is designed for maximum collection efficiency to collect light emitted in the +X direction. The spherical reflector 61 collects light emitted in the -X direction and images the light of the lamp back onto itself. The invention has been described in terms of a limited number of embodiments. Other embodiments within the scope of the invention and the appended claims are contemplated. For example, the dielectric layer of the extra capacitor may be silicon nitride formed on the conducting island, for example, by chemical vapor deposition. The pixels of the array need not be switched by a three terminal device such as a thin film transistor, but may also be switched by a two terminal device, for example, a non-linear switch such as a diode or MIM. In addition, the electro-optic medium need not be a nematic liquid crystal, but could be another liquid crystal material such as a super twist liquid crystal, or another electro-optic material such as a ferro-electric or electro-luminescent material. Furthermore, the display device need not be a light valve operating in the transmissive mode, but could also operate in the reflective mode.

Claims: An active matrix electro-optic display device comprising a pair of walls (41,42), at least one of which is optically transparent, an array of pixel electrodes (10), an array of switches (18) associated with the pixel electrodes, and an array of orthogonally related row (12) and column electrodes (14), the array of switches interconnecting the array of pixel electrodes and the array of row and column electrodes, the arrays being arranged on the inner surface of one of the walls, a counter electrode (44) on the inner surface of the other wall, orienting layers overlying the arrays and the counter electrode on the walls, and an electro-optic material (47) filling the space between the walls and contacting the orienting layers, the display including an array of storage capacitors (300) comprised of islands (301) of a first material, each in electrical contact with a pixel electrode (10), each island (301) underlying the row electrode (12) adjacent the pixel, and a layer of a dielectric material (302) between the island of the first material and the overlying row electrode, characterized in that the first material consists of a conductive material which is conducting under all operating conditions. The display device of Claim 1 in which the switches are TFT's, the row electrodes interconnect the TFT gate electrodes, the column electrodes interconnect the TFT source electrodes, and the pixel electrodes are each connected to an associated TFT drain electrode. The display device of Claim 2 in which the electro-optical material is a liquid crystal material. The display device of Claim 2 in which the conductive material is degeneratively doped polysilicon. The display device of Claim 4 in which the dielectric layer is formed by the thermal oxidation of a surface region of the conductive material. A method of forming an active matrix electro-optic display device comprising a pair of walls, at least one of which is optically transparent, an array of pixel electrodes, an array of switches associated with the pixel electrodes, and an array of orthogonally related row and column electrodes, the array of switches interconnecting the array of pixel electrodes and the array of row and column electrodes, the arrays being arranged on the inner surface of one of the walls, a counter electrode on the inner surface of the other wall, orienting layers overlying the arrays and the counter electrode, and an electro-optic medium filling the space between the walls and contacting the orienting layers, the method comprising the steps of: a) forming islands of a conductive material which is conducting under all operating conditions of the display device, b) forming layers of a dielectric material on the conducting islands, c) forming the row electrodes on the dielectric layers, at least above the islands of conductive material, and d) electrically connecting the conducting islands to the adjacent pixel electrodes, thereby to form a storage capacitor for each pixel of the array. The method of Claim 6, in which the switches are TFTs, the row electrodes interconnect the TFT gate electrodes, the column electrodes interconnect the TFT source electrodes, and the pixel electrodes are each connected to a TFT drain electrode. The method of Claim 7, in which the electro-optic material is a liquid crystal material. The method of Claim 8, in which the conductive islands comprise degeneratively doped polysilicon. The method of Claim 9, in which the islands of conductive material are formed by uniformly covering one of the walls with a layer of polysilicon, masking the regions in which the TFTs are to be formed with a photoresist, degeneratively doping the exposed portions of the polysilicon, stripping away the photoresist, and selectively etching the polysilicon to leave undoped polysilicon regions corresponding to the channel regions of the TFTs and doped polysilicon islands corresponding to the storage capacitors. The method of Claim 10, in which the channel regions and the doped island regions are simultaneously thermally oxidized to form a thin oxide layer corresponding to the gate dielectric for the TFTs and to the capacitor dielectric for the storage capacitors. A color projection apparatus including: three electro-optic display devices each comprising a pair of walls, at least one of which is optically transparent, an array of pixel electrodes, an array of switches associated with the pixel electrodes, and an array of orthogonally related row and column electrodes, the array of switches interconnecting the array of pixel electrodes and the array of row and column electrodes, the arrays being arranged on the inner surface of one of the walls, a counter electrode on the inner surface of the other wall, orienting layers overlying the arrays and the counter electrode on the walls, and an electro-optic material filling the space between the walls and contacting the orienting layers; means for creating separate monochrome components of a color image, each corresponding to one of the three primary colors red, blue and green, on the display devices; a projection screen; and means for projecting the image components onto the screen to form a full color display, characterized in that at least one of the display devices comprises a device according to claims 1-6.

Inventors: KHAN BABAR, STUPP EDWARD HENRY, KHAN, BABAR, STUPP, EDWARD HENRY
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: G02F1
IPC codes: H01L29, H01L27, G02F1, G09F9, H01L21