Active matrix liquid crystal display cell

Abstract: In an active matrix liquid crystal display cell having a switching active element (306), first (308) and second (310) transparent electrodes between which liquid crystal (316) is disposed, and a shading layer (312) for protecting the switching active element and enhancing the visibility of the display, the shading layer is partially laid under the first transparent electrode and separated therefrom by an insulating layer (314) interposed between the shading layer and the first transparent electrode.

Description: Background of the Invention The present invention relates to an active matrix liquid crystal display cell which serves to provide a single picture element, namely, a pixel, and an active matrix liquid crystal display panel comprising a plurality of the active matrix liquid crystal display cells arranged in a matrix fashion. In the manner which will later be described more in detail, a conventional active matrix liquid crystal display cell of the type described comprises a switching active element for driving liquid crystal, a capacitance element which stores charge for driving the liquid crystal, a pair of transparent electrodes the space between which is filled with the liquid crystal, and a shading layer for protecting the switching active element and enhancing the contrast of the image. It is a recent trend that each of the active matrix liquid crystal display cells becomes small in size so as to increase numbers thereof for getting an excellent resolution power of the active matrix liquid crystal display panel. However, the conventional active matrix liquid crystal display panel cannot provide a bright image, due to a structure of each active matrix liquid crystal display cell, when the active matrix liquid crystal display cell is small in size. EP-A-0,136,509 describes an active matrix type display apparatus comprising a thin film transistor, a pair of transparent electrodes the space between which is filled with liquid crystal, one of said electrodes being connected to the thin film transistor, and a light shielding layer provided on that one of the electrodes, a part of the light shielding layer opposing a part of that one of the electrodes through an insulation film to form a storage capacitor. Summary of the Invention It is therefore an object of at least the preferred embodiments of this invention to provide an active matrix liquid crystal display cell and a panel comprising a plurality thereof which can provide a bright image, even though the active matrix liquid crystal display cell is small in size. Other objects of this invention will become clear as the description proceeds. The present invention provides an active liquid crystal display cell, comprising: a switching active element including a gate connected to a first gate bus; first and second transparent electrodes between which liquid crystal is disposed, said first transparent electrode being connected to said switching active element for driving said liquid crystal and said second transparent electrode being positioned opposite to said first transparent electrode; a shading layer which is made of a conductive material, part of said shading layer being disposed under part of the first transparent electrode; an insulating layer which is interposed between said shading layer and said first transparent electrode; a capacitance element being defined by said shading layer, said first transparent electrode, and said insulating layer interposed therebetween; and an additional capacitance element positioned under said shading layer and separated therefrom by an insulating layer; said additional capacitance element being defined by a polycrystalline silicon layer, a gate insulating layer and a gate connected to a second gate bus. "Under" is to be interpreted with respect to the cell when oriented such that the liquid crystal is above the first transparent electrode. The shading layer may be made of aluminum and the insulating layer may be formed by an anodic oxidation of the aluminum. The present invention also provides an active matrix liquid crystal display panel comprising a plurality of active matrix liquid crystal display cells arranged in a matrix fashion, each of the active matrix liquid crystal display cells being as set forth above. Brief Description of the Drawings The invention will be described merely by way of example with reference to the accompanying drawings, wherein: Fig. 1 is a schematic vertical sectional view of a conventional active matrix liquid crystal display cell; Fig. 2 is a block diagram of an active matrix liquid crystal display panel according to a first embodiment of this invention; Fig. 3A is a plan view of an active matrix liquid crystal display cell; Fig. 3B is a schematic vertical sectional view taken across A-B-C of the active matrix liquid crystal display cell illustrated in Fig. 3A; Fig. 4 shows an equivalent circuit of the active matrix liquid crystal display cell illustrated in Figs. 3A and 3B; Fig. 5 is a schematic vertical sectional view of a capacitance element of an active matrix liquid crystal display cell) Fig. 6 is a schematic vertical sectional view of another capacitance element of an active matrix liquid crystal display cell; Fig. 7 is a schematic vertical sectional view of a modification of the capacitance element illustrated in Fig. 6; Fig. 8 is a schematic vertical sectional view of a capacitance element of an active matrix liquid crystal display cell; Fig. 9A is a plan view of an active matrix liquid crystal display cell according to an embodiment of this invention; Fig. 9B is a schematic vertical sectional view taken across X-Y of the active matrix liquid crystal display cell illustrated in Fig. 9A; and Fig. 10 shows an equivalent circuit of the active matrix liquid display cell illustrated in Figs. 9A and 9B. Referring to Fig. 1, a conventional active matrix liquid crystal display cell will first be described for a better understanding of this invention. In Fig. 1, the conventional active matrix liquid crystal display cell 100 comprises a principal glass substrate 102, an opposite glass substrate 104, a transparent pixel electrode 106 deposited at the side of the principal glass substrate 102, a transparent opposite electrode 108 formed under the whole area of the opposite glass substrate 104, and a liquid crystal 110 filling the space between the transparent pixel electrode 106 and the transparent opposite electrode 108, a switching active element 120, a capacitance element 130, and a shading layer 140. The switching active element 120 is formed by a thin-film transistor (TFT) of a bottom gate type, which consists of a gate 121, a gate insulating layer 122, an amorphous silicon layer 123, a doped amorphous silicon layer 124, a drain 125, and a source 126. The gate 121 is connected to a gate bus (not shown) and the drain 125 is connected to a drain or signal bus (not shown), while the source 126 is connected to the transparent pixel electrode 106. The capacitance element 130 consists of the transparent pixel electrode 106, a transparent electrode 132, and a dielectric layer 134 which is defined by a portion of the gate insulating layer 122. The shading layer 140 is partially deposited under the transparent opposite electrode 108. The liquid crystal 110 also fills the space between the switching active element 220 and the shading layer 140, as illustrated in Fig. 1. In the active matrix liquid crystal display cell 100, the transparent pixel electrode 106, the transparent electrode 132 and the transparent opposite electrode 108 are formed from ITO (Indium Tin Oxide) layer. As mentioned in the preamble of the instant specification, a plurality of the active matrix liquid crystal display cells 100 are arranged in a matrix fashion to form an active matrix liquid crystal display panel (not shown). In order to increase the number of active matrix liquid crystal display cells 100 to improve the resolution of the active matrix liquid crystal display panel, each active matrix liquid crystal display cell 100 is sometimes decreased in size to 50 x 50 micron meters. In such a case, two problems arise in the conventional active matrix liquid crystal display cell 100. First, since the shading layer 140 is deposited at the side of the opposite glass substrate 104, the shading layer 140 must be fully expanded to cover the whole area of the switching active element 120 and an edge portion of the transparent pixel electrode 106 in order to completely protect the switching active element 120 from light and to prevent a leak of light around the transparent electrode 132. However, an allowance of about 10 micron meters is generally required when the opposite glass substrate 104 is positioned to be fitted to the principal glass substrate 102. Further, an overlap of approximately 5 micron meters is required between the transparent electrode 132 and the shading layer 140. Accordingly, an allowance of about 15 micron meters is required at each side of the active matrix liquid crystal display cell 100. Thus a total allowance of 30 micron meters is required at both sides of the active matrix liquid crystal display cell 100, so in the case of a cell of 50 x 50 micron meters the effective pixel portion is only 20 x 20 micron meters. Thus, it is difficult to utilize light efficiency. Second, there is a problem because the capacitance element 130 is formed by the transparent pixel electrode 106 and the transparent electrode 132 both of which are made of ITO and between which the dielectric layer 134 is interposed. Since the transmittance of the each ITO layer is about 90%, the overall transmittance is about 80% when light is transmitted through the transparent pixel electrode 106 as well as the transparent opposite electrode 108. Furthermore, since the transparent electrode 132 has a poor conductivity, it is necessary that the gate 121' is connected to wiring (not shown) having low resistance. If the width of the gate 121' is 5 micron meters, the above-mentioned effective pixel portion further reduces to 15 x 20 micron meters. Thus, in the conventional active matrix liquid crystal display cell 100, it is difficult to utilize light efficiently, when the cell 100 is a minute size. As a result, an active matrix liquid crystal display panel which comprises a plurality of the cells 100 cannot provide a bright image. Referring now to Figs. 2, 3A and 3B, there is described an active matrix liquid crystal display cell and an active matrix liquid crystal display panel comprising a plurality of the active matrix liquid crystal display cells. In Fig. 2, an active matrix liquid crystal display panel 200 comprises a plurality of active matrix liquid crystal display cells 30 which are arranged in a matrix fashion, signal buses 150, and gate buses 152. As illustrated in Figs. 3A and 3B, each of the active matrix liquid crystal display cells 300 comprises a principal transparent glass substrate 302 having a principal surface 302a directed upwards of Fig. 3B, an opposite transparent glass substrate 304 having a lower surface 304a directed downwards of Fig. 3B, a thin-film transistor (TFT) 306 of top-gate type which is formed on the principal surface 302a, a first transparent electrode 308 connected to the TFT 306 for driving liquid crystal, a second transparent electrode 310 which is formed on the lower surface 304a and which is positioned opposite to the first transparent electrode 308, a shading layer 312 which is made of aluminum, and an insulating layer 314 which is formed by an anodic oxidation of aluminum. The shading layer 312 may alternatively be made of the other materials having a low transmittance. The principal transparent glass substrate 302 and the opposite transparent glass substrate 304 are made of quartz glass. The TFT 306 serves as a switching active element, which comprises a polycrystalline silicon layer 306a, a gate insulating layer 306b, a gate 306c, a drain 306d, and a source 306e. The drain 306d is connected to the signal buses 150, while the gate 306c is connected to the gate buses 152. A liquid crystal 316 fills the space between the first transparent electrode 308 and the second transparent electrode 310. The shading layer 312 has an upper and a side surfaces 312a and 312b. The insulating layer 314 is formed on the upper and the side surfaces 312a and 312b by the above-mentioned anodic oxidation of aluminum. The insulating layer 314 is therefore made of Al 2 O 3 . Preferably, the insulating layer 314 has a thickness between 100 and 200 nm. In order to form the shading layer 312 and the insulating layer 314, at first, an aluminum lamina is deposited in a known manner on a surface of an inter-electrodes insulating layer 315 to be patterned into the shading layer 312 having a predetermined pattern. The shading layer 312 is then anodically oxidized to have the insulating layer 314 of Al 2 O 3 on its upper and side surfaces 312a and 312b. Thus, the insulating layer 314 is interposed between the shading layer 312 and the first transparent electrode 308 with the shading layer 312 partially laid under the first transparent electrode 308 through the insulating layer 314. With this structure, a capacitance element 400 is defined by the shading layer 312, the first transparent electrode 308, and the insulating layer 314 interposed therebetween. As shown at segment D in Fig. 3A, an overlapping area of the shading layer 312 and the first transparent electrode 308 need to have a width between 4 and 8 micron meters. The portion where the source 306e is connected to the first transparent electrode 308 can not be covered by the shading layer 312, as illustrated in Fig. 3B. Accordingly, the source 306e serves as a shading layer at this portion. As symbolized in Fig. 4, two capacitance elements 400 and 410 are formed in parallel. One comprises the first transparent electrode 308, the second transparent electrode 310, and the liquid crystal 316 filled therebetween, while another comprises the first transparent electrode 308, the shading layer 312, and the insulating layer 314 interposed therebetween. Bias voltages V 1 and V 2 are applied to the capacitance elements 400 and 410, respectively. In this embodiment, since the insulating layer 314 is formed by the anodic oxidation method, there are less defects, such as pin holes or the like, in the insulating layer 314 than that formed by the other method carried out on a lower temperature condition, for example, a plasma CVD, sputtering, or the like. Consequently, the capacitance element 410 having a large dielectric area can be fabricated in a good yield. Referring to Fig. 5, description will proceed to an active matrix liquid crystal display cell, The active matrix liquid crystal display cell according to the second embodiment has a structure similar to that of the first embodiment except that the insulating layer 314 comprises primary and secondary insulating layers 314A and 314B. The primary insulating layer 314A is formed by an anodic oxidation of the shading layer 312, and thus is made of Al 2 O 3 , while the secondary insulating layer 314B is formed in another manner, for example the plasma CVD method. In this embodiment, the primary insulating layer 314A of Al 2 O 3 is formed by the anodic oxidation of the shading layer 312 to have a thickness of 100 nm, while the secondary insulating layer 314B of silicon nitride is formed by plasma CVD method to have a thickness of 100 nm. In this structure, since the insulating layer 314 comprises two stacked layers, namely, primary and secondary insulating layers 314A and 314B, it becomes possible that the above-mentioned defects, such as pin holes, in the insulating layer 314 is decreased in number, compared with the above-described first embodiment. Referring to Figs. 6 and 7, description will proceed to other active matrix liquid crystal display cells and modifications thereof. The active matrix liquid crystal display cells and the modification have structures similar to that of the first and the second embodiments except that the anodic oxidation method is not used to fabricate those insulating layers. In Fig. 6, a single insulating layer 314' is formed on the shading layer 312. The insulating layer 314' is made of silicon nitride and formed by a plasma CVD method to have a thickness of, for example, 200 nm. In Fig. 7, double insulating layers 314'A and 314'B are formed on the shading layer 312. The insulating layer 314'A is made of silicon nitride and formed by a plasma CVD method to have a thickness of, for example, 100nm. On the other hand, the insulating layer 314'B is made of silicon oxide and formed by a sputtering to have a thickness of, for example, 100nm. Referring to Fig. 8, description will proceed to another active matrix liquid crystal display cell. This active matrix liquid crystal display cell has a structure similar to that described with reference to Fig. 3A except that the shading layer 312 comprises double layers, one of which is a shading layer 312A of aluminum and another of which is a shading layer 312B of tantalum overlaid on the shading layer 312A. The shading layer 312B of tantalum is readily oxidized by the anodic oxidation to have an insulating layer 320 of Ta 2 O 5 which has a high dielectric constant. Accordingly, another insulating layer 321 of alumina is formed on the side surfaces of the shading layer 312A. With this structure, it becomes possible greatly to improve the storage capacitance compared with the above-mentioned structures. In this example, the insulating layer 321 of alumina is used to enhance a shading effect. Referring to Figs. 9A and 9B, there is described an active matrix liquid crystal display cell according to an embodiment of this invention. The active matrix liquid crystal display cell according to the this embodiment has a structure similar to the cell described with reference to Fig. 3A except that the active matrix liquid crystal display cell further comprises an additional capacitance element. As illustrated in Figs. 9A and 9B, the active matrix liquid crystal display cell 300' according to the embodiment further comprises the additional capacitance element 330 which is positioned under the overlapping section of the first transparent electrode 308 and the shading layer 312 through an insulating layer 314. The additional capacitance element 330 is defined by a polycrystalline silicon layer 330a, a gate insulating layer 330b, and a gate 330c. The gate 330c is electrically connected to the gate buses 152. The polycrystalline silicon layer 330a is connected to the polycrystalline silicon layer 306a. In order to fabricate the polycrystalline silicon layer 330a, a polycrystalline silicon lamina is, at first, formed on the principal transparent glass substrate 302. The polycrystalline silicon lamina is then patterned to a predetermined shape to form a patterned polycrystalline silicon layer. Second, the gate insulating layer 330b is deposited on the patterned polycrystalline silicon layer. Thereafter, a section of the patterned polycrystalline silicon layer overlaid by the gate 330c is doped by an ion implantation method to form the polycrystalline silicon layer 330a. In this embodiment, the additional capacitance element 330 is positioned under the overlapping section of the first transparent electrode 308 and the shading layer 312 through which light cannot pass originally. Accordingly, the additional capacitance element 330 makes it possible that the storage capacitance is greatly improved without reducing the efficient use of the light. As symbolized in Fig. 10, the additional capacitance element 330 is also formed in parallel to the two capacitance elements 400 and 410 which were mentioned in respect of Fig. 3A, as symbolized in Fig. 4. The additional capacitance element 330 is connected between the source 306e of a transistor Q 1 and the gate 306c of a transistor Q 2 as symbolized in Fig. 10. A TFT generally cannot avoid a leakage current between 0.1 pA and 1.0 pA. It is therefore desired that the storage capacitance is large enough to store a signal voltage without reduction while the active matrix liquid crystal display cell is scanned. When the active matrix liquid crystal display cell having such a TFT is small, in other words, is decreased in size into, for example 50 x 50 micron meters, the storage capacitance by the two capacitance elements 400 and 410 is 100 pF, at the most. The capacitance of 100 pF is not enough to store the signal voltage, as mentioned above, in view of the aforesaid leakage current of the TFT. Accordingly, it is very advantageous that the active matrix liquid crystal display cell 300' according to this embodiment comprises the additional capacitance element 330. While this invention has thus far been described in conjunction with only an embodiment thereof, it will readily be possible for those skilled in the art to put this invention into practice in various other manners within the scope of the appended claims.

Claims: An active liquid crystal display cell, comprising: a switching active element (306) including a gate (306) connected to a first gate bus (152); first (308) and second (310) transparent electrodes between which liquid crystal (316) is disposed, said first transparent electrode being connected to said switching active element for driving said liquid crystal and said second transparent electrode being positioned opposite to said first transparent electrode; a shading layer (312) which is made of a conductive material, part of said shading layer being disposed under part of the first transparent electrode (308); and an insulating layer (314) which is interposed between said shading layer (312) and said first transparent electrode (308); a capacitance element (410) being defined by said shading layer (312), said first transparent electrode (308), and said insulating layer (314) interposed therebetween; and an additional capacitance element (330) positioned under said shading layer (312) and separated therefrom by an insulating layer (315); said additional capacitance element being defined by a polycrystalline silicon layer (330a), a gate insulating layer (330b) and a gate (330c) connected to a second gate bus (152). An active liquid crystal display cell as claimed in Claim 1, wherein said shading layer is made of aluminium and said insulating layer is formed by an anodic oxidation of said aluminium. An active liquid crystal display cell as claimed in Claim 1, wherein said shading layer comprises tantalum and said insulating layer comprises a layer formed by an anodic oxidation of said tantalum. An active liquid crystal display cell as claimed in Claim 1, wherein said insulating layer comprises at least two stacked layers. An active liquid crystal display cell as claimed in Claim 4, wherein at least one of said stacked layers is formed by an anodic oxidation of said shading layer. An active matrix liquid crystal display panel comprising a plurality of active liquid crystal display cells arranged in a matrix fashion, each of said active liquid crystal display cells being as claimed in any of the preceding claims.

Inventors: MIKOSHIBA HIROAKI, MIKOSHIBA, HIROAKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: G02F1
IPC codes: G02F1, G09G3