Liquid crystal display device

Abstract: Two contact holes (21, 22) are formed through an insulating layer (10) at its portion corresponding to a drain (5D) of a switching thin film transistor (2) and a height of a wall portion (26a) between the two contact holes is selected to be smaller than that of a surrounding insulating layer (26), whereby the transparent electrode (1) is connected to the drain via the two contact holes with excellent coverage. Thus, a connection between the switching transistor and the transparent pixel electrode in the liquid crystal display device can be effected with improved reliability.

Description: BACKGROUND OF THE INVENTION The present invention relates to a liquid crystal display device according to the precharacterizing part of claim 1. Such a liquid crystal display device is known from "Liquid crystal TV-displays", E. Kaneko, KTK Scientific Publishers, 1987. Description of the Prior Art The structure of one pixel of a conventional active matrix type liquid crystal display device is shown in Figs. 1 and 2. In Fig. 1, reference numeral 1 designates a transparent pixel electrode constituting a pixel (a liquid crystal cell (LC)), and 2 a switching thin film transistor which drives the pixel. A selecting line 3 for selecting a row of the respective pixels (LC) is arranged between respective rows of the pixel electrodes 1, while a signal line 4 for supplying a video signal is arranged between respective columns of the pixel electrodes 1. Then, a drain 5D of the thin film transistor 2 is connected to the pixel electrode 1, its source 5S is connected to the signal line 4, and its gate is connected to the selecting line 3 (in Fig. 1, the gate 5G and the selecting line 3 are used in common). The specific structure thereof is as shown in Fig. 2, wherein a polycrystalline silicon layer 8 constituting the thin film transistor 2 is deposited at a required location on an insulating substrate 7 made of glass or the like, and on this polycrystalline silicon layer 8 there is formed the gate electrode 5G (or the selecting line 3) made of, for example, an impurity doped polycrystalline silicon through a gate insulating layer 9. A first insulating layer 10 made of, for example, a PSG (phospho-silicate glass) layer is formed over the entire surface of the gate electrode 5G so as to cover the same, and through a contact hole 12 formed through the first insulating layer 10 the signal line 4 made of, for example, Aℓ is connected to the source 5S. Further, a second insulating layer 11 made of, for example, a PSG layer is deposited on the entire surface. Through, the first and second insulating layers 10 and 11, there is formed a common contact hole 13 at a position corresponding to the drain 5D, and then a transparent conductive layer made of, for example, an ITO (indium tin oxide) layer is formed on the entire surface including the contact hole 13. A patterning is performed therefor to form the transparent pixel electrode 1 which is connected to the drain 5D through the contact hole 13. Then, another insulating substrate 14 made of glass or the like which has a light shielding layer 15 formed in its inner surface at positions corresponding to a wiring portion (a portion where the selecting line 3, the signal line 4 and so on exist) and the thin film transistor 2 and an opposite electrode 16 formed on its entire surface including the light shielding layer 15 is disposed in opposition to the insulating substrate 7. A liquid crystal layer 17 is then enclosed between both the substrates 7 and 14 to thereby constitute a liquid crystal display device 18. Fig. 3 shows another example of the conventional liquid crystal display device. In this example, the thin transistor 2 formed of the polycrystalline silicon layer 8, the gate insulating layer 9 and the gate electrode 5G (i.e. the selecting line 3) is formed on the insulating substrate 7 on which the insulating layer 10 is formed. In the next process, the transparent pixel electrode 1 is formed by the ITO layer so as to be connected to the drain 5D of the thin film transistor 2 via the contact hole 13 of the insulating layer 10, and the signal line 4 made of, for example, Aℓ is connected to the source 5S of the thin film transistor 2 via the contact hole 12 of the insulating layer 10. Further, in the liquid crystal display device, in order to improve the pixels, additional capacitance i.e. storage capacitance C S must be added to every pixel LC, and it is preferable that the value of the storage capacitance C S is as large as possible in order to prevent the occurrence of flicker. To this end, as, for example, shown in Fig. 4, it is proposed that a polycrystalline silicon layer 8 is elongated from the source 5D and the storage capacitance C S is constructed between an elongated portion 8A of this polycrystalline silicon layer 8 and an electrode wiring 19 for exclusive use of the storage capacitance C S interconnected via an insulating layer (not shown). One of the problems implied in the above-mentioned prior-art liquid crystal display device is the connection between the drain 5D of the thin film transistor 2 and the transparent pixel electrode 1. Due to the convenience for the manufacturing process, the polycrystalline silicon layer 8 of the thin film transistor 2 is formed in a lower portion, and the transparent pixel electrode 1 made of an ITO layer is formed on the topmost layer, whereby both are connected through the contact hole 13. However, the height h of this contact hole 13 has approximately 1 µm, which makes it difficult to connect the ITO layer 1 having a thickness of approximately 0.1 µm to 0.15 µm (film thickness because of the transparent pixel electrode) and formed of ITO layer with a favorable coverage. Therefore, the contact hole 13 is formed in a taper shape to provide an easier coverage, however, it cannot be said that the situation is 100% satisfactory. Further, if the contact hole is made smaller (e.g. 0.5 µm to 1.0 µm) in order to increase an opening ratio of the pixel, then it becomes difficult to interconnect the ITO layer 1 with an excellent coverage, and the contact resistance between the drain 5D and the ITO layer 1 is unavoidably increased by the amount corresponding to the reduced area of the contact hole. Particularly, since the ITO layer 1 contains oxygen, the silicon surface of the contact portion with the thin film transistor is naturally oxidized, thereby the ohmic contact being deteriorated. If the contact hole is made small, reliability of contact is considerably scattered, which results in the concentration of display pixel being scattered. On the other hand, in a liquid crystal display device in which the storage capacitance C S is provided, if the electrode wiring 19 used exclusively for the storage capacitance C S is made of the same conductive material as that of the gate electrode 5G, that is, the selecting line 3, then the opening ratio of the pixel is sacrificed for the provision of the electrode wiring 19. Accordingly, the contact hole must be made as small as possible but if the contact hole is made small, the contact between the transparent pixel electrode and the ITO layer is lowered in reliability. Then, there arise similar problems mentioned above. A defective contact between the transparent pixel electrode 1 and the thin film transistor 2 appears as a point defect on the liquid crystal display device, resulting in the image quality being deteriorated. Fig. 7.57 and the text of page 270 of the above cited "Liquid crystal TV-displays" disclose a liquid crystal display device comprising a plurality of pixels each arranged with a respective switching transistor and lined in a matrix form, each said pixel comprising a transparent pixel electrode (ITO) and an insulating layer between said transparent pixel electrode and an electrode of the respective switching transistor, and a contact hole in said insulating layer through which said transparent pixel electrode and said transistor electrode are connected. This known liquid crystal display device corresponds to the prior art device depicted in Fig. 2 of the present application and described above. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a high quality liquid crystal display device wherein the transparent pixel electrode is connected to the thin film transistor so that defective contacts, in particular point defects can be eliminated. The above object is, according to the invention, solved by a liquid crystal display device comprising a plurality of pixels each arranged with a respective switching transistor and aligned in a matrix form, each said pixel comprising a transparent pixel electrode and an insulating layer between said transparent pixel electrode and an electrode of the respective switching transistor, characterized by a plurality of contact holes in said insulating layer through which said transparent pixel electrode and said transistor electrode are connected (claim 1). Depending claims 2 to 4 are respectively specifying advantageous developments thereof. The above, and other objects, features and advantages of the present invention will become apparent in the following detailed description of illustrative embodiments thereof to be read in conjunction with the accompanying drawings, in which like reference numerals are used to identify the same or similar parts in the several views. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view of one pixel of a conventional active matrix type liquid crystal display device; Fig. 2 is a cross-sectional view taken along the line XIII to XIII of Fig. 1; Fig. 3 is a cross-sectional view of another example of the conventional active matrix type liquid crystal display device; Fig. 4 is a plan view of one pixel showing other example of the conventional active matrix type liquid crystal display device; Fig. 5 is a cross-sectional view illustrating a first embodiment of the liquid crystal display device according to the present invention; Fig. 6 is a cross-sectional view showing a method of forming contact holes; Fig. 7 is a cross-sectional view showing a main portion of a second embodiment of the liquid crystal display device according to the present invention; Fig. 8 is a cross-sectional view showing a main portion of a third embodiment of the present invention which improves the light transmittance in a contact portion of the transparent pixel electrode in the liquid crystal display device; Figs. 9A through 9D are process diagrams showing an example of the manufacturing method of Fig. 8, respectively; Fig. 10 is a graph showing the light transmittance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described with reference to the accompanying drawings. Fig. 5 shows a first embodiment of an active matrix type liquid crystal display device according to the present invention. Fig. 5 illustrates the structure of one pixel, wherein like parts corresponding to those in the foregoing Fig. 2 are marked with the same reference numerals. More specifically, reference numeral 1 designates a transparent pixel electrode made of, for example, an ITO layer, and 2 a switching thin film transistor. The thin film transistor 2 is constructed such that a polycrystalline silicon layer 8 is formed at a required location on an insulating substrate 7 made of glass, and a gate electrode 5G or a selecting line 3 made of, for example, an impurity doped polycrystalline silicon layer is formed on the polycrystalline silicon layer 8 through a gate insulating layer 9. The source 5S of the thin film transistor 2 is connected to a signal line 4 made of, for example, Aℓ through a contact hole 12 formed through a first insulating layer 10. The transparent pixel electrode 1 is formed on a second insulating layer 11 in a predetermined pattern, one end of which is connected to the drain 5D of the thin film transistor 2. Opposite to this insulating substrate 7, an insulating substrate 14 made of glass having a light shielding layer 15 and an opposite electrode 16 formed on an inner surface thereof is disposed, and a liquid crystal layer 17 is enclosed between both the substrates 7 and 14. Thus, in the first embodiment, in the connection of the drain 5D of the thin film transistor 2 with the transparent pixel electrode 1, a plurality of, in this embodiment, two contact holes 21 and 22 are formed through the two insulating layers 10 and 11 at a position corresponding to the drain 5D. The two contact holes 21 and 22 are respectively rectangular in plan view and adjacent to each other. A wall portion or an insulating layer 26a between both the contact holes 21 and 22 is formed to have a height t 1 lower than a height t 2 of a surrounding insulating layer 26 made up of the first and second insulating layers 10 and 11. For example, when the height t 2 of the surrounding insulating layer 26 formed of the two first and second insulating layers 10 and 11 is chosen to be 1 µm (the thickness of the first insulating layer 10 is 0.6 µm and the thickness of the second insulating layer 11 is 0.4 µm), the height t 1 of the insulating layer 26a between the contact holes 21 and 22 is chosen to be approximately 0.5 µm. Also, a length ℓ1 between the contact holes 21 and 22 is below 10 µm or 2 µm in this embodiment, and the width of each of the contact holes 21 and 22 is respectively chosen to be 5 µm in this embodiment. Then, the transparent pixel electrode 1 is made to be connected to the drain 5D of the thin film transistor 2 through these two contact holes 21 and 22. Such contact holes 21 and 22 can be formed by utilizing a side etching. More specifically, as shown in Fig. 6, the upper second insulating layer 11 is formed of a layer which is etched at a speed higher than the lower first insulating layer 10. When an etching is performed for the insulating layers 11 and 10 through a resist mask 25 having two openings 23 and 24 adjacent to each other, the insulating layer between both the openings 23 and 24 is removed by the side etching, with the result that both the contact holes 21 and 22 are formed by a single etching process, and the height t 1 of the insulating layer 26a between both the contact holes 21 and 22 is formed lower than the height t 2 of the surrounding insulating layer 26 made up of the two insulating layers 10 and 11. Further, by the side etching-process, the contact holes 21 and 22 are tapered in such a manner that the widths thereof become narrower as they become near the contact portions with the thin film transistor 2. More specifically, the first insulating layer 10 is assumed to be a PSG layer heat-treated at temperatures more than 800 °C, while the second insulating layer 11 is a PSG layer without heat treatment. Then, etching these layers through the resist mask 25 by an effervescent solution, the PSG layer heat-treated at temperatures more than 800 °C is etched at a speed faster than the PSG layer without heat treatment, which results in forming the contact holes 21 and 22 as shown in Fig. 6. Alternatively, assuming that the first insulating layer 10 is made of a PSG layer and the second insulating layer 11 a plasma SiN layer and they are etched through the resist mask 25 by a plasma etching method, a side etching is advanced in the plasma SiN layer at a speed approximately 10 times higher than the PSG layer, which results in forming the contact holes 21 and 22 as shown in Fig. 6. According to the liquid crystal display device 27 constructed as described above, the transparent pixel electrode 1 is connected to the drain 5D of the thin film transistor 2 through the two contact holes 21 and 22, whereby the probability of occurrence of defective contact can be reduced in comparison with the conventional case where they are connected through a single contact hole. Further, since the height t 1 of the insulating layer 26a between the two contact holes 21 and 22 is made lower than the height t 2 of the surrounding insulating layer 26 made up of the first and second insulating layers 10 and 11, the transparent pixel electrode 1 extends to the higher insulating layer 26 above the insulating layer 26a made lower. Stated other way, the step caused by the contact holes 21 and 22 is apparently softened, whereby the coverage of the transparent pixel electrode 1 becomes satisfactory, and the defective contact is eliminated. Thus, the occurrence of point defect on the liquid crystal display device is removed, and accordingly a high quality liquid crystal display device is provided. Although the contact dimension is made more or less large for forming the two contact holes 21 and 22, the polycrystalline silicon layer 8 constituting the thin film transistor 2 is approximately 0.04 µm which is thin enough to transmit a light, so that a loss of light is small. On the other hand, as shown in Figs. 1 and 2, the polycrystalline silicon layer 8 included in the contact portion of the transparent pixel electrode 1 causes an inferior light transmission therethrough in comparison with other portions of the transparent pixel electrode 1, which may incur a fear that a substantial opening ratio of the liquid crystal cell is decreased. Since a smaller dimension of the contact portion may cause inconveniences such as an increase in the contact resistance, a deteriorated coverage of the transparent pixel electrode 1 in the contact hole 13, and so on, the dimension of the contact portion cannot be reduced easily. A third embodiment for improving this point is shown in Fig. 8. In this embodiment, between a polycrystalline silicon layer 8 constituting a thin film layer 2 and an insulating substrate 7 made of glass, there is formed a layer having an intermediate refractive index n 3 between the refractive index n 1 (= 3.8) of the polycrystalline silicon and the refractive index n 2 (= 1.45) of the glass substrate (SiO 2 ), for example, an Si 3 N 4 layer 30 (refractive index n 3 = 1.9), wherein the transparent pixel electrode 1 is connected to the drain 5D of the polycrystalline silicon layer 8 through a contact hole 13. This Si 3 N 4 layer 30 becomes a so-called reflection preventing layer. The rest of the structure is similar to Fig. 2. An example of a manufacturing method thereof is shown in Figs. 9A to 9D. On the insulating substrate 7 made of glass, there is deposited an Si 3 N 4 layer 30 having a thickness of approximately 0.07 µm (Fig. 9A) and, the surface of the Si 3 N 4 layer 30 is slightly oxidized to form an SiO 2 layer 31 (Fig. 9B). A heat treatment, for example, in an oxidizing atmosphere at a temperature of 1000 °C for 20 minutes is performed to form the SiO 2 layer 31 of approximately 0.001 to 0.002 µm. This surface oxidization allows the next polycrystalline silicon layer 8 to be smoothly deposited. Next, after forming the polycrystalline silicon layer 8 having a thickness of, for example, approximately 0.04 µm on the SiO 2 layer 31 (Fig. 9C), the polycrystalline silicon layer 8, the SiO 2 layer 31 and the Si 3 N 4 layer 30 are patterned through a resist mask 32 by, for example, a CF 4 plasma etching (Fig. 9D). Thereafter, the thin film transistor 2, the transparent pixel electrode 1 and so on are formed by way of an ordinary process identical to Fig. 2. An optimal thickness of the Si 3 N 4 layer 30 is different depending on the thickness of the polycrystalline silicon layer 8. A curve I in Fig. 10 shows the light reflectivity when a light is incident from the substrate 7 side, wherein the thickness of the Si 3 N 4 layer 30 is assumed to be 0.07 µm, and the thickness of the polycrystalline layer 8 is assumed to be 0.04 µm. However, a curve II shows the reflectivity when the Si 3 N 4 layer 30 is not included. As is apparent from Fig. 10, when the Si 3 N 4 , layer 30 is formed beneath the polycrystalline silicon layer 8, the reflection by the polycrystalline silicon layer 8 is decreased by approximately 30 %. Thus, in the active matrix type liquid crystal display device, the formation of the Si 3 N 4 layer 30 beneath the polycrystalline silicon layer 8 constituting the thin film transistor 2 results in enhancing the transmittance in the contact portion with the transparent pixel electrode 1 and elevating the substantial opening ratio of the liquid crystal cell. It is needless to say that the present technique can be applied to the liquid crystal display device according to the present invention where the transparent pixel electrode 1 is connected through the aforementioned plurality of contact holes 21 and 22 as shown in Fig. 7. According to the liquid crystal display device of the present invention, it is possible to reduce the probability of occurrence of defective contacts by connecting a transparent pixel electrode to a thin film transistor through plurality of contact holes. Further, by making the height of an insulating layer between the plurality of contact holes lower than the height of a surrounding insulating layer, the coverage of the transparent pixel electrode in a contact portion becomes satisfactory, and defective contacts are eliminated. It is therefore possible to provide a high quality liquid crystal display device without point defects in the liquid crystal display device. Accordingly, the point defect inherent in the liquid crystal display device can be removed, and the liquid crystal display device of high display quality can be obtained. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that various changes and modifications thereof could be effected by one skilled in the art.

Claims: A liquid crystal display device comprising a plurality of pixels each arranged with a respective switching transistor (2) and aligned in a matrix form, each said pixel comprising a transparent pixel electrode (1) and an insulating layer (26) between said transparent pixel electrode and an electrode (5D) of the respective switching transistor, characterized by a plurality of contact holes (21, 22) in said insulating layer through which said transparent pixel electrode and said transistor electrode are connected. The liquid crystal display device according to claim 1, in which a height t 1 of the portion of said insulating layer that separates the respective contact holes of said plurality of contact holes is selected to be smaller than the height t 2 of the remainder of the insulating layer (26). The liquid crystal display device according to claim 1, in which said plurality of contact holes are tapered so as to reduce their widths toward the contact portion with each of said switching transistors. The liquid crystal display device according to claim 1, in which an insulating substrate and said switching transistors have a reflection preventing layer therebetween.

Inventors: HAYASHI HISAO, KAWAMURA AKIO, HAYASHI, HISAO, KAWAMURA, AKIO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G02F1
IPC codes: G02F1, H01L29, H01L21