JPH05150260A - Active matrix substrate - Google Patents

Abstract

PURPOSE:To reduce leakage current from a picture element additional capacity part as one of the causes of defective inhomogeneous contrast between picture element units of an active matrix liq. crystal panel. CONSTITUTION:This active matrix substrate has a picture element electrode for driving a liq. crystal and an active switching element for driving the liq. crystal. Additional capacity is formed between a thin silicon film 2 electrically connected to the picture element electrode and an electric conductive thin film electrode 4 disposed on the film 2 with a gate SiO2 film 3 in-between. This capacity is independent of the capacity of the liq. crystal between the picture element electrode and a counter electrode. The silicon film 1 under the electrode 4 is doped with impurity atoms.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a pixel additional capacitance of an active matrix substrate used in a liquid crystal display.

[0002]

2. Description of the Related Art In an active matrix substrate having a thin film active element used in a conventional liquid crystal display, when an additional capacitor is provided independently of a pixel electrode, the additional capacitor is formed at the same time when a thin film transistor which is an active element is formed. It was Particularly in the staggered transistor, as shown in FIGS. 3 and 4, the silicon thin film 1 is formed on the glass substrate 16.
2 and 13 are formed by the CVD method, and after photoetching, a SiO ₂ film is formed by the oxidation, the CVD method, or the sputtering method. After that, the gate electrodes 19 and 15 were formed, and the source / drain portion 12 was ion-implanted by the self-alignment method using the gate electrodes. At this time, the additional capacitance is formed at the same time. The lower electrode 18 of the additional capacitor is formed by using the conductive layer silicon 20, 13 of the transistor. As the insulating film between them, SiO ₂ is used at the same time when the insulating film of the transistor is formed. The upper electrode 19 is formed simultaneously with the formation of the gate electrode 20. After that, similar to the ion implantation for forming the source and drain portions of the transistor, a dose of impurity atoms was performed to the lower portion 18 of the additional capacitance lower electrode. The structure of the additional capacitor using the above process is shown in FIG. 3, but the thin film silicon layer of the lower electrode below the gate electrode is in an intrinsic state in which impurity atoms are not dosed. Therefore, an inversion layer is induced in the lower electrode surface layer to which a voltage is applied to the upper electrode 19 to establish electrical conduction with the dosed portion 18 of the lower electrode. In this state, the function as an additional capacitor is activated to supplement the leak of the electric charge held in the pixel electrode caused by the leak current of the transistor or the leak current through the liquid crystal, and suppress the drop of the pixel potential. There is. 18 of FIG.
Shows the portion where the impurity atoms of the lower electrode of the additional capacitance are dosed, but it is in a state of entering several μm below the gate electrode by the activation heat treatment after ion implantation. Until the voltage applied to the additional capacitor upper electrode 19 reaches the threshold value, only the portion diffused under the upper electrode 19 functions as a capacitor. When the voltage applied to the upper electrode 19 is larger than the threshold value, the capacity corresponding to the pattern area of the upper electrode 19 functions. That is, the additional capacitance has a voltage dependency on the upper electrode 19. Since the pixel electrode potential changes according to the liquid crystal drive potential, the voltage applied to the additional capacitance changes according to the liquid crystal drive potential.

[0003]

However, the above-mentioned prior art has a problem that the leak current through the additional capacitance insulating film causes the liquid crystal holding operation to be hindered. In a small-sized (diagonal screen size of 2 inches or less) liquid crystal panel or a liquid crystal panel whose pixel pitch is 50 μm or less due to high definition, the ratio of the additional capacitance is larger than the liquid crystal capacitance, and the leakage current through the additional capacitance is large. It becomes a problem.

The potential of the lower electrode of the additional capacitor is equal to the liquid crystal driving potential, and the positive and negative potentials are alternately repeated with respect to the potential of the counter electrode as in the AC waveform. Since the upper electrode of the additional capacitance must always have a potential difference equal to or higher than the threshold voltage with the lower electrode, when the lower electrode potential is negative with respect to the counter electrode potential, a voltage exceeding the threshold voltage is added. It is applied to the capacitor, which is a disadvantageous situation for the leakage current. AC driving of the liquid crystal is indispensable from the viewpoint of the life of the liquid crystal, and is inevitable in terms of driving. When the leak current becomes large, the contrast becomes non-uniform on a pixel-by-pixel basis on the display, resulting in a defect in the inspection after the panel display, which causes a decrease in yield. In addition, it is very difficult to manage the leak current flowing through this insulating film, and the ratio of liquid crystal capacity to additional capacity is optimized at the design stage, and the leakage current of transistors and liquid crystal is kept low to allow a little holding operation. There is no way to deal with it. In terms of product planning, it was a technical bottleneck that made miniaturization and high definition difficult. Therefore, the present invention solves such a problem, and an object of the present invention is to provide means for reducing the leak current through the insulating film of the additional capacitance and stabilizing the holding operation of the liquid crystal drive. It is in the place of providing.

[0005]

An active matrix substrate of the present invention is a silicon thin film having electrical continuity with a pixel electrode and a liquid crystal capacitance between the pixel electrode and a counter electrode, the silicon thin film and the gate SiO 2. Liquid crystal driving pixel electrode and liquid crystal driving active switching, characterized in that an additional capacitance is formed between the conductive thin film electrodes via _two films, and impurity atoms are dosed in the silicon thin film under the conductive thin film. An active matrix substrate having elements.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The active matrix substrate for a liquid crystal display of the present invention is intended to reduce the leak current of the additional capacitance portion provided in the pixel, stabilize the holding operation, and eliminate the non-uniformity in contrast in pixel units. I am trying. A detailed description of the means will be given below.

The structure of the additional capacitor of the present invention is shown in FIG. The capacitor lower electrodes 1 and 2 are formed on the glass substrate 5, the insulating film 3 is formed thereon, and the upper electrode 4 is further formed thereon. The feature is that a thin film transistor (hereinafter referred to as T
It is shown as FT-ThinFilmTransister. That is, the P-type and N-type impurity atoms are selected and dosed according to the polarity of the impurity atoms. FIG. 2 is a view of the structure of FIG. 1 as seen from above, and FIG. 1 is a sectional view taken along line AA ′ in FIG. Reference numeral 10 represents a source / drain portion of the transistor, and 9 represents a gate electrode of the transistor. The transistor portion is different from the additional capacitance portion in that the conductive layer silicon-channel portion under the gate electrode is intrinsic and not dosed.
The upper electrode 6 of the additional capacitance portion is always set to a potential equal to or higher than the threshold voltage of the inversion layer, but the structure according to the present invention eliminates the voltage dependency of the capacitance and can form a stable additional capacitance. In the case of the conventional intrinsic structure, the leakage current is 2 × 10 ^-12 A / μm ² per unit area.
(At the time of applying 16.5 V), the structure of the present invention is 2 ×
It was possible to lower the value by 10 digits to 10 ⁻¹⁴ A / μm ² (when 16.5 V was applied). Poor pixel contrast non-uniformity often occurs at 10 ^-14 A / μm or more, and the margin is 2
Yield improvement can be expected in order to make a profit. In addition, it is impossible to find pixel contrast non-uniformity defects in the TFT substrate manufacturing stage, and only after the opposite electrodes are bonded to each other as a liquid crystal panel, liquid crystal is injected, sealed, and voltage is applied to turn on the halftone. This defect is discovered. In other words, the man-hours for assembling the non-defective counter substrate and the panel may be wasted, and it can be considered as one of the defects that reduce the product cost. The prevention of this defect by the present invention can be expected to be effective in reducing the product cost. The reason why the leak current causes the uneven contrast defect will be described in detail. There are mainly four types of leak current. One is the leak current through the transistor, and the other is the leak current through the additional capacitance. In addition, there are a total of four types: current leaking to the counter electrode through the liquid crystal and current leaking to the source line arranged in parallel with the pixel electrode. In either case, the electric potential accumulated in the pixel electrode flows out from the electric potential of the pixel electrode, the electric potential difference applied to the liquid crystal is reduced by the electric charge accumulated in the pixel electrode, and the light transmission amount of the liquid crystal layer is expected by the applied signal voltage. It will be different from the one. Since the difference is different among tens of thousands of pixels, when all the pixels are simultaneously observed as an image, the contrast becomes non-uniform, and it looks as if there are countless halftone point defects. 4 mentioned above
Among the kinds of leak currents, the ones that are determined depending on the physical properties of the leaking material are the ones through the liquid crystal and the leak of the additional capacitance which is the subject of the present invention. Until now, even if it was improved, the only way to change the physical properties was to develop the material. However, according to the method of the present invention, the leak current through the additional capacitance can be reduced with a slight process change.

Now, the process for realizing the additional capacitance structure of the present invention will be described in detail. A first-layer silicon thin film is formed on the glass substrate (quartz glass or the like) 5 shown in FIG. 1 by the CVD method (reduced pressure), and is patterned into islands by photoetching. The thickness is about 800Å to 1200Å. This silicon layer is the transistor portion 10 and the additional capacitor lower electrodes 7 and 8 shown in FIG. After that, an insulating film (SiO ₂ ) is formed on the above-mentioned silicon thin film (hereinafter referred to as a first-layer silicon thin film) in a gate oxidation process. The thickness of the insulating film is 1
It is about 100 Å to 1300 Å, and the oxidation method is dry oxidation. After that, a photo step is performed to leave the resist pattern only in the transistor portion, and then phosphorus or boron ions are ion-implanted to the entire surface of the first-layer silicon thin film. After that, the silicon thin film to be the gate electrode is depressurized C
It is formed by the VD method, and is patterned by the photoetching method into the shapes of the transistor gate electrode 6 and the additional capacitance upper electrode 9 shown in FIGS. After forming the source and drain of the transistor by ion implantation, the interlayer insulating film is formed by CVD or TE.
After forming a contact hole by photo-etching by forming OS, ITO (oxide of indium and tin) which is a pixel electrode is formed by a sputtering method, patterning is performed by photo-etching, and aluminum of a source line is formed by a sputtering method. Patterning is performed by the photo etching method. When the first-layer silicon thin film in the TFT thus formed is doped with impurity atoms, the current flowing through the gate oxide of the additional capacitance portion is smaller than that when it is not doped, resulting in non-uniformity in pixel unit contrast. Difficult

[0009]

According to the invention as described above, in an active matrix substrate having a liquid crystal driving pixel electrode and a liquid crystal driving active switching element, independently of the liquid crystal capacitance between the pixel electrode and the counter electrode, A silicon thin film having electrical continuity with the pixel electrode, an additional capacitance is formed between the silicon thin film and the conductive thin film electrode via the gate SiO ₂ film, and impurity atoms are dosed to the silicon thin film below the conductive thin film electrode. By doing so, the leak current flowing through the additional capacitance portion is reduced by two digits compared with the conventional structure, so that it is possible to prevent a pixel contrast nonuniformity defect from occurring.

[Brief description of drawings]

FIG. 1 is a sectional view of a pixel additional capacitance structure of the present invention.

FIG. 2 is a plan view of a pixel additional capacitance structure of the present invention.

FIG. 3 is a sectional view of a conventional pixel additional capacitance structure.

FIG. 4 is a plan view of a conventional pixel additional capacitance structure.

[Explanation of symbols]

 1 Overlay portion with additional capacitance lower electrode gate electrode 2 First layer silicon thin film 3 Gate oxide film 4 Gate electrode 5 Glass substrate 6 Gate electrode 7 Overlay portion with additional capacitance lower electrode gate electrode 8 First layer silicon thin film 9 Gate electrode 10 Transistor Source / Drain Part 11 Transistor Channel Part 12 First Layer Silicon Thin Film 13 Overlapping Part with Additional Capacitance Lower Electrode Gate Electrode 14 Gate Oxide Film 15 Gate Electrode 16 Glass Substrate 17 Overlapping Part with Additional Capacitance Lower Electrode Gate Electrode 18 1st Layer Silicon thin film 19 Gate electrode 20 Transistor source / drain portion A cross section taken along one-dot chain line AA 'in FIG. 2 is shown in FIG. A cross section taken along one-dot chain line BB ′ of FIG. 4 is shown in FIG.

Claims (1)

[Claims] 1. An active matrix substrate having a liquid crystal driving pixel electrode and a liquid crystal driving active switching element, the silicon having electrical continuity with the pixel electrode independent of a liquid crystal capacitance between the pixel electrode and a counter electrode. An active matrix characterized in that an additional capacitance is formed between a thin film, the silicon thin film and a conductive thin film electrode via a gate SiO ₂ film, and impurity atoms are dosed to the silicon thin film below the conductive thin film electrode. substrate.